Biomechanical evaluation of the motor function of the thumb.

An experimental apparatus was developed to measure force production of a digit at various points of force application along the digit and in any direction within the transverse plane of the longitudinal axis of the digit. Eight normal subjects with asymptomatic hands were tested. Maximum voluntary isometric contraction forces were measured at the interphalangeal joint of the thumb in 16 directions that were evenly distributed within 360 degrees. Peak forces measured in all directions were used to create polar plots and to construct force envelopes using cubic spline interpolation. The areas of the force envelope and force quadrants were then calculated. The force produced by the thumb was dependent on the direction of force application. The highest force, 104.8 +/- 14.2 N, was generated in flexion, while the lowest force was generated in extension. The forces in extension, abduction and adduction were 24.8%, 57.2%, and 46.2% of the flexion force, respectively. Relatively high forces were generated in the directions of flexion combined with abduction, and flexion combined with adduction. The area of the entire force envelope was found to be 12,142 +/- 3,149 N-N. The percentage quadrant areas, relative to the total force envelope area, for extension-adduction, extension-abduction, flexion-abduction, and flexion-adduction were 7.8%, 11.4%, 39.3%, and 41.4%, respectively. The percentage quadrant areas for extension, abduction, flexion, and adduction were 4.9%, 23.6%, 52.7%, and 18.8%, respectively. The current study provides a more advanced and comprehensive method for quantification and investigation of the motor function of the thumb, which has potential in clinical applications for diagnosis of hand disorders, evaluation of deterioration or improvement of hand motor function, and guidance of therapeutic and surgical intervention.