Biomechanics of the distal radioulnar joint.

The distal radioulnar joint (DRUJ) is a complex joint involved in pronosupination and ulnocarpal motion and support. The ulnar head, in a rolling, sliding motion, moves from the dorsal to the volar rim of the sigmoid notch as the joint moves from pronation to supination. The triangular fibrocartilage (TFC) is taut first dorsally and then volarly in the same sequence. The ulnar carpus is supported variably, as a function of ulnar length relative to the radial articular surface, by the pole of the distal ulna through the TFC. The TFC does not resist the pistonlike movement of the DRUJ, which occurs under dynamic loading. The coronal alignment of the DRUJ minimizes shear stress on the articular surfaces because of its alignment with the rotational axis of the forearm. Ulnar variance is a factor in the development of several clinical conditions.