Atraumatic Knee Pain In A Cross Country Runner

HISTORY: 20F cross country runner with PMH of delayed bone age due to late menarche, L tibial stress injury, and anorexia nervosa, presented for atraumatic R knee pain. She developed pain and swelling 3 days prior to evaluation. She endorsed medial knee pain which increased with knee flexion. She reported no running for past 5 days due to URI. She was able to cross train. Physical exam: Patient arrived using crutches. She appeared well. She had a gross knee effusion with ballotable patella. No overlying erythema nor ecchymosis. There was severe reduction of active and passive knee flexion, causing pain. Mildly reduced active and passive knee extension. She was tender to palpation at distal medial femoral condyle. No tenderness with calf palpation nor significant edema. No discomfort nor laxity with valgus or varus stress. Lachman’s was negative, limited due to effusion. Sensation intact and temperature equal throughout lower extremities. Differential: acute internal derangement, hemarthrosis, femoral stress fracture, reactive arthritis TESTS AND RESULTS: XR R Knee: no acute fracture, large R knee joint effusion, and mild lateral patella subluxation. MR R Knee: lateral greater than medial distal femoral posterior physeal widening with fluid contiguity to subperiosteum, elevation of metadiaphyseal periosteum, marrow edema, and pericapsular edema. FINAL/WORKING DIAGNOSIS: Osteomyelitis Treatment and Outcomes: Patient was referred to the emergency room (ER) 7 days after initial visit when Radiologist communicated concerning findings. She then reported improvement of her R knee pain and swelling, and had self-discontinued crutches. CRP was 2.2, ESR 115, and blood cultures were obtained. After consultation with Pediatric Orthopedics, repeat contrasted MRI reported extensive edema and marrow abnormality in distal femur, small subperiosteal fluid, and moderate joint effusion. Follow-up with Infectious Disease noted subjective improvement. Inflammatory markers decreased. Subsequent CT and MRI demonstrated interval development of lucency with ill-defined margins and internal fragment at site of prior abnormality. Bone biopsy was obtained. Tissue culture resulted with Staphylococcus aureus resistant to Clindamycin. She was treated with Linezolid for 30 days with positive outcome.