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.

Hyaline, or articular, cartilage is highly organized tissue consisting of loops of type II collagen within a ground substance of proteoglycans produced by chondrocytes. The proteoglycans are hydrophilic and the tissue tension within the deep layers of hyaline cartilage is considerable. The structure can be compared with an inflatable building or a ‘blister’ over a tennis court: one is inflated by water drawn in by proteoglycans, and the other by air pumped in by an electric fan. In one, the pressure is contained by collagen loops, in the other by a plastic canopy (Fig. 3.12).

Fibrocartilage varies from place to place according to its elasticity, which depends on the relative amounts of elastin and collagen within it. The fibrocartilage of the ear and nasal cartilages, for example, is different from that of the intervertebral discs.

The importance of the relationship between muscles is illustrated by certain fractures. A fracture of the femur at the junction of its upper and middle thirds leaves the hip abductors and most of the flexors on the upper fragment, and all the adductors and most of the extensors on the lower. The upper fragment therefore swings up into unopposed abduction and flexion while the lower fragment stays on the bed and swings into adduction (Fig. 3.14). Similarly, a fracture of the forearm separates the supinators (biceps and supinator) in the upper fragment from the pronators (pronator teres and pronator quadratus). What happens? The upper fragment supinates as far as it can and the lower fragment pronates to produce a nasty rotational deformity that prevents supination. This means that the patient cannot collect change when shopping.

A variation on the same theme is seen in fractures of the upper end of the femur. With the femur intact, the iliopsoas internally rotates the hip because it lies lateral to the axis of rotation of the joint. If the femoral neck is broken, the iliopsoas cannot act on the hip, but spins the femur about its long axis to produce the characteristic external rotation deformity of a fracture at the upper end of the femur.

Fractures which separate the mechanical linkage between muscle groups are difficult to hold without internal fixation devices.

There are two compartments in the forearm, ventral (flexor) and dorsal (extensor).

Ventral compartment. The ventral compartment includes the median and ulnar nerves, and the radial and ulnar arteries. Compression of these structures within the fascial sheath can have disastrous consequences.

Dorsal compartment. The extensor compartment is less often damaged than the ventral and includes the posterior interosseous nerve but no major vessels. The consequences of a dorsal compartment syndrome are less serious than ventral compartment compression, partly because it contains fewer important structures and partly because its fascia is less dense.

In the leg there are four compartments and all can cause serious problems (Fig. 3.16).

Anterior tibial compartment. The anterior tibial compartment contains the anterior tibial artery and deep peroneal nerve after it has left the peroneal compartment.

Superficial posterior compartment. There are no important vessels or nerves in the superficial posterior compartment, which includes gastrocnemius and soleus only.

Lateral (peroneal) compartment. The lateral compartment contains the superficial peroneal nerve but it is seldom affected by compression.

Patients with persistent paraesthesia in the distribution of the median nerve are likely to have compression of the median nerve at the wrist (Fig. 3.20) (p. 387). The fluid retention of pregnancy is one cause but repetitive movements of the flexor tendons of the hand and wrist also lead to a localized flexor tenosynovitis, which in turn causes soft tissue swelling and compression of the median nerve as it enters the hand through the carpal tunnel.

As the median nerve is compressed, sensory symptoms are felt in thumb, index and middle fingers, half the ring finger on the palmar surface and on the back of the fingers. The palm itself is spared because the palmar branch arises before the nerve enters the carpal tunnel.

Variations in the distribution of the median nerve are common.

The ulnar nerve may be irritated at the elbow as it runs behind the medial epicondyle. The nerve can be damaged by trauma, osteoarthrosis, deformity from malunion (tardy ulnar palsy, p. 387), or even pressure on the medial side of the elbow as the arm rests on an operating table. The patient will experience paraesthesiae or numbness in the little finger and the ulnar half of the ring finger, weakness of the hand, or sometimes clumsiness due to weakness of the intrinsic muscles supplied by the ulnar nerve (Fig. 3.21).

The radial nerve is safe in the forearm but vulnerable in the upper arm. Pressure on the radial nerve from old-fashioned axillary crutches can lead to a troublesome drop wrist but few sensory symptoms (Fig. 3.22). Other causes, now almost historical, are the ‘Saturday night palsy’ of inebriated patients who fall asleep with their arms over the back of a chair and, in more recent and healthier days, pressure on the medial side of the upper arm from prolonged pressure on the back of a cinema seat.

In the lower limb, the commonest peripheral nerve lesion is damage to the lateral half of the sciatic nerve from posterior dislocation of the hip (Fig. 3.23) (p. 182).

A similar but not identical clinical picture is produced by a lesion of the common peroneal nerve as it passes around the fibular neck.