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 diagnosis of congenital versus traumatic RUS, and for differentiation between types 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 resulting haematoma or of the interosseous (IO) membrane. Congenital cases include those which result from incompetent differentiation of the single mesenchymal mass that gives rise to the radius and ulna (Type I) and those that result from radial head dislocation in utero, resulting usually from humeral or ulnar defects (Type II). Type II CRUS can be differentiated by the presence of radial overgrowth, among other factors. Following these guidelines, we describe and compare the morphology of three congenital cases from North America. Two present bony fusions and one a functional fusion with radial overgrowth. In each case, CRUS significantly limits both supination and pronation. Despite differences among the cases, several morphological accommodations are shared as a result of fixation in the neutral (semi-pronated) position. These changes include the ventral migration of the IO crests of the ulnae and enlargement of the dorsal tubercles of the radius. Understanding the pattern of accommodations made in the face of compromised function ultimately increases our understanding of the patterns of growth in normal functional regimes.