Abstract
Intraspinous and pedicle screw-based (PSB) dynamic instrumentation systems have been in use for a decade now. By direct or indirect decompression, these devices theoretically establish less painful segmental motion by diminishing pathologic motion and unloading painful disks. Ideally, dynamics should address instability in the early stages of degenerative spondylolisthesis before excessive translation occurs. Evidence to date indicates that Grade II or larger slips requiring decompression should be fused. In addition, multiple segment listhesis, severe coronal plane deformities, increasing age, and osteoporosis have all been listed as potential contraindications to dynamic stabilization. We reviewed the exclusion and inclusion criteria found in various dynamic stabilization studies and investigational drug exemption (IDE) protocols. We summarize the reported limitations for both pedicle- and intraspinous-based systems. We then conducted a retrospective chart and imaging review of 100 consecutive cases undergoing fusion for degenerative spondylolisthesis. All patients in our cohort had been indicated for and eventually underwent decompression of lumbar stenosis secondary to spondylolisthesis. We estimated how many patients in our population would have been candidates for dynamic stabilization with either interspinous or pedicle-based systems. Using the criteria for instability outlined in the literature, 32 patients demonstrated translation requiring fusion surgery and 24 patients had instability unsuitable for dynamic stabilization. Six patients had two-level slips and were excluded. Two patients had coronal imbalance too great for dynamic systems. Twelve patients were over the age of 80 and 16 demonstrated osteoporosis as diagnosed by bone scan. Finally, we found two of our patients to have vertebral compression fractures adjacent to the site of instrumentation, which is a strict exclusion criteria in all dynamic trials. Thirty-four patients had zero exclusion criteria for intraspinous devices and 23 patients had none for PSB dynamic stabilization. Therefore, we estimate that 34 and 23% of degenerative spondylolisthesis patients indicated for surgery could have been treated with either intraspinous or pedicle-based dynamic devices, respectively.