The Effect of Radial Shortening on Wrist Joint Mechanics in Cadaver Specimens With Inherent Differences in Ulnar Variance

Purpose: To assess load changes in the wrist and forearm resulting from surgically-induced radial shortening in wrists with inherent differences in ulnar variance. Methods: Eleven fresh cadaver upper extremities, 4 with ulnar-plus variance of 2 mm or greater and 7 with ulnar-minus variance of 2 mm or greater were used. The radius and ulna of each specimen were instrumented with load cells, a 15-mm segment of the radius was resected and replaced with aluminum blocks of various sizes, and the specimens were loaded with 143 N (32 lb) at 1-mm differences of radial length. The load distribution between the radius and ulna was measured. Results: The load distribution in the specimens with an ulnar-plus variance averaged 69% through the radius and 31% through the ulna. In the wrists with ulnar-minus variance, the load distribution averaged 94% through the radius and 6% through the ulna. The mean force in the ulna increased and the mean force in the radius decreased with incremental shortening of the distal end of the radius. The mean force through the ulna in the ulnar-plus-variance group was always higher than that of the ulnar-minus-variance group. When compared not by the number of millimeters of radial shortening but by the adjusted ulnar variance, there was no difference between the ulnar-plus-variance and the ulnar-minus-variance groups. Conclusions: Wrists with ulnar-minus variance could accommodate radial shortening without as much increase in the ulna load as wrists with ulnar-plus variance. The clinical relevance is that a patient with an ulnar-minus variance may accommodate more radial shortening after a wrist fracture without developing an ulnar impaction syndrome than a patient with an ulnar-plus-variant wrist.