Dense/Porous Layered Hydroxylapatite Ceramic for Orthopedic Device Coating Prepared by Tape Casting Technique

A possible surgical technique in the replacement of a traumatized hip joint by a prosthesis system is to connect the acetabular component of the implant directly with pelvis bone tissue, without use of bone cement. It is possible to improve the osteointegration process and to ensure a better connection with bone tissue by coating the outside implant surface with a biocompatible ceramic. The best choice for a bioceramic coating is porous hydroxylapatite because its surface shows bonding-osteogenesis properties much higher than other materials. Here, a double HAp layer has been made by tape casting technology. The first layer was a high porous HAp ceramic with high osteophilic-osteoconductive characteristics. Because the scale of porosity was relatively insensitive to slurry composition and sintering temperature such a microstructure was produced using a particular technique described here. The second layer was dense HAp ceramic that resulted a substrate able to improve the mechanical properties of the brittle porous HAp layer. Several microstructure-designed ceramic coatings having the porous part with a controlled porosity can be obtained by tape casting using the same technique.     

Bone remodeling adjacent to total hip replacements: A naturally occurring material design problem

Summary The reaction of bone to orthopedic implants is an example of a self-adjusting material which changes from a normal state to an altered state, based on the mechanical features of the implant and the loads applied to it. The changes in bone around cemented and uncemented femoral total hip components are well documented, and many numerical characterizations of the material reaction to stress have attempted to mimic the natural remodeling process. In this study we review the development of a simple material remodeling rule which yields a stable structure which is optimal and which allows a unique solution. We then use this algorithm to assess the effect of prosthesis stiffness and the presence of a compliant layer on bone remodeling around these implants. An axisymmetric model for axial loading is used to model changes in bone density through the thickness of the cancellous bone around the implants. With cortical remodeling left out of the simulation, the simulations showed density distributions that agreed in general with the results in the literature, and showed a marked difference in response if a compliant layer was added to the prosthesis.     

Improved mechanical long-term reliability of hip resurfacing prostheses by using silicon nitride

Although ceramic prostheses have been successfully used in conventional total hip arthroplasty (THA) for many decades, ceramic materials have not yet been applied for hip resurfacing (HR) surgeries. The objective of this study is to investigate the mechanical reliability of silicon nitride as a new ceramic material in HR prostheses. A finite element analysis (FEA) was performed to study the effects of two different designs of prostheses on the stress distribution in the femur–neck area. A metallic (cobalt–chromium-alloy) Birmingham hip resurfacing (BHR) prosthesis and our newly designed ceramic (silicon nitride) HR prosthesis were hereby compared. The stresses induced by physiologically loading the femur bone with an implant were calculated and compared with the corresponding stresses for the healthy, intact femur bone. Here, we found stress distributions in the femur bone with the implanted silicon nitride HR prosthesis which were similar to those of healthy, intact femur bone. The lifetime predictions showed that silicon nitride is indeed mechanically reliable and, thus, is ideal for HR prostheses. Moreover, we conclude that the FEA and corresponded post-processing can help us to evaluate a new ceramic material and a specific new implant design with respect to the mechanical reliability before clinical application.     

A numerical investigation of porous titanium as orthopedic implant material

Porous titanium is being developed as an alternative orthopedic implant material to alleviate the inherent problems of bulk metallic implants by reducing the stiffness to be comparable to bone stiffness and allowing complete bone ingrowth. However, a porous microstructure is susceptible to local permanent plastic strain and residual stress under cyclic loading which reduces damage tolerance and therefore limits their application as orthopedic implants. The mechanical properties of porous titanium are governed by the microstructural configurations such as pore morphology, porosity, and bone ingrowth. To understand the influence of these features on performance, the macroscopic and microscopic responses of porous Ti are studied using three-dimensional finite element models. The models are generated based on simulated microstructures of experimental materials at porosities of 15%, 32% and 50%. The results show the effect of porosity and bone ingrowth on Young’s modulus, yield stress, and microscopic stress and strain distribution. Importantly, simulations predict that the bone ingrowth reduces the stress and strain localization under cyclic loading so significantly that it counteracts the concentration condition caused by the increased porosity of the structure.     

Effect of strontium-containing hydroxyapatite bone cement on bone remodeling following hip replacement

It is uncertain whether the use of bioactive bone cement has any beneficial effect on local bone adaptation following hip replacement. In this study, twelve goats underwent cemented hip hemiarthroplasty unilaterally, with either PMMA bone cement or strontium-containing hydroxyapatite (Sr-HA) bioactive bone cement. Nine months later, the femoral cortical bones at different levels were analyzed by microhardness testing and micro-CT scanning. Extensive bone remodeling was found at proximal and mid-levels in both PMMA and Sr-HA groups. However, with regard to the differences of bone mineral density, cortical bone area and bone hardness between implanted and non-implanted femur, less decreases were found in Sr-HA group than PMMA group at proximal and mid-levels, and significant differences were shown for bone area and hardness at proximal level. The results suggested that the use of Sr-HA cement might alleviate femoral bone remodeling after hip replacement.     

A strut graft substitute consisting of a metal core and a polymer surface

In revision hip replacement surgery cortical strut grafts made of allograft bone are used to augment femoral bone stock and to fix periprosthetic femoral fractures. These struts are made from femoral bone as hemi-cylinders and are fixed to host bone with cerclage wires. We developed an artificial bone substitute for such strut grafts in order to overcome availability restrictions and potential infectious hazards with allograft bone. The partially biodegradable implant consists of a functionally-graded combination of titanium, polylactide, hydroxyapatite, and calcium carbonate. It is made by manual dip-coating of the metal (after chemical surface treatment) into solutions of polylactide with suspended calcium salts. In this way the titanium core is surrounded by an inner layer of slowly biodegradable poly(l-lactide) with calcium carbonate. The part of the implant that is in contact with the bone consists of rapidly biodegradable poly(d,l-lactide), hydroxyapatite and calcium carbonate. This method leads to an implant which is easily adaptable before the implantation to the geometry of the patient’s bone when moderately heated (70 °C), but has a sufficient mechanical strength to serve as support under physiologic temperatures. The implant is mechanically stable, biocompatible, partially biodegradable, and provides a scaffold for growing bone.     

Fertigung einer universellen Hüftprothesenpfanne mittels Hochdruckblechumformung

In recent years, the use of hip prostheses has become a routine procedure. The use of total hip replacement has evolved in recent years to a routine procedure. Despite this experience, it always comes back to complications. Especially the migration or loosening of the acetabular component because of the artificial load adaptive bone remodeling is still a current problem. This is due to the changing mechanical situation after the implantation of the prosthesis. Another problem is the high bone loss during implantation, which complicates a revision of the prosthesis. One solution is the use of patient‐specific prostheses. So far, the use of these prostheses is limited due to the time‐consuming and cost‐intensive production. The overall objective of the presented project in this publication is the development and establishment of an innovative concept for the production of patient‐specific hip prosthesis made of titanium plates by sheet metal forming. The idea of this concept involves the development of a two‐stage process. First of all a standardized hip prosthesis is made by high‐pressure sheet metal forming and then individually in the second step. This publication contains the derivation of the standard prosthesis. For this purpose, a design methodology was generated, based on the so‐called agglomerative clustering. In addition, presents the results of the numerical preliminary design of the first stage of the process and the tool developed concept in the course of this paper.     

基于SLM的梯度多孔牙种植体力学特性

目的 确定既满足强度要求又能够有良好长期稳定性的梯度多孔牙种植体最佳孔隙值。方法 设计4组不同孔隙率(G30、G40、G50、G60)的梯度多孔结构样件及均质多孔样件S30,选区激光熔化(SLM)成型后通过准静态压缩试验对其力学性能进行研究,测量出样件的弹性模量和屈服强度。通过有限元分析评估不同孔隙率种植体及对应下颌骨组织的应力分布。结果 相较于实体钛合金结构(110 GPa),多孔结构的弹性模量(13.47~15.88 GPa)已完全符合人体自然骨组织(2~20 GPa)范围,多孔结构屈服强度(484.81~834.47 MPa)远高于皮质骨(180.5~211.7 MPa);梯度多孔结构样件弹性模量相较于均质多孔结构略有提升,屈服强度(834.47 MPa)比均质多孔结构样件(730.56 MPa)提高了约14%。梯度多孔种植体周围皮质骨最大等效应力值分布在43.362 9~45.015 4 MPa之间,松质骨最大等效应力值分布在4.756 58~ 5.055 6 MPa之间,完全满足2~60 MPa范围内的最大应力,适合骨组织生长。种植体与下颌骨之间的应力差值随着孔隙率的增大而逐渐变大,孔隙率为30%的TPMS–G型梯度多孔牙种植体与下颌骨应力差值最小,生物力学特性最佳,有利于形成稳定的骨整合。结论 通过试验及仿真模拟,确定了适用于种植体的最佳梯度多孔结构,既满足强度要求,又具有良好的长期稳定性。     

Uniform porous and functionally graded porous titanium fabricated via space holder technique with spark plasma sintering for biomedical applications

Porous materials with low stiffness and high strength are sought as implant materials to prevent stress shielding and fracture during in vivo use. This study proposes a powder metallurgy-based space holder technique to fabricate porous titanium with mechanical performance suitable for implant materials. Mixed powders of titanium and sodium chloride were sintered at low temperature using spark plasma sintering, and then the sodium chloride was dissolved in water. As a result, uniform porous titanium (UP-Ti) with a wide range of microstructures: porosity from 26% to 80% and average pore size from 75 μm to 475 μm was successfully fabricated. Also, functionally graded porous titanium (FGP-Ti) was successfully fabricated, in which porous titanium with high porosity and dense titanium were placed at the inside and surface, respectively. The stiffness of UP-Ti was comparable to that of natural bones, but its strength was lower than that of natural bones, which would be insufficient for use as an implant. In contrast, the mechanical performance of FGP-Ti was improved, compared with UP-Ti with the porosity comparable to the average porosity of FGP-Ti: its strength was higher than that of natural bones and its stiffness was comparable to that of natural bones. These results imply that porous titanium, especially functionally graded porous titanium, is a candidate metal for implants used to replace heavily loaded natural bone.     

Osteoblast cell response to nanoscale SiO2/ZrO2 particulate-reinforced titanium composites and scaffolds by powder metallurgy

The strength of porous pure titanium (Ti) scaffold decreases dramatically with the introduction of porosity and might become lower than that of natural bone when with high porosity. To simultaneously meet the requirements of low-elastic modulus and appropriate strength for implant materials, it is necessary to develop new biocompatible Ti-based composites that are stronger than those currently available while providing low-elastic modulus and adequate strength when they are scaffolded into a porous structure. In this study, new particulate-reinforced Ti-based composites with nanoscale oxide particles of SiO₂ and ZrO₂ were prepared using a powder metallurgical method. The strengths of the new particulate-reinforced titanium composites were found to be significantly higher than that of a pure Ti. Cell culture results revealed that the articulate-reinforced titanium composites showed excellent biocompatibility and cell adhesion. Human osteoblast-like SaOS2 cells grew and spread well on the surfaces of the new titanium composites. The present study illustrated the feasibility of using the particulate-reinforced titanium composites as an orthopaedic implant material.     

Physical and mechanical characterisation of 3D-printed porous titanium for biomedical applications

The elastic modulus of metallic orthopaedic implants is typically 6–12 times greater than cortical bone, causing stress shielding: over time, bone atrophies through decreased mechanical strain, which can lead to fracture at the implantation site. Introducing pores into an implant will lower the modulus significantly. Three dimensional printing (3DP) is capable of producing parts with dual porosity features: micropores by process (residual pores from binder burnout) and macropores by design via a computer aided design model. Titanium was chosen due to its excellent biocompatibility, superior corrosion resistance, durability, osteointegration capability, relatively low elastic modulus, and high strength to weight ratio. The mechanical and physical properties of 3DP titanium were studied and compared to the properties of bone. The mechanical and physical properties were tailored by varying the binder (polyvinyl alcohol) content and the sintering temperature of the titanium samples. The fabricated titanium samples had a porosity of 32.2–53.4 % and a compressive modulus of 0.86–2.48 GPa, within the range of cancellous bone modulus. Other physical and mechanical properties were investigated including fracture strength, density, fracture toughness, hardness and surface roughness. The correlation between the porous 3DP titanium-bulk modulus ratio and porosity was also quantified.     

A comparative study of zirconium and titanium implants in rat: osseointegration and bone material quality

Permanent metal implants are widely used in human medical treatments and orthopedics, for example as hip joint replacements. They are commonly made of titanium alloys and beyond the optimization of this established material, it is also essential to explore alternative implant materials in view of improved osseointegration. The aim of our study was to characterize the implant performance of zirconium in comparison to titanium implants. Zirconium implants have been characterized in a previous study concerning material properties and surface characteristics in vitro, such as oxide layer thickness and surface roughness. In the present study, we compare bone material quality around zirconium and titanium implants in terms of osseointegration and therefore characterized bone material properties in a rat model using a multi-method approach. We used light and electron microscopy, micro Raman spectroscopy, micro X-ray fluorescence and X-ray scattering techniques to investigate the osseointegration in terms of compositional and structural properties of the newly formed bone. Regarding the mineralization level, the mineral composition, and the alignment and order of the mineral particles, our results show that the maturity of the newly formed bone after 8 weeks of implantation is already very high. In conclusion, the bone material quality obtained for zirconium implants is at least as good as for titanium. It seems that the zirconium implants can be a good candidate for using as permanent metal prosthesis for orthopedic treatments.     

Acoustical and Poromechanical Characterisation of Titanium Scaffolds for Biomedical Applications

Abstract:  Biocompatible materials are designed so as to mimic biological materials such as bone as closely as possible. As regards the mechanical aspect of bone replacement materials, a certain stiffness and strength are mandatory to effectively carry the loads imposed on the skeleton. In this paper, porous titanium with different porosities, produced on the basis of metal powder and space holder components, is investigated as bone replacement material. For the determination of mechanical properties, i.e. strength of dense and porous titanium samples, two kinds of experiments were performed – uniaxial and triaxial tests. The triaxial tests were of poromechanical nature, i.e. oil was employed to induce the same pressure both at the lateral surfaces of the cylindrical samples and inside the pores. The stiffness properties were revealed by acoustic (ultrasonic) tests. Different frequencies give access to different stiffness components (stiffness tensor components related to high-frequency-induced bulk waves versus Young's moduli related to low-frequency-induced bar waves), at different observation scales; namely, the observation scale the dense titanium with around 100  μ m characteristic length (characterised through the high frequencies) versus that of the porous material with a few millimetres of characteristic length (characterised through the low frequencies). Finally, the experimental results were used to develop and validate a poro-micromechanical model for porous titanium, which quantifies material stiffness and strength from its porosity and (in the case of the aforementioned triaxial tests) its pore pressurisation state.     

Preparation,microstructure and mechanical properties of porous titanium sintered by Ti fibres

Open-cell porous Ti with a porosity ranging from 35 to 84% was successfully manufactured by sintering titanium fibres. The microstructure of the porous titanium was observed by SEM and the compressive mechanical properties were tested. By adjusting the spiral structure of the porous titanium, the pore size can be controlled in a range of 150–600 μm. With the increasing of the porosity, compressive yield strength and modulus decrease as predicated. However, high mechanical properties were still obtained at a medium porosity, e.g. the compressive yield strength and the modulus are as high as 100–200 MPa and 3.5–4.2 GPa, respectively, when the porosity is in the range of 50–70%. It was suggested that the porous titanium be strong enough to resist handing during implantation and in vivo loading. It is expected to be used as biocompatible implant, because their interconnected porous structures permit bone tissues ingrowth and the body fluids transportation.     

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.     

Dynamics of bone tissue mineralization in porous titanium and the mechanical properties of a titanium-bone tissue composite

We studied the dynamics of bone tissue mineralization in the pores of a titanium matrix and the mechanical properties of the resulting titanium-bone tissue composite. It is shown that the mineralization process is virtually accomplished within six months after implantation of a porous titanium article into the organism. As the pores and channels in the titanium matrix are filled with bone tissue, the material becomes a composite with mechanical properties higher as compared to those of the porous implant or bone tissue.     

Direct visualization and quantification of bone growth into porous titanium implants using micro computed tomography

The utility of porous metals for the integration of orthopaedic implants with host bone has been well established. Quantification of the tissue response to cementless implants is laborious and time consuming process requiring tissue processing, embedding, sectioning, polishing, imaging and image analysis. Micro-computed tomography (μCT) is a promising three dimensional (3D) imaging technique to quantify the tissue response to porous metals. However, the suitability and effectiveness of μCT for the quantification of bone ingrowth remains unknown. The purpose of this study was to evaluate and compare bone growth within porous titanium implants using both μCT and traditional hard-tissue histology techniques. Cylindrical implants were implanted in the distal femora and proximal tibiae of a rabbit. After 6 weeks, bone ingrowth was quantified and compared by μCT, light microscopy and backscattered electron microscopy. Quantification of bone volume and implant porosity as determined by μCT compared well with data obtained by traditional histology techniques. Analysis of the 3D dataset showed that bone was present in the pores connected with openings larger 9.4 μm. For pore openings greater than 28.2 μm, the size of the interconnection had little impact on the bone density within the porosity for the titanium foams.     

The analysis of human femurs and prostheses using electronicspeckle pattern interferometry

The potential lifetime of hip replacements is reduced by aseptic loosening-an inadequate fixation of the implant. Increasing the life of the surgical procedure requires knowledge of the interaction between prosthesis and femur. The authors have used electronic speckle pattern interferometry (ESPI), a laser-based metrology technique developed for the analysis of displacements and strains, to develop an understanding of the biomechanics of the proximal femur whilst mounted in a compression test rig which also simulates muscle and tendon behaviour. Cadaveric femora were studied under physiological loads, before and after the implantation of the femoral component of a total hip replacement     

On the influence of shape and material used for the femoral component pegs in knee prostheses for reducing the problem of aseptic loosening

The integration of materials selection and design are essential to the success of new product development, especially when applied to biomedical devices. The knee prosthesis, like any other implant, is a product that still lacks satisfactory design solutions for solving the problem of aseptic loosening. Stress shielding is one of the main causes of aseptic loosening that is intimately related to the overall design of the knee prosthesis. The design of the location pegs in the femoral component of the knee prosthesis is seen to have a critical effect on the stress shielding. In this study, therefore, different combinations of location peg geometries and material designs were assessed using finite element analyses in conjunction with a design of experiments procedure. The materials considered were Co–Cr alloy (as reference material) and functionally graded material (FGM) for the main body of the femoral component, and various porous materials for the pegs (as promising new materials). The performance outputs (responses) were stress levels in the femoral bone to assess the stress shielding effect, and stress levels in the pegs to assess adequate peg strength. The result revealed conflicts in satisfying the design objectives. Therefore, a multi-objective optimization was carried out to find the optimal geometries of the pegs for the femoral component. Based on the findings of the optimization process, a set of candidate designs was generated and a multi-criteria decision making approach used to obtain the final ranking of candidate designs. The ranking order demonstrated the superiority of using a FGM femoral component with porous material pegs of conical geometry. By comparing the results with the standard Co–Cr design, it was shown that the new design of pegs can significantly increase the magnitude of stresses seen at the distal femur; hence reduce the stress shielding effect, without over compromising on the strength of the pegs.     

Does sodium fluoride in bone cement affect implant fixation? Part I: Bone tissue response, implant fixation and histology in nine rabbits

The addition of sodium fluoride to poly (methyl-methacrylate) (PMMA) bone cement may theoretically improve the fixation of joint replacement. This hypothesis was tested in an animal model using nine mature healthy lop-eared rabbits. A femoral prosthesis was inserted in both knees to resurface the patellofemoral articulation. The same acrylic cement, with and without sodium fluoride, was randomised between the two sides for prosthetic fixation. Two screw shaped implants machined from cured rods of either cement were also inserted bilaterally into the proximal tibia. Qualitative and quantitative histomorphometry of the bone tissue response surrounding the cement in the femur and the intact tibial implants revealed similar results regardless of sodium fluoride addition. Six weeks after surgery removal, torque did not significantly differ between the two sides. Our findings indicate that addition of sodium fluoride to PMMA has little effect on implant stability and bone remodeling in rabbits in the short-term.