Effect of interface conditions on the behaviour of a Freeman hip endoprosthesis

The femoral element of a total hip replacement is a composite structure of two, or perhaps three, components — the endoprosthesis, the bone and, where present, the cement. The interfacial conditions are such that complete structural continuity does not necessarily obtain. That this is so has often been suspected due to the observed loosening which can occur in vivo. In modelling the system, typically for finite element analysis, it has usually been considered to be monolithic, such that tensile and shear stresses could be transmitted across the interfaces as well as the normal compressive stress. Here the femoral component of a Freeman hip replacement is considered, implanted without bone cement, and analyses are carried out under monolithic, i.e. fully bonded, and non-bonded assumptions. Simultaneously the effect of retaining the neck of the femur, one of the features of using this particular prosthesis, is also examined.     

Finite element analysis of a cemented ceramic femoral component for the assembly situation in total knee arthroplasty]

The femoral components of the total knee replacements are generally made of metal. In contrast, ceramic femoral components promise improved tribological and allergological properties. However, ceramic components present a risk of failure as a result of stress peaks. Stress peaks can be minimised through adequate implant design, proper material composition and optimum force transmission between bone and implant. Thus, the quality of the implant fixation is a crucial factor. The objective of the present study was to analyse the influence of the cement layer thickness on stress states in the ceramic femoral component and in the femur. Two- and three- dimensional finite element analyses of an artificial knee joint with cement layers of different thickness and with an unbalanced cement layer thickness between the ceramic femoral component and the femur were performed. Higher stress regions occurred in the area of force transmission and in the median plane. The maximum calculated stresses were below the accepted tensile strength. Stresses were found to be lower for cement layer thickness of <2.0 mm.     

A three-dimensional non-linear finite element study of the effect of cement-prosthesis debonding in cemented femoral total hip components.

A three-dimensional non-linear finite element analysis of a cemented femoral component in which the component was partially debonded from the cement mantle was used to assess the effects of debonding on stresses in the cement. Three cases of partial cement-metal debonding were modelled with debonding of the proximal portion of the implant down to a horizontal plane which was 35, 62.5, or 82.5 mm below the prosthesis collar. Each situation was studied under loads simulating both gait and stairclimbing. Also, complete debonding between the implant and the surrounding cement mantle was modeled for loads simulating gait. Under stair climbing loads with partial cement-mental debonding, hoop stresses of 13-18 MPa were observed in the cement at the cement-metal interface at the proximal postero-medial corner of the implant. Similarly, in stair climbing, the maximum principal stresses in the cement were also adjacent to the proximal postero-medial region of the implant. These stresses were compressive and increased from 15 MPa with fully bonded interfaces to 48 MPa with debonding down to 82.5 mm below the prosthesis collar. Under gait loads, complete debonding caused high compressive stresses up to 34.9 MPa in the cement distal to the prosthesis tip. Thus, cement failure subsequent to prosthesis debonding is likely in the proximal region in a partially debonded implant due to stair climbing loads and is likely below the prosthesis tip in a fully debonded implant due to gait loading.     

A finite element study of the effect of diametral interface gaps on the contact areas and pressures in uncemented cylindrical femoral total hip components

Uncemented femoral total hip components rely entirely on contact with the prepared femur for their initial fixation. The contact areas and stresses between a straight tubular bone and a metal cylindrical prosthesis 12.5 cm long and 13 mm in diameter were calculated in a finite element model which includes uniform diametral gaps varying from 20 to 500 microns, using transverse loads from 100 to 2000 N. Frictionless three-dimensional contact elements were used between the bone and the prosthesis. Contact stresses were high and irregular in all cases, and the contact areas were small. Two regions of contact were apparent for lower loads and larger gaps. A third region of contact occurred near the distal tip of the implant at higher loads. This region of contact markedly increased the contact stresses at the distal tip of the prosthesis. A 20 microns overlap between bone and implant was modelled to assess a slight interference fit. The contact stress distribution in this case was markedly different from the stress distribution with a 20 microns diametral gap. The data collectively indicates that gaps of less than 20 microns between bone and implant can substantially change contact stress distributions.     

Stress analysis and failure prediction in the proximal femur before and after total hip replacement

An investigation was performed to determine the effects of the presence of two lengths of proximal Müller prosthesis on predicted failure loads, as compared to those for an intact femur. Three-dimensional stresses in a bone/cement/prosthesis system were determined using finite element methods, with both isotropic and transversely isotropic material properties used for the diaphyseal cortex. Significant increases in prosthesis stem stresses were found when the transversely isotropic material properties were employed in the diaphyseal cortex. This leads to the conclusion that accurate anisotropic material properties for bone are essential for precise stress determination and optimum design in prosthetic implants. Failure loads were also predicted for vertical compression and axial torque, similar to available experimental conditions, and were within the range of the experimental failure data found in the literature. The technique developed herein can be used to systematically assess existing as well as future implant designs, taking into account the complex three-dimensional interaction effects of the overall bone/cement/prosthesis system.     

3 种不同弹性模量钛合金股骨假体在羊股骨置换模型中应力分布的三维 有限元分析

目的:通过三维有限元分析方法来观察并比较3种不同弹性模量钛合金股骨假体在羊股骨置换模型中von-Mises应力分布的情况。方法:采用64排螺旋CT对一健康成年羊的下肢股骨进行全长的CT扫描,扫描层厚为0.5 mm,扫描所得的数据存储为DICOM文件。将得到的DICOM文件导入到CT图像分析软件Mimics 10.0,然后利用Mimics 10.0软件来生成股骨的骨质点云数据,再将生成的骨质点云数据导入到Simpleware分析软件,通过机械加工反求中的复杂曲面造型技术建立起精确的三维实体模型。对三维实体模型进行网格划分,确定了髓腔的形状,并根据羊下肢股骨髓腔的形状设计了作者实验用的羊股骨假体模型,然后在ANSYS 12.1软件中进行网格划分。给予加载缓慢行走载荷以及扭转载荷,分析并比较羊股骨以及3种不同弹性模量钛合金股骨假体在股骨置换模型中von-Mises应力分布的情况。结果:在缓慢行走载荷以及扭转载荷条件下,3种不同弹性模量钛合金股骨假体von-Mises应力分布变化趋势一致,假体的柄颈结合部以及假体柄上1/3为应力集中区域。3种不同弹性模量的最大应力集中点均位于柄颈结合部,60 GPa弹性模量的股骨假体植入后假体的最大应力最小(37.8 MPa、29.1 MPa),股骨的最大应力最大(12.6 MPa、24.5 MPa);80 GPa的次之,假体的最大应力(38.4 MPa、33.4 MPa),股骨的最大应力(12.5 MPa、24.5 MPa);110 GPa的股骨假体植入后假体的最大应力最大(38.9 MPa、38.1 MPa),股骨的最大应力最小(12.3 MPa、24.5 MPa)。60 GPa弹性模量的股骨假体植入后的假体最大位移和相对位移均最小(缓慢行走载荷下假体最大位移为0.551 mm、相对位移为0.008 mm,扭转载荷下假体最大位移为0.730 mm、相对位移为0.011 mm)。结论:较低弹性模量的钛合金股骨假体(60 GPa)由于其弹性模量更接近于骨组织的弹性模量,股骨假体与股骨间的"应力遮挡"效应较小,更有利于应力在股骨假体及股骨间的传递,增加了股骨假体的早期稳定性,延长了其临床寿命。     

Three-dimensional finite element analysis of glenoid replacement prostheses: a comparison of keeled and pegged anchorage systems

Glenoid component loosening is the dominant cause of failure in total shoulder arthroplasty. It is presumed that loosening in the glenoid is caused by high stresses in the cement layer. Several anchorage systems have been designed with the aim of reducing the loosening rate, the two major categories being "keeled" fixation and "pegged" fixation. However, no three-dimensional finite element analysis has been performed to quantify the stresses in the cement or to compare the different glenoid prosthesis anchorage systems. The objective of this study was to determine the stresses in the cement layer and surrounding bone for glenoid replacement components. A three-dimensional model of the scapula was generated using CT data for geometry and material property definition. Keeled and pegged designs were inserted into the glenoid, surrounded by a 1-mm layer of bone cement. A 90 deg arm abduction load with a full muscle and joint load was applied, following van der Helm (1994). Deformations of the prosthesis, stresses in the cement, and stresses in the bone were calculated. Stresses were also calculated for a simulated case of rheumatoid arthritis (RA) in which bone properties were modified to reflect that condition. A maximum principal stress-based failure model was used to predict what quantity of the cement is at risk of failure at the levels of stress computed. The prediction is that 94 percent (pegged prosthesis) and 68 percent (keeled prosthesis) of the cement has a greater than 95 percent probability of survival in normal bone. In RA bone, however, the situation is reversed where 86 percent (pegged prosthesis) and 99 percent (keeled prosthesis) of the cement has a greater than 95 percent probability of survival. Bone stresses are shown to be not much affected by the prosthesis design, except at the tip of the central peg or keel. It is concluded that a "pegged" anchorage system is superior for normal bone, whereas a "keeled" anchorage system is superior for RA bone.     

Pre-clinical validation of a new partially cemented femoral prosthesis by synergetic use of numerical and experimental methods

The present work reports the pre-clinical validation of an innovative partially cemented femoral prosthesis called cement-locked uncemented (CLU) prosthesis. The inventors of the device under investigation claimed that, when compared to a comparable fully cemented stem, the new stem would present various advantages. Two previous experimental studies confirmed that primary stability and stress shielding were comparable to those of cemented stems. Aim of the present study was to investigate if the remaining claims were confirmed as well. A complete finite element model of the bone-implant complex was created from CT data. The model was validated against in vitro measurements of bone surface strains as well as against primary stability measurements. The peak stresses predicted in the CLU cement mantle were not found significantly lower than those reported in other studies on fully cemented stems. However, once the cement inlet geometry is optimised and the associated stress risers are eliminated, the CLU cement mantle should be subjected to much lower stresses. The stress induced in the stems by both load cases was well below the fatigue limit of the Ti6Al4V alloy. Finite element models predicted for all load cases relative motion between cement and metal lower than 60 microm. This amplitude may be fully accommodated by elastic deformations of the cement micro-ridges. The experimental and numerical results showed the validity of the new fixation concept, although a further optimisation of the geometry of the cement pockets is needed in order to further reduce the stresses in the cement.     

The influence of cyclic loading on gentamicin release from acrylic bone cements

Antibiotic-loaded acrylic bone cement is widely used in total joint replacement to reduce infections. Walking results in cyclic loading, which has been suggested to stimulate antibiotic release. The goal of this study is to compare antibiotic release from cyclically loaded bone cement with the release from unloaded bone cement. Two models of the frontal aspect of a femoral stem were cemented with CMW 1 Radiopaque G, Palacos R-G and Palamed G. Both were immersed in water, and the gentamicin concentration in the water was monitored. One model was cyclically loaded at 5 Hz during immersion achieving physiological stresses in the bone cement mantle. After 10.8 x 10(6) cycles, initial release of gentamicin from Palamed G was increased significantly for loaded over unloaded, but not from CMW 1 Radiopaque G and Palacos R-G.     

Design considerations for ceramic resurfaced femoral head: effect of interface characteristics on failure mechanisms

Ceramic hip resurfacing may offer improved wear resistance compared to metallic components. The study is aimed at investigating the effects of stiffer ceramic components on the stress/strain-related failure mechanisms in the resurfaced femur, using three-dimensional finite element models of intact and resurfaced femurs with varying stem–bone interface conditions. Tensile stresses in the cement varied between 1 and 5 MPa. Postoperatively, 20–85% strain shielding was observed inside the resurfaced head. The variability in stem–bone interface condition strongly influenced the stresses and strains generated within the resurfaced femoral head. For full stem–bone contact, high tensile (151–158 MPa) stresses were generated at the cup–stem junction, indicating risk of fracture. Moreover, there was risk of femoral neck fracture due to elevated bone strains (0.60–0.80% strain) in the proximal femoral neck region. Stresses in the ceramic component are reduced if a frictionless gap condition exists at the stem–bone interface. High stresses, coupled with increased strain shielding in the ceramic resurfaced femur, appear to be major concerns regarding its use as an alternative material.     

A new class of potential anti-tuberculosis agents: Synthesis and preliminary evaluation of novel acrylic acid ethyl ester derivatives

Novel acrylic acid ethyl ester derivatives were synthesized and evaluated as potential agents against Mycobacterium species. A versatile and efficient copper-catalyzed coupling process was developed and used to prepare a library of substituted acrylic acid ethyl ester analogs. Minimum inhibitory concentration assays indicated that two of these compounds 3 and 4 have greater in vitro activity against Mycobacterium smegmatis than rifampin, one of the current, first-line anti-mycobacterial chemotherapeutic agents. Moreover, members of this new class of compounds appear to exhibit a specific anti-mycobacterial effect and do not inhibit the growth of the other Gram-positive or Gram-negative species tested.     

Mathematical shape optimization of hip prosthesis design

The long-term success of artificial-joint replacement depends partly on the chances for acrylic cement failure and interface disruption. These chances can be diminished by an optimal load-transfer mechanism, whereby stress concentrations are avoided. The present paper introduces a method for numerical shape optimization, whereby the finite element method is used iteratively to determine optimal prosthetic designs, which minimize interface stresses. The method is first applied in a simplified one-dimensional model of a cemented femoral stem fixation, using acrylic cement. The results show that 30-70% cement and interface stress reductions can be obtained in principle with an optimized design. Although the actual optimal shape is susceptible to the characteristics of the joint load, the stem length, stem modulus, cement modulus and bone properties, its general geometrical characteristics are consistent, featuring proximal and distal tapers, and a belly-shaped middle region. These general characteristics are confirmed in a more realistic two-dimensional FEM model. It is concluded that this method of shape optimization can provide a meaningful basis for prosthetic design and analysis activities in general.     

Polymères bactériostatiques : une nouvelle approche pour les ciments orthopédiques

The implantation of joint prostheses may lead to infection (about 2% of the cases) with more or less dramatic consequences as morbidity or even death. Several clinical and experimental studies showed the high interest of antibiotic prophylaxis to prevent such infections but also its relative inefficiency when the infection is established. Bioactive polymers bearing anionic groups have synthesized and showed to be able to inhibit bacterial adhesion. In this study, the bioactive polymers were mixed with commercial acrylic orthopaedic cement used in the surgery of total hip prosthesis. We demonstrated that mixtures cement/bioactive polymers may inhibit adhesion of Staphylococcus aureus (S. aureus) – strain resistant MRSA 88244) by diffusion process. Kinetics of diffusion of the bioactive polymers can be controlled by addition of hydrophobic units as methymethacrylate in the macromolecular chains of the polymers.     

Biomechanics of the femoral component of total hip prostheses with particular reference to the stress in the bone-cement

Two-dimensional finite element analyses were used to determine the normal and shear stress distributions at the prostheses-cement and cement-bone interfaces in the femoral component of a total hip replacement. Various combinations of stem, cement and bone stiffnesses were investigated. In particular the influences of stem taper, cement stiffness, prosthesis stiffness and the effect of a plateau, on the cement stresses were examined and compared. It was particularly noticeable that the normal direct stress across the cement in the proximal region of the stem, both literally and medially, as generally compressive. It was found that the more flexible the cement the more uniform were the stress distributions. Furthermore, these stresses increase as the stiffness of the stem decreases.     

Dependence of stem stress in total hip replacement on prosthesis and cement stiffness

Two dimensional finite element analyses were used to determine the direct tensile stress distributions along the lateral edge of the stem of the femoral component of a total hip replacement. The efficacy of using two-dimensional analysis for three-dimensional composite structures was assessed, and the two-dimensional model was found to be suitable for parametric studies. Various combinations of stem and cement stiffnesses were investigated. In particular the influence of stem taper, cement stiffness, prosthesis stiffness and the effect of a plateau on the prosthesis stresses, were examined and compared. The most significant factor was found to be the modulus of elasticity of the stem material. The tensile stresses decrease with decreasing modulus. Cement stiffness and stem shape appeared to have for less effect on stem stress.     

Design and synthesis of novel antimicrobials with activity against Gram-positive bacteria and mycobacterial species,including M. tuberculosis

The alarming increase in bacterial resistance over the last decade along with a dramatic decrease in new treatments for infections has led to problems in the healthcare industry. Tuberculosis (TB) is caused mainly by Mycobacterium tuberculosis which is responsible for 1.4 million deaths per year. A world-wide threat with HIV co-infected with multi and extensively drug-resistant strains of TB has emerged. In this regard, herein, novel acrylic acid ethyl ester derivatives were synthesized in simple, efficient routes and evaluated as potential agents against several Mycobacterium species. These were synthesized via a stereospecific process for structure activity relationship (SAR) studies. Minimum inhibitory concentration (MIC) assays indicated that esters 12, 13, and 20 exhibited greater in vitro activity against Mycobacterium smegmatis than rifampin, one of the current, first-line anti-mycobacterial chemotherapeutic agents. Based on these studies the acrylic ester 20 has been developed as a potential lead compound which was found to have an MIC value of 0.4 μg/mL against Mycobacterium tuberculosis. The SAR and biological activity of this series is presented; a Michael-acceptor mechanism appears to be important for potent activity of this series of analogs.     

Polymer composites for use in orthopedic surgery

Ever since Movin in (1950) and McKee in (1951) introduced the use of acrylic cement for fixation of hip prosthesis components a number of investigators have proposed various hip prosthesis designs using this cement fixation concept (Neale, 1967). This study was undertaken to support the hypothesis that certain dental materials could provide a more satisfactory bone-prosthesis bond than that presently possible with acrylic bone cement.^‡ Two restorative resins were found to have superior strength and resistance to thermal degradation when compared to acrylic bone cement. Tests of acrylic cement combined with apatite fillers suggest that restorative resin-anorganic bone composites would exhibit improved strength and toxicity properties and would also promote improved bonding due to resorption of the surface anorganic bone particles with subsequent bone infiltration and anchorage. Relatively high degradation of acrylic bone cement in accelerated aging tests suggests caution in using this material for implantation.     

Finite-element modelling of femoral shaft fracture fixation techniques post total hip arthroplasty.

The presence of a femoral prosthesis superior to a shaft fracture severely complicates fixation and treatment. This study uses two-dimensional, multithickness, plane stress finite-element models of a femur with prosthesis to investigate the stresses developed with the application of three popular fixation techniques: revision to a long stem prosthesis, lateral plating with a cortical bone allograft strut and cerclage wires, and custom plate application with proximal Parham band fixation with distal cortical screws (Ogden plate). The plate and bone contact as well as the fracture site contact were modelled by using orthotropic elements with custom-fit moduli so that only the normal stress to the interface was significant. A thermal analogy was used to model the cerclage and Parham band preloads so that representative preloads in the proximal fixation of the two types of plate treatments could be modelled. A parametric study was performed with the long-prosthesis model to show variations in stem lengths of one, two and three femoral diameters distal to the fracture site. The Ogden plate model showed a transfer of tensile stress near the proximal-most band, with the highest tensile stress being at the fracture site with evidence of stress shielding of the proximal lateral cortex. The cortical bone strut model showed a transfer of tensile stress to the bone strut but showed less shielding of the proximal cortex. The cerclage wires at the base of the bone strut showed the highest changes in load with the distalmost wire increasing to almost four times its original preload.(ABSTRACT TRUNCATED AT 250 WORDS)     

Status of Bone Mineral Density in Patients Selected for Cardiac Transplantation

ObjectiveTo determine the prevalence and correlates of low bone mineral density (BMD) in ambulatory outpatients with end-stage heart failure who were awaiting cardiac transplantation.MethodsFifty-five cardiac transplant candidates with end-stage heart failure were enrolled in this study. Bone mineral density at the lumbar spine and proximal femur was determined by dual-energy x-ray absorptiometry. Laboratory studies included serum alkaline phosphatase, calcium, intact parathyroid hormone, and 25-hydroxyvitamin D.ResultsThe mean proximal femur and lumbar spine Z scores were 0.3 ± 1.1 and 0.3 ± 1.5, respectively. The mean BMD was not lower than that of the age- and sex- matched reference population. Z scores were less than -1 in 23% at the lumbar spine and 15% at the proximal femoral neck. On the basis of T scores, osteopenia (T scores between -1 and -2.5) was present in 24% (confidence interval, 13% to 35%) of patients at the lumbar spine and in 20% (confidence interval, 10% to 30%) at the proximal femur; osteoporosis (T scores of less than -2.5) was present in 4% of the study population. Half of the patients in this study sample had elevated intact parathyroid hormone levels, and a third of the patients had low 25-hydroxyvitamin D levels.ConclusionLumbar spine and hip BMD measurements were not significantly low relative to age and sex in ambulatory patients with heart failure awaiting cardiac transplantation. (Endocr Pract. 2008;14:704-712)     

人工髋关节置换术在320 例股骨头缺血性坏死中的治疗体会

目的：探讨人工髋关节置换术在治疗股骨头缺血性坏死（ANFH）中的临床疗效。方法：选择2007 年2 月-2011 年2 月我院收 治的320 例（340 髋）股骨头缺血性坏死患者,均采用人工髋关节置换术对患者进行治疗,其中160 例（172 髋）患者应用骨水泥型 假体进行治疗,另外160 例（168）患者采用非骨水泥型假体进行治疗。采用Harris评分对患者手术前后的髋关节功能情况进行评 价,并比较骨水泥治疗组和非骨水泥治疗组的临床疗效。结果：患者均获得随访,随访时间为3～18 个月。全部患者手术后的 Harris评分明显高于手术前,差异有统计学意义（P<0.05）。骨水泥治疗组和非骨水泥治疗组在术后出血量、术后Harris 评分及住 院时间方面的差异无统计学意义（P>0.05）,但非骨水泥治疗组的并发症发生率明显低于骨水泥治疗组（P<0.05）。结论：采用人工 髋关节置换术治疗ANFH疗效显著,能明显改善患者的生活质量,骨水泥型假体与非骨水泥型假体的治疗效果相当,应根据患者 的具体情况进行合理的选择。