How do joints work?
Joints are movable connections between two bones. Each joint is made up of the surfaces of the bones involved, a joint cavity and a joint capsule.
The joint surfaces (articular surfaces) of the bones are covered with a layer of cartilage. This layer is between 0.2 and 0.5 mm thick, and up to 6 mm thick on the kneecap. It provides a smooth surface to stop the bones in the joint from rubbing against each other too much. The cartilage passes the pressure in the joint on to the bone underneath it. This is especially important in joints that bear heavier loads, such as hip or knee joints.