Dr. Ebraheim’s educational animated video describes the condition of bipartite patella - unfused ossification center.
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Bipartite Patella
Bipartite patella is a failure of the ossification centers of the patella to fuse. Common types of bipartite patella include inferior pole, lateral margin, and superolateral pole. The accessory ossification center at the superolateral pole remains unfused. In the cases of bipartite patella, you can see that the patella has two parts: the smaller part is rounded and usually located laterally. It can be confused with a fracture. The superolateral pole is the most common type (occurs in about 75% of patients). Try to get an x-ray of the other knee (it is bilateral in about 50% of the time). Skyline view x-ray with a squatting position (weight-bearing) may show displacement and increased separation of the fragment. The bipartite patella is usually asymptomatic, and it is usually an incidental finding on the x-ray. It may cause symptoms that mimic those of a fracture. Minor trauma or injury can cause the fibrous tissue between the two segments to become inflamed and irritated. Localized tenderness over the separated fragments, usually the superolateral part of the patella. The most common presentation is pain at the area of the separated fragments, especially during or after heavy physical activity or sports. It is usually asymptomatic and does not require treatment or surgery. In general, the most common error with bipartite patella is mistaking the condition for a fracture and fixing it (the patient really does not need surgery). Treatment is usually reassurance and observation, rest, knee immobilizer, physical therapy, and nonsteroidal anti-inflammatory medication (NSAID). Nonoperative treatment should be done for at least 6 months. These patients will improve without surgery. Surgery is rare, and it is excision of the fragment if the fragment is small. If the fragment is large, painful, and conservative treatment fails, then you can do lateral release of the retinaculum to reduce the traction force on the unfused, smaller fragment. Internal fixation and possible bone graft is rarely done if the fragment is large and painful.