CHAPTER SYNOPSIS:
Lumbar degenerative disorders are typically divided between those that cause mechanical back pain and those that cause radiating leg pain. Treatment decisions must recognize that each of these conditions represents a point on a continuum rather than a discrete pathophysiologic entity. Although the use of lumbar fusion is increasing, the precise indications continue to be intensely debated. The purpose of this chapter is to review the pathophysiology, clinical imaging, indications, surgical technique, and outcomes for fusion in lumbar degenerative disorders.
IMPORTANT POINTS:
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Degenerative disorders of the spine represent a point on a continuum rather than a discrete pathophysiologic entity.
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Degenerative changes are frequently seen on radiographic imaging of asymptomatic patients.
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Fusions should be considered at the time of decompression in patients with evidence of a spondylolisthesis or scoliosis.
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Fusion should be considered at the time of surgery if iatrogenic destabilization is noted at the time of surgery.
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Excessive facet or pars resection may lead to postoperative instability.
CLINICAL/SURGICAL PEARLS:
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The majority of patients with degenerative conditions of the lumbar spine can be treated without surgery.
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The decision to undergo surgery should be undertaken only after a careful assessment correlating the findings from the patient’s history, examination, and imaging.
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Surgical options include posterolateral intertransverse fusion, anterior lumbar interbody fusion, posterior lumbar interbody fusion, transforaminal interbody fusion, and circumferential 360-degree fusions.
CLINICAL/SURGICAL PITFALLS:
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The validity of spinal fusion in the degenerative spine relates directly to the risk/benefit ratio of the intended procedure.
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Evidence of a solid radiographic fusion does not guarantee a clinically successful outcome.
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Conversely, excellent results can be obtained in the setting of radiographic pseudarthrosis.
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Fusion outcomes in mechanical back pain of degenerative disc disease have been mixed.
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By 2000, the annual number of lumbar spinal fusion exceeded 150,000 and continues to grow. Most of these fusions are performed for degenerative disease. Understanding the indications for spinal fusion in degenerative disease requires:
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A basic understanding of the pathophysiology of the disease states involved
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Recognition that a variety of fusion techniques are undertaken, each with markedly different levels of surgical morbidity
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An understanding that despite the clearcut surgical goal, outcomes of surgery do not correlate with the fusion rate
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A rough idea of the risk/benefit ratio that might justify surgery
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Alternatives to fusion
Lumbar degenerative disorders are typically divided between those that cause mechanical back pain (i.e., degenerative disc disease) and those that cause radiating leg pain (i.e., disc herniation or spinal stenosis). Treatment decisions, particularly those regarding fusion, must recognize that each of these conditions represents a point on a continuum rather than a discrete pathophysiologic entity ( Box 17-1 ).
- 1.
At the time of decompression in patients with spondylolisthesis
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At the time of decompression in patients with scoliosis
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When iatrogenic destabilization is noted at the time of surgery
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Postoperative spinal instability, such as pars fracture and spondylolisthesis
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Postoperative recurrence of stenosis or disc herniation at the same level
- 6.
Incapacitating mechanical back pain originating in one or two lumbar segments
- 7.
Symptomatic adjacent level degeneration
- 8.
Painful nerve root compression not accessible without iatrogenic destabilization (e.g., some cases of foraminal stenosis)
This continuum spans from asymptomatic desiccation of the lumbar disc, to painful disc degeneration associated with mechanical pain, to various neurocompressive lesions including herniation and spinal stenosis, to marked lumbar instability with multiplanar spondylolisthesis and degenerative scoliosis. More importantly, similar radiographic changes may occur in asymptomatic patients and are nearly universal in the aging spine. Defining when a radiographic finding becomes a pathologic state, therefore, remains controversial, particularly in patients without neurocompressive symptomatology.
The goals of fusion in this patient population include stabilization of scoliosis and spondylolisthesis, decrease of pain by limiting mechanical stimuli to free nerve endings of a painful outer annulus and posterior longitudinal ligament, and decrease of the occurrence of laminar regrowth and recurrent stenosis. Although the use of lumbar fusion is increasing, the precise indications continue to be intensely debated.
PATHOPHYSIOLOGY
Spondylosis represents a sequence of progressive degenerative changes in the discs, bodies, facets, and ligaments of the spine. Degeneration typically begins with disc desiccation. The tripod relationship between the disc and facets accommodates segmental flexion, extension, and rotation without significant decrease in the volume available for the neural elements. This interdependent architecture reflects disc degeneration to the facets posteriorly. Facetal degeneration allows increased segmental motion precipitating a vicious cycle of increasing disc degeneration. In response to segmental microinstability, tissue strains increase and tissues hypertrophy.
Fusion procedures are occasionally indicated in the face of instability or intractable mechanical pain. In each patient with lumbar degeneration, a balance between destabilizing degenerative processes and restabilizing tissue reactions is established. In some patients, a stiff, hyperstable spine results. In others, pathologic motion may occur. In fact, one level may demonstrate hyperstability or autofusion while the subjacent segment becomes unstable. The appropriateness of fusion at a given spinal level depends on that level’s location on this spectrum of stability.
Degenerative spinal instability most commonly manifests as anterolisthesis. In woman older than 60, 10% have a degenerative slip, most commonly at L4-5. The iliolumbar ligament restrains L5, whereas while L4 slides over it anteriorly. Diabetes and estrogen depletion increase the risk for spondylolisthesis. Biomechanical factors increase the risk for slippage as well, including hemisacralization of L5, sagittal facet orientation, a lowered intercrestal line (passing through L5 instead of L4). Unlike isthmic spondylolisthesis, the posterior arch remains intact in degenerative slips and may contribute to canal stenosis.
In 6% to 10% of the older population, degenerative instability presents as a de novo scoliosis. As opposed to residual idiopathic curves, degenerative curves are smaller, exhibit less rotation, but have more segmental translation. Up to 3.3 degrees of progression per year has been reported.
When lumbar degeneration leads to mechanical pain syndromes, the term degenerative disc disease is used. Occasionally, they have been further subdivided: internal disc derangement , isolated disc resorption, and lumbar segmental instability. Patients undergoing fusion surgery typically exhibit reduced intervertebral disc height, end plate sclerosis, and osteophyte formation on radiographs. Patients with marked lumbar disc degeneration typically lose lordosis, but this finding, in isolation, has not been found to correlate with pain.
CLINICAL IMAGING
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is the study of choice for the evaluation of spinal infections and tumors, as well as chronic back and leg pain that has failed nonoperative management. Unfortunately, no pathognomonic findings have been reported for the painful axial degeneration syndromes. MRI evidence of disc degeneration is seen as a “black disc” on T2-weighted images, reflecting diminished water content. Modic et al. categorized the end plates changes seen in patients with disc degeneration. However, these radiographic and pathologic changes are also nearly universally present with aging. In a study of 600 autopsies, 90% had evidence of disc degeneration by age 50 years. Signs suggestive of lumbar degeneration were present in the plain radiographs of 90% of adults in one study, of which only 53% were symptomatic. Similarly, on MRI, more than 33% of asymptomatic patients had abnormalities. On the other hand, occasional studies report statistically significant correlations of degenerative changes such as traction spurs or disc-space narrowing with low back pain (LBP).
Discogram
Because of the difficulty in definitively identifying the “pain generator” in patients with degenerative disease and mechanical pain syndromes, provocative testing such as discography is occasionally recommended. Fluid is injected into the disc, increasing end plate pressure. Disc morphology can then be assessed fluoroscopically. Disruptions of normal disc morphology and dye leakage are found in up to 37% of asymptomatic patients. A concordant study implies radiographic changes accompanied by pain mimicking the patient’s typical symptoms ( Fig. 17-1 A ). Adjacent, control levels are also injected and should be pain free. Some surgeons avoid discography in patients with single-level disease on MRI, utilizing the study to select the pain generator in patients with multilevel or equivocal MRI changes. In the absence of MRI findings, fusion at discographically painful levels is associated with poor outcomes. Discography has not been proved to improve fusion outcomes. Discography is considered only in psychologically normal patients with positive MRI findings contemplating operative intervention for longstanding, severe, functionally limiting LBP that has not responded positively to nonoperative management.
INDICATIONS FOR LUMBAR FUSION
The majority of patients with degenerative conditions of the lumbar spine are best treated without surgery. Lumbar fusion surgery is indicated only when it can be demonstrated to improve on the natural history of the disease process. Unfortunately, this natural history is only partly known. Furthermore, patients with neurogenic symptoms are at much greater risk for progression than those with mechanical pain only.
Mechanical Low Back Pain
Some authors do not recognize disc degeneration as a disease process. Thus, lumbar fusion in younger patients with back pain remains controversial. This controversy is heightened by highly variable surgical outcomes and the possibility that fusion surgery may precipitate future problems (such as adjacent segment degeneration). On the other hand, in highly selected cases, lumbar fusion has been reported to improve on the natural history of mechanical back pain. In this setting, arthrodesis may be considered after failure of a lengthy trial of nonoperative treatment (see Fig. 17-1 B, C )
Multiple-level fusions for degenerative disc disease have poor outcomes. Fusions should not be undertaken for mechanical back pain in the presence of unresolved secondary gain issues, abnormal psychometrics, or exaggerated pain behaviors. Those off work longer than 3 months tend to have worse results. The ideal fusion candidate is professionally satisfied and psychological normal with a single or occasionally two painful discs. A history of successful discectomy for radiculopathy may result in better outcomes after interbody fusion for intractable back pain ( Fig. 17-2 ).
Radicular Syndromes
Most patients with intractable radicular or pseudoclaudicatory complaints are well served with decompression alone. However, inclusion of a fusion may be appropriate in cases in which the decompression occurs at an unstable segment, such as scoliosis or spondylolisthesis. Other indications for fusion in neurocompression patients include recurrent herniations, recurrent stenosis, adjacent segment degeneration, or after iatrogenic destabilization (e.g., facet resection).
Certainly not all patients with spondylolisthesis undergoing decompression require fusion. No prospective data support risk stratification; however, stronger consideration for fusion should be given to larger slips (more than 50%, or 3.5-mm slips), taller discs (>6 mm), and segmental hypermobility (>10-degree angular motion on flexion/extension radiographs) ( Fig. 17-3 ). If full decompression requires more than half of the facet to be removed, fusion should be considered. One series of nearly 50 patients with degenerative spondylolisthesis without evidence of hypermobility on flexion/extension radiographs underwent laminectomy alone. Greater degrees of displacement and taller disc heights predicted the need for subsequent fusion. However, another series reported that postoperative increase in olisthesis had no negative impact on outcomes.
Fusion is also routinely undertaken after decompression of stenosis in patients with degenerative scoliosis ( Fig. 17-4 ). The primary treatment goal in this patient group is adequate decompression of the neural elements. When decompression of the whole curve is undertaken, postoperative curve progression is often noted; therefore, concomitant posterior fusion is recommended. Other risk factors for progression include previously documented progression, a lower intercrestal line (though L5, not L4), flexibility (>50% correction on side bending), loss of lordosis (marginal sagittal balance), asymmetric tilt of an interspace, lateral olisthesis, or multiplanar instability (especially if >6 mm).
Occasionally, lumbar fusion is indicated in patients with degenerative scoliosis without neurogenic complaints, such as those with marked mechanical pain or progressive deformity. Typically, a degenerative scoliosis larger than 50 to 90 degrees will progress. If a fusion is planned, it may encompass all of the decompressed segments, selected segments, or beyond those levels to be decompressed. Strong consideration should be given to including subluxated, kyphotic, or coronally angulated levels within the fusion ( Fig. 17-5 ).
Related posts:
Rheumatoid Arthritis of the Cervical Spine
Instrumentation of the Lumbar Spine for Degenerative Disorders
Posterior Lumbar Interbody Fusion for Degenerative Disorders of the Spine
Failure of Cervical Spine Instrumentation
Failure of Lumbar Spine Instrumentation
Facet Joint Replacement for the Treatment of Degenerative Disorders of the Spine
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