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Traditional computational and experimental assessments of implant performance are determin- istic; each computational (usually finite element (FE) based) simulation or experiment describes a single situation. While useful information can be gained from these analyses, when the number of variables involved increases, experimental simulations becoming increasingly time consuming and complex. In these cases, computational simulations are increasingly relied upon to predict implant performance. However, even when employing computational simulations to look at the effect of a large number of variables via sweep simulations for example, the problem can become computationally expensive and unfeasible in terms of time required. In the present work, stochastic cornpu tational methods are employed to assess the effect of multi- ple variables on the performance of the cernentless hip replacement. To verify the computational simulations, at each stage of the project, selected scenarios were tested experimentally. To assess implant performance, the following metrics were used: (i) implant micromotion and migration: excessive micromotion and migration are believed to be related to the most common cause of implant failure, implant loosening, and (ii) bone strain; excessive bone strain can result in bone fracture. An initial study on a neutrally positioned stem showed good correlation between the experimental results and the computational predictions. Mesh morphing techniques were employed to allow implant position to change throughout the simulations and assess how this altered the output metrics; it was observed that micromotion and strains generated in the cortex were most sensitive to varus/valgus angle. To further reduce computational expense, a surrogate modelling technique was used to assess the effect of both loading and implant positioning, on micromotion. The surrogate model was verified by selected FE models, placing confidence in the model, and again highlighted that in addition to vertical load, the varus/valgus angle affected the micromotion of cementless implant. Experimental investigations were carried out to corroborate the results obtained computation ally. The novelty of the experimental tests was in the use of an optical system, called digital image correlation (DIC), to measure implant motions and bone strains. This technique enabled non- contact three dimensional measurements to be made. While some qualitative relationships were obtained with FE outputs, good quantitative corroboration between the strain gauge and DIC suggests that DIC is a promising technique for the evaluation of implant performance in vitro.

Total hip replacement is one of the most common surgical procedures performed both in the UK and worldwide, with aseptic loosening cited as the primary reason for revision. Aseptic loosening is attributed to the wear debris generated by wear of the components. Recently, as great progress has been achieved in reducing wear at the head–cup interface, there has been a shift of research interest to other load bearing surfaces. The main purpose of this thesis is to study fretting wear mechanisms at the polished femoral stem–bone cement interface. The initial studies have investigated the bond strength at the stem–cement interface using seven brands of bone cement and femoral stems with different surface finishes. It can be confirmed that debonding at this interface is inevitable, which subsequently facilitates generation of fretting wear on the stem surface. A new test methodology has been developed to reproduce fretting wear clinically seen on polished stems through in vitro wear simulations, and it shows great success in comparison with previous attempts. In addition, migration of the stem within the cement mantle has been investigated, and it has been indicated that the simulation setup more realistically mimics clinical situations. The influence of two factors on generation of fretting wear, i.e. the duration of in vivo service of the hip implant and bone cement brand, has been studied. A potential fretting wear initiator that is concerned with polymerisation of bone cement has been identified, with both experimental (the results of wear simulations) and theoretical (finite element analysis) evidence being provided. In summary, the overall contribution of this research is that it has gained a deep insight into the fretting wear mechanism between polished femoral stem and bone cement.

When there is debonding at a bone-implant interface, the difference in stiffness between the implant and the bone can result in micromotion, allowing existing gaps to open further or new gaps to be created during physiological loading. It has been suggested that periprosthetic fluid flow and high pressure may play an important role in osteolysis development in the proximity of these gaps. It has also been suggested that the periprosthetic flow may facilitate migration of wear polyethylene particles to the periprosthetic bone, which can also cause osteolysis. To explain these phenomena, the concepts of ’effective joint space’ and ’pumping stem’ have been cited in many studies. However, there is no clear understanding of the factors causing, or contributing to these mechanisms. It is likely that capsular pressure, gap dimensions and micromotion of the gap during cyclic loading of an implant as well as factors such as biological osteolysis threshold, the rate of wear generation and the degree of particle clogging in the periprosthetic tissue, play defining roles in periprosthetic flow, particle migration and osteolysis generation. In order to obtain a better understanding of the above mechanisms and factors, steady state and transient 2D computational fluid dynamic models of the lateral side of a stem-femur system including the joint capsule, a gap in communication with the capsule and the surrounding bone were studied. First, fluid velocities and pressures in the periprosthetic tissue were investigated. Then, particles were introduced to the continuum fluid at the gap entrance as a discrete phase and their migration to the bone was analysed. Lastly, the models were further refined by introducing algorithms and factors developed to simulate particle clogging and permeability variation caused by the fibrous tissue generation in osteolytic lesion and particle clogging in the periprosthetic tissue throughout postoperative periods. Simulations without particles showed that high capsular pressure may be the main driving force for high fluid pressure and flow in the bone surrounding the gap, while micromotion of only very long and narrow gaps can cause significant pressure and flow in the bone. At low capsular pressure, micromotion induced large flows in the gap region; however, the flow in the bone tissue was almost unaffected. The results also revealed the existence of high velocity spikes in the bone region at the bottom of the gap. These velocity spikes can exert excessive fluid shear stress on the bone cells and disturb the local biological balance of the surrounding interstitial fluid which can result in osteolysis development. High capsular pressure was observed to be the main cause of these velocity spikes whereas, at low capsular pressure, gapmicromotion of only very long and narrow gaps generated significant velocity spikes in the bone at the bottom of the gaps. Simulations with particles also showed that capsular pressure is the main driving force for particle migration to periprosthetic tissue. In contrast to common belief, the models showed that implant micromotion pumped out, rather than sucked in the particles to the interfacial gaps, except in long gaps in which, even at low pressure, particles that made it to the bottom region migrate to the bone tissue as a result of micromotion. Particles entered the periprosthetic tissue along the entire length of the gap with higher concentration at proximal and distal regions. However, particles mainly accumulated with an increasing concentration at the bottom of the gap because of the presence of the fluid spikes in this region. Therefore, focal osteolysis is more likely to develop in the gap bottom region, whereas linear osteolysis, which requires less particle concentration, is more likely to develop along the entire gap length. It was also shown that risk of osteolysis development was higher for shorter gaps since they experience higher particle concentration. In addition, the models showed that for osteolysis to develop, a constant supply of particles, as well as an access route to the endosteal bone must be available. When a particle clogging model was included, it was shown that the depth of particle penetration into the surrounding tissue reduced, leading to increased particle concentrations. Particle clogging and accumulation initially occurred at distal and proximal gap regions in the stem proximity. However, as time elapsed, this accumulation extended along the entire interface. The rate of particle accumulation was a function of particle wear generation. Accumulation of particles at the interface caused changes in the tissue permeability and periprosthetic flows. Partial clogging and particle accumulation in the gap bottom region caused increases in fluid spikes in that region. Once this region was completely clogged, the magnitude of fluid spikes reduced. In addition, there was a complementary relationship between particle concentration and the reduction of permeability; regions with lower permeability tended to experience higher particle accumulation. Models developed to simulate fibrous tissue generation in osteolytic lesions, presented this tissue by regions with increased permeability. It was showed that, as time elapses, particle concentrations become higher than the osteolytic threshold which leads to increased periprosthetic tissue permeability. In general, for lower osteolysis thresholds, regions with increased permeability progresses faster at the bottom of the gap. This results in increased permeability having a linear pattern along the interface and a focal pattern at the bottom of an interfacial gap. Higher osteolysis thresholds, generally, result in only a linear pattern of increased permeability along the gap except for the cases with high wear generation rates in which a focal pattern at the bottom of the gap can still be seen.

Total hip replacement is one of the most common surgical procedures carried out both in the UK and Worldwide. With an increasing number of younger patients undergoing the procedure, there is an emphasis on increasing the longevity of prostheses. The following reports on a number of component studies which, when combined give an insight into the mechanism of wear behind the loosening and failure of matt surface finish femoral stems. By examining stems which have been explanted from patients, a method of wear classification has been developed, and also 3D surface measurement techniques have been employed to quantify wear through parametric characterisation and also volume analysis. Initial findings suggested that the wear of matt finish femoral stems differs to that of smoother polished femoral stems. Studies also provide information regarding the nature of bone cement, its behaviour and the interaction between stem and cement following insertion of the stem. It was found that geometric change in bone cement occurred during polymerisation, and following curing. This geometric change presented itself in the form of differential shrinkage. This shrinkage of cement was observed initially through 3D surface topography analysis and later confirmed with geometric measurement techniques. The presence of voids between stem and cement give rise to the possibility of debris creation and transportation, adding to the evidence for a difference in wear mechanism between polished and matt surface finish femoral stems. Some progress was made towards replication of wear in vitro which has future possibilities for wear screening of materials and designs of future prostheses. The overall conclusion of the study suggests that the dominant wear mechanism which occurred between the stem and bone cement was abrasive in nature and this is likely to explain the accelerated wear of matt stems which has been reported by clinicians and researchers.

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Text in Afrikaans / In this quantitative study emphasis is placed on the role of the nurse as a member of the rehabilitation team. Furthermore, the role of the nurse in patient education as well as the importance of effective patient education during the rehabilitation process after hip replacement surgery is emphasised. Various factors that impacted on the rehabilitation process are highlighted. A structured interview schedule was used to interview a group of 20 respondents. All of the respondents participated voluntarily. Orem's Selfcare Theory was used as a theoretical basis for the study. The researcher determined which information respondents regarded as important to attain functional independence during the rehabilitation process. The data gathered would serve as recommendation for a planned information brochure that could in future be given to patients at their first visit to the surgeon before surgery. The brochure could then be used as a basis for patient education in the course of the rehabilitation process. / Advanced Nursing Sciences / M.A. (Verpleegkunde)

Text in Afrikaans / In this quantitative study emphasis is placed on the role of the nurse as a member of the rehabilitation team. Furthermore, the role of the nurse in patient education as well as the importance of effective patient education during the rehabilitation process after hip replacement surgery is emphasised. Various factors that impacted on the rehabilitation process are highlighted. A structured interview schedule was used to interview a group of 20 respondents. All of the respondents participated voluntarily. Orem's Selfcare Theory was used as a theoretical basis for the study. The researcher determined which information respondents regarded as important to attain functional independence during the rehabilitation process. The data gathered would serve as recommendation for a planned information brochure that could in future be given to patients at their first visit to the surgeon before surgery. The brochure could then be used as a basis for patient education in the course of the rehabilitation process. / Advanced Nursing Sciences / M.A. (Verpleegkunde)