The effect of cup orientation on stability of trapeziometacarpal total joint arthroplasty: a biomechanical cadaver study.

It has been suggested that the cup of a trapeziometacarpal total joint replacement should be positioned parallel with the proximal articular surface of the trapezium to align it with the centre of motion. This would diminish the chance of dislocation. The goal of this study was to test this idea biomechanically. A linked trapeziometacarpal prosthesis was implanted in seven cadaver hands and combined with three-dimensional printed trapezium cups in 17 different orientations. For every combination, stability of the prosthesis was assessed through its entire passive range of motion. Dorsal inclination of the cup relative to the proximal articular surface increased the risk of dislocation with thumb flexion and opposition. The risk of dislocation was also increased with lateral or medial inclination of the cup exceeding 20°. Our results demonstrate that cup orientation is an important factor in prosthetic joint stability. Cup placement parallel to the proximal articular surface is ideal.