Biomechanical interaction between the transverse carpal ligament and the thenar muscles.

The transverse carpal ligament (TCL) serves as the origin of the thenar muscles and is integral to thenar muscle contraction anatomically and biomechanically. TCL hypertrophy has been observed in patients with carpal tunnel syndrome and is potentially caused by repetitive hand use. The purpose of this study was to investigate the biomechanical interaction between the TCL and the thenar muscles. Specifically, the morphological changes of the carpal arch, formed by the TCL, in response to thenar muscle contractions were examined during isometric tip pinch between the thumb and index finger. Ultrasound videos of the carpal tunnel were recorded from 13 healthy subjects and were synchronized with the forces measured by a pinch dynamometer. The thenar muscles' ulnar point, trapezium, and hamate were tracked by a pattern-matching program. The pinch force significantly affected the carpal arch height, width, and area (P < 0.005). As the pinch force increased from 0 to 100% maximum voluntary contraction force, the carpal arch height increased from 1.8 ± 1.0 to 2.3 ± 1.3 mm, the carpal arch width decreased from 23.9 ± 2.4 to 23.1 ± 2.4 mm, and the carpal arch area increased from 22.2 ± 13.6 to 27.3 ± 16.3 mm(2). The TCL was pulled volarly during thenar muscle contractions, providing evidence for the biomechanical interaction between the ligament and muscles. Repetitive biomechanical stimulation on the TCL from thenar muscle contractions could lead to tissue remodeling and then TCL hypertrophy. This study sheds light on the potential cause of TCL hypertrophy, which may be an etiological factor for carpal tunnel syndrome.