Posterior cruciate ligament (PCL): The knee joint is surrounded by a joint capsule with ligaments strapping the inside and outside of the joint (collateral ligaments) as well as crossing within the joint (cruciate ligaments). These ligaments provide stability and strength to the knee joint.
The posterior cruciate ligament (cruciate means shaped like a cross) crosses behind the anterior cruciate ligament within the joint, as indicated here:
The knee is a joint which has three parts. The thigh bone (femur) meets the large shin bone (tibia) forming the main knee joint. This joint has an inner (medial) and an outer (lateral) compartment. The kneecap (patella) and the femur form a third joint, called the patellofemoral joint.
The meniscus is a c-shaped cartilage pad between the two joints formed by the femur and tibia. The meniscus acts as a smooth surface for the joint to move on. The knee joint is surrounded by fluid-filled sacs called bursae, which serve as gliding surfaces that reduce friction of the tendons. Below the kneecap, there is a large tendon (patellar tendon) which attaches to the front of the tibia bone. There are large blood vessels passing through the area behind the knee (referred to as the popliteal space).
The large muscles of the thigh move the knee. In the front of the thigh the quadriceps muscles extend the knee joint. In the back of the thigh, the hamstring muscles flex the knee. The knee also rotates slightly under guidance of specific muscles of the thigh.
The knee functions to allow movement of the leg and is critical to normal walking. The knee flexes (bends) normally to a maximum of 135 degrees and extends (straightens) to 0 degrees. The bursae, or fluid-filled sacs, serve as gliding surfaces for the tendons to reduce the force of friction as these tendons move. The knee is a weight-bearing joint. Each meniscus serves to evenly load the surface during weight- bearing and also adds in disbursing joint fluid for joint lubrication.