Subchondral insufficiency fracture of the femoral head in a patient with ankylosing spondylitis: Case report and literature review.

Subchondral insufficiency fracture (SIF) of the femoral head occurs in patients with osteoporosis, elderly women, and renal or liver transplant recipients. Although SIF has been reported in several patients with rheumatic disease, SIF of the femoral head has not been reported in patients with ankylosing spondylitis (AS), and the association between AS and SIF has not been determined. A 48-year-old man with AS presented with pain in his left hip for 2 months. He had been diagnosed with AS and radiographic bilateral grade 3 sacroiliitis 11 years earlier. He had been treated with subcutaneous adalimumab 40 mg biweekly for more than 10 years, during which time his condition remained stable. This patient was obese but had no other known predisposing conditions, such as old age, overexertion, osteoporosis, steroid use, or transplantation. He had never taken steroids. X-rays showed no specific findings, other than mild osteoarthritis in both hips. However, pelvic magnetic resonance imaging demonstrated flattening and subchondral irregularity with a large amount of bone marrow edema, confirming a diagnosis of SIF of the femoral head. Thus, even in patients with AS having no significant risk factors, SIF should be considered as part of the differential diagnosis of hip pain.