Metaphyseal dysplasia, or Pyle disease, is a disorder of the bones. It is a rare disease in which the outer part of the shafts of long bones is thinner than normal and there is an increased chance of fractures. Its hallmark feature is an abnormality of the long bones in the arms and legs in which the ends (metaphyses) of the bones are abnormally broad; the shape of the bones resembles a boat oar or paddle. The broad metaphyses are due to enlargement of the spongy inner layer of bone (trabecular bone). Although trabecular bone is expanded, the dense outermost layer of bone (cortical bone) is thinner than normal. As a result, the bones are fragile and fracture easily. The bone abnormalities in the legs commonly cause knock knees (genu valgum) in affected individuals.

Other bone abnormalities can also occur in Pyle disease. Affected individuals may have widened collar bones (clavicles), ribs, or bones in the fingers and hands. Dental problems are common in Pyle disease, including delayed appearance (eruption) of permanent teeth and misalignment of the top and bottom teeth (malocclusion).