Development of an optimum acetabular cup prosthesis.

Abstract

The aim of this researchis to use advanced materials and state of the art Finite Element Analysis to produce a novel acetabular cup prosthesis. An investigation into current static and fatigue testing procedures for acetabular cup prostheses has identified current strengths and weakness and proposes a cheap, fast method that is closer to anatomical loading conditions. The current trend of performing Total Hip Replacements(THR) on younger patients incurs an increasing number of revisions. Therefore, prolonging the life of a THR is of paramount importance and is the principal goal of this study. With this trend comes a new set of design goals driven by higher patient expectations of their THR. Younger patients not only require increased longevity from their prosthesis but also increased performances so as to be able to undertake more strenuous everyday activities, such as sports. Historically, the main objective of THR was to relieve pain and increase quality of life in the elderly and was not intended for young active patients. Previous research proposes that the acetabularcup design has far more impact on long-term survival of the THR than the femoral component. Optimising the acetabular cup prosthesis produces a highly complex problem where many of the individual design factors have massive impact on the system. A main aim is to develop a material or combination of materials to optimise the stress distribution in the system without sacrificing the service life of the THR. Of the 800,000 hip replacements carried out annually, many of the current acetabular cups have some form of polyethylene bearing surface. However, polyethy lenewear debris is seen as a major contributing factor to bone resorption and hence prosthesis loosening. Changes in stress values, even caused by initial primary fixation during the operation, can result in stresses being transferred in an unrealistic manner. The effect is that the bone grows to differing thickness and strengths. This is known as remodelling. For optimisation of the acetabular cup, the properties of the natural hip must be retained by minimising both remodelling and bone resorption. The present research, with collaboration from Orthodynamics Ltd., has produced novel composite acetabular cup prostheses with a ceramic on ceramic bearing surface that should last longer and perform better, thus reducing the necessity for costly and debilitating revisions later on in life. In addition, the use of the proposed novel anatomically orientated mechanical testing methods gave fast,cheap results and could also allow further research into fatigue failure of alumina bearing couples in THR.