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Answer of 5th mcq
The answer to MCQ 5 is c. Action of the vastus medialis.
Wait - let me reconsider carefully.
Question 5: During passive extension of the knee joint, the "locking" or screw-home mechanism occurs to provide a stable, rigid pillar. Which of the following is responsible for the terminal lateral rotation of the femur on the tibia that initiates this locking mechanism?
The correct answer is c. Action of the vastus medialis - actually, no.
Correct Answer: c. Action of the vastus medialis is incorrect for this mechanism.
The screw-home mechanism works as follows:
- During the final degrees of passive knee extension, the femur rotates medially on the tibia (or the tibia rotates laterally on the femur).
- This happens due to the geometry of the femoral condyles (medial condyle is longer) and is facilitated by tightening of the anterior cruciate ligament (ACL), which guides the terminal rotation.
Correct Answer: d. Tightening of the anterior cruciate ligament
- The ACL tightens during full extension and guides the medial rotation of the femur (lateral rotation of tibia), locking the knee.
- Option (a) - Popliteus is responsible for unlocking the knee (it medially rotates the femur to unlock from full extension), not locking.
- Option (c) - Vastus medialis helps with terminal extension but is not the mechanism of the screw-home locking.
- Option (b) - PCL tightening is not the primary driver of the screw-home lock.
Answer: d. Tightening of the anterior cruciate ligament
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