The same covariates used in modeling ordinal summed degenerative scores were used to construct the model for the percentage of mean disc height change (Table VII). Results showed that the preoperative mean disc height percentage and proximity to the fusion were the only two factors that significantly influenced the mean disc height percentage change at the time of follow-up (p < 0.0001). The length of the fusion (p = 0.15) and the postoperative sagittal alignment of the fused segments (p = 0.56) had no significant impact on the mean disc height percentage change, and neither did follow-up duration (p = 0.60).
The mean global lordosis decreased from 19.6° preoperatively to 13.6° at the time of follow-up (p < 0.001). A mean 8.1° of kyphosis developed at the fusion segments: 4.5° developed at 1 level away, 10.1° developed at 2 levels away, and 9.6° developed at 3 levels away. Classifying subjects into kyphotic and non-kyphotic groups based on postoperative fusion segment alignment, the mean global lordosis was 11.7° for the kyphotic group and 19.0° for the non-kyphotic group (p < 0.007).
Despite the conventional wisdom that radiographic changes in the cervical spine are generally asymptomatic, imaging findings can be associated with neck pain when studying patients over the longer term2,4. A 10-year study of 159 asymptomatic subjects showed that 15% (24 of 159 patients) developed neck pain at a mean onset of 4.9 years2. Subjects with initial degenerative findings at the C6-C7 level were 4 times more likely to develop pain than those without initial degenerative findings. In another longitudinal study of asymptomatic individuals, magnetic resonance imaging (MRI) scans were repeated over a 10-year period. Subjects who developed neck pain over this duration also developed more compression of the dura (12.4% compared with 5.8%; p = 0.015) and foraminal stenosis (25.0% compared with 8.4%; p = 0.04) on follow-up MRI scans4. Matsumoto et al.23 compared radiographic degeneration and neck pain between asymptomatic subjects and patients who had anterior cervical fusion. At the 10-year follow-up, the surgical group had more degenerative changes (posterior disc protrusion, disc narrowing and degeneration, foraminal stenosis) than asymptomatic subjects. Clinical symptoms at the time of follow-up were also higher in the surgical group, with more neck pain and numbness in the upper extremity. The present study carefully characterized the radiographic changes that occur in the cervical spine of subjects after arthrodesis, at a mean time of 12.7 years following the surgical procedure, and investigated factors that may contribute to these changes. We found a high incidence of radiographic degenerative progression at levels near previously fused segments. More than 90% of patients had worsening posterior osteophytes and facet arthrosis at all assessed adjacent levels and worsening anterior osteophytes at the immediately adjacent levels. This represents a higher incidence of degenerative progression than previously reported incidences over similar periods of time (70% and 65%) for non-surgical subjects1,4.
There has been debate over whether adjacent segment degeneration is a result of natural progression of spondylosis or whether the fused segments mechanically predispose the adjacent segments to faster deterioration9,14,15. In the present study, several findings from regression analysis support the theory that biomechanical effects play a role in the adjacent segment degeneration. First, the proximity of an adjacent level to the fused levels was a strong predictor for all categories of degenerative changes. The levels immediately adjacent to the fusion (more so at the inferior level) showed the most severe changes compared with levels further away from the fusion. Second, age was not a significant predictor for the progression of adjacent segment degeneration; instead, duration since the time of the surgical procedure was significantly correlated with the changes in the overall degenerative score. In a study of 181 patients who underwent anterior cervical discectomy and fusion, Goffin et al. also reported that, for 5 to 15.6 years following the surgical procedure, the increased degenerative scores were significantly correlated with the length of time after the surgical procedure but not with the patient age24. We found that a radiographically healthy disc adjacent to a contemplated fusion was not protection against adjacent segment degeneration. On the contrary, a strong inverse relationship was found between the preexisting degeneration at levels adjacent to the fusion and the severity of progression of degenerative changes at the time of follow-up. Matsumoto et al. reported similar findings in a 10-year follow-up MRI study of 64 patients who had undergone anterior cervical discectomy and fusion; in that study, the progression of degenerative change at the adjacent discs was much higher in patients without preexisting disc degeneration23. A comparable compensatory increase in motion was also seen at healthy discs adjacent to degenerative discs in human subjects25 and finite element models26. These findings suggest increased mechanical consequences at healthy discs adjacent to a fused segment.
In long-term studies of anterior cervical discectomy and fusion, some authors have reported a lower level of adjacent segment deterioration following multilevel cervical fusion27-29. Intuitively, a longer stiff segment should result in greater adjacent segment degeneration, and, given the absence of findings of increased severity27 or incidence28,29, those authors proposed that adjacent segment degeneration is less likely due to mechanical effects of the fusion and more likely a consequence of aging. The data in the current study also showed that the severity of adjacent segment degenerative changes did not increase with the number of levels fused. More specifically, patients who had C5-C6 included in the fusion (87% [145 of 166 patients]) had fewer degenerative changes than the group in whom C5-C6 was adjacent to the fusion (13% [21 of 166 patients]). The mean degenerative score change for anterior osteophytes was 1.56 in the C5-C6 included group compared with 1.88 in the C5-C6 adjacent group (p < 0.003); for end-plate sclerosis, 0.55 in the included group compared with 0.61 in the adjacent group (p < 0.02); and for posterior osteophytes, 1.78 in the included group compared with 1.93 in the adjacent group (p < 0.066). A subanalysis of patients with single-level fusion (n = 51) revealed a similar trend, with patients undergoing C5-C6 arthrodesis (n = 32) showing significantly fewer degenerative changes at adjacent levels than when C5-C6 was one of the adjacent segments. The mean degenerative score changes for anterior osteophytes was 1.36 for the included group compared with 2.00 for the adjacent group (p < 0.0001); for posterior osteophytes, 1.62 for the included group compared with 1.93 for the adjacent group (p < 0.004); and for facet arthrosis, 1.85 for the included group compared with 2.13 for the adjacent group (p < 0.03). We propose that data from multilevel fusion need to be carefully weighed, because most subjects with multilevel cervical fusion likely include the C5-C6 level. The inclusion of C5-C6, a level that is inherently predisposed to greater degeneration within the fusion segment, potentially skews the assessment of adjacent segment effects. A similar scenario may exist at the C6-C7 level, but the small sample size at that level did not allow for adequate analysis.
Prior investigations have shown mixed findings on whether degenerative changes in the cervical spine are predisposed to by sagittal alignment of the preoperative cervical spine or by kyphosis that frequently results in the year following the surgical procedure as settling occurs through the graft-vertebra interface17,18,30-32. Okada et al.30 studied MRI-based degenerative changes over a 10-year period in two groups of asymptomatic subjects: lordotic and non-lordotic. The non-lordotic group had a higher odds ratio of posterior disc protrusion (p = 0.033) than the lordotic group. Using a similar MRI grading system, Lundine et al.17 recorded degenerative findings in 106 symptomatic patients prior to the surgical procedure, and showed no correlation between cervical spine alignment and degeneration. In patients who had undergone arthrodesis, Katsuura et al.32 found a higher incidence of lordotic alignment of the cervical spine in patients who did not have adjacent level degeneration (86% [18 of 21 patients] compared with 43% [9 of 21 patients]; p = 0.0148), whereas Goffin et al.24, with a mean 8.3-year follow-up in 180 patients, found no correlation between the increase in the degenerative scores and the alignment of the fused segment (Spearman rs = 0.002; p = 0.983). Regression analysis from the present study demonstrated that, over the long term, sagittal alignment of both the global cervical spine and the fused segments did not affect the severity of adjacent segment degeneration.
The present study has several strengths. The incidence and severity of multiple radiographically relevant degenerative changes were carefully analyzed in a large group of subjects followed over the long term. Analysis of severity changes using regression models facilitated investigation of multiple potential covariates that may play a role in predisposing to degenerative changes at segments adjacent to a fusion. Labeling the location of adjacent segments with reference to their proximity to the fusion segment enabled the analysis to be conducted on the collective data of fusion procedures at different spinal level(s) to maximize the statistical power.
However, the retrospective nature of the study and the nonconsecutive inclusion of cohort cases were some inherent limitations. The radiographs were obtained for routine postoperative clinical evaluation, in the days before digital imaging. As such, several radiographs were suboptimal studies, in that not all levels, especially the inferior adjacent level, could be adequately assessed. This resulted in some variation in the number of disc levels available for evaluation. The absence of information on subject pain levels precluded us from drawing conclusions on clinical implications of the degenerative findings or sagittal alignment of the cervical spine. The absence of repeat surgical procedures in this subset of patients suggests that clinical manifestations were not substantial over the follow-up period.
In summary, we find that adjacent segment degeneration following cervical arthrodesis is likely a multifactorial process. The progression of these degenerative changes was significantly associated with proximity to the fusion and the time elapsed since the surgical procedure and was inversely associated with the preoperative degenerative condition of the disc. The C5-C6 level is most prone to degeneration and is also the most likely to show adjacent segment degeneration when not included in a fusion. Several other factors, including patient age, number of fusion levels, and lordosis of the fused segment, had no significant influence on the severity of degenerative progression at levels adjacent to prior fusion.
Investigation performed at the Department of Orthopaedic Surgery, Medical College of Wisconsin, Milwaukee, Wisconsin
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