Plated and Intact Femur Strains in Fracture Fixation Using Fiber Bragg Gratings and Strain Gauges

This paper presents an experimental methodology to determine plated and intact femur strains using fiber Bragg gratings and strain gauges. A plated and an intact synthetic femur were used and loaded under a simplistic static load of 600 N. A stainless steel (316L) plate was used to fixate a simulated 45° fracture on one femur. Strains were recoded at the same sites on both femurs. Strain shielding is shown to be more pronounced at the distal region of the plated femur. The experimental methodology based on fiber Bragg grating sensors is a novel approach to assess bone plate strains, which could also be used to obtain biologic tissue and implant surface strains in locations where conventional strain gauge use is not technically feasible.     

Experimental Measurement and Numerical Validation of Bone Cement Mantle Strains of an In Vitro Hip Replacement Using Optical FBG Sensors

Experimental pre-clinical tests associated with numeric models of cemented implants are important for screening of new implants in the market. The aim of this study was to measure strain profiles and maximum temperature polymerization inside a cement mantle of an in vitro cemented hip reconstruction using optical fiber Bragg grating (FBG) sensors. For this purpose, a hip femoral prosthesis was instrumented with 12 FBG sensors, three in each aspect of the femur, anterior, posterior, medial and lateral. These were positioned at the proximal, middle and distal part of the cement mantle relatively to the stem. Another sensor was placed in the lateral-proximal region of the mantle to measure the maximum temperature of cement polymerization. The strains measured were compared with those obtained with a Finite Element model, both for quaistatic mechanical loading. The results show that the experimental technique used can measure strains inside the cement mantle with good correlation, R²?=?0.970, with the numerical model results. The results present a maximum temperature of polymerization around 110°C inside of cement at proximal region. It was also observed strain concentration in lateral aspect of the femur in polymerization process. The procedure hereby explained can be used to improve experimental pre-clinical tests to measure the strain distribution inside the cement mantle as well as residual strain and temperature variation along with time, as a result of the curing process of cement.     

Failure analysis of the total knee prosthesis anchored with PMMA

Stress and failure analysis of the bone/cement/metal prosthesis of the femorotibial joint are made. The system is modelled by the finite element method in two dimensions while the strain energy density (SED) criterion is applied to locate the potential medial and lateral failure sites in the PMMA for five different prosthesis positions. Examined also is the influence of the PMMA cement that may or may not completely fill the ligament between the stems of the femoral prosthetic component.According to the SED criterion, the cement next to the base of the femoral component systems is the weakest, particularly when the implant is tilted locally resulting in an uneven spread of the cement lining. A 10° local misalignment of the metal implant with reference to the mechanical axis of the femur is more detrimental than the 20° valgus or 20° varus position. The precise locations of possible failure sites on the cement are predicted even though the overall structural integrity of the total knee prosthesis system is found to be adequate for the ideal case investigated. Loadings of a dynamic nature combined with imperfections such as voids and defects in the cement are additional factors that can enhance failure initiation.     

Hyperelastic modeling of location-dependent human distal femoral cartilage mechanics

Knee articular cartilage exhibits complex mechanical behavior, even under high strain rates, which poses a challenge to developing accurate and efficient cartilage models. In particular, the tissue׳s stress–strain response is non-linear and the stiffness of the response is location-dependent. Hyperelastic models such as those of Alan Gent and others have increasingly found use in soft tissue biomechanics. Recently, a hyperelastic statistical chain network model representing the transverse isotropy of the collagen matrix in the superficial tangential zone has been developed. The model successfully simulated the 100% strain/s unconfined compression response of human proximal tibial cartilage. Moreover, spatial variations in the tangent modulus to the nominal stress–strain curve taken at 10% strain were reflected in the variability of a single parameter of the model. Given the success of the model, we desired to determine whether these outcomes are equally applicable to healthy human distal femoral cartilage so that a complete model of tibiofemoral joint cartilage can be developed. The transversely isotropic model was employed along with two other hyperelastic chain network models to determine which model best simulated unconfined compression data for healthy distal femoral cartilage. The transversely isotropic model fit the data excellently (R²=0.999). The model was subsequently simplified to depend on a single parameter and reapplied to the dataset. The modified model maintained an excellent fit to the data (R²=0.999), and its single parameter varied in a statistically similar regional pattern (p<0.05) to the experimentally-obtained elastic modulus of the tissue. Outcomes suggest that this model is suitable for modeling the spatially-varying, non-linear mechanics of healthy human distal femoral cartilage. Implementation of this constitutive relation within computational models of the knee will provide novel insight into the relationship between joint mechanics, cartilage loading, and knee osteoarthritis development.     

利用分形相关法测量股骨头剖开面的面内位移

由于股骨头内松质骨结构的复杂性,对股骨头内骨小梁的应力和变形的分析与测试十分困难。在已经开展的这一方面的研究工作中主要是进行松质骨试样的宏观力学性能测试。随着计算机和图像技术的发展,图像及图像相关方法开始被用来进行松质骨的生物力学研究,这是非接触的测量方法,可以用来测量松质骨的表面位移场。本文利用图像相关方法在宏观尺度下测量了股骨头受外力作用时冠状面的面内的位移分布,作为进一步探索的开端。尽管松质骨表面的凹凸不平性对测量精度有影响,但是,还是可以分析出面内位移的分布特点。     

基于图像分析法的水稻茎秆力学性能测量与分析

水稻茎秆力学性能的准确描述是水稻抗倒伏力学分析的关键。由于茎秆是由生物活性材料组成的形状不规则柔性结构,传统的测试方法难以对其力学性能进行准确测量。以南方水稻为研究对象,采用拉伸试验机对水稻茎秆进行了拉伸试验。首先采用传统的应变片方法测量应变,继而探索采用图像分析法实现非接触、高精度地测量水稻茎秆的截面尺寸、拉伸变形。用数字摄像机记录不同荷载水平下试件表面的数字图像,采用数字图像相关分析法(DIC)进行分析,通过两种方法计算茎秆的轴向应变;将完成拉伸实验的试件切片,采集其横截面的数字图像,通过数字图像分析技术得到其几何特性,并对大量实验数据进行了统计分析,实现了对水稻茎秆弹性模量、抗弯刚度等的合理描述。实验结果表明,采用传统的应变片测量求得的水稻茎秆的弹性模量的离散性极大,超出样本差异性的范围,而采用数字图像分析方法得到的弹性模量、抗弯刚度的结果有一定的离散性,但分布比较合理,比较准确地反映了水稻茎秆相关的力学性能。     

Tissue level microstructure and mechanical properties of the femoral head in the proximal femur of fracture patients

This study aims to investigate the regional variations of trabecular morphological parameters and mechanical parameters of the femoral head,as well as to determine the relationship between trabecular morphological and mechanical parameters.Seven femoral heads from patients with fractured proximal femur were scanned using a micro-CT system.Each femoral head was divided into 12 sub-regions according to the trabecular orientation.One 125 mm~3 trabecular cubic model was reconstructed from each sub-region.A total of 81 trabecular models were reconstructed,except three destroyed sub-regions from two femoral heads during the surgery.Trabecular morphological parameters,i.e.trabecular separation(Tb.Sp),trabecular thickness(Tb.Th),specific bone surface(BS/B V),bone volume fraction(BV/TV),structural model index(SMI),and degree of anisotropy(DA) were measured.Micro-finite element analyses were performed for each cube to obtain the apparent Young's modulus and tissue level von Mises stress distribution under 1%compressive strain along three orthogonal directions,respectively.Results revealed significant regional variations in the morphological parameters(P0.05).Young's moduli along the trabecular orientation were significantly higher than those along the other two directions.In general,trabecular mechanical properties in the medial region were lower than those in the lateral region.Trabecular mechanical parameters along the trabecular orientation were significantly correlated with BS/BV,BV/TV,Tb.Th,and DA.In this study,regional variations of microstructural features and mechanical properties in the femoral head of patients with proximal femur fracture were thoroughly investigated at the tissue level.The results of this study will help to elucidate the mechanism of femoral head fracture for reducing fracture risk and developing treatment strategies for the elderly.     

乘员股骨在轴向压力-弯矩下的损伤生物力学机理研究

汽车前碰撞事故中在冲击力作用下乘员股骨经常产生骨折创伤.为研究乘员股骨在不同的轴向压力-弯矩作用下的损伤机理及其耐受限度值,首先建立了一个较为精细的50百分位乘员的坐姿下肢有限元模型,并通过模拟股骨动态三点弯曲及下肢的轴向膝部冲击实验对模型的有效性进行了验证.在此基础上,针对股骨在轴向压力-弯矩载荷下的断裂失效分别进行了曲梁力学模型分析及有限元虚拟实验研究.结果表明:股骨骨折位置依赖于膝部轴向压力及弯矩的载荷大小的变化,在预加载弯矩从0增加到676Nm时,股骨失效部位由股骨颈部转移到股骨干末端区域;失效部位发生在颈部及股骨干时的最大力矩分别为285 296Nm和381443Nm.股骨损伤机理的分析结果阐释了在膝部轴向冲击实验中失效部位位于股骨颈部,而在汽车前碰撞事故中仍有大量的股骨干骨折出现的原因.      

Fluid Flow Analysis of Integrated Porous Bone Scaffold and Cancellous Bone at Different Skeletal Sites: In Silico Study

The dynamic characteristic of bone is its ability to remodel itself through mechanobiological responses. Bone regeneration is triggered by mechanical cues from physiological activities that generate structural strain and cause bone marrow movement. This phenomenon is crucial for bone scaffold when implanted in the cancellous bone as host tissue. Often, the fluid movement of bone scaffold and cancellous bone is studied separately, which does not represent the actual environment once implanted. In the present study, the fluid flow analysis properties of bone scaffold integrated into the cancellous bone at different skeletal sites are investigated. Three types of porous bone scaffolds categorized based on pore size configurations: 1 mm, 0.8 mm and hybrid (0.8 mm interlaced with 0.5 mm) were used. Three different skeletal sites of femoral bone were selected: neck, lateral condyle and medial condyle. Computational fluid dynamics was utilized to analyze the fluid flow properties of bone scaffold integrated cancellous bone. The results of this study reveal that the localization and maximum value of shear stress in an independent bone scaffold are significantly different compared to the bone scaffold integrated with cancellous bone by about 160% to 448% percentage difference. Low shear stress and high permeability were found across models that have higher Tb.Sp (trabecular separation). Specimen C and femoral lateral condyle showed the highest permeability in their respective category.

     

OPTIMAL DISTAL SCREW ALIGNMENT IN THE GAMMA NAIL

The effect of stress distribution due to the changes of the distal screw alignment in relation to the Gamma nail and the femoral shaft is thoroughly studied in this paper. Failure of the Gamma nail composite occurs through the cranial aperture of the distal screws and the insertion hole for the lag screw due to nonunion, delayed-union and continued weight-bearing. A three-dimensional finite element model was used to study the fractured femur, the Gamma nail, the lag screw and the distal locking screws. The first and the second distal screws were inserted into the Gamma nail in four different configurations. We found that the stress of the Gamma nail composite was substantially reduced with the two screws configured in the anterior to posterior direction. This alignment can bear greater loading in the more demanding fracture types. In the subtrochanteric fracture or the comminuted fractures at the proximal femur, the optimal alignment of the two distal screws was in the anterior to posterior direction.     

股骨形态及应力应变的三维有限元分析

建立了一个针对高危女性人群股骨结构的三维有限元模型,分析了运动荷载作用下此类股骨的应力和应变分布以及股骨破坏的可能部位.结果表明:运动荷载使股骨产生较大的横向位移和弯曲变形;股骨颈处的应力和应变最大,容易在股骨颈处以及大转子与股骨干的连接部位附近发生破坏.模拟结果为研制适用于此类人群的高性能人工股骨提供了参考.     

Finite Element Studies on the Role of Mechanical Loading in Bone Response Around Oral Implants*

Inappropriate implant loading has been suggested as one of the most important factors for late failures of endosseous oral implants. In order to quantify peri-implant bone stresses and strains for a controlled load applied during an animal experiment, an anatomical finite element model was built. Marginal bone loss that occurred during the animal experiment could only partially be related to overload. Furthermore, it is hypothesised that relative motion between bone and implant could have also played a role in the marginal bone response. In order to further explore the relation between marginal bone resorption and overload, a computer algorithm was developed that simulates overload-induced marginal bone loss. It was found that marginal bone resorption can be arrested due to a redistribution of the peri-implant bone stresses.     

Numerical and Experimental Models of the Mandible

This study aimed to validate a numerical model of an intact mandible for further development of a new TMJ implant. Numerical and experimental models of the biomechanics of the mandible were elaborated to characterize the human temporomandibular joint and to approach the development of a condyle implant. The model of the mandible was obtained through the use of a polymeric replica of a human cadaveric mandible and through 3D geometry acquisition. The three-dimensional finite element model was generated as a tetrahedral finite element mesh. The level of mesh refinement was established via a convergence test and a model with more than 50,000 degrees of freedom was required to obtain analysis accuracy. The functional loading cases included muscle loading in four different load boundary conditions. The same boundary conditions were applied to the experimental model. The strains were measured with an experimental procedure using electric resistance strain gauges applied on the external surface of the mandible. The mechanical response is shown and discussed in terms of strains, principal numerical and measured strains. This study proved that FE models of the mandible can reproduce experimental strains within an overall agreement of 10%. The FE models correctly reproduced bone strains under different load configurations and therefore can be used for the design of a novel TMJ implant considering other load configurations and bone mechanical properties.     

Comparison of uniaxial and triaxial rosette gages for strain measurement in the femur

A major problem of hip prostheses and other bone fixation devices is that they alter the stress distribution in the femur. This phenomenon is named stress shielding, and derives from the bigger stiffness of the implant when compared to the bone. Strain gages are the most common tool used in in vitro stress-shielding investigations. There is a lack of agreement in the literature about which gage type is to be preferred (uniaxial grid, biaxial or triaxial rosette). Rosettes are required when complete information is needed. However, uniaxial gages can be advantageous because they reduce the number of measurement channels required. The aim of this work is to verify if (and under which hypothesis) uniaxial gages can replace triaxial rosettes.     

Optimal internal architectures of femoral bone based on relaxation by homogenization and isotropic material design

The influence of the loading conditions on the trabecular architecture of a femur is investigated by using topology optimization methods. The response of the bone to physiological loads results in changes of the internal architecture of bone, reflected by a modification of internal effective density and mechanical properties. The homogenization based optimization model is developed for predicting optimal bone density distribution, wherein bone tissue is assumed to be a composite material consisting of a mixture of material and void. The homogenization scheme treats the geometric parameters of the microstructures and their orientation as design variables and homogenizes the properties in that microstructure, which is generally anisotropic. The penalization of the optimal material density then leads to a classical optimal structure which consists of regions with bone material and regions without bone material. The IMD (Isotropic Material Design) approach is next applied to determine the optimal elasticity tensor in terms of the bulk and shear moduli for the present loading applied to the femoral bone sample. IMD is able to provide both the external shape and topology together with the optimal layout of the isotropic moduli. Both topology optimization methods appear to be complementary. Simulations of the internal bone architecture of the human proximal femur results in a density distribution pattern with good consistency with that of the real bone.     

A new nonlinear multibody/finite element formulation for knee joint ligaments

The focus of this investigation is to study the mechanics of the human knee using a new method that integrates multibody system and large deformation finite element algorithms. The major bones in the knee joint consisting of the femur, tibia, and fibula are modeled as rigid bodies. The ligaments structures are modeled using the large displacement finite element absolute nodal coordinate formulation (ANCF) with an implementation of a Neo-Hookean constitutive model that allows for large change in the configuration as experienced in knee flexion, extension, and rotation. The Neo-Hookean strain energy function used in this study takes into consideration the near incompressibility of the ligaments. The ANCF is used in the formulation of the algebraic equations that define the ligament/bone rigid connection. A unique feature of the ANCF model developed in this investigation is that it captures the deformation of the ligament cross section using structural finite elements such as beams. At the ligament/bone insertion site, the ANCF is used to define a fully constrained joint. This model will reflect the fact that the geometry, placement and attachment of the two collateral ligaments (the LCL and MCL), are significantly different from what has been used in most knee models developed in previous investigations. The approach described in this paper will provide a more realistic model of the knee and thus more applicable to future research studies on ligaments, muscles and soft tissues (LMST). Current finite element models are limited due to simplified assumptions for the spatial and time dependent material properties inherent in the anisotropic and anatomic constraints associated with joint stability, and the static conditions inherent in the analysis. The ANCF analysis is not limited to static conditions and results in a fully dynamic model that accounts for the distributed inertia and elasticity of the ligaments. The results obtained in this investigation show that the ANCF finite elements can be an effective tool for modeling very flexible structures like ligaments subjected to large flexion and extension. In the future, the more realistic ANCF models could assist in examining the mechanics of the knee to study knee injuries and possible prevention means, as well as an improved understanding of the role of each individual ligament in the diagnosis and assessment of disease states, aging and potential therapies.     

Reversible strain gage

A reversible strain gage was developed for accurately measuring thermal strains, especially for use on large structures where strain gages cannot be welded. These strain gages can be peeled after taking required apparentstrain measurements in a furnace and can be attached reverse-side-up at the points of interest on a test structure. After many trials, a polyimide strain gage was developed that is the same on both the base side and the cover side. The thermal characteristics of the reversible strain gage—repeatability of apparent strain, gage-factor change, creep, drift and the output for a given mechanical strain—were investigated. The repeatability of apparent strains for 100 reversible gages was within 60 microstrain of difference at 250°C. The output of reversible gages for mechanical strain, after 2 to 3 heat cycles which were peeled and cemented in the reverse-side-up position, almost coincided with those of virgin reversible gages.Paper was presented at Fourth SESA International Congress on Experimental Mechanics held in Boston, MA on May 25–30, 1980     

各向同性材料中微裂纹的最大屏蔽效应

通过对微裂纹屏蔽不同来源的分析及计算,发现在各向同性脆性材料中,残余应力释放引起的微裂纹对主裂尖产生最大屏蔽效应时该微裂纹的倾角与最大张应力的方向没有明显的对应关系．在Hutchinson[1]所指出的屏蔽效应的第二个来源中,还应计及微裂纹形成引起的远场应力在微裂纹处产生的应力场的释放从而导致应力场的再分布．     

Analytical elasto-creep model of interfacial thermal stresses and strains in trilayer assemblies

A two-dimensional model has been developed for thermal stresses, elastic strains, creep strains, and creep energy density at the interfaces of short and long trilayer assemblies under both plane stress and plane strain conditions. Both linear (viscous) and non-linear creep constitutive behavior under static and cyclic thermal loading can be modeled for all layers. Interfacial stresses and strains are approximated using a combination of exact elasticity solutions and elementary strength of materials theories. Partial differential equations are linearized through a simple finite difference discretization procedure. The approach is mathematically straightforward and can be extended to include plastic behavior and problems involving external loads and a variety of geometries. The model can provide input data for thermal fatigue life prediction in solder or adhesive joints. For a typical solder joint, it is demonstrated that the predicted cyclic stress–strain hysteresis shows shakedown and a rapid stabilization of the creep energy dissipation per cycle in agreement with the predictions of finite element analysis.     

A modified hole-drilling technique for determining residual stresses in thin plates

This paper presents a modified hole-drilling technique for measuring residual stresses in sheet and thin-plate materials. The primary advantage of the modification is that it eliminates the necessity for calibration of each experimental hole-gage assembly. The relaxation coefficients are calculated from theory, and the strain components which are extraneous to the true relaxation strains are determined and separated from the measured relaxation strains. Experiments were conducted on 0.050-in. (1.27-mm) and 0.125-in. (3.175-mm)-thickness aluminum-alloy specimens. Sources of extraneous strain components are analyzed and values for these strain components resulting from machining residual stresses and localized plastic yielding are determined. Finally, the recommended range of the nondimensional ratio of hole diameter to distance between hole center and strain-gage center is determined by the maximum permissible error in residual-stress estimates. The modified technique appears to be accurate within ±5 percent or better and is, therefore, comparable in precision with the X-ray technique.