There are 2 types of radioulnar synostosis: in type 1, there is a proximal, smooth fusion of 2 to 6 cm between the radius and ulna and the radial head is absent; in type 2, there is a fusion just distal to the proximal radial epiphysis in association with congenital dislocation of the radial head (Bauer and Jonsson, 1988). Both types result in a limitation of pronation and supination of the forearm, and in type 2 there is also a restriction of extension at the elbow. Dominant inheritance through several lines in several generations was demonstrated by a family reported by Davenport et al. (1924). Hansen and Andersen (1970) found a positive family history in 5 of 37 cases. Radioulnar synostosis is a feature of certain chromosome abnormalities, notably the triple X-Y syndrome (XXXY). See pronation-supination of the forearm, impairment of (176800). Radioulnar synostosis occurs in an autosomal dominant syndrome with amegakaryocytic thrombocytopenia; see RUSAT1, 605432. [from OMIM]