In-vitro fatigue fracture of hips stems - is the ISO standard valid?

摘要

Introduction: One of the most important aspects of the fatigue test method outlined in ISO 7206-4 for the endurance properties of hip arthroplasty stems is the peak load applied to the femoral head. This load assumes a mean patient weight of 78 kg (applied load = 2300 N). Whilst in the past this may have been representative of the population the aim of this study was to assess current trends in patient weight and to apply these in a fatigue test to a popular hip stem as a means to assess the validity of the ISO standard companies utilise for testing. Materials and Methods: ISO fatigue testing was completed using an Instron E3000(lnstron, UK) on 2 new femoral stem sizes; neither stem size had a published weight limit, all stems had a normal femoral offset. Stems were tested assuming a body weight of 90Kg and a force of 3.2 times body weight simulating walking (axial sinusoidal toad ranging from 300-2800 N) with proximal loosening (incomplete coverage). 10 million cycles of ISO cyclic fatigue was completed, followed by 50 further stumbling cycles at a toad of 10 times body weight applied in 0.1 seconds. Results: Under cyclic fatigue testing all components of the smaller size of the 2 stems fractured at -30000 cycles. Components of the larger stem size did not fracture following 10 million cycles of fatigue testing. The larger stems were found to permanently deform ＞5mm following the aggressive stumbling cycles, however, no subsequent fractures were observed. Discussion: The weight of 90kg used in this study reflected the 75th percentile of patient weight. BMI values of THR patients commonly exceed 30 and this corresponds well with studies published in the literature. Furthermore joint reaction forces of 3.2× body weight are representative of a hip replacement patient during normal walking. All implants chosen were above the implant size range where weight limits are applied. Preclinical fatigue testing is often carried out as a part of the design process to assess the endurance performance of the prosthesis. Consequently, ISO 7206 was developed to provide an international standard for the fatigue testing of the stemmed femoral component. Part 4 of the standard simulates proximal loosening and introduces out-of-plane bending and torsion a more severe loading case. Proximal loosening has been observed clinically in a very small number of prostheses raising a potential concern, however, no clinical fractures have been reported to the authors knowledge. Conclusion: In-vitro fatigue testing of a popular femoral stem simulating proximal loosening with a 75th percentile patient weight caused fracture in all components of a smaller stem size. The results suggest that the loading conditions applied by the ISO standard should be reviewed to reflect current weight trends in patients.