Effect of radial shortening on muscle length and moment arms of the wrist flexors and extensors.

The purpose of this study was to determine the effect of distal radial shortening on muscle length and moment arm of the wrist flexors and extensors. In eight cadaveric upper extremities, distal radius fractures were simulated by an ostectomy. The distal radius was progressively shortened by 2.5, 5.0, 7.5, and 10 mm. Changes in the resting length of the flexor carpi radialis and ulnaris, extensor carpi radialis longus and brevis, and extensor carpi ulnaris muscles were measured with rotary potentiometers at neutral position, flexion, extension, and radial and ulnar deviation of the wrists. The wrists were passively moved through flexion-extension and radioulnar deviation, and tendon excursions and wrist joint angulation were recorded simultaneously. Tendon moment arms were derived from tendon excursions and joint motion. The results showed that either muscle length or moment arm of the principal wrist flexors and extensors was significantly affected by the radial shortening. Muscle length decreased significantly after radial shortening in all the wrist flexors and extensors except for the extensor carpi ulnaris. The moment arm of the extensor carpi ulnaris tendon decreased significantly during either wrist flexion-extension or radioulnar deviation. The extensor carpi radialis brevis and flexor carpi ulnaris tendons also showed a significant decrease in their moment arms during radioulnar deviation of the wrist. Radial shortening of only 2.5 mm caused statistically significant changes in muscle length and moment arm of the wrist flexors and extensors. Increasing the extent of radial shortening exaggerated the biomechanical changes in the wrist motors. These results validate the importance of normal radial length for wrist kinetics and, from a biomechanical perspective, support complete correction of radial shortening after distal radius fractures.