Regional migratory osteoporosis in a patient with ankylosing spondylitis.

We report the first case of regional migratory osteoporosis (RMO) in a patient with ankylosing spondylitis (AS). This middle-aged man suffered from an acute onset of knee pain that increased on weight bearing, followed by ankle pain. The diagnosis of RMO was confirmed using magnetic resonance imaging (MRI), after exclusion of other causes of knee pain. MRI revealed a large area of bone marrow oedema without a zone of demarcation or subchondral fracture with a demonstration of shifting marrow oedema on the follow-up MRI scan from the medial femur condyl to the tibia plateau lateral and then to the distal tibia epiphysis. Treatment with the bisphosphonate ibandronate, however, was unsuccessful. RMO is characterized clinically by migrating arthralgia of the weight-bearing joints of the lower limbs, mainly in middle-aged males. Although the aetiology is unknown, the pathophysiology of RMO seems to be closely related to transient osteoporosis of the hip (TOH), which has been considered a reversible stage of avascular necrosis of the hip (AVN). There is no causal treatment for RMO. Avoidance of weight bearing and use of analgesics are effective in reducing symptoms. The combination of RMO and AS yielded diagnostic difficulties, as the clinical picture and the marrow oedema seen on MRI could be attributed to several AS-related causes such as enthesitis, early stadium of arthritis, osteonecrosis, or sterile osteomyelitis.