Bone Loss Is Detected More Frequently in Patients with Ankylosing Spondylitis with Syndesmophytes

OBJECTIVE: To define the relationship between bone growth (syndesmophytes) and bone loss (osteoporosis) in ankylosing spondylitis (AS). METHODS: Bone mineral density (BMD) at the spine, hip, and radius was measured by dual-energy x-ray absorptiometry (DEXA), dual-energy quantitative computed tomography (DEQCT), and peripheral quantitative computed tomography (pQCT) in 103 patients with AS. Radiographs of the lumbar spine were used to detect syndesmophytes. Patients were divided in 3 groups according to disease duration. RESULTS: Osteopenia at the hip and spine was found by DEXA in 56% and 41%, respectively, of the patients with disease duration < 5 years (n = 27), with an additional 11% and 15% having osteoporosis. In patients with a longer disease duration, > 10 years (n = 28), 29% were osteoporotic at the hip and only 4% at the lumbar spine. In contrast, using spinal DEQCT, 59% of patients with early disease were found to be osteopenic; 36% of patients with long-standing disease were osteopenic and 18% were osteoporotic. More than half the patients (55%) had syndesmophytes (n = 55). With spinal DEQCT there were more patients with syndesmophytes (63%) in the group with reduced bone density than in the group without (45%). This was similar with DEXA measurements at the hip, where 31% compared to 14% had osteoporosis, respectively. Osteocalcin was elevated in 34% of patients, but was not associated with disease activity or BMD. CONCLUSION: The majority of patients with AS had reduced bone density. The method of bone density measurement is critical and should be different depending on disease duration. The finding that more patients with syndesmophytes had reduced bone density than those without suggests that bone growth and bone loss occur in parallel, and the role of inflammation in this process warrants further investigation.