Abstract:The formation and development of wear is now widely accepted as one of the major concerns in the long-term survivorship of contemporary knee prostheses in vivo. This review examines the role of surface topography, third-body debris, load, contact mechanics and material quality in the wear process. Some of the kinematic and physiological issues that need to be modelled in the development of wear testing regimes for evaluation of material combinations and geometrical combinations in total knee implant designs are considered. Wear testing procedures and some of the results from wear tests are discussed and the need to consider the impact of rolling and sliding in the study of wear in total knee components is highlighted. The dominant wear mechanisms that occur in vivo are identified and the role of these mechanisms is currently being examined experimentally at the University of Limerick wear testing machine.Keywords: polymer wear debris (PWD), total joint replacement ( TJR), total knee replacement ( TKR), third body debris ( TBD) INTRODUCTIONtions in the surrounding tissues, causing macrophage activity and necrosis of the bone-prosthesis interface [14][15][16][17][18][19]. The biological response to loose PWD, is believed Over the last 30 years, ultra-high molecular weight polyto be a contributing factor to prosthetic loosening in TKRs. ethylene ( UHMWPE) has been the choice of material usedIn laboratory wear studies, it is important to know what as bearing surfaces in total joint replacements (TJRs). It is conditions can cause an increase in the wear rate of polyrenowned for its toughness, low friction and biocompatiethylene and hence an increase in the number of polymer bility [1][2][3]. Designs of total knee implants have a variety particles. Polyethylene wear has been related directly to its of contact geometries. Many now have relatively high conmolecular properties, and factors have been identified that formity whereby the radii of the curvature of both femoral affect the molecular characteristics of polyethylene and and tibial components are similar. A considerable number thereby its wear characteristics. [4][5][6][7][8][9][10][11][12][13]. The influence of surgical techniques on the these factors in more detail. long-term performance of TKRs has been well documented and will be discussed later. In vivo the generation of submicron-sized PWD can cause adverse cellular reac-2 IN VITRO STUDIES OF WEAR
Currently, abdominal aortic aneurysms (AAAs), which are a permanent dilation of the aorta, are treated surgically when the maximum transverse diameter surpasses 5.5cm. AAA rupture occurs when the locally acting wall stress exceeds the locally acting wall strength. There is a need to review the current diameter-based criterion, and so it may be clinically useful to develop an additional tool to aid the surgical decision-making process. A Finite Element Analysis Rupture Index (FEARI) was developed.Ten patient-specific AAAs were reconstructed, and the corresponding wall stress computed. Previous experimental work on determination of ultimate tensile strengths (UTS) from AAA tissue samples was implemented in this study. By combining peak wall stress along with average regional UTS, a new approach to the estimation of patient-specific rupture risk has been developed.Ten cases were studied, all of which were awaiting or had previously undergone surgical AAA repair. A detailed examination of these ten cases utilising the FEARI analysis suggested that there was a possibility that some of the AAAs may have been less prone to rupture than previously considered.It is proposed that FEARI, used alongside other rupture risk factors, may improve the current surgical decision-making process. The use of FEARI as an additional tool for rupture prediction may provide a useful adjunct to the diameter-based approach in surgical decision-making.
IntroductionThe objective of this study was to determine if a synthetic bone substitute would provide results similar to bone from osteoporotic femoral heads during in vitro testing with orthopaedic implants. If the synthetic material could produce results similar to those of the osteoporotic bone, it could reduce or eliminate the need for testing of implants on bone.MethodsPushout studies were performed with the dynamic hip screw (DHS) and the DHS Blade in both cadaveric femoral heads and artificial bone substitutes in the form of polyurethane foam blocks of different density. The pushout studies were performed as a means of comparing the force displacement curves produced by each implant within each material.ResultsThe results demonstrated that test material with a density of 0.16 g/cm3 (block A) produced qualitatively similar force displacement curves for the DHS and qualitatively and quantitatively similar force displacement curves for the DHS Blade, whereas the test material with a density of 0.08 g/cm3 (block B) did not produce results that were predictive of those recorded within the osteoporotic cadaveric femoral heads.ConclusionThis study demonstrates that synthetic material with a density of 0.16 g/cm3 can provide a good substitute for cadaveric osteoporotic femoral heads in the testing of implants. However we do recognise that no synthetic material can be considered as a definitive substitute for bone, therefore studies performed with artificial bone substrates may need to be validated by further testing with a small bone sample in order to produce conclusive results.
Degradation of tibial inserts in vivo has been found to be multifactorial in nature, resulting in a complex interaction of many variables. A range of kinematic conditions occurs at the tibio-femoral interface, giving rise to various degrees of rolling and sliding at this interface. The movement of the tibio-femoral contact point may be an influential factor in the overall wear of ultra-high molecular weight polyethylene (UHMWPE) tibial components. As part of this study a three-station wear-test machine was designed and built to investigate the influence of rolling and sliding on the wear behaviour of specific design aspects of contemporary knee prostheses. Using the machine, it is possible to monitor the effect of various slide roll ratios on the performance of contemporary bearing designs from a geometrical and materials perspective.
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