Prehistoric radio-ulnar synostosis: Implications for function

Proximal radio-ulnar synostosis (RUS) may occur congenitally, either as an isolated condition or as part of a syndrome, or it may occur as a secondary complication of forearm trauma. We provide a key for the differential diag nosis of congenital versus traumatic RUS, and for differentiation between t ypes of congenital RUS. Congenital cases (CRUS) include absence of fracture , radial head abnormalities, radial shaft bowing, and absence of a sigmoid notch, whereas post-traumatic RUS results from either radial neck fracture or evulsion of the biceps brachii, followed by ossification of the resultin g haematoma or of the interosseous (IO) membrane. Congenital cases include those which result from incompetent differentiation of the single mesenchym al mass that gives rise to the radius and ulna (Type I) and those that resu lt from radial head dislocation in utero, resulting usually from humeral or ulnar defects (Type II). Type II CRUS can be differentiated by the presenc e of radial overgrowth, among other factors. Following these guidelines, we describe and compare the morphology of three congenital cases from North A merica. Two present bony fusions and one a functional fusion with radial ov ergrowth. In each case, CRUS significantly limits both supination and prona tion. Despite differences among the cases, several morphological accommodat ions are shared as a result of fixation in the neutral (semi-pronated) posi tion. These changes include the ventral migration of the IO crests of the u lnae and enlargement of the dorsal tubercles of the radius. Understanding t he pattern of accommodations made in the face of compromised function ultim ately increases our understanding of the patterns of growth in normal funct ional regimes. Copyright (c) 2000 John Wiley & Sons, Ltd.