Anatomy of the ulnar tunnel and the influence of wrist motion on its morphology.

PURPOSE: To examine the anatomy of the ulnar tunnel, with emphasis on the pisohamate arcade and pisohamate hiatus, and study the influence of wrist kinematics on the morphology of these structures.

METHODS

Ten fresh-frozen cadaveric hands were dissected. The dimensions and relationships of the ulnar tunnel, pisohamate arcade, and pisohamate hiatus were recorded. The effect of 4 wrist positions on these dimensions and relationships was investigated.

RESULTS

The ulnar tunnel has 3 compartments--proximal, middle, and distal relative to the pisiform-with variable morphologies, dimensions, and boundaries. In wrist neutral position, the length of the ulnar tunnel was 45 mm (range, 42 to 51 mm). The middle compartment was found to be the narrowest; the proximal, the widest, averaging 5.0 mm; and the pisohamate hiatus that separates the middle and distal compartments, highly dynamic. The length of the pisohamate arcade was 21.5 mm (range, 18.0 to 26.0 mm), and the length and width of the pisohamate hiatus were 11.0 mm (range, 9.8-11.5 mm) and 6.0 mm (range, 5.3 to 7.2 mm), respectively. During wrist extension, the ulnar nerve was under tension. Wrist flexion was the position that caused the most change in ulnar tunnel and pisohamate arcade and hiatus anatomy, causing the width of the proximal compartment to increase from 5.0 to 10.0 mm. During this motion, the shape of the pisohamate arcade changed from a C shape to linear, and the length increased to 24.5 mm (range, 19.3 to 28.5 mm). The pisohamate hiatus narrowed, its width decreased to 1.5 mm, and the deep branch of the ulnar nerve was somewhat compressed.

CONCLUSIONS

The ulnar tunnel is a dynamic space with dimensions and relationships that are influenced by wrist motion. During ulnar tunnel surgery, all 3 compartments of the ulnar tunnel should be explored and decompressed, including the pisohamate hiatus, by releasing the pisohamate arcade.