Morphological and positional changes of the carpal arch and median nerve during wrist compression

Abstract

Background: The carpal tunnel is a fibro-osseous structure containing the median nerve and flexor tendons. Its cross-sectional area has been shown to increase during compressive force application to the carpal bones in modeling and in vitro studies. The purpose of this study was to investigate the morphological and positional changes of the carpal arch and median nerve while in vivo compressive force was applied in the radioulnar direction across the wrist.

Methods: Ultrasound images of the carpal tunnel and its contents were captured for 11 healthy, female volunteers at the distal tunnel level prior to force application and during force application of 10 and 20N.

Findings: With applied force, the carpal arch width significantly decreased, while the carpal arch height and area significantly increased (P<0.001). The median nerve shape became more rounded as the compressive force magnitude increased, reflected by decreases in the nerve's flattening ratio and increases in its circularity (P<0.001). The applied force also resulted in nerve displacement in the radial-volar direction.

Interpretation: This study demonstrates that noninvasively applying radioulnar compressive force across the wrist may potentially provide relief of median nerve compression to patients suffering from carpal tunnel syndrome.

Keywords: Carpal tunnel syndrome; Compression; Force; Median nerve; Wrist.

Figure 1

Experimental set-up for in vivo wrist compression and ultrasound imaging.

Figure 2

Ultrasound images at each force level (0, 10, and 20 N) for a representative subject. The images on the left are unlabeled and the images on the right reflect the selections made in ImageJ for each landmark of interest, specifically the hook of hamate (), ridge of the trapezium (), thenar muscles ulnar point (), and the median nerve (solid line).

Figure 3

The displacement of the hook of hamate (HH, ), thenar muscles ulnar point (TUP, ), and median nerve (MN, solid line) with centroid () for a representative subject at 0, 10, and 20 N of wrist compression relative to the anatomically defined coordinate system with its origin at the trapezium (TM, ). The area beneath the dotted lines bounded by the X-Axis represents the carpal arch area.

Figure 4

The (a) carpal arch width (CAW), (b) height (CAH), and (c) area (CAA) at the different magnitudes of compressive force applied across the wrist. * P < 0.05, *** P < 0.001