Cervical Facet Joint Ankylosis in Patients With Axial Spondyloarthritis Serves as a Surrogate Radiographic Indicator of Propensity for Bone Formation

Objective To investigate the involvement of facet joints and its clinical implications during the treatment course in patients with axial spondyloarthritis (axSpA). Methods This was a retrospective observational study. Radiographs of the cervical and lumbar spine were evaluated for 799 patients with axSpA using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) and the de Vlam methods. Differences in radiographic progression indices and initiation of TNF inhibitors between patients with and without facet joint fusion were investigated using propensity score matching. Results Facet joint involvement at the cervical spine was observed in a subset of patients with axSpA (20.6%). Patients with facet joint fusion ( n = 71) had higher mSASSS and a higher number of syndesmophytes at both cervical and lumbar levels compared with the matched patients without facet joint fusion. Dagger signs and trolley‐track signs were more commonly observed ( p < 0.001), and trolley‐track signs developed more frequently in patients with facet joint fusion during follow‐up (Δ9/49 vs. Δ1/68, p = 0.0038). Disease activity score and CRP level at baseline were also significantly higher in patients with facet joint fusion. The risk of initiation of TNF inhibitor was significantly higher in patients with facet joint fusion (hazard ratio [95% CI] 1.909 [1.187, 3.07], p = 0.007). Conclusions Facet joint involvement in the cervical spine was identified in a subset of patients with axSpA and was closely associated with advanced radiographic structural damage and the initiation of TNF inhibitors.