In trapeziometacarpal arthroplasty, correct implant position may be necessary to prevent complications such as dislocation, component loosening and premature wear. The metacarpal stem more easily fits anatomically. However, the cup in the trapezium is not anatomical and guidelines for its orientation are not uniformly defined. We determined the centre of the range of motion of the trapeziometacarpal joint in 30 healthy patients on postero-anterior and lateral radiographs and its relationship to the proximal articular surface of the trapezium. Our study suggests that in thumb carpo-metacarpal total joint arthroplasty, the prosthetic cup in the trapezium should be placed parallel to the proximal articular surface of the trapezium and combined with a metacarpal neck with 7° palmar offset. This should optimize arthroplasty ranges of motion and may minimize the risk of postoperative complications. Our study provides a reference for the surgeon to check correct cup alignment intra-operatively with fluoroscopy.