A normal knee joint consists of three bones: the lower end of the thigh bone (the femur), the upper end of the shin (the tibia) and the knee cap (patella). The surface of the bones is protected by cartilage. The joint cavity is surrounded by the synovial membrane or synovium, and the space is filled with a lubricant called synovial fluid. In an undamaged knee joint this synovial capsule does not contain blood.
