Case report 746: Osteopetrosis.

Osteoporosis is a disorder characterized by osteoclastic dysfunction. The bones of afflicted patients become sclerotic and show modeling defects resulting in either a decrease or obliteration of the marrow cavity and resultant pancytopenia. Other clinical manifestations include bony deformities, cranial nerve palsies from bony overgrowth, pathological fractures, osteomyelitis, and hepatosplenomegaly secondary to extramedullary hematopoiesis. In the skull, the diploic space is usually decreased or obliterated. Absent or decreased marrow space is observed on plain films. On MRI, the marrow cavity very likely will be obliterated with low signal intensity on T1- and T2-weighted images. We report a case of osteopetrosis in which the diploic space is markedly increased, giving a hair-on-end appearance resembling thalassemia major. Perhaps some local factor within the diploic space prevented the expected osteoclastic dysfunction, allowing remodeling and expansion of the cranial vault.