Is osteoporosis generalized or localized to central skeleton in ankylosing spondylitis?

Osteoporosis at the lumbar spine and at the femur is a well-established complication in ankylosing spondylitis (AS), but the exact mechanism and the distribution of osteoporosis are not known absolutely.

To determine whether the osteoporosis is generalized or localized to central skeleton and to examine the relation between bone mineral density (BMD) and disease activity and radiologic progression in patients with AS.

In this study, 26 patients with AS and 33 healthy controls matched for age and sex were recruited to the study. Hip and forearm BMD were measured by dual energy X-ray absorptiometry (DEXA). Laboratory and clinical disease activity parameters were documented, and anteroposterior sacroiliac radiographs were taken to determine the radiologic progression.

The urine deoxypyridinoline levels of the patients with AS were statistically significantly higher (P = 0.02) and the serum osteocalcin levels were significantly lower with respect to controls (P = 0.03). The femoral neck and femur BMD values and T scores were significantly lower in patients with AS compared with the controls (P = 0.019, 0.003, 0.01, and 0.01, respectively). The differences in BMD values and T scores of the distal 1/3 radius between 2 groups were not statistically significant. The relation between BMD and disease activity, and radiologic progression in patients with AS could not detected.

Sparing of distal regions such as the as radius suggests that osteoporosis might be due to localized effects of inflammatory activity or immobility rather than a systemic effect. Both increased resorption and decreased formation might be involved in the pathogenesis of osteoporosis. Radius BMD may not be appropriate to evaluate bone loss in patients with AS.