Transverse ultrasound assessment of median nerve deformation and displacement in the human carpal tunnel during wrist movements

Abstract

The symptoms of carpal tunnel syndrome, a compression neuropathy of the median nerve at the wrist, are aggravated by wrist motion, but the effect of these motions on median nerve motion are unknown. To better understand the biomechanics of the abnormal nerve, it is first necessary to understand normal nerve movement. The purpose of this study was to evaluate the deformation and displacement of the normal median nerve at the proximal carpal tunnel level on transverse ultrasound images during different wrist movements, to have a baseline for comparison with abnormal movements. Dynamic ultrasound images of both wrists of 10 asymptomatic volunteers were obtained during wrist maximal flexion, extension and ulnar deviation. To simplify the analysis, the initial and final shape and position of the median nerve were measured and analyzed. The circularity of the median nerve was significantly increased and the aspect ratio and perimeter were significantly decreased in the final image compared with the first image during wrist flexion with finger extension, wrist flexion with finger flexion and wrist ulnar deviation with finger extension (p < 0.01). There were significant differences in median nerve displacement vector between finger flexion, wrist flexion with finger extension and wrist ulnar deviation with finger extension (all p's < 0.001). The mean amplitudes of median nerve motion in wrist flexion with finger extension (2.36 ± 0.79 normalized units [NU]), wrist flexion with finger flexion (2.46 ± 0.84 NU) and wrist ulnar deviation with finger extension (2.86 ± 0.51 NU) were higher than those in finger flexion (0.82 ± 0.33 NU), wrist extension with finger extension (0.77 ± 0.46 NU) and wrist extension with finger flexion (0.81 ± 0.58 NU) (p < 0.0001). In the normal carpal tunnel, wrist flexion and ulnar deviation could induce significant transverse displacement and deformation of the median nerve.

Keywords: Carpal tunnel syndrome; Median nerve; Ultrasound.

Figure 1

Ultrasound examination setup. The transducer was placed at the proximal carpal tunnel. A custom-made transducer-fixing device was fastened at the subjects’ palm to keep the transducer stable during finger or wrist motion. A goniometer was fixed on the back of hand to measure the wrist angle during wrist movement.

Figure 2

The location of median nerve at the beginning of wrist flexion with finger extension: the median nerve (arrow) is located at the palmar midline position of the carpal tunnel.

Figure 3

The location of median nerve at the end of wrist flexion with finger extension: the median nerve (arrows) was located posteriorly in the carpal tunnel.

Figure 4

The location of median nerve at the end of wrist ulnar deviation: the median nerve (arrows) was located between the flexor pollicis longus tendon and flexor retinaculum.

Figure 5

Median nerve displacement vector for finger flexion, wrist flexion with finger extension and wrist ulnar deviation. (Solid ellipses represent standard deviation, dashed ellipses represent 95% confidence limits and the radial line represents the mean vector for each group.)

Figure 6

The amplitude of median nerve displacement. The amplitude of median nerve motion in finger flexion, wrist extension with finger extension and wrist extension with finger flexion (A) were significantly less compared to wrist flexion with finger extension, wrist flexion with finger flexion and wrist ulnar deviation (B) (p<0.0001).