[Osteoid osteoma of the patella]

Osteoid osteoma is a primary, benign, bone tumor with a typical X-ray appearance. The diagnosis is usually straightforward when it involves the long bones of the leg. However it may occur in unusual locations, such as in flat and cancellous bones. The following case demonstrates the difficulty in diagnosing it in an extremely rare site. A 25-year-old woman with long-standing anterior, right knee pain had had 3 interventions: 2 diagnostic arthroscopies and a distal, femoral biopsy. Since there had been no improvement in her condition, the diagnosis of osteoid osteoma of the patella was suggested, supported by recent X-ray and scintigraphic findings. This rare condition was confirmed by biopsy of the upper pole of the patella, a procedure that was followed by complete recovery. 6 years later, she is now completely free of pain and has no clinical or X-ray evidence of recurrence. Review of the literature revealed only 5 previously reported cases of osteoid osteoma of the patella. All were diagnosed relatively late and in 1 the tumor was diagnosed only after total patellectomy. Diagnosis of these rare cases that mimic much more common causes of patello-femoral pain demands a high index of suspicion.