# Ultrasound Shear Wave Speed of the Transverse Carpal Ligament During Pinching
###### Abstract  
The thumb and transverse carpal ligament (TCL) have an anatomical connection and biomechanical interaction. Understanding the in vivo mechanics of this interaction is valuable for the study of hand and wrist biomechanics. The objective of this study was to quantify this biomechanical interaction using shear wave elastography. The hands and forearms of healthy volunteers (\(n=11\)) were submerged in water. A pinch meter was placed between the thumb and index finger. An ultrasound transducer was placed at the distal carpal tunnel to image the cross section. Ultrasound and shear wave elastography images were taken for pinch forces of 0, 10, 20, and 30 N. The shear wave speed (SWS) was measured on the TCL. Repeated measures ANOVAs were used for comparisons (\(\alpha=0.05\)). SWS increased with increasing pinch force (\(p=0.007\)). The SWS changed by 0.27 m/s [95% CI: (\(-0.01\) m/s, 0.55 m/s); \(p=0.0572\)], 0.54 m/s [95% CI: (0.07 m/s, 1.01 m/s); \(p=0.0294\)] and 0.83 m/s [95% CI: (0.27 m/s, 1.39 m/s); \(p=0.0079\)] when the pinch force was increased from 0 to 10 N, 0 to 20 N and 0 to 30 N. TCL SWS increases with in vivo loading.  
t =1 [table]  
and the frequent loading of the ligament during common hand activity have motivated previous studies of TCL mechanics.  
Ex-vivo tensile testing of the TCL has shown the ligament to be capable of sustaining appreciable magnitudes of deformation [11]. The TCL is anisotropic, with greater elastic modulus in the radioulnar direction, as compared to the proximal-distal direction [12]. Regional analyses have shown the mechanical properties of the ligament to vary along the surface [13; 14]. In vivo loading of the TCL is further complicated by the distribution of the musculature attachments [2] and the diversity of hand and thumb kinetics. Investigating the in vivo response of the TCL to everyday hand activity involving the thumb can advance the understanding of the normative biomechanics of the healthy ligament and the biomechanical interaction with the thumb.  
One such method of analyzing in vivo tissue characteristics is shear wave elastography (SWE). SWE has been used for the analysis of a wide variety of anatomy in experimental and clinical settings, including for the carpal tunnel [15; 16; 17; 18]. Observationally, SWE has demonstrated capability in detecting shear wave speed (SWS) differences between healthy and pathological conditions [19; 20]. Mechanically, the imaging modality has exhibited increases in SWS correlating with increases in mechanical perturbations, such as pressure increases in the carpal tunnel [15; 16] and force application to ligamentous tissue [21; 22]. The proportionality of SWS and tissue mechanics is a well-established phenomenon [23]. When looking to evaluate the in vivo mechanics of the TCL resulting from thumb activity, SWE is a suitable imaging modality capable of detecting the biomechanical response as potential changes in SWS.  
Investigating the in vivo mechanics of the TCL, as a result of daily hand usage involving the thumb, can provide insight into the natural interaction between the thumb and the TCL. An understanding of this interaction can serve as a baseline for diagnostic analyses of common hand and wrist musculoskeletal disorders, such as carpal tunnel syndrome or trapeziometacarpal joint osteoarthritis, which may act to compromise this interaction. The objective of this study was to use SWE to measure in vivo TCL SWS during pinch tasks. The hypothesis was that TCL SWS would increase with increasing pinch force.