gms | German Medical Science

Objectives/Interrogation: Screw-home torque rotation is known to occur on carpometacarpal joint of the thumb during the final phase of opposition, but there is no criterion for quantitative measurement of the opposition of the thumb. Therefore, we evaluated the opposition by dividing it into flexion and abduction movements and examined when the movement occurs using CT images.

Methods: The subjects were 10 healthy males aged 23-65 years (mean, 37.3 years). CT images were obtained in 4 equally divided positions during the motion from maximum extension to maximum flexion of the thumb using a supportive device made of Styrofoam. Similarly, images were obtained in 4 positions during maximum adduction to maximum abduction. A three-dimensional model was constructed from the obtained images, and the models of each position were superimposed with reference to the trapezium. A straight line connecting the extension line of the bone axis of the first metacarpal bone at each position with the shortest distance was defined as the rotation axis, and we hypothesized that the first metacarpal bone is translated while rotating around the rotation axis and the bone axis. A movement section from the first limb position to the second limb position is defined as I, from the second limb position to the third limb position as II, and from the third limb position to the fourth limb as III. The amount of angular change around the rotation and bone axes and the translational movement between each position were evaluated.

Results and Conclusions: The amount of angular change around the rotation axis increased as the flexion angle increased during flexion and decreased as the abduction angle increased during abduction. The angular change around the bone axis involved internal rotation in both flexion and abduction, which increased as the flexion angle increased during flexion and was constant during abduction. In terms of translational movement, the first metacarpal bone shifted toward the palm side with respect to the trapezium during flexion, but then moved toward the dorsal side as maximum flexion was approached. During abduction, the first metacarpal bones moved toward the ulnar side with respect to the trapezium regardless of the abduction angle.

In this study, as the flexion angle increased, flexion of the first metacarpal bone and angular change in internal rotation also increased. Hence, the screw-home torque rotation appears to be more prominent in flexion than in abduction.