The knee joint is comprised of the distal end of the Femur (lower end of thigh bone) which articulates with the proximal part of Tibia (leg bone) both ends are covered with a layer of cartilage which provides smooth movement of joint surfaces against each other and act as shock absorber.
The patella is a triangular bone that covers the front of the knee joint making a joint itself with the distal end of Femur ( the patellofemoral Joint ). The posterior surface (back surface) of the patella is also covered by cartilage to ensure a smooth movement of the patella on the femur.
The knee joint is supported by strong ligaments: the Anterior Cruciate Ligament ACL, the Posterior Cruciate Ligament, Medial Collateral Ligament MCL and the Lateral Collateral Ligament LCL and is controlled by two group of muscles: the Quadriceps muscle which is situated on anterior (front) thigh and is responsible of extending (straightening) the joint and the Hamstring muscle which is located in posterior (back) thigh and is responsible of flexing (bending) the joint.
Between the femur and the tibia situated two cartilaginous discs: the medial meniscus and lateral meniscus. Both mensci act as cushions that absorbs shocks and provides a smooth movement between the joint surfaces.
Both menisci and the ligaments could be affected either by direct trauma or a twist at the knee joint which is common in athletes specially soccer players.
The Anterior Cruciate Ligament ACL function is to restrain anterior (forward) movement of the Tibia on the femur.
The Posterior Cruciate Ligament PCL is responsible of restraining posterior (backward) movement of Tibia on the Femur and it is twice as strong than the ACL which makes it less prone to injury compared to ACL.
The Medial Collateral ligament MCL passes on the medial (inner) aspect of the knee connecting the medial Femoral condyle to the upper medial aspect of the Tibia. Unlike the Lateral Collateral Ligament, part of the MCL is attached to the medial meniscus. MCL mainly supports the medial aspect of the knee resisting valgus stresses but also restrains lateral rotation of the Tibia and hyperextension.
The Lateral Collateral Ligament LCL runs on the lateral (outer) aspect of the knee and connects the lateral Femoral condyle to the head of Fibula bone. It supports the lateral aspects of the joint and restrains Varus stresses and helps also to control posterior translation and lateral rotation of the Tibia.