Modelling and simulation of alternative designs for the femur–implant interface of Journey patellofemoral prosthesis

The implantation of conventional patellofemoral arthroplasty solutions has been identified as a source of later femur weakness, which is associated with bone resorption and stress-shielding effect. In this work, the Journey PFJ® arthroplasty system, of Smith & Nephew (UK), is investigated. The purpose is to evaluate the structural effects of different femoral components on the bone–implant interface. The finite element method is used to evaluate the influence of the geometry and stiffness of alternative implants, for post-implantation and long-term scenarios. Firstly, the original bone–femur interface of Journey PFJ®, with four fixation pins, was modelled. Secondly, four alternative models were developed: (i) model with two pins; (ii) model without pins; (iii) model with new pin design; (iv) model with increased thickness. The behaviour of the five models was evaluated and compared. Validation against experimental data was also performed. Results showed that the elastic strain is two to three times lower on bone areas close to the pins, in the post-implantation scenario, which is most likely due to a stress-shielding effect. In a long-term scenario, bone rupture by fatigue may occur, associated with an increase in elastic strains. This effect is particularly noticed in the implants supported by fixation pins. In addition, contact analyses evidenced that implant stability is best promoted without fixation pins. Such outcomes suggest that the alternative implant without fixation pins on the bone–femur interface can improve the protection of the femur after patellofemoral arthroplasty, assuming good cement fixation throughout the implant’s lifetime.