The phenomenon of adjacent-segment degenerative disc disease of the cervical spine remains incompletely described. While it has been clear for many years that some patients do develop symptomatic degenerative disease and spinal stenosis at levels adjacent to surgical fusions, the role of the fusion itself has proven difficult to separate from the natural progression of underlying degenerative disease. Added to this, there are several mechanisms by which surgery may cause or accelerate adjacent-segment degeneration, including surgical trauma, direct impingement by spinal implants, and altered biomechanics of levels adjacent to spinal fusions; the relative importance of each of these has proven similarly difficult to isolate.
The paper by Garrido et al. contributes in a meaningful way to this discussion. The authors present a retrospective analysis of prospective, randomized data collected as part of a multicenter FDA trial comparing the Bryan cervical total disc replacement with one-level anterior cervical discectomy and fusion utilizing plate fixation. The purpose of the original study was to compare the clinical outcomes between these two surgical treatments. In contrast, Garrido et al. compared the radiographic severity of adjacent-level ossification development between the two treatment arms of patients enrolled from one center in the original study.
The methods included blinding of radiographic reviewers to the surgery performed as well as the enlistment of multiple assessors of the severity of adjacent-level ossification development in forty-six patients split fairly evenly between either a total disc replacement or an anterior cervical discectomy and fusion. The patient cohorts were well matched with regard to age, sex, pending litigation, Workers’ Compensation status, and alcohol use. The results showed a significant increase in the prevalence and severity of adjacent-level ossification development among patients undergoing plated anterior cervical discectomy and fusion as compared with total disc replacement at both two and four years of follow-up.
The authors can fairly conclude that there is an increased rate and severity of adjacent-level ossification development among patients managed with anterior cervical discectomy and fusion as opposed to total disc replacement. The causes for this effect are not clear, but the study design and statistical analysis strongly suggest that there is a real difference between the two procedures in terms of the effects on adjacent discs. While there was not a particularly strong correlation between individual raters’ assessment of adjacent-level ossification development severity, the blinded design and the use of multiple raters ameliorates this concern.
Several features remain unresolved despite these compelling results. This study gives little further information as to the cause of adjacent-segment degenerative disc disease. We are left with the same list of potential causes that we started with: surgical dissection, plate encroachment, altered biomechanics, and the natural history of degenerative cervical spine disease. Perhaps more importantly, the study does not demonstrate a clinical impact resulting from the radiographic effect observed.
Nonetheless, the authors do not overstate the conclusions that their work produces. The result lends further support to the argument that cervical total disc replacement may reduce the prevalence of adjacent-segment disease compared with anterior cervical discectomy and fusion. It can be hoped that this result, based on solid evidence at the time of the four-year radiographic follow-up, will lead to a more favorable insurance approval environment than currently exists for this promising new technology.