When determining what levels to fuse in ACDF, surgeons typically start by identifying the symptomatic levels with nerve root or spinal cord compression that correlates with the patients symptoms. Frequently, there can be adjacent levels without neural element compression that are degenerative or spondylolisthetic. These levels pose a conundrum, as it is known that degenerative adjacent segments left unfused are more likely to develop adjacent segment degeneration that healthy adjacent levels. However, it is unclear if an adjacent level spondylolisthesis is more likely to degenerate than an adjacent segment without a slip. In order to answer this question, Dr. Segar and colleagues at Thomas Jefferson University in Philadelphia retrospectively analyzed 264 patients who underwent ACDF for degenerative conditions who had at least one year of follow-up including patient reported outcomes and radiographs. They stratified the patients by the presence of baseline adjacent segment spondylolisthesis (AS), defined as at least 1 mm of listhesis on a lateral neutral, flexion, or extension x-ray. Twenty percent of patients had AS, with the remaining 80% classified as non-adjacent segment spondylolisthesis (NAS). At baseline, the AS patients were 6 years older, more likely to be female (68% vs. 50%), had a higher Charlson Comorbidity Index (2.5 vs. 1.9), and had a greater number of levels fused (2.4 vs. 2.1). Baseline patient reported outcomes were similar for the two groups. The average slip was 2.7 mm, and mean mobility at the listhetic segment on flexion-extension radiographs was 1.3 mm. At final follow-up, the slip was actually about 1 mm less than at baseline. Patient reported outcomes were similar for the two groups, and the reoperation rate was slightly lower for the AS group (6% vs. 9%). None of the reoperations in the AS group were for adjacent segment disease.
This study suggests that a spondylolisthetic level adjacent to the symptomatic levels to be fused with ACDF does not always need to be included in the fusion. Prior to drawing overly strong conclusions from this paper, one must consider its limitations. This is a retrospective study in which the treating surgeon determined if a level was to be fused or not. They likely included many spondylolisthetic segments in the fusion constructs, and it is unclear how often a degenerative, spondylolisthetic level was included even in the absence of neural element compression there. As such, the surgeons may have determined which spondylolisthetic levels were at greatest risk for adjacent segment disease and included these. The authors also did not note the level of baseline degeneration at the adjacent segment. Mild degeneration with a low grade, non-mobile listhesis is relatively common at C3-C4 or C4-C5 supradjacent to stiff, degenerative more caudal segments. This paper suggests that those segments likely need not be included in the fusion. In the case of the AS patients, the average fusion length was 2.4 levels, suggesting that many underwent a C4-C7 ACDF with a low-grade listhesis at C3-C4 which tended not to progress over the average 20 month follow-up in this study. The duration of follow-up was also quite short, so what happened to the AS levels over time is unknown. The NAS group also had fewer segments fused and they were younger, both risk factors for adjacent segment degeneration. This may explain the higher reoperation rate for adjacent segment disease in the NAS group. This paper suggests that it is reasonable for a surgeon to leave an adjacent segment with a low grade, stable listhesis unfused if they believe it is asymptomatic and unlikely to degenerate quickly. Concluding that all adjacent spondylolisthetic segments should not be included in the fusion is probably not appropriate.
Please read Dr. Segar's article on this topic in the September 1 issue. Does this change how you consider whether to fuse an adjacent, spondylolisthetic segment while planning ACDF? Let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
Associate Web Editor