Odd facet of patella

The odd facet of the patella is a distinct anatomical region located on the extreme medial aspect of the patellar articular surface. It is separated from the main medial facet by a subtle ridge and is not in contact with the femoral trochlea during normal knee extension or early flexion. The odd facet only articulates with the medial femoral condyle during deep flexion, typically beyond 135° of knee flexion, which is when it becomes functionally relevant in patellofemoral contact mechanics.

Morphologically, the odd facet is smaller and more steeply angled compared to the medial and lateral facets. It is covered by hyaline cartilage, similar to the rest of the patellar articular surface, but its cartilage may be thinner and more susceptible to degenerative changes due to its unique loading pattern.

The presence and shape of the odd facet can vary among individuals, but it is identified in the majority of adult patellae.