Which of the following best describes the screw home mechanism of the tibiofemoral joint during open kinetic chain movement?
a. Lateral rotation of the femur on the tibia occurring at 90o knee flexion to lock the tibiofemoral joint in its close-packed position
b. Medial rotation of the tibia on the femur occurring at 60o knee flexion to lock the tibiofemoral joint in its close-packed position
c. Lateral rotation of the tibia on the femur occurring at 0o knee flexion to lock the tibiofemoral joint in its close-packed position
d. Medial rotation of the femur on the tibia occurring at 45o knee flexion to lock the tibiofemoral joint in its close-packed position
ANS: C
There is an asymmetry in the size of the medial and lateral femoral condyles. During weight-bearing (closed kinetic chain motion), extension of the femur on the relatively fixed tibia results in a rotational moment necessary to complete the extension motion. The medial femoral condyle is longer and will continue to roll and glide on the tibia until all motion is taken up. The continued motion of the medial condyle results in medial rotation of the femur on the tibia. This rotation occurs at the very end of the extension motion.
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