The patella is the largest sesamoid bone in the body, and it functions to direct the forces exerted by the quadriceps muscles. Patients with patella fracture will have pain / swelling over the knee with large knee effusion and tenderness over the patella.
Fracture can be caused by direct or indirect forces.
Direct impact to patella can occur from a fall on a flexed knee landing directly on patella or motor vehicle crash. This can result in a transverse fracture pattern (more common in younger adults) or a comminuted fracture (more common in elderly patients and those with osteoporotic bone).
Indirect impact can occur with sudden flexion of the knee against a contracted quadriceps muscle (depending on the force and properties of the muscle / bone / tendon, something in the patient's extensor mechanism will break, whether it is the quadriceps tendon, the patellar tendon, or the patella itself). Avulsion fractures involve the superior / inferior pole.