Force variations in the distal radius and ulna: effect of ulnar variance and forearm motion.

PURPOSE

To better define normal wrist joint forces during wrist motion and forearm motion at specific wrist and forearm positions and to see if there is a relationship between these forces and the amount of ulnar variance. A secondary purpose was to determine the relationship between the thickness of the articular disk of the triangular fibrocartilage complex and the amount of force transmitted through the distal ulna.

METHODS

Multi-axis load cells were attached to the distal radius and ulna of 9 fresh cadaver forearms. The axial radial and ulnar compressive forces were recorded while each wrist was moved through wrist and forearm motions using a modified wrist joint simulator. During each motion, the tendon forces required to cause each motion were recorded. The ulnar variance and triangular fibrocartilage complex articular disc thickness were measured.

RESULTS

The axial force through the distal ulna and the wrist extensor forces were greatest with the forearm in pronation. No relationship was found between the amount of force through the distal ulna and the amount of ulnar variance. A strong inverse relationship was found between the triangular fibrocartilage complex thickness and the ulnar variance.

CONCLUSIONS

Wrists with positive ulnar variance have generally been thought to transmit greater loads across the distal ulna, which has been felt to predispose these wrists to the development of ulnar impaction. The results of this study appear to show that all wrists have similar loading across the distal ulna regardless of ulnar variance. By comparison, pronation relatively increases loading across the distal ulna.

CLINICAL RELEVANCE

Because these results suggest that within reasonable ranges of ulnar variance loading across the distal ulna is independent of ulnar variance, the clinically observed incidence of ulnar impaction is more likely the result of increased wear on a thinner and less durable triangular fibrocartilage complex than due to increased distal ulna loading in ulnar positive variant wrists.