Quantitative stiffness of the median nerve, flexor tendons, and flexor retinaculum in the carpal tunnel measured with acoustic radiation force impulse elastography in various wrist and finger positions

Abstract

Despite the high prevalence and clinical importance of soft-tissue disorders, objective methods for evaluation of the biomechanical properties of soft tissues are lacking. This study aimed to quantitatively evaluate stiffness, an important biomechanical characteristic of soft tissue, using acoustic radiation force impulse (ARFI) elastography. The shear wave velocity (SWV, m/s) values of soft tissue structures within the carpal tunnel (CT) were measured in various combinations of wrist and finger positions. Twenty-six healthy adults were enrolled in this study. We measured the cross-sectional area of the median nerve (MN) and the SWV values of several structures within the CT at the CT inlet level. Measurement of SWV of the MN, flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), and transverse carpal ligament (TCL) were conducted in six wrist/finger motion combinations. When the wrist and fingers were in neutral positions (position A), the mean SWV was lowest for the MN (mean +/- standard deviation, 2.3 +/- 0.5m/s), followed by the FDS (2.9 +/- 0.2), FDP (3.2 +/- 0.3), and TCL (3.3 +/- 0.4). The SWV was significantly different among the six different wrist/finger positions for all structures (P<.001). However, the MN cross-sectional area was not significantly different (P=.527). The SWV values for the MN, FDS, and FDP increased significantly as the wrist/finger positions the stress on the tendons increased (from position B to F) compared with a neutral position, while the SWV of the TCL was significantly higher for in all positions compared with neutral, except for wrist neutral, finger extension. The SWV values for the MN, FDS, and TCL gradually increased as stress increased. The intra-CT structures are under increased stress during wrist and finger motions than when the hand is in a neutral position. We have used ARFI elastography to gain insight into the pathophysiology of CTS.