Osteoid osteoma of the patella: a case report.

Though osteoid osteoma is a common primary benign lesion of the bones, intra-articular involvement is rare and poses diagnostic difficulties when it affects middle-aged patients. We present the case of a 51-year-old woman with a 2.5 year history of anterior knee pain that was misdiagnosed as osteochondritis dissecans. Radiological findings were absent, whereas MRI showed a well-circumscribed lesion. A local excisional biopsy was performed and microscopic appearance confirmed diagnosis.