[How to define the joint movements unambiguously: proposal of standardization for the trapezometacarpal joint]

In order to define the movements of a joint, clinicians usually use anatomic terms. These terms are clearly understandable for a simple movement, defined in an anatomic plane. However, these terms are ambiguous for complex movements or for movements out of an anatomic plane. This, for instance, is the case for the internal-external axial rotation of the trapezometacarpal joint. For the study of complex movements, engineers preferentially use methods such as Euler angles, which correspond to three angles about three axes chosen in a defined order or sequence. Thus, the International Society of Biomechanics has proposed a joint coordinate system definition where every axis is associated with a functional degree of freedom of the joint. The first and third axes are embedded in the proximal and distal segments whilst the second axis, called the "floating" axis, is always orthogonal to the other two. The present work deals with the application of this concept to the trapezometacarpal joint. The two principal degrees of freedom, of flexion-extension and of abduction-adduction are defined following classical anatomical axes of respectively the trapezium and first metacarpal. Conversely, internal-external axial rotation is defined about the "floating" axis which does not have anatomical definition but can be geometrically deduced from the two others.