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. 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.
The thickness of the cartilage is different in different joints. For instance, it is between 0.2 and 0.7 mm thick in the interphalangeal joints (in the middle of your fingers), and up to 6 mm thick on the kneecap.