Screw home mechanism
The knees have a "Screw Home" rotation that allows for full knee
extension and flexion. There is an observable rotation of the knee during
flexion and extension. 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. That might not seem all that
significant, but this rotation is important for healthy movement of the
knee. Whether it is tibial-on-femoral rotation, as in an open chain
exercise like in the leg extension machine, or femoral-on-tibial rotation,
as in a closed chain exercise like the squat, rotation must occur to
achieve full extension and then flexion from full extension.
This is due to factors including:
The shape of the medial femoral condyle. The condyles on the
femur are shaped a bit differently than those elsewhere in the
body. According to research, the articular surface (the end of the
bone where two bones join to allow movement) on the medial
femoral condyle has a 30 degree lateral curve. This curvature
allows the tibia and femur to follow this curve with tibial-on-
femoral rotation as well as femoral-on-tibial rotation.
The lateral pull of the quadriceps is a factor in the lateral rotation
during the later phases of extension. (The quadriceps insert into the
tibial tuberosity via the patellar tendon). When contracted, the
quadriceps cause an anterior translation of the tibia on the
femur. This creates a passive tension in the anterior cruciate
ligament (ACL) which contributes to external rotation. This
passive tension helps stabilize the extended knee by resisting
excessive anterior translation of the tibia or excessive posterior
translation of the femur.
In order to unlock the knee from extension,
the popliteus muscle must work to initiate
internal or external rotation. The tibia must
internally rotate slightly to allow for knee
flexion in an open chain, and the femur must
externally rotate for knee flexion in a closed
chain. This mechanism of rotation
contributes to proper movement at the knee.
Clinical significance
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.
In most of the knee problems generally there is an insufficiency
on terminal extension and vastus medialis obliquus function.
Loss of full knee extension range of motion (ROM) is a frequent
finding in the population with knee OA. Such loss of normal
terminal knee extension may have important effects on knee
mechanics during walking and standing.
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.
PT Orthopedic
Assignment: screw home mechanism
Dr. Labib Mousa
Name : Moaaz Abdallah Goda
ID: 4440
