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 arthroplasties in young patients, improving the long-term success of cementless implants is becoming a major challenge for the orthopedic community. This thesis proposed tools that can lead to improvements of implants survival, and a better understanding of the mechanisms behind aseptic loosening, reducing the need for implant revisions and their associated social and financial burden

Micromotion-induced peri-prosthetic fluid flow around a cementless femoral stem

Micromotion-induced interstitial fluid flow at the bone-implant interface has been proposed to play an important role in aseptic loosening of cementless implants. High fluid velocities are thought to promote aseptic loosening through activation of osteoclasts, shear stress induced control of mesenchymal stem cells differentiation, or transport of molecules. In this study, our objectives were to characterize and quantify micromotion-induced fluid flow around a cementless femoral stem using finite element modeling. With a 2D model of the bone-implant interface and full-factorial design, we first evaluated the relative influence of material properties, and bone-implant micromotion and gap on fluid velocity. Transverse sections around a femoral stem were built from computed tomography images, while boundary conditions were obtained from experimental measurements on the same femur. In a second step, a 3D model was built from the same dataset to estimate the shear stress experienced by cells hosted in the peri-implant tissues. The full-factorial design analysis showed that local micromotion had the most influence on peak fluid velocity at the interface. Remarkable variations in fluid velocity were observed in the macrostructures at the surface of the implant in the 2D transverse sections of the stem. The 3D model predicted peak fluid velocities extending up to 2.2 mm/s in the granulation tissue and to 3.9 mm/s in the trabecular bone. Peak shear stresses on the cells hosted in these tissues ranged from 0.1 Pa to 12.5 Pa. These results offer insight into mechanical stimuli encountered at the bone-implant interface

Distribution of muscular forces in a numerical musculoskeletal model of the shoulder

The shoulder is a complex joint with many muscles acting between the scapula and the humerus. Thus it is obvious that the mechanical system is indeterminate and an optimization method has to be used to find the muscular forces acting on the glenohumeral joint. In the LBO, an optimization method based on null space optimization had been developed (Aeberhard et al., 2009) and applied to a numerical musculoskeletal model of the glenohumeral joint (Terrier et al., 2007). The goal of the project is to use this optimization approach to determine the forces of the muscles acting on the glenohumeral joint during a simple abduction motion and activities of daily living