At what degree of knee flexion does the PFJ have the most contact area with the femur?

• A. 0-30 degrees
• B. 30-60 degrees
• C. 60-90 degrees
• D. 90-120 degrees

Answer: (C)

The main concept here is how the patellofemoral joint contact area changes as the knee moves through flexion. As the knee bends, the patella sits deeper in the femoral trochlea and more of its articular surface comes into contact with the groove. This contact area grows until it reaches a peak when the patella is most congruent with the groove, which is typically in mid-range flexion around 60 to 90 degrees. At this range, a broad portion of the patellar surface engages the femur, distributing load over a larger surface area and reducing stress for a given force. If the knee stays in shallow flexion, only a smaller portion of the patella contacts the trochlea, so the contact area is smaller. In deep flexion, the contact pattern shifts to different facets and the area does not continue to increase in the same way, so the maximum contact area isn’t reached again. Therefore, the greatest patellofemoral contact area occurs in roughly 60–90 degrees of knee flexion.