hinge joint that is formed by the meeting of the thigh bone ( femur) and the larger bone ( tibia) of the lower leg. The knee is the largest joint in the body and has to sustain the greatest stresses, since it supports the entire weight of the body above it. Consequently, the rounded ends, or condyles, of the femur and tibia that meet at the knee are massive. The rounded ends of the tibia move forward and backward on the corresponding ends of the femur; the kneecap, or patella, rests upon the ends of the femur and serves to prevent the tibia from moving too far forward when the leg is bent. The articulating (meeting) surfaces of the femur and tibia condyles are very smooth and are separated by a slight gap. The femur and the tibia are held together at the joint by a complex system of ligaments that run from the condyles of one bone to the condyles of the other. The two bones' possible contact with each other is cushioned by a synovial membrane and by layers of cartilage on the surface of each condyle. The entire knee joint, including the kneecap, is enveloped in a capsular apparatus that is large enough to allow for the movement of the tibia and also allows the kneecap to swing up and down freely on the front surface of the femur.

The quadriceps muscle (quadriceps femoris muscle) of the thigh causes knee extension (straightening of the leg), while a number of other upper leg muscles cause the complementary motion, flexion, or bending, of the leg. Some rotation of the lower leg is also possible when the knee is bent, but the tightening of strong lateral ligaments at the joint prevents rotation when the leg is straight. Although well-adapted for the downward transmission of the body's weight, the structure of the knee itself offers little resistance to the lateral displacement of the femur and tibia condyles during motion, so the knee's stability depends on the strength of the surrounding ligaments and muscles. Most common knee injuries, including bone dislocations and torn cartilage, reflect the susceptibility of the knee joint to the lateral displacement of its bones.