This study examined the displacement of the ulnar head with respect to the distal radius after distal radius malunion on the axial plane using computed tomography.

Twenty-four patients with a residual dorsal tilt > 10° or radial shortening > 2 mm than the normal side, and no limitation in wrist rotation after radius union were enrolled in this study. The bilateral CT scans were obtained in 70° pronation, neutral, 70° supination. The radioulnar ratio was measured and the ratios of the abnormal side were compared with those of the opposite normal side. The results were statistically analyzed using a two-tailed paired t-test.

The ulnar head of the patients with a malunited radius had a tendency for anterior translation compared with the normal side. In patients with a dorsal tilt deformity, the anterior translation of the ulnar head with respect to the radius was significant, particularly in the supination and neutral position. However, no significant difference was found in patients with radial shortening in terms of the ulnar head translation between the normal and abnormal side.

These results might explain the mechanism of distal radioulnar joint instability and low incidence of ulnocarpal impingement in patients with distal radius malunion.