The knee is a complex joint made up of bones, muscles, tendons, ligaments, and cartilage. It may be described as a hinge joint, similar to the hinge on a door. The knee not only bends back and forth but also has a complex rotational component that occurs with flexion and extension.

Each knee has two crescent-shaped menisci, or cushion pads. The lateral meniscus lies at the outer side of the knee and the medial meniscus lies at the inner side of the knee. The strong but flexible menisci act as shock absorbers between the femur (upper-thigh bone) and the tibia (shin bone), especially during weight-bearing activities such as walking or stair climbing. The menisci also stabilize and evenly distribute your body weight across the knee joint.

The ACL is one of a pair of crucial ligaments that forms a cross in the center of the knee joint. Both the ACL and the PCL function to stabilize the knee from front to back. The ACL limits the forward movement of the tibia, and the PCL limits the backward movement of this bone. Additional ligaments provide stability to other movements at the joint, including angulation (left/right motion) and rotation. Without such stability, you may feel that your knee is simply giving out from under you.