角膜黏弹性对喷气试验的影响

目的 探索不同条件下角膜黏弹性在喷气试验中的作用。方法 基于患者角膜地形图数据构建个性化全眼模型,加载喷气模型,分析不同条件下线弹性和黏弹性角膜材料喷气试验结果的差异。结果 在角膜刚度、眼内压、巩膜刚度都相同的情况下,线弹性材料的角膜顶点最大位移比黏弹性材料小0.01～0.03 mm;当角膜刚度和巩膜刚度一定,眼内压从12 mmHg增大到24 mmHg(1 mmHg=0.133 kPa)时,线弹性材料的角膜顶点最大位移绝对值的下降速率比黏弹性材料慢0.9μm/mmHg;当角膜刚度和眼内压一定,巩膜刚度从1.24 MPa变化到9.92 MPa时,线弹性材料的角膜顶点最大位移绝对值的下降速率比黏弹性材料慢1.1μm/MPa;当巩膜刚度和眼内压一定,角膜刚度0.827 MPa变化到2.790 MPa时,线弹性材料的角膜顶点最大位移绝对值的下降速率比黏弹性材料慢8μm/MPa。结论 喷气试验中的角膜顶点位移主要由角膜组织的弹性驱动,角膜黏弹性在喷气试验中的作用不明显。在临床上采用喷气法评估角膜生物力学响应时,可忽略角膜黏弹性影响。     

脂肪组织压缩实验中摩擦系数对力学响应的影响

目的针对当前摩擦力对脂肪组织无约束压缩实验结果影响的不确定性,研究压缩实验中合适的摩擦系数设置及适用于模拟脂肪组织生物力学响应的材料本构模型。方法构建低应变率(0.2s^-1)和中应变率(20s^-1)下的脂肪组织有限元模型,分别应用LS-DYNA中常用于模拟脂肪组织的线性黏弹性材料本构、Mooney-Rivlin超弹性材料本构、Ogden超弹性材料本构、软组织材料本构,在不同摩擦系数下进行无约束压缩实验,分析不同摩擦系数及本构模型对接触力大小的影响。结果4种材料本构模型在低、中应变率下,输出的接触力均与摩擦系数呈正相关,有摩擦时的接触力比无摩擦时的接触力大50%左右。中应变率下脂肪组织的力学响应对摩擦系数的灵敏度比低应变率下的更高,且不同材料本构模型输出的接触力差异显著。结论在脂肪组织无约束压缩实验中,静摩擦系数取0.1,动摩擦系数取0.05是合理的,在低、中应变率下Ogden超弹性材料本构能够良好地反映脂肪组织的生物力学响应。     

大腿残肢步态过程的非线性有限元分析

目的利用三维有限元分析方法研究大腿截肢患者在行走过程中3个不同时相下残肢的生物力学特性,为建立完整的大腿接受腔测量、设计与评估系统提供研究基础。方法首先根据CT图像三维重建大腿截肢患者的骨骼、肌肉软组织和接受腔的三维几何模型;定义软组织为超弹性和线弹性材料属性,并相应建立两个有限元仿真模型;定义残端与接受腔之间的接触关系,约束残肢近端,对模型的远端施加膝关节载荷,模拟步态周期中足跟着地时期、站立相中期、脚尖离地3个时相下大腿残肢-接受腔系统所受载荷;计算分析接触界面上的应力,并对比分析超弹性和线弹性软组织力学特性对接触界面力学行为特性的影响。结果无论线弹性还是超弹性模型,3个时相下大腿残肢-接受腔界面的最大接触压力均在残肢末端达到最大值。超弹性模型3个时相下接触压力峰值分别为55.80、47.63和50.44 kPa;而线弹性模型接触压力的最大值都增加2倍以上,其值分别为149.86、118.55和139.68 kPa。同时通过分析接触面间的径向剪切应力和轴向剪切应力发现,3个时相下接触界面间的应力在残肢末端较集中,在足跟着地到脚尖离地过程中,有部分力通过接受腔后侧缘传递转向接受腔前缘传递。结论不同时相下残肢与接受腔接触界面的压力和剪切应力分布情况不同,在设计接受腔时需要充分考虑其受力特点。     

颅-脑在头部受直接撞击时相对位移的模拟研究

颅脑相对运动所产生的相对位移是造成头部损伤的原因之一。以往的研究均是采用真实实验的方法，这样做不仅成本提高，而且也会引起一些社会伦理问题。本研究利用二维的颅脑弹簧．质量系统模型和2个二维颅脑有限元模型，模拟了颅脑在受到撞击时所产生的相对位移。在这些模型中，脑、颅骨通过脑脊液耦合成为一个整体。其中，在弹簧-质量系统模型中，脑脊液以弹簧和阻尼组成的黏弹性系统模拟；而在有限元模型中，脑脊液以线弹塑性模拟。3个模型都有效地求得了与实验相吻合的颅脑最大相对位移。对有限元模型与弹簧-质量系统模型的模拟结果也进行了比较分析。最终得出：在受到外界冲击时，颅脑之间产生的相对位移不超过5mm。     

基于可行方向法的脂肪组织本构模型参数反求优化

目的研究脂肪组织在中等应变率下本构模型及其参数反求。方法基于脂肪组织力学性能实验,通过有限元方法重构脂肪组织压缩实验,并对常见表征脂肪组织的本构模型进行参数筛选。结合最优化方法中的可行方向法(method of feasible direction,MFD),进行中应变率下脂肪组织本构模型相关参数的反求。结果中应变率(260 s~(-1))下黏弹性本构模型相比Ogden本构模型更适合表征脂肪组织的力学响应,并反求得到适用于仿真的本构模型参数。结论中等应变率下黏弹性本构模型更适合表征脂肪组织力学响应。研究结果为汽车碰撞有限元仿真中探究人体脂肪组织对人体损伤的影响提供参考。     

成骨细胞的体外剪切应力加载实验的变形分析及信号转导区域探讨

目的 根据已有体外培养鼠成骨细胞的参数实验数据,估算剪切应力加载实验中细胞整体剪切形变,借以研究细胞的主要转导区域.方法 计算过程采用黏弹性力学理论,对细胞运用了标准黏弹性模型,并简化其膜所受剪切力为均匀.结果 细胞剪切力产生的细胞变形大约是引起成骨细胞相同生物学响应的拉伸加载变形的十分之一.结论 从细胞总的力学刺激生物学响应来看,剪切应力加载实验中细胞的整体变形所产生的力学转导是可以忽略的,主要转导区域在承受剪切应力的细胞膜.     

利用三维有限元分析根尖切除对上颌中切牙生物力学性能的影响

目的研究根尖切除手术后牙体及牙周应力分布的变化以及牙齿的位移,为临床医师实施根尖手术提供数据支持,提高根尖手术治愈率。方法基于Micro CT图像数据建立正常上颌中切牙及其牙周组织的三维有限元模型。在此基础上仿真根尖周炎和根切治疗手术,并建立根尖周炎及不同根切长度(3、4、5、6、7、8 mm)上颌中切牙及其支持组织的三维有限元模型,施加咬合力,通过三维有限元仿真分析,研究术后愈合牙齿的生物力学行为特性,对比分析手术修复的生物力学效果,得到最佳的根尖切除长度。结果根尖切除手术术后,愈合模型降低患牙的应力水平(26.8%)及牙齿动度(7.3%);随着根切长度的增加,根切达8 mm时,牙齿颈部和牙周膜的应力分别增加11.14%和29.27%,牙槽骨的应力增加83.11%,切面形成的新的根尖处应力相较于正常牙齿相同部位整体上升;牙齿沿长轴的位移也逐渐增大,当根切超过5 mm时,位移水平明显增大(18.39%)。结论根尖切除手术对于难治性根尖周炎患牙的生物力学特性有明显改善,建议临床手术中根尖切除长度范围为3～5 mm,冠根比不低于0.84。     

Suture Endobutton与跖肌腱重建Lisfranc韧带的生物力学对比研究

目的通过Suture Endobutton与跖肌腱重建Lisfranc韧带的生物力学对比研究,评价两种方法对恢复Lisfranc关节稳定性的作用,为临床治疗Lisfranc损伤提供理论依据。方法选取10例成人新鲜尸体小腿标本,依次制备Lisfranc韧带完整模型、损伤模型及损伤后的Suture Endobutton或跖肌腱重建模型,并分别给予轴向或外展载荷,记录内侧楔骨(C1)~第2跖骨(M2)基底的平均位移变化。结果轴向载荷和外展载荷下,完整模型C1~M2的平均位移变化分别为(0.70±0.05)mm、(1.21±0.10)mm,损伤模型为(1.59±0.07)mm、(3.73±0.11)mm,Suture Endobutton重建模型为(0.84±0.04)mm、(1.29±0.06)mm,跖肌腱重建模型为(1.01±0.05)mm、(1.34±0.05)mm。轴向或外展载荷下,完整模型和重建模型C1~M2间位移变化明显小于损伤模型,差异具有统计学意义(P0.05);重建模型的位移变化略大于完整模型,但差异无统计学意义(P0.05);Suture Endobutton与跖肌腱重建模型之间位移变化相似,差异无明显统计学意义(P0.05);任一模型外展载荷时的C1~M2位移变化大于轴向载荷。结论 (1)在恢复Lisfranc关节稳定性上,Suture Endobutton与跖肌腱的重建方法提供的生物力学强度相似;(2)相对于轴向载荷的位移变化,外展载荷的实验结果差异更加明显,推荐使用外展载荷进行Lisfranc关节相关力学试验。     

基于整体角膜膨胀实验对兔眼角膜参数的确定

目的角膜一般被视为非线性弹性材料,用线弹性模型刻画角膜的力学特性难免存在一定误差。本研究基于体外兔眼角膜膨胀实验,探索用分段的线弹性模型描述角膜非线性弹性行为的可行性。方法选取5只新鲜兔眼,制备附带巩膜环的角膜试件,将试件固定在自制的加压装置上,用微量注射泵给试件加压以模拟角膜随内压力的增加发生形变的过程,同时利用显微镜获取相应压力下角膜的正侧面轮廓图像。预调后,记录加载过程中压力与角膜顶点及轮廓信息。分别选取角膜内压力为7mm Hg、14 mm Hg和21 mm Hg时的角膜正侧面轮廓图像,建立具有不均匀厚度的轴对称角膜几何模型,将角膜视为各向同性不可压缩材料,在7~14 mm Hg、14~21 mm Hg和21~28 mm Hg三个眼内压范围内视角膜为线弹性材料,通过有限元方法分段模拟体外角膜膨胀实验,将模拟结果与膨胀实验角膜外轮廓线位移结果比对,确定角膜在上述压力范围内的弹性模量。结果获得的兔眼角膜在7~14 mm Hg、14~21 mm Hg和21~28 mm Hg的弹性模量分别为(0.73±0.16)MPa、(2.20±0.43)MPa和(3.03±0.37)MPa。此结果与文献结果基本一致。结论用分段的线弹性模型近似描述角膜非线性弹性行为的方法是可行的。     

髂股韧带与股骨头韧带黏弹性实验研究

目的 研究新鲜成人尸体髋关节髂股韧带和股骨头韧带的黏弹性力学性质,为临床提供生物力学参数.方法 取正常国人髂股韧带和股骨头韧带各10个试样进行应力松弛、蠕变实验.结果 得出了应力松弛、蠕变实验曲线,以回归分析的方法处理实验数据,得出了回归系数.结论 髂股韧带7200s应力松弛蠕变量显著大于股骨头韧带.     

Factors influencing the accuracy of biomechanical breast models

Recently it has been suggested that finite element methods could be used to predict breast deformations in a number of applications, including comparison of multimodality images, validation of image registration and image guided interventions. Unfortunately knowledge of the mechanical properties of breast tissues is limited. This study evaluated the accuracy with which biomechanical breast models based on finite element methods can predict the displacements of tissue within the breast in the practical clinical situation where the boundaries of the organ might be known reasonably accurately but there is some uncertainty on the mechanical properties of the tissue. For two datasets, we investigate the influence of tissue elasticity values, Poisson's ratios, boundary conditions, finite element solvers and mesh resolutions. Magnetic resonance images were acquired before and after compressing each volunteer's breast by about 20%. Surface displacement boundary conditions were derived from a three-dimensional nonrigid image registration. Six linear and three nonlinear elastic material models with and without skin were tested. These were compared to hyperelastic models. The accuracy of the models was evaluated by assessing the ability of the model to predict the location of 12 corresponding anatomical landmarks. The accuracy was most sensitive to the Poisson's ratio and the boundary condition. Best results were achieved for accurate boundary conditions, appropriate Poisson's ratios and models where fibroglandular tissue was at most four times stiffer than fatty tissue. These configurations reduced the mean (maximum) distance of the landmarks from 6.6 mm (12.4 mm) to 2.1 mm (3.4 mm) averaged over all experiments.     

Biomechanical consequences of progressive marginal bone loss around oral implants: a finite element stress analysis

The purpose of this study was to explore the biomechanical effects of progressive marginal bone loss in the peri-implant bone. Finite element model of a Ø 4.1 × 10 mm Straumann dental implant and a solid abutment was constructed as predefined eight-layers around the implant neck. The implant-abutment complex was embedded in a cylindrical bone model to analyze bone biomechanics regardless of anatomical influences. Angular and circular progressive marginal bone loss was simulated by sequential removal of each layer, resulting crater-like defects and a total of ten finite element models for analysis. Each model was subjected to a vertical and oblique static load of 100 N in separate load cases. Principal stress minimum and maximum, displacement, and equivalent of elastic strain outcomes were compared. Under vertical loading, principal stresses minimum and maximum decreased remarkably as with the increase in bone resorption. Under oblique load simulations, decrease in principal stress maximum and minimum was evident. With progressive bone loss and under oblique load simulations, displacement and equivalent of elastic strain increased considerably in trabecular bone contacting the implant neck. The presence of cortical bone contacting a load-carrying implant, even in a bone defect, improves the biomechanical performance of implants in comparison with only trabecular bone support as a sequel of progressive marginal bone loss.     

Nonlinear finite element analysis of the vibration characteristics of the maxillary central incisor related to periodontal attachment

The vibration characteristics of a maxillary central incisor were investigated by using the finite element method (FEM) according to nonlinear behavior of the human periodontal ligament (PDL). The effect of alveolar bone loss was also studied to obtain the relationship between the vibration property of the tooth in the periodontal system and the level of periodontal attachment for assessing the condition of periodontium. Three-dimensional (3D) finite element model of the tooth was constructed using CT image-reconstruction, and the elastic face foundation constraint was applied to the surface of the tooth root where the PDL was attached to. Modal analysis was performed by using FEM. The nonlinear behavior of the PDL was assigned and approached by the piecewise linearized method. The results indicated that the vibration of the maxillary central incisor in the periodontal system could be described by several modal frequencies and modes. The first four modes were dominant, which varied with the deformation of the PDL or the force applied on the tooth. The vibration frequency of the maxillary central incisor decreased with the losing of the alveolar bone, but the ratio of decrease had no significant correlation with the nonlinear behavior of the human PDL. The vibration frequency of the maxillary central incisor can be used to describe the loss of the alveolar bone and the level of periodontal attachment, under physiological short-term loading.     

Biomechanical analysis of the keratoconic cornea

Keratoconus is a non-inflammatory disease characterized by irregular thinning and gradual bulging of the cornea, which results in distortion of the corneal surface that causes blurred vision. We conducted three-dimensional finite element (FE) simulations to analyze the biomechanical factors contributing to the distorted shape of a keratoconic cornea. We assumed orthotropic linear elastic tissue mechanical properties, and simulated localized tissue thinning (reduction from 0.5 mm to 0.35 or 0.2 mm). We analyzed tissue deformations, stresses and theoretical dioptric power maps predicted by the models, for intraocular pressure (IOP) of 10, 15 20 and 25 mmHg. The analyses revealed that three factors affect the shape distortion of keratoconic corneas: (i) localized thinning, and (ii) reduction in the tissue’s meridian elastic modulus or (iii) reduction in the shear modulus perpendicular to the corneal surface, whereas thinning showed the most predominant effect. Maximal stress levels occurred at the centers of the bulged regions, at the thinnest points. The IOP levels had little influence on dioptric power in the healthy cornea, but a substantial influence in keratoconic conditions. The present FE studies allowed characterization of the biomechanical interactions in keratoconus, toward understanding the aetiology of this poorly studied malady.     

具有详实解剖学结构的国人第5百分位女性胸腹部有限元模型开发及验证

目的 预测与评估汽车碰撞中小身材女性胸腹部的生物力学响应及损伤机制。方法 基于国人第5百分位女性志愿者CT图像,提取精确的胸腹部几何轮廓,借助相关软件构建具有详实解剖学结构的国人第5百分位女性胸腹部有限元模型,并重构3组胸腹部尸体实验,即胸部正面撞锤冲击实验、腹部正面棒击实验和胸腹部侧面撞锤冲击实验,对模型进行有效性验证。结果 仿真实验所得接触力-变形量曲线及胸腹部组织器官损伤生物力学响应与尸体实验结果吻合,验证了模型的有效性。结论 该模型可用于小身材女性胸腹部损伤机制的研究,也能应用于小身材女性乘员约束系统的研发及司法鉴定案例分析,并为开发中国体征第5百分位女性整人有限元模型奠定基础。     

Toward efficient biomechanical-based deformable image registration of lungs for image-guided radiotherapy

Both accuracy and efficiency are critical for the implementation of biomechanical model-based deformable registration in clinical practice. The focus of this investigation is to evaluate the potential of improving the efficiency of the deformable image registration of the human lungs without loss of accuracy. Three-dimensional finite element models have been developed using image data of 14 lung cancer patients. Each model consists of two lungs, tumor and external body. Sliding of the lungs inside the chest cavity is modeled using a frictionless surface-based contact model. The effect of the type of element, finite deformation and elasticity on the accuracy and computing time is investigated. Linear and quadrilateral tetrahedral elements are used with linear and nonlinear geometric analysis. Two types of material properties are applied namely: elastic and hyperelastic. The accuracy of each of the four models is examined using a number of anatomical landmarks representing the vessels bifurcation points distributed across the lungs. The registration error is not significantly affected by the element type or linearity of analysis, with an average vector error of around 2.8 mm. The displacement differences between linear and nonlinear analysis methods are calculated for all lungs nodes and a maximum value of 3.6 mm is found in one of the nodes near the entrance of the bronchial tree into the lungs. The 95 percentile of displacement difference ranges between 0.4 and 0.8 mm. However, the time required for the analysis is reduced from 95 min in the quadratic elements nonlinear geometry model to 3.4 min in the linear element linear geometry model. Therefore using linear tetrahedral elements with linear elastic materials and linear geometry is preferable for modeling the breathing motion of lungs for image-guided radiotherapy applications.     

The mechanical function of the periodontal ligament in the macaque mandible: a validation and sensitivity study using finite element analysis

Whilst the periodontal ligament (PDL) acts as an attachment tissue between bone and tooth, hypotheses regarding the role of the PDL as a hydrodynamic damping mechanism during intraoral food processing have highlighted its potential importance in finite element (FE) analysis. Although experimental and constitutive models have correlated the mechanical function of the PDL tissue with its anisotropic, heterogeneous, viscoelastic and non‐linear elastic nature, in many FE simulations the PDL is either present or absent, and when present is variably modelled. In addition, the small space the PDL occupies and the inability to visualize the PDL tissue using μCT scans poses issues during FE model construction and so protocols for the PDL thickness also vary. In this paper we initially test and validate the sensitivity of an FE model of a macaque mandible to variations in the Young’s modulus and the thickness of the PDL tissue. We then tested the validity of the FE models by carrying out experimental strain measurements on the same mandible in the laboratory using laser speckle interferometry. These strain measurements matched the FE predictions very closely, providing confidence that material properties and PDL thickness were suitably defined. The FE strain results across the mandible are generally insensitive to the absence and variably modelled PDL tissue. Differences are only found in the alveolar region adjacent to the socket of the loaded tooth. The results indicate that the effect of the PDL on strain distribution and/or absorption is restricted locally to the alveolar bone surrounding the teeth and does not affect other regions of the mandible.     

戴用口腔矫治器后阻塞性睡眠呼吸暂停低通气综合征患者舌骨力学的有限元分析

背景：采用三维有限元对下颌骨、气道、舌骨等共同建模进行研究的报道较少。 目的：通过建立下颌骨、气道、舌骨以及周围肌肉组织的模型,模拟戴用矫治器使下颌前伸改变气道形态的状况,在下颌骨上进行前伸加载,分析舌骨的生物力学表现。 方法：选取1名确诊为阻塞性睡眠呼吸暂停低通气综合征的男性患者,螺旋CT扫描眼眶下缘至甲状软骨图像,导入Mimics10.01软件分别提取各组织,Imageware10再对得到的文件点云进行处理,由ANSYS 10.0等软件生成三维有限元模型。在下颌骨上分别加载2,4,6,8 mm的前伸量,观察舌骨的生物力学表现。 结果与结论：上气道的应力主要集中在软腭及口咽部,舌骨上的应力主要集中在与下颌、气道相连的肌肉处。随着下颌前伸量加大,应力大小随伸长量增加而增大,舌骨随着肌肉的牵拉主要沿前上方移动。结果说明三维有限元法能够建立具有较高几何相似性及力学相似性的模型,是阻塞性睡眠呼吸暂停低通气综合征病理研究方法的扩充。     

角膜的弹性模量非均匀分布对其光学相干弹性成像的影响

目的 分析激励源位置、检测区域长度和检测深度对人眼角膜的剪切波光学相干弹性成像(optical coherence elastography, OCE)的影响。方法 结合人眼角膜的弹性模量的实际分布情况,构建角膜弹性模量非均匀分布的有限元模型。通过模拟剪切波OCE实验,对比分析有限元模拟结果和理论结果。结果 当激励源位置不同时,角膜前后基质的剪切波波速误差不同;当检测区域长度不同时,角膜前后基质的剪切波波速非线性变化;在超弹性材料模型下,当检测深度不同时,剪切波波速明显变化。结论 由于角膜弹性模量非均匀分布,在角膜前后基质不同激励源位置、不同检测区域长度和不同检测深度的有限元模拟剪切波波速的结果不同。将具有非均匀性的生物组织视作均质进行OCE实验会影响结果准确性。