The Burden of Spine Structural Damage on Function in Patients With Axial Spondyloarthritis: Adaptation-Mediated Uncoupling?

Objective To investigate the association between spinal damage and functional capacity in patients with axial spondyloarthritis (axSpA) and to compare the performance of 2 radiographic scores (modified Stoke Ankylosing Spondylitis Spine Score mSASSS and Combined Ankylosing Spondylitis Spine Score CASSS). Methods Radiographs from 101 patients with axSpA were scored for cervical facet joints (CFJ) and mSASSS for vertebral bodies. CASSS was calculated as the sum of both scores. Physical function was assessed by Bath Ankylosing Spondylitis Functional Index (BASFI); disease activity by Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and Ankylosing Spondylitis Disease Activity Score (ASDAS); mobility by Bath Ankylosing Spondylitis Metrology Index (BASMI); and quality of life by Ankylosing Spondylitis Quality of Life (ASQOL). Univariate and multivariate analyses were performed to investigate the association between possible explanatory variables and outcomes. Results BASFI correlated strongly with ASQOL (Spearman ρ 0.66) and BASDAI (ρ 0.70), moderately with BASMI (ρ 0.46) and ASDAS (ρ 0.59), and weakly with mSASSS (ρ 0.29) and CASSS (ρ 0.28). A best-fit multivariate model for BASFI, adjusted for symptom duration, age, sex, and smoking status, included BASDAI ( B 0.76, P < 0.001), BASMI ( B 0.62, P < 0.001), and history of total hip arthroplasty ( B 1.22, P = 0.05). Radiographic scores were predictors of BASFI only when BASMI was removed from the model (mSASSS: B 0.03, P = 0.01; CASSS: B 0.02, P = 0.01). Conclusion Spinal damage was independently associated with physical function in axSpA, but to a lesser extent than disease activity and mobility. Moreover, incorporating CFJ assessment in the mSASSS did not improve the ability to predict function.