Zygapophyseal Joint Fusion in Ankylosing Spondylitis Assessed by Computed Tomography: Associations with Syndesmophytes and Spinal Motion

Abstract

Objective.Because zygapophyseal joints (ZJ) are difficult to visualize on radiographs, little is known about the relationship of ZJ fusion to other features of spinal damage in ankylosing spondylitis (AS). We used computed tomography (CT) to investigate the concordance of ZJ fusion and syndesmophytes, and examined the contribution of both features to spinal motion.Methods.We performed thoracolumbar CT scans (T10–T11 to L3–L4) on 55 patients. Two readers scored scans for ZJ fusion, which were compared to syndesmophyte height and extent of bridging, measured by computer algorithm at the same levels. We used multiple regression analysis to evaluate the relative contributions of ZJ fusion and syndesmophytes to spinal mobility.Results.Fifty-one percent of patients had ZJ fusion in at least 1 vertebral level. Fusion was present in 129 of 652 individual ZJ. Syndesmophytes and bridging were often present in vertebral levels without ZJ fusion, suggesting that syndesmophytes most often develop first. ZJ fusion was present in 34% of vertebral levels with syndesmophytes and 55.9% of levels with bridging, suggesting a closer association with bridging. Syndesmophytes and ZJ fusion had similar associations with the modified Schober test, but syndesmophytes were more strongly associated with limitations in lateral thoracolumbar flexion. ZJ rarely showed new fusion over 4 years.Conclusion.Thoracolumbar ZJ fusion in AS is rarely present at vertebral levels without syndesmophytes. Syndesmophytes, therefore, likely appear before ZJ fusion at a given vertebral level. Both syndesmophytes and ZJ fusion contribute to limited forward lumbar flexion, but syndesmophytes contribute more to limited lateral flexion.