3-D finite element model of the impaction of a press-fitted femoral stem under various biomechanical environments.

Background: Uncemented femoral stem insertion into the bone is achieved by applying successive impacts on an inserter tool called "ancillary". Impact analysis has shown to be a promising technique to monitor the implant insertion and to improve its primary stability.

Method: This study aims to provide a better understanding of the dynamic phenomena occurring between the hammer, the ancillary, the implant and the bone during femoral stem insertion, to validate the use of impact analyses for implant insertion monitoring. A dynamic 3-D finite element model of the femoral stem insertion via an impaction protocol is proposed. The influence of the trabecular bone Young's modulus (E), the interference fit (IF), the friction coefficient at the bone-implant interface (μ) and the impact velocity (v) on the implant insertion and on the impact force signal is evaluated.

Results: For all configurations, a decrease of the time difference between the two first peaks of the impact force signal is observed throughout the femoral stem insertion, up to a threshold value of 0.23 ms. The number of impacts required to reach this value depends on E, v and IF and varies between 3 and 8 for the set of parameters considered herein. The bone-implant contact ratio reached after ten impacts varies between 60% and 98%, increases as a function of v and decreases as a function of IF, μ and E.

Conclusion: This study confirms the potential of an impact analyses-based method to monitor implant insertion and to retrieve bone-implant contact properties.