The joint formed by the FEMUR, TIBIA and patella (knee-cap). Its cavity consists of three joints fused into one, separated in part by ligaments and folds of the synovial membrane. The ligaments which bind the bones together are extremely strong, and include the popliteal and the collateral ligaments; a very strong patellar ligament uniting the patella to the front of the tibia; two CRUCIATE LIGAMENTS in the interior of the joint; and two fibro-cartilages, which are interposed between the surfaces of tibia and femur at their edge. All these structures give the knee-joint great strength, so that it is seldom dislocated. The cruciate ligaments, although strong, sometimes rupture or stretch under severe physical stress such as in contact sports or athletics. Surgical repair may be required, followed by prolonged physiotherapy.

A troublesome condition often found in the knee – and common among athletes, footballers and other energetic sportspeople – consists of the loosening of one of the fibro-cartilages lying at the head of the tibia, especially the one on the inner side of the joint. The cartilage may either be loosened from its attachment and tends to slip beyond the edges of the bones, or it may become folded on itself. In either case, it may cause locking of the joint when sudden movements are made. This locking persists until the cartilage is replaced by forcible straightening, and the accident may be followed by an attack of SYNOVITIS which can last some weeks, causing lameness with pain and tenderness especially felt at a point on the inner side of the knee. This condition can be relieved by an operation – usually by MINIMALLY INVASIVE SURGERY (MIS)– to remove the loose portion of the cartilage. Patients whose knees are severely affected by osteoarthritis or rheumatoid arthritis causing pain and stiffness can now have the joint replaced with an artificial one. (See also ARTHROPLASTY; JOINTS, DISEASES OF.)