What is the screw home mechanism in open chain vs closed chain?

• A. Open chain: tibial IR; Closed chain: femoral external rotation during flexion
• B. Open chain: patellar tilt; Closed chain: fibular rotation
• C. Open chain: the last 5-10 deg of tibial ER during terminal knee extension; Closed chain: femoral internal rotation on the tibia during terminal knee extension
• D. Open chain: tibial ER during flexion; Closed chain: tibial ER during extension

Answer: (C)

The screw-home mechanism describes how the knee achieves a stable, locked position as it finishes extending. In an open-chain situation (where the foot moves freely), the tibia must rotate externally by about 5–10 degrees on the femur during the last part of extension to lock the knee. In a closed-chain situation (where the foot is planted and the femur moves on a fixed tibia), the locking is accomplished by internal rotation of the femur on the tibia during terminal extension. This difference comes from the shapes of the femoral condyles and tibial plateau and their ligamentous constraints, which favor external rotation of the tibia in open chain and internal rotation of the femur in closed chain to reach the same locked, stable position. Unlocking for flexion reverses this pattern, typically involving the opposite rotation to begin moving out of extension.