screwhome

Screwhome, typically referred to in anatomy as the screw-home mechanism, is a biomechanical description of a coupled rotation at the knee that helps the joint move from near-extension to full extension and back. The term reflects a “locking” action that increases knee stability during standing and walking.

In open-chain movements, such as when the leg is straight and not bearing weight, the tibia externally rotates relative to the femur during the last degrees of extension. This rotation is produced by the rounded shape of the medial femoral condyle, the unequal articular surfaces, and the tension of ligaments such as the cruciate ligaments. The mechanism also contributes to a smoother transition into full extension. When unlocking the knee for flexion, the popliteus muscle initiates internal rotation of the tibia to begin movement.

In closed-chain movements, such as standing with the foot planted, the rotation occurs in the opposite direction: the femur internally rotates relative to the tibia during the early phase of flexion to enable knee bending. The interplay between joint surfaces and ligaments remains the foundation of the mechanism, supporting stability when the knee bears weight.

Clinical relevance centers on the mechanism’s role in gait, athletic performance, and rehabilitation. Alterations in the screw-home motion can arise from ligament injuries, meniscal damage, or arthritis, potentially affecting the ease of initiating flexion or maintaining stability during standing. As a concept, screwhome is widely described in anatomy and kinesiology texts under the screw-home mechanism, with individual variation common across people.