A bipartite patella develops from two or more ossification centers. These centers usually fuse to form a single bone. In some cases a separate second center of bone formation is present that results in a main and an accessory bone. The main and accessory bones are either connected or remain separate by fibrocartilaginous tissue. The following facts where found concerning the bipartite patella:
A congenital condition usually present by age 12 and may persist into adult life
Incidence ranges between approximately 2% -- 6% of the population
More present in males than females 8:1
Occurs bilaterally in about 43% of the cases
Accessory bone location is most commonly at the superolateral pole of the patella
Most cases are asymptomatic
After a traumatic event or repetitive micro trauma, most cases become symptomatic
There are usually no symptoms in the majority of individuals and the diagnosis of this is usually an incidental radiological finding by an X-ray or MRI. In 1943, Saupe proposed a radiologic classification of the bipartite patella based on the position of the accessory ossification center. They are as follows:
Type I ---- (5%) the accessory fragment is located at the inferior pole of the patella
Type II ---- (20%) the accessory fragment is located along the lateral margin
Type III --- (75%) the accessory fragment is located at the superolateral pole of the patella
Sometime the bipartite patella maybe mistakenly diagnosed as a patella fracture because it is also painful and ununited. Therefore, a comparison view of the opposite knee will confirm it or rule it out.
Most patients respond to non-operative treatment such as a cylinder cast or knee immobilizer for 4 to 6 weeks. In cases of persistent or recurrent pain surgical intervention such as excision of the painful fragment or release of the vastus lateralis tendon from the fragment maybe necessary. Nevertheless, most patients have excellent clinical long term results.
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