Causes / typical injury mechanism: Bipartite patella is a developmental variation of the ossification of the patella. In this condition, the patella develops as 2 pieces rather than 1 due to a failure of 2 segments to fuse together.

Classic history and presentation: Bipartite patella is typically asymptomatic, and most cases are discovered incidentally. Only 2% of cases are symptomatic and present with anterior knee pain following direct trauma, overuse, or aggravation by activities such as jumping, squatting, or climbing stairs. This condition may develop bilaterally or unilaterally, with unilateral involvement occurring in almost equal numbers in the left and right knees. The incidence of bilateral involvement in individuals with bipartite patella ranges from 25%-43%.

Prevalence: This condition has a low incidence, reported at 0.2%-1.7% of the population.

• Age – The age of onset of pain typically occurs at 12-14 years.
• Sex / gender – Bipartite patella is more common in males than females.

Risk factors: This condition is considered to be developmental. Risk factors for symptomatic bipartite patella include repetitive movements that stress the knee joint and direct trauma to the knee joint.

Pathophysiology: Ossification of the patella occurs initially between 3 and 5 years of age. As the ossification center expands, the margins may become irregular, and accessory or secondary ossification centers may form. If these 2 ossification centers fail to fuse, fibrocartilaginous tissue forms between the body of the patella and the bipartite fragment. This fibrocartilaginous zone may be disrupted following direct or indirect injuries that may lead to pain.

Grade / classification system: Bipartite patella is classified by the location of the fragment. The most commonly used system is the Saupe classification.

• Type I located at the inferior pole of the patella
• Type II located at the lateral margin of the patella
• Type III located at the superolateral pole is the most common

A new classification system was developed to include tripartite patella and medial bipartite patella that cannot be classified under the Saupe classification. The newer classification system uses the location and number of fragments to classify the patellae. Four types are classified:

• Supero-lateral bipartite type
• Lateral bipartite type
• Supero-lateral and lateral tripartite type
• Supero-lateral tripartite type