ANATOMIC CONSIDERATIONS

Synovial joints are classified according to axis of movement, as follows:

Gliding or plane joint (e.g., articular processes of the vertebrae): permits a sliding of one surface on the other. Joint motion is limited by ligaments and surrounding structures.

Hinge joint (e.g., knee): allows movement in only one plane. The movements of this joint are flexion and extension.

Pivot joint (e.g., radial head): permits rotational movement around a pivot in one axis.

Ellipsoidal or condylar joint (e.g., wrist joint): an oval head or condyle articulates with an elliptic cavity. This joint exhibits movements of flexion, extension, adduction, abduction, and circumduction. An ellipsoidal joint is incapable of axial rotation.

Saddle joint (e.g., carpometacarpal joint of the thumb): similar to ellipsoidal or condyloid joints in its movements. Structurally, its articular surfaces are convex in one direction and concave in the other, at right angles to each other.

Ball-and-socket joint (e.g., hip joint): formed by a concave socket that receives a ball-shaped head. It permits movement in three axes—flexion, extension, adduction, abduction, and rotation.

A summary of the various individual synovial joints and their types and movements is presented in Table 23-1.

Synovial joints take their name from the fluid contained within the joint space (Fig. 23-2). Synovial fluid is a clear viscous fluid that serves primarily as a lubricant to facilitate joint movement. This fluid, along with the specialized articular surfaces and intra-articular structures (the menisci, disks, and fat pads), allows for almost frictionless movement of the joint surfaces.

Synovial fluid resembles the white of an egg in consistency, and it nourishes the hyaline cartilage lining the articular surfaces. The synovial fluid is produced by the synovial membrane, the inner lining of the joint capsule.

The synovial joint space is enclosed in a fibrous layer called the joint capsule. The fibers composing the joint capsule are arranged in irregular bundles that make them sensitive to any tension from the joint. Nerve endings are also located within the joint capsule; they pass impulses to the spinal cord and brain to transmit information regarding position and movement of the joint. The joint capsule attaches to the articulating bones just beyond the joint space uniting the bones of the joint.

The inner surface of the fibrous joint capsule is lined with a connective tissue called the synovial membrane. This membrane covers all the structures within the joint except the hyaline articular cartilage, menisci, and intra-articular disks. It produces the synovial fluid that lubricates the joint surfaces.

Hyaline articular cartilage is located on the bearing surfaces of the bones composing the joint. It does not contain nerve endings or blood vessels.

The intra-articular joint structures include the menisci, fat pads, synovial folds, and intra-articular disks. These intra-articular structures aid in providing efficient lubrication of the articular surfaces.

Ligaments may be present within the joint space as well as in the joint capsule. The accessory ligaments limit motion in undesirable directions and function as sense organs of motion and position.

Because joints constitute a mechanical system, wear and tear can be expected even though friction has been greatly reduced, and this usually results in the destruction of the hyaline articular cartilage. Other factors culminating in deterioration of the joints are trauma and disease.