The shoulder, elbow, hip, knee and ankle are all synovial joints lined by synovium, which secretes synovial fluid. The articular surfaces are covered with smooth hyaline articular cartilage and movement is determined by the shape of the bones, ligaments, surrounding soft tissues and the joint capsule, which contains the proprioceptors which form the afferent segment of postural reflexes.

Synovial joints can be classified according to shape and the movement that occurs between the bones. Ball and socket joints such as the hip (Fig. 3.1) and shoulder allow movement in all planes, but a hinge joint such as the knee allows movement in one plane only. Condyloid joints such as the radiocarpal joint are elliptical and allow movement in two planes, whereas pivot, or peg, joints such as the superior radioulnar and atlantoaxial joints allow movement about one axis only (Fig. 3.2).

Some joints, such as the patellofemoral joint, have a very large range of movement (Fig. 3.3), but others have much less. The small bones of the tarsus, for example, are linked by plane joints with so little movement that groups of joints must work together to permit a useful range of motion.

The posterior intervertebral, or facet, joints are also plane joints, as is the sacroiliac joint.

Other joints are more complex and more interesting mechanically. The first carpometacarpal joint is saddle-shaped and similar to a universal joint in design (Fig. 3.4). Other joints work in pairs, including the superior and inferior radioulnar joints, which function as a single unit with two bones moving about an axis that passes through the centre of each (Fig. 3.5). The tibiofibular joints are similar; movement of the ankle produces movement at the superior tibiofibular joint (try it on your own leg). The subtalar joint also consists of two separate articulations between the same two bones, one convex upwards and the other convex downwards, but movement occurs about an oblique axis which does not pass through the articular surface of either.

There are two types of cartilage joints: primary, or synchondroses, and secondary, or symphyses (Fig. 3.6). Synchondroses link immature growing bone at the epiphyses in children and have no movement. Symphyses, which are only found in the midline of the body, have a mass of fibrocartilage linking the bones instead of a synovial cavity. The pubic symphysis is the best known but the intervertebral joints with their intervertebral discs and the manubriosternal joint are also symphyses.

The flat bones of the skull are linked at the suture lines by fibrous tissue, which prevents any appreciable movement. These linkages are, strictly speaking, joints, but their function is to limit movement rather than encourage it. The inferior tibiofibular joint is the largest syndesmosis in the body and is important for the stability of the ankle (Fig. 3.7).

The epiphysis in a growing long bone is separated from the hollow shaft, or diaphysis, by the epiphyseal plate, or physis. The part of the diaphysis next to the physis is the metaphysis. Any bone arranged like this is called a long bone, even if it is quite short – the phalanges of the fingers and toes are ‘long’ bones in structure. Damage to a growing epiphysis causes deformity.

Flat bones, such as the skull, pelvis and ribs, form in condensations of fibrous tissue and are often called membrane bones. Their function is the protection of soft viscera such as the brain and lungs.

Short square bones like those of the tarsus and carpus form in blocks of cartilage and ossify from the centre. They do not have epiphyses.

In addition to the normal bones, accessory ossicles occur as variants of normal. These are entirely innocent structures but can be mistaken for fractures and treated as such. The os trigonum behind the talus (Fig. 3.9a) and the accessory navicular (Fig. 3.9b) are among the most common.