Prediction of Hip Prosthesis Fatigue Properties : Influence of the Process

Recent studies have shown that more and more, patients who will receive a hip prosthesis, can be younger, more active and as well may have a body weight higher than previously. These evolutions must be taken into account during the development stage of new hip prosthesis. Fatigue testing must be performed as part of a new design approval. In most cases, a minimum level of fatigue performance is evaluated by testing at least 6 specimens of the worst case design in agreement with US Food and Drug Administration guidance (FDA) and applicable standards. It is well known that traditional fatigue test campaigns (e.g. staircase method) require large sample sizes and testing time to accommodate the scatter associated with component fatigue test results. For these reasons, high cycle fatigue (HCF) properties of orthopaedics implants are not often completely determined. This study aims at developing a testing tool for the fast prediction of hip prosthesis fatigue strength. For that purpose, we shall use a method developed these last years and for which the efficiency and the reliability have already been shown by several research teams. We found that the method so-called “self-heating” allows to determine with a single test tube the fatigue limit of certain orthopaedics materials. Nevertheless, in order to predict the fatigue life of finished goods, it is important to assess the state of the material in the course of the manufacturing process: strain, internal stresses, microstructures (grain size). In this work, we used finite element analysis to derive the strain/stress state after the forging process of a hip prosthesis. Once validated, this method will be used to calibrate the "self heating" measurements results for fast prediction of a hip prosthesis fatigue properties.