What does the "screw home mechanism" refer to in knee biomechanics?

The screw home mechanism refers to the close relationship between the femur and tibia during the last degrees of knee extension. Specifically, as the knee moves into full extension, there is a natural external rotation of the femur on the tibia in an open kinetic chain. This movement occurs because of the anatomical configuration of the knee joint, allowing for the femur to rotate laterally as it locks the knee into a stable position.

In open chain activities, when the tibia is fixed or moving relative to a stable femur, the tibia externally rotates on the femur to complete the extension. Conversely, if we consider closed chain movements, where the foot is planted, the femur will internally rotate on the tibia due to the alignment and shape of the joint surfaces.

Therefore, the option that accurately describes the screw home mechanism in the context provided is the external rotation of the femur in open chain movements. This understanding of knee biomechanics highlights the dynamics of how proper knee function and stability are achieved, particularly during activities that require full extension.