screwhomemechanisme

The screwhomemechanisme, commonly known as the screw-home mechanism, is a knee joint phenomenon describing a rotation that occurs in the leg as it moves toward full extension. The term appears in anatomy and biomechanics literature and is most apparent during the final portion of knee straightening.

In open-chain movement (for example, a leg hanging from the knee), the tibia externally rotates relative to the femur by roughly 10 to 15 degrees as the knee locks into full extension. In closed-chain movement (such as standing), the opposite rotation occurs: the femur medially rotates on the tibia. The mechanism is driven by the geometry of the articular surfaces, particularly the longer medial femoral condyle, and is aided by tension in the posterior cruciate ligament and surrounding soft tissues. The medial compartment contributes most to the locking, helping stabilize the knee in full extension.

Functionally, the screw-home mechanism increases joint congruency and stability when the knee approaches full extension, allowing the person to stand with less muscular effort. It is a characteristic feature of normal knee mechanics and is integrated into the late stance phase of gait.

Clinical relevance varies with injury and pathology. The mechanism can be diminished or altered after injuries such as anterior cruciate ligament tears or meniscal damage, or with osteoarthritis. In such cases, changes in mechanics can affect standing stability and gait, and rehabilitation may aim to restore or compensate for the natural locking action. Individual differences and activity can influence the degree and visibility of the mechanism.