Abstract
Thumb opposition is essential for grasping, and involves the flexion and abduction of the carpometacarpal and metacarpophalangeal joints of the thumb. The high number of degrees of freedom of the thumb in a fairly small space makes the in vivo recording of its kinematics a challenging task. For this reason, along with the very limited independence of the abduction movement of the metacarpophalangeal joint, many devices do not implement sensors to measure such movement, which may lead to important implications in terms of the accuracy of thumb models. The aims of this work are to examine the correlation between thumb joints and to obtain an equation that allows thumb metacarpophalangeal abduction/adduction movement to be estimated from the other joint motions of the thumb, during the commonest grasps used during activities of daily living and in free movement. The correlation analysis shows that metacarpophalangeal abduction/adduction movement can be expressed mainly from carpometacarpal joint movements. The model thus obtained presents a low estimation error (6.29°), with no significant differences between grasps. The results could benefit most fields that do not typically include this joint movement, such as virtual reality, teleoperation, 3D modeling, prostheses, and exoskeletons.
Highlights
IntroductionAt the CMC joint, the flexion/extension axis is in the trapezium and the abduction/adduction axis is in the metacarpal, their positions being constant throughout the range of motion of the joint, and not perpendicular to the bones [5]
The correlation between thumb joints during grasps was explored, and a prediction model was obtained that allows the thumb MCP abduction/adduction angle to be estimated from the other joint motions of the thumb during grasp postures and free movements
IP joint coordination is present in previous studies [8] and reflects the fact that grasping, in general, requires flexing the joints to place the tip of the thumb in the grasping position
Summary
At the CMC joint, the flexion/extension axis is in the trapezium and the abduction/adduction axis is in the metacarpal, their positions being constant throughout the range of motion of the joint, and not perpendicular to the bones [5]. As the axes are non-perpendicular and non-intersecting, it is difficult to assign classic planes to thumb movements and postures, leading to different ways of defining the thumb motion [6]. For this reason, some studies consider pronation at the CMC joint, the most widely accepted kinematic model is to use only five DoF: CMC flexion–extension and abduction/adduction, MCP flexion-extension and abduction/adduction, and interphalangeal (IP) flexion–extension.
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