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。     

膨胀试验测定猪眼角膜生物力学参数的研究

目的 通过膨胀试验测定猪眼角膜的非线性应力/应变关系,研究猪眼角膜弹性模量和眼内压之间的关系。方法 选取新鲜猪眼11只,剪下附带4~5mm宽度巩膜环的角膜,采取胶水黏附联合机械固定方式将猪眼角膜固定于自制的施压装置,缓慢逐步增加液压,液压范围：25mmH2O(1.8375mmHg) ~ 1000mmH2O (73.5mmHg),激光位移计记录角膜前表面顶点的前突位移量,根据板壳理论推导公式计算猪眼角膜的生物力学参数,包括应力、应变及弹性模量。结果 猪眼角膜前突位移量同眼内压的关系及应力/应变均呈指数函数关系,弹性模量与眼内压呈线性关系。结论 膨胀法测量猪眼角膜的弹性模量具有较好的重复性和精确性,此方法所测得的弹性模量可用于后期屈光手术模型的建立、人工角膜与角膜接触镜设计及其它方面的研究。     

压平式非破坏性技术在体测量角膜弹性模量与蠕变力学性能

临床由于缺少在体眼角膜生物力学性能参量检测技术,眼科医师对生理环境眼内压作用下角膜变形机制的理解存在一些问难。依据眼科临床采用的压平式眼内压测量时角膜受力变形情况,运用Young理论分析的角膜弹性模量与角膜变形压平面积、眼压测量值及真实眼内压之间的关系,提出将一项可同时动态获取角膜变形压平面积、角膜压平力和角膜变形压平位移的新型角膜变形压平眼压式测量技术,发展应用于角膜生物力学性能参数检测。在体兔眼球角膜的实验结果证明该技术能成功实现角膜弹性模量及黏弹蠕变力学行为的非破坏性测量。     

腹主动脉瘤直型覆膜支架的生物力学分析

应用数值模拟分析方法,研究腹主动脉瘤直型覆膜支架在径向压握、自膨胀释放及植入后平衡状态3种工况下的稳定性及各项生物力学指标。建立覆膜支架、压握工具以及目标血管的有限元模型。对圆柱状压握工具沿R轴负方向施加大小为6.50 mm的位移载荷,压握覆膜支架至外径为7.00 mm;将压握工具恢复到原始尺寸,建立血管/覆膜支架接触关系;支架完全释放后平衡状态下,对其内表面均匀施加50～150 mmHg动脉压。分析支架部分在压握与平衡状态下的最大主应变(MPS)峰值及分析后形态、释放状态下变形血管以及覆膜的等效应力峰值(VMS)。在径向压握过程中,金属支架最大压握主应变峰值为5.73%;在自膨胀释放过程中,造成血管壁应力集中的峰值为0.371 MPa,覆膜应力峰值为0.388 MPa;在植入后平衡状态下,支架平均应变为0.0859%,振荡应变为0.0486%,覆膜应力峰值为2.09 MPa,安全因子为8.23。支架部分在各工况下应变处于镍钛合金屈服强度之内,在圆角弯折处应变最为集中;覆膜部分在各工况下应力值也满足e PTFE膜材料的屈服强度。该研究结果可以为覆膜支架的结构优化设计以及覆膜材料选择提供一种分析方法, 可以提高覆膜支架的生物力学性能,并给工程设计和临床操作提供参考。     

坐姿下不同组织压力性损伤生物力学研究

目的 探讨坐姿下臀部压力性损伤易发部位以及不同软组织的生物力学响应,为有效预防深层组织压力性损伤提供参考。 方法 基于臀部 CT 扫描数据,建立坐位臀部有限元模型,包括骨骼、肌肉、脂肪和皮肤组织及坐垫模型,利用生死单元模拟组织损伤。 对比实验坐垫界面压力测量数据与有限元模拟结果,验证模型有效性。 模拟坐位力学状态,研究软组织的应力、应变情况,分析不同软组织中的压应力及超出极限值后可能造成的损伤情况。结果 通过对比坐垫模型仿真结果与实验界面压力测量结果,证明模型有效。 坐位时坐骨结节下方软组织区域出现应力集中现象。 其中,臀大肌组织中的横向压应力峰值约为 38 kPa,剪切应力峰值约为 3. 4 MPa;而脂肪组织中的最大压应力与剪切应力峰值分别为 22 kPa 与 4. 5 MPa,均未出现在坐骨结节正下方。 结论 软组织受到一定时间和大小的压力载荷作用,可能出现深层组织损伤。 当保持坐姿一定时间后,应及时变换体位,以降低压力性损伤出现的概率。 研究结果为预防压力性损伤提供生物力学依据,具有重要的临床研究价值。     

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。     

顿挫期圆锥角膜拟锥体参数与角膜动力学响应参数的关系

目的 探索圆锥角膜拟锥体的中心位置和力学性质对角膜动力学响应参数的影响。方法 基于90例顿挫期圆锥角膜(forme fruste keratoconus, FFKC)患者三维眼前节分析诊断系统给出的角膜形态数据,用锥体自动获取方法确定其拟锥体参数。依据所得拟锥体中心位置选取两组(水平组21例,下方组20例),分析组间角膜动力学响应(dynamics corneal response, DCR)参数的差异。基于健康眼球几何数据构建具有3种不同锥体位置的圆锥角膜有限元模型,改变锥体区域的力学参数,计算给出在空气脉冲作用下DCR参数的模拟结果,用于研究角膜锥体区域的位置和力学参数同DCR参数的关系。结果 FFKC的拟锥体中心位置不同,在DCR参数中,最大压陷曲率半径(radius)、最大凹陷曲率(maximum inverse radius, MIR)和积分半径(integration radius, IR)存在统计学差异。当FFKC拟锥体中心位于角膜旁中心区域时,只有与拟锥体区域呈中心对称的区域的角膜厚度存在统计学差异。有限元模拟结果显示,锥体区域位于角膜中心时,角膜顶点最大位移改变量最大...     

兔胸腹部爆炸伤后不同器官微血管通透性的变化

目的探讨爆炸性冲击伤后不同组织器官微血管通透性变化的特点及其可能的机理。方法采用雷管对兔的爆炸性冲击损伤模型 ,以及 125I-白蛋白标记的方法 ,观察爆炸性冲击波对兔心、脑、肺、肾、肝组织中的微血管通透性的影响。结果距爆源20cm处的爆炸冲击波平均压力峰值为174 .4kPa,可引起红细胞压积增加为14.9 % (与对照组相比 ) ,血浆丢失为对照的5.12倍 ;距爆源10cm时 ,爆炸冲击波的压力峰值可陡增至1000kPa以上 ,所致白蛋白漏出率为伤前的1.5倍 ,心、脑、肺、肾组织中的残留放射性较对照组增加约1 .6～1.9倍。结论爆炸冲击波能引起红细胞压积明显增加 ,血浆丢失 ,以及心、脑、肺、肾组织中微血管通透性的增加。     

眼部传输及生物流体力学

眼睛是光转换的视觉器官,因此通常不考虑将其作为一个生物力学结构。然而眼睛是眼内肌和眼外肌构成的压力系统,同时复杂的眼内血管系统构成流体和溶合物的传导系统。眼睛的生物力学研究包括很多内容,有近期和远期问题,以及常见的病症诸如青光眼、黄斑变性、视网膜病变、近视和老花眼等病症。该综述中将介绍有关眼睛的生物力学和传输问题,如眼内的血液流动,房水动力学,眼角膜的传输,透过Bruch膜的传输,巩膜的渗透性及药物在眼中的传输等问题,并提出了有待解决的一些问题。     

门静脉高压症猪门静脉的生物力学特性及其临床意义

目的:建立猪门静脉高压症模型,探讨门静脉高压症时门静脉的生物力学特性。方法:采用2月龄湖北白种猪,用四氯化碳、苯巴比妥、乙醇,配合高脂、低蛋白、低胆碱饮食进行混合饲养。通过脾静脉插管测压,取门静脉在生物软组织力学试验机上测定其压力-直径关系,横断取材,冰冻切片,H E法染色,用计算机图像分析系统测量其几何形态学指标。结果:实验组门静脉压为(4.17±1.03)kPa,对照组为(1.51±0.79)kPa(P<0.01),实验组门静脉的Einc、Ep和EV均随压力的上升而增大,在相同压力下明显大于对照组的Einc、Ep和EV。在0～4 kPa压力范围内实验组门静脉的顺应性(C)显著低于对照组,而在4～8 kPa的高压时两者顺应性差异并不明显(P>0.05)。结论:门静脉高压症时,门静脉的生物力学特性均发生了明显变化。肝移植时,移植材料间的生物力学特性也应考虑。     

Optical coherence tomography measurements of the fresh porcine eye and response of the outer coats of the eye to volume increase

Corneal and scleral thickness and anterior chamber dimensions are required for understanding developmental and pathological processes. Parameters of the eyeball are also required to calculate optical and material properties. As the eyeball resembles a pressure vessel, it has been suggested that elasticity of the cornea and sclera could be calculated from the measurements of thickness. Baseline corneal and scleral thicknesses and anterior chamber dimensions and how these change with incremental increases of intraocular fluid are measured in fresh porcine eyes using the Visante OCT (optical coherence tomography). At baseline, corneal thickness is almost constant. Anterior scleral thickness is variable, decreasing from 0.91+/-0.07 mm near the limbus to a minimum of 0.58+/-0.13. Posterior scleral thickness is more constant with an average of 0.78+/-0.09 mm. Near the optic nerve the thickness increases to 1.00+/-0.09 mm. Average baseline anterior chamber angle, diameter, and depth were found to be 33.15+/-4.91 deg, 13.60+/-0.38 mm, and 2.13+/-0.22 mm, respectively. After fluid injections, maximum changes in corneal and scleral thicknesses were 9 to 10 and 1 to 3%, respectively. Anterior chamber angle and depth decreased slightly but significantly. Changes in the eyeball coats with fluid injections, indicate that the pressure vessel model can be applied to the eye to calculate corneal and scleral elasticities.     

Full-field deformation of bovine cornea under constrained inflation conditions

The viscoelastic response of bovine corneas was characterized using in vitro inflation (bulge) experiments combined with spatially-resolved deformation mapping via digital image correlation. A complex fixture conforming to the limbal annulus was developed to hold the attached sclera rigid while allowing deformation only in the cornea. A statistical set of experiments was performed for a pressure range of 3.6-8 kPa (27-60 mmHg), representing nominal bovine intraocular pressure (IOP) to acute glaucoma conditions. A broader pressure range of 0-32 kPa (0-240 mmHg) was also examined to characterize the nonlinear finite deformation behavior of the tissue. Results showed that for pressures near and above IOP, the majority of the deformation was localized in the limbus and peripheral regions, which left the central cornea largely undeformed. This observation was consistent with the known preferred circumferential alignment of collagen fibrils outside of the central cornea. In general, the inflation experiments observed viscoelastic behavior in the form of rate-dependent hysteresis in the pressure-deformation response of the apex of the cornea, creep in the apex deformation at a constant inflation pressure, and relaxation in the pressure response at a constant inflation volume. The 3.6-8 kPa (27-60 mmHg) pressure range produced small viscoelastic deformations and a nearly linear pressure-deformation response, which suggests that for physiological pressure ranges, the cornea can be approximated as a linear viscoelastic or linear pseudo-elastic material.     

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.     

负压吸附式角膜夹持器设计与吸附参数仿真研究

目的 提出一种通过负压将角膜吸附提升至合适缝合深度的高度后进行缝合的方案,并对吸附参数进行仿真研究。方法 根据手术要求,设计负压吸附式角膜夹持器。建立不同长度、深度及位置的直型伤口角膜的吸附仿真模型,针对6种负压条件,利用流固耦合分析对负压吸附角膜过程进行仿真,求解角膜变形、应力及吸附力。在PDMS薄膜上测量6种负压条件所产生的吸附力,并与仿真结果对比分析。同时在离体猪眼球上进行吸附实验,评估所设计角膜夹持器的操作安全性。结果 -70～-40 kPa负压下,所有仿真模型角膜最大变形均大于角膜缝合深度要求的伤口最大提升高度。最大变形随负压减小而增大。所有仿真模型角膜最大应力均小于角膜基质破坏强度(5 MPa)。吸附力仿真值相对实验值比较吻合。负压高于-70 kPa时所设计夹持器进行吸附操作时角膜未出现破裂。结论 该角膜夹持器在-70～-40 kPa负压下能实现角膜缝合深度要求,且可通过改变负压精确调控缝合深度,高于-70 kPa时进行吸附操作不损伤角膜组织。在该角膜夹持器辅助下,医生可使用直针完成角膜缝合操作,降低临床操作的难度。     

A Finite Element Model for Ultrafast Laser–Lamellar Keratoplasty

A biomechanical model of the human cornea is employed in a finite element formulation for simulating the effects of Ultrafast Laser–Lamellar Keratoplasty. Several computer simulations were conducted to study curvature changes of the central corneal zone under various physiological and surgical factors. These factors included the combined effect of corneal flap and residual stromal bed thickness on corneal curvature; the effect of the shape of the lenticle on the surgical procedure outcomes and the effect of flap thickness on stress distribution in the cornea. The results were validated by comparing computed refractive power changes with clinical results. The effect of flap thickness on the amount of central flattening indicates that for flap thickness values 28% over the corneal thickness, central corneal flattening decreases. Moreover, the change in corneal curvature induced by subtraction of a plano-convex lenticle under a uniform flap, naturally imply a smaller change in the structure of the anterior layers of the cornea, but a bigger deformation in the structure of the posterior layers that are left behind the resection of the lenticle. In addition, the model also verified that the corneal curvature increased peripherally with simultaneous thinning centrally after subtraction of corneal tissue. This result shows that not only the treated zone is affected by the surgery, indicating the important role of the biomechanical response of the corneal tissue to refractive surgery, which is unaccounted for in current ablation algorithms. The results illustrate the potentialities of finite element modeling as an aid to the surgeon in evaluating variables.     

Assessment of Corneal Biomechanical Properties with Inflation Test Using Optical Coherence Tomography

Biomechanical properties are important for the cornea to maintain its normal shape and function. There is still a need to develop better methods for accurate measurement of corneal mechanical properties. In this study, we propose to introduce the optical coherence tomography (OCT) in inflation test for the imaging of corneal deformation in order to measure its biomechanical properties. Ten cornea-mimicking silicone phantoms with different stiffness and five fresh porcine corneas were tested using the proposed method. Intra-ocular pressure was changed from 10 to 90 mmHg using two different loading rates to observe the pressure-apex displacement relationship and calculate the apparent stiffness of the corneas. Stiffness of the corneal phantoms obtained by the inflation test ranged from 0.2 to 1 MPa, which was highly consistent with the results from the mechanical tensile test (y = 0.70x, p < 0.001). The porcine corneas showed highly viscoelastic behavior with obvious hysteresis in inflation. The apparent stiffness of the porcine corneas was 0.63 ± 0.07 and 1.05 ± 0.08 MPa with loading rates of 3.3 and 33 mmHg/min, respectively. Mapping of corneal surface displacement was also generated for both the phantom and porcine corneas. This study showed that it is feasible to incorporate the high resolution OCT imaging in inflation test to measure the biomechanical properties of the cornea.     

角巩膜生物力学2021年度研究进展

角巩膜为眼外壁组织,在维持眼球形状和屈光功能方面均发挥着重要作用。作为典型的黏弹性组织,角巩膜力学性能的变化会引起相应的眼部疾病,如近视和圆锥角膜等。临床上,可通过人为改变角巩膜组织的几何结构(如屈光手术、角膜接触镜的佩戴、角膜胶原交联术)改变生物力学行为,治疗一些与屈光有关的眼科疾病。本文通过对与角巩膜相关疾病及临床治疗方法中的生物力学研究进展进行综述,理出这些临床诊疗手段中仍需要解决的问题,以期为进一步临床诊疗策略的完善和新的潜在治疗手段的开发提供参考。     

Corneal splitting in the developing lamprey Petromyzon marinus L. eye

The cornea of the adult lamprey has both dermal (spectacle) and scleral components. These are separated by a thin mucoid layer that allows free movement of the globe. This study has shown that during the larval (ammocoete) stage, the lamprey cornea develops in a manner similar to that of other lower vertebrates. Just prior to the period of transformation to the adult parasite, the outer dermal portion of the ammocoete cornea (spectacle) consists of an anterior stratified columnar epithelium with goblet cells at the surface. The stroma of the dermal cornea consists of a thick outer layer of orthogonally oriented collagen with branching fibroblasts and a thin, loosely organized inner layer with slender elongated fibroblasts. The scleral cornea is lined internally by a flattened monolayer of mesodermal cells, the corneal endothelium. Its narrow stroma is composed entirely of thin, orthogonally aranged, collagen-fiber lamellae, and is bounded externally by a thin continuous mesothelial layer of cells that abuts directly onto the loose stromal component of the dermal cornea. During the early stages of transformation, the anterior epithelium of the dermal cornea becomes stratified squamous in type. Later, the inner loose stroma of the dermal cornea (spectacle) begins to separate from the scleral cornea components, and a third complete mesothelial layer forms a distinct inner border for the dermal cornea. A mucoid layer is formed between the dermal (spectacle) and scleral corneas and remains throughout the adult life.     

Correlation between central corneal thickness and intraocular pressure peak and fluctuation during the water drinking test in glaucoma patients

OBJECTIVE:

To investigate the correlation between central corneal thickness and outflow facility assessed by intraocular pressure peak and fluctuation during the water drinking test.

METHODS:

Fifty‐five newly diagnosed primary open‐angle glaucoma patients submitted to central corneal thickness measurements and water drinking test were enrolled in this retrospective study.;. Patients were divided into three groups according to their central corneal thickness. Pearson''s Correlation test was performed in the groups with lower and higher pachymetric values.

RESULTS:

The mean age was 65,65 ± 28,28 years; 63,63% were female and 52,72% were caucasian. The mean central corneal thickness was 544,32 ± 36,86 µm, and the mean baseline intraocular pressure was 23,36 ± 6,26 mmHg. During the water drinking test, the mean intraocular pressure peak and mean intraocular pressure fluctuation were 30,43 ± 8,13 mmHg and 31,46 ± 18,46%, respectively. No relevant correlation was detected between the central corneal thickness and the intraocular pressure peak (r² = 0,021) or between the central corneal thickness and the intraocular pressure fluctuation (r² = 0,011). Group 1 presented a mean central corneal thickness of 505,81 ± 13,86 µm, and Group 3 was 583,55 ± 27,87 µm (p = 0,001). The mean intraocular pressure peak was 31,05 ± 9,05 mmHg and 27,83 ± 4,92 mmHg in Group 1 and in Group 3, respectively (p = 0,193). The difference of intraocular pressure fluctuation was not statistically significant between Group 1 (mean 28,47 ± 16,25%) and Group 3 (mean 33,27 ± 21,27%) (p = 0,43).

CONCLUSION:

In our case series, no correlation was found between central corneal thickness and water drinking test results.     

Elastic modulus and collagen organization of the rabbit cornea: Epithelium to endothelium

The rabbit is commonly used to evaluate new corneal prosthetics and study corneal wound healing. Knowledge of the stiffness of the rabbit cornea would better inform the design and fabrication of keratoprosthetics and substrates with relevant mechanical properties for in vitro investigations of corneal cellular behavior. This study determined the elastic modulus of the rabbit corneal epithelium, anterior basement membrane (ABM), anterior and posterior stroma, Descemet’s membrane (DM) and endothelium using atomic force microscopy (AFM). In addition, three-dimensional collagen fiber organization of the rabbit cornea was determined using nonlinear optical high-resolution macroscopy. The elastic modulus as determined by AFM for each corneal layer was: epithelium, 0.57 ± 0.29 kPa (mean ± SD); ABM, 4.5 ± 1.2 kPa, anterior stroma, 1.1 ± 0.6 kPa; posterior stroma, 0.38 ± 0.22 kPa; DM, 11.7 ± 7.4 kPa; and endothelium, 4.1 ± 1.7 kPa. The biophysical properties, including the elastic modulus, are unique for each layer of the rabbit cornea and are dramatically softer in comparison to the corresponding regions of the human cornea. Collagen fiber organization is also dramatically different between the two species, with markedly less intertwining observed in the rabbit vs. human cornea. Given that the substratum stiffness considerably alters the corneal cell behavior, keratoprosthetics that incorporate mechanical properties simulating the native human cornea may not elicit optimal cellular performance in rabbit corneas that have dramatically different elastic moduli. These data should allow for the design of substrates that better mimic the biomechanical properties of the corneal cellular environment.