Diagnosis of distal radioulnar joint subluxation in patients with rheumatoid wrist by computed tomography.

Subluxation of the distal radioulnar joint (DRUJ) is associated with extensor tendon rupture in patients with rheumatoid arthritis (RA). However, it remains difficult to quantitatively evaluate DRUJ subluxation in RA wrist. We devised a new method for assessing DRUJ subluxation. This study investigated whether the new method, known as the RA subluxation ratio (RASR), or a conventional method was superior for detecting extensor tendon rupture in the RA wrist. Thirty-five RA wrists and 10 wrists of healthy volunteers were scanned using computed tomography. The RA wrists were divided into a tendon rupture group and a nonrupture group. The dorsal surface of the distal radius from Lister's tubercle to the ulnar aspect of the distal radius maintains a planar surface in the RA wrist. Therefore, we defined the RASR as the extent of dorsal subluxation of the ulna relative to this plane. We quantified subluxation of the DRUJ by using the RASR or the modified radioulnar line method, and compared the two methods. The RASR was 0.440 in the rupture group, 0.333 in the nonrupture group, and 0.106 in the healthy volunteers. The RASR was significantly higher than the modified radioulnar line method in the sensitivity of diagnosing tendon rupture.