20.3:
Joints
In the human body, joints are points of articulation, locations where bones meet.
There are three main types. The first, fibrous joints, are stable and allow for little to no mobility. For instance, the sutures of the skull are held together by fibrous connective tissue and do not allow movement between adjacent bones.
The second kind, cartilaginous joints, are bones connected through fibrocartilage, which makes them strong and flexible to allow for bending motions, for example, between each vertebrae of the intervertebral disc.
The third and most common group, synovial joints, allow for the greatest mobility. In these joints, the bony surfaces are not directly connected. Rather, they are covered by a thin layer of cartilage and surrounded by an articular capsule that is filled with synovial fluid to provide lubrication.
One example is the glenohumeral joint of the shoulder, also known as a ball and socket joint. The rounded part, the head of the humerus, is located within the glenoid cavity, the socket.
The surrounding capsule is then supported by ligaments and rotator cuff muscles to yield a wide range of motion in all directions, to enable reaching an object.
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.
Suggested Reading
Gasbarro, Gregory, Benjamin Bondow, and Richard Debski. “Clinical Anatomy and Stabilizers of the Glenohumeral Joint.” Annals of Joint 2, no. 10 (October 26, 2017). [Source]