Most research on the thumb kinematics has focused on the local trapezo-metacarpal movement independently of the hand. In the clinical area, these studies can be sufficient. However, when the simulation of the hand movement and prehensile tasks with numerical dummies is needed, the thumb has to be considered as an integral part of the hand so a study in this direction proves to be essential. The objective of the present study is to analyze the kinematics of the thumb in relation to the hand (i.e. from the wrist’s joint). This paper proposes a four-link kinematic model of the thumb with 5 degrees of freedom (DOF) for a better representation of the opposition of the thumb with the other fingers. The interphalangeal and the metacarpophalangeal joints of the thumb have 1 flexion/extension DOF each and the carpo-metacarpophalangeal has 3 DOF (flexion/extension, abduction/adduction and rotation). For this last joint, the axes are not perpendicular and not converging, according to the anatomical features. The movement of the carpal complex is not taken into account. This model is based on Denavit-Hartenberg convention, designed to define any joint axis, even if successive axes are neither perpendicular nor converging. The position and orientation of each axis is defined using 3 parameters.
A Motion Analysis system has been used to collect the spatial trajectories of markers glued on the thumb and on the hand’s palm. Six volunteers (males and females) have been asked to perform simple flexion-extension and abduction/adduction movements while the hand was stabilized on a plate. For the study of the trapezo-metacarpal joint movement, a splint is fixed on the distal joints of the thumb in order to immobilize them.
This study allowed an in vivo kinematical characterization of the thumb and especially of the carpo-metacarpal joint. The mean values for the angle between the abduction/adduction axis of the wrist and the flexion/extension axis of the carpo-metacarpal joint is 131 ± 7° and the angle between the flexion/extension axis and the abduction/adduction axis of the carpo-metacarpal joint is 25 ± 10°.
The validation of this kinematical model consisted of executing various realistic postures of the hand while integrated in the numerical mannequin MAN3D.