Modelling and in vitro evaluation of customised Ti6Al4V and PEEK hip implants for improving osseointegration and reducing stress shielding

Following total hip arthroplasty (THA), a considerable level of natural mechanical loading is shielded from the cortical bone and is transferred to the hip stem. This stress shielding effect caused by the underloaded femur post THA, over time, lead to bone loss. This consequently weakens the implant support and increases the risk of elevated micromotion at the bone-implant interface, leading to aseptic loosening and potentially femoral fracture, requiring a revision surgery. Many studies have investigated the development of different types of hip stems with various geometrical/structural designs and material properties to reduce the stress shielding. However, current approaches in the literature do not show a significant reduction in stress shielding and bone resorption, and often do not evaluate these parameters in Gruen zones, where such considerations are very important for clinicians when evaluating the performance of a hip implant. Furthermore, no fatigue performance has been performed on any of the suggested low stiffness hip stems in the literature. The overall aim of this thesis was to design and develop a low stiffness hip stem that can simultaneously minimize bone resorption and implant instability. Two custom tailored stems were developed and evaluated. One had a low material stiffness made from polyether ether ketone (PEEK) and one had a variable stiffness based on graded lattice structures made from 3D printed titanium (i.e. Ti6Al4V). In this study, stress shielding and bone resorption were evaluated across the Gruen zones using experimental and validated computational models. The overall stiffness and fatigue life of the developed hip stems were measured. Results demonstrated that the developed low stiffness porous Ti6Al4V and PEEK hip stems considerably reduced the level of stress shielding and bone resorption compared to a solid Ti6Al4V hip stem with identical geometry. This reduction was more evidenced in the proximal femur