Since the work by Herkowitz and Fischgrund in the 1990s suggesting that pseudarthrosis did not affect short-term outcomes in degenerative spondylolisthesis, debate has raged about the relationship between fusion status and outcomes in degenerative conditions of the lumbar spine. 1,2 In the March 1 issue, the group from the University of Louisville provided further information suggesting that obtaining a successful fusion might matter. Their study compared outcomes between 171 patients who had a successful fusion by 2 years to 22 patients who had a pseudarthrosis. The results were somewhat mixed, with no significant differences in the mean SF-36 PCS, Oswestry Disability Index (ODI), and back and leg pain change scores between the fused and pseudarthrosis groups but a significantly greater proportion of fused patients reaching the thresholds of minimum clinically important difference (MCID) and substantial clinical benefit (SCB). These results suggest that there is a likely benefit to a successful fusion, however, the magnitude of this benefit may not be substantial. Other factors likely play a much larger role in determining clinical success (i.e. psychosocial factors, underlying diagnosis, etc).
The markedly different conclusions one can draw depending on how the data are analyzed raise interesting methodological issues that have yet to be resolved. In this case, comparing mean change scores showed no significant differences between the fused and non-fused groups. On the other hand, there were significantly more fused patients who reached MCID and SCB. How should we interpret these differences? First, one must realize that this study was quite underpowered due to the fact there were only 22 patients in the non-fused group. While their 89% fusion rate is impressive, this success ironically limited their statistical power. As such, it is impossible to know if the lack of differences in mean change scores was due to the two groups having the same outcomes or simply a lack of power to detect a real difference. Second, the distribution of outcomes in the pseudarthrosis group must be quite skewed given that the mean ODI improvement was 16.5 points but only 32% of patients reached the MCID of 12.8 points. How would the results have differed if the MCID was defined as 10 points? My guess is that many of the patients had change scores in the vicinity of the MCID, so slight changes in its definition may have resulted in markedly different percentages reaching MCID, even though these small differences are clinically meaningless. The authors should be applauded for reporting both the continuous and dichotomous outcomes, though further discussion of how to interpret the discordant results would have been helpful.
So how should a spine surgeon use these data? Given that this is another paper suggesting that obtaining fusion might matter, the surgeon should stick to sound technical principles and do his or her best to get a successful fusion. Unfortunately, the question of what is the best fusion technique for each underlying diagnosis remains unanswered. This study included a mixed bag of underlying diagnoses, which further complicates interpretation of the results. While some will likely use these data to argue for the use of techniques that maximize fusion rates, the only way to truly answer the question is to perform sufficiently powered randomized trials using modern, patient centered outcome measures for strictly defined diagnostic categories. Until then, we will not know for certain if the benefits associated with the higher fusion rates obtained with instrumentation outweigh the potential risks associated with these techniques (i.e. higher complication rate, increased adjacent level disease, etc). How will you use these results in your practice? Please let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
1. Fischgrund JS, Mackay M, Herkowitz HN, Brower R, Montgomery DM, Kurz LT. 1997 Volvo Award winner in clinical studies. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective, randomized study comparing decompressive laminectomy and arthrodesis with and without spinal instrumentation. Spine 1997;22:2807-12.
2. Herkowitz HN, Kurz LT. Degenerative lumbar spondylolisthesis with spinal stenosis. A prospective study comparing decompression with decompression and intertransverse process arthrodesis. The Journal of bone and joint surgery 1991;73:802-8.