Forearm lengths in cases of unilateral congenital radioulnar synostosis, were analysed radiographically to evaluate the amount of length discrepancy according to different types.

We measured the ulnar length ratio (length of affected forearm/length of unaffected forearm) in 15 cases of uni-lateral congenital radioulnar synostosis. We observed two distinct radiographic patterns. Type I (Lamda type) appeared to be of gamma character grossly, and showed severe radial bowing and a severely hypoplastic radial head. Type II (Chi type) seemed x like in character, showed mild radial bowing and a mildly hypoplastic, or normal radial head.

Nine cases were type I and 6 were type II. The ulnar length ratio was 0.89 in type I and 0.98 in type II. Significant differences were formed in the ulnar length ratios between normal subject and type I, and between type I and type II (p<0.05). The equation determined by simple linear regression analysis in a patient of type I, who had been followed for 10 years, was'ulnar length ratio=0.959-0.013 X age (year)'.

Unilateral congenital radioulnar synostosis can be classified into two types according to radiological morphology. This classification can be helpful in the prediction of forearm length discrepancy, and in the determination of surgical treatment type.