Effects of interfacial conditions on shape optimization of cementless hip stem: an investigation based on a hybrid framework

The typical biomechanical failure mechanisms associated with cementless Total Hip Arthroplasty (THA) are initial micromotion, stress shielding and interface stresses. While the former is a short-term load-induced failure criterion affiliated with the post-surgery rehabilitation period, the other two mechanisms are relatively longer term phenomena. At immediate post-operative stage, biologic fixation through bone ingrowth is yet to occur and implant stability is determined by the initial micromotion. However, by the time the other failure mechanisms become prominent, sufficient bone ingrowth already prevails and consequently, the implant-bone interface characteristic changes. Therefore, any preclinical simulation aimed at designing femoral implants needs to account for this dual interfacial behaviour while dealing with these failure objectives. The present study implements a hybrid framework comprised of neural network (NN), genetic algorithm (GA) and finite element (FE) analysis for a multi-criteria 3-D shape optimization of cementless femoral implant by addressing the dual interfacial behaviour. Bonded interfacial condition was used to analyse the effects of stress shielding and interface stresses, whereas a set of contact models were used to develop an NN for faster prediction of the initial micromotion based on implant geometry. The final trade-off implant models were analysed and subsequently, compared with a generic design of femoral implant.