Median nerve shape and position in response to varied wrist postures during pinch.

This study investigated changes in median nerve flattening ratio (FR) and transverse position in the carpal tunnel across wrist flexion-extension (FE) postures during pinch in healthy adults. We hypothesized that deviated wrist FE postures from neutral would increase FR and alter nerve transverse position, and that pinch force would increase FR and amplify postural effects on position. Eleven healthy participants underwent ultrasound transverse imaging at the distal carpal tunnel of the right hand with the wrist splinted in three 30° extension, neutral, and 30° flexion with neutral radioulnar position. Participants performed tip-to-tip pinch from 0 to 20 N using a pinch dynamometer. Nerve FR was calculated as the ratio of the major to minor axis, and nerve centroid position was tracked using optical flow analysis. Two-way repeated measures ANOVAs were used to examine the effects of wrist FE posture and pinch force on nerve FR as well as radioulnar and dorsovolar position. FR of the nerve was significantly dependent on FE (p < 0.001), force (p < 0.005), and their interaction (p < 0.001). The greatest FR (7.7 ± 3.2) was observed in wrist flexion posture at 20 N. The nerve was at different radioulnar or dorsovolar positions at varying wrist FE postures and forces (p < 0.05). Wrist FE posture and pinch force independently and interactively influence median nerve morphology and displacement in the carpal tunnel, with the most pronounced effects in the flexion posture under load. These findings advance understanding of median nerve mechanical behavior in the wrist for healthy subjects and potential pathomechanics associated with manual tasks.