The combination of a trapeziometacarpal prosthesis and a metacarpophalangeal arthrodesis of the thumb was formally contraindicated by JY de la Caffinière. This is a landmark notion, but no theoretical, experimental or clinical study has confirmed it. Combined indications for arthrodesis and trapeziometacarpal prosthesis are rare. Isolated arthrodesis is more common. In both cases, it is important to avoid overloading the trapeziometacarpal joint.

The aim of this study is to verify whether metacarpophalangeal arthrodesis really does increase the load on the trapeziometacarpal joint, taking into account bone shortening and the angle at which the arthrodesis is performed. The second aim is to review the literature on series of metacarpophalangeal thumb arthrodeses.

Material & Methods: We used a previously developed 2-dimensional mathematical model to calculate stresses at the trapeziometacarpal joint during a key pinch grip. We calculated stresses at constant thumb column lenght, and after 6 and 8 mm shortening to account for the bone resection required to perform arthrodesis. We also tested several levels of flexion between the metacarpal and the first phalanx in the arthrodesis situation.

All other parameters being equal, we found a 118% increase in trapeziometacarpal stress after metacarpophalangeal arthrodesis. When arthrodesis is performed at 25 ° of flexion, the trapeziometacarpal stress level is comparable to the physiological situation.

Arthrodesis does not increase trapeziometacarpal stress when performed at 25 ° flexion. Under these conditions, metacarpophalangeal arthrodesis does not increase the risk of trapeziometacarpal osteoarthritis, and is compatible with a trapeziometacarpal prosthesis.