The screw-home mechanism of the knee joint occurs during the final degrees of active knee extension and involves which movement?
- A Internal rotation of the tibia on the femur, locking the joint
- B External rotation of the tibia on the femur (lateral rotation of tibia), locking the joint ✓
- C Forward gliding of the femoral condyles on the tibial plateau
- D Posterior horn of medial meniscus being pushed forward by the ACL
Correct answer: B. External rotation of the tibia on the femur (lateral rotation of tibia), locking the joint
Explanation
The screw-home mechanism is the terminal rotation that locks the knee in full extension. In open-chain extension the tibia rotates externally (lateral rotation) on the femur during the last 5–10 degrees; in closed-chain extension the femur rotates medially on the fixed tibia. This results from the larger articular surface of the medial femoral condyle and the oblique pull of the cruciate and collateral ligaments. Locking is facilitated by the popliteus muscle which initiates unlocking (by internally rotating the tibia) to allow flexion from the fully extended position.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.