Osteoid osteoma is a rare benign osteoblastic tumor that mainly occurs in the extra-articular segments of lower limb long bones. The elbow is an exceptional location for osteoid osteoma. The diagnosis is usually delayed since the elbow is a rare site for this tumor and unspecific clinical signs, such monoarthritis, may mimic other more common conditions. We report the case of a 16-year-old girl who presented with chronic monoarthritis of the elbow which was treated falsely as tuberculous arthritis and monoarticular rheumatoid involvement. The laboratory tests were negative. Computed tomography scan and magnetic resonance imaging showed the focal nidus at the coronoid process of the ulna with mild synovitis. The diagnosis of osteoid osteoma was not made until the disease had been progressing for eighteen months and the elbow had already become stiff. Arthroscopic excision of the lesion was performed, and histopathology of the nidus was consistent with osteoid osteoma. Since then, the patient has been pain-free with an improved elbow range of motion.
