A joint's function is to bear weight, perform work and exhibit a particular range of motion during movement where two or more bones come together for the purpose of movement. A joint moves when the muscles crossing it contract.
Joints are classified based on how much motion they allow. Joints that do not move are known as synarthrosis. Examples of these include sutures of the skull and the gomphoses that connect the teeth to the skull. Joints that permit slight movement are classified as amphiarthrosis. These include the intervertebral disks of the spine and the pubic symphysis of the hips. Diarthroses joints have the largest range of movement and include joints in the shoulder, knees, wrists and elbows.
In addition to their range of movement, joints may be classified based on the material present in them. These include fibrous, synovial, which is the most common joint, and cartilaginous. Several types of synovial joints exist. They are ball and socket, hinge, saddle and gliding. Each type of synovial joint has a different range of motion. Ball and socket joints have the greatest range of motion. Hinge joints move in one direction only. Saddle joints have a 360 degree range of motion, and gliding joints allow bone movement in any direction.