Review Article 792

During the past 2 decades, transpedicle spinalinstrumentation has been broadly accepted for

treatment of spinal instability. Chen(1) reported theclinical experience of back pain after thoracolumbarfracture treated by long instrumentation and shortfusion. Some surgeons use pedicle screws for anteri-or reconstructive spinal surgery for a metastaticmalignancy, even though the screws are fixed to thevertebral body instead of through the pedicles.(2) Theauthors used pedicle instrumentation for combinedanterior and posterior surgeries in the treatment ofspinal tuberculous spondylitis.(3) In a study of 116consecutive patients with symptomatic spondylolis-thesis treated with decompressive laminectomy, seg-mental fusion, and pedicle instrumentation at ChangGung Memorial Hospital, 88% of the patientsachieved solid fusion.(4) Some clinicians advocateautogenous bone grafts to achieve spinal fusion,while others suggest other modalities, including allo-grafts, ceramic bone grafts, or even hyperbaric oxy-gen therapy.(5)

Many patients with lumbar instability have been

treated with decompression and fusion with or with-out instrumentation. Pedicle screw instrumentationguarantees rigid fixation and increases the unionrate. High fusion rates and satisfactory clinical out-comes have been reported.(4,6) A retrospective studyanalyzed the survivorship of Dorsale KompressionsSpondylodes (DKS) instrumentation and the clinicaloutcomes in 185 patients with spondylolisthesis.(6)

Among those patients treated with Zielke DKSinstrumentation for a mean follow-up period of 3.5years, 14% experienced rod breakage, and 1.7%screw breakage, respectively. Adjacent instabilitydeveloped in 9.7% of patients.

However, an increasing number of patients whohave undergone lumbar fusion are now seeking fur-ther treatment. Instability of neighboring unfusedsegments is becoming common, because spinal sur-geons are enthusiastic about instrumentation of thelumbar spine. Harris(7) reported that acquiredspondylolysis is a sequel to spinal fusion. Patientswith recurrent back pain and sciatica after lumbarspinal fusion present a complex problem. Several

Adjacent Instability after Instrumented Lumbar Fusion

Wen-Jer Chen, MD; Po-Liang Lai, MD; Lih-Huei Chen, MD

The invention of pedicle screw instrumentation has greatly improved outcomes ofspinal fusion, which has become the treatment of choice for lumbar spondylolisthesis. Asresearchers accumulate experience, both theoretical and clinical advances are continuallybeing reported. A review of the literature and the experience of the authors show that thedevelopment of adjacent instability, as in the breakdown of a neighboring unfixed motionsegment, is a common consequence of an instrumented lumbar spine. This article reviewsthe risk factors and surgical treatment of adjacent instability. The authors believe that properpreoperative planning and complete surgical procedures are imperative to prevent adjacentinstability. For those who need revision surgery, meticulous surgical techniques can achievesatisfactory results. (Chang Gung Med J 2003:26:792-8)

Key words: adjacent instability, adjacent segment, failed back surgery syndrome, lumbar fusion,pedicle screw instrumentation.

From the Department of Orthopedic Surgery, Chang Gung Memorial Hospital, Taipei.Received: Feb. 12, 2003; Accepted: Aug. 26, 2003Address for reprints: Dr. Wen-Jer Chen, Department of Orthopedic Surgery, Chang Gung Memorial Hospital. 5, Fushing Street,Gueishan Shiang, Taoyuan, Taiwan 333, R.O.C. Tel.: 886-3-3281200 ext. 2163; Fax: 886-3-3278113; E-mail:chenwenj@cgmh.org.tw

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studies have reported the accelerated degeneration oflumbar segments adjacent to a previous fusion.(4,6,8)

Adjacent instability has been recognized as animportant type of failed back surgery syndrome.Previous investigations have reported a 7% incidenceof adjacent instability after 2.4 years and 45% after33 years.(4,8)

Failed back surgery syndrome causes distressfor both patients and spinal surgeons. Not only is thesyndrome difficult to diagnose and treat appropriate-ly, but the surgery also yields unpredictable results.(9-

11) Factors related to successful results are inconsis-tent. Following surgical procedures to the lowerlumbar spine, some patients continue to suffer backpain and sciatica. Favorable factors vary acrosspatient populations, operation indications, follow-upprocedures, and criteria for success. Conservativetreatment, including orthosis and medication, canhelp patients accept their impairments, minimizeaggravating factors, and improve their daily activitylevels. Pain may be reduced with improved trunkstrength and spinal mobility.(12) Surgery is the lastmethod considered to resolve the problem. Successrates of revision lumbar spinal surgery in the relevantliterature range from 60% to 80%.(10,13) The successrate of revision lumbar spinal surgery declines as thefollow-up period and the number of spinal proce-dures increase. Finnegan analyzed 65 back patientswho had multiple operations and concluded that therelief of mechanical compression, such as disc pro-trusions or bone impingement, yields satisfactoryresults.(14) Surgical treatment for adjacent instabilityafter instrumented lumbar fusion has rarely beenreported.

Diagnosis of adjacent instabilityThe period of the pain-free interval after the

operation can assist in the differential diagnosisbetween disc herniation and fibrosis. A symptom-free period of 12 months or longer indicates a lesiontype other than fibrosis.(10,13,14) The absence of anypain-free interval usually means that the previousoperative procedure did not properly target thelesion.

A detailed history and physical evaluation mustbe conducted for a diagnosis of adjacent instability.Patients always address their symptoms whenexposed to mechanical stimuli. Radiography can beused to clarify the status of the spinal fusion.

Although, pseudoarthrosis is usually correlated withpoor results, solid fusion does not guarantee a satis-factory outcome. A dynamic lateral image of thelumbar spine is essential in diagnosing adjacentinstability. Because of the distortion produced onMRI by metal artifacts, a myelogram is believed tobe a better choice for evaluating stenosis due to adja-cent instability after instrumentation.(15) A myelo-gram can clearly determine the status of the adjacentstenosis. The criteria for adjacent instability arewell-defined spondylolisthesis or dynamic instabilitywith slippage of more than 4 mm and/or an anglechange of more than 10˚ on flexion and extension.Spondylolysis of the neighboring level occurringafter spinal fusion is considered to result from opera-tive damage or a stress fracture of the pars interartic-ularis on both sides and can be diagnosed by lateralfilm of the spine.(7,16) The motion segment just abovethe fused segment is the most common level of adja-cent instability.(4,13,15,16)

Risk factors for adjacent instabilityRisk factors for developing post-fusion adjacent

instability have not been clearly defined. Fusionwith instrumentation exhibits a higher rate of devel-oping adjacent instability than does fusion withoutinstrumentation.(4,17) Pedicle screw instrumentationhas a supplementary effect on developing adjacentinstability after lumbar fusion. In a study investigat-ing the location of pedicle screws in relation to adja-cent superior segment facet joints using computedtomography and plain radiographs, facet joint viola-tion occurred in 30% of patients.(18)

Age has been considered a predisposing fac-tor.(19) Older patients have higher baseline degenera-tion in their adjacent segments, which are possiblymore susceptible to increased mechanical stress.(17,19)

Patients who have degeneration in their adjacent seg-ments are possibly more susceptible to developingadjacent instability. Young patients may have bettersupporting structures in their nucleus pulposus,annulus fibrosus, and facet joints than do olderpatients. However, older patients are less active intheir daily life and have less exposure to highmechanical stresses.

Levels decompressed but not included in fusionare theoretically more susceptible to stress and morelikely to undergo degeneration.(17) If the decompres-sion level extends beyond the fusion level, adjacent

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instability is more likely to develop. For a fused L4-L5 spine, if the decompression is localized to thelower L4 and upper L5, the chance of developingadjacent segment instability is less than with decom-pression which includes the entire L4 and upper L5.Removal of laminae and spinous processes of theentire L4 means sacrificing the anchoring point forthe L3-L4 supraspinous ligament, which jeopardizesthe stability of the spinous process-supraspinous lig-ament-spinous process posterior complex. Thisbone-tendon-bone model increases dynamic stabilityfor adjacent unfused segments. We believe that thebone-tendon-bone complex between the fused seg-ments and motion segment plays an important role inpostoperative spine stability. Sacrificing either thesupraspinous ligament or the anchoring points on thespinous processes will cause increased stress at thecorresponding disc level.

Gender is not a significant contributing fac-tor.(19,20) Even though the structure of the spine of amale is more rigid than that of a female, males aregenerally more active and their spines theoreticallyundergo more stress than do those of females.

The rigidity of an implant might affect adjacentinstability. An implant is more likely to developadjacent instability following more-rigid instrumen-tation.(4) However, other surgeons have challengedthis idea.(17,19,21) Some claim that the choice of bonegrafting technique is more important than the selec-tion of implants.(22) Instrumented lumbar fusion usu-ally achieves a high union rate. The relation ofpseudoarthrosis and adjacent instability is difficult toanalyze because of very small case numbers. To ourauthors' knowledge, there is no relevant report onthis issue. Nagata et al. reported that immobilizationof long segments caused increased facet force andadjacent segmental motion.(23) Clinically, it is notclear whether long instrumentation might accelerateadjacent instability more than short instrumentation.Shlegel et al. found no significant difference amongpatients classified as having short lumbar floatingfusion, lumbosacral fusion, and long thoracolumbarfusion.(24)

The authors retrospectively reviewed 270patients with spondylolisthesis who underwent alaminectomy, posterolateral fusion, and pediclescrew instrumentation between 1987 and 1991.(25)

Thirty-two (11.9%) patients developed adjacentinstability after a mean follow-up period of 4.7

(range, 3~8) years. Most of these (30/32) occurredproximal to the fusion. Neither the type of implant,the fusion level, nor the laminectomy level had a sta-tistically significant relation to adjacent instability.However, the occurrence of adjacent instability indegenerative spondylolisthesis was more commonthan in the spondylolytic type. This may beexplained by accelerated degeneration of preexistentdegeneration of adjacent segments in degenerativespondylolisthesis.

Adjacent instability may be a time-relatedprocess. The interval of the follow-up period is animportant factor. The longer the follow-up period is,the greater amount of adjacent instability whichdevelops.(4,6,11)

Biomechanics of adjacent motion segmentsafter spinal fusion

Degenerative changes follow alterations in bio-mechanics caused by the fused rigid segment, whichputs additional stress on normal segments above andbelow the spinal fusion. Quinnell et al. suggestedthat floating fusion of a single disc generated addi-tional localized loading on the disc immediatelyabove and below the fusion.(21) Lee and Langranastudied three types of lumbosacral fusion using amathematical model to analyze the stress distribu-tion. In their study, posterior fusion caused the great-est amount of stress on the facet joints of the adja-cent segment.(22) Since the advent of spinal instru-mentation, some surgeons have reported that adja-cent instability occurs within a shorter time.Increased angulatory and translational motions havealso been identified at the motion segments adjacentto a spinal fusion.

The effects of spinal alignment on load trans-mission at the levels adjacent to the fusion have sel-dom been reported. Hypolordosis in fused lumbarsegments causes increased loading on the posteriorcolumn of the adjacent segments. Oda et al. reportedthat a kyphotic deformity may have led to facet jointcontracture and facet arthritis and may have servedas the origin of low back pain at the cranially adja-cent level in a sheep model.(26) Ha et al. demonstrat-ed that after immobilization, the facet patterns of theadjacent segment changed, and segmental motionincreased.(27) A study, funded by the NationalScience Council of Taiwan, used a porcine spinemodel to study the effect of sagittal alignment on the

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instability of adjacent segments after lumbarfusion.(28) Twenty-four fresh porcine lumbar spineswere instrumented using posterior pedicle screwimplants between L2 and L4. Group I had kyphosisof 20˚, Group II was neutral, and Group III had lor-dosis of 20˚. The preliminary data revealed that thestress on the adjacent segment increased in thekyphotic group. However, no clinical study hasshown that the configuration of a short fused seg-ment is correlated with the development of adjacentinstability.

Pathologic changes in adjacent motion segmentsSegments proximal to the instrumentation might

be required to compensate for changes in alignmentto maintain a normal posture.(23) Lumbar malalign-ment might accelerate adjacent deterioration.(24)

Possible pathologic changes at the adjacent unstablesegment include instability of the motion segment,disc space narrowing, and stenosis caused by facetdegeneration and ligament flavum hypertrophy.Spinal canal stenosis at the adjacent level can be eas-ily demonstrated by myelography. Lee indicated thatthe most common pathologic condition at the adja-cent segment is hypertrophic degenerative arthritis ofthe facet joints.(22,29) Damage to the posterior liga-ment complex may also contribute to the develop-ment of lesions by reducing the resistance to shear-ing forces at the intervertebral level next to thefusion. Chow et al. reported that neighboringunfused segments must work more frequently towardthe extremes of their functional ranges of motionafter fusion, and that such an effect becomes moremarked after a 2-level fusion.(30) Under such circum-stances, a patient may experience back pain, sciatica,and intermittent claudication.

Pitfalls of revision surgeryThe major difficulty in revision surgery is ade-

quate decompression of spinal stenosis when scaradhesion is present. Dural tears are a potential prob-lem during revision surgery. Fritsch et al. and Kimand Michelsen reported dural tear rates of 5.1% and8%, respectively in revision spinal surgery.(10,13) Twoof 39 patients had dural tear in another study.(15) Itwas observed that scar tissue was less adherent in thelateral recess than in the midline area. The laminec-tomy was begun from the nonstenotic area, wherethe scar was less developed. Next, decompression

was performed through the bilateral lateral recess byan extensive medial facetectomy. Notably, a medialfacetectomy can prevent vigorous manipulation ofthe nerve root and decrease the rate of dural tearing.

Although successful arthrodesis does not alwaysdecrease symptoms, pseudoarthrosis usually leads toclinical failure. When a fusion is attempted betweena previous fusion and the adjacent motion segment,the resulting increase in stress on this area makespseudoarthrosis likely without instrumentation.Whitecloud et al. demonstrated that in a fusion to adegenerated segment adjacent to a lumbar fusion, thepseudoarthrosis rate of 80% fell to 17% with the useof supplemental instrumentation.(31) The authorreported a nonfusion rate of 5.1% in the surgicaltreatment of adjacent segment instability.(15)

Surgical techniquesDuring the operation, patients are positioned

prone on a 4-poster spinal frame. The hips areextended to maintain lumbar lordosis. A laminecto-my is initially performed through an adjacent virginsite and then it is performed in the stenotic area,which is always the motion segment of adjacentinstability. The main pathologic change of stenosisis at the cranial margin of the epidural fibrosis. Arisk of dural tear or nerve injury during nerve decom-pression is present. A laminectomy is performed byextensive removal of the medial facet to preventdural tearing. The inferior facet is carefully dividedfrom the pars interarticularis using an osteotome andis completely removed. Then the lateral recess isunroofed using small, sharp Kerrison rongeurs andosteotomes. This procedure always includesremoval of at least the medial 1/3 of the superiorfacet. After a medial facetectomy and foraminoto-my, the nerve roots of the stenotic segment becomedecompressed. If bone chips or scar tissue adhere tothe dura in the midline, they are carefully removedwith a scalpel. All patients must undergo extensionof the posterolateral fusion mass with a bone graft.Next, the old pedicle screws are removed andreplaced with new pedicle screws of a larger diame-ter. A pair of pedicle screws is then inserted throughthe virgin pedicles of the adjacent segment (Fig. 1).After the screws are tightened, the bone graft isplaced between the decorticated virgin transverseprocess and the decorticated old fusion mass.(15,19,31)

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ConclusionsFailed back surgery syndrome is a troublesome

problem. With the extensive use of instrumentedlumbar fusion, increasing instances of adjacent insta-bility cases occur. A thorough evaluation, includinga detailed history and physical examination, is essen-tial for making a correct diagnosis. Signs and symp-toms of neurological claudication and mechanicalpain should be noted. Revision is difficult in thosepatients and is fraught with complications. The

major difficulty in surgical treatment is adequatedecompression when scar adhesion is present. Theauthors follow the principles of (1) performing awide decompression by a medial facetectomy toreduce nerve root injury and (2) extending the fusionwith an autogenous bone graft and pedicle screwinstrumentation. When meticulous procedures arefollowed, satisfactory outcomes of revision surgeryfor adjacent instability can be achieved.

Fig. 1 (A) Preoperative myelogram of a62-year-old woman showing L4-L5degenerative spondylolisthesis with spinalstenosis. (B) Postoperative lateral viewshowing screws at L4 and L5. The L3-L4disc height is normal. (C) Adjacent insta-bility which developed at L3-L4 6 yearsafter the index surgery. The myelogramshows severe adjacent stenosis. (D)Lateral view after revision surgery. Theold L4 and L5 screws were removed, andnew screws were applied to L3 and L4.(E) Anteroposterior view after revisionsurgery showing successful extension ofthe fusion mass from L4-L5 to L3-L5.

A B

D E

C

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(pedicle screw instrumentation) (adjacent

instability)

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