Our skeleton is made up of bones which are rigid, inflexible objects that require help to allow us to move. This aid comes in the form of joints at the point where bones make contact.

The joint operates by connecting the bones with elastic band-type chords, called ligaments, which keep them in place. A protective coating, called cartilage, covers the bone surface at the joint to prevent bones from rubbing directly against each other. The joint is surrounded by a capsule also, and the space within the joint (joint cavity) contains a liquid, called synovial fluid, which provides nutrients to the joint and cartilage. It is produced by the synovial membrane (or synovium) which lines the joint cavity.

Movement of the joint is operated by the muscles attached to the bones as they lengthen and shorten. For example, when the bicep shortens and the tricep lengthens the lower part of the arm is pulled upwards towards the shoulder.

What goes wrong with joints varies from one kind of arthritis to another.