Osteoid osteoma of the femoral neck mimicking monarthritis and causing femoroacetabular impingement.

Different aetiologies including the femoroacetabular impingement (FAI) may cause a painful hip, especially in young pa - tients. Two general types of femoroacetabular impingement have been described, the pincer- and the cam type impingement. The latter is characterized by a femoral deformity, usually a bump on the head-and-neck junction that impinges on the acetabular rim. The authors describe the case of a 21-year-old male, bodybuilder, suffering from progressive hip pain with impairment of exercise tolerance, gait and other daily activities. Besides limitation of hip internal rotation physical examination was normal. He had a transitory response to non-steroid anti-inflammatory drugs. Initially performed MRI of the pelvis shows predominant inflammation of the hip joint. In external performed arthroscopy the biopsies of the capsule demonstrated chronic synovitis. In the follow up hip pain remains, however, diagnosis was still unclear. Re-evaluation of the formerly performed and a follow up MRI, and of an additional CT, the findings were compatible with an osteoid osteoma in the femoral cervico-cephalic transition causing itself a cam impingement and monarthritis. The adopted therapeutic strategy consisted on arthroscopic excision of the nidus and trimming of the femoral neck, with clinical recovery after surgical intervention.