[Osteoporosis in ankylosing spondylitis].

Bone formation is classically observed in ankylosing spondylitis, but osteoporosis can also occur. This condition has been recognized for years on radiographs. Compared with controls, osteoporosis in ankylosing spondylitis is responsible for increasing the incidence of vertebral compression fractures and also explains spinal fractures after trauma, mainly observed at the cervical level. Measurement of bone mass is useful in diagnosing osteoporosis commonly observed in the lumbar spine and the femoral neck but not in the appendicular skeleton. Osteoporosis is seen early in the disease whereas increased bone mass is observed later or due to syndesmophyte formation. Osteoporosis in ankylosing spondylitis is probably a multi-factorial condition. Contributing factors are spine immobility secondary to ankylosis, inflammatory cytokines which enhance bone resorption, prolonged use of nonsteroidal antiinflammatory drugs and a deficit in sex hormone secretion. Furthermore, there is no alteration in calcium or phosphorus metabolism in ankylosing spondylitis. Finally, a study of bone morphometrics in the iliac crest region is required to better explain osteoporosis in ankylosing spondylitis.