Why The Screw Home Mechanism Is Important In ACL Rehabilitation

One pivotal aspect of ACL rehabilitation is regaining full range of motion which is a key component to reducing second ACL injuries. While it may seem straightforward, it amazes me how often I see ACL reconstruction patients from other physical therapists who still don’t have full knee extension over four months from surgery. It happens far too often and their previous physical therapist will tell them it’s “good enough.” GOOD ENOUGH!? Perhaps, I have more of an emotional attachment to ACL patients as I’ve personally suffered two ACL tears but ‘good enough’ should not be an option. Lacking full knee extension leads to never fully contracting the quadriceps leading to strength deficits which significantly puts the patient at risk for a second ACL injury. The question is, what can we do better? This brings me to the screw-home mechanism and why it’s often overlooked in ACL rehabilitation. The screw-home mechanism is pivotal for knee stability and involves rotation of the tibia (shin bone) and femur (thigh bone). This movement occurs at full extension to the initial 20 degrees of flexion. In open chain movement (when the foot is NOT fixated to the ground) the tibia internally rotates in relation to the femur. In closed chain (foot IS fixated to the ground) the tibia externally rotates until the knee hits terminal knee extension which tightens the cruciate ligaments maximizing knee stability. Why does the screw-home mechanism even matter in ACL rehabilitation? The ACL originates from the posterior medial corner of the medial aspects of the lateral femoral condyle in the intercondylar notch. It inserts anterior to the intercondyloid portion of the tibia and bends with the medial meniscus. During ACL reconstruction surgery, they are replacing a ligament that attaches to both the femur and tibia so the screw-home mechanism can potentially be affected. In ACL reconstruction rehabilitation, I personally try to address the screw-home mechanism early and often. I use a manual technique to facilitate the rotation of the tibia and femur while simultaneously having the patient activate their quads. In addition to the rotational component, I apply a downward force to help them achieve true full terminal knee extension. I personally believe this is more of a facilitation technique or neuromuscular re-education than a true manual technique. I want their quads to consistently actively achieve full terminal knee. After performing some manual work, patients often report they can push their knees further down and it feels similar to the non-surgical knee. Regaining full terminal knee extension through addressing the screw-home mechanism is a key component to optimal recovery during ACL reconstruction rehabilitation. Thanks for reading. -Wesley Wang, PT, DPT