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The tribological performance of biomaterials used for artificial joints is of much importance, and require low coefficients of friction, resistance to wear and the ability to withstand many millions of cycles under a multitude of loading regimes. Currently used material combinations include Ti6A14V, 316L stainless steel and Co-Cr-Mo articulating against UHMWPE. Although typical wear rates are low (60 mm(^3)/10(^6) cycles), the UHMWPE wear debris produced during articulation has been linked to osteolysis, leading to loosening of prostheses and necessitating revision surgery. This study aimed to characterise the surfaces and quantitatively assess the tribological performance of such biomaterials when surface modified by N(^+) ion implantation. Beyond this, investigation of the physical effects of the N(^+) ion implantations were carried out with a view to determination of an optimum ion implantation protocol. The tribological performance of the materials, were quantitatively assessed using multidirectional pin-on-plate wear testing. Surface characterisation of the materials, were studied using a combination of optical microscopy, AFM, non-contacting interferometry, SEM, and XPS. A significant increase in the surface microhardness of the modified materials was measured post ion implantation. This was attributed to the formation of ion implantation induced lattice disorder and hard phase nitride precipitates on the metallic surfaces, and cross-linking incorporating new formed chemical bonds on the polymeric surfaces. N(^+) ion implantation with 5 x 10(^15) N(^+)ions/cm(^2) significantly enhanced the wear resistance of UHMWPE by ≈ 55 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted Ti6A14V; by ≈ 48 % when articulated against 2 x lO(^17) N(^+) ions/cm(^2) implanted stainless steel; and by ≈ 48 % when articulated against 2 x 10(^17) N(^+) ions/cm^ implanted Co-Cr-Mo. The technique of ion implantation offers potential as a modification method, to improve wear resistance of these biomaterials for articulating applications such as in total joint replacement.

Understanding the wear of the biomaterial/cartilage interface is vital in the development of more satisfactory materials for use in the clinical repair of worn or damaged synovial joints. The aims of this study were to investigate a wide range of biphasic hydrogels as potential chondroplasty materials and to further the understanding of natural joint tribology. The mechanical properties of each potential chondroplasty material were quantified and their tribological performance investigated by means of a series of simple geometry friction and wear studies in Ringer's solution and a proteincontaining lubricant. Also uni- and multi-directional continuous sliding tests in a protein-containing lubricant were conducted under various loading conditions to evaluate the friction and degradation of each material and that of the opposing articular cartilage surface. A number of potential chondroplasty materials were also evaluated as defect repair materials when implanted using a proposed clinical method. Selected biphasic hydrogel materials showed a marked reduction in dynamic friction, degradation and articular cartilage pin damage when compared with single-phase materials. Following continuous wear studies, alterations in opposing cartilage surface topography were found to be associated with increased levels of dynamic friction. The protocols devised in this study are the first to yield objective and quantifiable data demonstrating a reduction in friction and opposing cartilage surface degradation following the implantation of certain biphasic hydrogel defect repair materials. They also demonstrate the potential of biphasic hydrogels to act as superior chondroplasty materials compared with currently available materials. Future work will focus on the optimisation of biphasic hydrogel properties, including the long-term durability and immunogenicity of each material following implantation, in order that materials will more closely mimic the tribology of natural articular cartilage.

Damage to cartilage, caused either by disease or injury, affects a large number of people worldwide, severely reducing the patient's quality of life and generating a huge burden on healthcare systems. The limited success of treatment options such as tissue grafts has been the driving force behind much research into tissue engineering strategies for cartilage repair. One of the challenges associated with tissue engineering cartilage is that of generating constructs of clinically relevant sizes since the formation of a crust of tissue at the scaffold periphery restricts the supply of nutrients to the growing tissue. The hypothesis of this thesis was that a tissue engineering system incorporating scaffolds containing both random and anisotropic porosity and a novel flow perfusion bioreactor system would facilitate in vitro tissue formation by enhancing the supply of nutrients to the growing construct. This hypothesis was examined using cartilage as a model tissue. It was shown that scaffolds combining both random and anisotropic porosity (sparse knit scaffolds) had improved flow properties compared to scaffolds containing random porosity alone (needled felt scaffolds). Following studies to characterise the scaffolds and to determine the appropriate conditions for seeding cells into the scaffolds, cartilage formation within the different scaffolds was assessed over a four week culture period. It was found that the flow perfusion system was not as favourable for in vitro cartilage formation as either the commercially available Rotary Cell Culture System (RCCS) or static culture. One of the sparse knit scaffolds (sparse knit 4) and the needled felt were further compared for cartilage formation over an eight week culture period, using static and RCCS culture. With respect to collagen and glycosaminoglycan (GAG) production, cartilage constructs generated from the two scaffold systems were similar. Following static culture it was found that more viable cells were present at the centre of sparse knit 4 scaffolds than needled felt scaffolds. It was therefore concluded that scaffolds combining random and anisotropic porosity were advantageous for culturing tissues in environments where nutrient supply was reliant on diffusion alone.

Finite Element Analysis is used extensively to assess the design and variation of surgical parameters in joint prostheses, but the majority of these analyses are undertaken on a single sample. The variation in implant stability has been found to be as large between patients as the variation found between different loading scenarios. Recent investigations have suggested that single sample studies are unable to account for natural inter-patient variation in bone geometry and material property distribution. This thesis investigates two methods which can be implemented to account for the patient variation experienced over the population. The first method involves analysing a group of patients and examining the variation in the results to determine the spread of data over the population. Developments in computer tomography based analyses make multiple sample studies possible; the question remains how many femurs are required to perform a study which accounts for such variations. This work uses computer tomography based finite element analyses on a group of 16 femurs to determine the effect of the resurfacing arthroplasty and variables in the surgical procedure and implant design. Sample sizing techniques are developed in this work and implemented on these case studies to determine whether or not a suitable number of femurs had been analysed in the investigations. The second method utilises probabilistic techniques in the analysis of the implant, these methods are able to account for inter-patient variables along with multiple variables such as surgical parameters and implant design. Two case studies are undertaken on a single femur model; one varying only surgical parameters the second incorporating patient variables such as load and bone density. The thesis determines that probabilistic methods will prove to be invaluable in future implant design analyses, however there will need to be a substantial development of current techniques before this becomes feasible. A more immediate answer to the question of inter-patient variation is the modelling of multiple femurs. This thesis has developed a set of sample sizing calculations that can be implemented in multi-sample studies to determine if a suitable sample sizes have been analysed.

Total joint replacement is one of the most successful surgical procedures and is a proven treatment for arthritis. Despite low failure rates, the wide application of the treatment means that large numbers of prostheses fail and must be revised. Improved pre-clinical testing methods for these orthopaedic devices may assist in developing new prostheses with improved clinical results. Computational modelling of biological systems is becoming increasingly accurate and is a much quicker and cheaper alternative to physical testing, but continued development is necessary to ensure computational models produce accurate and reliable predictions of implant behaviour. Acrylic bone cements have been used as a method of fixation for over 50 years but despite improvements in cement handling techniques and numerous attempts to improve the mechanical properties of the cement in other ways, the cement is often highlighted as the weak link in the joint replacement system. Aseptic loosening is cited as the cause for the majority of revision operations and cement degradation has been shown to be a contributor to the loosening process. In-vivo, cement is subject to cyclic loads and these are the primary cause of cement damage. Residual stresses generated during the polymerisation of the cement are now thought to play a significant role in cement failure. This thesis examines the development of residual stresses as a result of thermal and chemical changes during polymerisation of the cement. Experimental techniques for characterising the evolution of materials properties during the polymerisation reaction are discussed. Differential scanning calorimetry was used to measure the reaction variables such as the activation energy of polymerisation. The development of an ultrasonic rheometry technique for monitoring the mechanical property evolution within a bone cement specimen is discussed. Computational models were generated to predict the reaction behaviour of the cement in terms of the heat produced and the evolution of the physical properties of the curing mass. Some advantages and disadvantages of candidate mathematical models have been evaluated and are discussed, along with applications in several implant fixation scenarios. The model compared well with experimental data and was used to predict thermal necrosis in the bone surrounding both a hip resurfacing implant and a knee replacement. Using the output reaction path produced by the thermal model a mechanical model was also produced simulating the shrinkage and mechanical property evolution exhibited by the polymerising cement. Two material models were compared with and without the effects of plasticity. Residual stress magnitudes were assessed in comparison with published values and showed better agreement when plasticity was included. Peak stresses were observed to occur during polymerisation. The location of the peak stresses were compared with experimental data on pre-load crack locations in the literature and showed good agreement.