Decompression Procedure Decompression alone may result in curve compression The type of decompression used depends on the extent of necessary decompres- sion. There is the option to decompress microsurgically the lateral recess and/or the foramen or to perform a more extensive canal enlargement by laminotomy, hemilaminectomy, or laminectomy to address the crucial compressive lesion. If two adjacent segments need to be decompressed, a laminectomy can be consid- ered, specifically when a surgical stabilization is foreseen. Whether maintenance of the integrity of the vertebral arches is necessary in a stabilized and fused spine is not clear, but it may prevent scarring of the dural sac. Besides the direct decompression as mentioned above, there is the possibility of indirect decompression occurring on correction of deformity and realign- ment of the spine. The older the patient and the longer lasting the degenerative scoliosis is, the more carefully this concept has to be applied. Adhesion of the dural sac due to scarring between the dura and the hypertrophied ligamentum flavum and facet joint capsules, and sometimes directly to the bone, may induce traction and/or compression of neural elements with consecutive neurological The pros and cons of direct of indirect decompressions must be carefully weighed deficit. The benefits of correction of the curve therefore have to be carefully weighed against the direct decompression. The idea that osteophytes and bony spurs may disappear over time in a stabilized and fused segment may leave the patient with sometimes persistent symptoms for quite a long time. The recom- mendation is to explore the crucial roots after a corrective measure by small fen- estration of the spinal canal in order not to miss a possible persistent compres- sion or traction of a neural element. Correction Procedures Sagittal balance is most important Whether or not a degenerative scoliosis should be corrected remains a crucial and complex question. The treatment of a degenerative scoliosis has different goals than the treatment of adolescent scoliosis. While in the latter the goal is pre- vention of curve progression and cosmetic improvement, degenerative scoliosis requires the relief of back and leg as well as claudication symptoms. Correction has to address spinal imbalance, which is mainly in the sagittal plane [1]. Whether a degenerative scoliosis should be corrected or not, depends on sev- eral factors: age cardinal symptoms coronal balance sagittal alignment curve rigidity rigidity of the adjacent spine Age The need for curve correc- tion decreases with age Theolderthepatient,thelessnecessitythereistocorrectthedeformity.Correc- tion may induce diffuse back pain in elderly patients, which may be due to the age-related inability to adapt to a new muscle balance. A correction may be nec- essary if there is a clear frontal imbalance. The correction may, however, rather consist in a localized osteotomy than in an overall correction of the curve. An additional sagittal imbalance needs to be corrected in most cases of chronic back pain in the context of a degenerative deformity [13, 20]. The correction has to reach the plumb line falling from the projection of the outer auricular canal onto the femoral head. 724 Section Spinal Deformities and Malformations Cardinal Symptoms and Imbalance Curve correction is indicated inthepresenceofsignificant coronal or sagittal imbalance A curve correction is indicated in patients with chronic back pain without a localized pathomorphology (e.g., painful facet joints) and a clear coronally and sagittally unbalanced spine. In younger patients, treatment consists of an overall curve correction. A localized osteotomy is more appropriate in elderly patients. Curve Rigidity Rigid severe curves require anterior release In a completely rigid curve, specifically in elderly patients, a correction usually is not necessary except if the back pain is related to the imbalance of the curve. The correction of a rigid curve may be achieved either by a localized corrective osteo- tomy (transpedicular reduction osteotomy) preferentially in elderly patients, or alternatively by a multilevel release and mobilization of the facet joints with oste- otomies in the joints and an overall correction through reduction of the mobi- lized spine to a pre-contoured rod. A rigid thoracolumbar curve >70° usually needs a combined approach [19, 20] ( Case Study 1). Rigidity of the Adjacent Spine Postoperative coronal imbalance is a risk In the case of a lumbar or thoracolumbar degenerative curve which is adjacent to a rigid (fused or ankylosed) idiopathic thoracic curve, any correction of the lum- bar spine has to be well thought through. Because of the rigid thoracic curve, the spine may fall completely out of balance following a lumbar correction. In youn- ger patients rarely it may be necessary to add a mobilizing osteotomy to the upper curve to effect a necessary lumbar correction. Surgical Techniques The armentarium of surgical techniques for the correction of degenerative scoli- osis consists of: posterior release anterior release wedge osteotomies transpedicular reduction osteotomies Posterior release canbeachievedthroughmobilizationandosteotomiesofthe facet joints. This procedure may be accompanied by an anterior release when sig- nificant osteophytes and intervertebral disc calcifications exist. If posterior release and facet joint osteotomies are not sufficient, wedge osteotomies of the arches ( Fig. 6) may provide further correction. For a significant localized correc- tion, a bilateral or unilateral transpedicular reduction osteotomy ( Fig. 7)maybe necessary at one, two or three levels. The correction of the lordosis in severe flat back syndrome can best be achieved by a pedicular reduction osteotomy when an anterior and posterior release is not sufficient. In all the above-mentioned methods a posterior pedicle-based instru- mentation is necessary [2, 8, 12, 22, 32]. The correction is done by contouring the rod in the desired shape and by pulling and/or pushing the pedicle anchorage toward the rod. One possibility is to adapt the rod to the curve – in the lumbar spine on the convex side – and to rotate the rod, which is inserted in the pedicle anchorage (screws or pedicle-based hook screws) into the lor- dosis. An alternative is to bend and adapt the rod in situ to the best possible contour. Degenerative Scoliosis Chapter 26 725 ab Figure 6. Smith-Peterson arch osteotomy This technique creates lordosis and is usually applied to one or multiple levels. a The interspinous ligament and the adjoining spinous process are resected with a rongeur and the interlaminar ligamentum flavum is removed in the mid- line, from where lateral osteotomies are carried out bilaterally, through the facet joints in the direction of the interspinal foramina. b These osteotomies are directed laterocranially, at an angle of 30 –40 degrees to the horizontal. The desired slot width of 5 –7 mm is obtained by using a suitably wide rongeur. If there is a lateral overhang, the osteotomies are made slightly larger on the convex side. The osteotomy gap is closed by a tension banding pedicular fixation one or two levels above or below. With one single osteotomy approximately 10 degrees of correction can be achieved. Unilateral cage insertion facilitates segmental correction A further methodology to achieve specifically short distance correction in the lumbar spine without performing osteotomies consists of complete mobilization of a deformed segment with complete removal of the disc through either an ante- rior or a posterior approach and using a unilateral cage or t ricortical bone graft by either an anterior lumbar interbody fusion (ALIF)oraposteriorlumbarinter- body fusion (PLIF) procedure. In the case of a uniquely posterior procedure, a posterolateral intertransverse fusion is done by autologous bone graft, either collected from laminar bone dur- ing the decompression procedure and/or the iliac crest, or by an allogeneic bone graft from a bone bank or a combination of autologous/allogeneic bone, which canstillbeaugmentedby,e.g.,granulartricalciumphosphate. An isolated anterior release and stabilization is seldom applicable and may work in younger patients at the thoracolumbar junction by sparing segments from inclusion into the fusion. In cases where anterior surgery is done, it is mostly a combined front and back procedure [19]. Avoid fusion to the sacrum in young patients Debatecontinuesontheindicationsforalumbosacral fusion. Only general recommendations can be given [9, 12, 30]. In young patients with secondary degenerative scoliosis, it is better to omit L5/S1 from fusion whenever possible in ordertopreventiliosacraljointdegenerationoranearlyhipproblem.Itisalso usually preferable to stop at L4 in a lumbar curve whenever possible. However, a fusion to the L5 vertebra is necessary when the condition of the L4/5 facet joint is poor ( Case Study 1). This obviously leads to an overload of the L5/S1 segment. However, it is difficult to predict the time when the secondary facet arthritis will occur, and possibly a good sagittal alignment will delay this substantially. The 726 Section Spinal Deformities and Malformations a b cd Figure 7. Pedicle reduction osteotomy a The osteotomy is started by removing the posterior arch including the facet joints until only the pedicle stump at the transition to the posterior wall ofthe vertebral body is left with also thetransverse process removed. b The pedicle stump is then excavated continuously into the vertebral body, which is emptied by means of an “eggshell” procedure. c The remaining posterior bridge between the two wholes of the pedicle stumps is then resected by a large Kerrison rongeur. d The created “empty” wedge is then closed under compression by means of a posterior pedicle-based tension banding system. Add an interbody fusion when fusion to the sacrum is intended patient, however,needs to be informed that secondary surgery may become nec- essary later [9, 12, 30]. When fusion to the sacrum cannot be avoided, it is important to add an interbody fusion to decrease the risk of a non-union. This caneitherbedonebyananterior(ALIF)oraposterior(PLIF)approach( Case Study 2 ). Degenerative Scoliosis Chapter 26 727 abcd e f Case Study 1 A young female teacher presented with progressive idio- pathic scoliosis. At the age of 35 years the curve measured 62° ( a).Threeyearslaterthecurvehadprogressedto75°(b). With curve progression, the patient developed incapacitat- ing back and leg pain and was unable to work. The major curve progression occurred during pregnancy. All conserva- tive treatment failed and the patient decided to undergo surgery. A left bending functional radiograph shows only some correction of the curve ( c). The patient presented with lumbar kyphosis which needed to be addressed ( d). Com- bined anterior/posterior surgery was performed. First, an anterior release through a minimally invasive thoracophreni- columbotomy from the left side was done and the interver- tebral disc spaces of T12/L1, L1/L2, L2/L3 and L3/L4 were released and filled with a hybrid of corticocancellous bone combined with beta-tricalciumphosphate ( -TCP) for an anterior fusion. Second, for posterior release and facet joint osteotomies, correction was done in conjunction with recon- struction of the lumbar lordosis and a posterolateral fusion from T9 to L5. Radiographs at 18 months follow-up show res- toration of lumbar lordosis and coronal balance ( e, f). 728 Section Spinal Deformities and Malformations a b c d Case Study 2 A 39-year-old female patient presented with incapacitating back pain due to a progression of adult idiopathic scoliosis (Type 2) ( a). There was no evidence of claudication symptoms or radicular pain. Non-operative treatment did not result in persistent pain relief. The preoperative lateral radiograph shows a significant loss of lumbar lordosis (3°) ( b). The postoperative radiographs show a restoration of lordosis to 22° and circumferential fusion with PLIF at the lumbosa- cral junction in order to avoid non-union. Frontal correction of the scoliosis was satisfactory ( c, d). Recapitulation Epidemiology. Primary degenerative scoliosis de- velops de novo after skeletal maturity and needs to be distinguished from the secondary degenerative changes of a curve already present at the end of growth. The prevalence of scoliosis in patients old- er than 50 years is about 6% including both types. Degenerative scoliosis is more prevalent in males than in females. The overall prevalence is increasing due to the aging population. Pathogenesis. Primary degenerative scoliosis results from segmental instability and degenerationof inter- vertebral discs and facet joints, often resulting in ante- rior and lateral displacement. The body counteracts the instability by a thickening of the ligaments, lum- bar spondylosis and facet joint hypertrophy causing central and foraminal stenosis. The clinical symptoms closely relate to the pathomorphological alterations. Secondary degenerative scoliosis results from asym- metric loading and dysbalance of the spine. Clinical presentation. The cardinal symptoms are back pain, claudication symptoms, radicular pain, neurological deficits and increasing deformity. Back pain is often related to spinal instability. Cosmetic aspects are not a predominant complaint in con- trast to adolescent scoliosis. Claudication symp- toms are very frequent but neurological deficits ap- pear late. The clinical assessment must focus on the sagittal and coronal balance as well as on the sagit- tal profile (flat back, thoracolumbar or lumbar ky- phosis). Concomitant osteoporosis must be as- sessed. Diagnostic work-up. Standing whole body anterior and posterior radiographs are indispensable for a clear understanding of the curve and the etiology. A differentiation of primary and secondary degen- erative scoliosis is difficult in advanced stages be- cause spinal rotation and lateral displacement can be present in both types. MRI is the imaging modal- ity of choice to show disc degeneration and neural compromise. CT and combination with myelogra- phy are sometimes necessary to better demon- strate the three-dimensional character of the curve and neural impingement. Provocative discography Degenerative Scoliosis Chapter 26 729 as well as facet joints, nerve root and epidural blocks often allow the identification of the source of the pain. Neurophysiologic studies and osteo- densitometry are helpful in selected cases. Treatment. Non-operative treatment consists of NSAIDs, physiotherapy, spinal injection studies and orthosis. However, conservative treatment cannot prevent progression of the curve. The general goals of surgery derive from the cardinal symp- toms: resolution of back pain and claudication symptoms, reversal of neurological deficits, and correction of deformity or prevention of curve pro- gression. In elderly patients, decompression may suffice if the main symptom is spinal stenosis. Care must be taken not to further destabilize the spine. The correction procedures consist of anterior, pos- terior or combined interventions. The choice of the technique depends on age, cardinal symptoms, coronal balance, sagittal alignment, curve rigidity, and rigidity of the adjacent spine. In elderly patients, posterior release is sufficient to realign the spine. A severely rigid curve in young individuals usually requires a combined anterior/posterior release. When anterior and/or posterior release is insufficient, wedge osteotomies or transpedicular reduction osteotomies are indicated to rebalance the spine. Posterior pedicle screw fixation is the standard fixation technique. Posterolateral fusion with autograft, allograft or bone substitutes accom- panies spinal instrumentation in almost all cases. Only in young individuals with short segmental curves is anterior release and instrumented fusion advisable. Sagittal and coronal rebalancing as well as reshaping the sagittal contours (flat back) are crucial for a good outcome. Fusion to the sacrum should be avoided whenever possible in young individuals. However, if fusion to the sacrum can- not be avoided, an interbody fusion is mandatory to reduce the risk of non-union. Key Articles Aebi M (2005)Theadultscoliosis.EurSpineJ14(10):925 – 948 A recent review article which allows for further reading. Bridwell KH (1996) Where to stop the fusion distally in adult scoliosis: L4,L5,orthe sacrum? Instr Course Lect 45:101 – 7 This articles highlights the many aspects which must be weighed and discussed with the patientbeforedecidingonalongfusiondowntothemiddleordistallumbarspine.Out- come of surgery is crucially dependent on how well the different aspects are addressed by surgery. Grubb SA, Lipscomb HJ, Coonrad RW (1988) Degenerative adult onset scoliosis. Spine 13:241 – 245 Theauthorsreviewed21patientswith the diagnosisof degenerativescoliosis.This review shows that patients can de novo develop progressive scoliosis and loss of lumbar lordosis with a resulting flat back deformity. These patients commonly present in the 6th decade of life with predominant claudication symptoms but often lack the classic feature of relief in a sitting posture. The number of male and female patients was approximately equal. Roentgenogram findings show a high angle deformity over a short number of spinal seg- ments and an absence of bony features associated with idiopathic scoliosis such as lateral vertebral wedging and alterations of the lamina. Grubb SA, Lipscomb HJ ( 1992) Diagnostic findings in painful adult scoliosis. Spine 17(5):518 – 527 Fifty-five adults with painful scoliosis were evaluated with regard to diagnostic findings. The curves were 49% adult degenerative onset and 44% idiopathic. The older degenera- tive patients had myelographic defects most commonly within the primary curve and multiple abnormal, not necessarily painful, discs throughout the lumbar spine on discog- raphy. The idiopathic group had myelographic defects most commonly in a compensa- tory lumbar or lumbosacral curve. On discography,all idiopathic patients had at least one abnormal, painful disc, and 88% had their pain reproduced. Pain-producing pathology was frequently identified in areas that would not have been included in the fusion area according to accepted rules for treatment of idiopathic scoliosis. 730 Section Spinal Deformities and Malformations Key Articles Swank S, Lonstein JE, Moe JH, Winter RB, Bradford DS (1981)Surgicaltreatmentof adult scoliosis. A review of two hundred and twenty-two cases. J Bone Joint Surg Am 63:268 – 87 Classical case series which predominantly deals with secondary degenerative scoliosis. Ponseti IV (1968) The pathogenesis of adult scoliosis. In: Zorab PA (ed) Proceedings of Second Symposium on Scoliosis Causation. E & S Livingstone, Edinburgh A comprehensive treatise on the pathogenesis of adult scoliosis by one of the pioneers of scoliosis surgery. References 1. Aebi M (2005) The adult scoliosis. Eur Spine J 14(10):925–948 2. Aebi M (1988) Correction of degenerative scoliosis of the lumbar spine. A preliminary report. Clin Orthop Related Res 232:80–86 3. Albert TJ, Purtill J, Mesa J, McIntosh T, Balderston RA (1995) Study design: Health outcome assessment before and after adult deformity surgery. A prospective study. Spine 20: 2002–2004; discussion p. 2005 4. Ali RM, Boachie-Adjei O, Rawlins BA (2003) Functional and radiographic outcomes after surgery for adult scoliosis usingthird-generation instrumentation techniques. Spine 28(11): 1163–1169 5. Ascani E, Bartolozzi P, Logroscino CA, Marchetti PG, Ponte A, Savini R, Travaglini F, Binazzi R, Di Silvestre M (1986) Natural history of untreated idiopathic scoliosis after skeletal matu- rity. Spine 11(8):784–789 6. Benner B, Ehni G (1979) Degenerative lumbar scoliosis. Spine 4:548 7. Boachie-Adjei O, Gupta MC (1999) Adult scoliosis + deformity. AAOS Instructional Course Lectures 48(39):377–391 8. Bradford DS, Tay BK, Hu SS (1999) Adult scoliosis: surgical indications, operative manage- ment, complications and outcome. Spine 24:2617–2629 9. Bridwell KH (1996) Where to stop the fusion distally in adult scoliosis: L4, L5, or the sacrum? AAOS Instructional Course Lectures 45:101–107 10. Deviren V, Berven S, Kleinstueck F, Antinnes J, Smith JA, Hu SS (2002) Predictors of flexibil- ity and pain patterns in thoracolumbar and lumbar idiopathic scoliosis. Spine 27(21): 2346–2349 11. Deyo RA, Cherkin DC, Loeser JD, Bigos SJ, Ciol MA (1992) Morbidity and mortality in asso- ciation with operations on the lumbar spine. The influence of age, diagnosis, and procedure. J Bone Joint Surg Am 74(4):536–543 12. Edwards CC, Bridwell KH, Patel A, Rinella AS, Jung Kim Y, Berra A, Della Rocca GJ, Lenke LG (2003) Thoracolumbar deformity arthrodesis to L5 in adults: The fate of the L5–S1 disc. Spine 28(18):2122–2131 13. Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F (2005) The impact of positive sagittal balance in adult spinal deformity. Spine 30(18):2024–2029 14. Grubb SA, Lipscomb HJ, Coonrad RW (1988) Degenerative adult onset scoliosis. Spine 13: 241–245 15. Grubb SA, Lipscomb HJ (1992) Diagnostic findings in painful adult scoliosis. Spine 17(5): 518–527 16. Grubb SA, Lipscomb HJ, Suh PB (1994) Results of surgical treatment of painful adult scolio- sis. Spine 19:1619–1627 17. Guillaumat M (1993) Les scolioses lombaires de l’adulte. In: SOFCOT, Chirurgie du Rachis de l’Adulte. Expansion Scientifique Fran¸caise, Paris, pp 199–222 18. Healy J, Lane J (1985) Structural scoliosis in osteoporotic women. Clin Orthop 195:216 19. Johnson JR, Holt RT (1988) Combined use of anterior and posterior surgery for adult scolio- sis. Orthop Clin North Am 19:361–370 20. Kostuik JP (1980) Recent advances in the treatment of painful adult scoliosis. Clin Orthop 147:238–252 21. Lenke LG, Edwards CC, Bridwell KH (2003) The Lenke classification of adolescent idio- pathic scoliosis: How it organizes curve patterns as a template to perform selective fusions of the spine. Spine 28(20S):S199–S207 22. Marchesi DG, Aebi M (1992)Pedicle fixation devices in the treatment of adult lumbar scolio- sis. Spine 17:S304–309 23. Ogilvie JW (1992) Adult scoliosis: evaluation and nonsurgical treatment. Instructional Course Lectures 41:251–255 Degenerative Scoliosis Chapter 26 731 24. Ponseti IV (1968) The pathogenesis of adult scoliosis. In: Zorab PA (eds) Proceedings of sec- ond symposium on scoliosis causation. E & Livingstone, Edinburgh 25. Reindl R, Steffen T, Cohen L, Aebi M (2003) Elective lumbar spinal decompression in the elderly: is it a high-risk operation? Can J Surg 46(1):43–46 26. RinellaA,BridwellK,KimY,RudzkiJ,EdwardsC,RohM,LenkeL,BerraA(2004)Latecom- plications of adult idiopathic scoliosis primary fusions to L4 and above: The effect of age and distal fusion level. Spine 29(3):318–325 27. SchwabF,el-FegounAB,GamezL,GoodmanH,FarcyJP(2005)Alumbarclassificationof scoliosis in the adult patient: preliminary approach. Spine 30 (14):1670–1673 28. Simmons ED Jr, Kowalski JM, Simmons EH(1993) The results of surgical treatment for adult scoliosis. Spine 18:718–724 29. Sponseller PD, Cohen MS, Nachemson AL,Hall JE, Wohl ME(1987) Results of surgical treat- ment of adults with idiopathic scoliosis. J Bone Joint Surg Am 69(5):667–675 30. Swank S, Lonstein JE, Moe JH, Winter RB, Bradford DS (1981) Surgical treatment of adult scoliosis. A review of two hundred and twenty-two cases. J Bone Joint Surg 63A:268–287 31. Takahashi S, Del´ecrin J, Passuti N (2002) Surgical treatment of idiopathic scoliosis in adults: An age-related analysis of outcome. Spine 27(16):1742–1748 32. Tribus CB (2003) Degenerative lumbar scoliosis: evaluation and management. J Am Acad Orthop Surg 11(3):174–183 33. Winter RB, Lonstein JE, Denis F (1988) Pain patterns in adult scoliosis. Orthop Clin North Am 19:339–345 732 Section Spinal Deformities and Malformations 27 Spondylolisthesis ClaytonN.Kraft,RüdigerKrauspe Core Messages ✔ Spondylolisthesis is the end result of various distinct causes ✔ Spondylolisthesis is a disorder of the entire lumbosacral junction and spondylolysis is a result of a stress fracture ✔ Not every radiographically evident slippage causes clinical symptoms ✔ Standard radiographs are the imaging modality of choice for a first assessment ✔ Oblique radiographs may demonstrate a pars defect not visible on the lateral view ✔ In the presence of neurologic deficit, MRI is the imaging modality of choice ✔ Multi-slice CT with image reformation detects pars defects not visible on standard radio- graphs ✔ Treatment rationale is based on the etiology of the disorder, degree of slippage, intensity of pain, and neurologic symptoms ✔ The vast majority of patients with spondylolis- thesis can be treated non-operatively ✔ The primary aim of all surgical options is to achieve stability, prevent progression and decompress neurologic structures ✔ The surgical technique (posterolateral fusion in situ, instrumentation and posterolateral fusion with or without interbody fusion) depends on the surgeon’s familiarity with the approach as well as on the deformity ✔ Reduction of low-grade spondylolisthesis is not the primary aim of surgery but may be neces- sary to decompress foraminal stenosis ✔ There is continuing debate on the subject of reduction of high-grade spondylolisthesis ✔ The L5 nerve root is at high risk when reducing high-grade spondylolisthesis because of a tether- ing effect ✔ Anterior buttressing (interbody fusion) is needed when a slipped vertebra is reduced and/or distracted ✔ Frequent complications after spondylolisthesis surgery are non-union and postoperative nerve root compromise Epidemiology There is a gender and ethnic factor to spondylolysis and spondylolisthesis Spondylolysis is not the only cause of spondylolisthesis, only the most intensively studied one. Lumbar spondylolysis occurs in the general population at the rate of around 5% [36, 49]. Based on data published by Fredrickson et al. [24], the rate ofspondylolysisislessthan4.4%forchildrenundertheageof6yearsand approximately 6% for adults. According to Grobler and Wiltse [27], Caucasian males are significantly more frequently affected than black females, indicating that there is a gender as well as an ethnic factor underlying the condition. This presumption is underlined by a recent study by Whitesides et al. [115], who were able to demonstrate that in different ethnic groups there is a genetically deter- mined difference in the upper sacral tilt, which again is associated with the occurrence of pars defects. Sports with intensive hyperextension and rotation of the spine may cause pars defects Numerous studies have shown that young athletes engaged in strenuous train- ing in sports that incorporate intensive hyperextension and rotation of the lum- bar spine have a predisposition to spondylolysis and subsequent spondylolis- Spinal Deformities and Malformations Section 733 . source of the pain. Neurophysiologic studies and osteo- densitometry are helpful in selected cases. Treatment. Non-operative treatment consists of NSAIDs, physiotherapy, spinal injection studies and orthosis visible on standard radio- graphs ✔ Treatment rationale is based on the etiology of the disorder, degree of slippage, intensity of pain, and neurologic symptoms ✔ The vast majority of patients. instability and degenerationof inter- vertebral discs and facet joints, often resulting in ante- rior and lateral displacement. The body counteracts the instability by a thickening of the ligaments,