Is spinal mobility in patients with spondylitis determined by age, structural damage, and inflammation?

OBJECTIVE: To explore the association between mobility, inflammation, and structural damage in ankylosing spondylitis (AS).
METHODS: Patients with AS were included in a cross-sectional study in which spinal mobility was measured by the Bath Ankylosing Spondylitis Metrology Index (BASMI) and by the University of Córdoba Ankylosing Spondylitis Metrology Index (UCOASMI), based on an automated motion analysis. Structural damage was measured by the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS), and activity by the Ankylosing Spondylitis Disease Activity Score (ASDAS) and the Bath Ankylosing Spondylitis Disease Activity (BASDAI). We analyzed the correlations between variables, as well as interaction by multiple linear regression models to reach a predictive equation.
RESULTS: Fifty AS patients, mainly men in their mid-40s and with moderate levels of disease activity and structural damage, were included in the study. BASMI and UCOASMI scores showed a strong correlation (r = 0.89). UCOASMI scores correlated stronger than BASMI with structural damage (r = 0.72 versus r = 0.67) and patient's age (r = 0.68 versus r = 0.56). Correlations of mobility were weaker with disease activity by the ASDAS (r = 0.38) and BASDAI (r = 0.49), and disease duration (r = 0.40). Multiple linear regression showed that factors associated to mobility by UCOASMI were age, the BASDAI, mSASSS, ASDAS (0:<2.1, 1:≥2.1), and disease duration >15 years. The largest weight in the equation corresponded to the mSASSS. The association between the ASDAS and UCOASMI is dependent on disease duration.
CONCLUSION: Mobility in AS is influenced by both structural damage and activity, but definitely also by age and disease duration. Improved mobility should be a relevant target in AS, even more prominently than activity, given its closer relation to structural damage.