The Patellar Facets

The patella is divided into 7 facets. On the medial and lateral sides, there are superior, middle, and inferior facets. The "odd" facet is medial to the medial facet and does not articulate with the femur until flexion exceeds 90 degrees. On deep flexion the patella rotates and the odd facet articulates with the medial femoral condyle. The odd facet is frequently the first part of the patella to be affected in premature degeneration of articular cartilage.

References

Goodfellow J, Hungerford DS, Zindel M. Patello-femoral joint mechanics and pathology. 1. Functional anatomy of the patello-femoral joint. J Bone Joint Surg Br. 1976 Aug;58(3):287-90.