Screw home mechanism is a special bio mechanical property of the knee joint. Every one involved in orthopaedics and physiotherapists as well as other personnel must be well acquainted with this simple phenomenon
Definition of screw home mechanism of the knee
It is an example of coupled and co-existent kinematic phenomenon where rotation is coupled with flexion and extension. This phenomenon entails the rotation of tibia or femur during the last 20 degrees of extension and depending upon whether the tibia is fixed on not to the ground (viz. open chain vs closed chain exercises)
A little bit anatomy to enlighten the phenomenon
Ball and socket joints are governed by the principle of co-existence of roll and glide depending on whether the ball is moving in the socket or the socket is moving over the ball. Knee is a ball and socket joint with distal femur being the ball and proximal tibia being the socket. Proper twisting of the knee during flexion and extension is essential for its health and proper function.
The tibio-femoral component of knee joint has unique anatomical shape. Unique shape of femoral condyle dictates rotational component for each flexion and extension. Lateral condyle of the femur is pretty much vertical straight. On the other hand, medial femoral condyle curves inward towards the central axis of femur.
In the tibia, the articular surface of the medial tibial condyle is longer than on the lateral side.
What’s the real screw home mechanism then?
During the last 20 degrees of extension and in open chain exercises (e.g., kicking, swinging), anterior tibial glide persists on the tibial medial condyle, because its articular surface is longer than that on the lateral side. Prolonged anterior glide on the medial side produces external tibial rotation (of about 10 degrees), the screw-home mechanism.
Similarly, during closed chain movement (when the tibia is stabilized on the ground / surface) the femur rotates internally and medially during the last 20 degrees of extension.
If screw home is all about extension, what about the initial 20 degrees of flexion?
When the knee begins to flex (0° of extension to 20° of flexion), posterior tibial glide begins first on the longer medial condyle, and produces relative tibial internal rotation, the reverse screw home mechanism.
What is the Helfet’s test ?
The Helfet’s test is a maneuver to assess the screw home mechanism.
Ask the patient to sit with legs hanging free. Mark the center of the patella and the center of the tibial tuberosity. Ask the patient to extend the knee and note the position of these two points in full extension. Normally, the tibial tuberosity moves laterally demonstrating external rotation of tibia.
Clinical significance of Helfet’s test
Failure of the marked tibial tuberosity to move signifies ligamentous instability, meniscal damage or muscular imbalance.
To Summarize, screw home mechanism is the external rotation of tibia coupled with terminal 20 degrees of extension.
During the screw home mechanism of the knee which of the following takes place
a. Femur internally rotates and tibia externally rotates
b. Femur externally rotates and tibia internally rotates
c. Femur and tibia internally rotate
d. Femur and tibia externally rotate