Appendicular and vertebral bone mass in ankylosing spondylitis. A comparison of plain radiographs with single- and dual-photon absorptiometry and with quantitative computed tomography.

OBJECTIVE. We assessed the prevalence, severity, and anatomic distribution of ankylosing spondylitis (AS)-related osteopenia (OP). METHODS. We studied 70 patients (60 males, 10 premenopausal females) with AS (according to the New York criteria) to determine the frequency of OP. Bone mass was measured by plain radiographs of the spine, by single-photon absorptiometry (SPA) of the distal and midshaft of the radius on the nondominant side, by dual-photon absorptiometry (DPA) of the lumbar spine (L2-L4), and by quantitative computed tomography (QCT) of the lumbar spine. RESULTS. SPA values for the radius mass were normal in males and in females, both at the distal and midshaft sites. In contrast, spine radiographs showed diminished density of the vertebral bodies in 69% of the males and 50% of the premenopausal females. Two male patients had had a vertebral compression fracture, and one female patient had had two. DPA values for the spine mass were significantly diminished in the male patients compared with the controls, but not in the female patients. Males with less severe AS also had the largest reduction in lumbar bone mineral content. In patients with more severe disease, lumbar bone mineral content was not statistically different from that in controls. QCT of the lumbar spine performed in 10 patients disclosed low density of the trabecular bone of the vertebral bodies, more so in those with more severe AS and syndesmophyte formation and/or apophyseal joint fusion, which contrasts with the normal values on DPA in these patients. CONCLUSION. Male patients with AS have axial osteopenia. In those who have very severe AS with new bone formation, DPA demonstrates normal values as a result of two opposite trends: central osteopenia (as assessed from QCT) and peripheral new-bone formation, which transforms vertebral bodies into long bones. This could modify the mechanical resistance of the spine and account for the propensity for anteroposterior transvertebral and transdi