Unusual presentation of an osteoid osteoma.

We present a case of osteoid osteoma of the humerus diagnosed after it presented clinically as impingement syndrome. Clinical and magnetic resonance imaging data supported impingement, but standard non-surgical treatment did not provide complete pain relief. Subacromial injection resolved the anterior shoulder pain and unmasked medial arm and axillary pain. Humerus X-rays revealed a lytic diaphyseal lesion, and en-bloc resection of the humeral mass diagnosed an osteoid osteoma. This case is unusual in that only 1% of osteoid osteomas present in the humeral shaft. The clinical features and presentation of impingement and osteoid osteoma are discussed.