knee

Joint between the thigh and lower leg, involving articular surfaces on the femur (thigh bone), tibia (shin-bone), and patella (knee-cap). The movement of the knee is in general that of a hinge-joint, although some rotatory movement is possible when the joint is moderately flexed.

The articular surface of the femur comprises the greater part of the surface of its condyles (knuckles), which are separated by a deep notch, while faint transverse grooves show the limit of their articulation with the patella. The articular surface of the tibia is deepened by two cartilages, one on each side, known as the semi-lunar cartilages. The patella is a heart-shaped bone; the wide upper part is concave and smooth, and is divided into two articular portions by a rounded ridge; the lower part is rough and non-articular. The joint is nearly surrounded by a series of ligaments of complex structure and movement. The ligaments are lined by a synovial (fluid-secreting) membrane, which is the largest in the body. In humans the mechanism of the joint is particularly adapted to maintaining the erect posture, which involves extension of the joint.

Injuries and diseases The knee is subject to the same injuries and diseases as other joints (see arthritis; dislocation; rheumatism; sprain; tuberculosis), but dislocation is not common. The patella is liable to fracture, but is rarely dislocated. The most frequent knee injury, and one to which athletes are particularly prone, is damage to one of the semi-lunar cartilages, most often the internal one. The cartilage is split and displaced, and the loose pieces interfere with the joint movement and cause sudden locking from time to time. The treatment is surgical removal of the damaged cartilage.