The patella (kneecap) is a subcutaneous bone within the quadriceps tendon that can be fractured by direct impact, or by forces generated by the quadriceps muscle. A typical mechanism of injury for a direct fracture is a fall on a hyper-flexed knee. An indirect fracture can be caused by an eccentric contraction of the quadriceps against resistance that pulls the patella apart. An indirect fracture might be seen after a fall from moderate height, in which the foot striking the ground forces the knee into flexion.
Displaced patellar fractures need to be fixed surgically to reconstruct the extensor mechanism of the knee. Without reduction and fixation, normal knee extension is not possible. Some non-displaced fractures are also fixed to obviate the need for prolonged immobilization.
A bipartite patella is caused by failure of fusion of a secondary ossification center of the patella during development. A bipartite patella may be mistaken for a patella fracture. A bipartite patella can be identified by the characteristic superior-lateral position of the “fracture line,” as well as the matching appearance in the contralateral knee in about half of all cases.
Although non-unions are rare after patellar fractures, anterior knee pain is frequently encountered after the fracture has healed. This knee pain may be present even if the articular surface appears normal on imaging.
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