Diagnóstico sobre los principales problemas que afectan a los estudiantes de la carrera de

By definition abrasive wear occurs when hard material particles or asperities move across a surface causing a loss of material [48]. This is one of the most important forms of wear to be considered for joint implant devices. During initial contact of the femoral head and acetabular cup, no third body particles are expected to be present between the surfaces. This is of course assuming that no debris exists from tooling or ground bone or human tissue. During the operating life of the components third-body mode abrasive wear (Figure 2.17) is a possibility. Within the hip articulating joints, the potential for 3rd _{body wear could however be lower than}

for an enclosed contact surface due to a separation of the device leading to the debris being carried away from the wear zones by synovial fluid. Two-body mode abrasive wear could also be significant to the wear of the device in the wear zones, which means that the surface damage and wear is caused by the contacting surface. By using electron microscope techniques, implant retrieval studies show evidence of abrasive wear following cyclic loading of the devices, including in the region of stripe wear [32, 134]. Fisher et al. [135] reports scratches on femoral heads and therefore used the finite element method to model third body debris in total joint replacements which highlights damage to the articulating surfaces. There appears to be no study using the finite element method to assess the combined effects of edge loading and third body interaction. From the review of current literature there is still much deliberation on the kinematics and wear modes which cause stripe wear, therefore more asperity contact modelling research could lead to a solution to the problem. The challenge of reducing the surface stress and wear loss as a result of abrasive wear is appreciated when observing the

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simplicity of a theoretical model, as developed by Pourzal et al. [136]. Despite the additional complexity, these theoretical models can be cumbersome and only provide very specific assessments in dealing with the problem of abrasive wear.

Figure 2.17. Third body abrasive wear

To understand abrasive wear further it is imperative to examine the shape of debris or grit between articulating surfaces. This can determine the resulting wear mechanism i.e. blunt debris leading to ploughing and sharp debris leading to scratching of the articulating surface with further debris particles being released. If debris particles are carried away from the bearing surface, then it is possible that they can be carried back there. By using computational fluid dynamic techniques it has been shown that a larger diameter bearing surface can lead to a high velocity influx of synovial fluid, which is ideal for improving the lubrication of the joint [117]. However, this could increase the amount of wear debris being transmitted back to the articulating surfaces, potentially leading to third body abrasive wear. An experimental study investigated ‘foreign’ body material leading to wear and damage of the articulating hip joint prosthesis caused by third body abrasive wear. Through the use of a scanning electron microscope, images of the surface and roughness from surface profiles revealed the severity and consequences of the problem [137, 138]. Unfortunately, this problem does not only occur on the softer bearing surface such as a UHMWPE, but also on a hard bearing surface such as CoCrMo. A study by Que and Topoleski [139] reports third-body wear on ASTM-F75 and ASTM-F799 CoCrMo material caused by poly-methyl methacrylate (PMMA) and bone particle surface ploughing. Therefore, ignoring the contributing factor of bone and other

1st_{body. e.g. acetabular cup}

2nd_{body. e.g. femoral head}

Lubrication

3rd_body

(debris from 1st_or

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foreign particles may not be a wise decision as this could account for a large amount of the total wear. More specifically a subjective study reports that 56.5% and 42% of the total wear debris collected after patients implantation surgery were bone fragments and cement fragments respectively, metal debris only account for the other 1.5% [140]. Past studies have also used chemical analysis techniques to observe debris from implantation tooling materials remaining within the implantation area.

For abrasive and debris modelling, the finite element method was used by Mirghany and Jin [141] in which a two dimensional axisymmetric finite element model was developed to simulate the entrapment of a 3rd _{body wear particle between polyethylene and CoCrMo alloy surfaces.}

The aim was to study the effects of the debris size on von Mises stress and deformation. The von Mises stresses could be used as a comparison to the materials yield strength and provide an indication of potential plastic deformation or scratching of the surface. The limitation of the study is that it only considers spherical wear particle morphology; also the study does not consider the effects of scratching through the increased number of wear particles between the bearing surfaces. One further study by McNie et al. presents results of the wear debris and near surface strain fields for UHMWPE using experimental and finite element methods, which also covers the assessment of scratch lip geometry on wear rate and debris morphology in UHMWPE. Although this is valuable research, not all causes of surfaces scratches were identified [135].

By reviewing the research carried out by Fisher et al. [75], for MoM and CoM bearing surfaces, a large portion of the wear debris assessed were “oval to round in shape”. Although the size of scratching within the contact area was between 0.5 µm to 5 µm and size of craters caused by pitting or carbide pullout, were between 0.7 µm and 3.5 µm, the size range of wear debris were in the nanometer scale. The mean maximum diameter for CoM and MoM debris were 18 nm and 30 nm respectively. The difficultly in assessing the ceramic debris was realised

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due to the low wear debris quantities and accuracy at the nanoscale. A separate study by Shishido et al. [142] reports the use of a scanning electron microscope to determine the wear debris size and morphology to be 3 µm to 4 µm in size and polygon in shape for ceramic-on- ceramic bearing surfaces. The study is based on a retrieval Mittelmier type prosthesis with stripe wear observed during inspection of the device. Shishido et al. [142] concludes this type of prosthesis to be a poor design concept even though the device is shown to have excellent wear resistance.

For assessing hip implant bearing surfaces at this level, it is clear that the method of inspection used has a major role to play in determining the classification of micro and even nano sized particles. Nano sized alumina wear debris between 5-90 nm were recorded through the use of a transmission electron microscope (TEM) and larger particle sizes between 0.05-3.2 µm were observed using a lower resolution scanning electron microscope [33]. Although this study by Ingham et al. [33] is based only on the Mittelmeier and Charnley hip replacements, the study concludes that the wear debris morphology of particles released during standard kinematic conditions is smaller than that released during rim contact due to microseparation and heal strike [33]. A continuation of this study offers further research into the effects of abrasive wear and debris within the stripe wear zone specifically. By using both the SEM (scanning electron microscope) and TEM (transmission electron microscope) techniques the shape of the debris were found to be mostly oval to round in shape, however some were polygonic [143]. For the larger particles, it is shown that high local stresses during rim contact formed stripe wear patterns by trans-granular fracture. Specifically for metal-on-metal contact, Brown et al. [144] used TEM equipment and observed nano sized particles which ranged in size from 51-116 nm with a mean of 81 nm, and were mostly round in shape. However, a very small portion were characterised as “needle like”. The wear debris assessments from standard and microseparation based experimental simulator testing could provide further information

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regarding the causes of stripe wear. Research carried out using a field emission gun scanning electron microscope (FEGSEM) found that both MoM and CoC wear particles were round and irregular in shape under standard and microseparation tests. No major differences in shape or size were found between particles from each test and the mean particle size was 50 nm [145]. By using the same inspection techniques Leslie et al. [146] reports that particles from a MoM 15 million cycle (mc) hip simulator study were oval or round in shape within a size range of 8- 108 nm, and no sharp particles were found. Energy dispersive X-ray equipment was used to confirm the presence of cobalt chrome wear debris. Even more interestingly for the purpose of this project, Leslie et al. [146] noted that an increased bearing surface size led to a decrease in volumetric bedding in wear and overall wear. However the bearing diameter did not affect wear debris profile or magnitude.

The form of adhesive wear is determined by many factors and it is known to be one of the most common forms of wear [136]. When describing the wear mechanisms of stripe wear caused by edge loading adhesive wear is sometimes described and referred to as “grain pull out” [27, 32].