The purpose of this study was to investigate the deformation and displacement of the normal median nerve in the carpal tunnel during index finger and thumb motion, using ultrasound. Thirty wrists from 15 asymptomatic volunteers were evaluated. Cross-sectional images during motion from full extension to flexion of the index finger and thumb were recorded. On the initial and final frames, the median nerve, flexor pollicis longus (FPL), and index finger flexor digitorum superficialis (FDS) tendons were outlined. Coordinate data were recorded and median nerve cross-sectional area, perimeter, aspect ratio of the minimal-enclosing rectangle, and circularity in extension and flexion positions were calculated. During index finger flexion, the tendon moves volarly while the nerve moves radially. With thumb flexion, the tendon moves volarly, but the median nerve moves toward the ulnar side. In both motions, the area and perimeter of the median nerve in flexion were smaller than in extension. Thus, during index finger or thumb flexion, the median nerve in a healthy human subject shifts away from the index finger FDS and FPL tendons while being compressed between the tendons and the flexor retinaculum in the carpal tunnel. We are planning to compare these data with measurements in patients with carpal tunnel syndrome (CTS) and believe that these parameters may be useful tools for the assessment of CTS and carpal tunnel mechanics with ultrasound in the future. The carpal tunnel contains nine flexor tendons and the median nerve. These structures are surrounded by the subsynovial connective tissue (SSCT), which functions as a sliding interface among these structures. 1 The major pathological finding in carpal tunnel syndrome (CTS) is fibrosis of the SSCT, which changes the motion characteristics of the SSCT, tendon excursion, and median nerve, as noted during intraoperative inspection in cases of carpal tunnel release. 1-4 These changes may also cause elevated strain and pressure in the carpal tunnel, which ultimately can lead to CTS. 2,5 We hypothesize that due to fibrosis of the SSCT, the kinematics of the nerve and tendons in the carpal tunnel change in patients with CTS. We further hypothesize that these changes are associated with the evolution of CTS and that these changes can be monitored noninvasively by ultrasound. A first step in testing our hypotheses is to identify the normal motion pattern of the tendons and the median nerve in the carpal tunnel. These data can then be used as a baseline against which to compare CTS patients' data. If, as we hypothesize, detectable differences exist in the SSCT and tendon and nerve kinematics in patients with CTS, then these differences could be sought in individuals at risk for CTS. If our hypotheses are supported, then ultrasound could be a useful noninvasive tool to study the genesis of CTS and to monitor at risk individuals.Ultrasonography is a good imaging technique for the structures in the carpal tunnel. Several parameters within the carpal tunnel have been assess...