Objective: To analyze whether inflammatory disease activity plays a substantial role in the loss of bone mass observed in ankylosing spondylitis (AS) patients who have not yet developed ankylosis.
Methods: A longitudinal cohort study of 34 patients with early AS (duration <10 years) without ankylosis was conducted. The mean followup was 19 months. Loss of bone mass in defined regions of the lumbar spine and femoral neck was analyzed by dual x-ray absorptiometry. Patients were grouped according to biologic parameters of disease activity (erythrocyte sedimentation rate or C-reactive protein level). Group 1 consisted of 14 patients with active disease; group 2 comprised 20 patients with inactive disease. Serum levels of interleukin-6 (IL-6) and of hormones (sex, thyroid, and calciotropic), vertebral mobility (Schober test), daily physical activity, and treatment administered were recorded every 6 months for all patients.
Results: At the end of the followup period, patients with active AS showed a significant reduction in bone mass in the lumbar spine (mean 1.01 gm/cm2 at study entry versus 0.961 gm/cm2 at followup [P = 0.005]) and femoral neck (0.849 gm/cm2 versus 0.821 gm/cm2 [P = 0.015]), which represented losses of 5% and 3%, respectively. In contrast, no significant reduction in bone mass was observed in patients with inactive AS. As expected, serum IL-6 levels were significantly higher in patients with active AS than in those with inactive disease (mean +/- SD 8.3 +/- 9 pg/ml versus 2.8 +/- 5 pg/ml [P = 0.008]). No significant differences were observed between the 2 groups in any of the other variables analyzed.
Conclusion: The observation that loss of bone mass in AS occurred only in patients with persistent active disease strongly suggests that inflammatory activity of the disease itself plays a major role in the pathophysiology of the early bone mineral disorders observed in these patients.