Influence of Dynamic Factors on Ulnar Impaction Syndrome: An Axial Compression Test Using Fresh-Frozen Cadavers.

PURPOSE: The dynamic evaluation of ulnar impaction syndrome has recently garnered interest; however, its underlying mechanism remains unclear. This research aimed to analyze the dynamic ulnar impaction biomechanically during axial loading.

METHODS

Ten fresh-frozen cadaveric upper limbs were tested. With the elbow at 90° flexion, forearm in neutral, and wrist at 0° flexion-extension, axial loads from 0 to 300 N were applied to the wrist. Posteroanterior radiographs were taken every 60 N (0, 60, 120, 180, 240, and 300 N). Ulnar variance was measured, and the relationship between the applied load and changes in variance was analyzed. Radiographs at 60 N and 300 N were overlaid to assess ulna displacement.

RESULTS

Mean ulnar variance measured 1.70 mm at 0 N and increased to 3.00 mm at 300 N. There was a positive correlation between load and variance change (r = 0.67, P < .001). The regression equation, Y (change in ulnar variance, mm) = 0.0044X (load, N), indicates an increase of approximately 0.44 mm for every 100 N of load. The ulna was displaced distally by 1.12 mm and radially by 0.81 mm, whereas the lunate moved proximally by 0.38 mm and toward the ulna by 0.43 mm.

CONCLUSIONS

Axial loading caused a linear increase in ulnar variance. The ulna and lunate shifted closer together, indicating a dynamic ulnar impaction.

CLINICAL RELEVANCE: These findings reinforce the biomechanical understanding of dynamic ulnar impaction and underscore the importance of considering load-dependent changes during assessment and treatment.