The Children's Hospital Boston describes a joint as the point where two bones meet. Components of joints include a sheet of tissue called the synovial membrane, which seals the joint and encompasses synovial fluid; ligaments, which encompass the joint and limit movement; tendons, which attach muscles to bone; and cartilage. The body has three main types of joints: synarthroses, amphiarthroses and diarthroses.
Synarthroses
Synarthroses are joints where there is no movement between the adjoining bones. The bones of synarthroses joints are in very close proximity to each other, separated by only a small layer of connective tissue. An example of a synarthrosis joint is the area where the bone plates meet in the skull. The skull is made up of separate bones so that the skull can grow in size as the head of a child grows. However, when fully grown, the joints where the skull bones of the head meet are immovable so as to provide maximum protection to the brain.
Amphiarthroses
Amphiarthroses are joints that demonstrate slight but limited movement. This type of joint is characterized by bones that are separated by a type of cartilage called hyaline cartilage, which is a very fibrous type of connective tissue. One type of amphiarthrosis joint is where the ribs are connected to the sternum.
Diarthroses
Most of the joints of the body are diarthroses, which are freely movable joints. One sub-type of diarthroses is a ball-and-socket joint, where the ball-shaped end of one bone fits into a cup shape formed by the other bone. The shoulder joint is a ball-and-socket joint. Another sub-type of diarthroses is the pivot joint, which enables bones to rotate; the neck is a pivot joint. Hinge joints, another sub-type of diarthroses, allow the bones to bend and straighten but not move sideways. Examples of hinge joints are fingers, toes, knees, and elbows. The only saddle joint in the body is the thumb, in which the bones that meet have curved regions that complement each other. A saddle joint allows a very wide array of movements in different directions. Gliding joints involve two flat, or nearly flat, bones that slide along each other. The carpal bones in the wrist and the tarsals in the ankles are gliding joints.
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