Assessment of dorsal instability of the ulnar head in the distal radioulnar joint: comparison between normal wrist joints and cases of ruptured extensor tendons.

In the present study, the adaptability of the distal radioulnar joint (DRUJ) was evaluated using conventional computed tomography (CT) evaluation methods. In addition, we investigated/compared a new method to evaluate dorsal displacement of the ulnar head. Our subjects consisted of 32 healthy volunteers (64 wrists) and 11 patients (13 wrists) with extensor tendon injuries related to dorsal displacement of the ulnar head. To diagnose instability in the DRUJ based on CT scans, the radioulnar line method and the modified radioulnar line method were measured. Instability was evaluated by the new method that the ulnar head was located on the dorsal side from a line involving the peak of Lister's tubercle in parallel to this baseline was regarded as showing abnormal dorsal displacement of the ulnar head. The diagnostic accuracy of each method was calculated. The sensitivities, specificities, false-positive rates, positive predictive values and the negative predictive value of new methods were better than other two methods. The new method that we recommend is simple. Based on the results of this study, an evaluation of normal/abnormal dorsal displacement of the ulnar head in the DRUJ using the new method may be useful for determining the timing of surgery.