20.3: Joints
Joints, also called articulations or articular surfaces, are points at which ligaments or other tissues connect adjacent bones. Joints permit movement and stability, and can be classified based on their structure or function.
Structural joint classifications are based on the material that makes up the joint as well as whether or not the joint contains a space between the bones. Joints are structurally classified as fibrous, cartilaginous, or synovial.
Fibrous Joints Are Immovable
The bones of a fibrous joint are connected by fibrous tissue and have no space, or cavity, between them. Thus, fibrous joints cannot move. Although the skull appears to be a single large bone, it includes several bones that are connected by fibrous joints called sutures. Syndesmoses, the second type of fibrous joint found in the fibula (calf bone), allow more movement than sutures. The third type of fibrous joint, gomphoses, connect teeth to their sockets.
Cartilaginous Joints Permit Modest Movements
The bones of cartilaginous joints are connected by cartilage and permit minimal movement. The two types of cartilaginous joints, synchondroses and symphyses, differ in the kind of cartilage that connects the bones (hyaline cartilage and fibrocartilage, respectively). The epiphyseal plates in the bones of growing children contain synchondroses. Symphyses connect vertebrae as well as the pubic bones.
Synovial Joints Enable the Greatest Range of Movement
Synovial joints, found in the elbows, shoulders, knees, and elsewhere, are the most common type of joint and allow the greatest range of movement. They are also the weakest joints and the only joints that contain a cavity between adjoining bones. The envelope of connective tissue that surrounds this synovial cavity is called an articular (or joint) capsule and contains fluid that reduces friction between bones.
