How the screw home mechanism works
The screw-home mechanism is a rotation between the tibia and femur occurring at the end of knee extension, between full extension (0 degrees) and 20 degrees of knee flexion.
If a patient cannot achieve the last few degrees of knee extension, they may not be able to stand on that leg unsupported.
Peer-reviewed paperQuote from peer-reviewed paper:
"As the medial femoral condyle is longer than the lateral condyle, the tibia rotates internally on the femur during the first stage of flexion....[this is called] the screw‐home movement......."
Citation: Zhang LK, Wang XM, Niu YZ, Liu HX, Wang F. Relationship between Patellar Tracking and the "Screw-home" Mechanism of Tibiofemoral Joint. Orthop Surg. 2016 Nov;8(4):490-495. doi: 10.1111/os.12295. PMID: 28032709; PMCID: PMC6584384.
How releasing the 'screw home' facilitates patellar tracking
As the muscles contract to unlock the knee, the change in direction of the 'screw' helps to pop the patella properly into the underlying groove of the femur in which is runs, so that it does not sublux or dislocate.
Peer-reviewed paperQuote from peer-reviewed paper:
"....During knee flexion, the patella moves into the femoral trochlear groove while rotational movement adjusts the patellofemoral joint stability; this is known as the capture mechanism of the femur....[and it] prevents abnormal patellar tracking in the early stage of knee flexion...the screw‐home mechanism is the insurance of the capture mechanism, and plays an important role in guiding patellar sliding into the....groove."
Citation: Zhang LK, Wang XM, Niu YZ, Liu HX, Wang F. Relationship between Patellar Tracking and the "Screw-home" Mechanism of Tibiofemoral Joint. Orthop Surg. 2016 Nov;8(4):490-495. doi: 10.1111/os.12295. PMID: 28032709; PMCID: PMC6584384.