Development of an Optimum Artificial Wrist Joint

Wrist joint prostheses have developed from simple silicone joint spacers to the current state of the art where design philosophy now follows that of large joint replacement. A variety of shortcomings identified in existing designs
through their clinical use led to the development of a new wrist joint prosthesis being designed. A combination of disciplines yields the necessary data and tools to develop the optimum solution to replicating the anatomical rotational centres of the healthy wrist whilst at the same time satisfying the requirements of stability, effective implantation and wear characteristics.

The key tool in drawing conclusions from the articulating surfaces selected in the design proposals is Finite Element Analysis. The use of an implicit, nonlinear static contact analysis model developed and solved using MSC MARC
software was used to examine the behaviour of the implant models' Ultra High Molecular Weight Polyethylene component over a large range of motion and under a compressive load.

The anticipated displacements and loads caused by performing daily living tasks fall comfortably within the range of motion specified for the analyses.
This, coupled with repeated analysis of the model for small geometrical alterations enabled an optimum model to be selected.

The information from these analyses was used directly in making decisions on the design of the prosthesis in conjunction with cadaveric trials. The initial cadaveric trial validated the primary geometrical features of the design regarding centres of rotation and stem location and identified clear areas for design refinement. The second trial fully validated the design decisions taken.

Following the analytical work on the new design, and the completion of the cadaveric trials, a comparative analysis was carried out with competing designs to further validate the design. The results of this show that the new design is competitive in terms of demonstrating low stress concentrations over a practical range of motion, and that it also has excellent stability. Beyond these features, the new implant design also facilitates secure implantation, in
conjunction with a range of sizes likely to accommodate the greatest range of patient anatomical variation.

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