Joints

The bones of the body come together to form joints. It is through these articulations that movement occurs. However, the type and extent of the movement possible depend on the structure and function of the joint; these latter can, and do, vary considerably. Nevertheless, the variation that exists in the form and function of the various joints of the body allows them to be grouped into well-defined classes: fibrous, cartilaginous and synovial, with the degree of mobility gradually increasing from fibrous to synovial.

Fibrous joints

Fibrous joints are of three types: suture, gomphosis and syndesmosis.

Suture(figure a)

This is a form of fibrous joint that exists between the bones of the skull. They permit no movement as the edges of the articulating bones are often highly serrated, as well as being united by an intermediate layer of fibrous tissue. Either side of this fibrous tissue the inner and outer periosteal layers of the bones are continuous, and in fact constitute the main bond between them.

The sutures are not permanent joints, as they usually become partially obliterated when age increases beyond 30 years.

Gomphosis(figure b)

In this form of fibrous joint a peg fits into a socket, being held in place by a fibrous ligament or band; the roots of the teeth being held within their sockets in the maxilla and mandible are such examples.

Syndesmosis(figure c)

In a syndesmosis the uniting fibrous tissue is greater in amount than in a suture, forming a ligament or an interosseous membrane. Examples in the adult are the inferior tibiofibular joint where the two bones are joined together by an interosseus ligament, and the interosseus membrane between the radius and ulna. Flexibility of the membrane or twisting and stretching of the ligament permit movement at the joint. However, the movement allowed is restricted and controlled.

Cartilaginous joints

In cartilaginous joints the two bones are united by a continuous pad of cartilage. There are two types of such joint, primary and secondary cartilaginous(synchondrosis and symphysis respectively).

Primary cartilaginous(figure a)

Between the ends of the bone involved in the joint is a continuous layer of hyaline cartilage. These joints occur at the epiphyseal growth plates of growing and developing bone, and obviously become obliterated with fusion of the two parts(diaphysis and epiphysis). Because the plate of hyaline cartilage is relatively rigid, such joints exhibit no movement. However there is one such joint in the adult, which is slightly modified because, by virtue of its structure, it enables slight movement to occur. This is the first sternocostal joint.

Secondary cartilaginous(figure b)

These joints occur in the midline of the body and are slightly more specialized. Moreover, their structure enables a small amount of controlled movement to take place. Hyaline cartilage covers the articular surfaces of the bones involved in the joint, but interposed between these hyaline coverings is a pad of fibrocartilage. Examples are the joints between the bodies of adjacent vertebrae, where the fibrocartilaginous pad is in fact the intervertebral disc, and the joint between the two bodies of the pubic bones.

Synovial joints

Synovial joints are a class of freely mobile joints, with movement being limited by muscles, ligaments and the associated joint capsules. The majority of the joints of the limbs are synovial. In synovial joints are articular surfaces of the bones involved are covered with articular(hyaline) cartilage, which because of its hardness and smoothness enable the bones to move against each other with minimum friction. Passing between the two bones, either attaching at or away from the articular margins, is a fibrous articular capsule. The capsule my be strengthened by the blending of ligaments or the deeper parts of muscles crossing the joint. Lining the deep surface of the capsule is the synovial membrane, which covers all the surfaces within the capsule except the articular cartilage(figure a). The synovial membrane secretes synovial fluid into the joint space(cavity) enclosed by the capsule, and serves to lubricate and nourish the articular cartilage as well as the opposing joint surfaces. During movement the joint surfaces either glide or roll past each other.

If the bones involved in the articulation originally ossified in membrane, then the articular cartilage has a large fibrous element. In addition there is enclosed within the capsule an intra-articular disc, which may not be complete(figure b).

Bursae are often associated with synovial joints, sometimes communicating directly with the joint space.

Because of the large number of synovial joints within the body and their differing forms they can be subdivided according to the shape of their articular surfaces and the movements possible at the joint.

Plane joint

The joint surfaces are flat, or at least relatively flat, and of approximately equal extent. The movement possible is of a single gliding type or a twisting of one bone against the other, usually within narrow limits. An example is the acromioclavicular joint.

Saddle joint

The two surfaces are reciprocally concavoconvex, as a rider sitting on a saddle. The principal movements possible at the joint occur about two mutually perpendicular axes. However, because of the nature of the joint surfaces there is usually a small amount of movement about a third axis. The best example in the body is the carpometacarpal joint of the thumb.

Hinge joint

The surfaces are so arranged to allow movement about one axis only. Consequently, the “fit” of the two articular surfaces is usually good, but in addition the joint is supported by strong collateral ligaments. The elbow is a typical hinge joint. The knee joint is considered to be a modified hinge joint, as it permits some movement about a second axis. In this case the movement is possible because of the poor fit of the articular surfaces.

Pivot joint

Again movement occurs about a single axis, with the articular surfaces arranged so that one bone rotates within a fibro-osseous ring. The atlantoaxial joint is a good example of a pivot joint.

Ball and socket joint

As the name suggests the “ball” of one bone fits into the “socket” of the other. This type of joint allows movement about three principal mutually perpendicular axes. The hip joint is a ball and socket joint.

Condyloid joint

This is a modified form of a ball and socket joint, which only allows active movement to occur about two perpendicular axes. However, passive movement may occur about the third axis. The metacarpophalangeal joints are examples of such joints.

Ellipsoid joint

This is another form of a ball and socket joint, although in this case the surfaces are ellipsoid in nature. Consequently, movement only occurs about two perpendicular axes. The radiocarpal joint is an ellipsoid joint.