The burden of spine structural damage on the 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 two radiographic scores [modified Stoke AS Spine Score (mSASSS) and Combined AS Spine Score (CASSS)]. METHODS:Radiographs from 101 axSpA patients were scored according to de Vlam et al. for cervical facet joints (CFJ) and mSASSS for vertebral bodies. CASSS was calculated as the sum of both scores. Physical function was assessed by BASFI; disease activity by BASDAI and AS Disease Activity Score (ASDAS); mobility by BASMI; and quality of life by AS 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) and CASSS (B 0.02; P = 0.005). 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.