7 It has been suggested by Haines 8 and Edmunds 4, 5 that the screw-home torque occurring during thumb opposition leads to a congruent joint with tightening of the dorsal ligaments, offering a stable configuration for powerful gripping and pinching.ĭespite this detailed appraisal, a quantification of the entire joint chain kinematics, including not only the TMC but also the scaphotrapezial (ST) joint, remains lacking. This can be compared to the screw-home mechanism of the knee, where external rotation of the tibia occurs at the end of knee extension. 2, 4- 6 Important in the functioning of the thumb is the existence of a screw-home mechanism, 4, 5 characterized by an axial rotation of the MC1 during which soft-tissues surrounding the TMC joint generate a stabilizing screw-home motion at the end phase of thumb flexion and thumb opposition. The biomechanics of the thumb have been extensively described in previous studies. During opposition of the thumb, which is typically used to score thumb mobility and functionality (i.e., Kapandji score 3), flexion, adduction, and pronation are combined. In addition, the thumb can rotate around its long axis, a movement also referred to as pronation-supination (Pro-Sup). 1 This saddle-shaped joint has two principal axes of motion 2 allowing extension–flexion (Ex–Fl) and abduction–adduction (Ab–Ad) of the thumb. The wide range of motion of the thumb is mainly achieved through motion at the basal thumb joint or trapeziometacarpal (TMC) joint. The thumb is the most critical digit to overall hand function and its high mobility is important for the dexterous capacity of the human hand. In addition, our results point to a potential motion coupling between the TMC and ST joints and underline the complexity of thumb kinematics.

The coupling of axial rotation of the first metacarpal with flexion and abduction during thumb flexion supports the existence of a screw-home mechanism in the TMC joint. A wider range of motion is observed for the first metacarpal, which displays a clear axial rotation. The analysis shows that flexion and adduction of the thumb result in a three-dimensional rotation and translation of the entire articular chain, including the trapezium and scaphoid.

The corresponding rotations angles, translations, and helical axes were calculated for each sequence. The CT images were segmented and three-dimensional surface models of the radius, scaphoid, trapezium, and the first metacarpal were created for each thumb motion. The hand of 16 asymptomatic women without signs of thumb osteoarthritis were CT scanned in positions of maximal thumb extension, flexion, abduction, and adduction. In addition, we want to evaluate the potential coupling between the TMC and ST joints during these tasks. The primary objective of this study is to quantify the in vivo kinematics of the trapeziometacarpal (TMC) and scaphotrapezial (ST) joints during flexion and adduction of the thumb. Such investigation is essential to improve our understanding of thumb function and to develop better strategies to treat thumb joint pathologies. However, the kinematics of its entire articular chain have not yet been quantified.

The thumb plays a crucial role in basic hand function.

Thumb abduction