The knee joint is composed of the top of the tibia (shin bone) and the bottom of the femur (thigh bone). The patella (kneecap) is also an integral part of the knee joint. Where these three bones contact each other there is a layer of very smooth articular cartilage lining.
These bones are held together by ligaments. Ligaments essentially function as living pieces of rope. They prevent the movement of the bones in relation to one another.