SCREWED TRAPEZIUM CUP: ANATOMICAL, BIOMECHANICAL AND MULTICENTER CLINICAL EVALUATION OF THE RIGHT IMPLANTATION

Abstract

Introduction: Trapeziometacarpal prosthesis allows to reach faster mobility and usefull thumb than trapeziectomy. But successfull Implantation of the trapezium cup depends on the bone stock and the jig. An anatomical and biomechanical study is presented followed by a prospective clinical evaluation of the impllantation af a screwed trapezium cup to define the best way to reach the center of the trapezium.

Matériel et méthodes: Cadaver study: 11 screwed trapezium implant have been implanted on cadaver (age > 70 yo, alcool conservation). 5 implants with 5 spires and 6 implants with 3 spires have been tested. Extraction tests have been performed after Xray evaluation of the position of the implant. 2 series of test were done in the subgroup of trapezium with 3 spires. Ergonomic and dynamometric jig to implant the trapezium cup has been invented to avoid fracture during implantation.

Clinical study: 58 trapezium cup have been implanted in 6 months with evaluation of the position of the implant in the center of the trapezium.

Résultats: Best bone stock was identified on medial border of trapezium. Only 1/5 trapezium cup with 5 spires was extracted (120N). In the subgroup of trapezium with 3 spires, if only 2 spires were screwed (first serie of test) the extraction load reached 103, 24N (57–133). If 3 spires were screwed (second serie of test) the extraction load reached 89,5 N (45–137). Trapezium was stronger in male than in female No trapezium fracture have been pointed, but slight fissuration of the lateral border were observed in 4 cases after é series of test. The multicenter study allowed to validate the the operative technique of implantation: key point were reported as: optimal view on the borders of the trapezium, implantation of a pin in the center of the trapezium under Xray control and preparation of the bone with approproate jig around the well positionned pin. 2 fractures of the trapezium were observed explained by the implantation of the trapezium cup without help of the Xray control. Each time the surgeon has pinned with no fluoroscopic assistance, the pin was never in the center of the trapezium.

Discussion: The two main complication of trapezio metacarpal prosthesis remain the instability with dislocation and loosening. Such implant is not recommended if trapezium is less than 8 mm. The key point of such procedure remain the implantation of the cup in the trapezium. Bone stock is more important on medial side and implantation of the cup in the center of the bone needs fluorocopic even if the surgeon is an experimented one. 3 spires in the bone of the tested screwed cup remain efficient to reach sufficent extraction load.