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Metal on metal (MoM) hip replacements consist of a cobalt-chromium-molybdenum alloy femoral head which articulates against an acetabular cup manufactured from similar material. MoM hip replacements were introduced in the 1980s. It was thought that the overall reduction in volumetric wear as well as the avoidance of polyethylene would lead to greater longevity of these prostheses. There had been isolated reports of adverse tissue reactions with previous generations of MoM devices but it was thought that improved manufacturing technology would eliminate these problems. In the 1990s, the Birmingham Hip Resurfacing (BHR) was developed. The positive mid-term results of this device led to a rapid increase in the use of the BHR throughout the world. For obvious reasons, the enhanced stability large diameter bearings provided proved extremely attractive to surgeons and patients. Manufacturers therefore began to develop total hip replacement systems for patients unsuitable for the resurfacing procedure. These systems used bearings of size 36mm and greater, in Contrast to the existing 28mm Metasul device. From 2005 onwards there began to emerge increasing numbers of reports of local complications in the tissues adjacent to MoM prostheses. These reactions included sterile masses, tissue destruction and osteolysis. The incidence of these tissue reactions was unknown, as were the risk factors for their development. This piece of work sought to quantity the volumetric and linear wear rates of failed MoM hips and to investigate the relationship these wear rates and a number of clinical parameters. These parameters included blood, serum and hip fluid chromium and cobalt concentrations, and the macro and microscopic appearance of periprosthetic tissue at revision surgery. In this way it was hoped that component design, host and surgical factors leading to adverse tissue reactions could be identified and potentially eliminated.

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Success of total hip replacements is well reported however, failures as a result of wear processes and the biological response to these products continue to challenge the orthopaedic community. Lately, corrosion of metal surfaces as well as wear particles have seen particular interest with elevated blood cobalt levels widely reported in patients receiving metal-on-metal (MoM) hip replacements. Some instances have also reported this in patients with metal-on-polyethylene (MoP) components and these corrosion products are believed to contribute to hypersensitivity reactions reported. This thesis considers wear and cobalt release in MoP and MoM hip bearings tested under standard and challenging hip simulator conditions and includes an exploration of novel bearing coatings to reduce cobalt release. The incorporation of silver into these coatings may be sufficient to produce an antibacterial response, reducing the risk of mid-term infections, another reported cause of failure. Polyethylene wear was low under standard and clinically relevant adverse conditions in 28mm and 52 mm diameter MoP bearings (less than 35 mm3/mc). Cobalt release was measurable in 28 mm diameter MoP bearings (51 ppb/mc) with higher levels produced in large 52 mm diameters (123 ppb/mc), the first time this has been reported, although cobalt release was substantially less than that observed in MoM bearings (6909 ppb/mc). Alumina abrasives introduced in the lubricant substantially damaged MoP bearings, increasing the cobalt release to 70,690 ppb after 1 mc, greater than found after edge loaded MoM bearings (19,240 ppb). The removal of these particles still produced elevated cobalt levels compared to standard conditions and increased polyethylene wear to 435 mm3/mc. A chromium nitride (CrN) coating in MoP bearings was resistant to this abrasive damage showing no delamination in the coating, with negligible cobalt released after 7.04 mc (153 ppb) and maintained a polyethylene wear rate below 20 mm3/mc. Silver CrN coatings on both bearing surfaces of MoM components prevented cobalt release under standard conditions, with silver release after 0.17 mc up to 3,720 ppb in high silver surface coatings, although the wear was relatively high (5.24 mm3/mc). A silver CrN coating with a low concentration of silver at the surface reduced wear and was resistant to 5 mc of edge loading. It generated 241 ppb of cobalt and maintained comparable steady state wear rates (0.65 mm3/mc) to the uncoated metal while releasing 18,786 ppb silver which may be sufficient to be an effective anti-microbial agent. These coatings may provide potential clinical benefits in MoP and MoM bearings by reducing both wear and cobalt release in ideal and adverse conditions. There may also be beneficial wear products in the form of silver, although further testing of optimised coatings is required.

Corrosion at the head-neck junction of total hip replacements is a poorly understood phenomenon with an incidence of 1 - 2 %. Concerns around taper junction corrosion have focused on design factors including changes in taper surface topography and geometry as well as operating conditions such as high bearing surface friction and fluid ingress-egress at the taper junction. Hence, this thesis considered 3 aspects of the head taper junction namely: (1) frictional torque at the bearing surface and below the taper junction for varying head sizes and bearing material combinations, (2) Cobalt and Chromium ion release from CoCr/Ti taper junctions, (3) FE analysis of tapers utilising variables including taper length, material, angle, and clearance under loading conditions representative of walking, hip simulator profiles and stair climb. Bearing friction and the torque about the taper axis beneath the taper junction were positively correlated with the head size (R2 = 0.57 bearing friction, R2 = 0.88 torque) and average surface roughness (Ra) (R2 = 0.66 bearing friction, R2=0.79 torque) of the femoral head. Torque generated on large MoP bearings (0.93 ± 0.2 Nm) was found to be comparable to MoM (0.81 Nm). The median cumulative Cr release rate was at least 2 times greater than that of Co (0.0220 ppb/cycle Cr relative to 0.0109 ppb/cycle Co) due to the acidic environment utilised in the accelerated tests. No statistically significant difference in ion release was found, between the trunnions of different surface finishes. Finite element analyses showed that the largest gaps generated at the mouth of the taper, were associated with smaller taper contact areas. Clearances within ±0.1° enabled the tapers to engage over comparable lengths and therefore did not show differences in taper opening, showing this was influenced by the taper engagement length rather than location (proximal or distal) of contact. Stair climb loading generated the largest taper gaps (80 m) and surface stresses on the head taper (1200 MPa); these were greatest on the shortest trunnion. Although the stair climb loading condition is not currently mandated in testing THR devices, its use could provide a more accurate prediction of taper performance in vivo and may be beneficial to 'beyond compliance' initiatives to improve implant performance.

Hip replacement surgery has experienced a dramatic evolution in recent years supported by the latest developments in many areas of technology and surgical procedures. Unfortunately complications that follow hip replacement surgery remains the most challenging dilemma faced both by the patients and medical experts. The thesis presents a novel approach to segment the prosthesis of a THR surgical process by using an Active Contour Model (ACM) that is initiated via an automatically detected seed point within the enarthrosis region of the prosthesis. The circular area is detected via the use of a Fast, Randomized Circle Detection Algorithm. Experimental results are provided to compare the performance of the proposed ACM based approach to popular thresholding based approaches. Further an approach to automatically detect the Obturator Foramen using an ACM approach is also presented. Based on analysis of how medical experts carry out the detection of loosening and subsidence of a prosthesis and the presence of infections around the prosthesis area, this thesis presents novel computational analysis concepts to identify the key feature points of the prosthesis that are required to detect all of the above three types of complications. Initially key points along the prosthesis boundary are determined by measuring the curvature on the surface of the prosthesis. By traversing the edge pixels, starting from one end of the boundary of a detected prosthesis, the curvature values are determined and effectively used to determine key points of the prosthesis surface and their relative positioning. After the key-points are detected, pixel value gradients across the boundary of the prosthesis are determined along the boundary of the prosthesis to determine the presence of subsidence, loosening and infections. Experimental results and analysis are presented to show that the presence of subsidence is determined by the identification of dark pixels around the convex bend closest to the stem area of the prosthesis and away from it. The presence of loosening is determined by the additional presence of dark regions just outside the two straight line edges of the stem area of the prosthesis. The presence of infections is represented by the determination of dark areas around the tip of the stem of the prosthesis. All three complications are thus determined by a single process where the detailed analysis defer. The experimental results presented show the effectiveness of all proposed approaches which are also compared and validated against the ground truth recorded manually with expert user input.

A challenge in engineering coupling design is the understanding of performance of contact geometry for a given application. “Wear” is one of a number of mechanical failures that can occur in mechanical coupling design. “Fretting wear” occurs where surfaces in contact are subjected to oscillating load and very small relative motion over a period of time. Fretting has been observed in many mechanical interactions and is known to be a reason for failure in many designs. Recent evidence suggests that fretting wear occurs at the taper junction of modular total hip replacements and leads to failure of the implants. Experimental testing to determine the wear behaviour that occurs in mechanical devices is time consuming, expensive and complicated. Computational wear modelling is an alternative method which is faster and cheaper than real testing and can be used in addition to testing to help improve component design and enhance wear characteristics. Developing an algorithm that can accurately predict fretting wear considering linear wear, volumetric wear and surface wear damage is the main focus of this thesis. The thesis proposes a new computational methodology incorporating published wear laws into commercial finite element code to predict fretting wear which could occur at the taper junction of total hip replacements. The assessment of wear in this study is solely based on mechanical wear (fretting) as being the primary mechanism causing surface damage. The method is novel in that it simulates the weakening of the initial taper ‘fixation’ (created at impaction of the head onto the stem in surgery) due to the wearing process. The taper fixation is modelled using a contact analysis with overlapped meshes at the taper junction. The reduction in fixation is modelled by progressive removal of the overlap between components based on calculated wear depth and material loss. The method has been used for three different studies to determine surface wear damage, linear and volumetric wear rates that could occur at taper junction of total hip replacements over time. The results obtained are consistent with those found from observation and measurement of retrieved prostheses. The fretting wear analysis approach has been shown to model the evolution of wear effectively; however, it has been shown that accurate, quantitative values for wear are critically dependant on mesh refinement, wear fraction and scaling factor, wear coefficient used and knowledge of the device loading history. The numerical method presented could be used to consider the effect of design changes and clinical technique on subsequent fretting wear in modular prosthetic devices or other mechanically coupled designs.

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