ACHONDROPLASIA

Achondroplasia is a congenital affection in which there is marked shortness of the limbs, with consequent dwarfing. It is of autosomal dominant inheritance but many cases arise from a fresh gene mutation.

Pathology. There is a failure of normal ossification in the long bones, which may be only half their normal length. Growth of the trunk is only slightly impaired.

Clinical features. Achondroplasia is apparent at birth, the child being strikingly dwarfed, with very short limbs that are out of proportion to the trunk: shortness is especially marked in the proximal segments of the limbs (Fig. 6.1). Adult achondroplasts are seldom more than 130 cm (4 feet 3 inches) in height. The hands are short and broad, the central three digits being divergent and of almost equal length (‘trident’ hand). The head is slightly larger than normal, with a bulging forehead and depressed nasal bridge. There is marked lumbar lordosis, often with thoracic kyphosis, which may occasionally lead to compression of the spinal cord. There is no mental impairment and life expectancy is usually normal.

Fig. 6.1 Achondroplasia, showing the typical features in a child. The dwarfing is due to shortness of the limbs, especially in their proximal segments; development of the trunk is but little impaired. Note the ‘trident’ hands.

Radiographic features. Apart from striking shortness of the limbs, there are characteristic changes in the pelvis, the inlet being widened from side to side but narrowed in the antero-posterior diameter, with notably small greater sciatic notches. The spinal canal is also narrowed in the antero-posterior diameter, which predisposes to spinal stenosis. The calvarium of the skull is large, but the base short.

Treatment. With improved techniques of leg lengthening, there may be increasing scope in the future for worthwhile gain in stature.

OSTEOGENESIS IMPERFECTA (Fragilitas ossium)

Osteogenesis imperfecta is a congenital and inheritable disorder – or more probably a heterogeneous group of disorders – in which the bones are abnormally soft and brittle, on account of defective collagen formation. In addition to the bones, other collagen-containing tissues such as teeth, skin, tendons, and ligaments may be abnormal. It is usually transmitted as an autosomal dominant, but in a severe variant of the disease the parents are normal and a fresh gene mutation or autosomal recessive inheritance is postulated.

Clinical features. In the worst cases, which occur sporadically rather than from inheritance and probably represent a distinct entity, the child is born with multiple fractures and does not survive. In the less severe examples fractures occur after birth, often from trivial violence. As many as fifty or more may be sustained in the first few years of life. The fractures unite readily, but in the more severe cases marked deformity often develops, either from malunion or from bending of the soft bones (Fig. 6.2), and such patients may be badly crippled. In the milder cases there is a tendency for fractures to occur less frequently in later life.

Fig. 6.2 Recent and old fractures of the bones of the upper limbs in an infant with osteogenesis imperfecta (fragilitas ossium).

Additional features, not always present, are a deep blue colouration of the sclerotics of the eyes, deafness from otosclerosis (which becomes worse in later life), and ligamentous laxity.

Treatment. Fractures are generally treated in the ordinary way, but in a severe case intramedullary nailing of affected long bones should be considered as a means of preventing crippling deformity and permitting earlier resumption of activity. Newer techniques have been developed with telescoping rods, to obviate the need for multiple operations. Protective appliances, such as walking calipers, may be required in older children and adults.

MULTIPLE HEREDITARY EXOSTOSES (Diaphyseal aclasis)

This is a congenital affection characterised by the formation of multiple exostoses (osteochondromata) at the metaphysial regions of the long bones. It is transmitted by an autosomal dominant mutant gene without any gender predisposition.

Pathology. The fault is in the epiphysial cartilage plate. Nests of cartilage cells become displaced and give rise to bony outgrowths, which are capped by proliferating cartilage. Growth of the exostoses may cease at skeletal maturity. These exostoses, or osteochondromata, constitute one type of benign bone tumour (p. 109). The number of outgrowths varies; often there are between ten and twenty, of varying size. In severe cases the process of remodelling by which a bone attains its normal adult shape is impaired, and there may be marked deformity, with reduction of longitudinal growth and consequent shortness of stature. Malignant change in the cartilaginous cap of one of the tumours can lead to the development of a chondrosarcoma (the lifetime risk is approximately 5%).

Clinical features. Usually the only symptoms and signs are those caused by the local swellings, or by their pressure effects. The patient is often of short stature, and there may be marked deformity of the limbs. Malignant change – often lethal – is suggested by rapid enlargement of one of the swellings in a patient, often at the age of skeletal maturity.

Radiographs show the bony outgrowths. In severe cases the bones are broad and ill modelled (Fig. 6.3).

Fig. 6.3 Diaphysial aclasis (multiple exostoses). A shows stunting of the upper end of the femur due to failure of the remodelling process that accompanies normal growth. Defective remodelling is a characteristic feature of the disease in its more severe forms. B shows typical exostoses (osteochondromata) projecting from femur and tibia. Such outgrowths are always directed away from the end of the bone.

Treatment. An outgrowth that is causing symptoms should be excised at its base and sent for routine histological examination to exclude any malignant change. Some deformities, particularly when they interfere with function, may require corrective osteotomies. Leg length discrepencies may also require correction, by either epiphyseal arrest or lengthening procedures.

MULTIPLE ENCHONDROMATOSIS (Dyschondroplasia; Ollier’s disease¹)

In dyschondroplasia masses of unossified cartilage persist within the metaphyses of certain long bones, and the growth of the bone is retarded. The condition becomes evident in childhood, but the cause is unknown. In contrast with multiple hereditary exostoses, just described, heredity plays no part.

Pathology. The fault is in the epiphysial cartilage plate. In this respect dyschondroplasia resembles multiple exostoses, but this is the only resemblance between them: in other respects they are entirely distinct. Nests of cartilage cells are displaced from the epiphysial plate into the metaphysis, where they persist as enchondromata.

Any bone formed in cartilage may be affected. The more rapidly growing ends of the femur and tibia (that is, the ends near the knee) and the small long bones of the hands and feet are particularly common sites. There is a tendency for the disorder to affect one side of the body predominantly, or even exclusively. When a major long bone is affected interference with growth at the epiphysial cartilage adjacent to the lesion may lead to serious shortening and distortion of the bone (Fig. 6.4A). When skeletal growth ceases the masses of cartilage may ossify. Rarely one of the tumours undergoes malignant change, to become a chondrosarcoma (p. 117).

Fig. 6.4 Multiple enchondromatosis (Ollier’s disease). A Masses of proliferating cartilage occupy the metaphyses of one tibia. Growth is retarded and uneven. The normal tibia is shown for comparison. B Multiple enchondromata in the metacarpals and phalanges.

Clinical and radiographic features. A limb affected by dyschondroplasia is usually short and may be markedly deformed. The hands may be grotesquely enlarged by multiple cartilaginous swellings or outgrowths. Radiographs show multiple areas of transradiance in the affected bones (Fig. 6.4).

Treatment. Osteotomy may be required to correct deformities resulting from uneven growth of bone. If there is marked discrepancy in the length of the lower limbs a leg equalisation procedure (p. 46) may be advisable. Lesions in the hand may be curetted and packed with bone chips.

PAGET’S DISEASE¹ (Osteitis deformans)

Paget’s disease of bone is a slowly progressive disorder of one or several bones. Affected bones are thickened and spongy, and show a tendency to bend. The disease is one of the commonest general affections of the skeleton. The cause is unknown. It has been suggested that infection with a slow virus of the paramyxovirus family may be responsible, on the evidence of inclusion bodies that have been found consistently in the osteoclasts of affected bones.

Pathology. The basic abnormality is thought to be defective function of osteoclasts, with consequent irregular bone resorption and increase of bone turnover. The bones most commonly affected are the pelvis, vertebrae, femur, tibia, and skull. The disease may be confined to a single bone at first, but it often spreads to involve other bones later. The cortex of the bone loses its normal compact density and becomes spongy. At the same time the cortex is widened by the formation of new bone on both its outer and inner surfaces. The whole bone is thus thickened, but the usual sharp distinction between cortex and medulla is blurred. The marrow spaces are filled with fibrous tissue. Whereas the bone is soft and vascular at first, in the later stages there is a tendency for it to become denser and very hard.

In the spongy state the softened bones are liable to bend. Pathological fracture may occur. In rare instances osteosarcoma develops in the diseased bone.

Clinical features. The affection seldom begins before the age of 40, but a childhood form is recognised. Often there are no symptoms, the condition being discovered incidentally during routine radiographic examination. When long bones are affected pain is sometimes complained of, but it is by no means an invariable feature. Bending of the softened long bones leads to deformity, usually in the form of anterior and lateral bowing of the femur or tibia (Fig. 6.5). Thickening may be obvious clinically, especially in the case of the tibia or the skull. Thus if the patient wears a hat he may notice that he requires progressively larger sizes. More importantly, bone thickening may also cause constriction of foramina in the skull, with risk of damage to a nerve – for instance the optic nerve or the auditory nerve.

Fig. 6.5 (left) The typical appearance of a patient with widespread Paget’s disease. Note the bowing of the legs and shortening of the trunk from collapse of softened vertebrae. The head was also enlarged.

Imaging. The main radiographic features (Figs 6.6 and 6.7) are:

Fig. 6.6 (right) Paget’s disease. A Tibia, showing bending, coarsening of trabeculae, and thickened cortex which, however, is not sharply demarcated from the medulla. B Femur, showing broadening of the bone with loss of the normal demarcation of cortex and medulla.

Fig. 6.7 Paget’s disease. Half pelvis, side by side with a normal one shown for comparison. Note the coarse trabeculae and slight distortion of the softened pelvic ring, with deepening of the acetabulum.

The long bones are often shown to be bowed, the pelvis may be deformed, the vertebrae may be compressed and the vault of the skull thickened. Radioisotope scanning with ^99mtechnetium shows markedly increased uptake in the affected bones.

Investigations. The alkaline phosphatase content of the plasma is increased, often to a high level if several bones are affected. Urinary hydroxyproline is increased, reflecting the increased resorption of bone matrix.

Complications. The important complications are pathological fracture, compression of cranial nerves, and occasionally – but most importantly – osteosarcoma.

Treatment. Very often treatment is not required because the disability is negligible. Treatment is needed mainly for bone pain and for complications. Two drugs are known to affect the outcome – calcitonin and bisphosphonates.

Both drugs inhibit bone resorption. They reduce bone turnover, with lowering of plasma alkaline phosphatase, and both are effective in relieving bone pain in a high proportion of cases. Treatment must, however, be prolonged. Unlike calcitonin, which must be injected (though nasal spray and suppository forms are being tried), a bisphosphonate compound may be taken by mouth. Moreover its benefit often persists for up to six months after the drug has been discontinued; so it is the preferred drug in the first instance, on the basis of cost and ease of administration.

POLYOSTOTIC FIBROUS DYSPLASIA

Replacement of bone by fibrous tissue forms a conspicuous part of several unrelated bone diseases. In two conditions in particular, fibrous replacement is the predominant change. In one of these – parathyroid osteodystrophy – the changes are associated with hyperparathyroidism (p. 73). In the other, now to be described, there is fibrous replacement without any evidence of excessive parathyroid secretion.

Polyostotic fibrous dysplasia, then, is a condition in which parts of several bones are replaced by masses of fibrous tissue, but in which there is no evidence of hyperparathyroidism. The condition is rare and the cause is unknown. There is no evidence to suggest a genetic basis.

Pathology. The number of bones involved varies from two or three to twelve or more. The major long bones are those mainly affected – especially the femur. The skull and mandible are also commonly involved. Affected bones are liable to bend or break.

Clinical features. The onset is in childhood but the condition is often not recognised until adult life. The main features are deformity, from bending or local enlargement of bone, and pathological fracture. The disease progresses for years and may eventually lead to severe crippling.

Radiographs of the affected bones show well-defined transradiant areas which often have a characteristic homogeneous or ‘ground-glass’ appearance: these changes may be localised and patchy rather than uniform. The lesions are in the shaft and metaphyses rather than the epiphyses. When the lesion is extensive the cortex is expanded and thin, and the bone is bent when weight-bearing, producing deformities such as the ‘shepherd’s crook’ in the proximal femur (Fig. 6.8). Sometimes the lesion has a honeycombed appearance.

Fig. 6.8 Polyostotic fibrous dysplasia affecting the radius A and the tibia B. Parts of the skeleton are replaced by fibrous tissue, giving in places a ‘ground-glass’ appearance. Unlike hyperparathyroidism, this disorder is not associated with any known endocrine dysfunction. C Extensive fibrous dysplasia affecting the proximal femur with softening of the bone resulting in a typical ‘shepherd’s crook’ deformity.

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