Clinical Manifestations. The major signs are failure to thrive, with resultant short stature in infancy and persistence of an open anterior fontanel even into adulthood.

The head is large in relation to the body, with protrusion of the frontal and occipital bones. The major cranial sutures and anterior fontanel often remain open, giving the impression of hydrocephalus. The face is small in proportion to the cranium and is characterized by bulging or prominent eyes, parrot-like nose, receding chin, and an obtuse angle of the jaw. Dental anomalies include premature or delayed eruption of teeth, persistence of the deciduous dentition, malocclusion, and hypoplasia of the enamel. The vault of the palate is highly arched and sometimes deeply grooved. The sclerae may be blue.

Because of increased bone density, even such mild trauma as tooth extraction can cause fractures. Deformities of the long bones, often due to fractures and malunion, may exacerbate the short-limb dwarfism. Arm span tends to be less than normal, and the terminal phalanges of the fingers are short and wide. Kyphosis, scoliosis, and exaggerated lumbar lordosis may develop. In some patients, the thorax is narrow and long. Adult height varies from 51 to 59 inches.

Radiographic Findings. Sclerosis is seen throughout the skeleton. The cranium is large, shortened, and brachycephalic with separation of sutures and an open anterior fontanel. Multiple sutural (wormian) bones are often present, and the facial bones, particularly the jaw, are underdeveloped. There is variable cortical thickening of the long bones with moderate metaphyseal undermodeling, with or without evidence of fractures. In the hands and feet, partial aplasia of the tufts and distal portions of the phalanges creates a bizarre drumstick appearance on radiographs. The acromial ends of the clavicles are dysplastic and hypoplastic.

Differential Diagnosis. Pycnodysostosis is easily distinguished from cleidocranial dysostosis (see Plate 4-28) and osteopetrosis (see Plate 4-26).