Background: The metacarpophalangeal joint has a unique morphology with a high degree of freedom. However, few studies have analyzed the kinematics of fingers owing to the rapid movement of the small bones involved. The in-vivo kinematics of metacarpophalangeal joints were analyzed by four-dimensional computed tomography (4DCT) and associated with its morphology. Methods: The flexion motion of the fingers of bilateral hands in 10 volunteers were examined using 4DCT. Iterative surfaces were registered to trace the surface of the proximal phalanges with respect to metacarpals. Rotation angles were calculated using Euler/Cardan angles. Findings: In the index finger, the proximal phalange supinated to a maximum flexion of 40° and then pronated, and its range of rotation was larger than the previous reports. In the other fingers, the proximal phalanges continued to supinate during flexion. The helical axis of the proximal phalange passed a point extremely close to the center point of bilateral condyles, and it moved toward the proximal and palmar directions until the middle stage of flexion and toward the proximal and dorsal directions during the late stage of flexion. The translation of the rotation axis was larger in the ring and little fingers. Interpretation: The rotation in the index finger was larger than previously reported. The helical axes moved in the dorsal direction and proximally during the latter phase of the flexion. These results can be employed to better understand the causes of implant failure of the metacarpophalangeal joints.
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