Articulations or Joints

A joint is a junction between two or more bones. Joints may be classified as immovable, slightly movable, and freely movable; but perhaps it is more helpfu to consider them as:

• The skull type: Immovable; or temporary joint.
• The vertebral type: Slightly movable; or secure joint.
• The limb type: Freely movable; insecure; or synovial joint.

The Skull Type

The skull type is either a suture or a synchondrosis depending on whether the bones concerned ossify in membrane or in cartilage; it is immovable (fig. 20).

Fig 20: A suture and a synchondrosis

Suture

When the growing edges of two bones (or ossific centers) developing in membrane come together, a residual film of membrane may persist unossified between them for some years, till middle age, or indefinitely. Such a union is called a suture; and sutures are confined to the skull (fig. 20). The edges may interlock in jig-saw fashion, or like the teeth of a saw. They may be bevelled and overlapping or relatively flat and abutting. A ridge may fit into a groove as between sphenoid and vomer, or a tooth may fit into a socket.

Synchondrosis

Similarly when the growing edges of two bones (or ossific centers) developing in a single mass of cartilage come together, a residual plate of carilage may persist unossified between them for a number of years. Such a union is a synchondrosis.

Sites

Synchondroses occur at the base of skull between the basi-occipital and basi-sphenoid, sphenoid and ethmoid, petrous-temporal and basi-occipital, and petrous-temporal and jugular process. Their distribution however is widespread, for is it not by synchondroses (epiphyseal plates) that epiphyses are united to bones? (fig. 13)

Synostosis

Synostosis is the obliteration of a suture or a synchondrosis by bone. It is associated with cessation of growth locally.

The Vertebral Type

The vertebral type is either a symphysis or a syndesmosis. Intervertebral joints are built for strenth and security; so, their opposed bony surfacesare firmly bound together. This mnimises the risk of dislocation, which here would be disastrous; it also greatly restricts mobility.

Symphysis

A symphysis is a joint where two opposed bony surfaces are coated with hyaline cartilage, are united by fibrocartilage, and are further united in front and behind by ligamentous bands. There is no joint cavity, but a small cleft may be present. Symphyses are not directly controlled by muscles.

Sites

Symphyses occur between (a) the bodies of vertebrae, (b) between the pubic bones, and (c) between the manubrium and body of the sternum. They are all situated in the median plane.

Symphysis between two vertebral bodies

The body of a vertebra is a miniature long bone. Each epiphysis, the upper and the lower, is a peripheral bony ring (fig. 22), surrounding a central unossified plate of hyaline carilage.

Fig 21: Scheme of an intervertebral disc

Fig 22: A vertebra in childhood

Intervertebral discs

Adjacent bodies are united by a fibro-cartilaginous disc whose peripheral part is composed of about a dozen concentric layers of fibers, the annulus fibrosus. The fibers in alternate layers cross like the fibers of the External and Internal Intercostal muscles. The center of the disc is filled with a fibrogelatinous pulp, the nucleus pulposis, which acts as a cushion or shock absorber and, being under pressure, bulges when the disc is cut accross.

Longitudinal ligaments of the bodies

Longitudinal ligaments of the bodies, an anterior and a posterior, extend from sacrum to basi-occiput: the one is attached to the intervertebral discs and adjacent margins of the vertebralbodies anteriorly; the other is attached to them posteriorly, i.e., within the vertebral canal. The anterior ligament is a broad strong band, except above, where it becomes a cord that gains attachment to the anterior tubercle of the atlas and extends above it to the basi-occiput. The posterior ligament is weak and narrow, but it widens in a denticulate manner at its attachments to the discs. Its upper end is the membrana tectoria (p. 718).

Vessels and nerves

Small blood vessels from the marrow spaces pass through the hyaline plate to supply the disc until the 8th year, and some of these may persist until the 20th or 30th year. (Coventry et al.; and others). Branches of the spinal nerves have been traced to the longitudinal ligaments and to the annulus (Roofe; and others).

Note

(a) The hyaline plate is apt to crack and the nucleus pulposus to herniate through it into the cancellous body of the vertebra; and (b) the pulp may burst through the annulus fibrosus posteriorly, where it is thinnest, and protrude under cover of the posterior longitudinal ligament. These accidents happen commonly.

Syndesmosis

A syndesmosis is a union by ligamentous fibers, the bony points united being some distance apart.

Sites

Syndesmoses occur between the vertebral arches, and between the lever-like processes of the bertebrae, also between coracoid and clavicle (coracoclavicular ligament), and between the bones of the forearm and of the leg (interosseous membranes) including the inferior tibio-fibular joint.

Vertebral syndesmoses

The laminae of adjacent vertebrae are united by yellow elastic bands, called the ligamenta flava. These broad bands unite the upper border and posterior surface of one lamina to the lower border and anterior surface of the lamina above. In virtue of their elasticity, the ligamenta flava serve as “muscle sparers”, i.e. they assist in the recovery to the erect posture after bending forwards and they are particularly strong in the lumbar region.

The adjacent borders of the spinous processes are united by weak interspinous ligaments and their tips are united by stronger supraspinous ligaments, which in the neck become the ligamentum nuchae (p. 616). The transverse processes may be connected by weak intertransverse ligaments.

The articular processes of the vertebrae are united by articular capsules to form Joints of the Limb Type. These have synovial folds and fat-pads. In the lumbar region lateral extensions of the plate-like ligg. flava close the joint cavities anteriorly.

So, typical vertebrae are united to each other by 3 types of joints: symphyses, syndesmoses, and synovial joints.

The Limb Type

The limb type is known as a Synovial Joint or Articulation. The limbs being primarily organs of locomotion have joints that permit free movement; so, the site which in a symphysis is occupied by fibro-cartilage becomes a synovial joint cavity. This makes for insecurity.

At the limb type of joint the ends of two (or more) bones are capped with hyaline cartilage and are united by an articular capsule (fig. 23). This consists of a short sleeve of fibrous tissue, called the fibrous capsule, which extends well beyond the articular (hyaline) cartilage, and it is lined with an inner sleeve of synovial membrane (the synovial capsule). The synovial membrane is reflected from the fibrous capsule on to the bone, and this it covers right up to the articular cartilage.