Prevalence and risk factors of osteoporosis in patients with ankylosing spondylitis: a 5-year follow-up study of 504 cases

Objective The objective of this study was to assess the prevalence and risk factors of osteoporosis (OP) in patients with ankylosing spondylitis (AS). Methods Demographic and clinical data of 504 AS patients were collected. Bone mineral density (BMD) measurements of the lumbar spine, proximal femur and forearm were performed by dual-energy x-ray absorptiometry at baseline and follow-up. 106 cases of sex- and age-matched healthy volunteers were enrolled as normal controls. Results In contrast to normal controls, AS patients displayed a higher prevalence of both OP (9.7% vs. 0%) and osteopenia (57.5% vs. 34.9%). The prevalence of OP was significantly higher and the BMD were significantly lower in patients with elevated erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) than patients with normal ESR and CRP. Juvenile onset, morning stiffness lasting over 0.5 hours and elevated ESR levels were risk factors for bone loss at the lumbar spine; Male gender, older age, hip involvement and lack of regular treatment were risk factors for bone loss at the femur. 173 cases were followed up for 1 to 5 years, BMD changes per year at the lumbar spine, femur and forearm were 4.8%, 2.7%, and 2.6% respectively. There was no significant difference in annual BMD change between patients treated with or without low dose glucocorticoids (GCs). Hip involvement and persistent elevated ESR levels, but not GCs treatment, were associated with decreased BMD at both the lumbar spine and the femur during follow-up in longitudinal regression analysis. Conclusion High disease activity and hip involvement are risk factors of bone loss in patients with AS. Low-dose GCs treatment in AS does not increase the risk of OP.