In this article, Rao et al. present radiographic changes in the cervical spine following anterior arthrodesis. This is a retrospective review of 166 patients over a mean follow-up of 12.7 years, although there was a wide variation in follow-up, ranging from 5 to 30 years. Good interobserver reliability was demonstrated by having two assessors grade 20 sets of radiographs, but the other 146 sets of radiographs were graded by only one assessor. The study’s strengths include that the arthrodesis was performed by a single surgeon and that no instrumentation was used, which could be seen as contributing to adjacent-level ossification disease1.
It is important to note that this is a study assessing radiographic changes, which do not reliably correlate with clinical symptoms2. Radiographic changes were noted to correlate with time since the index surgical procedure, but not with age. This may point to the surgical procedure as having contributed to degeneration. Rao et al. showed that the C5-C6 level tended to wear more often. Thus, patients who had this level included in the index arthrodesis had less degeneration at the time of follow-up because a common culprit had been eliminated3.
Rao et al. noted a correlation between degeneration of a disc and its proximity to the level of fusion. Most surgical procedures were performed in the distal cervical spine, so the more proximal segments tended to degenerate less frequently. However, it is important to note that this simply implies correlation and not causation. Readers must be cautious in interpreting these results as implying a protective effect of distance from the level of fusion, because proximal segments tend to degenerate less frequently in the native spine.
The authors state that their findings suggest an increase in mechanical stress across non-fused segments as a result of additional compensatory motion. They further imply expectations that a radiographically normal disc should be relatively protected from adjacent degeneration. This line of thinking assumes that a normal disc is better equipped to deal with the possible increased stress of being adjacent to a fused level. However, Rao et al. found an inverse relationship between preoperative degeneration of an adjacent disc and severity of progression at the time of follow-up. This should not be a surprise, because a normal disc has nowhere to go but down.
A traditional approach to adjacent segment degeneration has been one in which we show concern for a degenerated segment next to the level that we are about to fuse. Our concern is that this level may further break down and should be included in the index surgical procedure. However, this study may ask us to reverse such thinking, because those degenerative discs do not appear to wear considerably and may have already worn during their natural history to a point of relative stability. This is an area that will continue to challenge the judgment of every spine surgeon.
↵* The author indicated that no external funding was received for this work. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article.
- Copyright © 2016 by The Journal of Bone and Joint Surgery, Incorporated