[Biomechanical aspects of arthrosis of the carpometacarpal joint of the thumb and its treatment with a special endoprosthesis].

The joint between the trapezium and the first metacarpal is anatomically like a saddle. In function, however, it is used like a ball and socket joint with the following possible movements: 1. Flexion - extension, 2. abduction - adduction, 3. opposition and circumduction. The movements in the two first planes are completely congruent, whereas in the third an incongruent motion of the joint is created by the rotation of the metacarpal on the trapezium. In this situation there is joint contact at one point only, being - biomechanically - a prearthrotic deformity as described by HACKENBROCH. Accordingly, the carpometacarpal joint of the thumb in normal use is exposed to special stress which can end in degeneration. When in arthrosis of the carpometacarpal joint of the thumb conservative measures fail, we replace the deformed joint by prosthesis having the construction of a ball and socket joint. The prosthesis (designed by FRANCOBAL) has a metallic part with body, neck and a sphereshaped head, and a polyethylene socket, which can be coupled to the head by a snap mechanism. Both parts are inserted with polymethylmetacrylate cement into the prepared bone of the trapezium and the first metacarpal. The advantages of this method are: no pain, stability of grip, and motion similar to that of a ball and socket joint. Because this motion is physiological, loosening of the implants and their insertions are avoided. The first 8 patients treated with this method show satisfying functional results. The longer follow-up time is 6 years.