Micromotion and fluid flow around cementless femoral components in total hip arthroplasty

Cementless total hip arthroplasty is a highly successful and reliable procedure to restore joint function and reduce pain in patients with severe osteoarthritis. Nevertheless, despite the technical advances over the last decades in the development of cementless implants, between 5 % and 10% of cementless femoral components have been revised at 15-year follow-up. Revision procedures are less successful, require longer hospital stays, and are associated with higher mortality rates than primary procedures. The main cause for revision of cementless femoral components is aseptic loosening. The mechanisms behind aseptic loosening remain unclear, but the initial local mechanical environment is thought to be critical. In particular, both excessive micromotion and fluid flow at the bone-implant interface during early peri-implant healing have been related to aseptic loosening. In this thesis, micromotion was measured in vitro and fluid flow was predicted from measured micromotion using numerical modeling. The thesis is divided into three studies. First, a micro-computed tomography (micro-CT) based technique using radiopaque markers to measure full-field local implant micromotion around metallic cementless stems was developed. The technique was highly reliable, with a bias and repeatability similar to that of linear variable differential transformers (LVDTs), which are the current gold standard for micromotion measurement. It provided the first full-field map of micromotion around a cementless femoral stem. This technique offers promising developments in the area of pre-clinical testing of orthopedic implants, and paves the way towards the validation of patient-specific preoperative planning tools. Then, the developed micro-CT technique was used to compare the primary stability of the collared and collarless versions of the same cementless femoral stem. Local micromotion was measured in two groups of cadaveric femurs implanted with either version of the stem. We found no significant difference in primary stability between collared and collarless stems for activities of daily living. Finally, a poroelastic finite element model of the initial bone-implant interface around a cementless stem was developed. The model predicted micromotion-induced fluid flow based on local micromotion determined experimentally with the micro-CT based technique. We obtained the distribution of fluid velocity in the granulation tissue between the implant and bone, and within the bone that surrounds the implant. From fluid velocity, we inferred the range of shear stress experienced by the cells hosted in each tissue. These results offer new prospects to understand the interplay between mechanical and biological aspects that leads to aseptic loosening. Indeed, the mechanical stimuli experienced by cells in the peri-implant space could be related to results obtained in vitro with cells cultured in flow chambers. With the aging population and the continual increase of arthroplasti