Review Question - QID 216282

As the knee moves from full extension to flexion, patellofemoral joint contact pressures increase. An increased Q-angle causes a laterally directed force across the patellofemoral joint that further increases pressures in the lateral patellofemoral joint, specifically.

Patellofemoral joint biomechanics play an important role in supporting the stability of the patellofemoral joint during knee range of motion (ROM). Contact forces through the patellofemoral joint are some of the highest in the body. The medial patellofemoral ligament (MPFL) is the primary restraint to lateral translation from 0-20 degrees, up to the point where maximum bony contact is achieved between the patella and trochlea around 45 degrees. Contact pressures continue to increase with flexion up to about 90 degree - up to 7x body weight with squatting. In cases where a higher Q-angle is present in the lower extremity, the lateral vector of pull is increased through the patella. This has been noted to most pronounced between 40 and 90 degrees of flexion.

Ramappa et al. examined the effects of medialization and anteromedialization osteotomies on patellofemoral kinematics in cadavers. They noted that compared to normal knees, increased Q angles increased the contact pressures in the lateral facet, which were able to be partially corrected by both osteotomy types. They concluded that both osteotomies were similar in their ability to correct abnormal patellofemoral biomechanics with increased Q angles.

Huberti et al. evaluated the changes in patellofemoral contact pressures at different degrees of flexion and different Q-angles. They noted that during "normal" patellofemoral motion, contact pressures were evenly distributed between the medial and lateral patellar facets, with maximum forces being seen at 90 degrees of flexion, decreasing to 1/3 of the force by 130 degrees of flexion. They also concluded that a 10 degree increase in Q angle increased the peak pressures during flexion, particularly in the lateral facet.

Incorrect Answers:
Answer 1: The MPFL is the primary constraint to lateral patellar translation from 0-20 degrees. Bony stability becomes more important after this point.
Answer 2: While the vastus medialis is a dynamic stabilizer, the MPFL provides the primary constraint to lateral patellar translation between 0-20 degrees of flexion, as the patella enters the trochlear groove.
Answer 3: Patellofemoral contact forces are highest at about 90 degrees of flexion, not full extension.
Answer 4: Contact pressures are typically higher in the proximal patella as the knee reaches deeper degrees of flexion based on the fact the patella glides inferiorly with knee flexion.