Multi-objective design optimization of functionally graded material for the femoral component of a total knee replacement

The optimal design of complex systems in engineering requires pursuing rigorous mathematical modeling of the system’s behavior as a function of a set of design variables to achieve goal-oriented design. Despite the success of current knee implants, the limited life span remains the main concern of this complex system. The mismatch between the properties of engineered biomaterials and those of biological materials leads to insufficient bonding with bone, stress shielding effects and wear problems (i.e. aseptic loosening). The use of a functionally graded material (FGM) for the femoral component of knee implants is attractive because the properties can be designed to vary in a certain pattern to meet the desired requirements at different regions in the knee joint system, thereby decreasing loosening problem. However, matching the properties does not necessarily guarantee the best functionality of the knee implant and there is a need for developing the optimal design of an FGM femoral component that is longer lasting. In this study, therefore, a multi-objective design optimization of a FGM femoral component is carried out using finite element analysis (FEA) and response surface methodology (RSM). The results of using optimized FGM are then compared with the use of standard Co–Cr alloy in a femoral component knee implant to demonstrate relative performance.     

Material selection in the design of the tibia tray component of cemented artificial knee using finite element method

Aseptic loosening of the tibial component; which may be caused by mechanical stress shielding in the bone and may require revision surgery; is the primary concern of total knee replacement (TKR). The stiffness of the implant material had a marked influence on the stresses developed in the constituents and surrounding bones of the artificial knee and then will affect the bone stress shielding. Therefore, the functionally graded materials had been developed as a potential tibia tray material of TKR due to its improved capability of stress distribution. In the current investigation two dimensional finite element models have been developed to study bone and interface stresses for six different tibial prothesises (titanium, CoCrMo and four functional graded materials “FGM” models). The utilization of FGM tibia tray with elastic modulus changing gradually in vertical direction downwardly showed a favorable stress distribution outcome. Furthermore, the results has revealed that the FGM tibia tray will reduce the stress shielding in the surrounding bones of the artificial knee which will increase the life of the total knee prosthesis.     

Aseptic loosening of femoral components – Materials engineering and design considerations

Aseptic loosening is one of the main reasons for the revision of a total knee replacement (TKR). The design of the key component of a TKR, the femoral component, is particularly problematic because its failure can be the result of different causes. This makes the development of new biomaterials for use in the femoral component a challenging task. This paper focuses on the engineering design aspects in order to understand the limitations of current materials and design deficiencies. The paper describes the introduction of a new biomaterial for a femoral component and justifies the recommendation to use multi-functional materials as a possible solution to aseptic loosening. The potential advantages of applying functionally graded biomaterials (FGBMs) in prosthetic femur are explained by reducing the leading causes of failure including wear, micro-motion and stress-shielding effect. The ideas presented in this paper can be used as the basis for further research on the feasibility and advantages of applying FGBM in other superior implant designs.     

Material selection for femoral component of total knee replacement using comprehensive VIKOR

The increasing trend of total knee replacement (TKR) revision surgery, which is associated with aseptic loosening, makes it a challenging research subject. The concern of loosening can be partially improved by selecting the optimal materials for TKR components. Therefore, this paper considers selection of the best material among the set of alternatives for femoral component of TKR through the multi-criteria decision making approach. The comprehensive VIKOR method was used to select the optimum material, and a systematic technique for sensitivity analysis of weights was introduced to find more reliable results. The obtained ranking order suggested the use of new materials over the existing ones. Porous and dense NiTi shape memory alloys were ranked first and second respectively.     

Properties of open-cell porous metals and alloys for orthopaedic applications

One shortcoming of metals and alloys used to fabricate various components of orthopaedic systems, such as the femoral stem of a total hip joint replacement and the tibial plate of a total knee joint replacement, is well-recognized. This is that the material modulus of elasticity (E′) is substantially larger than that of the contiguous cancellous bone, a consequence of which is stress shielding which, in turn, has been postulated to be implicated in a cascade of events that culminates in the principal life-limiting phenomenon of these systems, namely, aseptic loosening. Thus, over the years, a host of research programs have focused on the synthesis of metallic biomaterials whose E′ can be tailored to match that of cancellous bone. The present work is a review of the extant large volume of literature on these materials, which are called open-cell porous metals/alloys (or, sometimes, metal foams or cellular materials). As such, its range is wide, covering myriad aspects such as production methods, characterization studies, in vitro evaluations, and in vivo performance. The review also includes discussion of seven areas for future research, such as parametric studies of the influence of an assortment of process variables (such as the space holder material and the laser power in the space holder method and the laser-engineered net-shaping process, respectively) on various properties (notably, permeability, fatigue strength, and corrosion resistance) of a given porous metal/alloy, innovative methods of determining fatigue strength, and modeling of corrosion behavior.     

A Method of Material Optimization of Cementless Stem Through Functionally Graded Material

Ideally, a bone implant should be such that it exhibits an identical response to loading as real bone and is also biocompatible with existing tissue. A stiff stem, which is usually made of titanium, shields the proximal bone from mechanical loading (stress shielding). On the other hand, decreasing the stem stiffness increases the proximal interface shear stress and the risk of proximal interface failure. Therefore the purpose of this study is to solve these conflicting requirements in order to have more uniform interface shear stress distribution and less stress shielding through the concept of functionally graded material (FGM). FGM is a kind of advanced composite materials, which changes its composition and structure gradually over one or two directions of its volume, resulting in corresponding changes in the properties of the material. This study is divided into two parts; in the first part, the finite element analysis and optimization technique are used to design the stem as one-dimensional FGM, while in the second part, the stem is designed as two-dimensional functionally graded material. The aim of both designs is to overcome the above mentioned problems. In the case of part one (one-dimensional FGM), the gradation of elastic modulus is changed along the vertical direction (model 1) and along the horizontal direction (model 2), in order to find the optimal gradation direction. It is found that the optimal design is to change the elastic modulus gradually from 110 GPa (Hydroxyapatite) at the top of the stem to 1GPa (Collagen) at the bottom (model 1). This optimal gradation decreases stress shielding by 83%, while reduces the maximum interface shear stress by 32% compared to homogenous titanium stem. However, in the second part (two-dimensional FGM, model 3) the materials of optimal design are found to be hydroxyapatite, Bioglass, and collagen. This design leads to the same stress shielding reduction as in model 1, while at the same time, the maximum interface shear stress is reduced by 45% and 63% compared to the optimal one-dimensional FGM design and homogenous titanium stem, respectively.     

A comparative study of the performance of metallic and FGM tibia tray components in total knee replacement joints

Finite element analysis has been used to predict knee implant biomechanical behavior under various loading conditions. Two-dimensional finite element models have been developed which include the artificial knee and portions of the surrounding biological materials to investigate the required design as a functionally graded material (FGM) tibia tray. The gradation of elastic modulus is changed vertically and horizontally in order to find the optimal gradation direction. The investigation has shown that the use of FGM tibia tray will give excellent performance, and will increase the life of the total knee prosthesis.     

Numerical investigation of the effect of porous titanium femoral prosthesis on bone remodeling

Porous titanium is a promising orthopedic implant material. As a potential use in total hip replacement, the effect of a porous titanium femoral prosthesis on bone remodeling is investigated in this paper. The stress and strain fields of a post-operative femur with a hip replacement are calculated by applying the three-dimensional finite element method. The effect of the implant material on the bone remodeling is evaluated by analyzing the loss of bone density following a strain magnitude based bone remodeling theory. Different implant materials, including currently used solid cobalt–chrome and solid titanium, potential porous titanium with different porosities, are considered in this study. This investigation confirms that bone loss around the implant strongly depends on the value of the elastic modulus of the prosthesis. There will be a sharp drop of the volume of the bone with density loss if a cobalt–chrome implant is replaced by a porous titanium implant. The numerical results show that both of the bone volume with density loss and the bone density loss rate decrease linearly with the increase of the porosity. However, increasing porosity will reduce the strength of porous titanium. With regard to material design for porous titanium-based femoral prosthesis, stress analysis is required to meet the strength requirement.     

Numerical optimization of the design of a coated,cementless hip prosthesis

A numerical optimization technique was used to improve the design of a coated, cementless hip prosthesis. The prosthesis was represented by a simple one-dimensional finite element model, and its diameter and coating thickness at various points were altered so as to minimize stress shielding while keeping implant-bone interface stresses within realistic limits. The resulting design showed a very large reduction in both stress shielding and interface stresses compared to conventional designs.     

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.     

Optimal design of customised hip prosthesis using fiber reinforced polymer composites

The need for new materials in orthopaedic surgery arises from the recognition of the stress-shielding effect of bone by high-modulus implants presently made of engineering alloys. A lower modulus implant material will result in the construction of a more biomechanically compatible prosthesis. In this respect, composite materials are gaining importance because they offer the potential for implants with tailor-made stiffness in contrast to metals. In the present study, the bending stiffness of composite prosthesis is matched with that of bone in both the longitudinal and radial directions by choosing optimal carbon fiber reinforced polyether ether ketone (PEEK) matrix lay-up. A numerical optimization algorithm is developed to deduce the optimal composite femoral prosthesis lay-up that matches the stiffness properties of the femoral bone in both the transverse and longitudinal directions. Effective bending moments and compressive forces acting on the hip joint are considered in the design of the optimal length and diameter of the prosthesis. The optimization algorithm was implemented, by using MATLAB(R)™ for designing the composite prosthesis to a patient’s specific requirement. Finally the efficiency of the composite stem is compared with that of metallic alloy stems in terms of stress shielding using a finite element program.     

Exact solutions for characterization of electro-elastically graded materials

The present paper deals with a class of functionally graded materials (FGM), called active FGM that has electro-elastically graded material phases. An active FGM system leads to minimization of stress concentration that arises due to mismatch in the electrical and elastic properties of the constituent phases. This work focuses on the characterization of the through thickness stresses of an active FGM subjected to electrical excitation. The structure is comprised of a substrate, an electro-elastically graded layer and an active layer. A formulation for exact solutions of the system based on Euler–Bernoulli theory is presented. Power-law variation of the composition of the two phases in the graded layer is considered. Performance of linearly gradient FGM for a range of stiffness and electrical property ratios of the active and substrate materials have been studied. It is observed that the electrical strain component and the compositional gradation significantly influence the stress characteristics of the active FGM.     

梯度结构烤瓷的成分优化及性能

以镍铬合金为金属基底,在白榴石为主晶相的烤瓷粉中按不同比例加入Ni、Cr、NiO、Cr₂O₃作为梯度调节物,制备具有成分梯度结构的金属烤瓷。采用材料抗弯强度测试和热膨胀系数测试的方法筛选优化梯度调节物的配比,发现质量比为m(NiO)∶m( Cr₂O₃)∶m(Ni)∶m(Cr)＝0∶50∶40∶10时能得到抗弯强度为101.217 MPa的最佳梯度层。用弹性应力模型模拟计算梯度层内的热应力曲线,研究了梯度层中富陶瓷端到富梯度调节物端热应力缓和的情况。抗弯强度、扫描电镜、XRD测试表明,减小烤瓷粉粒度可增强梯度层的性能,降低烧结温度。梯度层的抗弯强度呈现各向异性的特点。     

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.     

No-slip crack model for damaged bone/cement interface

Cemented hip and knee prostheses are glued to cancellous bone with polymethylmethacrylate (PMMA) adhesive. The PMMA fills the cavities of the porous bone leading to a rough interface. It has been postulated that flaws at the bone/PMMA interface lead to loosening of the prosthesis. Several researchers have modeled these flaws, but unfortunately they have not calculated the stress intensity factor (SIF) for all values of applied external loads. We redefine the model for the pressure acting on the crack so that this may be possible and calculate the effect on the SIF due to the rough nature of the bone/PMMA interface.     

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.     

论传统建筑装饰艺术符号门簪

门簪原为锁合传统宅门中槛和连楹的连接构件,后逐渐演变成装饰小品。门簪起源于"阀阅",其外形有圆形、方形、六角形、八角形和多瓣形等样式,一般为2～4枚,其纹样雕刻手法主要有平雕、浮雕和凹雕等,其纹饰可分为图案型、文字型和图文结合型。门簪外形简洁却内涵丰富,其纹样创作一般运用借喻和象征的手法,体现了古人对万物有灵的崇拜,折射出人们对幸福美好生活的追求和向往。     

Wie sicher sind keramische Kugelköpfe für Hüftendoprothesen

How safe are ceramic femoral heads for hip endoprotheses? This is a review of the basic properties, standards, and regulations that are correlated to the application and safety of ceramic femoral heads for hip endoprotheses. The investigation of more than 150 retrieved femoral heads (revisions and autopsies) provides information about the reasons of the failures and was basis for projects to improve ceramic femoral heads. The revision on a hip endoprothesis is mostly caused by aseptic loosening of the stem or the socket. Failures because of materials properties, e.g. fracture of a stem or because of an infection are quite seldom. Because of this investigation it can be concluded that the Biolox femoral head is a safe component in a hip endoprothesis. Based on 20 years clinical experience the in vivo failure rate is less than 0.02% for Biolox femoral heads, 0.01% for heads wite 28mm diameter.     

Materials evolution of bone plates for internal fixation of bone fractures:A review

Bone plates play a vital role in bone fracture healing by providing the necessary mechanical fixation for fracture fragments through modulating biomechanical microenvironment adjacent to the fracture site.Good treatment effect has been achieved for fixation of bone fracture with conventional bone plates,which are made of stainless steel or titanium alloy.However,several limitations still exist with traditional bone plates including loosening and stress shielding due to significant difference in modulus between metal material and bone tissue that impairs optimal fracture healing.Additionally,due to demographic changes and non-physiological loading,the population suffering from refractory fractures,such as osteoporosis fractures and comminuted fractures,is increasing,which imposes a big challenge to traditional bone plates developed for normal bone fracture repair.Therefore,optimal fracture treatment with adequate fixation implants in terms of materials and design relevant to special conditions is desirable.In this review,the complex physiological process of bone healing is introduced,followed by reviewing the development of implant design and biomaterials for bone plates.Finally,we discuss recent development of hybrid bone plates that contains bioactive elements or factors for fracture healing enhancement as a promising direction.This includes biodegradable Mg-based alloy used for designing bone screw-plates that has been proven to be beneficial for fracture healing,an innovative development that attracts more and more attention.This paper also indicates that the tantalum bone plates with porous structure are also emerging as a new fracture internal fixation implants.The reduction of the stress shielding is verified to be useful to accelerate bone fracture healing.Potential application of biodegradable metals may also avoid a second operation for implant removal.Further developments in biometals and their design for orthopedic bone plates are expected to improve the treatment of bone fracture,especially the refractory fractures.