Relationship between thumb laxity and trapezium kinematics

To investigate the relationship between thumb laxity (passive mobility), shape of the trapezium and trapezial mobility relative to the second metacarpal.

Sixty normal volunteers were assessed for the amount of thumb laxity by measuring the shortest distance of the thumb nail to the radius when the thumb was forcefully approximated to the forearm with the wrist in flexion. The inclination of the distal surface of the trapezium (angle β) and the mobility of the trapezium relative to the II metacarpal (Δangle α) were assessed using dynamic X-rays in maximal radial and ulnar deviation.

There was no statistical correlation between thumb laxity and shape of the trapezium (angle β). However, trapezium mobility (Δangle α) and thumb laxity were strongly correlated (P=0.018), with the more lax individuals registering higher trapezium mobility.

This investigation does not support the concept of thumb hypermobility being associated to a trapezium with more pronounced inclination of its distal articular surface. However, it has been found that the higher the thumb mobility, the more the trapezium tilts under load.