Causes

An abnormality of development may be caused by:

Studies of families and twins have helped geneticists to determine the influence of genetic and environmental factors, alone or combined, in the causation of many of the recognised malformations.

Genetic causes include mutation of a whole chromosome, as in Down’s syndrome (mongolism), and mutation of a small part of a chromosome or of a single gene, as in achondroplasia. The defect is not necessarily always inherited from an affected parent: it may arise from a fresh mutation in the germ cell.

Environmental causes are not well understood. Experiments in animals have shown that many different types of environmental influence – dietetic, hormonal, chemical, physical, or infective – may cause abnormalities of development, and the system of the body that is mainly affected depends upon the timing of the environmental ‘insult’. But except for a few specific agents acting early in pregnancy there is no conclusive evidence that similar influences are important causes of malformations in man. The specific agents whose influence in man is well attested include radiation, the virus of rubella, and certain drugs (notably aminopterin and thalidomide).

Combined genetic and environmental factors seem to be the usual cause of the more common congenital malformations in man, on the evidence of twin and family studies. It is thought probable that developing embryos react differently to environmental influences: some have a natural resistance whereas others are susceptible. A malformation is therefore likely to arise when an environmental ‘insult’ is inflicted upon cells that have a genetically determined lack of resistance to it.

Practical significance

Many of the recognised congenital abnormalities of the musculo-skeletal system have little practical importance, either because they are very rare or because there is little that can be done for them: these will not all be considered further in this book. There are others, however, that present major problems to the orthopaedic surgeon and may demand energetic treatment. These include developmental or congenital dislocation of the hip (p. 343), congenital club foot (p. 434), spina bifida (p. 171), congenital scoliosis (p. 218), osteogenesis imperfecta (p. 62), and cervical rib (p. 202).

Inborn predisposition to disease in adults

It is well recognised that, quite apart from the overt congenital anomalies discussed above, there exists in some patients a genetically determined predisposition to abnormalities developing in later life. Examples of orthopaedic conditions to which a susceptibility may exist include certain types of osteo-arthritis (especially of the hips), ankylosing spondylitis, gouty arthritis, rheumatic fever, idiopathic scoliosis, osteochondritis dissecans, and Dupuytren’s contracture.

Causes

The causes of deformity arising at a joint may be summarised under the following headings (Fig. 5.1):

Fig. 5.1 Seven causes of deformity arising at a joint. 1. Dislocation. 2. Muscle imbalance. 3. Tethering of muscle or tendon. 4. Soft-tissue contracture. 5. Arthritis. 6. Posture 7. Idiopathic (cause unknown).

Dislocation or subluxation

This is usually caused by injury, but it may occur as a congenital deformity, or it may follow disease of the joint (pathological dislocation).

Muscle imbalance

Unbalanced action of muscles upon a joint may hold it continuously in a particular arc of its range. In time, secondary contractures occur in the dominant muscles or in the soft tissues, preventing the joint from returning to the neutral position (Fig. 5.1 (2)). The two fundamental causes of muscle imbalance are:

Thus equinus deformity at the ankle may follow paralysis of the dorsiflexor muscles (for instance, from damage to the lateral popliteal nerve) because the action of the plantarflexors and of gravity is unopposed. Or a similar deformity may be caused by spasticity of the calf muscles, which overpower their antagonists. This occurs commonly in cerebral palsy (p. 168).

Tethering or contracture of muscles or tendons

If something happens to muscles or tendons that prevents their normal to-and-fro gliding, or their elongation and retraction, the joint may be held in a position of deformity. Thus a muscle or tendon may be tethered to the surrounding tissues in consequence of local infection or injury (Fig. 5.1 (3)). An example is the anchoring of a flexor tendon of a finger within its fibrous sheath as a result of suppurative tenosynovitis, with consequent flexion deformity at the interphalangeal joints. Or a muscle may lose its elasticity and contractile power from impairment of its blood supply. An important example is Volkmann’s ischaemic contracture of the forearm flexor muscles (p. 299) from occlusion of the brachial artery or from increased intra-compartmental pressure, with consequent flexion deformity of the wrist and fingers.

Contracture of soft tissues

Apart from any disturbance of the muscles, contracture of other soft tissues alone can account for joint deformity. An example is the common condition of Dupuytren’s contracture (p. 321), in which the thickened and contracted palmar aponeurosis pulls the metacarpophalangeal and proximal interphalangeal joints of one or more fingers into flexion. Similarly, a flexion deformity of the knee or elbow, or indeed of any joint, may occur from contracture of the scarred skin after burns of the flexor surface of the limb (Fig. 5.1 (4)).

Arthritis

The various types of arthritis will be discussed in a later chapter. Any type of arthritis may lead to joint deformity. In some cases the joint is firmly fixed in a deformed position by bony or fibrous ankylosis. In other instances the joint retains some movement but is prevented from reaching the neutral position. Thus flexion and adduction deformity is common in osteoarthritis of the hip, flexion deformity is common in arthritis of the knee, and the deformity of ulnar deviation of the fingers (Fig. 16.11A, p. 305) is a well-known feature of rheumatoid arthritis of the metacarpo-phalangeal joints.

Posture

The habitual adoption of a deformed position of a joint often leads in time to permanent deformity. A common example is the lateral deviation of the great toe at the metatarso-phalangeal joint – hallux valgus – so common in women who cramp their feet into narrow pointed shoes (Fig. 5.1 (6)). Another postural deformity that is still seen occasionally – though it should never be allowed to occur – is fixed flexion of the knees in a patient confined to bed for a long time with the knees bent over a pillow.

Unknown causes

In some cases deformity occurs at a joint for no apparent reason. Thus many children develop knock-knee deformity between the ages of 3 and 5 years without demonstrable cause. It is usually unimportant because it tends to correct itself spontaneously. Another common and more serious example is the inversion deformity of the foot known as talipes equino-varus or congenital club foot. A more sinister deformity that is equally ill explained is the idiopathic scoliosis of adolescents (p. 215).

Causes

There are three causes of deformity arising in bone:

Fig. 5.2 Three causes of deformity arising in a bone. A Fracture. B Bending of softened bone. C Uneven epiphysial growth.

Fracture

This is by far the most common cause. Unless a fracture is reduced so that the fragments are perfectly aligned, deformity will result. Examples are the genu valgum (knock knee) that is often the consequence of compression fractures of the lateral condyle of the tibia, the cubitus valgus that may follow displaced fractures of the lateral condyle of the humerus, and the common ‘dinner-fork’ deformity of an unreduced fracture of the lower end of the radius.

Congenital tibial pseudarthrosis may mimic an ununited fracture, but is often present at birth and may be linked to neurofibromatosis (seep. 70).

Bending of softened bone

Many unrelated conditions can cause softening of bone, with liability to bending and consequent deformity. They are mostly generalised disorders in which several or all of the bones are affected. The following are examples. Metabolic disorders: rickets, osteomalacia. Endocrine disturbances: parathyroid osteodystrophy, Cushing’s syndrome. Affections of unknown cause: Paget’s disease (osteitis deformans), fibrous dysplasia of bone, idiopathic osteoporosis. The main features of these disorders will be described in a later chapter.

Uneven growth of bone

In children any disturbance of the growing epiphysial cartilage (growth plate) may lead to uneven growth and consequent deformity. The usual effect of interference with a growing epiphysial cartilage is that its growth is retarded; occasionally it is accelerated. Deformity will follow only if the growing cartilage is affected more in one part than another, or if the interference with growth affects only one bone of a pair, as in the forearm or leg (Fig. 5.2C). The most frequent causes of retarded epiphysial growth are:

In the relatively uncommon cases in which epiphysial growth is accelerated the usual cause is local hyperaemia induced by an adjacent focus of infection or by a vascular tumour such as a haemangioma.