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joints ‘…the thigh bone's connected to the knee bone, the knee bone's connected to the leg bone … O hear the word of the Lord.’ … or marvel at the evolution of the superb engineering and biological ‘servicing’ throughout a human lifetime whereby these connections are combined with mobility, and the moving parts kept intact and well ‘oiled’ — if we are lucky.

Joints evolved in animal bodies along with skeletons, whether external (insects) or internal (vertebrates). Clearly a bony core would not be compatible with movement without joints, not only for running, climbing, and flying but also for the limbless propulsion of snakes and fish.

A study of the bare bones of the human skeleton reveals the shape of the surfaces of the bones where they move on each other at joints, and gives some indication of the direction and range of movement which those shapes will permit. Also, in the living joint, the structures that connect the one to the other vary in being appropriately loose or tight to allow or restrict movement. The movement itself also depends on the arrangement of the muscles and of their tendons which span the joint.

Joints first appear during the sixth week of fetal life, and develop from embryonic mesoderm, the precursor of connective tissue. By the ninth week discernable joints are present, which by the twelfth week are well developed and can be moved by muscle contractions. By the sixteenth week such fetal movements are just perceptible to the mother.

This account will describe the features common to all joints in the body, the different types in mechanical terms, the tissues that form them, and the means whereby their living structure is maintained and repaired. As well as the examples which illustrate these features, details about the main individual joints and the muscles which control movement at them can be found under their own names.

General features of joints

Freely movable joints are referred to as synovial joints. The whole joint region is enclosed by a ‘capsule’ of connective tissue, which is firmly attached to the bones beyond the surfaces involved in the joint itself. The opposing surfaces of bone have a covering of cartilage, a tough material but with more resilience than bone itself, which acts as a ‘shock absorber’. (This cartilage is different from the various specially shaped ‘cartilages’ which are present, for example, in the knee). In most joints the lubrication necessary for these covered surfaces to move smoothly against each other is provided by synovial fluid, so called because it resembles egg white (derived from ‘syn’ meaning ‘with’ and ‘ovium’ meaning ‘egg’). This is secreted from a thin lining layer on the internal surface of the capsule. This forms the completely enclosed ‘cavity’ of the joint — though cavity is not quite the right word, since it is really a potential space lined by a thin film of synovial fluid. This fluid, which is derived from the blood flowing to the joint, has another important function of providing nutrition to cartilage, which is devoid of blood vessels. When the joint is injured or inflamed excess synovial fluid may be formed (an effusion), leading to swelling of the joint.

The joint capsule contains sensory receptors which, when appropriately stimulated, give rise to nervous impulses which the brain interprets as a sensation. Pain is an important sensation which is associated with most forms of joint injury or disease (arthritis). Although normally pain plays an important protective role to warn of tissue injury or damage, it is debilitating in chronic arthritis. A less obvious but ever-present sensation derived from joints is the ‘sixth’ sense called proprioception — the conscious awareness of joint position and movement. This sensory feedback is vitally important in the development of the control of the musculature which is reflected in the smoothness and co-ordination of movements.

Types of joint

A few personal observations reveal the different mechanical nature of the main joints of the limbs: elbows and knees have no side-to-side movement and a range approaching a half-circle; these and the finger and toe joints work like hinges. The restrictions are imposed mainly by the shape of the bones at the elbow, and by ligamentous connections between them at the knee. At the shoulder, by contrast, the arm can be wielded in all directions, implying something like a ‘ball-and-socket’ — though the socket has to be shallower than that in a car-to-trailer joint which allows movement only in the horizontal plane. This arrangement makes the shoulder more liable than any other joint to dislocation — the ball at the top of the humerus slipping out of its socket on the scapula (shoulder blade). The hips also have a ball and socket structure, with a somewhat deeper socket in the ilium (pelvic or hip bone) for the head of the femur (thigh bone). The wrists have an intermediate freedom of movement; the ‘hinging’ is wider in range than side-to-side movement. The ankles are essentially hinge joints; the side-to-side movement of the foot relative to the leg is achieved at joints beyond the ankle joint itself.

In both the hands and the feet there are joints among the small bones of the carpus (beyond the wrist joint) and tarsus (beyond the ankle joint), and between these and the five metacarpals and five metatarsals, each of which is jointed to the bones of the digits (fingers and toes) themselves. Within each digit, there are a further two joints between the three bones (phalanges) — crucial of course in the hand for fine manual skills, but of only vestigial importance in the toes.

Some individuals have a larger than normal range of movement at their joints and are sometimes referred to as being ‘double-jointed’. Of course, this is not literally the case, it is simply that these individuals have ‘looser’ joints. This is more properly called joint hypermobility, and some individuals with this condition find employment as contortionists.

Movements at many other joints are less easy to observe, though at least as important for posture and movement. Each of the separate bones of the spine (the vertebrae) has joints with its neighbours above and below, allowing bending and rotation; at the top end, the highest of the cervical (neck) vertebrae (the ‘atlas’ — compare he who supported the world) has joints with the skull which support it and allow nodding movement; also the head can turn from side to side because this ‘atlas’ forms a rotating joint with an upward protuberance on the next below — the ‘axis’ vertebra. Near the lower end, the sacro–iliac joints link the spinal column to the pelvis — in this instance, forming a relatively rigid connection rather than allowing a range of movement. Since the two iliac bones are bound strongly together at the centre front, the bony pelvis (sacrum and iliac bones) can only be tilted as a whole by movement at the hip joints and by bending the spine. There are thus strong links between the lower limbs, via the hip joints, to the spinal column — very necessary for upright posture and movement. The upper limbs by contrast are not linked by joints to the spine. The shoulder ‘girdle’ from which the arm is suspended, incompletely encircles the upper torso and consists of clavicles (collar bones) jointed to the sternum (breast bone) at the front, and scapulae (shoulder blades) which are relatively freely mobile.

There are joints in the chest which are crucial for breathing movements. The ‘rib cage’ is lifted and enlarged by muscle action when we breathe in, assisting the filling of the lungs. It can change shape in this way because of the small joints which link the back end of every rib to the vertebral column, and the front end of all but the lowest ribs to the sternum.

Problems with joints

Minor injuries are often called ‘sprains’: this may involve tearing of some fibres of the joint capsule or ligaments; there may ‘be swelling’ due to soft tissue damage outside the joint itself or to an increase in the production of synovial fluid inside the cavity (effusion).

Dislocation occurs when the bones become displaced from their proper position in the joint, due to a strong jolt which forces an unnatural direction of movement. Weakness of the relevant muscles can make it more liable to happen. Shoulders, fingers, and toes are the most liable to this type of injury, and the displacement can usually be reversed without serious damage. Dislocations in other joints, and ones which are more serious, can happen along with fractures of the limb bones.

Displacement of other components of joints include the common sports injury to the knee, when the flat horseshoe-shaped cartilages which lie on the upper end of the tibia slip out of place or are torn. More seriously in the knee, the cruciate ligaments can be damaged; these join the tibia to the femur, at angles which prevent over-extension (forward bending), so the knee becomes unstable if they are ineffective. Then there is the notorious ‘slipped disc’ in the spine: the joint between the main bony bodies of adjacent vertebrae consists of an ‘intervertebral disc’ which has a soft but not liquid centre. ‘Slipping’ means that this soft substance bursts through the fibrous membrane around it, protruding to where it can irritate a nearby nerve root where it leaves the spinal canal, leading to pain being felt in the territory innervated by that root. The pain of ‘sciatica’ can be felt in the leg or even the foot, although the actual site of nerve irritation is close to the spine.

Arthritis is a general term for different types of disorders affecting joints. The commonest, osteoarthritis, often referred to as being a ‘degenerative’ condition is associated with wearing down of the cartilage, particularly of the weight-bearing joints, which becomes progressively more common with increasing age. Other types include rheumatoid arthritis which is an autoimmune disease process affecting any joint, and which occurs in younger individuals but has a long time course. It is often associated with severe deformity of the hands. Infections with bacteria or viruses can give rise to an acute inflammation of a joint (septic arthritis).

When joints are badly damaged and painful, usually as a result of chronic disease, a very successful treatment performed by orthopaedic surgeons involves complete replacement of the joint with an artificial one (arthroplasty). However, these never last as long as the joints we are born with, and further, more difficult, operations (revisions) may be necessary.

William R. Ferrell, and Sheila Jennett

See also ankle; elbow; hip; knee; spinal column; wrist.