Achondroplasia, the most common form of dwarfism, is the result of diminished proliferation of cartilage in the growth plate (decreased enchondral bone formation). Because membranous bone formation is not affected, the patient has short limbs that contrast with the nearly normal length of the trunk. Other characteristic physical features include a large head with frontal bulging, saddle nose, a prognathous jaw, and prominent buttocks that give the false impression of lumbar lordosis.
The radiographic hallmark of achondroplasia is symmetric shortening of all long bones. The proximal segments of the extremities are usually more severely involved than the distal portions, so that the humerus and femur are disproportionately shorter than the radius and tibia (rhizomelia). Continued appositional growth at the metaphyses causes the ends of the long bones to be splayed. However, appositional growth at bone ends is often deficient in the center of the metaphyseal regions, producing a V-shaped notch surrounding and partially burying the epiphyseal center (ball-and-socket epiphysis). The bones of the hand are short and thickened, and the fingers tend to be of the same length. Separation of the ring and middle fingers results in a “trident hand.”
In the lumbar spine, the interpedicular distances narrow progressively from above downward, the opposite of normal. Narrowing of the spinal canal in both dimensions may lead to neurologic signs of cord compression and nerve root encroachment, especially in patients with pronounced kyphoscoliosis. Anterior wedging of lumbar vertebral bodies and scalloping of their posterior margins may also be seen. The pelvis is typically short and broad, the ilia short and square, and the acetabular angles decreased.
The skull is enlarged in achondroplasia, usually because of communicating hydrocephalus due to deficient brain space in the posterior fossa caused by early closure of the intersphenoid and spheno-occipital synchondroses. Upward brain growth causes frontal bulging; the relatively larger size of the normally growing mandible gives the appearance of prognathism.
Ollier’s disease (multiple enchondromatosis) is a bone dysplasia affecting the growth plate in which an excess of hypertrophic cartilage is not resorbed and ossified in a normal fashion. This causes proliferation of rounded masses or columns of decreased-density cartilage within the metaphyses and diaphyses of one or more tubular bones. The involvement is usually unilateral, and the affected bones are invariably shortened and often deformed. In long bones, the columns of radiolucent cartilage may be separated by bony septa, producing a striated appearance. In the hands and feet, the lesions are globular and expansile, often with stippled or mottled calcification. Malignant transformation of an enchondroma into a chondrosarcoma is infrequent and almost never occurs with lesions in the hands or feet. The combination of enchondromatosis and multiple cavernous hemangiomas of the soft tissues is termed Maffucci’s syndrome. The radiographic demonstration of calcified thrombi (phleboliths) is pathognomonic of the vascular lesions.
Multiple Exostoses (Diaphyseal Aclasis)
Multiple exostoses represent an inherited bone dysplasia in which multiple osteochondromas arise from the ends of the shafts of any bone preformed in cartilage. The individual exostosis is a broad-based bony overgrowth, consisting of a cortical shell surrounding a core of cancellous bone, that arises from the metaphyseal cortex close to the epiphyseal line. The apex of the exostosis points away from the nearest joint, and the lesion is covered by a cartilaginous cap that may contain small punctate calcifications. Exostoses are most common at the sites of greatest growth, such as the knee, shoulder, elbow, and wrist. They are often asymptomatic, producing symptoms only if they interfere with the function of a joint or tendon or cause compression of a nerve. A frequently associated finding is deformity of the forearm due to shortening and bowing of the ulna.
Exostoses begin to grow in childhood and stop enlarging when the nearest epiphyseal center fuses. Sudden, painful enlargement of an exostosis long after growth should have ceased suggests sarcomatous degeneration.
Fibrous dysplasia is a disorder that usually begins during childhood and is characterized by the proliferation of fibrous tissue within the medullary cavity. The disease may be confined to a single bone (monostotic) or the bones of one extremity, or it may be widely distributed throughout the skeleton (polyostotic). Patients with polyostotic fibrous dysplasia frequently have a unilateral predominance, often with localized pigmentations (cafe au lait spots) that tend to be on the same side as the bone lesions and have an irregular (“coast of Maine”) outline in contrast to the smoothly marginated (“coast of California”) pigmented macules seen in neurofibromatosis. About one-third of females with the polyostotic form of fibrous dysplasia also demonstrate precocious puberty (Albright’s syndrome); sexual precocity is very rare in males with this condition.
The monostotic form of fibrous dysplasia primarily involves the long bones (especially the femur and tibia), ribs, and facial bones. Fibrous replacement of the medullary cavity typically produces a well-defined radiolu-cent area, which may vary from completely radiolucent to a homogeneous ground-glass density, depending on the amount of fibrous or osseous tissue deposited in the medullary cavity. Irregular bands of sclerosis may cross the cystlike lesion, giving it a multilocular appearance. The bone is often locally expanded, and the cortex may be eroded from within, predisposing to pathologic fractures. In severe and long-standing disease, affected bones may be bowed or deformed (e.g., the characteristic shepherd’s crook deformity of the femur).
Skull lesions of fibrous dysplasia may be either lucent or sclerotic. Involvement of the base of the skull produces a sclerotic ground-glass appearance. Disease of the facial bones causes marked sclerosis and thickening, often with obliteration of the sinuses and orbits, that creates a leonine appearance (leontiasis ossea). In the calvarium, there may be multiple irregular areas of lucency with expansion of the outer table of the skull and only minimal involvement of the inner table.
Fibrous dysplasia of a rib characteristically produces an expansile lesion with a ground-glass or soap-bubble appearance; indeed, the most common cause of an expansile focal rib lesion is fibrous dysplasia.
Although reported, malignant degeneration in association with fibrous dysplasia is extremely rare.
Osteogenesis imperfecta is an inherited generalized disorder of connective tissue involving bone, sclera, inner ear, teeth, skin, ligaments, tendons, and fascia. The major clinical features include blue sclerae, multiple fracdeath in utero or soon after birth is usually caused by intracranial hemorrhage. In the less severe form (osteogenesis imperfecta tarda), the disorder is first noted during childhood or young ad ulthood because of the unusual tendency for fractures; loose-jointedness; and the presence of blue sclerae, which is caused by intraocular pigment that shows through the thin, translucent outer membranes of the eyes.
Osteopetrosis (marble bone disease of Albers-Schon-berg) is a rare hereditary bone dysplasia in which failure of the resorptive mechanism of calcified cartilage interferes with its normal replacement by mature bone. This results in a symmetric, generalized increase in bone density. Osteopetrosis varies in severity and age of clinical presentation from a fulminant, often fatal, condition involving the entire skeleton at birth or in utero to an essentially asymptomatic form that is an incidental radiographic finding.
The dense sclerosis of osteopetrosis obliterates the individual bony components of cortex, medullary cavity, and trabecular pattern. Lack of modeling causes widening of the metaphyseal ends. Alternating dense and lucent transverse lines in the metaphyses often occur in the long bones and vertebrae, probably reflecting the intermittent nature of the pathologic process. In the vertebrae, this may produce a miniature bone inset within each vertebral body (bone-within-a bone appearance) or increased density at the end plates (“sandwich vertebrae”). Although radiographically dense, the involved bones are brittle, and fractures are a common complication, even with trivial trauma. Encroachment of bone on the marrow cavities causes a myelophthisic anemia that may lead to extramedullar hematopoiesis and enlargement of the liver, spleen, and lymph nodes. Sclerotic changes at the base of the skull may encroach on the cranial foramina and produce cranial nerve palsies, blindness, and deafness. The paranasal sinuses and mastoids may fail to develop or may become obliterated.
Melorheostosis is a rare disorder that appears as an irregular, sclerotic cortical thickening, which is usually confined to one side of a single bone or multiple bones of one extremity. The condition usually occurs in childhood, with a presenting symptom of severe pain that is sometimes associated with limitation of motion, contractures, or fusion of an adjacent joint. The sclerotic process involves the inner cortex and projects into the medullary cavity. It begins at the proximal end of the bone and extends distally. The resemblance of the appearance to the flowing (Greek rheos) of wax down a burning candle is responsible for the name.
Osteopathia striata is a rare, asymptomatic bone disorder in which an error in internal bone modeling causes the characteristic dense longitudinal striations in tubular bones. Involvement of the ilium produces linear densities that radiate out from the acetabulum and fan out to the iliac crest in a sunburst pattern.
Osteopoikilosis is an asymptomatic hereditary condition characterized by the presence of multiple small, well-circumscribed, round or oval areas of increased density in cancellous bone.The sclerotic focy vary in size from 2 mm to 2 cm and produce a typical speckled appearance that primarily involves the small bones of the hands and feet, the pelvis, and the epiphyses and metaphyses of long bones.
fibrous dysplasia · melorheostosis · achondroplasia ·