Understanding initiation and propagation of fretting wear on the femoral stem in total hip replacement

The femoral stem–bone cement interface in total hip replacement is supposed to experience low amplitude oscillatory micromotion under physiological loading, consequently leading to fretting wear on the stem surface, which nowadays is considered to play an important part in the overall wear of cemented prosthesis. However, initiation and propagation of fretting wear has been poorly documented and a better understanding concerning this issue has not been established as yet. This present study, on the basis of a profound surface investigation of a polished Exeter V40? femoral stem and Simplex P bone cement obtained from an in vitro wear simulation, demonstrated that the edges of the micropores in the cement surface matched pretty well to the boundaries of the worn areas on the stem surface. This would indicate that these micropores contributed significantly to the fretting process at the stem–cement interface.     

The contribution of the micropores in bone cement surface to generation of femoral stem wear in total hip replacement

Although cemented total hip replacement has long been recognized as a situation that can lead to wear, the wear generated on the femoral stem has not been well documented, especially with regard to how this wear is initiated and propagated. This present work aimed to further investigate this issue based on a comprehensive study on surface morphology of the femoral stem and the bone cement, which were collected from seven in vitro wear simulations. It was shown that the wear locations on the stem surface compared well with the results of retrieval studies, and the boundaries of the worn areas matched well the edges of the micropores present in the bone cement surface. This indicated that the micropores could potentially contribute to the generation of femoral stem wear. In addition, metallic debris was detected around the micropores from the simulation with increased loading cycles. However, no evidence of macro-cracks was observed across the cement mantle in spite of the presence of micro-cracks initiated at the edge of the micropores. This study demonstrated a possible cause for progression of femoral stem wear and it may have an important bearing on the long term durability of cemented hip prosthesis.     

Femoral stem wear in cemented total hip replacement

The great success of cemented total hip replacement to treat patients with endstage osteoarthritis and osteonecrosis has been well documented. However, its long-term survivorship has been compromised by progressive development of aseptic loosening, and few hip prostheses could survive beyond 25 years. Aseptic loosening is mainly attributed to bone resorption which is activated by an in-vivo macrophage response to particulate debris generated by wear of the hip prosthesis. Theoretically, wear can occur not only at the articulating head-cup interface but also at other load-bearing surfaces, such as the stem-cement interface. Recently, great progress has been made in reducing wear at the head-cup interface through the introduction of new materials and improved manufacture; consequently femoral stem wear is considered to be playing an increasingly significant role in the overall wear of cemented total hip replacement. In this review article, the clinical incidences of femoral stem wear are comprehensively introduced, and its significance is highlighted as a source of generation of wear debris and corrosion products. Additionally, the relationship between femoral stem surface finish and femoral stem wear is discussed and the primary attempts to reproduce femoral stem wear through in-vitro wear testing are summarized. Furthermore, the initiation and propagation processes of femoral stem wear are also proposed and a better understanding of the issue is considered to be essential to reduce femoral stem wear and to improve the functionality of cemented total hip replacement.     

Biomechanical finite element analysis of bone cemented hip crack initiation according to stem design

The purpose of this investigation was to determine the specific fracture mechanics response of cracks that initiate at the stem-cement interface and propagate into the cement mantle. Two-dimensional finite element models of idealized stem-cement-bone cross-sections from the proximal femur were developed for this study. Two general stem types were considered; Rectangular shape and Charnley type stem designs. The FE results showed that the highest principal stress in the cement mantle for each case occurred in the upper left and lower right regions adjacent to the stem-cement interface. There was also a general decrease in maximum tensile stress with increasing cement mantle thickness for both Rectangular and Charnley-type stem designs. The cement thickness is found to be one of the important fatigue failure parameters which affect the longevity of cemented femoral components, in which the thinner cement was significantly associated with early mechanical failure for shot-time period.     

A novel test method for evaluation of the abrasive wear behaviour of total hip stems at the interface between implant surface and bone cement

After total hip replacement, some cemented titanium stems show above-average early loosening rates. Increased release of wear particles and resulting reaction of the peri-prosthetic tissue were considered responsible. The objective was to develop a test method for analysing the abrasive wear behaviour of cemented stems and for generating wear particles at the interface with the bone cement. By means of the novel test device, cemented hip stems with different designs, surface topographies and material compositions using various bone cements could be investigated. Before testing, the cemented stems were disconnected from the cement mantle to simulate the situation of stem loosening (debonding). Subsequently, constant radial contact pressures were applied on to the stem surface by a force-controlled hydraulic cylinder. Oscillating micromotions of the stem (+/- 250 microm; 3 x 10(6)cycles; 5 Hz) were carried out at the cement interface initiating the wear process. The usability of the method was demonstrated by testing geometrically identical Ti-6A1-7Nb and Co-28Cr-6Mo hip stems (n= 12) with definite rough and smooth surfaces, combined with commercially available bone cement containing zirconium oxide particles. Under identical frictional conditions with the rough shot-blasted stems, clearly more wear particles were generated than with the smooth stems, whereas the material composition of the hip stems had less impact on the wear behaviour.     

钛合金球与骨水泥界面之间的微动磨损机制研究

采用钛合金球与自制骨水泥试样以球/平面接触方式,在自制的微动摩擦磨损试验机上开展干摩擦和25%小牛血清介质中切向微动磨损试验研究,考察钛合金球与骨水泥界面之间的微动运行特性,并采用S-3000N型扫描电镜观察磨痕形貌来分析其微动磨损机制。结果表明:随着微动振幅的增加,微动运行由部分滑移区向混合区转变。随着接触载荷的增加,试样接触面之间更容易发生黏着。与干摩擦相比,在小牛血清溶液中部分滑移区向较大振幅区扩展。部分滑移区摩擦因数值较低且保持稳定,混合区的摩擦因数先增大后保持不变。稳定摩擦因数随着接触载荷的增加而减小,随微动振幅增大而增大。骨水泥试样的磨损量在小牛血清介质中比在空气中大,并且随接触载荷增大而增大。骨水泥在小牛血清介质中微动磨损的损伤机制主要为黏着磨损和疲劳磨损,溶液分子在应力作用下对骨水泥基体有削弱作用。     

人体皮质骨/纯钛接触副径向微动损伤研究

对新鲜人股骨皮质骨对纯钛球(直径10mm)接触副,在12mm/min加载速度下,分别以最大法向载荷100,200,300,400N进行径向微动实验研究。在动力学特性分析的基础上,用激光共聚焦显微镜和扫描电镜观察皮质骨表面的磨痕形貌。结果显示:随着载荷的增加,股骨皮质骨表现出不同的界面变形行为,损伤随之加重,而且裂纹的产生存在一个临界载荷;裂纹在骨表面主要有4种生长方式,以沿粘合线生长最常见。讨论了减缓骨/种植体界面径向微动损伤的措施,表明降低界面应力和种植体表面改性有利于减缓骨/种植体界面径向微动损伤。     

人股骨皮质骨轴面微动摩擦磨损特性研究

采用高精密微动试验台外加体液恒温循环装置,在一定法向载荷和不同位移幅值条件下,研究了天然活性人股骨皮质骨对纯钛的微动摩擦磨损行为。试验结果显示：随位移幅值的增加,股骨皮质骨的微动运行状态从部分滑移向完全滑移状态转变,详细讨论了不同位移幅值下摩擦因数随循环次数的演变规律。微观观察表明：接触表面处于部分滑移状态时损伤轻微,而在完全滑移状态下磨损较严重。人股骨皮质骨的磨损机理主要表现为磨粒磨损和微骨折导致的剥层剥落。微动磨痕的深度随位移幅值的增加而增加,而且磨痕深度与摩擦因数有很好的对应关系。研究认为控制植入体/骨界面的微动幅度有利于提高皮质骨抗微动损伤的能力。     

The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel

Studies of explanted hip prostheses have shown high wear rates of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups and roughening of the surface of the metallic femoral head. Bone and bone cement particles have also been found in the articulating surfaces of some joints. It has been proposed that bone or bone cement particles may cause scratching and deterioration in the surface finish of metallic femoral heads, thus producing increased wear rates and excessive amounts of wear debris. Sliding wear tests of UHMWPE pins on stainless steel have been performed with particles of different types of bone and bone cement added. Damage to the stainless steel counterface and the motion of particles through the interface have been studied. Particles of bone cement with zirconium and barium sulphate additives and particles of cortical bone scratched the stainless steel counterface. The cement particles with zirconium additive produced significantly greater surface damage. The number of particles entering the contact and embedding in the UHMWPE pin was dependent on particle size and geometry, surface roughness and contact stress. Particles are likely to cause surface roughening and increased wear rates in artificial joints.     

微动疲劳参数对钢丝微动疲劳磨损演化的影响*

微动疲劳易引起钢丝表面磨损和横截面积损失,进而造成钢丝断裂失效并缩短钢丝绳使用寿命。不同微动疲劳参数（接触载荷、疲劳载荷、钢丝直径和交叉角度）引起差异的钢丝微动疲劳磨损特性,故研究微动疲劳参数对钢丝微动疲劳磨损演化规律影响至关重要。基于摩擦学理论和Marc仿真软件构建钢丝微动疲劳磨损模型,探究接触载荷、疲劳载荷、交叉角度和钢丝直径对钢丝微动疲劳磨损演化的影响规律。结果表明：钢丝微动疲劳磨损体积主要与接触载荷和疲劳载荷有关;疲劳钢丝的磨损深度、磨损率及磨损体积随着接触载荷的增加而增大,且不同接触载荷下疲劳钢丝磨损体积均随着循环次数的增加而呈线性增加;随疲劳载荷幅值的增加,疲劳钢丝的磨损深度、磨损率及磨损体积均呈增加趋势;在不同疲劳载荷范围下疲劳钢丝的磨损体积均随着循环次数的增加而呈线性增加;当接触载荷、疲劳载荷及钢丝间摩擦因数相同时,不同交叉角度和不同加载钢丝直径下疲劳钢丝的磨损体积相同。     

Frequency effects in fretting wear

The effect of frequency of vibration on fretting wear has been investigated in the 10 – 1000 Hz range with additional experiments at 20 000 Hz. Fretting tests were performed with two materials, a low carbon steel (AISI 1018) and an austenitic stainless steel (AISI 304). The experiments showed that two cases of fretting contact can be distinguished and related to the displacement amplitude. If the amplitude is low, the contact situation is characterized by partial stick at the interface. At these conditions the wear rate (measured as the volume of material removed per cycle) is little affected by frequency. However, in low amplitude fretting material damage by surface degradation and fatigue crack initiation is usually of more concern than the actual wear itself. Both of these parameters are found to be greatly accelerated by an increase in frequency. In high amplitude fretting, in contrast, gross slip occurs at the interface and wear becomes the dominant damage mode. At these conditions variations in frequency appear to have little effect on fretting wear and related mechanisms. Therefore, in the case of fretting at high displacement amplitudes, it may be possible to apply high frequency fretting to obtain accelerated testing conditions.     

Fretting wear of carburized titanium alloy against ZrO2 under serum lubrication

Fretting wear of carburized titanium alloys was investigated on the universal multifunctional tester (UMT) with the ball-on-flat fretting style under bovine serum lubrication. The tangential load and friction coefficient during the fretting process were analyzed, and the evolution of fretting log during the fretting process was investigated to understand the wear mechanism of the titanium alloy and carburized titanium alloy. Furthermore, the wear scar was examined using a SEM and three-dimension surface profiler. It was found that the friction coefficient of the titanium alloy increased faster than that of carburized titanium alloy in the first stage under serum lubrication, and then remained steady with a similar value in the second stage. The Ft-D curve indicated that there was wear mechanism transition from gross slip to mixed stick and slip. Finally, it was observed that there was a slight damage of the titanium alloy and carburized titanium alloy showed excellent performance during the fretting wear process under serum lubrication. All of the results suggested that carburized titanium alloy was a potential candidate for the stem material in artificial joints.     

Evaluation of fretting wear based on the frictional work and cyclic saturation concepts

It is time consuming or even impossible to simulate the whole process of fretting wear, since it always involves millions of cyclic loadings. This paper focuses on the modeling and evaluation method of fretting wear for the typical bridge type fretting test with a flat pad. The frictional work on the contact interface is chosen as the parameter to evaluate the fretting wear. To verify the fretting wear model, the predicted wear profile is compared with that obtained by the experimental results. Fretting wear always includes plastic deformations due to the edge stress singularity. The effect of cumulative plastic deformation is also taken into account in the wear model. The role of the coefficient of friction at the contact interface on the fretting wear has also been discussed.     

The role of plastic anisotropy deformation in fretting wear predictions

The deformation occurring under fretting conditions occurs over length scales of the same order as the grain size, so the plastic anisotropy plays a significant role in the very local region near the contact edge during fretting process. The present study first describes plastic anisotropy by unified anisotropy plastic model coupling with Archard's wear law on the fretting behavior incorporating the effect of wear debris into such a quantitative model. The finite element method, utilizing this model, is used to analyze gross slip fretting conditions. The implementation of the wear simulation tool together with anisotropy cyclic plasticity analysis during fretting process is applied to the wear depth simulation. The present study validates the experiment phenomena from numerical simulation that failure location of the specimens under the flat-on-flat configuration is very close to the trailing edge. The scar at the trailing edge is much deeper than any other locations and the larger relative slip range resulted in considerably deeper surface damage. Another interesting discovery is that when material with different orientations the degree of wear also develops differently and the quantitative prediction is given.     

Different aspects of the role of wear debris in fretting wear

Two different aspects of the role of oxide wear debris in fretting wear are studied by allowing them to escape from the interface during sliding. This is accomplished by laser surface texturing that forms regular micro-pores topography on the friction surfaces which enables this escape. It is found that the role of oxide wear debris depends on the dominant fretting wear mechanism. Their presence in the interface protects the friction surfaces when the dominant wear mechanism is adhesive and harms the friction surfaces when this mechanism is abrasive. The escape of oxide wear debris into the micro-pores results in up to 84% reduction in the electrical contact resistance of the textured fretting surfaces.     

Comparison between radial fretting and dual-motion fretting features of cortical bone

Fretting tests were carried out to compare the effect of radial and dual-motion modes on wear of cortical bone. A titanium ball was rubbed against fresh human cortical bone under controlled load and motion test conditions. Dual-motion fretting mode produced more severe damage than radial fretting, with more debris occurring at the worn surface. Cracks were abundant in radial fretting and they were significantly affected by the microstructure of cortical bone; this effect was not obvious in the case of the dual-motion fretting mode. Radial fretting test could be used to evaluate the anisotropy of material and crack propagation of brittle cortical bone.     

Fretting corrosion of orthopaedic implant materials by bone cement

Debonding of bone cement from the stem of the femoral component of a hip prosthesis can result in local tangential oscillatory movement, i.e. fretting, between the two contacting materials as the limb is moved. Patches where such rubbing has occurred are frequently seen on removed implants. Fretting fatigue experiments have been carried out in Hanks solution on austenitic stainless steel and Ti-6Al-4V (IMI 318) with bridges of bone cement clamped to the specimens. Fretting appears to have little effect on the fatigue life of either material but the scanning electron microscope reveals the formation of thick oxide layers which subsequently give rise to loose debris particles by a process of delamination. Further experiments carried out in Hanks solution in an electrolytic cell have shown that there are potential changes when a bone cement rider is fretted against a stainless steel or titanium alloy plate although the change in potential is only one tenth that obtained with a metal-on-metal contact. Fretting by bone cement appears to be producing damage to the metal surfaces which manifests itself as mild wear rather than a diminution in fatigue strength.     

On the fretting wear mechanism of Zr-alloys

Zirconium alloys are highly desirable in nuclear applications due to their transparency to thermal energy neutrons and for their high corrosion resistance. The main objective of this study is to investigate the fretting wear mechanism of Zr–2.5%Nb alloy. The experimental work was carried out in air at 265 °C, using a specially designed fretting wear tribometer. The transfer of material, the change in the wear volume and the maximum wear depth with the number of cycles were measured through 3D mapping of the topography of the fretted surface. SEM and Fourier Transform Infrared Interferometry methods were used to examine the microspall pits and to measure the distribution of the thickness of oxide layer in the fretting region. For relatively small slip amplitude, the results showed that the fretting wear mechanism is initially dominated by adhesion and abrasion actions and then by delamination and surface fatigue. The time variation of the wear losses was shown to be cyclic until a steady state value is reached. At high slip amplitudes, however, abrasion and delamination are the only dominant wear mechanisms. The volumetric wear losses were found to decrease monotonically with the number of cycles. A novel approach was introduced, whereby the thermal and electrical contact resistances of the fretting interface are simultaneously measured. The results demonstrated the potential use of this non-intrusive approach for real-time monitoring of the fretting wear mechanism.     

An in-vitro investigation into the cement pressurization achieved during insertion of four different femoral stems

Adequate cement pressurization during stem insertion improves the interdigitation of cement into bone. This increases the strength of the cement-bone interface, thus contributing to the reduction of the incidence of aseptic loosening, the commonest cause of revision surgery. This in-vitro study compared the cement pressurization achieved during insertion of four different stems of equivalent sizes: the Elite Plus (DePuy, UK), C-Stem (DePuy, UK), Exeter (Stryker, USA), and CPS-Plus (Plus Orthopedics, Switzerland). The maximum pressures attained at the time of stem insertion were recorded at proximal, mid and distal stem levels. The Elite Plus generated significantly higher distal pressures than the other stems. The CPS-Plus generated significantly greater proximal cement pressures than the Elite Plus, C-Stem, and Exeter prostheses. The triple taper of the C-Stem increased the cement pressurization medial to the stem. The stem shape and the presence or absence of a proximal stem centralizer affect cement pressurization. The presence of a proximal stem centralizer, a large stem volume, and a lateral-medial taper are all factors associated with increased cement pressurization during stem insertion.     

TC4合金微动疲劳损伤研究

研究了TC4合金在柱面-平面接触务件下的微动疲劳行为，分析了其微动疲劳损伤机制。结果表明：在试验务件下，微动区边缘的损伤特征以粘着磨损为主，而微动区中部则以磨粒磨损和接触疲劳为主。疲劳裂纹易于在微动区．特别是在蚀坑处萌生和扩展。促使微动疲劳裂纹萌生的因素：一是法向应力和切向摩擦力引起的材料表层塑性变形，二是微动磨损破坏了材料的表面完整性，造成了缺口应力集中效应。