Abstract
In brief: A serious knee injury can result in dysfunction and can thwart the promising career of a gifted young athlete if not accurately diagnosed and properly treated. This article focuses on the bloody effusion (hemarthrosis) that usually signals the presence of a serious internal derangement. The most frequent causes of hemarthrosis are patellar dislocation/subluxation, epiphyseal injury, disruption of the anterior cruciate ligament, and chondral/osteochondral fracture. It is essential to establish a definitive diagnosis and to begin treatment as soon as possible to avoid any complications. The author provides guidelines for diagnosis and treatment of the injuries that often cause hemarthrosis.