Bipartite patella

A bipartite patella (two-part patella) is a patella with an unfused accessory ossification center, typically at the superolateral aspect.

Epidemiology

The superolateral accessory ossification center of the patella is usually present by 12 years of age and may persist into adult life. Bipartite patella occurs in approximately 2% of the population, and occurs bilaterally in about 43% of cases. It is 9 times more common in males than in females ².

Clinical presentation

A bipartite patella is usually discovered incidentally in asymptomatic individuals. Only 2% of patients with bipartite patella experience symptoms. It may cause anterior knee pain, especially after trauma, sports injury, or overuse.

Classification

The Saupe classification describes the bipartite patella according to the location of the secondary ossification center:

• type I: inferior pole ~1%
• type II: lateral margin ~20-25%
• type III: superolateral portion ~75% 

Radiographic features

Plain radiograph

A weight-bearing skyline view may demonstrate separation of the accessory fragment, which may indicate a symptomatic bipartite patella ⁴. 

MRI

Bone marrow edema adjacent to the accessory fragment may indicate a symptomatic bipartite patella ⁴.

Treatment and prognosis

In the majority of cases, symptomatic bipartite patella improves without surgery. Surgical excision of the small fragment is recommended if conservative management fails and has been reported to give good results ³.

Differential diagnosis

• patellar fracture: the superolateral accessory ossification center is characteristic of a bipartite patella. With a patellar fracture, the volume of the fractured components is equivalent to that of a normal patella.
• multipartite patella: the volume of the true patella plus that of the smaller ossification centers is greater than that expected of a normal patella.