Screw Home Mechanism of The Knee Joint
Screw home mechanism (SHM) of knee joint is a critical mechanism that play an important role in terminal extension of the knee.
- There is an observable rotation of the knee during flexion and extension.
- This rotation is important for healthy movement of the knee.
- During the last 30 degrees of knee extension, the tibia (open chain) or femur (closed chain) must externally or internally rotate, respectively, about 10 degrees.
- This slight rotation is due to inequality of the articular surface of femur condyles.
- Rotation must occur to achieve full extension and then flexion from full extension..
Relevant Anatomyhinge joint with the main movement of flexion-extension. However, the radius and the length of the articular surface of the femur and tibia differ at the knee joint ie Articular surface of medial condyle of femur is greater than the articular surface of lateral condyle.
- As a result, a complex movement (includes "sliding") occurs during the last 30 degrees of knee extension, in addition to rolling between the two bones this allows the knee joint to move smoothly.
- At terminal extension, the knee joint is slightly hyperextended and stabilized with the tightening of the cruciate and collateral ligaments.
- As the length of the medial femoral condyle is longer than the length of the lateral condyle, the tibia rotates externally about 15° on the femur during the last 20° of extension.
- This kinematic phenomenon is well known, and it is called the screw-home movement
The shape of the condyles are not what brings about the movements.
- Tibial-on-femoral rotation occurs in an open chain exercise like in the leg extension machine (tibia externally rotates).
- Femoral-on-tibial rotation, as in a closed chain exercise like the squat (femur internally rotate).
- During knee extension, tibia rolls anteriorly, elongating the PCL and the PCL’s pull on tibia, causes it to glide anteriorly.
- During knee flexion tibia rolls posteriorly, elongating the ACL and it is the ACL’s pull on tibia, that causes it to glide posteriorly.
- Popliteus - unlocks knee with open chain motions
- Hip external rotation - unlocks knee with closed chain motions.
Restoration of terminal extension is an important goal of rehabilitation program. Terminal extension exercises and terminal extension mobilisations have important contributions on restoration of screw home movement /terminal extension. As does strengthening and stretching relevant muscles.
- Loss of full knee extension range of motion (ROM) is a frequent finding in the population with knee OA.
- The "screw-home" mechanism is considered to be a key element to knee stability for standing upright. The tibia rotates internally during the swing phase and externally during the stance phase.
- External rotation occurs during the terminal degrees of knee extension and results in tightening of both cruciate ligaments, which locks the knee.
- The tibia is then in the position of maximal stability with respect to the femur.
- In most of the knee problems generally there is an insufficiency on terminal extension and vastus medialis obliquus function.
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