Bone density measurement by dual photon absorptiometry (DPX) and single energy-quantitative computed tomography (SEQCT) in ankylosing spondylitis (AS)

A unique observation in ankylosing spondylitis (AS) is the unusual combination of the pathological basic phenonema of inflammation and ossification, whereas secondary osteoporosis is still an unclarified concomitant symptom. The aim of the study was [1] to show the frequency of changes in the direction of osteopenia/osteoporosis in AS depending on the duration and stage of the disease, and [2] to assess the ranking of two different methods of bone density measurement in this clinical pattern. We used two different methods of bone density measurement in 58 male and female patients, namely, Dual Photon Absorptiometry (DPX) and Single Energy-Quantitative Computed Tomography (SEQCT). In the initial and advanced stages of the disease, a high-percentage decrease of the axial bone density could be verified (DPX: osteopenia in 13.8% - osteoporosis in 5.1%; SEQCT: osteopenia in 10.3% - osteoporosis in 32.8%). A peripheral bone density decrease in the sense of osteopenia could be proven in 20.7% by DPX measurement with SEQCT. With SEQCT, a decrease in vertebral trabecular bone mineral density could be observed already in the initial stage decreasing continually during the process of the disease, whereas the cortical bone displayed the same trend up to stages of ankylosis. Using DPX, valid statements are more likely to be expected in less marked ankylosing stages of AS. However, since projective methods mainly measure the cortical bone and since a loss of trabecular bone occurs first of all during the initial stage of osteoporosis, we prefer the method of SEQCT due to the selective measurement of trabecular and cortical bone. Particularly, in stages of advanced ankyloses in the vertebral region, priority should be given to SEQCT measurement. The DPX method often yields values that are too high, and the replacement of vertebral trabecular bone by fatty bone marrow is not usually recorded as standard. There may already be an increased risk of bone fracture in AS in DPX osteopenia a longside an already established SEQCT osteoporosis.