Basic Biomechanics of the Musculoskeletal System-3rd Edition

, 78.3 E 7.9 J\\I\\I~I~\\~I\\ ~ llmlm 1111111 Ill\\/II\\ T~!! !i~i I~i~ -·32.4 -12578.1.6-, '::======::;:======::;====================~ 40.9 21.6 ;111111 III I1II1 1/111/1111\\ 2.3 +--.,--- 15.4 '''''1-'--,'--'1-r,'-'' I-'-'I--''--'--Ir-,TI--...,--1,-+-i-.....-r-1T,-,I--\"--\"\"I-,rl 0.1 9.3 18.4 27.5 36.6 45.7 54.7 A ROlation Torque and Position vs. Time 4.3,.-------------------------------, , 2.3 zE, \" 1.1 ~ !! .0... -0.6 -2.1 19.1 ,w 13.3 \"c~o '\"i' 7.0 c .Q .~ 0 0.7 Q. - 5. 1 -!--,----,-.-,.--...,--,-.,..--,----,-.-,.--...,--,-.,..--,----,-'-.-,.--...,--,,-.,..--,----,--I 0.1 9.3 18.4 27.5 36.6 45.7 54.7 B Dynamic (isoinertial) flexion-extension trunk testing until Data for lateral flexion was similar to axial rotation a exhaustion for one subject. Torque and position data is de- not shown here. Adapted with permission .from Parnianp picted for two planes, flexion-extension (A) and axial rota- M .. Nordin. M.. Kahclnovitz, N.. er al. (1988). The rriaxial tion (B). Note that flexion-extension torque production is diminishing as is the amount of performed extension of coupling 0; torque generarion of trunk muscles during iso the trunk (A). Rotational torque and movement amplitude, increased accessory motion, and torque is shown in B. ric exertions and the effect of fatiguing isoinercial movem on the motor outpur and movemeor pc1tterns. Spine. 13(9 982-992

Investigation into the effect of back bells on on the spine to bc minimized during lifting, the d muscle activity has revealed no significant EMG tance bL'lween the trunk and thc object lifted shou activity differences in the back extensors during be as short as possible. Iifling with or withoul a back bell (Ciriello & Snook. 1995; Lee & Chen. 1999), while McGill el al. 10 lAP and co-contraction of trunk musculatu (1990) showed slightly increased EMG aClivily in increases the slability of the spinal column. the abdominal (except for the intel'l1al oblique l11uscles) and erector spinae muscles. Thomas et '.11. 1/1' Trunk muscle fatigue may expose the spine (1999) have verified a slight increase in EMG ac- increased vulnerability as a result of loss of' mot tivity (2(10) in the erector spinae during symmetric control and thereby increased stress on the su lifting wilh a back belt. Back bells have not been rounding ligaments, disc, and joint capsules. shown to significantly increase Iifling capacity (Reyna el aI., 1995). ,12 vValking is an excellent exercise thai poses low load on the lurnbal' spine. Summary REFERENCES 1 The lumbar spine is a highly intricate and com- Abt.:nhairn, L., Rossignol. ~'t., V~dal. J.P., l't a1. (2000). Thc r plex structure_ of acti\\'ily in the tht.:r:q1clllic management of back pa Report of thl: Inll:rn:l1ion~1 Paris Task Force on Back Pa 2 A vertebra-disc-vertebra unit constitutes a mo- 5pinc. 25. IS. lion segment, the [unctionalunit of the spine. Ad~lillS, M.A.• Dolan, P.. & 1·lutlon W.e. (1988). The lum , 3\" The intervertebral disc scrves a hydrostatic spin\\.' in b~tChnlrd bl·nding. Spille. 13. 1019 function in the motion segmcnt, storing energy and distributing loads. This function is reduced with Atbms. ~L\\. & Hulton, W.C. (19831. The 1lll.:ch;lIlicill fu disc dcgeneration. tions of Ihe lumbar apophyst.\\ljoints. Spil/e. 8, 327. ~:\"4 The primary function of the facet joint is to Alkn, C.E.L (1948). i\\lusdc action potenlials used ill guide the motion of the mOlion segment. The orien- sludy of dynamic analolll~', 13r J Phy.~ Jtcd, 11,66. tation of the facets determines the type of motion possible al any level of the spine. The facels may Anders()Jl, C.K., Chaffin, D.B., Hcrrin, G.D .. ct :11. (1985) also sustain compressive loads, particularly during biolllcchanical model of the lumbosacral joint during l hyperextension. ing ~\\cli\\'ities. J BiollH'dllllll·I.:S. 18, 57. S Motion between two venebrae is small and :\\lHkl'sson. E.A .. Oddsson, L.I.E .. Grundstrom, 1-1., ct docs not occur independentl~y in vivo. Thus, the functional motion of the spine is always a combined (1996). E;\\'lG acti\\'ities or thl' qlladnHlIs lumborum ~ action of sevel',,1 motion segments. erector spinae musdes during flexion-relaxation and ot /6;- The instantaneous center of motion for the mo~ motor lasks. CJinial! Bio1llcdUlllics, JJ, 392. lion segments of the lumbar spine lIsually lies within the lumbar disc. Andersson, G.B.J. & l:l\\·cnd\"r. S.A. (1997). Evalu;ltion The trunk muscles play an important role in Muscle Function. In J.W. Frymoycr (Ed.). Tilt' A(/1I11 Spi providing extrinsic stability to the spine; the liga- Pril1c.:ip{es (lilt! Praclict: (2nd cd., pp. 341-3$0). New Yo ments and discs provide intrinsic stability. Li ppi ncot t-Ra\\·l:n. Andersson. G.B.J .. Ortengrcn. R.. 0:: Nachcmson. r\\. (197 8 Body position affecls the loads on the lumbar spine. Any deviation from upright relaxed standing orQuantitati\\-c studies b~lck loads in lifting. Spillt:. I. 1 increases the load. Forward flexion and simultane- ous twisting of the trunk produce high stresses on Andersson. G.B.J .. Oncngren. R., & Nachcmson. r\\. (197 the lumbar spine. lntr'ldiscal pressure. intr~\\·... bdomin ...1 prl.'SSUrl' ilnd m 9 EXlernally applied loads lhal are produced. 1'01' det:tric bad lllusck :ll:tivity rdated to pOSl\\lrc <lnd lo example. by lihing or carrying objects Illay subject ing. Clill OrtJlOp, 129, 156. the lumbar spine to very high loads. For the loads Andersson, G.B.J .. Onengl'cn, R., Nachclllson, A., et (1974). Lumbardisc pressure and myoelectric b\"H:k mus ~\\clidty during sitting. 1. Stu,dics 011 an experimen chair. SC(/I/d J Rclwhil ,Ht.:d, 6, 104. ,\\sl11usscn, E. & Klauscn, K. (1961). Form and fUllction or (.'rcc( human spinc. eJill OrlllOp, 25. 55. Axlel\". c.T. & McGill. S.M. (1997). Low back loads o\\·cr'l \\' ...'tv of abdomin.;\\1 e.'\\en.:ises: SeillThing for th..., s;lfcst d~l1lin;l1 challeng(·. .\\lcdiefl/I: & Sciem\"\":c ill Sports & E cise. 29, 804 B:II·tclink, D.L. (1957). Thi.: role of i.lbouminal prC'ssure in lieving the pressure on thl.' lumbar inu:n'crlcbral disc BOlle }oilll Slfr~, 39B. 718. B:ISIlWji.lll. J. V. (t958). Electromyography of iliopso'ls. :l Rcf.:, 131. 127. B~sl1laji~n. J.V. & DeLlll;;l, C.J. (1985). ;\\1l1sdc:s Alive. Ba mort: Williams &. Wilkins.

BicrilH!·SorenselJ. F. (1984). Plwsical nh.'<lsurcmenls as ri~k Eklwllll. J ....\\rhorelith. L·. F~lhlcr:Ill!I: ...\\ .. l·t :11. (lYiY \\';lli(11I (If :,hd(llHin:lllllu:-clc:- during SOli\\\\,.' ph.\\-sihl!ll, indlcalors for I()\\\\'-b~\\ck lrol;bll.' over a on<:-yl.'ar period. Spill!.'. 9, 106. til,.' c\\l,.'l\"ci .. l,.·!-. SnlJld) I?dwhil.l1t:d. II. 7.~. Callaghan. J.P.. P'IIla, .\\..E .. & McGill, S.\\\\. (1999). Low back 101 U\"hy. ,1.,.1, \" K;n~, ..1.1. (1 'J~6j. Inlcn,'rlchr,d d; ·f. thrcc--dimellsional joinl forces. killl.'malics. and kinetics f:l(\"l'! l,.·ollta(\"\\ prl,.·ssufl· in a:\\i~tll(lr!-i(1l1. In S.:\\. l.anl during walking. Clill;cal BioIJlC(://(lllics, I';', 203. King (Ed:-.). /9SfJ :\\d\\'/I/H\"\".~ ill Uilh·lI:.;illt'I'l'ill.:: (pp. or:\\l,.'\\\\' York: . \\!l1l'l\"icall SOl,.·iL·\\\\ \\In:h:\\lliL':d Ellginl:( Cappozzo, A. (1984). Compressive loads in the lumb'lr vent:- fir,..;.' br.1! column during llorlllal le\\'el walking. J Or/hop Res, l, r:ad~ln.ll.F. (19751. \\ltlsl,.·III~II' Jlh: .... h;llli .. 1lI the lUJ11k 192. ~tl\\d I Ill,.' P(,:>ilioll of Pfl\\\\\"l'!\" and dficil·/lL·..... On/H Carlsoo. S. (1961). Thl.' st .. tic muscle load in differelll work Xorth . \\IlI. 6, 135. posilions: All dL'ctromyographic study. Ergollolllic.,·. .J.. 193. Flinl . .\\1..\\1. (1965J. Ahdominal ll1u:>dL' in\\'uln'l1ll'lll Chaffin. D.B. (1969). r\\ cornputcriz.cd biolllechanical modcl- thl' pl,.-rfnrm:lllt.·l,.· of \\·.lriol1~ forms 01 :.it-up l,.·\\l,.·n; Dc\\'dopmcl11 of and usc in stud~'ing gross body <lctions. ) l,.·k('tr(Jm~·hgr'lphic :-llld~·. _'\\1lI J Phys .\\I,·d. ';'4. 114. '*BiOiI/Cc!wl/ics, 2, 429. Fln\\'d. \\\\'.F. <.\\: Silv ... r, P.fl.S. (193.:;). Thl' fUJll,.'linll (Ii lll lors spinal,.' Inllsl·k\" in cnl~lill 111()\\clll ... n\\S ~llld post Chaffin, D.B. Andersson. G.BJ. (1991). Occup(1tiol/al Bio- I/wcJ/(lIli<:s (2nd cd .. pp. 171-263\". New York: 10hn Wile.\\' 6.: man. J NI.niol. 129. 1$4. Sons. Inc. Fri(·dd)old. G. (195~J. Dil,.· ..\\kti\\·iL'1 nOI·lIl.da Rih:h·n: Cheng. c.1\\: .. Ch..:n. H.H .. Chen, C.S., et 31. (1998). Influcnces rnuskublur illl Ek,ktfom ....ogralllln ullll'r n'rschi of w~dking spce:d change' on the lumbosacral joint fon:c !·Iallllllgsik·dillgungl,.·n: l,.·illc Studil' distribulion. BiOl1lt'd Mara £11::', S, 155. Skl·kllllltlskdnwchanik. I Of/hop. 90. I. Choh:wicki. 1.. Juluru. K.. &: ~-IcGill, S.l\\-l. (1999a). Intra-ab- rukll\\:tlJl:t, S .. :\\,tbmur:l, T. nl·d~l. T.. l't ,d. (1995). dominal prt:ssun: ll1(.'chanisrn for stabilizing tll(.' lumbar k,(t of Itlc(!1:ulic.t! slrl's;,,; 011 hvpntroph\\' o!\" 11k spine.) Bio/lwch, 32. 1-13. li,\\;;II11l'!lIUIll Il:l\\'tllll. ) .')pilluIIJl'.-nn/. S. 126. Cholcwicki, 1., luluru, K .. Radt:bold. :\\., I:t <II. (1999b). Lum- Gatlilk. J.O. (1<)67). h'n:>ilc propl'nil's 01 11K hUIll;11l b.ll· spinc stability (;'til be :lllgmcnted wilh an abdominal :lIl1lUIIlS fihrh:->II:-, ..lew O,.lIwJ' S,'lmd SII1'''!. 100, 1- bcli and/or increased inlra-abdominal presslIrt:. f.:'llropcall G;lrdll~'r·.\\I(.tr~l,.·. .\\I.G. & S!('Kl,.· .... LA. (19%l. Till,.' dfl'({~ SJlill~ )ollrJ/al. 8, 388. dominal lllu:-dl' CI,·;ll·ti\\:ttic.n on lumbar :-pilll' :- Choh.'wicki, J., P.mjnbi, ~L\\L & f\\.h;lChntr~·all. A. (199i). Sta· S/Iill... II::!,'), ~9. bilizing function of trunk f1e.'l:or·('.'I:tcnsor llHlsch. s around G(·rtl.lkill. S.D .. SI.·liglllt,IIl,.I. Hl.lltln, R....... l .d. (1%5 <I neutral spine posture. Spine. 22(19), 1207. trndl' P~ltlL'l\"llS :lnd sl'glllL'Il!,lI il1Sl~d)i1il.\\ ill ,kgL'l Ciriello, \\'_,\\I....\\: Snook, S.H. (1995). The dfl.-·Cl of b<lck belts di:,\\.· disl':l:-e. S\"i!/,·. 10. 257. 011 lumbar muscle fatiguc_ Spillt', 20, 1271. Granat'l. K.P. .i: Sanford. :\\.1-1. (19991. LUll1har-pd\\'k n~tli(ln i:> influl'IH.·l,.·d b~ lifting l~\\sk par~\\nh.'kr:>. COSSl.'ltc. J.W., Farfan, H.F.. Robertson, G.H., e[ at. (19il). The instant~1I1l.'OUS <:enttr of rot~l[ion of Iht.' third Illlllb,lf 25(111.1413. inll.'ITl.'nebral joint. J Biol/m.:It . .;.( 149). 19i 1. Grl,.'gCl\":>l,.'Il, G.G . .i Luc~t~. D.l3. (1967). An in \\'inl slud.\\ Creswell, A.G. (1993). Rl.'sponses of intnl-abdorninal pressure lUlllbar a,xi:\\1 rot;lliU!l (If thl' hlllll~\\n Ilwrac(Jllllllb:\\ and ,Ibdomin~ll muscle activity during dynamic trunk load- .l 8011,' .luil!! Sur:: . .,19:\\. 2-+7-232. ing in man.l::ur) Appi Physiol, 66,315. llaht.:r. T.IC. O·llrit:lI ..\\1.. Dr.\\·a, .1.\\\\'\".. d 'II. (l\\FHI. TIll Crt.·sswcll. A.G .. Blakt.'. P.L.. &. Thofstcnsson. A. (1994<1). The till' IUlllb.\\I· f,ll,.\"l,.'! joinl::- ill spinal :-tahilil~·. IllcntiliGt drect of an ahdomin.d muscle Iraining program 011 ill[ra- :illtTn:niH' palh~ of 1(1~\\{lillg. Sl'ilh·. /9. 26-67. abdominal pressure. Scant!) RC/Ulbil ;\\ld. 26, 79. Hannan. E...\\., Fr~\"klll:lll. P.:\\ .. Clagclt. E.R ....... t ~tl. (19SS Cr~ss\\\\\"dl. kG .. Grundstr6m. 1-1 .• 6.: Thorslcllsson. :\\. (1992). ,.hd(llllinal ,tlld in!r:l·[hor~lL'ii.' prt'-,,~uro during lift Observations on intr<I-.llnlomillOlI preS$ur..: and p~lllcrns of jumping . .\\led Sci S;10/\"0 Erac 20. 195. abdominal intra-muscular acti\\'it~· in man . ..lcta Physiol Hasq:;\\\\\\:l. K.. T~\\bhashi. IU_:. \"Og~l. Y.. l,.'1 :1I. (199 Scaml, 1';'4. 409. Ch'Hlic:d propl,.,rti(·s or ostc'olwnic \\,t:rtdn:d hodk's Crcsswdl. A.G .• Oddsson. L.. & Thorstcnsson. A. (1994h). Thl.' torl,.·d hy ;Il,.·ou:-;tie l.'llIb:-ioll. /Jeme. /-1-. 7.'ii. influcnce of sudden perturbations on ll'unk muscle: aCli\\'- l-laugh!oJl, \\'..\\1.. Schmidt. T.A .. Kl,.·l,.-k, K .. l,.'1 ;.1. (200()) ity and intrn.- .. bdominal prcssU!\"t' while: sl<\\nding. 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psoas and lhe abdominal wall during a wid\\: \\'ariel,v of ~Ioll. .I.M.H. &. Wright. V. (1971). Normal range of spin;. bility. An objcclh'e clinic:l! study. AIlII UhClIIll Dis, 30 lasks. }fed S,:i SporlS f.XCfC. 30. 30 I. Morri!'O. J.!\"..l.. Bl:lllli:r. G., ~ Lucas. n.B. (1962). An orA.I .. Pr:\\sad. P.. ~ Ewing. C,1.. (1975). ~·h·dl<lnism tf(Jl1lyographic study of Ihe intrinsic muscles of the in man. J :111t1l l.mul. 96. 509-520. spinal injur~\" du(.' to l:<\\udoCl.:phabd accderalion. Onlw/J CIi\" North :1m, 6, 19. ~-lorris. J.~L, Lucas. D.B .. &. Bn:sier, B. (1961). Role o Kkin. J.A., Hickey, D.S., 0& Huskins. D.W. (I983). Radial trunk in st.:tbility of the spin<.'. } BUIIi: JOil1f Slll~~ . .:31\\ bul!:!in!: of lhe annulus fibrosus and the fUlll.:lion and fail· :\\-losekilck. L. (1993). Vertebral struclllr\\.' :lIld strength in llr~-of ~he inlt:ITerlL'hr~t1 dbc. J Bi01llt'ch. 16. 211-217. and in vitro. Cah:i{Tissllc 1111.53.5121. ~'1.H .. S~:mllssi, R.E., Wilder, D.G .. el ,d. (1987). Inh:r· N;'lChl'llIson. A. (1960). Lumbar inlrndisc'll pn:SSllrl.' ..-k/ nal displacelll~nl dislribution from in vilro loading of hu· man lhoracic and lumb'll\" spinal motion segments: Expl:r· [hop Sculld SlIp,.,l. n. 1-140. ill1l:ntal results and lhl'orelical predictions. Spill/.', 12, Nachclllson, A. (1963). Thl' influence of spinal rno\\'erni..' 1001-1007. till.' lumbar intrndiscal pri.'SSUl'l.' and on till' tensile str Krajc;Hski, S.R .. POI\\'in, J.R .. & Chi;1I1g:, J. (1999). The in d\\'O in IIIi.' ;tnlltllus fibrosus . ..lela Off/WI' SClIIul. 33, 183. r~sponse to the pl.:rlttrbations c:lllsing rapid f1cxion: Ef· Nachcmson, :\\. (1966). EIeCiromyographic studies Oil th f~cts of pre-load and step inpul magnitude. Cii,l BiO/lll/- td}ral portion of the PSO:IS llluscl~. With special rcr dUlU;CS. 14, 54. to its stabilizing function of the lumbar spine. lkw O Scalld, 37, Iii. Kuhlk. R.E. Schuhz. A.B .. BdYlschko. 1., el <II. (1975). Bin· Nach<.:mson, A. (1975). Towards a beller understandi back pain: A review or the mechanics of the lumbar ll1echank;tl charactcristics of \\'crICbral motion segments and intern:rtcbral discs. Or/hop Cfill North AI/I. 6, 121. RlJellllwlO! RelUlhi!. I.J, 129. L1nd~r. J.E .. B~ltes, B.T.. & Devita. P.(1986}. Biolllt.'chanics of th\\.' squat (.'xen:isc using. a modifil.'d Ci.'Ill(.'r of mass bar. Nach(.·m;.;on, A. & Elfstrom. G. (1970). IlIlravital [)\\'I .\\lcd Sci Sports E.un:, IS. 469. Pre:ssure: .\\lC(ISllrCJ1h.'lllS ill Llflllh(lr Dis,;:,: ,.\\ Sllui.\\' r.;{ Ll\\'cnder. $.:\\., ;\\lirka. G.:\\ .. Schol.'I1ITI.lrklin. R.\\\\'., el al. IlIOIl .\\IOl·i'J11CIlfS. :\\Ialli:III'crs allll Exercises. Stock (1989). Thi.' l:rrCC1S of pre\\'iew :lnd t<l.sk sym!lll:lry on trunk Almquisl & \\\\\"iksc-lt. ~achelll;.;on, r\\.L. .& E\\':lns. J.H. (1968). Some mech mllscli.' response to sudden loading. lIwlIlm Fat-tors. J I, propcnies of the third human lumbar intalaminar 101. ment (IigameJltulll flavum). J BiomcclulIIh's. I. 211. [.:\\\\'coder, S.A .. Tsuang. 1\".1-1 .. AIH.li.'rsson, G.B .. ~t a1. (1992). Nachclllson, A. & Morris, J.\\l. (1964). In \\'in) mCaSUri. Trunk nlusch.' co·contraction whitt: resisting applied mo· of inlradiscal pressure. Discolnl'lry. a mcthod iol' th ments in a Iwisted poslure. Ergollolllics. 36, 1145. termination of presslll\"i.' in the lower lumbar discs. J 1ui1l/ SI/rg, -16:\\, 1017. Lt.'c. Y.H. &. Ch('ll. C.Y. (1999). Lumbar vcrtebr.d angles and b.lck muscle loading with bells. Illd Health. 37, 390. or!\\alional Institute for Occupational Safety .tnd Health (1 WOl'kplclCe Use S(lck Be/IS. DHHS (NIOSH), Nu Lucas. D.B. &. Brcsier.~B. (1961). SIal,ilil.'· of ,ht: Ligamt.'I1fOUS 94-122. SpiltE.'. Biomechanics Laborator~', University of Cnliforni:\\, Ndson, J.M .. \\blmsky, R.P.. 6.: St(.'n:nsoll, J.,\\'I. (1 San Francisco and Berkeley. Technical Repon 40. San Francisco: Th~ Ltbor.llor\\,. RcI'lti,·c IUlllbm' ;tnd pl.'!\\·ic motion during londcd Lumsden. R.M. & Morris. J.~'1. (1968), :\\n in dvo study of ax- fk'\\ion/extcllsion. Spinc, 20. 199. inl rotation and immobilization at the lumbos:\\cral joint.} Nemeth. G. (1984). On hip and lumbar biomechanics. A of joint load and muscular ;lCtivity. SC(/l/(! J Uelwhi Heme Joill/ SlIrg. 50A. 1591. Manas, \\V.S. & Granata. K.P. (1995). A biomechanical assess· Stlppl, 10. Nonon, P.L. & Brown. T. (1957). The immobilizing c-ffi ment and model of axial t\\...·isling in the thoracolumbar spine. Spil1c? 20, 1440. of h'ICk braces. Their eHecl on th~ posture and mot :\\larras. \\V.S. & Mirb. G.A. (1992). :\\ comprehcllsivl.· c\\'alll:\\- thl..' 11l1lloosacr;.\\1 spine. 1 BOIIt' }oill1 S/lrg. 39:\\. Ill. Dncngrcn, R. And~rsson. G.B.l., & 1'\\achcmson. A.L. (1 lion of trunk respon$c 10 asymmL'tric trunk lllOlion. Spill/!. SllIdies of rd:lliollships bel'\\'cen lumb,lr disc pre myoelectric b<'H:k muscle acti\\'it~,. and intra-abdominn 17. 318 . trag;:\\slric) pr~SSllre. Spille, 6. 98. .\\larras, \\V.S .. Rang<lrajuili. S.L.. & Lan'nder. S.:\\. (1987). Osval<k'r. A.L.. Ncumann, P., Lo\\\"sund. P.. et a1. (1990) maII..' strength of the lumb.\\r spine ill fle.xion-.1I1 in Trunk loading and ('xpectation. Erf,:ollolllics. 30, 351. McGill, S.~vl. & Norman, R.\\V. (1987). Reasscssm~nt of the study.} Bio1l1t.:clull/ics, 23, 453. Panjabi. ,\\13\\-1., Gael, V.I-\\. .. ..\\: Tabtn, 1-\\.. (1982). Ph~'si role of inlra·abdominal prcsstlrt.' in spinal compression. strains in the lumbar spinal ligamC'1l1s. An in vilro b Ergollomics. 30, 1565. chanical slUdy. Spi1/i', 7. 192. McGill. S.M .. NOrllmn. R.W., &. Sharratt. M.T. (1990). The d· Parnianpour. M., Nordin, \\1.. K<thnnol\"iti'.. N., l.'t al. ( fret of an 'Ibdominal b~1t on trunk muscle acti\\'ity and intra·abdominal prl.'sslln..' Juring squal lifts. Er~ol/olllic.s. The tri<.lxia! clHlpling or torque gcneration of trunk 33, In. cles during isometric e.'\\cnions and the effect of f:tl McGill, S.~1.. Yingling, V.R .. &. Pl..'ach, J.P. (1999). Tlm.'I..'- isoincrtial mo\\,emenlS on till.' motor output and 1ll0\\ dimcnsion:d kin~rnatics and trunk muscle my()d~ctric ac· p~IIt('rns. SpiJlt', 13(9). 982-991. ti\\'it~' in the ddcrly spinc-<l dawb,lse compared to young Partridge. M.J. 6.: Walters, C.E. (1959). ~articip.llion people. CJiIl Biolllcclu/llics. 1'+. 389. abdominal IlHlsdes in \\'arious mOVt'lllc-lltS of Ih ..~ tru IlWIl. An L'!cl::trol11yographic study. Phys Tll!'r Rei', 39 ,\\'111es, M. & Sull ivan. W. E. ( 1961 ). L:lti:nd bi.'llding .11 Ihe hlln· bar and lumbosacral joints. .·\\11111 U~C, /39. 38i. ~liller. l.A.A., Hadersp\\.'ck. K.:\\., 6.: Schuhz, A.B. (1983). Pus- tl'rior (.'lelllenl loads in lumbar motion segmcnts. Spille. S, 331.

Paul.\\', J.E. (1966). :\\n dcclromyographic '\\llaly~is of cl:nain SIlI(ImOIlO\\\\\" \\\\., ZIHIlI. B.-II.. Bar:llia. R.\\'., 1.'1 itl. (19 nlO\\'('Il1(:nls <lilt! ex('rcises. I. SOllle c!('cp muscles of Lhl' llh.'l'h:lnic. tlf incrl'a:-I.:d expUSlIl\"(,.· to hlilihar inju back. All(/( Ret:. /55. 223. h.l.· l'~\"l'lic loadillg: Pan I. lo;.;s of rdl,:.\\i\\·(,.· 1Il1l:,(,: Perkins, ~I.S. & Bloswid. D.S. (1995). Th,· usc of h'ICk belts liz;llioll. Spillt'. 2·H23). 2-l26-243-l. f 10 increase inlr~wbdominal pressure as a means of pn:- Sldkn. T., B;II·alllki. II.C .. Ruhin, R.. d :11. (19t.1~J_ venting lo\\\\' back injuries: A survey of lhe lih:r.lIun:. 1m J inlradisl'al pr(,.'~:-.ur(,.· Ilh:a::-urcd in IhI.' :I11t(,.·riol\" itllt OCCIlP EUl'imll rlt.:lllih, I, 326. bler:!l anllul;,r rcgi(l/IS during :i='.\\\"Inml.'lricll I(I;l Pope, M.H., Andersson, G.B.J., Brolll.lIl, 1-1., L't .d. (1986). Biol/f(·clu/!/ic.\\. 13, 495. Electromyogr:q1hit studies of the ]lImb'H' trunk muscula. Sllln:ssoll, B\" Sl'h·ik. Go, & L\"dCIl, A. (1'J89). i\\!on tun: during thl: dcvelopmcnt of a.xi.1I torques. J (Julio!) thl.' s:lcroili:ll' joints. ,\\ rt)I.'rllgt·ll sLerl'oplHllOg Res, cJ, 288-297. :lIwlysis. Spi!lc, 1-1, 162. Ranu, B.S. (1990). Measuremctlt of pr(.'SSllrcs in the nucleus Thomas, ./.5., Lavelltk'r, S.:\\ .. Corcos, D.M., 1.'1 :t1. (1 and within the annulus of the hurn'lll spinal disc due 10 l.:X' fl'ct 01' lifting bl.'l!:- Ol1 ll'unk Illusck :1l\"lil':Lli()[l trcme loadilll.!. Pmc Illst Jleclt Ef/J,!, (llJ. 204, 14l. sudlk'111.\\\" appli<:d In.td. /l1t'WIII Faclo!\"s. ·iI. 6iO. Rcichm;t1Hl, S.~ (1971). Motion or thc lumbar anicular luong, N.H., {),lll:-l,.·r(,.·:HI. J .. \\latlr~tis, G.. 1.'1 ~d. (199S processes in fh:xiull-cxtcnsion and lateral flexion of the dillll..'llsional I.·\\·alu.ltion of lumhar urthosis d :-pinal bL'h;l\\\"i\\11\". J Uclwbil Il.·... Del'. 35. 3-1. spine. ~1c[(t .\\!orp!lo/ Nt'crl SCfllld 8, 261. Reichmann, S., Bl.'rglllnd, E.,..\\: Lundgr(,.·n, K. CJ972). Das B(,.·. lrb'lIl, J.P,G. & :\\kMllllin, J.r. (1985). Swdlillg prl wegungszcntrum in der LendenwirbclsAule bei Flc.xio[l thl' inleJ\"l.·crtd.u·al di!'L'; Infllh:llc(,.' 01 pmIL'oglyc;1I1 lind Extension. Z A/llIl EIII\\\\·ickltlll;;';;;':I!.\\·(.·h. /38,283. I:q;l'n COlllt:'IlI:-. Bifl,.h.,o!o.t:..\\·, 22. 1-45. Reid, J.G. &. Costigan, P.A. (1987). Trunk llluscle balance and \\',111 Dici:n. l,tI., llUCt\"l.I.'lllan:-, M.L\\!.. &. TUllssai muscular force. Spill/!, /2, 783. (1999J. SH,op or squal: ,.\\ rl.'\\·it'\\\\· of bilJlll(,.·<:hallil:a on lifling 1I.:(hniqu(,.'. CliJl Bifl/lft'c}wuic:•. /4. 6$5. Reyna, J.R. Jr., Leggdl, S.I-I., Kenney, ~., \"I .d. (1995). The ef. feCi of Ilimbar bells on isolated IlImb,lr musl'!e. Slrength \\bnfl~.~.l.l\\l1a.k & DUlllHS. G.:\\. (199S). \\kdl;lIlh,:al hdl;l\\'i s:l(roilial.· joinl [llld inflllL'nCt' of t!l(,., :l1l11 and dynamic (;\"padt)'. Spilll.', 20, 68. Rolandcr, S.D. (1966). ~'lotion of lhe lumb'lf spine wilh spe- poskrinr s:II.::roilia<: Iigamcnts under sagittal In cial reference to the stabilizing effect of posterior fusion. Hiw!lccltullio. /3, 193. An experiment\"l study 011 autopsy specimens . .'\\elll Ortho}l \\\\'hi!l.', :\\..-\\. (1969). An:d.\\\"sis of thL' Illl'I'!l,lnit:- 01' 8colld, .)11]1]11. 90, 1-144. spint:' ill Illan. An expl.·rillll.'llt,t1 stud.\\' (II' :lllIOPS Sato, K., Kikuchi, S\" & '{onez;Jwi1, 1'. (1999). In vivo intradis. mens . .·le/ll ()rt/wp ,\"jcl/ud, SIIp/i! il7, 1-105. cal pressure llle'lsuremCllt in healthy individuals and in Whilt:, A.A. l\\:. P:lnjahi. :\\I.N. (19701. Cliniud HiulIlt'cI p<iticnts with ongoing back probll.'ll1S. Spille, 24, 2468. Iht'Spilit'. I'hil:ldl.'lphia: J.B. Lippincott. Schuhz, A., ~l ~\\1. (1982). Loads on lhe lumbar spine. Valida. \"\"ilkl.', \"1.1., Ned. Po, C:lillli. :\\1.. <'I al. (19991. ;\\I.'W lion of a biolllcchanicaJ analysis by mC<lsurenH.'IllS of in. Ir~ldisc.d prc:sslIn:s <1I1d myol.'!l.·ctric sign ... k J BOIlt' Joil/! ml.·asur(,.·IIK·nts of pr(,.'ssurl.'S in Ihl.' inl<.'l'\\·(,,·rlChra Surg. 64.·\\,713. dail~' life. Spill.·, 24. 755. Wilder. D.G., POpl.· ..\\1.1-1 .. &. Fr.\\\"Ino~·L'r. J. W. (1980). T Shir4lzi-t\\dl. ,.\\. (1994). Biomt'ch4lnics of lhe lumbar spine in sagittalJlatcral moments. Spi/h:, /9, 2407. tional lopogr,tphy of the s;Jcroiliac joint. Sp;/It'. 5,

I· Biomechanics of th • Cervical Spin :1( Ronald Mosko il1Lh\"'d,~.,%>. mLdi. Introduction Component Anatomy and Biomechanics Anatomy Osseous Structures Intervertebral Discs Mechanical Pmperties Vertebrae Intervertebral Discs Ligaments Muscle Neural Elements Kinematics Range of Motion Surface JOint Motion Coupled Motion of the Cervical Spine Atlantoaxial Segment Subaxial Spine Abnormal Kinematics Spinal Stability Occipitoatlantoaxial Complex $ubaxial Cervical Spine Applied Biomechanics Decompression Arthrodesis Cervical Spine Fixation Biomechanics of Cervical Trauma Airbag Injuries Whiplash Syndrome Summary References

Introduction arc five t.'·pical cervical vertebrae, C3-C7, which arc sirnilar in structure and function. Knowledge of spinal biomechanics advanced expo- nentially during the second half of the twentieth The spine has four curves when viewed in the centlll)'. A two-column model of the spine was de- sagiual plane. The celvical and lumbar regions arc scribcd by Sir Frank HoldswOIth (1963) and, latel; a convex anteriorly (lordotic), while the thoracic and lhree-column model by Denis (1983), further refin- sacra) regions are convex posteriorly (k)iphotic). The lordotic curves develop after birth as the in- ing the principle or spinal stability. The computer fant's spine straightens out, which facilitates devel- opment of a bipedal posture. Although there is a age has produced powerful methods For modern hiomcchanical modeling, the promise being the Cl (atlas) ability to assess the slnbility of n construct prior to implantation. Today. the application of cendcal bio- C2 (axis) mechanical knowledge spans many inclustl'ies and supports improved medical diagnoses and treaL- Facet ment that is more effective. Future technological joint and electronic advances will continue to build on basic biomechanical principles, many of \\\\:hich will C7 be outlined in this chnpter. Component Anatomy and Biomechanics ANATOMY A The exquisite design of the cervical spine uniquely Uncovertebral contributes to the structure of the human body and joint profoundly enhances its function. The cervical Intervertebral spine supports the skull and acts as a shock ab- disc sorber for the brain. It also facilitates the transfer Tracheal of weights and bending moments of the hend. It air shadOw protects the brainstem, spinal cord, and various C7 neurovascular SU'UClures as they transit the neck nnd when they enter and exit the skull. The verte- B bral column also provides a multitude of muscle and ligamentous attachments for complex move- A, lateral roentgenogram of the cervical spine. Note the ment and stability. The neuromuscular control af- lordosis. The facet joints are aligned obliquely only to the forded by the muscle ntlnChmenls combined with frontal or <oronal plane; hence, their excellent visualiza- the numerous articulations of the cervical spine al- tion in the lateral view. a, Anteroposterior view of the <er· lows for a wide range of physiological motion that vical spine. maximizes the range of motion of the head and neck and serves to integrate the head with the rest of the body and the environment. The spine consists of 33 vertebrae divided into five regions: cervical (7) (Fig. II-I), thoracic (12), lumbar (5), sncml (5 fused segments), nnd coccvgeal (approximately 4). The two most cranial vertebrae, Cl (atlas) and C2 (axis). [Ire atypical. with a unique structural role in the articulation between the head and the cervical spine. The atlanta-occipital joint, bel ween CI and the oecipital bone of the skull, is also a functional part of [he cervical spine. There 287 •> QJ- ._

harmonious progression of these curves from one to sagittal range of motion of the cervical spine. another, which may help distribute stresses and Cl-C2 articulation is the joint primaril.\\-' respon strains, injuries occur more commonly at the jllnc~ for rotation in the cel'Vical spine. tional areas because of differences in the relative stiffness of each anatomical segment of the spine. The atlas, or C 1, is a bon.\\-' ring consisting o The physical structure of the anatomical elements anterior ;:\\lld posterior arch that is attached to modulates from the cervical to the sacral region in two !;:\\terallnasses of the atlas (Fig. 11~2). Th relation to the segmental function. pcrior surfaces or the latent! rnasses, which crani<:dly <:md inward, form an <:\\rticulation with The lordosis in the cervical spine, like that in the caudally and clutward-facing occipital cond.\\-'l lumbar spine, is maintained predoI1linantl:v b.v the skull (Fig. 11-3). E.xtension ()f the occipit slightl)' wedge-shaped intervertebral discs that arc vical joint is limited by' the bon.\\-' <:uwt()!ll.v; fle larger anteriorly' than posteriorly. In contradistinc- is lirnited prirnaril.\\-' by ligarneluous structures tion, thoracic kyphosis is maintained largely.' by the tectorial mernbrane, and the longitudinal fibe vertebral bodies themselves; because the posterior the crucifonll ligarnent as well as by the post portion of the thoracic vertebral body is larger than ligaments (Figs. 11 ~4 and 11 ~5). The anterio the anterior portion, there is a relative kyphosis of berek' on the <:llTh of C I serves <:lS <:In attachm the thoracic spine. for the longus colli muscle, a flexor of the n The posterior arch of the atlas is a modified lar The conceptual biomechanical building block of that is grooved on its superior surface for the the spinal colunln is the Functional spinal unit or sage or the vertebral arteries as the.v enter motion segment. It consists of two adjacent verte- the fClrarnen magnum after piercing tile post brae and the intervening intervertebral discs and lig~ ntlanto~(lccipital nlelllbl\"ane. aments between the vertebrae. These ligaments arc the anterior and posterior longitudinal ligaments; Simi!;:\\r to the occipitocel'Vical junction, the the intertransverse, interspinous, and supraspinous 11() intervertdJral disc betwecn Cl and C2. Stab ligaments; and the facet capsular ligaments. As a re~ at this levcl is thus predicated on intact ossco suit of the different functional demands of the vari- l1lentclus structurcs. The articulation between ous parts of the spinal column, segmental variation and C2 is primaril.\\-' specialized for rotation. is expressed by! changes in the size and shape of the bod.\\-' of C2 projects superiorly to fOlTn the odon vertebrae, the anatomy of the discoligamentous process, or dens (Fig. 11 ~6). The projection o structures, and the alignment and structure of the bod.\\-' or C2 and the dens has a characteristic ob facet joints. appearance on lateral cervical radiographs an Biological structures behave differently than do Superior articular Facet for dens. common engineering materials. Collagenous tissues facet on anterior arch exhibit both viscoelastic and anisotropic behavior. Viscoelastic properties are rate~dependent (time- Orig dependent) behaviors under loading that are seen in tran both bone and soft tissues; mechanical strength in- liga creases \\vith increased rates of loading. Anisotropy is the alteration in mechanical properties that is seen \\vhen bone is loaded along different axes. Anisotropic behavior occurs as a result of the dis- similar longitudinal and transverse microstructure. Osseous Structures Groove vertebral artery The occipul-CI-C2 complex comprises the upper cervical spine and is responsible for approxilnately 1cm 40% of cenrical flexion and 60% of cervical rotation. The occipital condyles articulate with the slightly D m I ' - - - - \" \"__\"I on posterior arch concave lateral masses of the atlas. The primary ~ Illotion permitted by this articulation is flexion and extension, accounting for a large portion of the 1Bony architecture of the atlas. Bar 1 C111.

Occipital Occiput·C1 joint , r - -__condyle Dens Transverse process Cl-C2 facel joint Lateral mass of Cl Lateral mass ofC2 The open-mouth radiograph demonstrates the occiput-(l and the atlantoaxial articula- tions. Note the symmetric spacing between the lateral masses of (1 and the dens. Asym- metry or widening of these spaces may occur after rotatory disturbances or fractures of .- - - - - - - - - - - - - - - - - - - - - - - - - -the (1 ring. Anterior Posterior Flexion \",,\",\"u Superficial layer of Apical tectorial membrane -~~- Iligament Tectoria! membrane . @0 01 dens Posterior )~'., -.:,::(-'\\~~.. ! Ant:~~~ longitudinal ligament Extension I of alias Tracings of lateral flexion and extension radiographs I ing the occiput, el, (2. and (3. The substantial relati Dens of axis -t;~ffB~ll motion between the occiput, (1. and (2 can be seen Transverse Large arrows indicate the direction of motion_ Small ligament ~~~~y.. rows indicate that approximation of the posterior ele Anterior ments limit occipitocervical extension. In contradistin maximum flexion is controlled by tautness of the Iiga I atlantoaxial ments. Reprinred with permission {rom MoskoviclJ, R. & J D.A (1999). Upper (eNica/spine instrlJmellt<11ion, Spine: I ligament of tIle Art Reviews. 13(2), 233-253. I interv~~:bral I fibrocartilage Anlerior 1 longitudinal ligament Body of C3 Median sagittal section through the occipital bone and the first three cervical vertebrae showing the articulations and surrounding ligaments.

Dens disc, The cCryiC~IJ \\'crtebral hod,\\\" is o\\'ul-slJaped (odontoid is widl.:l\" mediolaLerally Lhan <:Illtcropostcriorl:\", process) transverse pnlcL'ssCS or the cervical spin\\,.' arl' uni Spinal canal in thaI Lhe,Y all contain a lr~Hlsn:rse foramen for Superior Spinous passage of thc vertebral a ncr:', Tl\\\\\"~ transv facet process orprocesses the suba:xial c(~r\\'ical \\'('ncbr~\\e h Foramen Inferior facet lransversarium two projections, lhe alllerior and p(Jstcl'ior tu cit's. which serve as allilChl11\\,.'nt points for ante lcm and poslcriOl- lTluscles, respectively, The large a rior tubercle of C6, rt'fcrrcd to as the carotid tu The axis vertebra, or (2. The superior facet articulation cle, can be an important surgicallandmal'k, The permits multiplanar motion while the inferior facet is peric>!' surract;.' 01' the lrans\\,\\,.'l'se proc~ss provid aligned to articulate with a more typical cervical facet, groove for the exiling nerve root. which is more constrained. There is a smooth surface on Each pedicle connecLs the vl\".·ncbral bod~! to a the front of the dens for articulation with the anterior eral mass, that portion of bone containing lh~ s ring of (1. Bar>:: 1 em. rior and inferior facets, The facct joints n.::gulatc • ormO\\'l\".'ll1ents the spine and pla~- a c.:ritical rol often a helpful anatomical landmark. The dens ar- spinal slabilit~\" Those 01' the cervical spine are orie ticulates with and is restrained within a socket at approxirnatd~' 45'~ to the coronal pbne and arc formed by the transverse ligament of CI and the an- terior arch of C1. The transverse ligaments run from cated in the sagittal plane (Figs, 11-8 and 11-9), the anterior arch of C I. behind the dens, and pre- orientation allows grc(l(('r anlounts of flexion t vent anterior translation of Clan C2. The other lig- aments at the CI-C2 articulation are the alar, apical, does latl\".'ral bending or rotation in the cl.;.'lyical sp and accessory alar ligaments. The alar ligaments, The facet joims resist most of lhe shear forces and which are symmetrically placed on both sides of the dens, attach the dens to the occiput to prevent ex- proximalL'!:' 16l.:!r; of lhe compressi\\\"c forces acting cessive rotation. The left alar ligament prevents right rotation and vice versa, To some extent, the the spine (Adams & I-lutton, 1980). The laminae alar ligaments also act to limit motion during side arise from the lalcral masses, Tht' IaLeral rnHSS\\,.'S h bending (Dvorak & Panjabi, 1987). The apical liga- ment also connects the dens to the occiput. important surgical implications in tht: subaxial ce Unlike the 1\\\\'0 most cranial vertebrae, the cal spine because they conwin a relativel:' l anatomy of the third through the sixth cervical ver- amount 01\" bone and are easil~' accessible for the pl tebrae is similar (Fig. I 1-7). These fOUl\" cervical ver- tebrae consist of a body, two pedicles, two lateral Groove Inferio masses, two laminae. and a spinous process, The for spinal facet seventh cervical vertebra is slightly different in that Super it has a transitional form, It is called the vertebra nerve facet prominens and has a larger spinolls process that is not bifid like those of C3-C6. Uncinate Pos process tube The anterior components of a subaxial cervical Anteri motion segment are the vertebral bodies and the luberc lcm Superior view of a typical cervical vertebra, representa of (3-(6 ((7. the vertebra prominens. diHers slightly in that it has a prominent nonbifid spinous process),

C4 CS C6 Orientation of the facets of a typical cervical vertebra in C7 three planes. The facets are oriented at a 45\" angle to the transverse plane and the frontal plane, and are at right Lateral photograph of a fourth and seventh cervical ver angles to the sagittal plane. Y indicates the craniocaudal bra. The facet joint alignment is fairly close to 45° from the transverse plane. Note also the difference in size of axis. z the anteroposterior axis. and x the mediolateral the spinous processes, which are a reflection of the size axis. Adapted '.t'lieh permission from White, A.A. 111 & Panjabi, and importance of the muscle attachments_ M.M. (1990). Cfinical Biomechanics oi the Spine. Pili/adelphia: 1. B. Lippiflcort • I e menl of screws. as opposed lO the pedicles. which arc difficult to cannulate sarelv in the neck. The superior surfaces of the cervical vertebrae are saddle-shaped because of the uncinate processes, bony protuberances that aJ-ise from the lateral mar- gins or the superior end plates (Fig. I 1-10). The un- covertebral joints Uoints of Luschka) develop during spinal maturation and pla~,/ an important biome- chanical role with respect to kinetics and stability. Intervertebral Discs 1cm The intervertebral discs are highly specialized st11.1C- Anterior view of a sixth cervical vertebra. The shorr arro tures that contribute up to one-third of the height of indicate the uncinate processes and the Ic;mg arrow ind the vertebral column and fmm specialized joints be- cates the pathway of the sixth cervical nerve root. The tween the cartilaginous end plates of the adjacent facet joints are located posteriorly. Bar = 1 em. vertebral bodies. Activities such as running and jumping apply short-duration, high-amplitude loads to the intervertebral discs, whereas normal physical activity and upright stance resuh in the application of long-duration, low-magnitude loads to the disc. Discs are able to withstand greater than normal loads when compressive forces arc rapidly applied based on the biomechanical principles of viscoelasticity. This propeny protects the disc /i'om catastrophic failure until extremely high loads are applied.

The nucleus puiposlis is centrally locmed within ~~~~~~~~?-I)( ,./ pulposus the disc and consists of almost 90% water in young Nucleus individuals. The water content is highest at birth and decreases to approximately 700/0 as the disc de- A generates with age. The rest of the nucleus pulposus consists of protcoglycan and collagen, which is ex- I Disc - ' An clusively type II collagen. Type II collagen fibrils are fibe thought to be able to absorb compressive forces bet- ~~~ ter than type I collagen fibrils. I Prolcoglycans consist of a protein core auached to polysaccharide (glycosaminoglycan) chains. The Schematic drawings of an intervertebral disc showing pol.vsaccharicles are either keratin sulfate or chon- droitin sulfate. The core protein, with its attached criss-cross arrangement of its fibers. A, Concentric lay polysaccharides, is aggregated to hyaluronic acid of the annulus fibrosus are depicted as cut away to s through a link protein. The proteoglycans in th(: in- the alternating orientation of the collagen fibers. B, tervertcbral discs are similar to those in articular cartilage, except that the protcoglycans present in layers of the annular fibers are oriented at a 30'\" ang [he intcl\\'ertcbral discs have shorter polysaccharide the vertebral body and at 1200 angles to each other. chains as well as shorter core proteins. The nucleus Acf,;pred '.-Vlil! permission l,om W;uw, AA. 111 c~ Pan,iabl. M pulposlls contains more protcoglycan than does the (l990} Clinical B!Drn~'dl,Hll(<' of the Sp:rl(' Pili/dcle/ph/a: I annulus fibroslis. \\¥ith increasing age and disc de- Lippincor[ generation. the lotal protcoglycan content decreases. •! The annulus fibrosus is the OtHer portion of the disc. Its water content is slighl1y less than that of the Cortical bone is stilll.'!\" than cancellolls bone nucleus, being only approximately 78£10 \\vater in can withstand greater stresses before failure. \\V younger individuals. \\-\\lith age, the water content the strain in \"h'o excl.;cds 2t:'(; of the originallen falls to approsimalely 70%, like that of the nucleus conic::d bone l\"1\"<.tclLtrcs, but c<.tllcdlous bone pulposus in older persons. The annulus consists of withSland somewhat greater strains before fra collagen that is arranged in approximately 90 con- ing. The gr('..ltl.~r abilit~· to withstand strain i centric lamellar bands. The collagen fibers in these cause of\" the stl\"uctUrt: of cancellous bon ... : its p shccts nll1 at approximately 300 to the disc or 1200 it,v varies from 30 t() 9CY>\" compared with con to each other in the adjacent bands. This unique ori- bone with valuL's or 5 to 30%; (C~\\rtcr & H entation confers strength to the annulus while per- 1977). Vertebral corn pression strength incre milling some nesibility (Fig. I I-II). The composi- tion of the collagen in the annulus is approximately from lhe upper cervical lo th(' lower lumbar k\\ 60% type II collagen and approsimately 40% type I Bone strength decreases with age as a resu collagen. As the disc ages, the collagen undergoes irreducible cross-linking and the relative amount of [he de\\'c1oplllcnt of osteoporosis. The mineral type 1 collagen increases, replacing type [[ collagen tenl (If' vertebr~\\e decreases with increasing age in the disc. relatively con::'1ant rale (Hansson & RODS, Hansson et 411., 1980).;\\ 25cj{; «(..crease in osseou MECHANICAL PROPERTIES suc results in a 1110re lhan a 500'0 tkcrcasc in Vertebrae strength of the \\\"Crlchrac (Bell. 1967). BeC'lllSC cortical shell of a venchra is responsible for on1 The mechanical properties of the bone and soft tis- proximatcl.v 1(Y;',; 01\" its strength during com sues diffe!: Strength and stiffness and the relation of 'sion, good quality c~lIlcellotis bone is criticall~ stress to strain are the key IT'lCchanical properties. !Jorlant (McBroom e:t aI., 1985). Stress-strain curves are used to determine the rela- tive loading behavior of bone. Stress is the load per \\Vht,:.~n bone is loatk.'d in \"h'o, contraction o unit area of a perpendicularly applied load. Strain is musclt:s attached [0 the bone can alt0r [he s [he change in length per unit of original length, usu- ally expressed as a percenlage. :,lj ). -~-\"'>-- .- ~.\" I.ZiZY>_ .~\"

distribution in the bone. Bending moments are ap- in a neutral position or somewhat extended, and th plied to the vertebral bodies during motions. During pre-stresses the disc to some degree and provid lie.\"\"\", tensile stresses are applied to the posterior some intrinsic support to the spine (Nachemson Evans, 1968; Rolandcl~ 1966). The elastic properti and compression to the anterior cortex of the also assist in limiting the in\\varcl buckling of the body. To perform lifting tasks, the back ligaments during extension, which could potential compress the neural clements. are required to develop considerable forces IS\"hltltz et aI., 1982). Stresses in a typical cClvical Muscle change from tensile to cornpressive in a re- Muscular strength and control is imperative gion approximately 0.5 to I cn1 anterior to the pos- maintain head and neck balance. In the cervic ~ longitudinal ligament (Pintar et al., 1995). As spine, muscle strength also has a role in reduci stresses on bones. Bending n10ments are applied is weaker and fails earlier in tension than in the vertebral bodies during various motions. Duri compression, posterior paraspinal muscle contrac- flexion, tensile stresses are applied to the posteri cortex and compression to the anterior cortex of t can decrease the tensile stress on bone by! pro- vertebral body. Substantial loads on the eel-vie ducing a compressive stress that reduces or neutral- spine have been calculated during neck flexion, pa izes the posterior cortical tensile stresses, allowing ticularly! in the lower cenrical motion segments. the vertebrae to sustain higher loads than would othenvise be possible. However, bone will often fail Harms-Ringdahl (1986) calculated the bendi prior to damage occurring to the intervertebral disc moments generated around the axes of motion under compressive loading. Finite element model- the atlanta-occipital joint and the C7-TI moti ing of the cervical spine indicates that the increase segment in seven subjects with the neck in five po in end plate stresses may be the initiating factor for tions: full tlexion, slight flexion, neutraL head u failure of this component under compressive loads right with the chin tucked in, and full extensio ('loganandan et al., 1996a). The load on the junction between the occipital bo and C I was lowest during extreme extension (ran Intervertebral Discs ing from an extension moment of 0.4 Nm to a fle ion moment of 0.3 Nm). It \\vas highest during e Intenlertebral discs exhibit viscoelastic properties treme flexion (0.9 to 1.8 Nm), but this was only (creep and relaxation) and hysteresis (Kazarian, slight increase over that produced when the ne 1975). Creep occurs more slowly in healthy discs was in the neutral position. The load on the C7- than in degenerated or herniated discs, suggesting motion segment was low with the neck in the ne that degenerated discs are less viscoelastic in nature tral position but bccame even lower when the he (Kazarian, 1972). \\vas held upright with the chin tucked in (rangi from an extension m0111cnt of 0.8 Nm to a flexi Ligaments moment of 0.9 Nm). The load increased somewh during extreme extension (ranging from 1.1 to 2 Clinical stability of the spine depends primarily on Nm) and substantially during slight flexion (reac the soft tissue components, especially in the cervi- ing 3.0 to 6.2 Nm). The greatest loads \\vere p cal spine. The spinal ligaments are functional duced during extreme extension, with momen rnainly in distraction along the line of their fibers. ranging from 3.7 to 6.5 Nm. Ligament strength and limited extensibility help maintain stability, especially around the craniocer- In the same study, surface electrode electromyo vic'll junction. The alar ligaments have an in vitro raphy was used to record activity over the erec strength of 200 N, and the transverse ligaments spinae muscles of the cervical spine with the neck have an in vitro strength of 350 N (Dvorak et aI., the same five positions described above. Intere 1988a). The strength of the ligaments is related to ingl}', the values obtained showed very low levels both the anatomical demands and the flexibility re~ muscle activity for all positions, even during quired, \\vhich is a classic example of form follow- treme flexion in which the flexion moment on t ing function. C7-Tl motion segment increased n10re than thr fold over the neutral position. The fact that the e1 The ligaments all have high collagen content ex- cept for the ligamentum flavurn, which is exceptional in having a large percentage of elastin. The ligamen- tum flavum is under tension even when the spine is

tromyographic levels over the neck extensors were lateral cervical radiograph of a 68-year·old 1Noma low in this and other studies (Founlain et aI., 1966; presented with severe torticollis. She denied any h Takebe et aI., 1974) suggests that the nexing mo- injury and did not have evidence of a suuctural ve ment is balanced by passive connective tissue SlILIC- abnormality, infection. tumor, or inflammatory dis tures, such as the joint capsules and ligaments. This Pseudosubluxations (arrows) are evident subaxiall phenomenon is seen in many other joints in which consequence of the marked kyphosis. Her neck wa passive support is provided by the ligaments. twisted only because she had a severe cervical flex formity. and she was unable to see forward excep The values for the moments computed by turning her head to one side. She was neurologica Harms-Ringdahl (I986), howcvcr, are appmxi- tact. It was possible to extend her neck to a relativ mately 10% of the maximal values measured by tral position using gentle traction. Following post lvloroney and Schullz (1985) in 14 malc subjccts sion from (2 to (7, she returned to a normal inde who resisted maximal and submaximal loads life. Muscle biopsy was consistent with senile myo against the head while in an upright siuing posi- Reprinred with permiSSion from tvioskovlCh. R. (1997). tion. The mean maximal voluntary moments were mSiabihw (rheumarold. d\\·varfism, degeneraCtV£!. Oihers 10 Nm during axial rotation or the cervical spine, Bridwell &- RL DeW,11d fEds), The Te;(lbook of Spln,ll S 12 to 14 Nm during flexion and lateral bending, and (D.o. 969- i009). Phdacie!pfJJd. Lippincorr-R,wfdl Publishe 30 1m during extension. Calculation of the maxi- mum (compressive) reaction forces on the C4-C5 edge base is growing. To date, certain bas motion segment ranged from 500 to 700 N during meters have been established. The cervica flexion, rotation, and lateral bending. and rose La undergoes significant changes in length 1,100 N during extension. AnteroposLerior and lal- f1cxion and extension (Breig el \"I\" 1966 eral shear forces reached 260 Nand lION, respec- 1960). Thus, while there is some longitudin tively. Calculated moments and forces generally ticity to the spinal corel. it tolerates axial correlated well with mean~ll1easured m:voelectric tion poorl~\". It is the translator~' forces llw activities at eight sites around the perimeter of the cally result in neurological in.illr~·. A comp neck al the C4 level. tolerance betwecn 2.75 and 3.44 kN is cs for the adult c~rvical spine before significa Muscles playa cr-iLical role in basic postural rological injury occurs (i'Vlyers &. \\Vink homeostasis, as can be observed in both historical 1995). and present-day clinical settings. In unique obser- vational studies in lhe 1950s of severclv affecled po- liomyelitis patients, improvements in respiratory assistance for those with respiratory paralysis led to higher survival rates and a large number of pa- tients who sustained complete paralysis of the cer- vical musculature. Patients with completely nail cervical spines were unable to support their heads unless adequate support was provided and actually remained in bed despilc the good function of their cxtrcmities (Perry & Nickel, 1959). Similarly, severc cervical kyphosis occasionally is seen in elderly pa- tients \\vho do not have an obvious stntetural ctiol~ ogy when investigated radiologically. Some of these patients were found to have marked cenrical exten- sor muscle weakness that has been attributed to se- nile cervical myopathy (Simmons & Bradley, 1988) (Fig. 11-12). Neural Elements Biomechanics of the neural elements have noL been as well studied as the biomechanics of the os- teoligamentous vertebral column, but our know!-

Spinal cord injuries can also result from extreme N \" C1 or sudden flexion-c:\\tcnsion movernCl1ts, especially in the face of a $hallow spinal canal. I-lead Oexion 9i•• alone has been shown to result in significant in- 3. creases in the illlramcdullary spinal cord pressure Oa ~. a_ mm in dogs (Kitahara et aI., 1995). Neurological injuries ,i\",··· ..~ result from anteroposterior compression of the ., I': 17 20 23 26 29 corel and arc I110re common if the spinal N is stenotic. Flexion motions can result in in- when the spinal cord makes contact with cer- ...l2\"e vical osteophytes, and cxtcnsion mOl ions may resull ::\"':3'-. in a pincer-like compression of the cord between 10 12 14 16 18 20 mm (anterior) ostcophytes and (posterior) invaginated ligamentum Oavum. Anterior or central spinal cord N injuries may ensue. Although a diagnosis of spinal stenosis may be e. made based on the absolute size of the spinal canaL 4; imaging of the neuraxis itself may be of greater 0.__ value. Contrast-enhanced computerized tomogra- phy, myelography, and magnetic resonance imaging 10 12 14 16 18 20 mm can demonstrate actual impingement or distortion of the spinal corel. Studies performed in flexion and \".IlN 16: C4 extension may enhance the value of the information ,e.· by demonstrating the contribution of any dynamic O~. ____ .. soft tissue component to the impingement. The ex- act size of the bony cervical spinal canal and the ':.a.1. 10 12 1': 16 18 20 mm vertebral body \\vas measured in 368 cadaveric adult male vertebrae (Moskovich el aI., 1996). This sludy N C5 used well-validated parametric statistical methods to determine that the mean sagittal diameter of the ., spinal canal for C3-C7 was close to 14 mm (14.07 ± 0.. . 1.63 mm; N ~ 272) (Figs. 11-13 and 11-14). The 10 12 14161820 rnfn mean ratio of the sagittal canal diameter to the ver- tebral body diameter (canal to body [cob] ralio) was N 16 C6 86.68 ± 13.70. Thirly-one percent of subaxial verte- \".&..e brae would be diagnosed as having spinal stenosis if .;. a c-b ratio of less than 80% was considered abnor- 0.. .___ mal. Another study also found a high false-positive 81 error rate for the e-b ratio, with 49% of 80 asympto- : ; ...t..-.10 12 14 16 18 20 mm matic football players having a c-b ratio of less than 80O,b at one or morc cervical levels (Herzog et aI., N C7 1991). Yet another group evaluated the reliability of o4't. _ the c-b ratio using plain lateral radiographs and CT 10 12 14 16 18 20 mm scans (Blackley el al., 1999). Results confirmed lhat a poor con-elation exists between the true diameter Histograms of the C1-(7 sagittal canal diameters. Apart of the canal and the ratio of its sagittal diameter to from the (1 plot, the same scale is used on all of the hori- that of the vertebral body. The variab.ility in anatom- zontal axes so that the distributions of the diameters can ical morphology means that the use of ratios h'om be compared, Reprinted with permission (rom Moskovich, R.. anatomical measurements within the cervical spine et at. (996). Does rhe cervical canal [0 body rario predict spina is not reliable in delermining the true diameter of the cervical canal. stenosis? Bull Hasp Joint Dis, 55, 61-71. Spinal cord injul)' without radiographic abnor- malities (SCIWORA) have also been described, especially in children (Dickman el aI., 1991; Osen bach & Menezes, 1989: Pang & Pollack, 1989). Th etiology of such injuries is unknown, but one mech anism may be longitudinal traction. The unusuall elastic biomechanics of the pediatric bony spine a lows deformation of the l11usculoskeletal structure beyond physiological extremes, perm·itting direc cord trauma followed by spontaneous reduction o the bony spine (Kriss & Kriss, 1996). The isolate spinal cord resists tension poorly; axial tensil

rotation or translation of a body ~l!ong one a.\\ consistently' assclciated with a simultaneous tion or translation ~dong another axis, the mot are coupled. Coupled motions are normally' pressed as displacements in the X, Y, or Z d tions and rotations about the three orthog axes. Further analysis involving whole spine more complex, and while it has y'iclcled interes results, motion segment analysis remains im tant for basic understanding. Posterior longitudinal ligament Superior facet Axial computerized tomographic scan of a sixth cervical Capsular ligament, l\\v.-o-od{L Anterio vertebra, which was not part of the study detailed in the Spinous process . / longitud text. The anteroposterior diameter of the spinal canal ligame measured 13.96 mm in this specimen. \"'\" II Interverte • disc Interspinous forces to failure For three adult spinal cord speci- and, mens arc reported to be 278 N ± 90 (Yoganandan et aI., 1996b). Lower forces may result in direct neural supraspinous J injury or vascular disruption. ligaments //// . I I p osten.or Kinematics L' fl Inferior facet IJ longitudinal Intertransv Kinematics is the studv of motion of rigid bodies Igamentum avum ligament without taking into consideration other relevant ligamen forces. Kinematics of the spine descrihes the physi- ological and pathological motions that occur in the I various spinal units. The traditional unit of stud)! in kinematics is the motion segment, or the functional A Superior facet I Interlransv spinal unit. As described earlier, each motion seg- ligamen ment consists of two adjacent vertebrae and their Capsular ligament I intervening soft tissues (Fig. 11-15). Spinal canal \"- ' Anterio Basic biomechanical testing involves the applica- longitu tion of forces to a vertebral body and the subse- Spinous process \"- ligame quent measurement of the movemcnts that occur (Fig. 11-16). Movements can be either rotational or ~ translational. A degree of freedom is def-lned as a motion in \\vhich a rigid body can cither translate Interspinous ~ \"' Interverte back and forth along a straight line or rotate and supraspinous disc around a particular axis. Thus, each vertebral body may either translate or rotate in each of three or- ligaments Transv thogonal planes, for a total of six degrees of free- dom (Panjabi et aI., 1981) (Fig. 11-17). When either \"\"_~/ forame Ligamentum J flavum Posterior ~Ls_te_r_io_r '!Iongitudinalligament I A_n_t_e_ri_o_r Schematic representations of a cervical motion segmen composed of two typical cervical vertebrae ((4 and (5) intervertebral disc, and surrounding ligaments. The bro line divides the motion segment into anterior and pos components. A, Lateral view. B, Superior view. Adapied permission from White, A.A. 1/1, Johnson, fUvr, Panjabi, lvUv al. (7975) Biomechanical analysis of clmica! stability in rhe c cal spifJ(l. (lin Orthop. \\09,85··-96

n ,, aled is the fact that a considerable amount of fle:x 9m c..; ion and extension occurs at the CI-C2 articulation Diagram of a test rig to evaluate a functional spinal unit 5 to 20° or flcxion and extcnsion occur with mean using videophotogrammetry. This technique facilitates ac- of 12\" (aetive) to 15° (passive) (Dvorak et aI., 1988b) Approximately 90° of axial rotation takes place curate measurements of motion without the measure- in the subaxial eel\"vic'll spine (C3-C7), about 45° to ments themselves having an effect on the displacements of cach side of ncutral. Even greater lateral flexion i possible: approximately 49° to each side of ncutral the mobile vertebra. A. light-emitting diodes (LEOs); Band giving a total of about 98°. The range of flexion and extension is approximately 64°: about 24° of cxten C. Guide bars for application of tensile and compressive sion and 40° of flexion. The motion in each plane i fairly evcnly distributed lhroughollt the motion fOfces. D. Pulley for application of torques. Weights are at· segments (Lysel!, 1969). Thc mean total range o anteroposterior translation in subaxial spinal mo tached to guide wires (w), which go around pulleys to pro- tion segments is 3.5 ± 0.3 mm divided unequally 1.9 mm for anterior shear and 1.6 111m for posterio duce torque on the upper vertebral body (v). n. interverte- shear. Lateral shear loading results in a mean tota range of lateral motion of 3.0 mOl :!:: 0.3 mm, di bral disc; 9. acrylic cement that attaches lower aluminum vided equally between right and left; tension result in 1.1 mm of distraction and compression, 0.7 mIl plate (m) to test rig, which is rigidly bolted to the support of loss of vertical height (Panjabi et aI., 1986). frame. The upper plate (p) and upper vertebral body (v) The great Ilexibility of the eel'vical spine allow the head to be positioned in a wide variety of ways : are the moveable elements to which the loads and torques ;·Fy i are applied. LEOs are rigidly attached to the upper plate c1l1Y and their movement is recorded by two video cameras. I Reprinted with permission {rom Raynor. R.• Moskovich. R.• Zide/, I P. et a/. (1987). Alteration in primary and coupled neck motio1J5 l__a_f_,e_'_fa_,_e_,e_,_,o_n_'Y_._N_,e_\"_,o_S_\"'_9_e_ry_,_2_1(_5_),_6_8_'_-_68_7_. _ RANGE OF MOTION +Fz 0~i---le. orMeasurements of cervical range motion are based ~Fx on radiographic studies or postmortem investiga- A vertebral body showing the three primary Cartesian tions. Inclinometers and various optoelectronic and electromagnctic dcvices L1sed clinically for noninva- axes, x, y. and z. Along each axis a positive force, ~ F. is de sive evaluation of cervical spinc motion are not as accurate; in particular, coupled motion is poorly noted by the direction of the arrow. The curved arrows in- quantified (Roozmon et aI., 1993). The established range of active axial rotation to one side at CI-C2 is dicate the direction of a positive torque. +..... Reprinted will 27 to 49° (mean = 39°); passive rotation is 29 to 46° permission from Raynor, R.• Moskovich. R., Zidel, P., el af. (mean = 41°) (Dvorak et aI., 1987; Dvorak et aI., 1988b; Penning & Wilmink, 1987). These measure- (/987). Alteration in primary and coupled neck motions aller ments account for approximately 50r1'o of the lotal facete<:tomy. Neurosurgery. 21 (5). 681-687. cervical rotation. Another stereoradiographic study of neck motion in adult men found a mcan of 105° axial rotation be- tween the occiput and the C7 vertebra. Seventy per- cent of the total axial rotation OCCUlTed between the occiput and the C2 vertcbra. Eaeh motion segment between the C2 and C7 vertcbrae avcraged fTom 4 to 8° rotation (Mimura et aI., 1989). Less well appreci- ; i l-\",~~=~,...,.-.,-,~~-=-=-=--~;==;r.----,.--------:=,,~,'.\"\"'\".-.&f \"\" .. \",-\"_\",,,,,\"\"£1 - .~\"\"\"'\"- .-~- ~.~. •__ ' -,

permitting one, with equal ease, to gaze at an air~ \\ C5 plane overhead, glance over one's sho111dcI~ or look for an object under a table. An analysis of the com~ Analysis of surface motion of the facet joints of the bined motion of the cervical spine Llsing an eleclro- motion segment during flexion-extension. The sche goniometer produced a remarkably large range of drawing represents superimposed roentgenograms motion (Feipel et aI\" 1999): 122' ;;; 18' of nexion and motion segment in the neutral position and in sligh extension, 144°:!: 200 of axial rotation, and 880 ::': 16° ion. The upper vertebra ((4) is considered to be the of lateral nexion, All primal)' motions were reduced ing body and the subjacent vertebra ((5) is the bas with age. Sex had no influence on cervical motion bra. Two points have been identified and marked o moving body in the neutral position (sofid outline o range. and the same two points have been marked on the The active range of cervical spine motion re- roentgenogram with the motion segment slightly f (dashed outline of e4). li\"es connecting the twO se quired to perform daily functional tasks was studied points have been drawn and their perpendicular bi in healthv adults (Bennett et aI\" 2000), A cervical have been added. The intersection of the perpendi spine range of motion device was fastened to the sectors identifies the instant center of motion (larg subject's head with a Velcro strap and a magnet was ! dot) for the degree of flexion under study. The per placed on the patient's shoulders to calibrate the in- II ular bisector (arrowed fine) of a line drawn from th strument for measurement of cervical rotation and ter of motion to the contact point of the facet join motion, Of the 13 daily functional tasks performed, faces indicates tangential motion. or gliding. tying shoes (flexion-extension 66.7°), backing up a car (rotation 67.6°), washing hair in the shower .-,-~~~~~--~-~~- (flexion-extension 42.9°), and crossing the street (1'0· tat ion head left 31.7° and rotation he4:KI riglll 54.3°) required the greatest active range of motion of the cervical spine. Of interest, several tasks were not found to produce the degrees of lnotion expected, and these included reading a newspaper (19.9° Ocxion-cxtcnsion), writing at a table (26.2° flexion- extension), and reaching 1'01' objects overhead (4.3° flcxion·extension). Side·bending was not found to be a significant movement in completion of the tasks but was coupled \\vith rotation in one of the tasks (looking left and right to cross a street). SURFACE JOINT MOTION 30° of extension, r('spccti\\'c1~'. Convl'rsel~\", in lh~r~ \\\\'cre statistically significant increases o The motion between the joint surfaces of two adja- 10% at 20 and 30° 01\" fll.:'xion, l\"l.'spcclin~ly (Yoo cent vertebrae Illay be analyzed by means of the in· 1992), One p\"\"clical application of this dala stant cenlct· techniquc of Rculeaux, described in to cervical collars lIsed for the rdief of neck Chapter 6. The method may be used to analyze sur· Conventional foam collars tend to place pati face motion of the cen:ical spine during flexion· slight extension, which IlU\\.\\' aggravatc the extension and lateral flexion. toms. Turning a foam collar around. with thl.:' and narrow parl anterior, puts (he neck in In a normal cel'vical spine, the instant center of flexion-ex lens ion is located in the anterior part of flexion, which may increase the size of the in the lower vertebra in each motion segment. Instant tcbral foramina and thereb~1 relieve some center analysis indicates that tangential motion pressure on an inflamed nerve root. (gliding) takes place between the Facet joints as the cel\"ical spine is flexed and eXlended (Fig, 11-18), A orThe instant center motion of' the cClvica consequence of these rnotions is that the size of the intervertebral foramina increases with flexion and may be displaced as a result of patho decreases wilh extension (Fielding, 1957), These al- processes such as disc d~gcncrati()nor ligam terations have been quantified in a cadaver study pairment. In such cases, instant center analys that found there were statistically significant reduc- reveal distraction and jamming (compress tions of 10 and 13% in foraminal diameter, at 20 and the facel joint surfaces during flexion·extens sl~ad or gliding (Fig, 11-19),

Post. C4 there are 2° of coupled axial rotation for every 3 AnI. lateral bending, which gives a ratio of 2:3 or 0\".67 C7, there is lOaf coupled axial rotation for ev C5 7.50 of lateral bending, which gives a ratio of 2: 1 0.13 (White & Panjabi, 1990). Results of finite Schematic drawing representative of a roentgenogram of ment rnodeling indicate that the facet joints and the C4-CS motion segment of a patient injured in a rear- covenebral joints are the major contributOl\"S to c end auto collision. The instant center of flexion·extension pled motion in the lower cervical spine and thal at this level (represented by the large solid dot) has been uncinate processes effectively reduce motion c displaced from the anterior to the posterior part of (5 as a pling and primal)1 cer'vical motion (in the same result of the injury process, which impaired the ligaments rection as load application), especially in respo (compare with Fig. 11·18). The analysis of surface motion to axial roLation and lateral bending loads. I shows compression and distraction of the facet joints with covcrtcbral joints appear to increase primal)' ce flexion and extension. cal motion, showing an effect on cervical mot opposite to that or the uncinate processes (Clau • et aI., 1997). Coupling or nexion-exLension w transverse translation may be visualized roentge graphically (Fig. 11-23). During flexion, the VC bral body normally shifts rorward: the racets g up and over one another with pseudosubluxali Changes in normal coupling patterns occur foll iog palhological changes or surgical intervention Tensile load application to the cervical sp occurs in therapeutic traction, but more co monly so during trauma. Deployment of pass COUPLED MOTION OF THE CERVICAL SPINE Atlantoaxial Segment The coupling characteristics of the atlantoaxial A [ spinal motion segments arc particularly important, ~' as this area of the neck is extremely' mobile. The dens is constrained within the osteoligamentous \\ ring of the atlas, causing the Cl-C2 lateral masses to articulate similarly to the condyles of the knee, \\ with some sliding and rolling during Oexion and ex- tension. The instant centers of both rotation and \\1' n~xion-extension lie in the center of the dens itselr. i: Rotation at CI-C2 is coupled with both vertical B translation along the Y axis (Fig. 11-20) and a de- gree of anteroposterior disphlcenlent (\\,Verne, 1957). This implies that the CI-C2 joint is most stable in the neutral position, and, if rotated, allempts should be made to return it to the reduced position when performing an arthrodesis. Subaxial Spine Coupling of rotation and axial translation is depicted schematically. A, (1 and (2 are in the neutral position. The coupling patterns in the lower cervical spine arc S, C1 rises fractionally on C2 (arrow) as the head is rota slIch that in lateral bending to the left, the spinous away from the midline. Adapted with permission [rom processes move to the right, and in lateral bending to the right. they move to the left (Lysell, 1969: ofFielding, J.W (1957). Cineroentgenography the normal !vloroney et at.. 1988) (Figs. I 1-21 and 11-22). AL C2 . cervical spine. J Bone Joint Surg. 39A, 1280-/288. . _----~\" -:__ I'

vehicular n:.'S!l\"airH s.\\'stCtllS, such as airbags, induce tensile forces in the neck. Isolatcd vertebral discs fail at 569 N 54. and intac man cadaver cervical spines rail at 3373 N , (Yoganandan cl aI., 1096h1. Active muscular traction, 110\\\\'C\\'i..'r. is liki.'I~' 10 raise these fi cons i dl'J\"~1 hi v. Left lateral Neutral Righi lateral Abnormal Kinematics flexion flexion Abnonnal kinematics gCl1crall.\\\" refers to l'.'\\c Coupled motion during lateral bending is depicted motion within rUl1etional spinal units; howeve schematically. When the head and neck are flexed to the norrnal kinClnatics Ina..· also refer to at.\\vica left, the spinous processes shift to the right, indicating ro- terns of motion such as abnormal coupling or tation. The converse is also illustrated. Adapted with permis- doxicalmotion. Par~\\do.\\icallllotionis seen whe sion from White, AA 1// (~Panjabi, MM (1990). Clinical Bio- overall pattern of motion of one ~lSPCCI of the mechanics of the Spine. Philadelphia: 1.8. Lippincott. is in one direction while the local pattern is th posite. For instancc, parado'\\ical llc'\\iol1 is when llc'\\iol1 occurs at a single functional s unit, although the SpillL' as a whole is (''\\te AB A, This diagram illustrates some of the other coupled mo- results in motion of the spinous processes to the right tions, which occur in response to a torque (liZ) about the Z- left. B, The subject is bending to her right, demonstrat axis (lateral bending). lateral translation (Rx) and vertical the large range of normal cervical motion possible (ap motion (Ry) occur, as well as horizontal rotation (l!lY), which mately 50°).

Coupling of flexion-extension with transverse translation of (1-(2 articulation during flexion-extension; no translati the cervical spine is visible roentgenographically. A, Ouring evident in this example (black arrow). B, During extensio flexion the vertebral body shihs forward (small white ar- the reverse occurs, and the spinous processes limit motio row); the facets glide up and over one another with moder- they touch at full extension (arrow). The size of the inte tebral foramina increases with flexion and decreases wit II ate subluxation at full flexion (large white arrow). Up to 2.5 extension. mm of transverse translation may normally occur at the • These types of abnormal motions describc a pattern logical loads to maintain its pattern of displacem of mOVCIl'lCllt known as instability. so that there is no initial or additional ncurolog dcficit, no major defornlity,' and no incapacita Spinal Stability pain (1990). Using this definition, physiolog loads are those incurred during normal activity, The concept of spinal stability is an intriguing and pain is not felt to be incapacitating if it can be c sometimes confusing notion. Medical practitioners trolled by non-narcotic drugs. are frcqucnlly asked to look at a sel'ies of radi- ographs and make a dctermination whether the Stability is detcrmined by many factors. There spinc is stnblc. Exnclly what is stability, how is it de- different anatomical considerntions in different termincd, \"'ld what happens if it is not present? The gions or the spine. Certainly, ligamcntous anato term spinal stability has acquired diffcrent mcan~ plays a large part in the stability or the spine, but ings, depending on the setting in which it has been muscular and bony clements of the ~pine also used. White and Panjabi describe the term clinically important roles. as the loss of the ability of the spine undcr physio- Instability can be analyzed by considering kinem instability and structural or component instabi

Kinematic instability focuses on either the quantity of _ Conceptual Types of Instabilit motion (too much or too little) or the qualit..., of motion present (alterations in the nOimal pattenl), or both. Kinematic Instability Component instability addresses the clinical biome- chanical role of the various anatomical components of Motion increased the functional spinalunil. In this type of instability, los..o:; lm.tantaneous ax(,>s of rotation altered or alteration of various am.llomical portions detennines CDlipiing characteristics changed the presence or instability (Box II-I). Parddo;O:I(d\\ mOllon present Sir Frank 'Holdsworth's account of a simple two- Component Instability column concept of spinal stability provided a con- structive basis for describing and analyzing the ba- i\"raun1C1 orsic biornechanics the spine: TUIl10! The synal1hroscs bClwcen Ihe \\'cl1cbral bodies rely for Surgery theil' slabilit~, upon the immensely slrong annulus fi~ Degenerative changes brosus. ThL: clim'lhrodial apophys(.';:d joints arc sla- Developmental changes bilised by the capsule, by rhc interspinous and Combined Instability sllpraspinous Iigamenls and the ligamcnta Ila\\\"a. This K:nemiitK group of ligaments [ call the 'postedol' ligament com- Cornponen! plex.' It is upon this complex (hal (he swbilily of lh~ spine largdy dq:>ends (Holdsworth, 1963). ing I..'t aI., 1974). Antcrior displaceml... nt or CIo Subsequently, Denis (1983) described a classifica- or 3 to 5 mm is usuall:' indil';:ltive or a ruptu tion system for thoracolumbar fractures that can till' tr;:\\Ils\\\\:rse ligament. while disphlCl'Il1ClHS o also be applied to a biomechanical analysis of spinal 10 mill suggest nCCl.:'Ssol'\\ lig<.\\JlK'llt d~l11lage; stability. In this description, the spinal elements are plncC'tlll'lll greater than !O lllill occurs with ru divided into three regions that form three spinal columns: or all lhe ligal1lL'lllS (Fil.'lding et :.II .. 1(76). An I. The anterior column consists of the anterior translations or displacemcnts of Cion ('2 :J, longiludinalligament, the anterior annulus fi- sessed I'adi()graphicall~' by measuring tilt..' dis broslls, and the anterior half of the vertebral from lI1L' anteriur ring of the alias In the back o bock dc'J1' (atlaJ1todent,,1 intcly,,1 or ADI) (Fig. 1 (CasL' Stud.'\" 11-1). Poslt.:'rior subluxation of lhe 2. The middle column consists of the posterior C<.lll onl~' OCCUI\" if lhe dens is fractured or if tllL longitudinal ligament, the posterior half of all OS-OdOlllOidL'ulll or hypoplastic dellS. the venebrnt body, and the posterior nnnulus librosus. SOl11e diseases can wcakc'll or dL'S{nl~' th\\.' n.'l'sc ligament. :\\,10S1 nOlahl...-. s.vllo\\'itis in rhL 3. The posterior column consists or the pedicles, !oid arthritis can crL'~ltc a pannus. which hel facet joints, lam inn, spinaliS processes, as d(;SII'O,\\' the atlantoa.\\:ial ~1J'liClllalion as well a well as the interspinous and supraspinous lig- ll'ansn'!'SL' Iigamcnt (Figs. 11-25 and 11·26L aments. tients with Do\\\\'n s.\\'ndroll1c <.l1'C also susceptib \\\\,c;:lketlcd tmnsvcrsc ligaments and must be 4. The anterior and middle columns form the full.\" assessed clinicall.\\\" and radiographicnll .., b primary weight-bearing column of the spine, bdng allowed 10 pankipalc in sporting events with the posterior column providing the guid- as the Special OI~'l1lpics. ing and stabilizing elements. Stccrs \"Rule of Thirds\" is a guidt: 10 the am Occipitoatlantoaxial Complex or allanloaxial displw':cl11cl1t that can OCCllr b The transverse ligament of the atlas completes the socket into which the dens is inserted. The liga- spinal cord comprcssioll ensues (Ste~1. 1968) ment allows the dens to rotale, but limits its ante- illt~rnal anteropO-SIL'l\"inr diameter 01\" thL' atlas rior translalion. The ligament is inelastic and will pl'o.,\\jJl1alel~· 3 cm: of thal. the dens occupie not permit more than 2 to 3 mm of anterior sub- proximately I cm and thl' spinal cOI\"(1 approxim luxation of the first on the second vertebra (Field- I cm. kadng I CI11 of space for soh tisSlh.' an normal mo\\\"cnwnt to OCClll\" (Fig. 11-27).

subaxial Cervical Spine Atlantoaxial Instability Without Fracture (1963) stated that the musculature of the f\\:},9;Ye'a.r~oldwoman had a traumatic injllry as a result spine and the intervertebral discs were the most .'co\"J;t!.of a forced flexion movement in a car acodent. Con- significant anatomical structures is providing cer- 'vie'll stability. Holdsworth (1963) emphasized the hinuous and severe neck pain occurred after the accident. imparlance or the supraspinous and interspinolls ligaments as well as the nuchal ligament. The (s.,he visited the emergency room and after a careful ex· nuchal ligament is thought to playa major role in ~~~~rr,ination and x-ray evaluation, an anterior displacement SAC (2~'ZR.f:Cl on was discovered (Fig, cs11-1·1). ~:(E:::;A severe amerior dislocation of the atlas on the axis ~~:/~~as confirmed after measuring the atlantodental interval /':\"__i\"::\",'\" '.,1' . (~'rnrn). ,~or this case, no fracture of the atlas or the axis \\i\\I~~\"~:7,:te~,t,~d and thus a defiCiency of the transverse liga- lfIent,:'r;na'y, be assumed. Clinical instability of the spine depends mainly on the soft tissue components. The cervical spine is a very mo· ,bile area, especially at the atlanlOaxiallevel. Cervical sub- luxations and dislocations resulting from injuries of the osteoligamentous complex affect spinal stability and rna- ~ility at the cervical area. In addition, it may narrow the spin,al canal and cause neurological impairment. Pure lig. am~ntOtJs injuries (as in this case) are less likely to heal and become stable, and therefore surgical procedures are lik,ely to be considered. SAC I _ -rease. Study' Figure 11-1·1. lateral radiograph demonstrating . ,,~,?j}.n:~,~!~?sed atlantodental interval of 6 mm after trauma. I proprioception and correct functioning of the ~I L.- erector spinae muscles. Experimental section o The atlantodental interval (AD!) is inversely related to the the ligaments in sequence from either anterior to posterior or posterior to anterior suggests L1lat i I space available for the spinal cord (SAC), which is denoted a functional spinal unit has all or its anterior ele , by the dotted lines. Anterior atlantoaxial subluxation ments plus one additional structure or all or it causes a reduction in the SAC. Normal measurements for posterior elements plus one anterior structure in the ADI are less than 3 mm in adults or 4 mm in children. Reprinted V'lirh permission from Moskovich, R. (1994). Atlanro- axial instability Spine: Stare of the Art Reviews. 8(3), 531-549.

Flexion lateral radiograph of a patient with rheumatoid arthritis. The dens (0) has been eroded and the transverse ligament is incompetent, resulting in atlantoaxial subluxa- tion. The markedly widened atlanto·dens interval is indi- cated by the arrowed line. tact, it will probably remain stable under normal 1DmI, physiological loads. To provide for some clinical margin of safety, any malion segment should be ! considered unstable in which all of the anterior Postmyelogram cervical radiograph in a patient wi clements or all the posterior elements arc de- standing rheumatoid arthriris and fixed atlantoaxi stroyed or are unable [0 function (Panjabi eL al.. luxation. The space available for the cord has been 1975; White et al.. 1975). The clinical sLability of duced to 6.5 mm and compression of the proximal various injuries must be assessed individually. cord is evident by examining the subdural space. w The importance of clinical evaluation cannot be outlined in white by the injected contrast medium underestimated because significant spinal corel damage may occur after trauma even in the ab- , tient developed cervical myelopathy. which necess sence of fractures or ligamentous injuries (Gosch et al.. J972; Schneider et aI., 1954). Valuable 11 ...d._e_co_m.pression by transoral resenion of the dens c guidelines ror the detenllination of clinical insta- ,.. terior stabilization. bility in the lower cervical spine have been pro- vided in the form of a scoring system checklist orIII C~lS(.' Cjl1l...·slicJl1able in.iury. when (Box 11-2). ~Hl(1 extension ill~lI)Cll\\'l'rS should Ilol b Using this scale. the measurement of transla- formed, a stretch Lest l'~lll b~ d01l1:.' 10 asse tion takes into account variations in magnifica- deal inlq.!rit~·. A la(I.'ral CI.·ITiGll spine lions and is based on a tube-ta-film distance of ograph is taken al a standardized lube d 183 em. The 110 rotation is defined as 11 0 greater of 180 em. ~\\lld incrclll(,.'nud .i-kg weights than the amount of rotation that exists at the mo- plied as traction \\n lhe Sklllilisillg LT<.lllia tion segment above or below the functional spinal (Fig. 11-28). RadiogT~\\phs arc tak~n ark unit in question. The 3.5-mm value represents the addilion~d wcight has been npplicd. An radio-graphic measurement of the maximum per- missible translation when the radiographic magni- fication is taken into account (Panjabi et aI., 1986). /'i,' hl~~ ItM'~'\"'~\"~~M\" .' A~_.~' ,~;;;Sk0. .''''''~'iPi •.,••. ·W·,·_<· ,,,-- ',<>'< ' . \".....

Dens 1 \"3 Space available for the cord (SAC) is approximately two- Stretch test. A, Lateral radiograph of the cervical spine o thirds of the anteroposterior diameter of the spinal canal. 19-year-old boy who was admitted with multiple injuries One-third is taken up by the dens, one-third by the cord it- and neurological deficit compatible with an anterior cor self, and one-third is free space. Reprinted with permission syndrome. The radiograph shows increased angulation a ;rom MoskDvich, R. (994). AtlanrD-axial in5tclbility- Spine: State C3-C4 and a block vertebra at (5-(6. Flexion-extension r Df (he Art Reviews. 8(3). 531-5/19. diographs were contraindicated for fear of exacerbating his neurological injury. B. The stretch test was performed • to ascertain whether significant instability existed. No ab normal distraction occurred at the interspace in question mal Slretch tesl is defined as differences of His other injuries and fractures were treated routinely an ·'greater than 1.7 mm 'interspace separation or he was given a soh collar to wear for 6 weeks. He made slow but steady recovery with almost complete resolutio greater than 7.5 0 change in angle between pre- of his upper extremity deficit. with no evidence of instab stretch condition and the application of Ot1C- ity 1 year after the accident. Reprinted with permission (rom third body weight. Moskovich, R. (1997). Cervical insrability (rheumatoid, c!warfism degenerative, others). In K.H. Bridwell & R.i. DeWald (Eds.), T Checklist for the Diagnosis Textbook of Spinal Surgery (pp. 969-1009). Philadelphia: Lipp of Clinical Instability in the cott-Raven Publishers. Middle and Lower Cervical Spine •II Element • Point I ,A,nteri~r elements destroyed or unable to function Value- Applied Biomechan.ics I fJostenor elements destroye or unable to function 4, A thorough understanding of biomcchanical princ ples is an important aspect of the treating phy II Positive strelch test cian's knowledge base because the normal structu Radiographic criteria and function of the spinal ,column is frequent altered during surgcly. \\'''hether treatment is a d ! Flexion and extension x-rays c:omprcssive cervical laminectomy, a posteri ! Sagittal plane translation> 3.5 mm or 20% (2 pts) I-oraminotomy with partial facetcctolllv, or an ant rior cervical fusion, all of these inlen~'cnlions ha I Saginal plane rotatiDn > 20° (2 pts) certain ramifications with which onc must be fmn iar. This kno\\vledgc not only benefits patient care b OR also is valuable in planning and executing treatmen I Resting x-rays DECOMPRESSION Sagiltal plane displacement> 3.5 mm or 20%. (2: pts) Cervicallaminectom)' often is performed to decom Relative sagittal plane angulation> 11° (2 pts) press the spinal corel. The compression may caused by a stenotic process and can result I Developmentally narrow spinal canal II .:\\bnorrnal dISC narrowing 2' Spinal cord damage 1 ! Nerve root damage I Dangerous loading anticipated 1 -\"' Total of 5 or more == clinically unstable. I blodifiecJ from While. A.A. III & Panjabi. M.M. (1990). Clinical Biome<h- dnrcs oi the Spine (2nd ed., p. 314). Philadelph;ej: 1.B. lippincott. •

neurological sy'mptoIlls such as racliculopath.v or A nUlllber of studies using three-dimens my'clopath)'. Other posterior decompressive proce- nite element models showed that f::\\Cetectom dures such as partial or full facctcctomics for visu- greatcr errc'ct on annulus stress th<:ll1 (Hl in alization or decompression of nerve root pathology bral joint stillness. Based on these models also arc comnlonly performed. Development of concluded that a signific<:lnt increase in postlaminectol11)' kyphosis is well known in chil- strcsses and scgmentalrnobility nlaY ()ccur dren and may develop in 17 (Miyazaki et a!., 1989) lateral facet resection c:\\:ceeds S(Y'·'r (Kuma to 25% of adults (Berkowitz, 1988). [,ostlaminec- a!., 1997; VClO et a!., 1997), Deccnnpression tomy spinal deformity may..' occur in up to 50% of cal laminoplasty, in which the facet joints children who undergo laminectomies for spinal sacrificed and the laminae are reconstructed cord tumors (Lonstein, 1977). Simulated finite ele- in maintenance ()f fle:\\:ic)n-e:\\:tension and ment anal.'.'sis on cervical spines indicates that the bending stability', \\vith a nwrginal increase primary cause of postlaminecto!l1yr deformity is re- torsion. Iatrogenic injur.v is less likel:' to res section of one or more spinolls processes as well as capsules of the rernaining facet joints and the posterior ligamentous structures, such as the elements remain intact. ligamentum flavulll or interspinous or supraspinous ligaments. The removal of these structures causes Cervical subluxations and dislocations the tensile forces normally present in the cervical from injul':' may narrow the spinal canal an spine to become unbalanced and place extra stress neurological imp::lirmenl. In SOllle cases, a on the facet joints. Results indicate that either a reduction and re::dignment or the verteb kyphotic or a hy'periordotic cervical defonnity' may' lowed b~' stabilizatkm, may' decompress th ensue, depending on the center of balance of the elements without resecting bone (Fig. 11-29 head (Saito et aI., 1991). ARTHRODESIS Although reports exist of patients undergoing multiple-level cervical laminectomy with no evi- Spin::d arthrodesis is indicated in man:' dise dence of clinical instability or deformity' on long- cesses such as spinal instability, neoplas term follow-up (Jenkins, 1973), most biomechani- traumatic and degcnerative conditions of th cal studies indicate some degree of instability when Thc goa! of arthrodesis is to achicve a solid the posterior elements arc resected. Multilevel cer- sion in which two or more vcrtebrac solidi vical lan1inectom.v induces significant increases in In 1l1an:' cases, internal fixation is ust::.'d to total column flexibility associated with increased initial st::\\bilization as well as to correct defc segmental flexural sagittal rotations. In a cadaveric necessary. laminectomy model. the mean stiffness of the in- tact ce,·vical column was significantly' greater than An important principlc regarding v the mean stiffness for the laminectomized speci- arthrodesis is that the stabilit.\\' established men, and there were consistently greater rotations nal fixation is a prelude to the biological p as compared with the intact specimen (3.6 versus fusion. Although fusic)l\\ can occur without 8.0°) at ever)' cendcal spine level (Cusick et al., fi:\\ation, its appropriate USl' helps to increas 1995). sion rate and maintain structur::ll alignmen ware b:' no means supplants the necd for The loss of facet joints alone causes a significant gecm to perfol'ln a thorough and careful pre decrease in coupled motions that result from lateral of the vertebrae and add bone graft tCl ach bending. A moment about the anteroposterior axis sion. \\Vith few exceptions, Imrdw::lre that is results in a significant reduction in lateral displace- matel.v supported and protected by a soli ment, a decrease in vertical displacement, and a de- will fatigue and fail after a finite number o crease in rotation about the vertical axis. Partial There is ~l race b.\" the body to achieve a sol facetectomy «50% ) did not, however, significantly mass before a fatigue failure (If the implan alter flexion and extension movements (Raynor et tion device occurs, a!., 1987). Another anatomical study demonstrated that progressive laminectomy with resection of The choice of a surgical approach !() the more than 25% of the facet joints resulted in signif~ spine, as well as whether an anterior, poste icantly increased cervical flexion-extension, axial combined arthrodesis should be performe torsion, and lateral bending motion when compared pendent on the particular patholog.\" presen \\vith the intact spine (Nowinski et a!., 1993). performing a fusion, it is important for the to understand the biomeclwnical propertie

Unilateral facet dislocation in a 24-year-old woman who was involved in a motor vehicle accident. The degree of vertebral subluxation is less than half the AP diameter of tbe vertebral body. Her spinal cord was compressed and she presented with an incomplete spinal cord injury. A, Computerized tomographic scans and reformatted images demon- strate the canal compromise at (5-(6. An attempt to realign the spine was made using longitudinal traction of up to approximately one-third of her body weight. B, The lateral radiograph with traction applied shows persistent malalignment. An open reduction was performed using a posterior exposure of the vertebra. Once the spine was realigned, the posterior tension band was recreated using interspinous wiring, and autogenous bone graft was inserted. C, The radiograph taken after the operation demonstrates restora- tion of normal vertebral relationships. The fixation provided good stability and enabled the patient to mobilize early. She had a good clinical outcome. Reprinced with permission from Mos,l:ovich. R. (1997). Cervical instability (rheumatoid. dwarfism, degenerative. others). In K.H. Bridwell & R.L. DeWc1/d (Eds.). The Textbook of Spinal Surgery (pp. 969-1009). Philadelphia: Lippincott-Raven Publishers.

fcrent t)'PCS of fusion constructs. Cervical arthrode- tems have become available that can satis sis also has an affect on adjacent motion segments. stabilize the cervicul spine from either app Thcorcticall.',/, there is an increase in motion at nearby' unfused levels. Subsequent degeneration of \\Vhen an anterior approach is used, a dis other motion segments requiring additional levels or a vertebrectOlny at one or more levels is co of arthrodesis has been demonstrated in multiple studies (Cherubinc) et aI., 1990; Hunter et aI., 1980). performed, usuaIl-v followed bv. an anterior A recent study by Fuller ct al. (1998) evaluated the arthrodesis. The excised disc or bone mu distribution of motion across unfused cervical mo- placed with a structural graft or prosthesis t tion segments after a simulated segmental arthrode- anterior column support to the spine. Oss sis in cadaver cenrical spines. The authors simulated placement ma:v be in the fm'm of autogenou 01112-, t\\\\'o-, and three-level fusions in human cervical genic bone, comrnonly from the iliac crest i spines. They then moved the cervical spines through nous or from t.he fibula or iliac crest if alloge a nondestructive 30° sagittal range of motion and compared this range of motion with that of unfused 11-30). The more cortical nature or flbula gr cervical spines. The findings of this study were inter- esting in that sagittal plane rotation was not in- result in dela.yed incorporation compared\"\" w creased dispropc)J'tionatel)! at the cervical motion crest grafts, which, therefore, are preferred c segments immediately adjacent to a segmental The immediate postoperative strength of an.v arthrodesis. Although the authors ackno\\vledged bone grafts under axial compression on a certain limitations of the study, they proposed that a testing machine reveals that they \\\\'ill adequa cenrical fusion causes a fairly.' uniform increase in port the loads required in the cervical sp motion across all remaining open cervical motion ph.\\'sical properlies of human bank bone se segments; therefore, an increased potential for de- preferable to autologous bone grafts, espe generative change may exist at all cervical levels. Another stud.y was performed descdbing the inci- dence, prevalence. and radiographic progression of symptomatic adjacent level disease after cervical arthrodesis (Hilibrand et aI., 1999). Adjacent level disease \\vas defined as the development of a new radiculopath,Y or myelopathy that was referable to a motion segment adjacent to the site of a prior ante- rior cervical arthrodesis. The findings revealed that symptomatic adjacent level disease occurred at a relatively constant incidence of 2.9% per yeac A sur- vivorship analysis revealed that approximately 26(Y£) of patients \\vha had an anterior cervical arthrodesis would have new disease at an adjacent Icvel within lO y\"cars of the operation. The study also demon- strated that more than two-thirds of the patients who developed adjacent level cervical disease expe- rienced failure of nonoperative treatment and needed an additional procedure performed. Cervical Spine Fixation Lateral radiograph of the cervical spine of a 35-y male who had an anterior cervical discectomy at Arthrodesis of the cervical spine Illay be indicated interbody arthrodesis using autogenous iliac cres for a number of reasons, most commonly for bone graft. Note the maintenance of lordosis at trauma and degenerative diseases. lVluch research segment and the integration and remodeling of hus been performed to analyze the biomechanical advantages of anterior approaches, posterior ap- proaches, or a combined procedure. \"Vith the ad- vent of newer technologies, internal fixation s.\\'s-

Compressive Strength of Various Interbody Graft Materials Graft Type Mean ~ Standard Deviation Fibular strut 5.070\" 3.250 N Fibular strut significantly 1.150\" 487 N stronger than crest or rib Anterior iliac crest Posterior iliac crest 667,,311N grafts: P < 0.05 Rib 452 \" 192 N 1,420\" 480 N Pore size of 200 ~m signifj· Hydroxylapatite cantly stronger [han 500 j-Lm 200 f-l.m pore size 338\" 78 N pore size: P < 0.05 Hydroxylapatite 500 f.lm pore size Adapted with permission itQm Wittenberg, R.H.. Moeller. J., P. White, AA Ill. (1990). Compressive strenglh of autogenous (lnd alfogenolls bone grafts for thOf.lcolurnb.lf and cervical spine fusion. Spine. 15(0), 1073-1077. older patients (Wittenberg et aI., 1990). The biological Posterior arthrodeses of the ccndcal spine incOIvonuion or allograft appears to be satisfactory commonl:v performed after trauma but may also and its USe obviates the need to harvest autologous L1sed to treat degenerative, inflammatory, or n bone from the iliac crest, sparing patients additional plastic conditions. Unlike anterior fixation devi surgical trauma and potential complications. which arc solelyl lIsed in the subaxial cervical sp posterior fixation devices can extend up to the Calcium phosphate (bone) ceramics form a strong ciput (Fig. 11 ~32). Posterior fi.'\\ntion has commo bond with host bone because of a zone of apatite mi- consisted of various wiring methods, which crocrystals deposited perpendicular to the hydroxy- used as a tcnsion band posteriorly (Sutterlin et lapatite ceramic surface (Jarcho, 1981). Synthetic 1988). Within the past decade. however, techniq hydroxylapatite blocks used for interbody cervical utilizing screws in both thc atlantoaxial and sub arthrodesis in goalS produced similar fusion rates ial spine have become rnorc popular. and biomechanical stifFness when compared with autogenous bone (Pintar et aI., 1994). 11 is interest- Techniques for aLiantoaxial fixation have evol ing to note that the goat holds its head creCl, and from onlay bone grafting alone to cver~more sop thus loads the cervical spine similarly to human ticated methods of internal fixalion. Interlam bipeds. Goat models for cervical spinebiologicallbio- clamp fixation is clinically reliable and mechanical testing are, therefore. populal: (iVIoskovich & Crockard, 1992). Screw fixation te niques may confer increased translational and a A can inc thoracic anterior al1hrodcsis model rotational stability, but with greater surgical ri yielded contrary results when tested biomcchanically: A biomechanical evaluation of four commonly u autogenolls iliac crest grafts were stiffer in all motions techniques for posterior atlantoaxial fixation than \\\\lcre ceramic graft substitutes (Emery et aI.. performed (Grob et aI., 1992\\ Cadaver CI-C2 f, 1996). aturally grown coral is also a useful bone graft tional spinal units were tested in nexion~cxtensi substitute once it has been processed. Jt is available lateral bending, and rotation, while intact as we con1rncl'cially in t\\\\-'o different porosities (200 ~m and after a complete ligamel1lous injul)/. The fixa 500 lJ.m pore size). The grafls of lower porosity had a techniques used wcrc: compl\"Cssive strength comparable \\vith biconical iliac crest grafts, although they were much more briule. I. Sublaminar wire with one median graft They can therefore be considered appropriate for clini- (Gallie type) cal lise with rcspect to their compressive strength (Table 11-1). An increasing variety of prosthctic intcr- 2. \\Vire fixation with two bilateral grafts body cages ~\\re becoming available (Shono ct aL, 1993); (Brooks lype) long-tellll clinical results arc eagerly awaited. Addi- tional anteJior (Fig. 11 ~31) or posterior intcI11al ft:-.:atio!l 3. Transarticular screw fixation (Mager!) using plates and screws for added support ma.\\' be lIsed. 4. Two bilateral posterior Halifax clamps (Fig 11-33)

str<:ltcd similar 01' irnproved fusion rates com with stand-alolle bone grafts. Anolher group <:lssl.\"s;ed lilt, biollH.:L·hallical s it~· 01\" sen.'n different cervical n:construction ods using 24 c(lif cClyieal spinl' segments (KotC al.. 1994). Thl'~' studied Ihn:\\:.' posk'rior-nnl,\\' niques. anlcrior-onl.\\' u...·lyical platL·s. antcrior graft alone. and a combined <:lIlterior plate wilh (erior triple wiring. The results dcmollstnlled for onL'-lc\\\"L'1 instrlbilit\\', the stiffncss \\\"alues consllliciion with an a;1terior procedure alOlle si!.!nificanth- smaller than those of all the othL' st;ucts tcst~'d. Intcrl'slin!!lv, stiffness w<:\\s n.:sto prc-in,iul-y le\\'cb in specin'1L'nS \\\\'itlt lwo-lc\\'L'! i patterns reconstrucled using an~' of the three riot\" 1l'lL,thocls. TilL' findings do not SlIpport the C':\\clusi\\'L' ank'rior methods in either posteri An example of an anterior cervical plate applied over two motion segments on a model of the cervical spine (Peak cervical plate, DePuy Acromed, Inc., Raynham, MA). The screws do not penetrate the posterior cortex and are locked to the plate to prevent backing out. E I I E IThe Gallie technique allowed signincanLl.y morc rotation in Oexion, extension, axial rotation, and lat- eral bending than did the other three fixation tech- Posterior occipitocervical fixation using a highly mod niques, No significant differences were noted in the stainless steel loop (Ransford loop, Surgicraft; Redditc amount of rotation between the other three fixation Worcs\" U.K.) wired into place on a skeleton model. T techniques, although the Magerl transarticular cranial loop is attilched to the occiput Llsing wires tha screw fixation tended to permit the least amount 01' pass through paired full,thickness burr holes. The wir rotation, as might be expected, not passed around the foramen magnum. The loop is bent to conform to the posterior craniocervical angle A variety of anterior cervical plating systems are may be adjtlsted intraoperatively. The limbs of the lo currently available, and many have been subject are attached via sublaminar wires at each level. Note to biomechanical analyses. Investigators demon- the axis laminar wires are tightened below the flare strated that in a single-le\\'e1 procedure, an anterior loop, which serves to maintain distraction between th cel\"Vical plate serves as a load-sharing device rather ciput and the axis vertebra. Sublaminar wire use is on than as a load-shielding device, enabling graft con- many techniqlles available to achieve segmental post solidation as observed in other clinical studies fixiltion. Reprin:ec! (''1iIh permission from tvioskovirh R.. e (Rapoff el aI., 1999). Bone lInion may be expecled to (2000i. OcopitorervJCaJ stabi/izallon for mj'eJopathj' In pat occur at a lower rate if the bones are shielded from : ','Itil rlleumdtoid (1f!hritI5 ImpliratlotlS 0; /lO! ball£' gr(lf;lflg compressive forces; however, clinical experience BOllc JOlni Sur£). a2A. 349-365 with anterior cervical plates has generally demon- ! •-r

C1 equipped vehicle (King & Yang, 1995). Flowever, long after airbag devices becan'lC available, air C2 injuries involving front seat passengers began to described and by late 1997, 49 child deaths and Diagram of a skeletonized atlantoaxial motion segment seriolls injuries had been attributed to passeng viewed from its posterior aspect. The method of applica~ side airbags (Marshall et aI., 1998). tion of the interlaminar clamps can be seen. The bone grafts lie under the Halifax clamps and allow the posterior A retrospective review was performed by three construct to be locked tightly, providing immediate stabil- diatric radiologists to determine if a pattern of inj ity. Reprinted with permission from rvloskovich, R., & (rockard, was common to this new mechanism of pedia HA (992). Atlantoaxial anhrodesis using inrcr!aminc1{ clamps. trauma (Marshall et aI., 1998). Their findings, as w An improved technique. Spine, 17, 261-267 as the findings of other studies, demonstra that passenger-side airbags pose a lethal threat threc-colurnn instabilit.y. In three-column or multi- children riding in the front seat of an automo level instability, combined front and back proce- (Giguere et aI., 1998; McCaffrey et aI., 19 dures utilizing an anterior plate and posterior triple Mohamed & Banerjee, 1998). Many of the child wiring demonstrated clear biomechanical advan- killed in these accidents \\vere front seat passeng tages. Clinical use of anterior plate fixation without involved in low-speed collisions in \\vhich the dr posterior fixation for three-column cen/ical injuries, sustained no or only minor injuries; the pattern however, resulted in satisfactory clinical outcomes injury' seen is different in the rear-facing infant (Ripa et aI., 1991). These results underscore the seat versus the fOl\\vard-facing child car seat. In need for good clinical evaluation and studies that formec the injury' to the infant \\Vas often mas take into account the fourth-dimension-time. Fuw skull injury and cerebral hernatoma as a result of sion is a biological process that occurs over time proxirnit)' of their heads to the airbag, while in the and supercedes the importance of in vitro or ter, children sustained man)' more cervical injur computer-simulated biomechanical studies. '1\\\\'0 of the older children had autopsy findings o lanto-occipital dislocation and one sllstained a \"n BIOMECHANICS OF CERVICAL TRAUMA decapitation\" injury, demonstrating the vulnerab Airbag Injuries of the pediatric cervical spine to the explosive fo Motor vehicle accidents continue to be the leading of an expanding airbag that hyperextends the ch cause of injury-related deaths in the United States. fragile neck. Recent guidelines from the NHTSA ( [n 1984, the National Highway Traffic Safety Ad- 11-3) were prompted by reports of airbag injurie rninistration (NHTSA) required that automatic oc- children and emphasize that children of an}' cupant protection devices (airbags or automatic should be properly secured in the back seat (Natio scat belts) be placed in all automobiles in the Highway Traffic Safety Administration, 1996). 1987-1990 model years. In 1993, the passengelcside airbag was introduced. Studies generally' concluded A mathematical simulation was performed that front seat occupants are adequately! protected study the potential of head and neck injury to an against frontal impact if belts are worn in an airbag- belted driver restrained by an airbag (Yang et Nai;ional HighwayTraffic Safety Administration .Guidelines Regarding Airbags & Children The back seat is the safest place for children of any age to ride. Never put an infant (less than 1 year old) in the front of a c\"ar with a passenger-side airbag. Infants must always ride in the back seat, facing the rear of the car. Make sure everyone is buckled up. Unbuckled occu- pants can be hurt or killed by an airbag.

1992). It was found lhal when the standard 20° an- elude inlerspinous ligamcllt tears, spinous pr gie steering wheel was used, neck joint torques were fractures. disc rupture. ligamcntum f1avulll nlp facd joinl disruptiun, and stretching of 11K.' ~\\I decreased by 22%. The resultant head acceleration musdl's. The diagnosis and managcll1clH of lash injuries an.. oflel1 t.:ol1l\"oUIl(!L·d b~r concom increased 41% fTom the baseline study when a ver- psychosoci;':il ,lilt! IlH.'dicolcgal isSlIl...\"S':IS well (\\ tical steering \\vheel \\\\\"las used. \"If the verlical (limen- et ,Ii., 1998j. sian of the airbag was reduced by 100/0, neck joint torques were increased by l4%, while head acceler- One of the earlv (unpublishcd) biomceha ation showed a slight decrease of 90/0. Although studies or whiplash injury was clone b.v the la ideal dimensions and inflation rates for airbags re- Irving Tuell. nil orthopuedic surgeon in Se main elusive, their use has resulted in a significant \\V~\\shillgton. I-h.: uscd a cinl' camenl to photog reduction in head and neck injuries. himsdf driving as he was rammed from behin his surgical J'csidL'nt driving anothLT car. Fr Whiplash Syndrome drawn frolll Lhat movit.' ckarly demonstrate til pen:xtension of his neck over the sl.'alback of h Whiplash syndrome is a complex se' of symp,oms ,hal (Fig. 11-34). One C;:\\l\\ also scc thl' dlect of in may present after an acceleration hyperextension in- l\"orces on tht.: mandible as his jaw snaps opt'n jury. These injllIies typically occur when a Car is struck the Hcccicratioll forces his !lead backwards. From behind, bUl may also be caused by lalcml or Fron'al collisions (Barnslev el aI., 1994) (Case Study mechanism Illi.l\\ explain the tL'mporomandi 11-2). The acceleration of the car seat pushcs the torso injuries thal (In,.' a COllllllon accompanilllL'nt o of the occupant forward with the result that the un- \\'ical \\\\'hipbsh injuries_ supp0l1ed head falls backward. resulting in an exten- sion strain to the neck. A secondaJ)' Aexion injlll)' may r\\ recent sLud~' of rcar·end collisions qual occur if the vehicle just struck then strikes another ve- !i\\'d~' elucidated the m.:tllal neck movements hicle in front and just as suddenly decelerates again, wke place (Castro CI aI., 1997). A1kr throwing the occupant forward once more. Crowe ramllled from behind, the Illcan accl'ler<:\\tion coined the tellll 'whiplash' in 1928 in a lecture on neck target \\'chicles was from 2.\\ to 3_6 g. Maxim injllIies caused by rear-end automobile collisions in the tension was n.'ached when the head contacte headrest; the angle b('!wt...·L·n the head and t United States but latcr reported that he regretted using bOth' varied From 10 to 47\" (meall = 20\"). In th the term (Breck & Van Norman, 1971) because il de- scribes only the manner in which a head was moved orSCIlCL' a headrest, tht..., maximal r('cordcd e suddenly to produce a sprain in the neck and not a spe- cific injUl}' pattern. Therefore, we refer to this clinical sion was 80\". FollOW-LIp clinical and rvIRI cxam entity as whiplash syndrome, not as whiplash injllry. tions were pcrformt.:d_ The sttlcl~! concluded Allhough whiplash injuries are a common trau- the \"limit or hannlessllcss\" for stresses ar matic event, the pathology is poorlv understood. 01'- len the severity of the whiplash trauma does not from rear-end impacts with regard to the ve changes lies bd\\\\'cen 10 and 15 km/h, In stud con-elate with the seriousness or the dinical prob- reproducible \\\\-hiplash trauma mode! using w lem, which can include neck and shoulder pain, cervical spinc specimens 1ll00Jnh.:d on a belH.: dizziness, headache, and blurring of vision (Brault sled was used to simulate rear-end collision et al., 1998; Ettlin Cl aI., 1992; Panjabi el aI., 1998a; increasing horizontal accderations applied t Sturzenegger el 'II., 1994). Apart Ii'om a frequently sled (Panjabi ct 'II\" 1998a.b). Bo,h sled and observed loss of physiological lordosis, a radio- graphic examination of the cervical spine is often kinematics CHIl be measur'cd using potentiom normal. Even newer technology such as MRI is not and acceleroll1l.~tcrs. Using this whiplash modt always able (0 I-eveal a soft tissue injul}'. MRI exam- S-shapL'd cun:c was (kscrihed in whiplash in ination of the cerebrum and cervical spinal column in which the lowt.:'r cervical spinC' h.\\'perext perrormed 2 days after a \\vhiplash neck sprain in- ~JIld the upper cCITical spine flexed (Grauer JUI)' in 40 patients did not deli~cl an)' pathology con~ 19(7). The invesLigators r~h that Ihe in.iur~' w currcd during the hyperextension phase i '0nected lO the injlll)', nor was the MRI able predicl knvcr cervical spinc, symptom development or outcome (Borchgre\\fink d Correct positioning or adjustable headn..'s aI., 1997). Injuries thaI have been documented in- hind the skull. not hehind the Ileck, is impo Tests using a H~'gc sk~d and a Hybrid III dll wcre pcrrorl11L:d by other iJ1\\'estigalOrs to deter

fi .•,;;;;: _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ------------ ------------------------.- ,, ;\" '.. ,~ from Fr(1nkel, VH. (1972). Whiplash injuries to the neck. In C. Hirsh & Y Zotterman (Eds.), Cervical Pain (pp. 97~111). if ! Whiplash Syndrome New York: Pergamon Press. ;: Case Study Figure 11-2-1. ::;:':»',::'i:::)'//.:::,,':;)'::-' P,}\\ 32-year·old man was injured in a car accident. The trau- ::~9>:~;:_~i~;~ matic event happened after being struck from behind. [n /~~{~:d{t:ih.iS case. the inertia of the head and the flexible spine re- \"?g;'sulted in marked hyperextension movement of the head. The .¥a~celeration of the car seat pushed the torso of the occupant $Jo~va{d with the result that the unsupported head fell back- lr -;.'ft~~ard, resulting in an extension strain to the neck. :,~?;~ The palient presented with severe neck and shoulder pain ;7'''':; j- accompanied by sleep disturbance. The right sternocleido- mastoid muscle was approximately twice as large as the left t, sternocleidomastoid muscle. After muscle testing against re- >.: 'f.. sjstance, the pain increased (Fig. cs 11-2-1). ..t. ~> 'To decelerate the posterior rotating head, a moment and g>:a.force must be developed by active and passive stabilizers, i.-?int surfaces, and the intervertebral disc. The moment and force give rise to tension, compression, and shear stresses .',: .find strains in various parts oi the neck Cdusing, damage. fl., During the collision, rhe muscle tension increases with ihe , velocity of lengthening. A tension inappropriate for the length4 tension curve occurs and partial rupture of the sterno- cleidomastoid muscle is produced. Reprinted with permission '.- -.. I I I IID' IIDI B _ 1~_H.a_nd_-_d_r.aw_n.c.e.I .,• (A) Before and (B).tra.a.hc.eedr_bf'.oe.min_9t_wr_aom_f.mr_aemd_ef_r'o_om_f_ba.eC.hini.ne.d-.mb_oy_Va_nieo_ot.hf.De.rr_.cI.raVr.i_dnr9i_vTe_un_eb_ylI_dh.,i.i,V.rine.9,i_dhe.i'n.ct.a_.r- ,~. ; ..,.- ...

biomechanical responses for the various conditions plied, which protects the disc from catastrophic observed in normal driving (Viano & Gargan. 1996). lIrL' ulltil e ... trl\"lllcl~· high loads are applied. Risk of injury was assumed to be proportional to neck extension. A low headrest position carries a 5 Vi\".'I·lL\"bl·al bod~' cUlllpr~ssioll str~ngth incre relative injl1r~' risk of 3.4 in rear-end crashes. com- from upper ct.'n:ical 10 IO\\\\'L'r lumbar lev-cis. pared with 1.0 for the favorable condition. If all ad- justable headrests were placed in the up position, 6 TIlL' mean sagillal diametL'r of the malt.- a the relative risk would be lowered to 2.4, a 28.30/0 re- spinal canal for C3-C7 is close LO 14 Illm; the sp duction in whiplash injury risk. cord diameter is about 10 111 Ill. Significantly more complex acceleration injuries 7 LigamclltuJ11 flavlIJ11 is undt..'1' tension ascribed to high G-force activities are now occurring. when the spine is in a Ill:1Itral position or somew There is a report in the Htel·ature of cervical injury in cxtended. prestressing the disc and pro\\'iding s pilots of F- t 6 fighter planes, citing a I-year preva- intrinsic support 10 tht..' spine. lence of neck injury of 56.6% and a career prevalence of neck injury of 85.4% (Albano & Stanford. 1998). S iVlllscles pla~' a critical role in b~lsic post homeostasis. Patients with paral~'zL'd ct..'lyical m Maintenance of neurological homeostasis and cles arc unable to support thdr heads. protection of the spinal cord. nerves. and vessels and support and protection of the skull are the ulti- 9 The spinal cord has some longilUdinal c1a mate tasks of the cervical spine. An appreciation of it.'· blll it tole-ratL's axial translation pond.\". It is the principles presented should afford a greater un- translatolY forces {hat t.\\'picall,\\· rcsult in neurol derstanding to physicians and allied health profes- cal injul·.\\', sionals involved in the treatment of cervical spine pathology. 'vVith increasing technological advances 10 Instant c('ntl.'!' anal.\\'sis indicates that tange in our society, \\ve remain vulnerable not only to lnotion (gliding) wkcs placl.· bel\\\\,(.'c·]1 lltt::.' facet jo common types of cervical trauma but also to idio- as lItc cervical spine is flexed alld extended. The syncratic methods of injury' related to these new technologies. Acceleration injuries have been OCCUI'- or the intelTcrtebr~il !\"()ramina inc.TeDscs with fle l'ing for decades but cervical spine injuries caused by airbags have only recently been described, and and deCl'l:ascs \\\\'ith extension, aliI' involvement in increasingly greater speeds and higher risk activities places LIS in increasing jeop- 11 Kinematic instability' refl'rs to the quantit ardy, As our body' of knowledge expands, we con- motion (too much or too little) or till.' quality of tinue to discover new methods of injury. It is critical tion present (alterations in lile normal pattern to pursue rational treatments for these disorders both. Component instahilil.\\' addrcsses the clin based on sound biomechanical principles, biomechanic~ll role of till.' \\'ariolls anatomic st tures of the functional spinal unit. 5 11111111 a Iy 12 An.\" motion s~~glllent in which all of the a 1 A functional spinal unit or motion segment rior ek'llkllts or all the posterior ekments m·e consists of t\\\\'o adjacent vcnebrae and the interven- stn>yed or are unabk' to function should be con ing intervertebral discs and ligaments between the ered unstable. vertebrae. 13 A significant incn:asc in annulus stresses _2 Whcn eithcr rotation or translation of a body scgrnclltal mobility' may' OCCUI' when bilateral f along one axis is consistently associated with a si- resection exceeds 50(*;. multaneous rotation or translation along another axis. the motions arc coupled. 14 Appropriate use or internal fi:G1lion helps L (:reasc the fusion rate and maintain structural a 3,; Intervertebral disks cxhibit viscoelastic proper- ment. ties (creep and relaxation) and hysteresis. 15 Front seat occupants are adcqLH.llcl~' prote ~lgain.s{ frontal impact if scat belts arc worn in (4;' Discs arc able to withstand greater than nor- airbag equipped ,·chick. Passenger-side air pose a lethal threat to children riding in the f l1l~r loads when compressive forces arc rapidly ap- scat 01\" an automobile. 16 \\'Vhiplash s.\\\"ndrome is a complex set of s~ tOI11S thal may present aher an (\\(\"ccleration hy extension injury.

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Herkowitz. H.N. ( 1988). .-\\ comparison of ~ln\".:rinr cer\\'ic~ll fu· .\\li.\\:uaki, K.. T'I(b. h .. \\lahIHb. Y.. l'[ :11. / 19ii9J. I'o:- sion, (:cr\\'ic~t1 bmilll..'ctollly, and cc:rviC<11 hllninopl:lsty for l·\\ll·n:>i\\'\\.· silJlllll:llIl'(JIl~ lllulti:>l'gllll'lll:tl 11I:-lllll fC'I- l'l'!T the surgical managi:nH:nt of Illultipli: kvd spondyloti\\'\" 1JI.\\dl'P~lthy widl l-l'l\"\\'il':c! ill~l:JhiliIY :tlltl k~phlJlil' :t1 S·:-hapnl ddorlilitil·~. Spi!ll'. 1,1{ 11 L 1160-1170. radicul()path~'.5pillt', 13, 774-780. \\I<oh:Il11nl. :\\.:\\. & Ball\\.·rin·. ,\\. (199$1. P;lltl:l\"lI:> of injun Herzog, R.J .. Witns. J.J .. Dillingham. M.F.,l'l al. (1991). Nor- 1:>oL\"i;'lll'd wilh :llltullH,hik ;'lirh;lg Il~l·. J1o.'1.~nl/l .\\I•.'d J1l~d (:elyiGII spinc morphomelry iwd ce.:rviL\"ul spin:lI slcno· 4:15--i38. si:-: in ,lsymplOmatic professional foot hall players. Phlin .\\({lrolll·\\. S.P. &. Sl,.·llUh/, :\\.B. 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Acta Orr/IO{J Sea lui, No, 146, h:lg akn. Allil LJllc'I~!,: ,\\It'd, 2S, 14~. King, A,I. &: Y.Hlg, K.H. (1995), Research in biolncchanic:-I of \\owinski. G.P.. Vi:-ariu:-. II.. \\olk, I..P.. l·t al. (1993).:\\ occupant pn.Hcclion, J hal/IIUI, 33. 570-576. lIIl\"l-klilicll comp:lri:»un or Cl'l\"\\'k:t1 lalllinopbsly :lIal K.il:dl~lra, Y.. Uda, B .• & Tachihana. S. (1995), Effect of spin:d \\·il·:III:tlllin\\.'i,·lomy with pn1grl'::.:-i\\·l' bn·I\\,·d(llll.\\. Spine cord stn:Lching uuc to head fk.\\ioll on intr~lnH:duliary 1995-1004. pn:ssul'c. NI.!/lrol ,VIed Chi,., 35. 285-288. Os,,-·nbach. R.t\\.. l\\.;. .\\kne\"lt.'~, A.II. {1\"JS9). Spinal l'(}rd in Kot;'llli. Y., Cllllningh~un, B.W\" Ahumi, K., el a1. (1994). l3io· WilllllUI I\"i\\diog:raphit:: ... hl1()rfll<.llit~' ill (hildrl'n. !'e .\\\"('IIHhCi. 15. 168-174, mechani<.:.d anaiysis of cervical st:ibiliz<\\lioll systems, :\\11 Pang, D. &: f'ollad, I.E (19S~)' Spill:t!l..\"ord ill.i\\lr~· withou i1SSCSSll1Cllt of tl',l(lspedicllhlr screw fixation in the l.,'\\\":r\\'ic;~\\l diogr:lphil' ,dmorlll'llil.\\ in L\"lIildrl'JI-Thl' SCJ\\\\·OR.,\\ drollll·.l li\"lllllllll, 19. 6~-I-(l6-1,' spinc. SpiJlt·. /9, 2529-2539. P:lni\"bi, ,\\I ..\\L Cholc\\\\\"id,i, J.. i\\ihu, K.. l't;,)1. (I99~h). S lation (If \\\\\"hipla:.;h Ir:tlllll~t using w!l(,k (l'rdr:,1 :.;pinl' S Kriss, V.M, I.~ Kriss, T.e. (1996). SCIWORA (Spill~ll cord injury illll·n:.;. Spillc', 23. 17-14. without rlldiogr;'lphil; <lhnormality) in infnnts and chil· Panj:lhi. ,\\1.:\\1.. Chn]\\.'wil\"ki. J.. \\!ihll, K .. l'1 :t1. {199:-bi. Ca br lig~1Il1l'1l1 strt'lchl's during ill \\'Itro whiplash :o;i dren. CJiIl Pt,·dii/II\", 35, 119-124. li(IIl:-.l Spimd Dison/. 1/, 21i-l.'1. KUlll:ln:san, S., Yo~an;'lIldan. N.. PimaI', EA., et al. (199;). Fi- Panjabi. \\L\\1., I\\.r~lig. .\\1.11 .. & God. V.I\\.. (19~I). A 1l'.... hn nih..' ,·lell11.'!H modeling of cervical Iamillt:ctonw with for m\\.'i.t:'url·IlH.'llt alld dl'sl\"l'iptioll of lhrl:('-diml·ll::.iolla gr'l(k'd f.ICCtcClOI1lY. 1 S~)ill(J! Dison!, 10, -10--16. . dl..'g:rl'I.'-of-frl'l'dolll motioll 01':1 body joint \\d[h :1Il app lion totltl' hum,1l1 ~Jlinl:. 1 WIlIJh'dltJlJics. 1-1. -i-l7--i60 Lonstcin, J.E. (1977). Posl.laminc:clomy k.... phosis. CliI, Or· P:llIjahi, M.M,. SUlllllll'r:-, 0.1.. Pdkt'r, R.R .. l't al. (19 t/lOp, /28,93-100_ Thr('l..'-d;l1ll'lISion:,lloitd-displ:'I.'l,.'l1ll'lll CIll\"\\\"('S dui..' LO fo Lysdl. E. (1969). ,\\-Iotion in thc cer\\'ic~\\1 spine. An eXIH.'rilllcn· (111 till' l,.'l'l\"\\·il'al spilll' . .f On/lOp Uf.\", -I, IS~-161. 1<11 sludy on autopsy specimcns . .'lcla OnllOl' S,:cwd, SlIpp! /23. 1-61. Marshall. K.II'.. Koch. B.L.. & Egelhoff. J.e. (1998). Air bag. rcl<\\lcd dC<.lths and seriolls injuries in childrl.'n: Injury p;,H· terns ~Ind imaging findings. A'/I J Neuwnlt!;o!, 19, 1599-607. j\\·kBroom, R.J., Ha\\'t's. W.c., Edwards, W.T.. d nl. (1985). Prl.'diction of \\·c'-·Il.'bral body cOl1lprcs~in' fn.lClurc using quantitath'c compuled tomogr'lphy. 1 BOlle loil1f Surf,:, 67A. 1106-1114. McC.11'frcy, M., Gl.'rman, A., Lalondt:, F., (.'t al. (1999). Air bags ~IIH.I children: A pOlcntinlly I!.'lhal (ombin:ltioll. 1 I'l·dillir On/lOp, /9, 60-64. ~'lil1lllnl, M., tvtoriya, I-I.. Watanabe, T., ct al. (1989). Threl.'· dimension,11 motion anal~-'sis of lhe cer\\'ic:d Spillt: with spl:cial n.:fcrcnct: to lhe axial rotation. Spille, 14, 113;-1139.

Paniabi, ,\\1.,\\-1., \\Vhile, A.A. III. & Johnson, R.,\\1. (1973). Cer- Simmons, E.H. & Bradlev, D.D. (1988). Nturo-myorat \\:ical spine mechanics ,IS a I\"unction 01\" transection of corn- flexion deformities (/ the cervical spine. Spille, pOneJllS. 1 HiOlllt'c!/(lIIics, 8, 327-336. Penning, L. & Wilmink, J.T. (1987). Rotation of the cCl'vical 756-762. spine. A study in normal people. Spille 12, 732-738. Perry, J. & Nickel. V.L. (1939). Total cervical-spine fusion for Steel. H.ll. (1968). Anatomical and mechanical conside neck par,t1ysls. 1 BOIIt, loilll SIIJ'g, .Jl:\\, 37-60. Pintar, FA., \\himan, D.J., Hollowell. J.P., L't 'II. (1994). Fu- tions of atlanto-axial articulation. In Proceedings of sion rak and biorHechanical stillness of hydroxylapatite American Orthopaedic Association. J BOlle Joil/I SlIrg, 5 versus autogenous bone grafts for antnior discectorny. An in vivo animal study. Spille, 19, 2324-2328. 1481-1482. Pintar, F.A., Yoganandan, N., Pesigan, ~'1., L'l a!' (1995). Cervi- cal vertebra! strain llleasuremellts under axial and eccen- Sturzenegger, .\\1., DiStel\"ano, G.. Radano\\', B.P., et a!' (199 tric loading. 1 Bioll/cch Ellg, /17, 474--478. Presenting symptoms and signs after whiplash injury: T RapolT, A.J., O'Brien, T.J., Ghanayem, A.J., L'l al. (1999). An- influence of accident lllechanisms. NCllrology, .J.J, 688-6 terior cervical graft and plate load sharing. J Spillill Dis, 12,45-49. Sutterlin, C.E., McAfee, P.e., \\Vardell, K.E., ct al. (1988) Raynor, R.B., Moskovich, R., Zidel. P., et a!' (1987). Alteration biomechanical evaluation 01\" celYlcal spinal stabilizat in primary and coupled neck motions after fncell.'ctomy. NellnlslIl'.i.iery, 2l, 681-687. methods in a bovine model. Static and cyclical loadi Ri:ill. J. (1960). Effects of flexion-exti:lIsion movement of thi: Spillt', 13, 793-802. head and spine on the spinal cord and nerve roots . .I ,VCII- ml ,VClII'OSlIr.i.i Psychi(/IJ', 23, 214-221. Takebe, K\" Vitti, :v1., & Basrnajian, J.\\!. (1974). The functi Ripa, D.R., Kow,dl, \\I.G., \"'!eyer, P.R. Jr., et al. (1991). Series of semispinalis capitis and spli:nitls capitis muscles: of ninety-t\\\\·o traumatic cervical spine injuries stabilized with anterior ASIF pl,lte fusion technique. Spille, 16(3), electromyographic stud.v. Allat Rec, f79, 477-480. 46-55. Viano, D.e. &. Gargan, il,'l.F. (1996). Headrest position dur Rolander, S.D. (1966). \\-lotion of the lumbar spine with spe- cial refi:rence to thi: stabilizing (.'ffect of posterior fusion. normal driving: Implication to neck injury risk in r An experimental study on autopsy specimcns. /lcw Orthop Snllld,90, 1-144. crashes. Accidellt Allal fln'I', 28, 663-674. Roozmon, P., Gracovetsky, S.A., Gouw, G.J., ct a!' (1993). Ex- Yoo. I..M .. Kumar\"san. S .. Yoganandan. N.. 01 at. (1997). amining motion in the cervical spini:. I: Imaging systems and measurement ti:chniqui:s. 1 Bio/lled Ellg, 15,5-12. nite elemi:nt anal~'sis of ci:rvical facctectomy. Spille, Saito, '1'., 't'amamuro, '1'., Shikata, J., et a!' (1991). Analysis 964·-969. and prevention of spinal column deformit.;.' following cer- vical laminectomy. I. Pathogenetic analysis of post- Wallis, B.J., Lord, S.M., Barnsky, L., et al. (1998J. The p lamini:ctomy deformitii:s. Spilll.', 16, 494-502. chological profiles of patiellts with whiplash-associa Schneider, R.e., Chi:rry, G., & Pantek, H. (1954). The syn- drome of acute ci:ntral cervical spinal cord injury with headache. Cephalalgia, IS, 101-105. special reference to the mechanism involved in hyperex- tension injuries of the cervical spinto J /\":I.'ZlroslIJ'j;, 11, WertH.', S. (1937), Studies in spontaneous alIas dislocati 546-577. Acta Ort/lOp Sewld, 23 Suppl. Schultz, A., Anderson, G., ()rtengri:n, R., et al. (1982). Loads White, A.A. Ill, Johnson, R.~L Panjabi, :\\1.M., et al. (197 on the lumbar spine: Validation of a biol1ledwnical anal\\'- Biornechanical analvsis of clinical stabilitv in the cerv sis by measurements 01\" intl'adiscal pressure and myoell'c- tric signals. J BOllI.' .Ioillt SlIrg, 6\",.1, 713-720. spill(:. Clill Orl/top, l(N, 85-96. . Shono, Y., ivlcAI\"ee, P.C., Cunningham, B.\\V\" et at. (1993), A White, A.A. III & Panjabi. M.\\1. (1990). Clillical Bio/llechal biomeclwnical analysis of decompression and reconstruc- tion methods in the cervical spine. Emphasis on a carbon- oj' tlze Spille. Philadelphia: J.B. Lippincott. fiber-compositc cagt. } BOllI.' .loilZI S/lrg, 7504, 1674-1684. Wittenberg, R.l-I., i\\loeller, J.. &. \\Vhite, A.A. III. (1990). C pressive strength 01\" autogenous and alJogenolls b grafts 1\"01' thoracolumbar and cervical spine fusion. Spi 15(101. 1073-1077. Yang, K.H., Latour. B.K., & King, A.1. (1992). Computer s ulation of occupant neck rtsponse to airbag deploymen I\"rontal impacts. 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ik Biomechanics ~: ~ YJ,j i>' of the Shoulder f/, Craig 1. Della Va/Ie, Andrew S. Rokito, Maureen Gallagher Birdzel Joseph D. Zuckerma Introduction Kinematics and Anatomy Range of Motion of the Shoulder Complex Sternoclavicular Joint Acromioclavicular Joint Clavicle Glenohumeral Joint and Related Structures Glenoid labrum Joint Capsule Glenohumeral and Coracohumeral ligaments Additional Constraints to Glenohumeral Stability Scapulothoracic Articulation Spinal Contribution to Shoulder Motion Kinetics Muscular Anatomy Integrated Muscular Activity of the Shoulder Complex Forward Elevation External Rotation Internal Rotation EXiension Scapulotr,oracic Motion loads at the Glenohumeral Joint The Biomechanics of Pitching Summary References It ----._------------------------~--------~._-._-~-------_._- \"\"\" :;;;Cor

Introduction anatomy of the various aspeclS or the shoulder com shoulder links the LIpper extremity to the trunk plex and shows how their structure allows ror eff acts in conjunction with the elbow to position cient biotllcchanical function. hand in space for crfkient function. [t consists of the glenohumeral. acromioclavicular, stcrnoclav· Kinematics and Anatomy iculat: and scapulolhoracic articulations and the musculature structures that act on them La produce To produce the intricate motions necessary for no the most dynamic and mobile joint in the bodv (Fig, n1al functioning of the shoulder complex, the fou 12-J). An absence of bony constraints allo\\I.'5 a wide aniculations with their associated components ac range of motion at the expense of stability, which is pro- togcther in a way that produces mobility greate than that afforded by anyone individual aniClda for instead by the various ligamentous and mus- tion, The ability of the shoulder complex to positio stl1.!cturcs. the humerus and the remainder of the LIpper cx The biomechanics of the shoulder arc complex, trcmit~· in space is further augmented by movemen and a complete discllssion necessitates an analysis of the spine. A discussion follows of the t\\'pcs an of the four aforementioned articulations that make ranges of motion for the shoulder complex as up the shoulc!eI- complex. This chapter describes the whole, with subsequent sections discussing th manner in which motion is achieved at each of th Acromioclavicular @Stern1i~:~tCular articulat ions, /. /' ~-~iOin1 7' RANGE OF MOTION OF THE SHOULDER COMPLEX Acromion o f . scapula \\ Clavicle '\\ Sternum Shoulder range of motion is traditionally measure Scapula in terms of flexion and extension (elevation o \\, _\\ r movcrncnt of the humenls away from the side of th thorax in the sagittal plane), abduction (elevation i I the coronal plane), and intemal·external rotalio (axial rotation of the humerus with the arm held i ,)\" I ($ an aclducted position) (Fig, 12-2), Although durin functional activit.ies these pure motions are rarel i\"-(l--I \\ t?r ~,\\ seen, we can better understand the complex mo tions of the shoulder by anal~/zing the separate com \\ ponents needed to achieve anyone position, ,,, , Although forward elevation of 180° is theOl'et cally possible, the average value in men is 167° an Humerus in women it is 171°. Extension or posterior eleva lion averages 60· (Boone & Azen. 1979). These va I~0L-nScapulothoraclc _ ues are Iimiled by capsular torsion. Abduction i the coronal plane is limited by bony impingemen I Schematic depiction of the bony structures of the shoulder of the greater tubcr()~ity on the acromion, Forwar i and their four articulations. The circular insets show front elevation in thc plane or the scapula, thereFore, , views of the three synovial joints-sternoclavicular, II acromioclavicular, and glenohumeral-and a lateral view considered to be more Functional because in th plane the inferior portion of the capsule is no of the scapulothoracie joint, a bone- twisted and the musculature of the shoulder is op ,I muscle-bone articulation. Adapted with permis5ion from De timally aligned for elevation of the arlll (Fig. 12-3 Although shoulder range of motion !10nl1all~' d JI. Palma, A.F. (1983). Biomechallin of the shoulder. In Surgery of creases as part of the aging process, physical activ the Shoulder (3rd 00.. pp.6S-85J. P!I;/adelp!l;d: J.8. Upp;ncorr ity can counteract lhis process (Murray ct al 1985). 31 \"~._--Ii!_'., ~'~\"~T\"\"'-_\"\"_'O'''''='''r. =\".... \"~\",.,..,.\",-,,_: \"_,.. '''-:'''-:''~-,'''.,,~,•.=..\"'.=.\"..--\"...,.-..,.-\"_\"',.\"'_,\"',_J\"'.;./\"',~. -7\"+-r;\" ':,:-o\"'-:;'?\"'~_P.. ~\"'\"'T\":::;\"':..

Forward flexion Extension AbduClion !:::xternal and Internal rolation , ~ lf --c:+cs. ..- _........\"...'..........-=>1 II f I, I Humerus I, ,it, at the \\\\ side ,,,I I c D :,,, I ,,:,I I \":'1,1 AB A, Forward flexion. The humerus is in the sagittal plane. B, the humerus at the side, Internal rotation is shown with Extension. The humerus is in the sagittal plane. C, Abduction. arm behind the back, which is a functionally important The humerus is in the frontal plane. D, Rotation around the of this motion. long axis of the humerus. External and internal rotation with STERNOCLAVICULAR JOINT first rib. This is a truc s\\TH)vial joint that has a dle-like shape and contains a fibrocartilaginou The sternoclavicular joint consists 01\" the enlarged ticular disc or meniscus that divides it into medial end of the clavicle and the most superolat- Cotlllxlrtmcnts. eral aspect of the manubrium, linking the upper ex- Although the joint itself has little intrinsic st tremity directly to the thorax. In addition, a small facet is present inFeriorly that articulates with the ity, the articular disc in conjunction with ante posterior. C()stoclavicular, and intL't\"clavicular Scapular plane elevation ments maintains joint apposition (Fig. 12-4). anterior and posterior sternocla\\'icular ligam Long axis Long axis resist anterior ;:md posterior translatiolls (as we superior displacement), while the costoclm'i c3~~ lig;:Hllenl, which rUIlS betwcen the undersurfac the medial end 01\" the chlviclc and the first rib Elevation in the scapular plane, which is midway between sists upward as Wi..'l1 as posterior displacemen forward flexion and abduction. The humerus is in the the clavicle (\\'ia its anterior'portion) and is tho plane of the scapula. to be the major constraint in limiting stcrnoclav br motion. The interclavicular ligament conn the superollledial aspect or the clavicles ,:Hld as in restr;:lining the joint superiorl:v. '1'he post portion 01\" the interclavicular ligament also. as with anterior restraint of' the sternoclavicular j Speciflcall)-', the interclavicular ligament tigh with arm depression and is lax when the ann is vated (i\\'lorrey & An, 1990). The disc prevents ordial disphlcelnent the clavicle, which m<l!' o when C<\\IT)-'ing objects at the side, ;:IS well as infe

Sternoclavicular Joint ally directed coroid and the anterolaterally d rected trapezoid ligaments, which arc distin Ant. sternoclavicular Articular structures that serve different biomechanic Jig. disc functions. The smaller coroid ligament acts limit superior-inferior displacement of the clav cle. The quadrilaterally shaped trapezoid is t larger and stronger of the two ligaments and found lateral to the coroid; it resists axial com pression or motion about a horizontal axis. T Protraction-retraction Post. Demonstration of the anatomy of the sternoclavicular I • joint. Reprinted with permission from DeLee J. & Drez, D. (/9911). Orthopaedic Sports Medicine. Principles and Practice p~ (p. 464). Phi/adelfJhl~): WB. Saunders Co. Sternum \\ Clavicle displacement via articular contact. Although these structures act as important stabilizers, they still al~ A Ant. Meniscus Imv for significant motion including up to 50° of ax- ial rotation and 35° of both superior-inferior cleva· Elevation-depression l:7==....... lion and anteric)I'-posterior translation (Fig. 12-5). ACROMIOCLAVICULAR JOINT B The acromioclavicular joint (Fig. 12·6) lies between Rotation the lateral end of the clavicle and the acromion of the scapula (the lateral and anterior extension of the c scapular spine) and is subject to the high loads transmitted from the chest musculature to the up- Motion at the sternoclavicular joint. A, Top view showin per extremity. It too is a synovial joint, but has a pla- the clavicular protraction and retraction (anteroposterio nar configuration. A wedge-shaped articular disc, gliding) in the transverse plane around a longitudinal ax whose function is poorly understood, is found (solid dot) through the costoclavicular ligament, not \\vithin the joint originating from the superior as- shown. Motion takes place between the sternum and th pect. Both sides of the articular surface are covered meniscus. B. Anterior view showing clavicular elevation with fibrocartilage and the joint itself slopes infero- and depression (superoinferior gliding) in the frontal pla medially, causing the lateral end of the clavicle to around a sagittal axis (solid dot) through the costoclavic slightly override the acromion. lar ligament. not shown. Motion occurs between the cla cle and the meniscus. C, Anterior view depicting the clav A weak fibrous capsule encloses the joint and is reinforced superiorly' by' the acromioclavicular lig- ofular rotation around the longitudinal axis the clavicle ament. The acromioclavicular ligament acts pri- marily to restrain both axial rotation and posterior • translation of the clavicle. The majority of the joint's vertical stability is provided by the coraco- clavicular ligaments that suspend the scapula from the clavicle (Fukuda et aI., 1986). The cora- coclavicular ligaments consist of the posteromedi-

Coroid ligament Trapezoid Coracoclavicular vasclll~ll' slrlH.:lllrl\"S and SL'IYL'S as an altachmcn li)7ment ligamenl for Inan\\ of Iht.' l1luscles dl41t ~Ict on tilL' sho Tht.' clavicle' also pro\\'ides for the normal ap Acromioclavicular ance 4\\[H.! contour or tIJL~ llPP(~J' chest. Elevati ligament IhL' upper extrcmil,\\' is aCCOl11lXlnicd b.\\· rotati - Acromion wdl as ck'\\-',<1lion or the c1a\\'idL.', with approxim Coracoacromial -1-0 of c1m·icular dL.'\\·ation for evcr~· 10'> of arm \\·~ltion, \\\\·ith the majorit.\\, of this motion occu at the stL'I\"llocla\\'indar joint {Inman, Saunde Ahhot!. 1944 J. Intertubercular Humerus GLENOHUMERAL JOINT AND RELATED synovial sheath -1'-----') STRUCTURES Scapula Although motion at the acrol'nioclavicular, st chlvicular, and scapulothoracic articulalions is The coracoclavicular ligament complex consists of the larger and heavier trapezoid ligament, which is oriented laterally, 10 the on:rall function or Ihe shoulder comple and the smaller coroid ligament, situated more medially. Reproduced with permission from Hollinshead, WH. (/969). cClltral pla.n?r is the glt.'llohullleraljoinl. Tht..' a Anatomy for Surgeons (Vol. 3), New York: Harper & RO'll, lar surface of lhe proximal humerus forms a arc and is CO\\·cfL.,d with hyalin(' cartilag~ as coracoacromial ligament lies on the lateral side of glenoid fossa. The hUllleral head is rctnwt..'rlc the acromioclavicular joint and runs rrom the postt..'riorl.\\' dirt..'ctcd 3D'\" wilh rt..'speci 10 tilL> most lateral aspect of the coracoid to the medial cond.\\'lar plan..: of the distal hUlllerus and has a aspect of the acromion. ward or llledial indination of 45°; this configur gives the humerus an o\\'eral! more anterior an Although clavicular rotation does occur with arm eral orienl~\\liol\\ (Fig. 12-7). The greater and elevation, Rockwood (1975) found little relative mo- tuberosities lie lateral to Ihe articular surface o tion between the clavicle and acromion. This has proximal hUIl1L'rus thal SCIY('S as tht.' all<:lchlllL'1 been attributed to synchronous clavicular and for IhL' rotator culT IllUsctdalUrc. The long he scapular rotation with little resullant relative mo- Lhe biceps lendon trii\\vcrsL.'S till> bicipilal g tion at the acromioclavicular joint: the majority of scapulothoracic motion occurs via the stcnlOclavic- ular joint. Thus, rigid fixation or fusion of the acromioclavicular joint produces little loss or over- all shoulder function while sternoclavicular fixation leads to restricted motion or, more commonly. im- plant failure. CLAVICLE The two-dimensional orientation of the articular sur the humerus with respect to the bicondyrar axis, Rep The clavicle lies between the two aforementioned ......rth pemllC;SIOfl from Rockwood, C. & Matsen, f (J 990). articulations, acting as a strut connecting the tho- Shoulder (p. 219). Pillladelphia. ~'V.B. 5.:1lmderc; Co rax to the upper extrell1it~'. It is an S-shaped, double-curved bone: the medial two thirds of the body convex anteriorly while the lateral end is con- cave. It protects the lInderl~,..ing brachial plexus and

lies between the tuberosities) beneath the A I tr\",nsve,,'se humeral ligament (Fig. 12~8). B The proximal humerus articulates with the glen- fossa, which itselF is retroverted 7° and superi~ inclined 5° relative to the plane of the scapula 12-9, A & B). This slight superior inclination n,-OV\\C!,'5 a significant degree of geometric stability, to resist inferior subluxation or dislocation et al., 1992). The glenoid fossa is shallow and is to contain only approximately one third of the di,wlet\"r of humeral head. The bony architecture is by the cartilaginous surface, which is thicker peripherally than it is centrally, acting to slightly but significantly increase the depth of the Greater - - - - 1 ' '' ; . . - 1 A, The glenoid is retroverted r with respect to the p tuberosity perpendicular to the scapular plane. B, The glenoid f - - - +Transverse superiorly approximately 5°. Reprinted ,·vith permission humeral from Simon, S,R, (Ed). (1994). Orthopaedic Basic Science (p. 526-527). Rosemont. IL: AtlQS ligament glenoid as a whole. Although the congruity betw Long head of - - - t - the articular surfaces of the proxin1al humerus glenoid was previously' thought to be somewhat the biceps precise, stereophotogrammetric studies (\\vhere fine grid is projected onto an object that is then p Short head of -----,f-tf-- tographed From two different directions in the text of known reference targets, allowing for a th the biceps dimensional reconstruction) have shown articulation to be precise, \\vith the deviation f sphericity' of the convex humeral articular sur and concave glenoid articular surface being than 1% (Soslowsky et aI., 1992). Less than 1.5 of translation of the humeral head on the gle surface has been demonstrated in normal subj during a 300 arc of 1110tion (Poppen & Walker, 19 thus, motion at the glenohumeral joint is alm purely rotational. Given the paucity 01' bony straint, stability is instead provided by the caps ligamentous, and muscular structures that round the glenohumeral joint. The long head of the biceps goes in the bicipital groove Glenoid Labrum between the greater and lesser tuberosities. The transverse humeral ligament helps stabilize the biceps tendon in the The glenoid labru111 (Fig. 12-10) is a fibrocarti groove. Reprinted with permission from Neer, C. (1990). Shoul- nous rim that acts to deepen the glenoid, provi der Reconstruction (p. 29), Philadelphia: WB. Saunders Co. 50°;(; of the overall depth of the glenohumeral j (\\Varner, 1993). It has a triangular configura \\'.'hen viewed in cross-section and has firm att

Coracoacromial ligament Glenoid --',--'\\l;--i. Coracoid labrum process Glenoid --+--~: Coracohumeral cavity ligamenl Tendon of long head of A biceps brachii Long head of tTiceps brachll A, The glenoid labrum is attached to the underlying bony when vie'Ned in cross·section and serves to effectively glenoid and is confluent at its area with the long head of the deepen the glenoid, increasing the stability of the glen biceps tendon. B. The labrum has a triangular configuration hllmeral joint. menls inferiorly to the underlying bone, having DEmll.- morc variable and looser atlachments in its superior and anterosuperior portions. The superior portion Oblique coronal image showing a tear involving th or the glenoid labrum is connucnt with the tendon sertion of the long head of the biceps tendon and of the long head of the biceps and. along with the superior labrllm (arrow). adjacent slIpraglenoid tubercle. serves as its site of insertion (Moore. 1999). ~---------- iVlcasurcmenlS of the force needed to dislocate the humeral head under constant compressive pressure have shown that with an intact labrum, the humeral head resists tangential forces of ap- proximately 60% of the compressive load; resec- tion of the labrum reduces the effectiveness of compression·stabilization by 20% (Lippitt ct aI., 1993). Detachment of the superior labrum with an- terior-posterior extension (i.e., \"SLAP lesion\") can occur from traction (repetitive overhead activities or a sudden pull on the arm) or compression (a fall onto an outstretched arm). This lesion can be a cause of severe pain and shoulder instability (hoi, Hsu, & An, 1996) as a result of significant in- creases in glenohumeral translation whcn com- pared with the intact shoulder (Pagnani et al., 1995). (Fig. 12-11).

Capsule functional significance of the coracohumeral lig rne-nt, however, seems to be related to the overa '/fZ;/-:- The ~:dcnohLll11cral joint capsule has a significant de- development of the glenohumeral ligaments in $<:\"W-'~,,,~~,,~,\"\"~'-\"~t\"-\\t;:l;w>\"irecce-otfh given individual, having a larger role in those with inherent laxity with a stll-face area llull is less-developed superior glenohumeral ligame at of the humeral head (Warner, 1993), This (Warner et aI., 1992), ::~jZkJLredtindancyallows for a wide range of mOL ion. Mc- The middle glenohumeral ligament originates i ferior to the superior glenohumeral ligament (at th ,;,;\" \"idiallv, the capsule attaches both directly onto (an- I o'clock to 3 o'clock position, right shoulder) an inserts fl.1rther laterally on the lesser tuberosit :~;~:;.:terotnferior1y)and beyond the glenoid labrum and Great variability in the anatomy of this structu has been demonstrated, being absent in as many ~~-- laterally it reaches [Q the anatomical neck of the 30% of shoulders (Curl & Warren, 1996), It ma ;'~lh~humerlls.Superiorly, it is attached at the base of the originate from the anterosuperior portion of th labrum, the supraglenoid lubercle, or the scapul 'itrtY:' coracoid. enveloping the long head of the biceps neck, Morphological variants have been descl'ibe including a cord-like variant (clearly distinct fro ~'7F~;~tendon and mnking it an intl'n-articular structure the anterior band of the inferior glenohumeral lig ment) and a sheet-like variant (blending with th dfB\"HFig, 12-1 I), anterior band of the lnferior glenohumeral lig ment). Functionally, the middle glenohumeral lig -!:%ihi The capsule nlso has a stabilizing role, tightening ment acts as a secondary restraint to inferior tran lations of the glenohumeral joint with the arm ,-._. with various arm positions. In adduction, the cap- the abducted and externally rotated positio (vVarner el aI., 1992). It also sen/cs as a restraint sule is Lattl superiorly and lax inferiorly; with ab- anterior translation, having its maximal effect wi the arm abducted 45°. duction of the upper extremity, this relationship is The inferior glenohumeral ligament originat ,'ev'er<;ect and the inferior capsule tightens. As the from the inferior aspect of the labrum and inser on the anatomical neck of the humerus. It has bee is externally rotated, the anterior capsule tight- shown to have three distinct components (O'Brie et ai., 1990): an anterior band originating from 2 while internal rotation induces tightening pas- 4 o'clock (right shoulder), a posterior compone originating fTom 7 to 9 o'clock (right shoulder), an teriOl'lv, The posterior capsule in particular has been an axillat), pouch (Fig. 12-12), The inferiOl· glen humeral ligament has the greatest rll11ctional signi to be crucial in maintaining glenohumeral icance, acting as the primm)' anterio.' stabilizer the shoulder with the arm in 90' of abduction, A stability, acting as a secondary restraint to anterior the arm is abducted and externally rotated. the a terior band of the inferior glenohumeral ligame dislocation (particularly! in positions of abduction) tightens, resisting anterior tninslalion. \\·Vith inte nal rotation of the abducted arm, the posterior ban as well as acting as a primar~y posterior stabilizing becomes taut and posterior translation is resiste The inferior gleno·humeral ligament complex al structure (Itoi, Hsu, & An, 1996), serves to resist inferior translation of the glen humeral joint with the arm in the abducted pos Glenohumeral and Coracohumeral tion. Variability as to the size and attachnlent sit ligaments of the gle-nohumeral ligaments has been demo strated (Warner et aI., 1992); howe vet; the clinic The three glenohumeral ligaments (SUPCriOI~ mid- significance of this has yet to be fully elucidated a dle, and inferior) are discrete extensions of the an- though it has been suggested thal absence of t terior glenohumeral joint capsule and al'e critical to middle glenohumeral ligament may predispose shoulder stability and function (Fig, 12-12), The su- instability (Steinbeck et aI., 1998), perior glenohumeral ligament originates from the anterosupcrior labrum, just anterior to the long head of the biceps, and inserts onto the lesser tuberoSity. It is present in the majority of shoulders bllt only well developed in 50%, The superior gleno- humeral ligament acts as the main restraint to infe- rior translation with the arm in the resting or ad- dllcted position (Warner el aI., 1992), The coracohumeral ligament originates from the lateral side of the base of the coracoid to insert on the anatomical neck of the humerus (Fig, 12-6) (Cooper et aI., 1993). This structure lies anterior to the superior glenohumeral ligament and reinforces the superior aspect of the joint capsule, These liga- ments span the rotator interval belween the sub- scapularis and supraspinatus, Some research incH- cates thal these strUClures have a secondary role in preventing 'inferior translation of the shoulder while in the adducted, neutrall.v rotated posilion. The

A- AB ) AS cD A, Schematic drawing of the shoulder capsule illustrating the humeral ligament complex remain lax, C, The anterior b location and extent of the inferior glenohumeral ligament is the primary restraint resisting inferior translation of t complex (IGHLC). A, anterior; P, posterior; B, biceps tendon; shoulder at 45<> abduction and neutral rotation, In this SGHL, superior glenohumeral ligament; MGHL, middle glenoo tion, the superior glenohumeral ligament, the middle g humeral ligament; AS, anterior band; Ap, axillary pouch; PS, humeral ligament, and posterior band are lax, D, At 90° posterior band; Pc, posterior capsule. Reprinted with pertnis· duction, the anterior and posterior bands of the inferio glenohumeral ligament cradle the humeral head to pre sian from O'Brien, SJ, Neves, M.e, Amoczky, S.P, et at. (1990) inferior translation, The posterior band is more significa external rotation, whereas the anterior band plays a gr The anatomy and histology of the inferior glenohumeral ligament role in internal rotation. Reprinted wirh permission from W complex of the shoulder. Am J Sports Med. 18, 579-584. B. The IP, Deng, X.H.. Warren, R,F., et al. (1992). Staric capsuloliga superior glenohumeral ligament is the primary restraint to inferior translation in the adducted shoulder at neutral rota- taus restraints ro superior-inferior translations of rhe glenohu tion. In this position, the middle glen?humeral ligament and joint. Am J Sports Med, 20, 675-678. the anterior and posterior bands of the inferior gleno·

Experimental Techniques: Shoulder Instability Ligament Cutting Studies A 21-yea:\"0Id ma.n fell while skiing onto his right upper Ligament cutting studies have been instrumental in fur- thering our knowledge regarding the contribution of a . extremity, cauSing forceful abduction and external ro\" given anatomical structure to overall glenohumeral sta- tation. He noted acute pain in the arm and was unable to bility (Curl & \\JVarren, 1996). In this technique, cadav- IYlQveit Physical examination revealed loss of the normal eric specimens are biomechanically tested before and contour of the shoulder and painful range of motion. Ra\" after selectively cutting sequential structures. A force is dlogr~phs showed an anterior dislocation with posterior then applied in a given arm position, and the transla- superi.()rhumeral head impaction fracture. The patient un- tion that occurs is measured. From this information, the gerwent closed reduction. Postreduction radiographs con\" relative contribution that a given structure provides to firlYledreduction of the humeral head with a small bony overall stability can be determined. When a particular ayulsion fracture of the anterior-inferior glenoid rim, pattern of shoulder instability is then identified, the whichrepresents a detachment of the anterior labrum in physician can infer which anatomical structures may be the area of the superior band of the glenohumeral liga- deficient or disrupted so that a rational plan of physical ment insertion (Case Study Figure 12\" 1\" 1). therapy or surgical repair can be implemented. Structural alteration of bony geometry, ligaments, and • labrum resulted in shoulder instability. The detachment of the anterior labrum and the superior band of the liga- Additional Constraints to Glenohumeral ment insertion affected primarily the resistance to anterior Stability translation of the humeral head, resulting in anterior dis- location. !n addition, a concomitant capsule lesion af- Sy'l1ovial fluid acts via cohesion and adhesion to fected the intra\"articular negative pressure necessary to further stabilize the glenohumeral joint. Synovial pull the humeral head inward. After conservative man- fluid adheres to the articular cartila~e ovcrh;in cr the agement. the patient did well for 6 months until he sus\" glenoid and proximal humerus, c;using theb two tained a recurrent dislocation. The patient subsequently opted for operative treatment that included repair of the surfaces to slide along one another: Th~ synovial fracture and capsule and a complete period of rehabilita\" fluid provides a cohesive force bet\\veen the~e two, tion~vithernphasis on joint stability and proprioception. making it difficult to pull them apart (Simon, Case Study Figure 12-1\"1. 1994). Under normal conditions, the intra-articular pressure \\vithin the gknohumeral joint is negative, acting to pull the overlying capsule and gleno- humeral ligan1cnts inward. If the integrity of the glcnohun1cral joint capsule is cornpro~1is~d (e.g., venting the capsule) or if a significant effusion ex- ists (normally the glenohumeral joint contains less than 1 cc of fluid), significant increases in transla- tion are observed (Kumar & Balasubramaniam, 1985). Specifically, venting the capsule reduces the force needed for anterior humeral head translation by 55 0ft;, for posterior translation bv 430c), and for inferior translation by 57% (Gibb et ·al., 1991) (Case Study 12-1). SCAPULOTHORACIC ARTICULATION The scapula is a Flat, triangular bone that lies on the posterolateral aspect of the thorax between the second and seventh ribs. It is angled 30° anterior to the coronal plane of the thOl;X and is rotated slightly toward the midline at its superior end and tilted anteriorly with respect to the sagittal plane

(Fig. 12-13) (Laumann. 1987). The spine of the Serra Ius scapula gives rise laterally to the acromion process anterior that articulates with the distal clavicle at the acromioclavicular joint. The coracoclavicular liga- Anterior view of the scapulothoracic articulation, a ments and muscular attachments help to suppon I11llscle·bone articlJlation between the scapula and the scapula and stabilize it against the thorax (Fig. During scupular motion, the subscaplJlaris muscle. Io 12-6). There is, however, no osseous connection attaches broadly to the costa! surface of the scapula with the axial skeleton. This allows for a wide on the serratus anterior muscle, which originates or first eight ribs and inserts into the cost<ll surf..'!ce of range or scapular Illotion, including protraction, scapula along the length of its vertebral border. I\"etraction, elevation, depression, and rotation. .---------------- The scapulothoracic articulation involves gliding SPINAL CONTRIBUTION TO SHOULDER of the scapula on lite posterior aspect of the thorax. MOTION Interposed between the scapula and the thoracic \\vall lie the subscapularis (arising from the costal Allhaugh often O\\'crlookcd, motion 01\" the th surface of the body of the scapula) and the serratus and lurnlmr spine contributes to thL' :.\\bilit:,· t anterior, which help to stabilize the scapula against tion the upper c.\"tl'(.'l1lit~· in space, {ht.T('b~· l.' the chest wall and thus prevent \"scapular winging\" ing the on.:r<.111 Illotion and functioll of the sh (Fig. 12-14). These two muscles glide along one an- cOlllpk.·x. f'kxiol1 01\" tile spine away frolll an ,- other to provide greatly enhanced mobility of the il~' altempting ttl reach an object o\\'crhcacl t.'n shoulder complex as a whole, the range of motion <.lllainahlt: (Fig. 12·15), T Elevation of the arm involves rnotion at both the ponancc or spinal motion in O\\·l..'rlH.':'ld act glenohumeral joint and the scapulothoracic articu- lation. Although the contribution from each varies such as throwing ~Illd r<.tcqul.'l sports h<.ts als according to arm position and the specific task bc- c1emonst rated. lng performed. the average ratio of glenohumeral to scapulothoracic moL ion is 2: I (Tibone el aI., 1994). Elevation of the arm also induces complex rotatory motion of the scapula, \\vith anterior rotation during the first 90° followed by posterior rotation with a to- tal arc of approximately 15° (Morrey & An, 1990). Acromion ' r-.J 30 Scapular I I spine ~l I. Scapula Kinetics Scapular orientation on the chest wall. Left. 30~ anterior, Numerous llluscll..'s aCI nn the \\'arious comp Right. 3\" upward, Reprinted with permission from Warner JJP: or the shoulder complex to provide both m The gross anatomy of the joim surfaces, ligaments, labrum, and capsule. In: Matsen. F.A., FtJ, F.H., Hawkins, R.I (Eds.) The Shoul- and d\\'namic $t~lbilit.\\·. Dynamic stabilizati der: A Balance of Mobility and Stability. Rosemont, IL: American cllrs via sl.~\\'cral possible lllcch:'lnisms (Mo Academy of Orthopaedic Surgeons, J993. An, 1990), including p~lssi\\'(' musch.' lensi via a b:,uTic:r effect of the contracted Illuscle prl..'ssi,·L' forces broughl ahout h~' llHISCU!:'U traction, .ioint lllotion that induces lighten

\"fmm'------ activated differently for specific activities. The lateral bending of the spine enhances the ability to posi- toid originates from the lateral third of the clav acromion, and scapular spine and inserts on the I tion the upper extremity. Reprinted with permission from terolateral aspect of the humerus. The anterior h Rockwood, C. & Matsen, F (1990), The Shoulder (p. 2' 9). acts as a strong flexor and internal rotator of Philadelphia: WB. Saunders Co. humerus, the middle head as an abductor, and posterior head as an extensor and external rot the passive or ligamentous constraints, or via a The pectoralis major lies over the anterior c redirection of the joint force toward the center of wall and has two heads, a clavicular head orig the glenoid. ing from the side of the clavicle and a sternoco head originating from the sternum, manubri To understand rnusclc function and force trans- and the upper costal cartilages. The two heads mission. one must consider a given muscle's orien- verge at the sternoclavicular joint. The pecto tation, size, and activity. Given the multiple articu- major inserts at the intertubercular groove of lations present in the shoulder complex, an:y given humerus between the tuberosities and act muscle may\" span several different joints, and de- adduct and internally rotate the humerus. Se pending on the position of the upper extremity', its darily, the clavicular head acts as a flexor or fon relationship with regard to anyone articulation elevator of the humerus while the sternocostal may change, altering its effect on that joint and the extends the humerus. The pectoralis minor lies resultant forces or motions produced. to the pectoralis major, functioning as an impor scapular stabilize!: The pennate subclavius mu MUSCULAR ANATOMY lies inferior to the clavicle and Illay assist in cl ular motions. It has a tendinous origin from the The shoulder musculature can be thought of in lay- teromedial aspect of the first rib and insert ers. The outermost la)-!er consists of the deltoid and pectoralis major muscles (Fig. 12-16). The deltoid Deltoid Pectoralis major forms the normal, rounded contour of the shoulder Posterior Clavicular and is triangular in shape, with anterior, middle, Middle Sternal and posterior heads. Each portion of the deltoid is Anterior Serra ante Anterior sheath Pectoralis of rectus minor Anterior view showing the superficial muscles (left sh der) and the deep muscles beneath the deltoid and p toralis muscles (right shoulder).

the undersurface of the medial clavicle (Morrev & Anterior View An, 1990). Biceps Subscapularis BcnL~alh this ouler layer lies the rotator cuff A musculature: the supraspinatlls. infraspinatus. Posterior View subscapularis, and teres minor (Fig.12-17). These Supraspinatus four muscles act to abduct and rotate the humerus and act as important glenohumeral sta- \" bilizers via both passive muscle tension and dy- namic contraction. The supraspinatlls originates Infraspinatlls from the supraspinatus fossa of the scapula and inserts on the greater tuberosity of the prox.imal B humerus. It forms a force couple with the deltoid during abduction of the humerus. The infraspina- A, Anterior view. The \"rotator interval\" is a term duced in 1970 to indicate the space between the lllS and teres minOt- originate from the inferior as- supraspinatus and subscapularis tendons. The cor humeral ligament lies superficially along its ante pect of the scapula and insert on the greater where it is readily available for release as indicat (uberosity. These muscles act as external rotators long head of the biceps lies deep along its poste of the humerus. The subscapularis lies on the and serves as a guide to this interval during surg costal surface of the scapula and inserts on the Reprinted ~'llth permission from Neer. C. (1990). Shou lesser tuberosity of the proximal humerus. It construetiofl (D. 29). Pllild(/e~ohia: !NB. Saunders (0_ functions as an important internal rotator of the rior view_ The two external rotators of the hume humerus. The subscapularis. along with tlte mid~ ..infraspinatus and teres minor muscles. which are dIe and inferior glenohumcral ligaments, has also posterior wall of the rotator cuff. Note the medi been shown to act as an important anterior stabi- of the infraspinatus, which is often mistaken at s lizer of the glenohumcral joint, particularly with the border between the infraspinatus and the te lJ1(.> arm held at 45° of abduction. The teres major Reprinred with permission [rom Rock\\'-Ioocl, C. & Mat muscle (Fig. 12-18), while not part of the rotator (/990). The Shoulcier (p. 2) 9). Philadelphia: \\>';1 B. Sau culT, also originates from the scapula, but at its inferior angle coursing inferior to the teres minor and then passing anteriorly to insert on the humerus at the intertubercular groove. h func- tions to assist with arm adduction and internal rotation. The biceps muscle is also involved with motion of the shoulcleI· complex. It is composed of two heads: a short head that originates fTom the tip of the cora- coid process of the scapula and a long head that originates I'Tom the superior glenoid labrum and supraglenoid tubercle (Fig. 12-8). The tendon of the long head of the biceps lies within the glenohumeral joint and descends between the greater and lesser tuberosities. joining the short head to insert on the bicipital tuberositv of the radius. The biceps fl1nc- lions to Oex and supinate the forearm and elevates the humerus. The long head of the biceps also acts as a humeral head depressor and, as such, plays a role in maintaining glenohumeral stability (hoi et aI., 1994). Several muscles lie on the back and act directly on the scapula (Fig. 12-18). The outermost layer consists of the trapezium that covers the posterior neck and uppermost portion of the trunk, inserting on the superior aspect of the lateral one third of the clavicle, acromion, and scapular spine. The tl'apez-