A reliable in vitro approach to assess the stability of acetabular implants using digital image correlation

The main cause of failure of the hip acetabular component is aseptic loosening. Preclinical test methods currently used to assess the stability of hip acetabular implants rely on crude simplifications. Normally, either one component of motion or bone strains are measured. We developed a test method to measure implant 3D translations and rotations and bone strains using digital image correlation. Hemipelvises were aligned and potted to allow consistent testing. A force was applied in the direction of the load peak during level walking. The force was applied in 100‐cycle packages, each load package being 20% larger than the previous one. A digital image correlation system allowed measuring the cup‐bone relative 3D displacements (permanent migrations and inducible micromotions) and the strain distribution in the periacetabular bone. To assess the test repeatability, the protocol was applied to six composite hemipelvises implanted with very stable cups. To assess the suitability of the method to detect mobilisation, six loose implants were tested. The method was repeatable: the interspecimen variability was 16 μm for the bone/cup relative translations, 0.04° for the rotations. The method was capable of tracking extremely loose implants (translations up to 4.5 mm; rotations up to 30°). The strain distribution in the bone was measured, showing the areas of highest strain. We have shown that it is possible to measure the 3D relative translations and rotations of an acetabular cup inside the pelvis and simultaneously to measure the full‐field strain distribution in the bone surface. This will allow better preclinical testing of the stability of acetabular implants.     

Development of cementless metal-backed acetabular cup prosthesis using functionally graded material

Metal backing has become widely used in acetaular cup design. A stiff backing for a polyethylene liner was initially believed to be mechanically favorable. Yet, recent studies of the load transfer around acetabular cups have shown that a stiff backing in fact generates higher stress peaks around the acetabular rim than full polyethylene cups, while reduces the stresses transferred at the central part of acetabulum causing stress shielding at the dome of acetabulum. To overcome these two problems, the aim of this study is to improve the design of cementless metal-backed acetabular cup using the two-dimensional functionally graded material concept through a finite element analysis and the optimization techniques. It is found that the optimal 2-D FGM model has three bioactive materials of hydroxyapatite, Bioglass and collagen. This optimal material reduces the stress shielding at the dome of acetabulum by 40% and 37% compared with stainless steel and titanium metal backing shell, respectively. However, using 2-D FGM model reduces the maximum interface shear stress in bone by 31% compared to titanium metal backing shell.     

超高分子量聚乙烯人工髋臼材料压缩性能的红外辐射研究

采用先进的试验设备和方法对超高分子量聚乙烯(UHMW PE)人工髋臼材料在不同的加载速度下进行压缩力学性能试验的同时,对其压缩变形过程进行了热红外辐射温度的监测。对超高分子量聚乙烯人工髋臼材料试验过程中的应力与应变、平均红外辐射温度与应变、平均红外辐射温度与机械功之间的关系进行了分析。试验结果为超高分子量聚乙烯人工髋臼材料在特殊的人体环境中的应用提供了参考。     

基于SLM的髋臼杯多孔结构设计与力学性能分析

目的 获得长期稳定性更优的医用多孔髋臼杯。方法 利用三周期极小曲面方法设计出更适合应用于髋臼杯的多孔结构,采用选区激光熔化方法进行加工成形。对样件进行测量得到其力学性能参数。对得到的弹性模量、屈服强度进行分析,得出不同结构的变形模型及设计参数对多孔结构力学性能的影响,选择力学性能和生物相容性均符合髋臼杯多孔需求的一组多孔结构。结果 制造出的多孔结构弹性模量为3.27~7.44 GPa,屈服强度为164.84~407.21 MPa。试验结果表明,TPMS设计的多孔结构力学性能优良,除了70%与75%孔隙率的P结构之外,均可以在力学性能上满足髋臼杯的制造。从变形模式看,P结构变形模式以拉伸为主,G结构与D结构为混合模式变形。其中,G结构设计参数对多孔样件的力学性能影响最大。结论 得到最适合制造髋臼杯多孔结构的是TPMS的G结构,并将G结构应用于髋臼杯的三维模型中。     

The use of photoelastic coatings to investigate the pelvic strain variations resulting from the introduction of cementless metal backed acetabular cups

An in vitro photoelastic coating study was initiated to investigate the influence of metal backed cementless acetabular cups on pelvic strain conditions in the immediately post operative state, for comparison with those related to normal hip joint conditions. A single dried bone hemipelvis was loaded to simulate the pelvic orientation at both the 47% stance and single leg stance. The problems associated with use of the photoelastic coating technique are discussed, particularly in relation to the analysis of the anisotropic pelvic bone. A number of critical areas were highlighted on the pelvis where high shearing strains occurred after the introduction of cementless metal backed acetabular cups.     

超高分子量聚乙烯摩擦学性能研究进展

综述了超高分子量聚乙烯(UHMWPE)在摩擦学领域的研究进展,着重评述了UHMWPE材料在人工关节方面的应用以及在减摩耐磨材料方面的研究,并提出了UHMWPE作为减摩耐磨材料在研究与应用方面几个亟待解决的问题.     

Wear tests in a hip joint simulator of different CoCrMo counterfaces on UHMWPE

The objective in this work was to study the effect of different material counterfaces on the Ultra High Molecular Weight Polyethylene (UHMWPE) wear behavior. The materials used as counterfaces were based on CoCrMo: forged with hand polished and mass finished, CoCrMo coating applied on the forged CoCrMo alloy obtained by Physical Vapour Deposition (PVD). A hip joint simulator was designed and built for these studies. The worn surfaces were observed by optical and scanning electron microscopy. The results showed that the hand polished CoCrMo alloy caused the higher UHMWPE wear of the acetabular cups. The CoCrMo coating caused the least UHMWPE wear, while the mass finished CoCrMo alloy caused an intermediate UHMWPE wear. It is shown that the wear rates obtained in this work are closer to clinical studies than to similar hip joints simulator studies.     

Corrosion behaviour and mechanical properties of functionally gradient materials developed for possible hard-tissue applications

Artificial hip joints have an average lifetime of 10 years due to aseptic loosening of the femoral stem attributed to polymeric wear debris; however, there is a steadily increasing demand from younger osteoarthritis patients aged between 15 and 40 year for a longer lasting joint of 25 years or more. Compliant layers incorporated into the acetabular cup generate elastohydrodynamic lubrication conditions between the bearing surfaces, reduce joint friction coefficients and wear debris production and could increase the average life of total hip replacements, and other human load-bearing joint replacements, i.e. total knee replacements. Poor adhesion between a fully dense substrate and the compliant layer has so far prevented any further exploitation. This work investigated the possibility of producing porous metallic, functionally gradient type acetabular cups using powder metallurgy techniques – where a porous surface was supported by a denser core – into which the compliant layers could be incorporated. The corrosion behaviour and mechanical properties of three biomedically approved alloys containing two levels of total porosity (>30% and <10%) were established, resulting in Ti–6Al–4V being identified as the most promising biocompatible functionally graded material, not only for this application but for other hard-tissue implants.     

Observations of residual sub-surface shear strain in the ultrahigh molecular weight polyethylene acetabular cups of hip prostheses

Analysis of bearing surfaces of explanted cups can help to determine the wear mechanisms that are responsible for generation of wear debris. In this study a microscope polariscope was used to detect residual subsurface shear strains, deformation and subsurface cracks in explanted Charnley acetabular cups. The wear surfaces were compared to an acetabular cup from a hip joint simulator test. The six explanted cups that were studied had all failed after long periods of implantation, with penetrations ranging from 2.1 to 3.8 mm. The explanted and simulator cups both had a smooth, high-wear region. High residual subsurface shear strains were found in the high-wear region of most cups, with certain cups possessing subsurface cracks running parallel with the surface 5–10 m deep, close to the areas of high residual subsurface shear strain. This was caused by plastic deformation and subsurface fatigue of the polymer surface.     

Design and development of a new acetabular cup prosthesis

Historically, the main objective of Total Hip Replacements (THR) was to relieve pain and increase quality of life in the elderly and not normally intended for young active patients. With the current trend of performing THR on younger patients, the requirement is not only the increased longevity of the prosthesis but also increased performance for more strenuous activities. The acetabular cup design has far more impact on long-term survival of the THR than the femoral component. Optimising the acetabular cup produces a complex problem where the individual design factors have massive impact on the system. Changes in stress values, even caused by initial primary fixation during the operation, can result in stresses being transferred unfavourably. For optimisation of the acetabular cup, the properties of the natural hip must be retained by minimising both remodelling and bone resorption. This work has produced a novel acetabular cup prostheses with a ceramic on ceramic bearing surface that should last longer and perform better, thus reducing the necessity for costly and debilitating revisions. In addition, the use of the proposed novel anatomically orientated mechanical testing methods gives fast results and can also allow further insight into fatigue of alumina bearing couples in THR.     

Pre-clinical evaluation of the Cambridge acetabular cup

It is postulated that the stiffness of current acetabular designs compromises long-term component stability. We present a novel acetabular component design that is horseshoe shaped and has a large diameter bearing. It is made from composite materials and is designed to match the stiffness of subchondral bone. It is intended that stress shielding will be minimised and that the distribution of stress will be improved. The mechanical and biological suitability of the composite has been confirmed. A range of standard and non-standard, pre-clinical, tests have established the robustness and safety of the new component. The efficacy of the new design has been evaluated by clinical trial on 50 patients. Optimal results were obtained using the hydroxyapatite (HA) coated cups. Our results support the new design concept, with the caveat that biological fixation is imperative. Minor design modifications are recommended.     

Ceramic Femoral Heads for Total Hip Arthroplasty

Femoral heads and acetabular sockets made of alumina ceramics are used successfully in total hip replacement. The in vivo fracture rate of present, third‐generation, ceramic heads is less than 0.01 %. Ceramic femoral heads are one example of the applications of ceramics that give reliable long‐term use despite the permanent tensile stress to which they are exposed.     

Appropriate analysis of CIREN data: Using NASS-CDS to reduce bias in estimation of injury risk factors in passenger vehicle crashes

The Crash Injury Research Engineering Network (CIREN) database contains detailed medical and crash information on a large number of severely injured occupants in motor vehicle crashes. CIREN's major limitation for stand-alone analyses to explore injury risk factors is that control subjects without a given injury type must have another severe injury to be included in the database. This leads to bias toward the null in the estimation of risk associations. One method to cope with this limitation is to obtain information about occupants without a given injury type from the National Automotive Sampling System's Crashworthiness Data System (NASS-CDS), which is a probability sample of towaway crashes, containing similar crash information, but less medical detail. Combining CIREN and NASS-CDS in this manner takes advantage of the increased sample size when outcomes are available in both datasets; otherwise NASS-CDS can serve as a sample of controls to be combined with CIREN cases, possibly under a sensitivity analysis that includes and excludes NASS-CDS subjects whose status as a control is uncertain. Because CIREN is not a probability sample of crashes that meet its inclusion criteria, we develop a method to estimate the probability of selection for the CIREN cases using data from NASS-CDS. These estimated probabilities are then used to compute “pseudo-weights” for the CIREN cases. These pseudo-weights not only allow for reduced bias in the estimation of risk associations, they allow direct prevalence estimates to be made using medical outcome data available only in CIREN. We illustrate the use of these methods with both simulation studies and application to estimation of prevalence and predictors of AIS 3+ injury risk to head, thorax, and lower extremity regions, as well as prevalence and predictors of acetabular pelvic fractures. Results of these analyses demonstrate combining NASS and CIREN data can yield improvements in mean square error and nominal confidence interval coverage over analyses that use either the NASS-CDS or the CIREN sample alone.     

Failure analysis on retrieved ultra high molecular weight polyethylene (UHMWPE) acetabular cups

Retrievals obtained after revision surgery, provide valuable information on the rate of wear and the causes of wear that take place in vivo. The aim of this study was to perform a root cause failure analysis to determine the principle reason for mechanical failure on failed acetabular cups, retrieved during revision surgery, by making use of sound engineering failure analysis techniques. The bearing couples varied between steel on UHMWPE and ceramic on UHMWPE. The information gained during this analysis brought to the four that the principle mode of mechanical failure for acetabular cups is overheating of the UHMWPE. The resulting overheating is mainly due to a lack of good and/or sufficient lubrication.     

Herstellung von hochporösen,endkonturnahen Titan‐Formkörpern für biomedizinische Anwendungen

Near‐net‐shape manufacturing of highly porous titanium parts for biomedical applications The production of highly porous titanium parts is attractive for biomedical applications. Preferrentially, these parts are produced by powdermetallurgical means using suitable spacer materials. Porosities up to 75 % and well defined pore sizes in the range of 0.1 to 2.0 mm are achieved adjusting the amount and the particle size of the spacer material. Up to now, near‐net‐shape manufacturing of highly porous parts was hindered by the plastic deformation of the sintered network during machining leading to a partial or complete closing of the open porosity. A new manufacturing route is presented, where the shaping is already done in the unsintered state starting from pressed compacts. The stability of the compacts was found to be sufficient to machine the compacts without additional binders. The manufacturing route was successfully applied to the prototype of an acetabular cup. Additionally, some investigations are presented characterizing the highly porous titanium.     

Influence of a foreign body on the wear of metallic femoral heads and polyethylene acetabular cups of total hip prostheses

Excessive wear of polyethylene in total replacement hip prostheses elicits deleterious biologic reactions and may be thus a limiting factor that compromises the long-term performance of these devices. This study is based on the report of two clinical failures of total hip prostheses with metallic femoral heads and polyethylene acetabular cups. The investigations reveal that foreign bodies (titanium fibermesh pieces) can migrate into the joint space of total hip prostheses and participate in abrasive third-body wear of the polyethylene cups. This excessive wear of polyethylene enhanced by the modification of the metallic counterface roughness is likely to induce the early loosening of the devices.     

Assessment of wear on the cones of modular stainless steel Exeter hip stems

The wear on the stem cones of retrieved Exeter Universal hip stems has been assessed using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and surface profilometry. The in-service life of these prosthetic stems varied, up to a maximum of 7 years. A combination of SEM, EDX and visual assessment indicates that the stem cones have not suffered from any corrosion. SEM scans indicate that damage to stem cones (excluding extraction and post-removal damage) can be categorised into insertion marks and fretting marks. In some cases there are signs of material being deposited on the cone surface. Surface profilometry suggests that the levels of debris generation at the cone/internal head interface are very low relative to those that are likely to be associated with head articulation against the acetabular cup. A total of 20 stem cones underwent SEM scans. From these, 10 subsequently have undergone surface profilometry along with the corresponding internal head surfaces. There is a good correlation between surface roughness measured by surface profilometry and the topography observed in the SEM images. The surface roughness of each stem cone is similar to that of the corresponding internal head surface.     

A new tribological approach on metal cup with optimized pits model using spark discharge machine

ABSTRACT

An interference friction causes tear and wear in metal-on-metal (MoM) hip joint. The purpose of this research is to examine the optimization of pit embedded in the acetabular cup using a spark discharge machine. Tribology tests on cup with 8, 21, and 40 pits embedded produced promising results. A modified pin-on-disk tribometer was used to measure the effects of the coefficient of friction and wear on a 28-mm-diameter acetabular cup. Microscopy image analysis was used to examine particle debris and surface disfigurement. This study revealed that the more pits were produced in the hemispherical or curvature cup, the more lubricant was confined inside the pits, and the easier the contact was for MoM. The results also show that the curvature surface modification with pits can positively influence friction and wear and stability optimization of MoM implants.     

Advanced Stellite alloys with improved metal-on-metal bearing for hip implants

Stellite 21 is a low-carbon Co–Cr–Mo alloy that has been used for hip implants for decades. The Stellite 21 implants can fail when the femoral head and the acetabular cup loosen because of limited metal-on-metal bearing. Two modified Stellite 21 alloys with better bearing capacities are proposed as replacements in this study: Cr-modified Stellite 21 with additional 10 wt.% Cr, and N-modified Stellite 21 with addition of 0.5 wt.% CrN. The wear and corrosion resistances of these alloys were investigated in simulated conditions experienced by hip implants in human bodies. The experimental results show that the proposed alloys all exhibit better wear resistance than the conventional hip implant material, but only Cr-modified Stellite 21 displays better corrosion resistance, thus this alloy should be considered for use in future hip implants.     

An evaluation of the tribological performance of zirconia and CoCrMo femoral heads

Five new zirconia, five new CoCrMo and three explanted CoCrMo femoral heads were wear-tested in bovine serum for five million cycles using the Durham Hip Joint Wear Simulator. Wear was measured gravimetrically and surface topography with a 3D non-contacting profilometer. This allowed an evaluation of the different head types on UHMWPE acetabular cup wear rates and the effect of roughening of the femoral head on acetabular cup wear. The mean acetabular cup wear rate against the five CoCrMo femoral heads was 40.8 mm³/10⁶ cycles which was significantly higher (p = 0.03) than against zirconia (33.3 mm³/10⁶ cycles). The initial surface roughness of the CoCrMo femoral heads (R_a = 4.6 nm) was statistically significantly higher than for the zirconia heads (R_a = 3.1 nm). Over the wear test the CoCrMo heads got statistically significantly rougher (R_a = 10.5 nm) whilst the zirconia heads showed no statistically signficant change. The three explanted CoCrMo femoral heads had initial mean surface roughness, R_a, values of 19, 24 and 55 nm with corresponding cup wear rates of 97.6, 131.2 and 148.4 mm³/10⁶ cycles respectively. The very high wear rates against the explanted heads highlight the need for scratch resistant femoral head surfaces.