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Iatrogenic Spinal Instability: Causes, Evaluation, Treatment, and Prevention
Introduction
Instability in the lumbar spine is a well-recognized entity for both degenerative and traumatic processes. Less well defined is instability following an index decompressive or fusion procedure. This complex issue can exist as iatrogenic destabilization at either the same or adjacent level or could even be unrecognized preoperative instability in the setting of a patient who does not show improvement after their index procedure. During this chapter, we will provide a framework for categorizing, identifying, and treating postoperative instability. Additionally, there will be a section outlining preventative strategies during index lumbar procedures.
Epidemiology
Postoperative instability can be viewed in two broad categories with subdivisions within those categories. First are the patients who undergo a decompressive procedure and exhibit instability at the same operative level postoperatively. Second are patients who develop evidence of adjacent segment instability adjacent to a fusion procedure. Both of these categories can have acute postoperative instability or a more gradual, insidious onset many years after the index procedure.
Postdecompression
In a case series and literature review, Ramhmdani et al. identified a 9.5% rate for postoperative instability requiring reoperation at the same level of a previous decompression. The average time to reoperation identified within their series was approximately 32 months. Additionally, they highlighted a number of studies with heterogeneous patient populations and found that the rate of reoperation varied widely (1%–32%) following decompression laminectomy. Within these series, postoperative instability was generally defined in the same terms as degenerative spondylolisthesis.
In one long-term series of patients after primary microdiscectomy with 12 years of follow-up, 16 of 69 patients developed segmental instability at the operative level. Interestingly, five patients showed segmental instability either above or below the operative level, but not at the operative level. The majority of postoperative segmental instability showed translation in excess of 5 mm.
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