The knee is one of the complex joints in the body. It connects the thighbone (femur) and the shinbone (tibia). The knee joint is actually two joints. The major joint is between the thigh bone (femur) and the shin bone (tibia). The smaller joint is between the kneecap (patella) and the thigh bone (femur).
A smooth, tough tissue called articular cartilage covers the ends of the bones, allowing them to slide smoothly over each other. The synovial membrane is a bag that covers the surfaces of the knee joint and produces synovial fluid that lubricates the joint. If the articular cartilage becomes damaged, the ends of the bones rub together, causing pain and difficulty in moving the knee joint. A severely damaged joint may become stiff, and difficult to move.