Screw Home Mechanism of the Knee

In this video, I describe the screw home mechanism of the knee (tibiofemoral joint). The screw home mechanism is defined as concurrent external rotation of the knee during the last 25-30 degrees of extension. This is driven primarily by the shapes of the condyles of femur and secondarily by passive tension in the ACL near the fully extended position. The locked position of the knee is a very stable position, but there is nothing that prevents the knee from going back into flexion. Any muscle or external force pulling or pushing into flexion can "unlock" the knee. Popliteus specifically is not required to unlock the knee, but it may plan a small role maintaining joint congruency during active knee flexion. However, gracilis, semimembranosus, semitendinosus, and the lateral head of the gastrocnemius have some ability to guide internal rotation of the knee when coupled with knee flexion when starting from full extension. I do not think sartorius has the ability to help, as I incorrectly stated in the video, as it does not gain the ability for internal rotation until the knee has been flexed. I did not address knee extensors' roles in rotation in this video, but vastus medialis can contribute to medial rotation of the knee and vastus lateralis can contribute to lateral rotation. Please note that this information is based on looking at images of cadavers. Many artistic images of these muscles have slight variations that are misleading.