Endurance testing of hip prostheses: a comparison between the load fixed in ISO 7206 standard and the physiological loads

Background. Pre-clinical endurance validation of innovative hip prostheses femoral components are carried out following the ISO 7206 standard. The in vitro fatigue test must simulate accurately the physiological load to correctly define the section of the stem. Objective. This study investigates if the loads defined in the ISO 7206 standard simulate correctly the physiological load that occur in vivo. Design. Simulation of the damage induced by in vitro test and in vivo loads is performed. Different designs of the stem are examined. Materials with different fatigue limit are considered. For the in vivo loads, different body weights are modelled. Methods. The minimal stem dimensions required to stand 20 years of patient use and to pass the fatigue test are calculated based on linear damage accumulation. Results. The results show that the ISO load simulates reasonably well the physiological load that is likely to occur in active patients with a low or normal body weight. Conversely, the ISO test underestimates the physiological load that heavy and active patients are likely to apply to the implant. Conclusions. Different minimum requirements for endurance strength should be fixed for hip prostheses designed for patients of different weight. This solution would reduce the risk of undersizing or oversizing the stem.