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This study investigated the expansion of the carpal tunnel resulting from the application of palmarly directed forces to the transverse carpal ligament (TCL) from inside the carpal tunnel. Ten fresh-frozen cadaveric hands were dissected to evacuate the carpal tunnel and thus to expose the TCL. A custom lever device was built to apply forces, ranging from 10 to 200 N, to the TCL. Without force application, the carpal tunnel area was 148.4 ± 36.8 mm2. The force application caused the TCL to form arches with an increase in cross-sectional areas of 33.3 ± 5.6 mm2 at 10 N and 48.7 ± 11.4 mm2 at 200 N, representing respective increases of 22.4% and 32.8% relative to the initial carpal tunnel area. The TCL length remained constant under the applied forces. It was found that the TCL arch formation was due to the narrowing of the arch width, which resulted from the migration of the bony insertion sites of the TCL. A geometrical model of the carpal tunnel was then developed to elucidate the relationships among the arch width, TCL length, arch height, and arch area. The model illustrated the effectiveness of carpal tunnel expansion by TCL elongation or arch width narrowing.

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The objective of this study was to investigate three-dimensional thumb joint movements produced by individual extrinsic thumb muscles. Ten cadaveric arms were dissected to expose the musculotendinous junctions of the flexor pollicis longus (FPL), abductor pollicis longus (APL), extensor pollicis brevis (EPB), and extensor pollicis longus (EPL). Each muscle/tendon was loaded to 10% of its maximal force capability whereas three-dimensional angular movements of the carpometacarpal (CMC), metacarpophalangeal (MCP), and interphalangeal (IP) joints were obtained simultaneously. We found that each extrinsic muscle produced unique joint angular trajectories in multiple directions. The FPL, APL, EBP, and EPL generated two, two, three, and six movements, respectively. The extrinsic muscles all together generated eight movements among the multiple thumb joints. High interjoint coordination was shown between the MCP joint flexion and IP joint flexion by FPL loading, as well as between the MCP joint extension and IP joint extension by EPL loading. High intrajoint coordination was observed between extension and supination at the CMC joint by the APL, EPL, and EPB. We concluded that each muscle produces movements in multiple joints and/or in multiple anatomical directions. The findings provide a novel insight into the biomechanical roles of the extrinsic muscles of the thumb. The human thumb is exceptionally mobile, strong, and dexterous partially due to its sophisticated musculature. Kinematically, thumb motion results from complicated combinations of flexion/extension, abduction/ adduction, and pronation/supination (i.e., axial rotation) at the carpometacarpal (CMC), metacarpophalangeal (MCP), and interphalangeal (IP) joints. 1,2 The movement capability of the thumb was previously determined by measuring the linear distance between the thumb tip and the fingertips or other anatomical landmarks, 3,4 but thumb pathology can be characterized by examining the angular motion at the individual joints. Manual goniometry is commonly used in the clinic to evaluate statically joint mobility in a single plane. Electrogoniometers can be used to monitor dynamic joint motion, 5 but they can be too bulky to record simultaneous movements from multiple joints within the thumb and can interfere with delicate thumb movements. X-rays, 1,6 and CT 7 have been used to study in vivo thumb kinematics, but the amount of kinematic information is limited. Recently, noninvasive, marker-based video methods have been used to study advanced kinematic features thus providing a more accurate representation of thumb motion. 2,[8][9][10][11] Marker-based motion analyses were applied to study thumb pathokinematics in individuals with cervical myelopathy 12 and lower median nerve block; 13 the authors reported aberrant thumb kinematics during pinch movements.Thus far, our understanding of thumb motion by each muscle remains dominated by basic functional anatomy using unidirectional, simplified nomenclature such as flexors or extensors. Therefore, this ...