Treatment of Degenerative Disc Disease and Degenerative Spondylolisthesis of the Lumbar Spine - Fusion

Spinal Instability

A myriad of biomechanical and clinical definitions have been developed to describe spinal instability 2,14. Frymoyer describes segmental instability as ''a loss of spinal motion segment stiffness such that force application to that motion segment produces greater displacements) than would be seen in a normal structure, resulting in a painful condition, the potential for progressive deformity and neurotoxic structures at risk'' 14.

Radiographic signs of instability include traction spurs, spinal misalignments, such as retro or anterolisthesis, and/or degenerative scoliosis, and excessive angular ( > 250) or translational motion ( > 3 mm) on flexion–extension radiographs.

Figure 1 demonstrates translational instability in a patient with degenerative spondylolisthesis. These radiographic signs of segmental instability must be correlated with mechanical back pain unresponsive to non–operative treatment in order to be a factor in surgical decision–making. Although radiographic instability does not correlate strictly with clinical symptoms it is important in deciding on surgical treatment in the small group of patients who fail to respond to non–operative management.

Indications for Fusion – Degenerative Conditions of the Lumbar Spine Segmental Instability:

Mechanical low back pain secondary to segmental instability is classically seen in degenerative spondylolisthesis. A review of current literature indicates that patients with degenerative spondylolisthesis with > 3 millimeters of translation on flexion/extension radiographs who have failed conservative treatment should be considered for spinal fusion. Slippage most often occurs at L4–5 and rarely progresses to more than one third of the vertebral body width. Figure 1 (A – B) demonstrates radiographic instability at L4–L5 secondary to degenerative spondylolisthesis.

Beyond degenerative spondylolisthesis the indications for spinal fusion in the treatment of degenerative disc disease are controversial because of the difficulty in defining instability based on traction spurs, retrolisthesis and abnormal angular motion. Further, there is no consensus on ''classic'' symptoms of lumbar instability although there is a consensus that selective fusion of an unstable motion segment has a high likelihood of improving symptoms if the correct diagnosis can be made.

Discogenic Back Pain:

Spinal fusion for discogenic back pain based on discography remains one of the most controversial topics in spine surgery 20. Although the acceptance of discography as a diagnostic tool for discogenic pain has improved greatly in recent years there is still considerable controversy as to whether discography is a specific enough test upon which surgical decisions can reliably be made.Recent evidence by Weinstein and Rydevik suggests that it is but whether fusion will always alleviate discogenic pain remains unknown at this time 28. Weatherly has demonstrated that there continues to be anterior column motion after solid posterior arthrodesis, which may result in continued discogenic back pain and advocates providing internode support 27.

Determinants of Success in Patients Undergoing Spinal Fusions:

A. The most important factor in achieving success following a lumbar spinal fusion is patient selection. Patients who are obese, poorly motivated, abuse nicotine or have psychological problems have a much lower success rate. It is important to screen patients properly and correct any comorbidities that are correctable if surgery is being considered.

B .Fusions should always be limited to one or two levels at most to avoid the increased likelihood of pseudoarthrosis and poor clinical outcomes.

C. At least a portion of the graft material should be autogenous, preferably from the iliac crest. Allograft alone has a very poor fusion rate.

D. The type of operation may affect the outcome. The use of threaded ''stand alone'' internode cages have proven effective for narrowed discs but have a high failure rate for ''tall'' degenerative discs. The use of vehicle fixation appears to increase the rate of fusion and should be considered in healthy patients. Discogenic back pain is best relieved with the use of structural internode support.