Valgus knee instability in an adolescent: ligament sprain or physeal fracture?

A 15-year-old boy was hit on the lateral aspect of his left knee while playing football. The injury was initially diagnosed as a medial collateral ligament sprain, and radiographs were negative. Stress views, however, demonstrated medial widening of the physis consistent with a Salter-Harris type 1 injury to the distal femur, and magnetic resonance imaging (MRI) demonstrated a type 3 injury extending through the epiphysis. Stress radiographic views or MRI is diagnostic of distal femoral physeal fracture, and a positive diagnosis should prompt referral.