What is the functional benefit of the screw home mechanism?

• A. Increases knee flexion ROM
• B. Reduces contact between femur and tibia during flexion
• C. Increases patellar tracking
• D. Provides increased stability during weight bearing

Answer: (D)

The screw home mechanism mainly provides stability when the knee is in near-full extension, especially during weight bearing. As the knee approaches full extension, the knee joint locks through a small rotational movement between the femur and tibia, increasing congruence of the articular surfaces. This locking makes the leg more stable when you stand or load the knee, so you don’t have to rely on active muscle activity to maintain stance.

This is not about increasing how far you can bend the knee (flexion ROM), and it doesn’t primarily change patellar tracking. It also isn’t about reducing the contact between the femur and tibia; instead, it enhances joint stability through better congruence at near extension. When flexion resumes, a small unlocking movement occurs (often via the popliteus) to allow bending again.