Microsoft Word - DR GORE SYSTEM book 2018 may

Image60 Deep muscles of the back are cut and denervated in posterior midline access, that includes multifidus and long muscles as well as short segment muscles that are important for facet proprioception. Image on right side shows the small muscles of the back. Our transforaminal access avoids all this. The muscles are not cut but dilated and nerves are avoided. 50

Image61 Image above is placement of cannula in cadaver foramen on right. We are just right of the midline. Hemi laminotomy has been done exposing the dural sac and the axilla of the traversing root. Pars inter articularis is shown to right of the traversing root. Metal cannula is lying in the disc under pars, both facet joints are seen well. 51

Image62 CADAVER dissection done to show the normal lay of the roots. We can see exiting root coming out of foramen under pars and looks brownish. We note that superior facet of lower vertebra is hidden by inferior facet of the upper vertebra and facet joint normally is just behind [dorsal] but in cross plane of the disc. When ligamentum flavum changes with degeneration; the symptomatic part of the change is directly involving the axilla or the dorsal root ganglion of exiting root in upper foramen and not only interlaminar. It is this lateral part of the ligamentum that is more important in symptom and claudication generation. 52

Image63 Our working cannula is in axilla of the exiting nerve root. It is always on shoulder of the traversing root. We can see the transverse process. Groove between tp and lateral wall of pedicle houses the medial branch of dorsal ramus. 53

Image64 OMIT Commonly artery branches off the aorta and comes dorsal towards transverse process and foramen. Generally, the vessel is cephalad to dorsal root ganglion in the upper part of the foramen, so if we start along lower pedicle and then go towards the tip of sap we are safe, and in upper part of foramen we first see the DRG first and then more cephalad the artery. This artery gives pulsations of the DRG during surgery. image65 54

Image26 GORE thumb rule: In case of left sided access to foramen for better mental visualization; left hand is looked @ as above where thumb is exiting root, index finger is traversing, and first web space is axilla of the root. The metacarpophalangeal joint of thumb is representing the dorsal root ganglion of the exiting root and important part of the lateral wall of kambin’s triangle. [image 26] This aids in better visualization. Image [183] showing upper foraminal area with hypertrophied tissue covering roof of the foramen. This tissue is responsible for symptoms in claudication. This may be subarticular. Or over tip of sap. A2. PAIN GENERATOR: correlating pain with anatomy and structural failure Simplistic ideas and artists impressions about tear show it to be a radial straight-line tear etc. The reality is different where it may be wavy. It may be assessed by discography. Annulus is found in certain cases to have annular flaps and gaps. Discography gives important morphometric information. According to north American spine society we have following indications for discography where a needle is inserted in the affected disc 55

and after injecting dye we try and elicit and evoke pain and look for structural change in the posterior annulus. Discography is indicated mainly for back pain and not leg pain if • noninvasive images have not provided sufficient diagnostic info. • images need to be correlated with clinical symptoms. • Preoperatively: persistent pain where disc abnormality is suspect: • Intraoperatively: Consider disc for fusion so find symptomatic if considering one out of segment for fusion. adjacent disc be normal • Postoperatively: Previous surgery with significant pain, failure to relieve pain, and to differentiate postop scar and recurrence at interface of disc and the root. • We may additionally do epidurogram instead if pathology is more extradiscal. During discography, we note • Volume injected, pain response, location and concordance, dye distribution and follow up CT is highly recommended. • Radial annular tears extend into peripheral circumferential annular tears - lead to detached fragments of annulus- free in centre of disc. [seen in grade 4] • Peripheral circumferential annular tear detached - “bucket-handle tear”-with detached flap -lie free within centre. • The annular gap and fragments may not be seen and appreciated on MRI. 56

• Whole spectrum of inflammatory sciatica and backache may be lost clinically by neglecting this simple investigation resulting in failure to correlate inflammation and pain. • 1 2 3 1. Axial post diskogram CT shows circumferential annular tears (arrows), multiple free annular fragments (arrowheads), and a peripheral annular tear (curved arrow) without leakage. Distinction between annulus and nucleus is basically lost as disc is internally disrupted. The nucleus becomes more fibrous. 2. Axial post diskogram CT: wide annular gap (curved arrows a peripheral annular tear (arrows). 57

3. free annular fragments present in the annular gap (arrowheads). Diskographic contrast leakage positive. Similar changes in MRI seen. Below This image shows in addition to annular gap peripheralization of nucleus. The patient is a m 28 having severe back pain and chronic sciatic list. WE try “evoking the patient’s response of pain as Negative, concordant, similar, or discordant. Discography facilitates interpretation of the disc degenerative pattern and labels the degenerative disc for guided endoscopy. Degenerative disc is stained by a 10% mix of indigocarmine dye with a non- ionic radio opaque dye. Indigo carmine a vital dye helps in identification of nuclear fragments at surgery as it stains acidic symptom causing fragments blue. As the degenerative acidic blue stained nucleus is removed, the inner annulus is visualized for trapped remaining loose degenerative nuclear material caught in the torn annulus. Staining helps in proper directed and adequate removal of the trapped nucleus. 58

1 Normal looking MRI, discogram shows the annular tear and in post disco CT we see a leak at the right paracentral tear. Patient had only acute onset severe low back pain. 59

2 Patient above had severe acute onset low back pain. Normal looking MRI, discogram shows tear at L5S1 and post disco CT shows a right paracentral tear with leak. As a treatment strategy, we must plan for sealing this symptomatic tear. Restoring an annular barrier helps in full resolution of symptoms. We have started using Der1 plug for sealing annular opening with positive results. 48 We commonly see change of domain of pain from back to leg, but only isolated leg pain is also possible as it is a chemical radicular involvement and sensitization of nerve due to leaking cytokines and detected by GORE SIGN. It many times manifests as pain at posterolateral aspect of knee or heel. If the pain is not due to trauma, infection or ischaemia then it almost always 60

is neuralgia. Changes in posterior annulus with leak thus can lead to chemical inflammation and pain along leg or pain in back. Main change in discogenic back pain is disc loses its posterior concavity. Bulge in a disc denotes a likely intra annular change of tear with trapped fragment. In summary, degenerative disc change of annular tear can manifest as patchy pain spots or areas in lower limb, can come up as low back pain or sciatica if additional herniation is present. A3. Sciatica due to annular tear leak and fragment mechanically compressing nerve : DETECTION AND CONFIRMATION Image29 61

Pain along distribution of the L5 or S1 roots is sciatica; commonly seen with an annular tear or disc herniation. Pain can be present ONLY if inflammation is present. Priming of tissue by inflammatory cytokines is must for mechanical compression to be painful. Sciatica can be staged in 100% patients as 1. stage of inflammation and sensitization 80% [detected by gore sign], 2. additional mechanical compression [detected by straight leg raising] 15% and 3. progressing to neuro deficit [detected by routine neurological exam and electrophysiological studies]5%. These stages may not be necessarily linear in progression. With likely proportion of the stages of sciatica as 1 2 3 being 80-15-5, stage 1 is dominant so a new separate treatment modality in form of “pain management” has emerged. Masking pain is some-times proposed as substituting surgery and other hardware-based interventions. Masking pain can help in better diagnosis but may not be durable relief in presence of mechanical compression. Indian and other patients who sit on ground or in cross legged position, in very early stages of physical compression, sitting cross legged induces pain and tingling along the root path as sciatic nerve has reduced stretchablity. This can be taken as early stage of “mechanical” origin sciatica and reduced root mobility and stretchablity sign. [unpublished work] Sitting cross legged and its effect on pain in sciatica can be used to assess moderate mechanical compression. In symptomatic stenosis or claudication, we are yet to have dependable etiological studies. 62

Sciatica traditionally is assessed by checking movements of the spine namely flexion, extension and its effect on elicitation of pain. Flexion is known to be limited with disc herniation. In early stages of compression when the stretchablity of the nerve is reduced we have noted that sitting cross legged increases pain and numbness and tingling along the nerve root specially L5 at knee. If the McKenzie test indicates a peripheralization of pain, and gore sign is negative but SLR is positive and neuro deficit is present with added image evidence of a fragment or compression; surgery is definitely indicated. A4 Target in Canal stenosis Since most of the traditional interventions lack evidence 17 about their effectiveness we propose conceptually more direct decompression of the DRG under the facet and sap rather than a midline central interlaminar only decompression. Even surgeries done “over the top” may not be able to reach the real symptom causing tissue. 18 Traditional hemi laminotomy and over the top access. 63

The amount of bone removed to reach the target area and subsequent instability is serious issue. This also shows a lack of appreciation for the simple anatomical fact that ligamentum flavum lateral part is “lateral” to the medial pedicle line and in roof of the foramen. Stenosis should be tackled in upper foramen in stenosis plane, in plane of pars, lamina and body. Tackling Stenosis by transforaminal access under local anesthesia is easier. It will vary as per a. hypertrophied tissue over the sap and b. presence or absence of collapse of the disc height. Image196 image197Image198 image199 Image200 64

B: STITCHLESS SURGERY UNDER LOCAL ANESTHESIA FOR DISC HERNIATION TECHNIQUE Important objective in gore system is precise needle placement for accessing the pain or symptom generator. Operative procedure is carried out in an operating room, using local anesthesia and conscious sedation attended and monitored for vital signs by an anesthesiologist. The patient is prone, positioned on a radiolucent cushion frame, with the position of the imaging equipment, instrument table, and operating room personnel in relation to the surgeon shown in image. Image66 65

The conscious patient is asked to report any painful sensations while the procedure is in progress. The prone position allows a biportal (bilateral) approach if necessary. It is more common to use uniportal approach as it is sufficient to address most common issues. We can do biportal, unilateral biportal too. The starting point for needle entry is calculated by “line drawing” on the skin adapted by gore system from Yess system. We are between longissimus and psoas in sagittal plane. On average we go about 12.5 cm from midline. Since the needle has cm markings it is easy to do. The technique is always performed under local anesthesia. Use of Discography facilitates interpretation of the disc degenerative pattern and treatment endoscopically in select cases. Chronic non-healing annular tears and nuclear material caught in tears is a common cause of chronic persistent low back pain. Inner annulus is visualized for loose degenerative disc material trapped in the torn annulus. The annulus is inspected for tears and modulated as needed. Intradiscal visualized thermal modulation then treats the tear from the inside out. The endoscope is withdrawn from center of disc to foramen to inspect the foramen, the exiting nerve, and the epidural space as needed. at1 66

at2 at3 at4 at5 67

Extruded disc fragments are reached with use of a bendable hook which helps in mobilizing the fragment that has moved away from disc level. HOOK introduced by me is one of the most important advances in transforaminal spine surgery improving reach. Up migrated fragments herniate through annular tear between upper endplate and annulus and down migrated herniate through tear between lower endplate and the annulus. As we find the tear, using the hook through the tear facilitates mobilization and removal of the said fragment easily. Hook has made managing migrated fragments very easy without any bone cutting. In select cases using medialization of annular entry towards central tear site helps in loosening fragment and its removal. IT is also important to note healing of annulus chronically gives collagenization and hardening and sometimes calcification or ossification so it may need cutting, excision, ablation, modulation. . 68

Use of hook in a down migrated fragment mobilization. The patient had a previous surgery and it was necessary to gently mobilize the nerve and release the adhesions too. um3a um2a um1a um3 um2 69

Probing ventral and dorsal to the traversing nerve and the axilla of the exiting nerve confirms adequate removal of all mechanical lesions. Tissue in the axilla between the exiting and traversing nerve is cleared if a foraminal osteophyte or residual disc fragment is suspected. 41 42 43 44 45 46 47 48 Herniated material is of three types depending on the texture. It can be soft; pure nuclear fragment that may be phagocytosed naturally over 2-3 months’ time. It can be firm; nucleus and annular tissue that does reduce or vanish even over long time namely 6-8 months but may be uncertain and third type that may be a hard piece of nucleus and annulus with an endplate fracture fragment that may never ever get resolved naturally in periods less than 2 years. Even then the pain may reduce but mechanical compromise may persist. These two latter herniations need be removed and cannot be managed with non-operative treatment. Natural resolution of inflammation almost always fails in my experience in these later 2 scenarios. It is also important to keep in mind this natural regression and do an imaging study immediately prior to intervention to confirm the local pathology and its likely regression and the present status if imaging study available is old. 1 In herniations in patients less than 30 years of age, there is significantly higher percentage of nucleus pulposus than in the older group, whereas anulus fibrosus is found in significantly higher percentages in patients ≥30 years. Impaired reflexes more often in patients with ≥20% of cartilage and If nucleus pulposus is <30%, sensory impairment is more severe before treatment. It is therefore likely that histologic composition of the herniated 2 disc fragments may affect pain and clinical symptoms. In acute and sub- acute cases the policy to change from images to symptoms also can be taken 70

as an opportunity to educate patient. More than 80% patient population roams around with pain that is not classical sciatica but related to early disc changes and pain generators and is in spots or patches. This may summate over space and time and then manifest as full sciatica. Our philosophy of awake and aware surgery and pre-op analysis has given us unique ability to actively intervene in early stages of degeneration. In future with biological solutions and regenerative treatment strategy this ability may gain significant value. Only area on the disc annulus free of significant structures is the kambin’s triangle, our natural target zone. We have an illustrative case depicting access, basic line drawing to increase precision of the access in this case of L45 disc herniation. If we follow the line drawing method we would be landing precisely in the foramen close to area of our interest, chances of nerve injuries and landing in inappropriate places can be avoided. The patient is a female aged 28 years with Severe bilateral sciatica, left side symptoms are more prominent. Duration of pain was 8 months. Severity was 8/10 on VAS and activities of daily life were affected. Patient used to take up to 1 hour to get up from bed in morning. WE diagnosed a central; herniation at L45. Adequate non-operative treatment was given but resolution of inflammation failed resulting in lack of relief of her symptoms. Her scan showed a central herniation at L45 and additionally a posterior annular tear seen at L5S1. Plan of treatment was targeted fragmentectomy at L45. In L5S1 an injection of anti-inflammatory was given with hope of healing of the leaking tear. 71

B2 ILLUSTRATIVE CASE: Image67 This is a common meeting point of inclination lines of L45 and L5S1. MRI scan sagittal image shows the L45 disc is slightly lordotic and L5S1 is more inclined towards the head. But both discs inclination marking meets at one point. The L34 seems to be vertical. This understanding helps in planning trajectory for the disc or fragment. The cross section below shows a large fragment at centre of the disc and its down migrated indicating annular tear at lower endplate. The location can explain the bilateral symptoms of this patient. The plan is to go inside the disc under the fragment and land in the space which is vacated by this fragment. If we extrapolate this image analysis on an X-ray we need to be at the centre of the disc in AP and with a slight downward direction in lateral. 72

A left-sided approach was chosen as symptoms were more prominent on the LEFT.Image68 image69 This RED arrow in image below shows how we will be lodging under the facet from the left side and target the fragment. 73

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image70 image71 In my method, we can predict beforehand what we will find on scope insertion, this is targeted fragmentectomy and not trial and error. This image 71shows the final assembly of instruments to remove the offending fragment from L45. Our technique at basic level needs minimum 7 instruments, access instruments and disc surgery instruments. Puncture needle, guide wire, 75

dilation sleeve, operation sheath, punching sleeve, grasping forceps, mallet, in addition to scope, camera and light source. Patient is lying prone on table, head on left side of image C arm is lined up to exactly centre over the affected area. Exact AP image is assured by confirming that both pedicles at L45 are equidistant from the midline spinous process. The pedicles are seen end on. All areas of body likely to have pressure like anterior superior iliac spines, patella and feet are properly padded and also guarded against radiation. Patient is awake and aware throughout the procedure to interact and give real time feedback about pain and additional safety. In short line drawing involves 4 steps as below, details shown later. Line 1 midline, 2 disc space in AP as is seen, 3 lordosis considered in lateral and depth upto anterior border, 4 move line 2 to intersect line 3. That cross is entry point. 76

Image72 A metallic marker is used to mark the midline by feeling spinous processes or under C arm. Skin is marked with a permanent marker ink pen. Midline marking is essential for proper orientation. Non-alcohol soluble ink may be used as marking should stay. 77

Image73 Patient is prone on 2 bolsters or padding material, procedure lasts for about 30 minutes. We stand on the left side of the patient as the left side is the more symptomatic side. The video trolley is on the left, C arm monitor is seen on the right, C arm is in the centre on other side of table guiding the surgery. Image74 78

Here below the disc space is being marked as is seen in the AP image. It is known that lower lumbar discs are tilted towards the head due to the lordosis but at this stage of marking we mark disc space as is seen in this AP view. Image75 This image shows the marker seen at disc level. We prefer the image in such a way that the marker is seen in the left of the screen as it is our side of approach. [as seen below] This avoids any confusion in surgery and access. Making the marker go up and down is easy by rolling the pin on the skin. 79

Image76 Then C arm is lined up in lateral. image 77 First step is to line it up exactly parallel to disc space by removing any parallax between the C arm and disc end plates by moving c arm side to side. Ensure that both end plates are parallel to each other. Then marking is done for disc inclination. Here the disc is slightly tilted towards head and since the fragment is migrated downwards we will go a little above at entry on skin. Disc anterior border is where the tip of the marker is kept. Our target is the posterior annulus and not the disc centre. 80

Image77 image78 As can be seen here, disc marking is done with a marker lined up in line with the disc seen in C arm. The marker is tilted towards the head to incline it 81

with disc inclination of lordosis. Marker seen overlapping disc with its tip at anterior border of disc. Note we are aiming for a downwards migrated fragment so we will be starting our skin entry a little above the disc. Image79 This image shows we are marking the length of the marker from the back to its tip and also making it inclined towards the head with lordosis this disc inclination may be bigger eg: at L5S1.and that will allow us a targeting of the downwards migrated fragment. Once this distance is measured we proceed. 82

Image80 The same distance is taken on a line just above [for the inclination] our first line [disc space as is seen line], the tip of this line will be our point of skin entry. image81 83

The point of entry is marked with a cross on the skin. This generally is 12.5 or 13 cms from midline at L45 and L5S1. The point is above line in L45 and L5S1 levels and in line of L34 as it is vertical and in L12 and L23 it is below line. In most cases L45 and L5S1 have the same entry point. The accompanying scanogram image shows the marker lines for inclination meeting for L45 and L5S1.Line for D12L1 is tilted towards leg. image82 We can also target D12L1 where entry is generally the same as L12. This point is 6 fingers [of 7.5 glove size hand] from midline in most lower lumbar cases about 5 for upper discs. image83 Lateral view shows cross, our entry point, just above line marking L45 disc space. In general, rule of thumb is we should never go on the side of the patient, but stay on the back. Red line marks boundary between back and side of the body. 84

image84 Vertical arrow shows midline and horizontal arrow shows the point of entry. In image below we have taken a cross section on MRI with access lines from skin entry points as 1.13.87 cms 2 13.34 cms 3.12.17 cms 4.12.87 cms from midline. The image 85 shows cross cut in MRI with markings of likely access after skin entry at distances marked from midline. As can be seen 12.87 is ok but 13.87 goes through peritoneum so not advised. Red arc shows the peritoneal margin on left side of image. 85

image85 L4-5 image86 1. Cadaver dissection image with cut at disk level 45, L4 root is already out L5 is in foramen. We see ON obturator GN genitofemoral nerves. PM 86

th psoas muscle is seen having all nerves in its dorsal 1/4 . Common iliac veins are seen in front of the disc.[civ] Ascending lumbar vein is seen on left of the disc. This is an important ventral to foramen structure to be avoided from accidental injury during access. This picture shows the marker on skin along the likely path and will be seen in lower pic on C arm image. We note that here we are just above the disc space so we can target the downwards migrated fragment in the centre of the disc. Image87 Image88 87

The skin is prepared and the patient is draped. We will not be seeing any structure until we land on the annulus and put an endoscope inside the cannula. Until then this surgery is entirely guided by C arm images. The 3- dimensional targeting is done by our ability to think and visualize in 3D. The left of the camera image or top of c arm image is the head side. image89 Leg side . Local anesthetic 2% plain lignocaine without adrenaline is injected to numb skin and subcutaneous tissue. A weal is raised by the injection. Lignocaine may be injected along the needle path up to annulus. Image90 88

Image91 Confirmatory images with needle on projected path and checked with images on C arm. This will be the path which the needle and then in turn all instruments will take so it is important to mark and visualize the likely path. We take C arm images AP for direction and Lateral for depth of our access. Image92 89

The needle is inserted with an angle of 25 degrees to the horizontal. The tip is advanced to the intervertebral foramen and is visualized all along before proceeding. If the angle is too shallow it will hit the posterior bony structures namely superior facet and if too vertical it may enter the peritoneal sac. Violet line shows plane of transverse process. image93 HEAD image94 We try and hit the facet, confirm on c arm, withdraw the needle and raise our hand to go below the facet and land on the annulus. The AP C arm picture shows needle tip in the medial pedicle line which we are not supposed to cross. The medial pedicle line is the lateral edge of the dural sac. In patients who have chemical radiculitis or who have an annular 90

tear only or when we need to do a discogram, we follow these steps to target the symptom generator. Image of disc in lateral we should be touching the annulus. Image95 image96 91

Traversing root DURAL SAC Exiting root image97 Left hand to show thumb as exiting root and index as traversing root and 1 web space as th3e axilla of the roots, metacarpophalangeal joint of thumb as blue circle, DRG for better visualization. Image98 Improved visualization of the kambin’s triangle is achieved by doing a epidurogram or a radiculogram in cases like failed surgery, canal stenosis, mri demonstrated root anomaly. 92

Image99 A discogram is also warranted for a better understanding of changed morphology of the disc. Image above shows a central tear with trapped nuclear fragments in patient with back pain. Discogram is primarily not indicated for sciatica. Normally we enter skin at around 12.5 cms from midline. The tip of the needle in images above is just lateral to the medial pedicle line and in lateral C arm pic it is on the posterior annulus. This is an ideal placement. We prefer to go in lower part of foramen so we are away from nerve root that lies below the pedicle in upper foramen. It is also important to note that exiting root slopes down ventrally so cannot be in path of the needle. Once we are on the annulus and sure about the tip then we push the needle right up to the centre. The angle being 25 degrees or so, we are sure that when we look in lateral image, the tip of the needle will be in the posterior 93

quadrant of the disc. We are not in the centre in lateral view of the disc and we are not planning central debulking. We are targeting our efforts to removal of the fragment that is lying in posterior annulus. The fragment has a tail inside the disc and we will be landing in the space under the herniation which it has vacated. As we are close to the fragment base we try and pull it inside the disc and then remove it. Image100 Once we are sure about the placement of the needle in foramen and disc we proceed. The stylet of the needle is replaced by the guide wire. To increase our safety, we must always measure the guide wire against the stylet so the correct length is pushed in. Once the guide wire is placed properly we remove the needle by just pushing our guide wire with one finger. Rotatory movement of the needle hub is used and the needle is pulled out. Stylet and guide wire should glide in needle well or when we pull needle out the guide wire also may back out. It may be better to use a fresh guide wire for a case. 94

Image101 Image102 The guide wire is seen inside the disc. It has come back a little and needs to be corrected. Now this guide wire will guide all our activities in the disc. Skin incision of about 8 mm is now taken with a number 11 blade. Dilator which will gently dilate the muscles and is introduced over the guide wire. The tip of the dilator is blunt about 15 mm taper and is very safe. The dilator is introduced up to the annulus by rotatory movements and manipulating it by holding its roughened top portion. On reaching annulus, the patient may experience pain. Patient may get hypotension and bradycardia and needs monitoring for same and awareness of this for the treating surgeon. WE 95

need to give pre op glycopyrrolate or atropine for avoiding this and vasovagal syncope. Image103 Image104 When the dilator is going over the guide wire, a precaution is not to allow it to angle the wire and bend it. It must be in line with the guide wire to better reach the annulus. The lower picture shows the corrected aligned dilator over the guide wire. Here it seems to have reached the intertransverse area. The left image below shows a mal aligned dilator over the guide wire, which may bend it. IT IS CORRECTETD. This situation where a guide wire can bend is common for L5S1 when we dissect out the dorsolumbar fascial attachment from iliac crest during access 96

to L5S1. Image105 image106 97

The second side hole of the dilator may now be used to insert the long needle and use lignocaine over the annulus to numb it. The second hole also gives us orientation to the axilla of the nerve root and other structures. At this stage, depending upon the target, the long dilator may be pushed down towards the floor or lifted towards the ceiling with facet acting as fulcrum, so that the tip will ultimately lodge near the fragment. Our efforts are entirely directed to go as near the fragment as is possible and below it. We consider direction of instruments in AP view on c arm and depth in lateral view. Image107 The tip is on the posterior annulus and is in line with the disc space. A mallet is used to tap this blunt dilator inside the disc dilating the annulus and making a blunt entry inside the disc. It is advanced until we reach below our target fragment. The image below shows the dilator in the posterior quadrant of the disc. We have no intention to go to the central part of disc and destabilize it. Black part of the dilator is 15 mm and with perfect placement tip of dilator would be at centre of disc in AP and that black part just inside the posterior annulus in lateral image. 98

Image108 image109 99