Beyond 90 degrees of flexion, the patella slips off the trochlear groove of the femur and the lateral and medial facets of the patellar make contact on the femoral condyles.

Then at 135 degrees the lateral femoral condyle protrudes anteriorly to such a point that a transverse rotation of the patellar occurs. This rotation allows the Odd Facet to articulate on the medial condyle.

NB: Pressures change through out loaded flexion. In the squat for example, the compressional forces experienced during this excursion increase and the superior patellar articulations experience the greatest forces.

CLINICAL SIGNIFICANCE:
1. This facet is suspected to be responsible for causing osteochondritis dissecans.
Why? The area of patellar contact (on the lateral margin of the medial condyle) matches the site where the lesion is commonly found.
How? Damage most likely occurs during the transition from the medial facets to the odd facet. Between these facets is a ‘peak’ or ‘crest’… this causes pressure to intensify.

2. It is also suggested that this anatomical feature may account for cartilage lesions typically found in patellofemoral syndrome.

Article: http://www.bjj.boneandjoint.org.uk/content/58-B/3/287.long