Ultrasound assessment of the displacement and deformation of the median nerve in the human carpal tunnel with active finger motion

Abstract

Background: Peripheral nerves are mobile structures, stretching and translating in response to changes in the position of adjuvant anatomic structures. The objective of this study was to develop a novel method to characterize the relative motion and deformation of the median nerve on cross-sectional ultrasound images of the carpal tunnel during active finger motion.

Methods: Fifteen volunteers without a history of carpal tunnel syndrome or wrist trauma were recruited. An ultrasound scanner and a linear array transducer were used to evaluate the motion of the median nerve and the flexor tendons within the carpal tunnel during motion from full extension to full flexion by the four fingers (fist motion) and by the long finger alone. The displacement of the median nerve relative to the long-finger flexor digitorum superficialis tendon as well as the perimeter, cross-sectional area, circularity, and aspect ratio of a minimum enclosing rectangle of the median nerve were measured. The data were compared between single-digit motion and fist motion and between extension and flexion positions.

Results: The distance between the long-finger flexor digitorum superficialis tendon and the median nerve with isolated long-finger flexion was decreased in the ulnar-radial direction and increased in the palmar-dorsal direction as compared with the distance with four-finger flexion (p < 0.01). Compared with the values with fist motion, the aspect ratio was decreased and the circularity was increased with long-finger motion (p < 0.01).

Conclusions: This report presents a method with which to assess displacement and deformation of the median nerve on a cross-sectional ultrasound image during different finger motions. This method may be useful to assess pathological changes within the carpal tunnel, and we plan to perform a similar study of patients with carpal tunnel syndrome on the basis of these preliminary data.

Fig. 1

Setup for ultrasound examination. 1 = rubber band to secure the hand, 2 = protractor, 3 = transducer holder, 4 = transducer, and 5 = adjustable arm.

Fig. 2-A Fig. 2-B Fig. 2-C

Figs. 2-A, 2-B, and 2-C Image analysis of the median nerve. Fig. 2-A Cross-sectional image of the carpal tunnel in finger extension. FDS = flexor digitorum superficialis. Fig. 2-B and Fig. 2-C Aspect ratio of the minimum enclosing rectangle (Fig. 2-B) and circularity (Fig. 2-C). P = nerve perimeter, A = nerve area.

Fig. 2-A Fig. 2-B Fig. 2-C

Figs. 2-A, 2-B, and 2-C Image analysis of the median nerve. Fig. 2-A Cross-sectional image of the carpal tunnel in finger extension. FDS = flexor digitorum superficialis. Fig. 2-B and Fig. 2-C Aspect ratio of the minimum enclosing rectangle (Fig. 2-B) and circularity (Fig. 2-C). P = nerve perimeter, A = nerve area.

Fig. 2-A Fig. 2-B Fig. 2-C

Figs. 2-A, 2-B, and 2-C Image analysis of the median nerve. Fig. 2-A Cross-sectional image of the carpal tunnel in finger extension. FDS = flexor digitorum superficialis. Fig. 2-B and Fig. 2-C Aspect ratio of the minimum enclosing rectangle (Fig. 2-B) and circularity (Fig. 2-C). P = nerve perimeter, A = nerve area.

Fig. 3

Tendon and nerve displacement. a: Total displacement of the tendon was significantly larger with fist motion than it was with long-finger motion (p = 0.018). b: The tendon moved in an ulnar direction with fist motion and in a radial direction with isolated long-finger motion. There were significant differences in ulnar-radial motion of the tendon between the fist and long-finger motions (p < 0.01). c: The tendon moved in a palmar direction with fist motion and in a dorsal direction with long-finger motion. There was a significant difference in palmar-dorsal motion of the tendon between the fist and long-finger motions (p < 0.01). The I-bars indicate the standard deviation.

Fig. 4

Distance between the median nerve and the long-finger flexor digitorum superficialis tendon. The flexion ulnar-radial distance with long-finger motion was significantly smaller than that with fist motion (p < 0.01). In addition, in the test of long-finger motion, the ulnar-radial distance in flexion was significantly smaller than it was in extension (p < 0.01). The flexion palmar-dorsal distance associated with long-finger motion was significantly larger than that associated with fist motion (p < 0.01). The I-bars indicate standard deviation, the top and bottom of each bar indicate the range, and the horizontal line within each bar indicates the mean.

Fig. 5

Illustration summarizing our findings.