The effect of tendon excursion velocity on longitudinal median nerve displacement: differences between carpal tunnel syndrome patients and controls

Abstract

The subsynovial connective tissue (SSCT) is a viscoelastic structure connecting the median nerve (MN) and the flexor tendons in the carpal tunnel. Increased strain rates increases stiffness in viscoelastic tissues, and thereby its capacity to transfer shear load. Therefore, tendon excursion velocity may impact the MN displacement. In carpal tunnel syndrome (CTS) the SSCT is fibrotic and may be ruptured, and this may affect MN motion. In this study, ultrasonography was performed on 14 wrists of healthy controls and 25 wrists of CTS patients during controlled finger motions performed at three different velocities. Longitudinal MN and tendon excursion were assessed using a custom speckle tracking algorithm and compared across the three different velocities. CTS patients exhibited significantly less MN motion than controls (p ≤ 0.002). While in general, MN displacement increased with increasing tendon excursion velocity (p ≤ 0.031). These findings are consistent with current knowledge of SSCT mechanics in CTS, in which in some patients the fibrotic SSCT appears to have ruptured from the tendon surface.

Keywords: carpal tunnel; flexor tendons; median nerve; ultrasound; velocity.

Figure 1

A–B Shown is a macroscopic (A) and electron microscopic (B) image of the SSCT between tendons. Normally, the SSCT is a flimsy structure that envelops the structures within the carpal tunnel. The SSCT consists of layers with interconnecting fibers. These interconnecting fibers are also connected to the tendon, stretching the interconnecting fibers between consecutive layers when the tendon moves. Copyright permissions will be asked for upon acceptance of the manuscript (Plast Reconstr Surg. 2006 Nov;118 6):1413–22).

Figure 2

Placement of the probe on the wrist when making ultrasound scans of the median nerve and tendons at the carpal tunnel.

Figure 3

Representative example of a longitudinal plane ultrasound scan of the median nerve with placement of the region of interest (ROI).

Figure 4

Relative MN displacements at different tendon excursion velocity levels for CTS patients and controls. Significant differences exist for relative MN motion between the different tendon excursion velocity levels for CTS patients and controls.