Background: The biomechanical interaction between the median nerve and the flexor tendons is an important consideration in Carpal tunnel syndrome (CTS). We aim to quantify the displacement and compressive deformation pattern of the median nerve in various stages of finger flexion in the normal population at the inlet of the carpal tunnel. Methods: Transverse ultrasounds images were taken at the carpal tunnel inlet during full-extension, mid-flexion and full flexion. The displacement, distance, Feret's diameter, and perimeter of the median nerve were calculated and compared between each position. Results: Biphasic median nerve motion was observed, with a displacement of 2.84 ± 3.49mm in the ulnar direction from full-extension to mid-flexion (Phase I) and a further 0.93 ± 3.04 mm from mid-flexion to full flexion (Phase II). Of 49 hands, 37 (75.5) exhibited ulnar displacement in Phase I while 12 (24.5) exhibited radial displacement. Feret's diameter (5.95 ± 1.08mm) and perimeter (13.28 ± 2.09 mm) of the median nerve were greatest in the mid-flexed position. Conclusion: In a healthy Asian population, the median nerve has a biphasic motion during finger flexion, with maximal deformation in the mid-flexed position.
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