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The 'chef's hat' appearance of the femoral head in cleidocranial dysplasia.

Cleidocranial dysplasia (CCD) is inherited as an autosomal dominant disorder characterised by failure of membranous ossification. The condition is due to a mutation of the cbfa1 gene on chromosome 6 which has a role in the development of osteoblasts from the mesenchymal cells. In their growing years, these patients have an unusual shape of the femoral head reminiscent of a 'chef's hat'. In order to confirm the consistency of this sign, we have reviewed the radiographs of 28 patients with CCD. All except three had this appearance. The sign was also seen in patients with coxa vara associated with a variety of other conditions. The chef's hat sign may occur secondary to the particular mechanical environment created by coxa vara as well as abnormal cellular function in patients with CCD. Although coxa vara has some influence on the shape of the femoral head, it is not entirely responsible for its morphology since it was present in only six of the 28 patients with CCD.