HISTORY: A 15 year old competitive female tennis player presented with right knee pain and swelling of one month's duration. The initial pain began suddenly and was apparently without trauma. Since that episode, she has had intermittent pain, swelling and stiffness. Three days prior to presentation, she was on the court and developed sudden lateral knee pain with a further increase in pain and swelling. She was unaware of any trauma with this new increase in pain. She now noticed a “cracking” sensation within the knee and could not fully extend the knee secondary to something“blocking” her from doing so. PHYSICAL EXAMINATION: Well developed, well nourished white female in no acute distress. The right knee showed an obvious effusion and a bluish hue to the skin. No discrete ecchymosis was present. There was no warmth or erythema. Range of motion was complete for flexion, but there was a 5-10° loss of extension with a “hard end” feel. Palpation revealed tenderness at the inferior, medial aspect of the lateral femoral condyle and along the lateral facet of the patella. No tenderness was present along the joint lines. McMurray's testing was painless. All ligaments appeared of normal integrity without pain to testing. Gait was relatively normal with the exception of greater right pronation than left and an inability to fully extend the knee during late stance phase. The knee was aspirated of 10cc of a serosanguinous fluid. Ice and compression were applied. Radiographic testing was next ordered. DIFFERENTIAL DIAGNOSIS: Bucket-handle meniscal tear Osteochondral defect of patella or femoral condyle Patellar subluxation Malignancy Lymes or other inflammatory arthropathy Gonococcal arthropathy TEST AND RESULTS: Analysis of knee effusion fluid: WBC-1,200/mm3 (4% polys, 96% lymphs), RBC-26,100/mm3, no crystals seen. AP, lateral, notch and sunrise views of right knee: Large lucency of lateral femoral condyle. Faint calcific density seen near the intercondylar notch. Magnetic resonance imaging right knee: Revealed a bony defect of the lateral femoral condyle trochlear groove. FINAL/WORKING DIAGNOSES: Osteochondritis dessicans of the right femur with loose body in joint space. TREATMENT: Arthroscopic retrieval of loose body trochlear debridement, lateral retinacular release. Physical therapy protocol designed to enhance quadriceps (and VMO, in particular) strength, patellar tracking and flexibility through straight leg raises (advanced with weight as tolerated). Return to sports-specific activities as tolerated Patient has resumed skiing, playing tennis, and remains pain free.