The patella is normally one bone, but in approximately 1% to 2% of the population the patella develops as two unfused ossification centers. This condition, known as bipartite patella, is a variant of normal and affects men more than women. These two bones are not separate, but are connected by thick fibrous tissue. The patient with a bipartite patella is usually asymptomatic but can experience pain with standing or jumping. The classic bipartite patella appears as a small unfused fragment of the upper outer margin of the larger, main patellar fragment. On radiographs, inexperienced physicians can mistake the bipartite patella as a patellar fracture. Sometimes stress fracture superimposed on bipartite patella can occur and is a potentially difficult clinical entity to recognize (Fig. 8). MRI can aid with diagnosis by showing increased signal within the marrow on fat-suppressed T2WI compatible with marrow edema in cases of stress fracture.
Rest and strengthening exercises are usually sufficient treatment for uncomplicated knee pain in patients with bipartite patella; however, if there is avulsion at the fibrous connection, or a stress fracture, immobilization may be necessary.
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