Lumbar spinal fusion may be followed by juxta-fusion degeneration or Adjacent Segment Pathology (ASP). Systematic reviews have noted wide variations in its reported prevalence. A review by Harrop et al in 2008, calculated that 34% of fusion patients developed radiological signs of degeneration (ASDegen) and 14% developed symptomatic disease (ASDis). A 2011 study of 1000 consecutive PLIF procedures, noted an annual incidence of 2.5% of further surgery for ASDis, with age and the number of levels fused being significant risk factors.
The pathophysiology of ASP is complex. Identical twin and epidemiological studies have shown that genetic factors play a substantial role in the evolution of degenerative spine disease and it remains controversial whether the rigid fused segment(s) predispose to ASP or whether it is solely the result of the natural history.
In vitro biomechanical studies have reported increased range-of-motion or intra-discal pressures adjacent to simulated fusions but results have varied and the validity of test methodologies has been challenged; consequently, the conclusions remain in doubt and aspects of fusion procedures such as damage to adjacent structures may be important.
High-level clinical studies are now suggesting a contribution to ASP from the fusion. A 2014 systematic review and meta-analysis of ASP following lumbar fusion vs. motion-preserving devices, analysed the results of 1,270 patients from 13 studies. Significant differences appeared when follow-up exceeded 5 years (P = 0.001). Several cohort studies and three RCTs of posterior motion-preserving devices vs. fusion, have been reported. These results, as well as a current, non-randomised trial by the author, of ‘topping-off’ fusions with a flexible coupler, suggest a trend towards reduction in ASP but study numbers are small.
The controversy regarding the role fusion plays in the pathophysiology of ASP will hopefully soon be resolved and perhaps provide surgeons with ways to mitigate its development.