Thanatophoric dysplasia and cloverleaf skull.

Nine infants with thanatophoric dysplasia (TD) and cloverleaf skull (CS) are reported. Twenty-two previously published CSTD cases are reviewed. These CSTD cases are compared to cases of TD without CS. It is concluded that there are two types of TD: type 1, with curved femora and very flat vertebral bodies; and type 2, with straight femora and taller vertebral bodies. Consistent but subtle histopathological characteristics differentiate the two types. Only a very few type 1 cases have CS, and the CS is mild. Almost all type 2 cases have severe CS.