Article

Lattice Ti structures with low rigidity but compatible mechanical strength: Design of implant materials for trabecular bone

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Abstract

The development of porous metals to alleviate the effects of stress shielding in bone will help improve the function of metallic biomaterials in orthopaedic applications. A critical step in advancing this technology is to design metallic structures with low rigidity that is comparable with bone tissue, but with good mechanical strength. In this study, porous titanium (Ti) structures with periodic cell topologies were designed to achieve tunable mechanical properties. The versatility of the design scheme was demonstrated by examining lattice designs with different stiffness properties achieved by using the Selective Laser Melting (SLM) technology. The fabricated porous Ti exhibited a low modulus of 1.05 GPa but a high compressive strength of 55 MPa. Large deformation analysis using digital image correlation (DIC) technique indicated uniform strain patterns at micro-trusses, suggesting the overall high quality of the structure with absence of local flaws. A functionally-graded stiffness design was further investigated by varying the diameters of micro-trusses within the structure. A stiffness graded material may be favourable for anatomical site that has strong depthdependent variations, such as in trabecular bone microstructures.

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... Specifically, TNTZ is a relatively new alloy, and to-date, there is no international standard to follow. Nevertheless, it is possible to compare the mechanical properties of the as-printed TNTZ lattice with those of CP-Ti, Ti64, and Ti2448 (Ti-24Nb-4Zr-8Sn), as shown in Table 4 [69][70][71][72][73][74][75][76][77][78]. As shown in the table, the compressive strength and Young's modulus of the as-printed lattices evidently vary from study to study based on a high variety of factors, including the SLM processing parameters and adopted lattice structures [69][70][71][72][73][74][75][76][77][78][79]. ...
... Nevertheless, it is possible to compare the mechanical properties of the as-printed TNTZ lattice with those of CP-Ti, Ti64, and Ti2448 (Ti-24Nb-4Zr-8Sn), as shown in Table 4 [69][70][71][72][73][74][75][76][77][78]. As shown in the table, the compressive strength and Young's modulus of the as-printed lattices evidently vary from study to study based on a high variety of factors, including the SLM processing parameters and adopted lattice structures [69][70][71][72][73][74][75][76][77][78][79]. It is difficult to determine as to whether the currently developed TNTZ is more advanced than other Ti materials (e.g., CP-Ti and Ti- 24Nb-4Zr-8Sn) in terms of mechanical properties. ...
... Three-dimensional interconnected hole scaffolds can also enable the transfer of body fluids and nutrients in the implant, thereby promoting tissue regeneration and reconstruction, and accelerating the healing process [21]. With respect to the potential application of the asprinted lattice, the mechanical properties of the cancellous bone of human patients are close to those of the BD group samples developed in this study in terms of Young's modulus, strength, and fatigue limit (see Table 4 for details) [54,[69][70][71][72][73][74][75][76][77][78]. From a Young's modulus perspective, other possible bone replacement applications can include the human cortical bone (E = 3.8 GPa), proximal tibia (1.30 ...
... Using a biocompatible artificial material to fill the bone loss requires solving of several engineering problems. In particular, mechanical properties of the implant should be similar to the ones of the real bone tissue, firstly to assure the demanded carrying capacity of the whole bone after reconstruction, secondly to avoid stress-shielding phenomenon that leads to the local weakening/strengthening and causes bone resorption (bone loss), aseptic loosening and/or develop- [1][2][3][4][5][6][7][8]. Moreover, an implant should have a high fatigue resistance and crack-development resistance. ...
... Mechanical properties of designed scaffolds can be assessed by performing mechanical tests, i.e. static compression test (to assess Young modulus and yield strength) [7,21,25,26], flexural test (bending test) [27,28]. Designing a test, one should consider a scaffold porosity that has a big impact on the mechanical properties of the tested scaffold [8,10,26,29,30]. ...
... GPa reported in [26], minimum value 0.788 GPa reported in [39] and 0.76-18.2 GPa presented in [7]. ...
Article
This work demonstrates that an artificial scaffold structure can be designed to exhibit mechanical properties close to the ones of real bone tissue, thus highly reducing the stress-shielding phenomenon. In this study the scan of lumbar vertebra fragment was reproduced to create a numerical 3D model (this model was called the reference bone sample). New nine 3D scaffold samples were designed and their numerical models were created. Using the finite element analysis, a static compression test was performed to assess the effective Young modulus of each tested sample. Also, two important metrics of each sample were assessed: relative density and surface area. Each new designed 3D scaffold sample was analyzed by considering two types of material properties: metal alloy properties (Ti-6Al-4V) and ABS polymer properties. Numerical analysis results of this study confirms that 3D scaffold used to design a periodic structure, either based on interconnected beams (A, B, C, D, E and F units) or made by removing regular shapes from base solid cubes (G, H, I units), can be refined to obtain mechanical properties similar to the ones of trabecular bone tissue. Experimental validation was performed on seven scaffolds (A, B, C, D, E, F and H units) printed from ABS material without any support materials by using Fused Deposition Modeling (FMD) technology. Results of experimental Young modulus of each printed scaffold are also presented and discussed.
... To the best of the author's knowledge, only a few DIC studies on lattice materials are reported in the literature (Brenne et al., 2013;Gorny et al., 2011;Huynh et al., 2016;Chen et al., 2016;Warmuth et al., 2017). Some works make use of digital imaging to follow mechanical tests, but do not derive quantitative data from the images (Hernandez-Nava et al., 2016;Cheng et al., 2012). ...
... The Tresca strain values were averaged over unit cells and revealed strain heterogeneities along bands before failure. The validity and reliability of the DIC technique applied to porous structures were better discussed in (Chen et al., 2016) on the base of compression tests on cubic specimens recorded at 30 frames per second with a camera attached to a stereomicroscope. The post-processing of the film allowed precise strain concentrations along the diagonal failure bands to be measured and local tensile strength in some struts of the structure to be documented. ...
... In none of the above cited papers, use is made of the whole set of recorded images. Rather, images and strain maps are presented at selected moments of the tests, in quasi-static or cyclic conditions, showing strain localization along failure bands, or at the scale of the struts (Huynh et al., 2016;Chen et al., 2016). It appears that the temporal evolution of the strain patterns has never been reported. ...
... Solid metals and alloys such as stainless steel and titanium (Ti) have been successfully employed as bone implants owing to their excellent mechanical and bio-logical properties [1]. However, stress-shielding and impermeability have limited their applications [2]. To satisfy the biomechanics requirement of orthopedic scaffolds, a porous structure mimicking natural bone should be taken into consideration. ...
... Several attempts have been implemented to achieve gradient hierarchical arrangement of pores or random microarchitecture. Typical design strategies include computer aided design (CAD), image-based design, implicit surface modeling and topology optimization [2,[12][13][14][15][16]. CADbased design is obtained by using modern CAD tools which enables the redistribution of the density of structural parts. ...
... Therefore, the seven structures of the third layer were considered as the primary objective functions to construct the cellular structures, as summarized in Table V. By considering the volume fraction, DA, and mechanical properties as objective functions, topological optimization by equations (1)(2)(3)(4)(5) was used to calculate the density unit distribution of the materials. Moreover, the isolated unit points were filtered using the threshold value and the optimized unit cell models were shown in Fig. 5. ...
Article
Porous metals are found to be suitable as orthopedic scaffolds. However, only limited control over the internal architecture can be achieved using conventional design methods. The architecture with porosity variation strategy mimicking natural bone is critical to gain favorable combination of mechanical and biological properties for orthopedic implants. In this regard, a topology optimization method with customized morphology and mechanical properties derived from the trabecular bone was proposed to design three-dimensional architectures with gradient porosity resembling the porous structure of bone. In particular, the elastic constants for the trabecular bone were better predicted when the bone volume fraction was supplemented with a three-dimensional structural parameter, i.e., degree of anisotropy. These constants were set as the optimization constrains for morphology control. Then the porous titanium structures were manufactured by selective laser melting technology (SLM). The physical characteristics, mechanical properties of the scaffolds were compared systematically. The experimental results revealed that the as-built samples with the proposed method lead to a good match of morphological accuracy and mechanical properties to that of the bone. It demonstrates that the proposed topology optimization method with controlled morphology and mechanical properties provides an efficient manner for the biomimetic design of orthopedic implants.
... Lattice structures have many engineering applications [1][2][3][4][5][6][7][8]. They can be designed to have mechanical, thermal, and other properties that fit their intended applications. ...
... w [8] = w 0, ...
... Rights reserved. [6] and w [8] , i.e., w [6] − w [8] ...
Article
Full-text available
In this study, the effective out-of-plane rigidities of several 2D lattices, consisting of Euler beams, with different common unit cell topologies are investigated. The effective out-of-plane rigidities per weight density of these lattices, normalized by those of the full solid plates with the same material and thickness, are determined. The effective out-of-plane rigidities are computed by the homogenization method based on equivalent strain energy and the Kirchhoff plate theory. Particularly, the homogenization-related equations, including the equivalent strain energy equation itself, are not taken from their corresponding equations for 3D solids, but are derived directly using the Kirchhoff plate equations. Moreover, the exact forms, having some dimensionless factors, of the effective material constants for 2D-lattice plates are analytically derived. By using exact curve fitting, these exact forms, in most cases, yield the closed forms of the effective material constants. Finally, the efficiency of the considered unit cell topologies, in terms of the normalized effective rigidities per weight density of their resulting lattices, is discussed.
... Structural optimization of unit cells according to the desired application [6] allows a full exploitation of the features of additive manufacturing. Applications for such lattice structures can be found in various fields like medical engineering [7,8] , structural design [9] , heat conduction [10,11] or chemical reactors [12] . With regard to structural mechanics, Ashby [13] differentiates between two types of cellular materials: bending-dominated and stretch-dominated. ...
... In order to gain a deeper understanding of the local deformations, digital image correlation (DIC) was successfully used for lattice structures [8,19,26,[30][31][32][33] . So far, only 2D DIC was used and sometimes not the individual struts but a more diffuse surface is captured. ...
Article
The rapid progress in additive manufacturing enables the generation of complex structures that can be customized according to the application. For instance, lattice structures show potential in medical and lightweight applications as their mechanical properties can be scaled by the volume fraction of the cells according to the local requirements given by the load paths. In order to use lattice structures in design of structural parts, the mechanical properties need to be characterized. Due to the complex nature of the selective laser melting process, manufacturing imperfections as well as the microstructure play an important role and their effects can differ depending on volume fraction, building direction and especially load case (tension and compression). The aim of this study is to analyze these effects. In addition, a deeper understanding of the failure process is necessary which is gained by 3D digital image correlation and finite element simulations incorporating progressive damage. We found that surface defects are larger for horizontal struts printed directly on the powder bed and thus show a higher influence for specimens where building and loading direction are not aligned. Moreover, porosity leads to significantly different mechanical properties contingent on the load case. Depending on the volume fraction, different failure modes are observed which are captured and explained by finite element simulations allowing to avoid stress concentrations or undesired buckling in future designs. Finally, simulations of lattice structures are compared to computationally inexpensive simulations of unit cells with periodic boundary conditions. Good agreement is found and further insights into the influence of the load introduction are gained.
... The use of metals in trauma implants has been traditionally successful due to the desirable strength and biocompatibility they possess [6]. Titanium, in particular, is a commonly used biomaterial in human bone implants because of its ductility, resistance to corrosion, and fracture toughness [7,8]. However, titanium alloys are still rigid compared to bone; for example, the common biomedical implant material Ti-6Al-4V Grade 5 has an elastic modulus of 114-120 GPa [9]. ...
... One study demonstrated the capacity to adjust moduli between 1.05-12.01 GPa [7]. This is comparable to reported modulus values for trabecular and cortical bone of 18 and 19.9 GPa [10]. ...
Conference Paper
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Rib fractures and chest flail injuries are life threatening injuries that often require surgical treatment using metal (e.g. titanium) fracture reconstruction plates and screws. Current implant designs do not account for the variable stiffness present in human ribs and are much stiffer than the native bone, causing undesirable clinical outcomes. In this preliminary study, groups of latticed test plates were designed with a body centered cubic (BCC) lattice and porosities ranging from 36–86%. Porosity was altered by changing lattice strut thickness between 0.225–0.425 mm and unit cell length between 1, 2, and 3 mm. The test plates were fabricated using an established laser powder bed fusion additive manufacturing process. Flexural strength (4-point bending) tests were performed at a strain rate of 1.3 mm/min to characterize changes in bending stiffness and strength. It was found that implant stiffness could be decreased by 15.7% (p = 0.068) by decreasing strut thickness from 0.425 to 0.225 mm and increasing unit cell length from 1 to 3 mm. The results of this preliminary experiment serve as guidelines for the design of full-sized rib fracture reconstruction plates that contain a gradient lattice with varied mechanical properties to better match the behavior of intact ribs.
... In the design of our gradient samples, we used high stiff core for bearing the load, and porous exterior layers for reducing the overall stiffness of the implants. The approximation of the elastic modulus of the implant and bone would enhance the osseointegration [20,21]. We succeeded in the construction of the open interconnected J Mater Sci gradient porous lattice with a dense core and a porosity of about 50%. ...
Article
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This study aims to 3D print titanium alloy constructs incorporating gradient of porosities, from the fully dense core to the porous outer surface. Gradient porous specimens were prepared using selective laser melting (SLM). Fully dense specimens fabricated by SLM were used as the control group. Characterization of samples was done using X-ray tomography, uniaxial compression testing, and optical and scanning electron microscopes. The biocompatibility of fabricated samples was investigated using human periodontal ligament stem cells via assessment of cell attachment, viability, and proliferation by direct and indirect assays. The data were analyzed using ANOVA and Tukey’s post hoc test. Characterization of constructs reveals interconnected gradient porosities and higher contact angle in porous samples. The introduction of porosity leads to a significant decrease in compression strength. However, Young’s modulus of the samples with gradient porosity was more similar to the natural bone modulus. The surface microstructure consists of loosely bonded spherical particles. Biocompatibility of the dense and porous samples is appropriate. Although the porosity size led to a reduced cell proliferation rate in the gradient sample, the extract of the gradient sample results in more cell proliferation than the dense sample’s extract. The study demonstrates that a biocompatible functionally graded porous titanium structure can be well fabricated by SLM, and this structure leads to a good match of Young’s modulus to that of the bone.
... The literature [23,134,136,181,182] on AM manufactured metal lattices raises the repeatability of highly porous scaffolds as a major concern that affects both reproducibility and predictability of their mechanical performance. Although FEM could predict the stiffness and strength of porous scaffolds, the accuracy of the prediction reported in most cases is low [183][184][185]. This has been primarily due to the incomplete melting of the metal powders and the significance of the surface roughness at high porosity. ...
... This finding may be due to the presence of a prominent bending moment, since the length of the stem and diaphysis provide a considerable moment arm of the distally-located forces at knee joint force. Although the stresses at these locations were below the yielding threshold, it is nonetheless suggested that future osseointegrated implant designs mitigate these dynamic bending-generated loads, for example, using alternative implant geometry such as support ribs or radiused edges, or by using less-stiff biomaterials such as 3D printed lattice structures (Chen et al., 2016). Although the locations of the observed peak stresses in the cortical bone tended to occur at the muscle attachments (Figure 6), this result was likely due to the small attachment areas assumed in the finite element model (Polgar et al., 2003). ...
Article
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Background: Osseointegrated implants for transfemoral amputees facilitate direct load transfer between the prosthetic limb and femur; however, implant loosening is a common complication, and the associated implant-bone loads remain poorly understood. This case study aimed to use patient-specific computational modeling to evaluate bone-implant interface loading during standing and walking in a transfemoral amputee with an osseointegrated implant prior to prosthesis loosening and revision surgery. Methods: One male transfemoral amputee with an osseointegrated implant was recruited (age: 59-yrs, weight: 83 kg) and computed tomography (CT) performed on the residual limb approximately 3 months prior to implant failure. Gait analyses were performed, and the CT images used to develop a finite element model of the patient's implant and surrounding bone. Simulations of static weight bearing, and over-ground walking were then performed. Findings: During standing, maximum and minimum principal strains in trabecular bone adjacent to the implant were 0.26% and -0.30%, respectively. Strains generated at the instant of contralateral toe-off and contralateral heel strike during walking were substantially higher and resulted in local trabecular bone yielding. Specifically, the maximum and minimum principal strains in the thin layer of trabecular bone surrounding the distal end of the implant were 1.15% and -0.98%, respectively. Interpretation: Localised yielding of trabecular bone at the interface between the femur and implant in transfemoral amputee osseointegrated prosthesis recipients may present a risk of implant loosening due to periprosthetic bone fracture during walking. Rehabilitation exercises should aim to produce implant-bone loading that stimulates bone remodelling to provide effective bone conditioning prior to ambulation.
... Unfortunately, the quality of the SLM-built is a major concern that affects the scaffold reproductions. Although FEA could predict the modulus of cellular scaffolds, the accuracy of the prediction is low [46]. Poor correlation between FEA and physical testing results from an incomplete melting of the metal powders. ...
... Des moyens de contrôle adaptés sont en cours d'étude, notamment pour les réseaux de structures architecturées, utilisant des techniques ultrasonores ou de mesure de résistivité. [86], et utiliser les porosités pour favoriser l'ostéo-intégration [87]. Des porosités induites par des conditions de lasage imparfaites ont même été envisagées [88]. ...
Thesis
L’utilisation des Dispositifs Médicaux Implantables (DMI) en alliage de titane, principalement en Ti-6Al-4V, s’est fortement développée ces dernières décennies. D’un point de vue biologique et chimique, certains éléments de cet alliage tels que le vanadium et l’aluminium sont considérés comme potentiellement toxiques. D’un point de vue des propriétés mécaniques, le Ti-6Al-4V est significativement plus rigide que l’os cortical sur lequel les prothèses/implants sont fixés. Cette différence de rigidité entre l’os et l’implant est à l’origine d’une déviation des contraintes (appelée « stress-shielding ») qui peut finalement aboutir au descellement du dispositif dû à des pertes osseuses. Le travail de cette thèse a pour objectif d’apporter des solutions tant sur le choix du matériau que sur le choix de la géométrie des DMI, pouvant permettre l’amélioration du transfert de charge à l’interface. Le domaine dentaire où les efforts mis en jeu sont importants constitue un environnement adéquat à la mise en œuvre de ces solutions matériaux et géométriques. Deux types de dispositifs sont ainsi envisagés dans cette étude : les implants endo-osseux et les implants supra-osseux à plaque d’ostéosynthèse. Le positionnement des implants classiques, dits endo-osseux, est évalué au regard de l’os péri-implanté par une étude numérique multiparamétrique, prenant en compte des critères mécaniques. Les implants supra-osseux sont eux destinés à la réhabilitation d’arcades dentaires complètes de patients aux pathologies plus atypiques, notamment lorsque la pose d’implants endo-osseux n’est pas envisageable. La nécessité de disposer de formes individualisées aux patients justifie pleinement une réalisation par fabrication additive SLM (Selective Laser Melting). Cette approche nécessite cependant de s’intéresser à tous les aspects de la chaîne de valeurs de réalisation de DMI par fabrication additive métallique. Dans l’étape de dimensionnement il ne s’agit plus seulement de tenir compte de la tenue mécanique du dispositif implanté mais d’optimiser topologiquement sa géométrie au regard des sollicitations appliquées à l’os, afin d’optimiser le transfert de charge en l’os et l’implant. L’élaboration de ces dispositifs, sur-mesure, doit nécessairement tenir compte des opérations de parachèvement post-fabrication, par des procédés soustractifs. Ces étapes de fonctionnalisation constituent encore à ce jour un verrou qui limite l'utilisation de la fabrication additive. Les problématiques de reprise en usinage de pièces sur-mesure sont liées au transfert entre la machine de fabrication additive et le centre d’usinage. L’idée développée consiste à l’utilisation des supports de fabrication additive comme montage d’usinage sur-mesure. Pour anticiper les potentielles instabilités pendant cette opération de parachèvement, un outil numérique est développé, couplé à un modèle analytique de détermination des efforts de coupe en fraisage périphérique. Enfin, le contrôle de l’état de surface des DMI issus du procédé SLM par sablage et tribofinition, en lien avec une étude de biocompatibilité, est présenté.
... Was the size and structure of the lattice a constraint of design or the manufacturing process? Were attempts made to simulate the lattice structure of trabecular metal (Chen et al. 2016)? What concerns do they share about accelerated wear of titanium articulating with conventional chrome cobalt prosthesis, as in their pelvic recon-structions (Moharrami et al. 2013)? ...
... This technology allows to realize the components layer-afterlayer through the selective melting of a metallic powder using a Laser. Through this technique is possible to construct complex geometries such as trabecular structures [13] in a single component without the necessity of welding or junctions providing a reduction of part number of the entire system and the production and assembly costs [14,15]. ...
Conference Paper
One of the most challenging problems that engineers must face during the design phase of any single component of space systems is weight. The more is reduced, the less the launch will cost. Another critical aspect to consider is the overall dimensions of any object together with the coupling and compatibility with the surrounding parts. Via progressively integrating more functions into a single element, is it possible to optimize the final configuration of a complex object, addressing the multi-purpose role required for future applications. Although in the past this integration was not always possible, nowadays the emerging additive techniques are enhancing the design freedom with novel solutions. In fact, the present trend in all industrial sectors is to introduce additive manufacturing, not only for rapid prototyping, but also for producing components to be used for real applications. Due to safety reason, the usage of this production technique for space application is very limited, but several increasing test campaigns are rapidly rising its Technology Readiness Level (TRL). A novel concurrent design approach that integrates not only the thermal and structural performances, but also the technical feasibility, could be a great turning point for the project methodology. The purpose of this work is to cast some lights into the multidisciplinary design of the next generation of space components introducing these novel aspects. In as much as 3D printing is the future production trend, Selective Laser Melting (SLM) technology has been adopted for the analysis presented. The case study presented in this paper outline a new design method applied for a multifunctional panel to be used as cold plate as well as structural panel, able to cope with the hostile ambient conditions of space missions. The primary function of the proposed part is to withstand the loads, from the launch to the operative phases and, instead of having a dedicated and separated loop for thermal regulation, the panel is designed to internally incorporate the working fluid.
... Selected laser melting (SLM), a typical additive manufacturing technique with a super ability to fabricate 3D complicated architecture with customized pore, has drawn a lot of attention [27][28][29][30][31][32][33]. With high power laser energy, SLM printer melt selected area of powder directly to manufacture objects layer by layer, therefore objects can be manufactured accurately [34][35][36]. ...
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The Ti6Al4V alloy is one of the most commonly used in orthopedic surgery. Mechanical property of implant contributes important biological functions for load-bearing bone tissue reconstruction. There is a significant need for design and fabrication of porous scaffold with customized mechanical properties for bone tissue engineering. In this paper, bionic design and fabrication of porous implants were studied by using finite element analysis (FEA) and 3D printing techniques. Novel porous architectures were built up with diamond lattice pore structure arraying units. With finite element analysis, the structure weak points under pressure were simulated so that the mechanical properties of the implants were optimized. Porous implants with different porosities and mechanical properties were precisely fabricated by selected laser melting (SLM), one of powder bed fusion additive manufacturing techniques. The biocompatibility and repair effect were studied by in vivo experiments. Animal results indicated that the damaged load-bearing bones were well reconstructed. New generated bones embedded and fitted into the designed porous implants. The optimized design and precisely manufactured implants are conducive to bone tissue repair and reconstruction.
... The images are then postprocessed to evaluate the local strain on the lattice beams. 52,65,66 In this study, the DIC system uses a PointGrey Ò Grasshopper3 camera with a SONY Ò IMX174 sensor, with an acquisition frequency of 10 Hz. Ten points on the front face of the sample are identified to define five virtual extensometers aligned with the loading direction (Fig. 4b). ...
... However, they also involve high machining costs due to their superior mechanical properties. 1 - 9 ...
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Titanium alloys are widely used in high value-added applications such as aerospace and automobile products. However, their superior mechanical properties also make them difficult-to-cut materials. Various machining methods have been developed for machining difficult-to-cut materials. One of these methods, the trochoidal milling method, combines a linear tool movement with a circular movement. It has several advantages, including low cutting force and better tool life, and it can also be applied to high speed machining. Laser-assisted machining (LAM) is another hybrid machining method for difficult-to-cut materials. When the material is locally preheated by a laser heat source prior to machining by the cutting tool, cutting resistance is reduced, and processing efficiency is increased. The purpose of this study is to develop a laser-assisted trochoidal milling process by combining the two machining methods. The processing efficiency of the trochoidal milling method was verified through experiments and comparison with conventional milling. Also, the cutting force and energy efficiency by considering specific cutting energy during the laser-assisted trochoidal milling of Ti-6Al-4V were analyzed for various machining conditions and compared with trochoidal milling.
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A lattice plate was designed based on topological optimization and finite element modeling. The simulation of solid plate system and lattice plate system was carried out. The lightweight design of plate was realized. The results show that the weight of the plate can be reduced by about 40%. The lattice plate is sensitive to the thickness. By reducing the thickness of the plate in a small range, the stiffness of the plate can be significantly reduced. The application of lattice plate improves the average stress of the skeleton by about 4% and reduces the stress shielding effect of skeleton.
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The interest of manufacturing complex devices is steadily growing for high technological application fields, such as the aerospace and biomedical ones, thanks to the possibility of coupling structural and functional properties. Additive Manufacturing (AM) allows to produce 3D complex geometries, like lattice structures, offering lightness together with good mechanical properties. In the present work static and dynamic mechanical properties of Ti6Al4V lattice structures, produced by Selective Laser Melting, were investigated and compared with fully dense material, as reference. In details, the effects of heat treatment on mechanical properties and damping performance were investigated through tensile testing and dynamic compression measurements at different excitation frequency and deformation amplitude. The lattice structure can express a damping capacity of an order of magnitude higher than the full dense material, correlated to good mechanical behavior. In the prospective of newly designed parts for vibration suppression in aerospace applications, the opportunity of enhancing damping behavior by means of light structural components is allowed by the use of lattice structures. This article is protected by copyright. All rights reserved.
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Filigree lattice structures are sensible to geometrical imperfections and the scatter of material parameters which all depend on the stability of the manufacturing process. The aim of this study is to analyze these effects for polymer lattice structures and incorporate them in a finite element model for robust design. Micrographs of lattice structure slices show a smaller diameter for vertical struts. Basic mechanical tests on bulk material exhibit a tension-compression asymmetry which is captured with a Drucker-Prager material model in simulations. Digital image correlation measurements allow to determine true material properties. Plateau stress and failure strain are a result of the biggest flaw in the specimen. Hence, a new model to determine their probability distribution is proposed. This model outperforms standard approaches deriving the probability distribution from the central moments. A spatial correlation of geometric deviations and scatter of the material is investigated with variography subsequently allowing to model the varying properties with random fields. Simulations of dog-bone specimens show that the probability distributions of material properties are captured well. Also simulations of lattice structures are able to represent the probability distributions of their homogenized mechanical properties. The whole stress-strain response and the failure progression agree well with experimental results.
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This review analyses the trends and future opportunities of additive manufactured lattice structures and its fatigue behaviour. Additive manufacturing, a layer-by-layer process to fabricate components offers wide opportunities to produce complex lattice structures compared to conventional manufacturing techniques. The interest in lattice structure has increased with the breakthrough in additive manufacturing. Fatigue behaviour of lattice structure is important in ensuring structural integrity of the additively manufactured parts. A systematic review was conducted to analyse the trends in lattice structure and its fatigue behaviour in terms of articles published, citations, citations per article, journals, countries, field categories, and frequently used keywords. Based on Scopus's online database, the bibliometric analysis evaluates the trends in lattice structure research from 1965 to 2020. It can be concluded that the number of publications on lattice structures has increased significantly over the past decade and plays an important role in producing lightweight high-strength structures. Through keywords analysis, it is found that the term cellular structure or cellular solid and porous structure were commonly used until the early 2000, later, the term lattice structure became more popular with the introduction of metal additive manufacturing technology. Research patterns shifted from static analysis to dynamic analysis as its application is widen to various industries. Parameters such as unit cell topology, porosity, heat, and surface treatment have an influence on the fatigue behaviour of lattice structures.
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The development of additive manufacturing technology has facilitated the production of cellular structures such as lattices. Topology optimization is a tool for computing the optimal geometry of an object within certain conditions, and it can be used to increase the stiffness and decrease the weight. In this study, a “double-optimized lattice structure” was designed by applying the solid isotropic material with penalization method for topology optimization twice, first to optimize the unit cell of the lattice and then to grade and insert the cells into a global model. This design was applied to a Messerschmitt–Bölkow–Blohm beam and produced via material extrusion additive manufacturing. Subsequently, it was evaluated by a three-point bending test, and the results indicated that the double-optimized lattice beam had a 1.6–1.9 fold greater effective stiffness and a 2 fold higher ultimate load than the values obtained for the beam designed with conventional methods. Thus, the double-optimized lattice structure developed herein can be an effective material with regard to its low weight and high stiffness. Contrarily, the penalty factor p of the solid isotropic material with penalization did not affect the properties. This finding suggests that p can control homogeneity while maintaining the strength of the structure.
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We tested in compression specimens of human proximal tibial trabecular bone from 31 normal donors aged from 16 to 83 years and determined the mechanical properties, density and mineral and collagen content. Young’s modulus and ultimate stress were highest between 40 and 50 years, whereas ultimate strain and failure energy showed maxima at younger ages. These age-related variations (except for failure energy) were non-linear. Tissue density and mineral concentration were constant throughout life, whereas apparent density (the amount of bone) varied with ultimate stress. Collagen density (the amount of collagen) varied with failure energy. Collagen concentration was maximal at younger ages but varied little with age. Our results suggest that the decrease in mechanical properties of trabecular bone such as Young’s modulus and ultimate stress is mainly a consequence of the loss of trabecular bone substance, rather than a decrease in the quality of the substance itself. Linear regression analysis showed that collagen density was consistently the single best predictor of failure energy, and collagen concentration was the only predictor of ultimate strain.
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Porous titanium and Ti6Al4V alloy, biomedical candidate materials for use in orthopedic and dental implants, were manufactured by sintering the powders at various temperatures in loose condition. The characteristics of the corresponding powders and utilized sintering temperatures limited the final porosities in the range 30-37.5 vol. %. Similar to wrought alloys, compression stress-strain curves of porous samples exhibited 3 distinct deformation regions containing an elastic region, subsequent to yielding strain hardening region up to a peak stress and fast fracture after small straining. The mechanical properties of porous samples of both types were observed to obey minimum solid area (MSA) models in which the bond regions between particles perpendicular to loading direction are assumed to dominate in transmission of stress. A linear relation was obtained between yield strength and square of neck size ratio, (X/D) 2 , where X and D represent the average neck and particle diameters, respectively.
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This study concerns an investigation of the corrosion behavior of 316 stainless steel, CoCrMo and Ti6Al4V alloys in simulated body conditions (ringer lactate) at 37°C by the use of Tafel plots, mixed potential and electrochemical impedance spectroscopy (EIS). Ti6Al4V alloy has the highest corrosion resistance followed by CoCr alloy. Ti6Al4V–CoCrMO was the best couple for galvanic corrosion with the minimum galvanic potential and current values according to mixed potential theory and Tafel method. It was concluded that Ti6Al4V was the most suitable material for implant applications in the human body.
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The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life and the science and technology associated with this field has now led to multi-million dollar business. The paper focuses its attention mainly on titanium-based alloys, even though there exists biomaterials made up of ceramics, polymers and composite materials. The paper discusses the biomechanical compatibility of many metallic materials and it brings out the overall superiority of Ti based alloys, even though it is costlier. As it is well known that a good biomaterial should possess the fundamental properties such as better mechanical and biological compatibility and enhanced wear and corrosion resistance in biological environment, the paper discusses the influence of alloy chemistry, thermomechanical processing and surface condition on these properties. In addition, this paper also discusses in detail the various surface modification techniques to achieve superior biocompatibility, higher wear and corrosion resistance. Overall, an attempt has been made to bring out the current scenario of Ti based materials for biomedical applications.
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For many years, the solid metals and their alloys have been widely used for fabrication of the implants replacing hard human tissues or their functions. To improve fixation of solid implants to the surrounding bone tissues, the materials with porous structures have been introduced. By tissue ingrowing into a porous structure of metallic implant, the bonding between the implant and the bone has been obtained. Substantial pore interconnectivity, in metallic implants, allows extensive body fluid transport through the porous implant. This can provoke bone tissue ingrowth, consequently, leading to the development of highly porous metallic implants, which could be used as scaffolds in bone tissue engineering. The goal of this study was to develop and then investigate properties of highly porous titanium structures received from powder metallurgy process. The properties of porous titanium samples, such as microstructure, porosity, Young's modulus, strength, together with permeability and corrosion resistance were investigated. Porous titanium scaffolds with nonhomogeneous distribution of interconnected pores with pore size in the range up to 600 μm in diameter and a total porosity in the range up to 75% were developed. The relatively high permeability was observed for samples with highest values of porosity. Comparing to cast titanium, the porous titanium was low resistant to corrosion. The mechanical parameters of the investigated samples were similar to those for cancellous bone. The development of high-porous titanium material shows high potential to be modern material for creating a 3D structure for bone regeneration and implant fixation.
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In this study, the unit cell approach, which has previously been demonstrated as a method of manufacturing porous components suitable for use as orthopedic implants, has been further developed to include randomized structures. These random structures may aid the bone in-growth process because of their similarity in appearance to trabecular bone and are shown to carry legacy properties that can be related back to the original unit cell on which they are ultimately based. In addition to this, it has been shown that randomization improves the mechanical properties of regular unit cell structures, resulting in anticipated improvements to both implant functionality and longevity. The study also evaluates the effect that a post process sinter cycle has on the components, outlines the improved mechanical properties that are attainable, and also the changes in both the macro and microstructure that occur.
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We reviewed at a minimum elapsed time of five years a consecutive series of 143 primary Exeter hip replacements in which matt-surfaced femoral stems had been used. Twenty-five patients had died and six stems and two sockets had been revised before follow-up. The remaining 110 hips were all examined clinically and radiographically. In 15 hips there were radiographic signs of definite loosening of the stem and in eight suspected loosening. The acetabulum was loose in four hips. In another eight hips localised bone resorption was present without signs of loosening. Half the patients with loosening or localised bone resorption had mild pain or no pain at all. The late complication rate with the matt-surfaced Exeter femoral stem is unacceptably high.
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We tested in compression specimens of human proximal tibial trabecular bone from 31 normal donors aged from 16 to 83 years and determined the mechanical properties, density and mineral and collagen content. Young's modulus and ultimate stress were highest between 40 and 50 years, whereas ultimate strain and failure energy showed maxima at younger ages. These age-related variations (except for failure energy) were non-linear. Tissue density and mineral concentration were constant throughout life, whereas apparent density (the amount of bone) varied with ultimate stress. Collagen density (the amount of collagen) varied with failure energy. Collagen concentration was maximal at younger ages but varied little with age. Our results suggest that the decrease in mechanical properties of trabecular bone such as Young's modulus and ultimate stress is mainly a consequence of the loss of trabecular bone substance, rather than a decrease in the quality of the substance itself. Linear regression analysis showed that collagen density was consistently the single best predictor of failure energy, and collagen concentration was the only predictor of ultimate strain.
Chapter
Metallic biomaterials continue to be used extensively for the fabrication of surgical implants primarily for the same reason that led to their initial selection for these devices many decades ago. The high strength and resistance to fracture that this class of material can provide, assuming proper processing, gives reliable long-term implant performance in major load-bearing situations. Coupled with a relative ease of fabrication of both simple and complex shapes using well-established and widely available fabrication techniques (e.g., casting, forging, machining), this has promoted metal use in the fields of orthopedics and dentistry primarily, the two areas in which highly loaded devices are most common although similar reasons have led to their use for forming cardiovascular devices (e.g., artificial heart valves, blood conduits and other components of heart assist devices, vascular stents), and neurovascular implants (aneurysm clips). In addition, the good electrical conductivity of metals favors their use for neuromuscular stimulation devices, the most common example being cardiac pacemakers. These favorable properties (good fracture resistance, electrical conductivity, formability) are related to the metallic interatomic bonding that characterizes this class of material. While the purpose of this chapter is to focus on the important issues pertaining to the processing and performance of metallic biomaterials and to review the metals that are currently used for implant fabrication, a brief review of fundamental issues related to the structure-property relations of metals in general follows.
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The cyclic stress in lithium-ion battery electrodes induced by repeated charge and discharge cycles causes electrode degradation and fracture, resulting in reduced battery performance and lifetime. To investigate electrode mechanics as a function of electrochemical cycling, we utilize digital image correlation (DIC) to measure the strains that develop in lithium-ion battery electrodes during lithiation and delithiation processes. A composite graphite electrode is cycled galvanostatically (with constant current) in a custom battery cell while optical images of the electrode surface are captured in situ. The strain in the electrode is computed using an in-house DIC code. On average, an unconstrained composite graphite electrode expands 1.41 % during lithiation and contracts 1.33 % during delithiation. These strain values compare favorably with predictions based on the elastic properties of the composite electrode and the expansion of graphite-lithium intercalation compounds (G-LICs). The establishment of this experimental protocol will enable future studies of the relationship between electrode mechanics and battery performance.
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It is no secret that the laser was the driver for additive manufacturing (AM) of 3D objects since such objects were first demonstrated in the mid-1980s. A myriad of techniques utilizing the directed energy of lasers were invented. Lasers are used to selectively sinter or fuse incremental layers in powder-beds, melt streaming powder following a programmed path, and polymerize photopolymers in a liquid vat layer-by-layer. The laser is an energy source of choice for repair of damaged components, for manufacture of new or replacement parts, and for rapid prototyping of concept designs. Lasers enable microstructure gradients and heterogeneous structures designed to exhibit unique properties and behavior. Laserbased additive manufacturing has been successful in producing relatively simple near net-shape metallic parts saving material and cost, but requiring finish-machining and in repair and refurbishment of worn components. It has been routinely used to produce polymer parts. These capabilities have been widely recognized as evidenced by the explosion in interest in AM technology, nationally. These successes are, however, tempered by challenges facing practitioners such as process and part qualification and verification, which are needed to bring AM as a true manufacturing technology. The ONR manufacturing science program, in collaboration with other agencies, invested in basic R&D; in AM since its beginnings. It continues to invest, currently focusing on developing cyber-enabled manufacturing systems for AM. It is believed that such computation, communication and control approaches will help in validating AM and moving it to the factory floor along side CNC machines.
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Introduction Making Metal Foams Characterization Methods Properties of Metal Foams Design Analysis for Material Selection Design Formulae for Simple Structures A Constitutive Model for Metal Foams Design for Creep with Metal Foams Sandwich Structures Energy Management: Packaging and Blast Protection Sound Absorption and Vibration Suppression Thermal Management and Heat Transfer Electrical Properties of Metal Foams Cutting, Finishing and Joining Cost Estimation and Viability Case Studies Suppliers of Metal Foams Web Sites Index .
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Using pure Ti powder with particle sizes from 300 to 500 mum prepared by the plasma rotating electrode process (PREP), porous pure Ticompacts for biomedical applications were synthesized by powder sintering, and microstructures and mechanical proper-ties of the compacts were investigated in this study. Porous compacts having porosity of 19-35 vol% are successfully fabricated by controlling sintering condition, It is found that Young's modulus and compressive yield strength decrease linearly with increasing porosity, and porous Ti compacts having porosity of about 30-35 vol% exhibit identical Young's modulus of human bone.
Article
The design of custom or tailored implant components has been the subject of research and development for decades. However, the economic feasibility of fabricating such components has proven to be a challenge. New direct metal fabrication technologies such as Electron Beam Melting (EBM) have opened up new possibilities. This paper discusses the design and fabrication of titanium implant components having tailored mechanical properties that mimic the stiffness of bone to reduce stress shielding and bone remodeling. Finite Element Analysis was used to design the tailored structures, and results were verified using mechanical testing.
Article
Porous Ti compacts for biomedical applications are successfully fabricated in the porosity range from 5.0 to 37.1 vol% by controlling sintering conditions and Ti powder sizes. YoungÕs modulus and bending strength at the porosity of around 30 vol% are found to be similar to those of human cortical bone.
Article
New low modulus β-type titanium alloys for biomedical applications are still currently being developed. Strong and enduring β-type titanium alloy with a low Young's modulus are being investigated. A low modulus has been proved to be effective in inhibiting bone atrophy, leading to good bone remodeling in a bone fracture model in the rabbit tibia. Very recently β-type titanium alloys with a self-tunable modulus have been proposed for the construction of removable implants. Nickel-free low modulus β-type titanium alloys showing shape memory and super elastic behavior are also currently being developed. Nickel-free stainless steel and cobalt-chromium alloys for biomedical applications are receiving attention as well. Newly developed zirconium-based alloys for biomedical applications are proving very interesting. Magnesium-based or iron-based biodegradable biomaterials are under development. Further, tantalum, and niobium and its alloys are being investigated for biomedical applications. The development of new metallic alloys for biomedical applications is described in this paper.
Article
Titanium sandwich panels with cellular cores of a uniform 1–5 mm diameter open cell size are well suited for impact energy absorption and cross flow heat exchange applications. Periodic cellular structures (lattices) made from high specific strength, high temperature alloys are preferred for these multifunctional uses. A diffusion bonding method has been applied here to make cellular lattice structures from a Ti–6A1–4V alloy. To illustrate the approach, lattice structures with both square and diamond collinear topologies, a 2 mm open cell size, and a relative density of 15% were made from 254 µm diameter titanium alloy wires. These structures were found to have a compressive strength of 40 ± 5 MPa that was controlled by plastic yield followed by buckling of the struts. The cellular structures have been brazed to titanium alloy face sheets to create sandwich panel structures that appear well suited for multifunctional applications up to 420 °C.
Article
Digital image correlation is finding wider use in the field of mechanics. One area of weakness in the current technique is the lack of available displacement gradient terms. This technique, based on a coarse-fine search method, is capable of calculating the gradients. However the speed at which it does so has prevented widespread use. Presented in this paper is the development and limited experimental verification of a method which can determine displacements and gradients using the Newton-Raphson method of partial corrections. It will be shown that this method is accurate in determining displacements and certain gradients, while using significantly less CPU time than the current coarse-fine search method.
Article
This paper addresses foams which are known as non-stochastic foams, lattice structures, or repeating open cell structure foams. The paper reports on preliminary research involving the design and fabrication of non-stochastic Ti–6Al–4V alloy structures using the electron beam melting (EBM) process. Non-stochastic structures of different cell sizes and densities were investigated. The structures were tested in compression and bending, and the results were compared to results from finite element analysis simulations. It was shown that the build angle and the build orientation affect the properties of the lattice structures. The average compressive strength of the lattice structures with a 10% relative density was 10 MPa, the flexural modulus was 200 MPa and the strength to density ration was 17. All the specimens were fabricated on the EBM A2 machine using a melt speed of 180 mm/s and a beam current of 2 mA. Future applications and FEA modeling were discussed in the paper.
Article
The property profile exhibited by cellular metals identifies several applications, especially in technologies requiring multifunctionality. Their specific property attributes suggest implementation as: ultralight panels/shells, energy absorbing structures and heat dissipation media as well as for vibration control. Connections between the properties that govern these performance benefits and the cellular architecture, cell morphology and density have been made. Such structural relations facilitate choices of optimum cell characteristics for defined multifunctional applications.
Article
Periprosthetic bone resorption after tibial prosthesis implantation remains a concern for long-term fixation performance. The fixation techniques may inherently aggravate the "stress-shielding" effect of the implant, leading to weakened bone foundation. In this study, two cemented tibial fixation cases (fully cemented and hybrid cementing with cement applied under the tibial tray leaving the stem uncemented) and three cementless cases relying on bony ingrowth (no, partial and fully ingrown) were modelled using the finite element method with a strain-adaptive remodelling theory incorporated to predict the change in the bone apparent density after prosthesis implantation. When the models were loaded with physiological knee joint loads, the predicted patterns of bone resorption correlated well with reported densitometry results. The modelling results showed that the firm anchorage fixation formed between the prosthesis and the bone for the fully cemented and fully ingrown cases greatly increased the amount of proximal bone resorption. Bone resorption in tibial fixations with a less secure anchorage (hybrid cementing, partial and no ingrowth) occurred at almost half the rate of the changes around the fixations with a firm anchorage. The results suggested that the hybrid cementing fixation or the cementless fixation with partial bony ingrowth (into the porous-coated prosthesis surface) is preferred for preserving proximal tibial bone stock, which should help to maintain post-operative fixation stability. Specifically, the hybrid cementing fixation induced the least amount of bone resorption.
Article
EBM (Electron Beam Melting) technology can be used successfully to obtain cellular solids in metallic biomaterials that can greatly increase osseointegration in arthroprothesis and at the same time maintain good mechanical properties. The investigated structures, called Trabecular Titanium, usually cannot be obtained by traditional machining. Two samples: (A) with a smaller single cell area and, (B) with a bigger single cell area, were produced and studied in this project. They have been completely characterized and compared with the results in similar literature pertinent to Ti6Al4V EBM structures. Relative density was evaluated using different methods, the mean diameter of the open porosities was calculated by Scanning Electron Microscope images; the composition was evaluated using Energy-Dispersive X-Ray Spectroscopy; the microstructure (alpha-beta) was investigated using chemical etching and, the mechanical proprieties were investigated using UMTS. The mean porosity values resulted comparable with spongy bone (63% for A and 72% for B). The mean diameter of the single porosity (650 mum for A and 1400 mum for B) resulted compatible with the osseointegration data from the literature, in particular for sample A. The Vickers micro-hardness tests and the chemical etching demonstrated that the structure is fine, uniform and well distributed. The mechanical test proved that sample (A) was more resistant than sample (B), but sample (B) showed an elastic modulus almost equal to the value of spongy bone. The results of this study suggest that the two Ti6Al4V cellular solids can be used in biomedical applications to promote osseointegration demonstrating that they maybe successfully used in prosthetic implants. Additional implant results will be published in the near future.
Article
Patient specific porous implants for the reconstruction of craniofacial defects have gained importance due to their better performance over their generic counterparts. The recent introduction of electron beam melting (EBM) for the processing of titanium has led to a one step fabrication of porous custom titanium implants with controlled porosity to meet the requirements of the anatomy and functions at the region of implantation. This paper discusses an image based micro-structural analysis and the mechanical characterization of porous Ti6Al4V structures fabricated using the EBM rapid manufacturing process. SEM studies have indicated the complete melting of the powder material with no evidence of poor inter-layer bonding. Micro-CT scan analysis of the samples indicate well formed titanium struts and fully interconnected pores with porosities varying from 49.75%-70.32%. Compression tests of the samples showed effective stiffness values ranging from 0.57(+/-0.05)-2.92(+/-0.17)GPa and compressive strength values of 7.28(+/-0.93)-163.02(+/-11.98)MPa. For nearly the same porosity values of 49.75% and 50.75%, with a variation in only the strut thickness in the sample sets, the compressive stiffness and strength decreased significantly from 2.92 GPa to 0.57 GPa (80.5% reduction) and 163.02 MPa to 7.28 MPa (93.54 % reduction) respectively. The grain density of the fabricated Ti6Al4V structures was found to be 4.423 g/cm(3) equivalent to that of dense Ti6Al4V parts fabricated using conventional methods. In conclusion, from a mechanical strength viewpoint, we have found that the porous structures produced by the electron beam melting process present a promising rapid manufacturing process for the direct fabrication of customized titanium implants for enabling personalized medicine.
Article
The microstructure and mechanical behavior of simple product geometries produced by layered manufacturing using the electron beam melting (EBM) process and the selective laser melting (SLM) process are compared with those characteristic of conventional wrought and cast products of Ti-6Al-4V. Microstructures are characterized utilizing optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and included alpha (hcp), beta (bcc) and alpha(') (hcp) martensite phase regimes which give rise to hardness variations ranging from HRC 37 to 57 and tensile strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of layered manufacturing utilizing initial powders in custom building of biomedical components by EBM and SLM in contrast to conventional manufacturing from Ti-6Al-4V wrought bar stock are discussed.
Article
Young's modulus as well as tensile strength, ductility, fatigue life, fretting fatigue life, wear properties, functionalities, etc., should be adjusted to levels that are suitable for structural biomaterials used in implants that replace hard tissue. These factors may be collectively referred to as mechanical biocompatibilities. In this paper, the following are described with regard to biomedical applications of titanium alloys: the Young's modulus, wear properties, notch fatigue strength, fatigue behaviour on relation to ageing treatment, improvement of fatigue strength, fatigue crack propagation resistance and ductility by the deformation-induced martensitic transformation of the unstable beta phase, and multifunctional deformation behaviours of titanium alloys.
Article
Bone resorption around hip stems is a disturbing phenomenon, although its clinical significance and its eventual effects on replacement longevity are as yet uncertain. The relationship between implant flexibility and the extent of bone loss, frequently established in clinical patient series and animal experiments, does suggest that the changes in bone morphology are an effect of stress shielding and a subsequent adaptive remodeling process. This relationship was investigated using strain-adaptive bone-remodeling theory in combination with finite element models to simulate the bone remodeling process. The effects of stem material flexibility, bone flexibility, and bone reactivity on the process and its eventual outcome were studied. Stem flexibility was also related to proximal implant/bone interface stresses. The results sustain the hypothesis that the resorptive processes are an effect of bone adaptation to stress shielding. The effects of stem flexibility are confirmed by the simulation analysis. It was also established that individual differences in bone reactivity and mechanical bone quality (density and stiffness) may account for the individual variations found in patients and animal experiments. Flexible stems reduce stress shielding and bone resorption. However, they increase proximal interface stresses. Hence, the cure against bone resorption they represent may develop into increased loosening rates because of interface debonding and micromotion. The methods presented in this paper can be used to establish optimal stem-design characteristics or check the adequacy of designs in preclinical testing procedures.
Article
An animal model of anterior and posterior column instability was developed to allow in vivo observation of bone remodeling and arthrodesis following spinal instrumentation. After an initial anterior and posterior destabilizing lesion was created at the L5-L6 vertebral levels in 63 adult beagles, various spinal reconstructive surgical procedures were performed--with or without bilateral posterolateral bone grafting, with or without bilateral oophorectomies, and with or without spinal instrumentation (Harrington distraction, Luque rectangular, Cotrel-Dubousset pedicular, or Steffee pedicular implants). Observation 6 months after surgery revealed a significantly improved probability of achieving a spinal fusion if spinal instrumentation had been used (X2 = 5.84, P = .016). Nondestructive mechanical testing after removal of all metal instrumentation in torsion, axial compression, and flexion revealed that the fusions performed in conjunction with spinal instrumentation were more rigid (P less than .05). Quantitative histomorphometry showed that the volumetric density of bone was significantly lower (ie, device-related osteoporosis occurred) for fused versus unfused spines. In addition, a linear correlation occurred between decreasing volumetric density of bone and increasing rigidity of the spinal implant (r = .778); ie, device-related osteoporosis occurred secondary to Harrington, Cotrel-Dubousset, and Steffee pedicular instrumentation. Oophorectomized dogs became more osteoporotic than their surgically matched controls (posterolateral bone grafting alone, Cotrel-Dubousset pedicular instrumentation, and Steffee pedicular instrumentation); device-related osteoporosis added to the degree of hormonally induced osteoporosis (t = 5.0, P less than .0001). This is the first study to date documenting the occurrence of stress shielding in the spine secondary to spinal instrumentation.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
Individual trabeculae, rodlike in form, were excised from bovine femora and tested in tension to obtain stress-strain plots. Tensile grips were constructed to permit such small specimens to be tested and to avoid slippage during the test. Data were collected for 38 specimens. The results of these tests show that rodlike trabeculae obtained from the femora of young bovine animals have an average Young's modulus in tension of approximately 1 GPa. This value is an order of magnitude lower than the corresponding value for cortical bone in the diaphysis of the femur.
Article
The purpose of this study was to design a method to produce and test mechanically microspecimens of trabecular and cortical tissue from human iliac crests, and compare their measured moduli. Rectangular beam specimens were prepared on a low-speed diamond blade saw and a miniature milling machine. The final specimen dimensions ranged from approximately 50-200 microns for base and height. The modulus of each specimen was measured using three-point bending tests across a span length of 1.04 mm and performed at a constant rate of displacement. A subset of specimens was recovered for a radiographic estimation of degree of mineralization. The results showed the mean trabecular tissue modulus of all iliac crest specimens to be 3.81 GPa, whereas cortical tissue specimens averaged 4.89 GPa. This was a significant difference according to a two-way analysis of variance that controlled for differences between donors. No strong correlations were found between modulus and mineral density. Future investigations that consider other microstructural characteristics and their contributions to modulus, and specimen size effects, are indicated.
Article
In 1961, Evans and King documented the mechanical properties of trabecular bone from multiple locations in the proximal human femur. Since this time, many investigators have cataloged the distribution of trabecular bone material properties from multiple locations within the human skeleton to include femur, tibia, humerus, radius, vertebral bodies, and iliac crest. The results of these studies have revealed tremendous variations in material properties and anisotropy. These variations have been attributed to functional remodeling as dictated by Wolff's Law. Both linear and power functions have been found to explain the relationship between trabecular bone density and material properties. Recent studies have re-emphasized the need to accurately quantify trabecular bone architecture proposing several algorithms capable of determining the anisotropy, connectivity and morphology of the bone. These past studies, as well as continuing work, have significantly increased the accuracy of analytical and experimental models investigating bone, and bone/implant interfaces as well as enhanced our perspective towards understanding the factors which may influence bone formation or resorption.
Article
An ultrasonic technique and microtensile testing were used to determine the Young's modulus of individual trabeculae and micro-specimens of cortical bone cut to similar size as individual trabeculae. The average trabecular Young's modulus measured ultrasonically and mechanically was 14.8 GPa (S.D. 1.4) and 10.4 (S.D. 3.5) and the average Young's modulus of microspecimens of cortical bone measured ultrasonically and mechanically was 20.7 GPa (S.D. 1.9) and 18.6 GPa (S.D. 3.5). With either testing technique the mean trabecular Young's modulus was found to be significantly less than that of cortical bone (p < 0.0001). However, the specimens were dried before microtensile testing so Young's modulus values may have been greater than those of trabeculae in vivo. Using Young's modulus measurements obtained from 450 cubes of cancellous bone and 256 cubes of cortical bone, Wolff's hypothesis that cortical bone is simply dense cancellous bone was tested. A multiple regression analysis that controlled for group membership showed that Young's modulus of cortical bone cannot be extrapolated from the Young's modulus vs density relationship for cancellous bone, yet the Young's modulus of trabeculae can be predicted by extrapolation from the relationship between Young's modulus vs density of the cancellous bone. These results suggest that when considered mechanically, cortical and trabecular bone are not the same material.
Article
An experimental investigation was undertaken to measure the intrinsic elastic properties of several of the microstructural components of human vertebral trabecular bone and tibial cortical bone by the nanoindentation method. Specimens from two thoracic vertebrae (T-12) and two tibiae were obtained from frozen, unembalmed human male cadavers aged 57 and 61 years. After drying and mounting in epoxy resin nanoindentation tests were conducted to measure Young's modulus and the hardness of individual trabeculae in the vertebrae and single osteons, and interstitial lamellae in the tibiae. Measurements on the vertebral trabeculae were made in the transverse direction, and the average Young's modulus was found to be 13.5 +/- 2.0 GPa. The tibial specimens were tested in the longitudinal direction, yielding moduli of 22.5 +/- 1.3 GPa for the osteons and 25.8 +/- 0.7 GPa for the interstitial lamellae. Analysis of variance showed that the differences in the measured moduli are statistically significant. Hardness differences among the various microstructural components were also observed.
Article
To evaluate the biocompatibility of a new titanium-tantalum alloy, with qualities superior to titanium alone, for use in oral implantology, fibroblast and epithelial cell lines were grown on plastic, titanium, copper, and titanium-tantalum supports. Studies using scanning electron microscopy, flow cytometry, and cytotoxicity assays were conducted to compare the different supports. Scanning electron microscopic observations showed high densities of fibroblasts and epithelial cells with well-developed attachment systems in the form of cytoplasmic projections. Cell densities were lower on titanium and titanium-tantalum surfaces than on plastic. Cell numbers, as determined by cytotoxicity assays, were significantly higher on plastic than on titanium or titanium-tantalum surfaces while fibroblasts proliferated better than epithelial cells on both metal surfaces. Flow cytometric analyses of cell cycles did not reveal any significant variations in the distribution of cells among the cycle phases on the three materials. We found no differences with regard to the parameters studied between titanium and the titanium-tantalum alloy.
Article
Increased use of titanium alloys as biomaterials is occurring due to their lower modulus, superior biocompatibility and enhanced corrosion resistance when compared to more conventional stainless steels and cobalt-based alloys. These attractive properties were a driving force for the early introduction of alpha (cpTi) and alpha + beta (Ti-6A1-4V) alloys as well as for the more recent development of new Ti-alloy compositions and orthopaedic metastable beta titanium alloys. The later possess enhanced biocompatibility, reduced elastic modulus, and superior strain-controlled and notch fatigue resistance. However, the poor shear strength and wear resistance of titanium alloys have nevertheless limited their biomedical use. Although the wear resistance of beta-Ti alloys has shown some improvement when compared to alpha + beta alloys, the ultimate utility of orthopaedic titanium alloys as wear components will require a more complete fundamental understanding of the wear mechanisms involved. This review examines current information on the physical and mechanical characteristics of titanium alloys used in artifical joint replacement prostheses, with a special focus on those issues associated with the long-term prosthetic requirements, e.g., fatigue and wear.
Article
Acoustic microscopy (30-60 microm resolution) and nanoindentation (1-5 microm resolution) are techniques that can be used to evaluate the elastic properties of human bone at a microstructural level. The goals of the current study were (1) to measure and compare the Young's moduli of trabecular and cortical bone tissues from a common human donor, and (2) to compare the Young's moduli of bone tissue measured using acoustic microscopy to those measured using nanoindentation. The Young's modulus of cortical bone in the longitudinal direction was about 40% greater than (p<0.01) the Young's modulus in the transverse direction. The Young's modulus of trabecular bone tissue was slightly higher than the transverse Young's modulus of cortical bone, but substantially lower than the longitudinal Young's modulus of cortical bone. These findings were consistent for both measurement methods and suggest that elasticity of trabecular tissue is within the range of that of cortical bone tissue. The calculation of Young's modulus using nanoindentation assumes that the material is elastically isotropic. The current results, i.e., the average anisotropy ratio (E(L)/E(T)) for cortical bone determined by nanoindentation was similar to that determined by the acoustic microscope, suggest that this assumption does not limit nanoindentation as a technique for measurement of Young's modulus in anisotropic bone.
Article
Periprosthetic adaptive bone remodelling after total hip arthroplasty can be simulated in computer models, combining bone remodelling theory with finite element analysis. Patient specific three-dimensional finite element models of retrieved bone specimens from an earlier bone densitometry (DEXA) study were constructed and bone remodelling simulations performed. Results of the simulations were analysed both qualitatively and quantitatively. Patterns of predicted bone loss corresponded very well with the DEXA measurements on the retrievals. The amount of predicted bone loss, measured quantitatively by simulating DEXA on finite element models, was found to be inversely correlated with the initial bone mineral content. It was concluded that the same clinically observed correlation can therefore be explained by mechanically induced remodelling. This finding extends the applicability of numerical pre-clinical testing to the analysis of interaction between implant design and initial state of the bone.
Article
The mechanical properties of bone tissue are determined by composition as well as structural, microstructural and nanostructural organization. The aim of this study was to quantify the elastic properties of bone at the lamellar level and compare these properties among osteonal, interstitial and trabecular microstructures from the diaphysis and the neck of the human femur. A nanoindentation technique with a custom irrigation system was used for simultaneously measuring force and displacement of a diamond tip pressed 500 nm into the moist bone tissue. An isotropic elastic modulus was calculated from the unloading curve with an assumed Poisson ratio of 0.3, while hardness was defined as the maximal force divided by the corresponding contact area. The elastic moduli ranged from 6.9 +/- 4.3 GPa in trabecular tissue from the femoral neck of a 74 yr old female up to 25.0 +/- 4.3 GPa in interstitial tissue from the diaphyseal cortex of a 69 yr old female. The mean elastic modulus was found to be significantly influenced by the type of lamella (p < 10(-6)) and by donor (p < 10(-6)). The interaction between the type of lamella and the donor was also highly significant (p < 10(-6)). Hardness followed a similar distribution as elastic modulus among types of lamellae and donor, but with lower statistical contrast. It is concluded that the nanostructure of bone tissue must differ substantially among lamellar types, anatomical sites and individuals and suggests that tissue heterogeneity is of potential importance in bone fragility and adaptation.
Article
Although trabecular bone structure has been evaluated, variation with knee compartment and depth from joint surface is not completely understood. Cadaver knees were evaluated with microcomputed tomography analysis for these variations. Objective differences were compared between: medial vs. lateral compartments; femoral vs. tibial bone; and normal vs. arthritic knees. Depth dependent changes in the parameters were observed for the first 6 mm of the cores in normal knees: BV/TV, Tb.N and Conn.D gradually decrease, while Tb.Sp and SMI increase. In the first 6 mm of the normal tibia BV/TV, Tb.N, and Tb.Th are greater than in the femur on both the medial and lateral compartments while Tb.Sp, SMI, and Conn.D are lower. The medial compartment values for BV/TV, Tb.N, Tb.Th and Conn.D are generally greater than for the lateral in both the femur and tibia while Tb.Sp and SMI are lower. In comparison of normal vs. arthritic knees significant differences are observed in the first 6 mm of the medial tibia. With arthritis BV/TV and Tb.Th are lower, while SMI and Tb.Sp are higher. Tb.N and Conn.D show no statistically significant difference. The bone structure variations are, thus, most prominent in the first 6 mm of depth and medial compartment bone is generally more structurally sound than lateral. Severely arthritic bone changes are most prominent in the medial compartment of the tibia and bone structure is less sound in severe arthritis.
Article
The radiographs of 64 patients with 70 humeral head replacements were reviewed for signs of stress shielding. Of these, 49 were implanted for rheumatoid arthritis and 21 for osteoarthritis. The radiographic follow-up averaged 5.3 years. Measurements of cortex thickness were performed in 4 regions along the stem of the implant, and the differences between the postoperative radiograph and the radiograph at follow-up were calculated. The size of the stem in relation to the diameter of the humerus was calculated with the use of validated measures, resulting in the relative stem size. In 6 patients (9%) a significant reduction in cortical thickness was observed in the proximal-lateral region of the humeral stem, 5 in rheumatoid patients and 1 in an osteoarthritic patient. In the stress shielding group, the relative stem size was found to be significantly higher than that in the non-stress shielding group (0.58 vs 0.48). Osteoporosis, especially present in rheumatoid arthritis, could well be a risk factor. It was concluded that stress shielding is a long-term complication of shoulder arthroplasty and that the relative stem size is an important factor in its genesis.
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