Corneal Material Research Articles

Three-dimensional biomechanical modeling and simulation of trephine cutting cornea for keratoplasty.

Trephination is one of the basic operations of keratoplasty, and the biomechanical mechanism of the operation can be revealed based on three-dimensional modeling and simulation of trephine cutting cornea. Based on the analysis of the physical and biomechanical characteristics of corneal trephination, a three-dimensional numerical model of corneal trephination is built, where the cornea can be simplified to two layers structure including stroma and epithelium, and the trephine cuts the cornea under the vertical motion load and the rotational motion load. A three-dimensional failure criterion of corneal material is proposed based on the yield strength theory. On this basis, trephination simulation is carried out, and the units of corneal material are removed from the model when they meet the defined failure criterion. Under the given parameters including the velocity, the angle and the angular velocity, the trephine force curves, include the linear cutting force and the rotary cutting force are obtained, and show the change of the forces with displacement during the process of trephination simulation. The maps of the equivalent stress show the destruction and deformation of the cornea. Then, the experiment of robotic trephination is carried out under the same parameters and the effectiveness of the simulation is evaluated. Based on mechanics theory and finite element method, the process of trephine cutting cornea has been reproduced, and the interaction mechanism is revealed, which lays the foundation for the development of real-time simulation and virtual system of the corneal surgery.

The development of tissue engineering corneal scaffold: which one the history will choose?

Since the 21st century, the development of corneal tissue engineering technology has been developing rapidly. With the progress of biomaterials, cell culture and tissue engineering technology, tissue engineering cornea has gained great development in both basic scientific research and clinical application. In particular, tissue engineered corneal scaffolds are the core components of tissue engineered corneas. It is the focus of current research on tissue engineering cornea to search for scaffolds with good biocompatibility, high safety and good biomechanical properties. In this paper, the recent research progress of tissue engineering corneal materials is reviewed.

Material Properties from Air Puff Corneal Deformation by Numerical Simulations on Model Corneas

ObjectiveTo validate a new method for reconstructing corneal biomechanical properties from air puff corneal deformation images using hydrogel polymer model corneas and porcine corneas.MethodsAir puff deformation imaging was performed on model eyes with artificial corneas made out of three different hydrogel materials with three different thicknesses and on porcine eyes, at constant intraocular pressure of 15 mmHg. The cornea air puff deformation was modeled using finite elements, and hyperelastic material parameters were determined through inverse modeling, minimizing the difference between the simulated and the measured central deformation amplitude and central-peripheral deformation ratio parameters. Uniaxial tensile tests were performed on the model cornea materials as well as on corneal strips, and the results were compared to stress-strain simulations assuming the reconstructed material parameters.ResultsThe measured and simulated spatial and temporal profiles of the air puff deformation tests were in good agreement (< 7% average discrepancy). The simulated stress-strain curves of the studied hydrogel corneal materials fitted well the experimental stress-strain curves from uniaxial extensiometry, particularly in the 0–0.4 range. Equivalent Young´s moduli of the reconstructed material properties from air-puff were 0.31, 0.58 and 0.48 MPa for the three polymer materials respectively which differed < 1% from those obtained from extensiometry. The simulations of the same material but different thickness resulted in similar reconstructed material properties. The air-puff reconstructed average equivalent Young´s modulus of the porcine corneas was 1.3 MPa, within 18% of that obtained from extensiometry.ConclusionsAir puff corneal deformation imaging with inverse finite element modeling can retrieve material properties of model hydrogel polymer corneas and real corneas, which are in good correspondence with those obtained from uniaxial extensiometry, suggesting that this is a promising technique to retrieve quantitative corneal biomechanical properties.

Finite Element Analysis of Descemet’s Stripping Automated Endothelial Keratoplasty (DSAEK) Surgery Allograft to Predict Endothelial Cell Loss

ABSTRACTPurpose: To develop high fidelity finite element (FE) models of the Descemet’s stripping automated endothelial keratoplasty (DSAEK) allograft to estimate the stress distributions generated on the allograft during its deformed state in popular allograft insertion configurations and qualitatively correlate the stress distributions to postsurgical endothelial cell (EC) loss.Materials and Methods: Corneal allograft simulation was performed using ANSYS (Canonsburg, PA, USA) utilizing isotropic nonlinear hyperelastic corneal material properties to evaluate the stress distributions generated on the DSAEK allograft during popular allograft insertion configurations, namely forceps, taco, and double-coil insertion configurations. The gathered FE simulation results were qualitatively compared with published clinical studies to verify the simulation results.Results: The FE simulation results demonstrate that high stress regions predicted by FE model results correctly predict the areas of postsurgical EC loss as published in the studies available in open literature. The FE simulation stress magnitude results suggest that highest EC loss due to mechanical bending trauma occurs in double-coil configuration followed by forceps and then taco configuration.Conclusions: The results of the presented FE simulation study highlight that allograft regions with high stress distribution demonstrate postsurgical EC loss in clinical studies. The modeling procedures presented in this research can be utilized to develop novel surgical devices/techniques that can modulate the postsurgical EC loss due to mechanical bending trauma and facilitate allograft unfolding inside the AC, thereby improving the results of the DSAEK surgical procedure.

Theoretical and numerical analysis of the corneal air puff test

Ocular analyzers are used in the current clinical practice to estimate, by means of a rapid air jet, the intraocular pressure and other eye's parameters. In this study, we model the biomechanical response of the human cornea to the dynamic test with two approaches. In the first approach, the corneal system undergoing the air puff test is regarded as a harmonic oscillator. In the second approach, we use patient-specific geometries and the finite element method to simulate the dynamic test on surgically treated corneas. In spite of the different levels of approximation, the qualitative response of the two models is very similar, and the most meaningful results of both models are not significantly affected by the inclusion of viscosity of the corneal material in the dynamic analysis. Finite element calculations reproduce the observed snap-through of the corneal shell, including two applanate configurations, and compare well with in vivo images provided by ocular analyzers, suggesting that the mechanical response of the cornea to the air puff test is actually driven only by the elasticity of the stromal tissue. These observations agree with the dynamic characteristics of the test, since the frequency of the air puff impulse is several orders of magnitude larger than the reciprocal of any reasonable relaxation time for the material, downplaying the role of viscosity during the fast snap-through phase.

Simulation of non-contact tonometer-Ocular response analyzer

As per World Health Organization, Glaucoma is considered as the second leading cause of irreversible blindness worldwide. An accurate assessment of Intraocular Pressure (IOP) is crucial for diagnosis and management of a chronic eye disease called Glaucoma. The elevation of IOP in eye leads to optic nerve damage and hence causing visual impairment. Thus, IOP measurement in tonometry has become an essential part of routine eye examinations for the diagnosis, screening and managing response to treatment in patients. Simultaneous explosion of ophthalmic knowledge and medical instrument, being made in the 19th century, has led to the invention of tonometers of varied designs and principles, and Non-Contact Tonometers (NCTs) are among them. Glodmann Applanation Tonometer (GAT) is considered the ‘gold standard’ in measuring IOP; however, IOP measurement using GAT is now known to be affected by various factors like corneal thickness, curvature and material properties as demonstrated by Khan [1]. Due to inaccuracies in measuring IOP by GAT, this ‘gold standard’ has been challenged. Therefore, the present research aims to develop a multi-parametric correction equation to determine the True Intraocular Pressure (IOPT) using Non-Contact Tonometer and the current article focuses on evaluating the influence of individual parameters on IOP by NCT.

Clinical effect of bioengineered cornea and in vivo confocal microscopy observation after human lamellar keraplasty

Background Xenotransplantation is arousing more attention of researchers because of the lack of corneal donors. Biological corneal scaffolds constructed by porcine corneal acellular stroma appears to have an acceptable biocompatibility. However, its clinical effects and the histomorphological features in the corneal tissue of receipts' are still unclear. Objective This study was to evaluate the viability of bioengineered corneas as a new material of human lamellar keratoplasty and observe the in vivo biological features after human keratoplasty under the laser confocal microscope. Methods A prospective serial cases observational study was carried out. Fifteen eyes of 15 patients with infectious keratitis were enrolled in Henan Eye Hospital from February to August 2014 under the approval of Ethic Committee of Henan Eye Hospital and informed consent of each patient, including 4 eyes of fungal keratitis, 1 eye of bacterial keratitis, 9 eyes of mixed infectious keratitis and 1 eye of leucoma. Corneal lamellar transplantation was performed on the eyes with the bioengineered corneas as grafts and the follow-up time was one year. The survival of grafts was assessed and scored, and the indices including corneal transparency, neovascularization, graft lysis or inflammatory recurrence and visual acuity (logMAR) were evaluated. The morphology and density of corneal epithelial cells and endothelial cells, corneal stroma and subepithelial neural fibers were examined by laser confocal microscope 3, 6, 9 and 12 months after surgery. Results Postoperative inflammatory response was seen 3 days and disappeared 7 days after surgery. The grafts were clear 1 month after surgery, and no corneal dissolution was found during the follow-up duration. Glaucoma occurred in 1 eye at 6 months and graft rejection occurred in another eye 12 months after surgery. The logMAR, corneal transparency scores and corneal neovascularization scores improved after surgery in comparison with before surgery, with a significant difference among various time points (χ2=92.63, 59.37, 10.50, all at P<0.05). Complete epithelization of grafts was seen. Compared with the contralateral eyes, the morphology of epithelial cells was similar and endothelial cells were enlarged in the operated eyes. In addition, no stromal cell structure was seen in the corneal stroma in the operative eyes. Subepithelial nerve fibers appeared in 8 eyes at 6 months, but the fiber density was lower in the operated eyes than that in the contralateral eyes. Significant difference was found in epithelial cell density among different time points (F=1.48, P=0.22). The endothelial cell densities were (2 542±119), (1 895±129), (1 869±135), (1 854±101) and (1 844±103)/mm2 before surgery and 3, 6, 9 and 12 months after surgery, showing significant differences between preoperation and postoperative time points (all at P<0.05). The subepithelial nerve fibers densities were (1.26±0.13), (3.62±0.81) and (5.98±0.44) mm/mm2 at 6, 9 and 12 months after surgery, with significant differences between adjacent two time points (t'=-8.16, -7.24; both at P=0.00). Conclusions Bioengineered corneal grafts survive well in human eye after lamellar transplantation, which can reconstruct ocular surface and improve the visual acuity. Biological cornea can replace human corneal materials. Key words: [Key words] Corneal transplantation; Transplantation, heterologous; Graft survival/immunology; Microscopy, confocal; Humans; Bioengineered corneas; Porcine corneal acellular stroma

Assessment of in vitro dynamics of pathogenic Acanthamoeba strains originating from contact lens wearers with infectious keratitis

Recently, incidents of Acanthamoeba keratitis, the vision-threatening eye disease, are reported with increasing frequency worldwide, particularly in contact lens wearers. In our study, the retrospective assessment of in vitro dynamics of subsequent pathogenic Acanthamoeba isolates cultured at 24°C, detected in Polish contact lens wearers with keratitis is presented and results compared with those of environmental A. castellanii Neff strain. There were delayed the proper diagnosis that influenced prolonged and severe course of this eye disease and treatment difficulties. The corneal material was examined directly to visualize developmental amoeba stages for diagnose verification, microbiologically tested for the specific identification of bacteriae and fungi, and in vitro grown in culture medium in temperature 24°C. Among twenty-six keratitis incidents analyzed, eleven were cases of Acanthamoeba keratitis; in the six of them, Acanthamoeba strains and concomitant bacterial and/or fungal infectious agents were detected. In vitro assays showed variability in population density of several clinical strains in the exponential growth phase expressed in various range of overall amoeba number and different proportion between trophozoites and cysts. The clear influence of temperature on the in vitro cultivation of the amoebae was observed: statistically significant lower population dynamics was revealed by most of pathogenic clinical isolates in comparison with those showed by environmental strain. The in vitro monitoring of dynamics of Acanthamoeba strains isolated from infected eyes may be helpful for diagnostics verification, especially in mixed infectious keratitis.

Biomechanical simulation of needle insertion into cornea based on distortion energy failure criterion.

This paper is mainly about biomechanical behavior of needle insertion into cornea, and proposes a failure criterion to simulate the insertion process which has attracted considerable attention due to its importance for the minimally invasive treatment. In the process of needle insertion into cornea, tiny and complex insertion force is generated due to contact between needle and soft tissue. Based on the distortion energy theory, there is proposed a failure criterion of corneal material that can solve contact problem between rigid body and biological tissue in insertion simulation, where Cauchy stress of corneal material is the key to numerical calculation. A finite element model of in vivo cornea is built, and the cornea constrained by sclera is simplified to two layers containing epithelium and stroma. Considering the hyper-viscoelastic property of corneal material, insertion simulation is carried out. By insertion experiment, the insertion force increases with insertion depth accompanying obvious fluctuations. Different insertion forces are generated at different speeds. The punctured locations are obvious in the force-displacement curves. The results of insertion simulation are generally consistent with experimental data. Maps of von Mises stress reflect the tissue injury of the cornea during insertion process, and punctured status corresponds to the point in the curves. The ability of this study to reproduce the behavior of needle insertion into cornea opens a promising perspective for the control of robotic surgery operation as well as the real-time simulation of corneal suture surgery.

Corneal hyper-viscoelastic model: derivations, experiments, and simulations.

The aim of this study is to propose a method to construct corneal biomechanical model which is the foundation for simulation of corneal microsurgery. Corneal material has two significant characteristics: hyperelastic and viscoelastic. Firstly, Mooney-Rivlin hyperelastic model of cornea obtained based on stored-energy function can be simplified as a linear equation with two unknown parameters. Then, modified Maxwell viscoelastic model of the cornea, whose analytical form is consistent with the generalized Prony-series model, is proposed from the perspective of material mechanics. Parameters of the model are determined by the uniaxial tensile tests and the stress-relaxation tests. Corneal material properties are simulated to verify the hyper-viscoelastic model and measure the effectiveness of the model in the finite element simulation. On this basis, an in vivo model of the corneal is built. And the simulation of extrusion in vivo cornea shows that the force is roughly nonlinearly increasing with displacement, and it is consistent with the results obtained by extrusion experiment of in vivo cornea. Conlusions: This paper derives a corneal hyper-viscoelastic model to describe the material properties more accurately, and explains the mathematical method for determination of the model parameters. The model is an effective biomechanical model, which can be directly used for simulation of trephine and suture in keratoplasty. Although the corneal hyper-viscoelastic model is taken as the object of study, the method has certain adaptability in biomechanical research of ophthalmology.

Особливості реорганізації структур рогівки кролів з її хімічним опіком на тлі мерказоліл-індукованого гіпотирозу = Peculiarities of reorganization of the rabbits’ corneal structure with its chemical burn at the background of merkazolil-induced hypothyroid

Savchuk Z . L . , Klishch I . M ., H eras y miuk I . Ye . Особливості реорганізації структур рогівки кролів з її хімічним опіком на тлі мерказоліл-індукованого гіпотирозу = Peculiarities of reorganization of the rabbits’ corneal structure with its chemical burn at the background of merkazolil-induced hypothyroidism. Journal of Health Sciences. 2014;4(14):190-200. ISSN 1429-9623 / 2300-665X. http://journal.rsw.edu.pl/index.php/JHS/article/view/2014%3B4%2814%29%3A190-200 https://pbn.nauka.gov.pl/works/513118 DOI: 10.5281/zenodo.13360 http://dx.doi.org/10.5281/zenodo.13360 The journal has had 5 points in Ministry of Science and Higher Education of Poland parametric evaluation. Part B item 1107. (17.12.2013). © The Author (s) 2014; This article is published with open access at Licensee Open Journal Systems of Radom University in Radom, Poland Open Access. This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. This is an open access article licensed under the terms of the Creative Commons Attribution Non Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non commercial use, distribution and reproduction in any medium, provided the work is properly cited. This is an open access article licensed under the terms of the Creative Commons Attribution Non Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non commercial use, distribution and reproduction in any medium, provided the work is properly cited. Conflict of interest: None declared. Received: 15.11.2014. Revised 05.12.2014. Accepted: 10.12.2014. Особливості реорганізації структур рогівки кролів з її хімічним опіком на тлі мерказоліл-індукованого гіпотирозу Peculiarities of reorganization of the rabbits’ corneal structure with its chemical burn at the background of merkazolil-induced hypothyroidism Савчук З. Л., Кліщ І. М., Герасимюк І. Є. Savchuk Z . L . , Klishch I . M ., H eras y miuk I . Ye . ДВНЗ «Тернопільський державний медичний університет імені І.Я. Горбачевського МОЗ України» SHEE Ternopil State Medical University named after I . Ya . Horbachevskii of the Ministry of Public Health of Ukraine Резюме. У статті наведено результати патоморфологічних змін і морфометричних відхилень структур рогівки кролів з її хімічним опіком на тлі попередньо змодельованого гіпотирозу. Встановлено, що при опіку рогівки лугом у ранньому періоді (до 7-ми діб) виникають деструктивні зміни переднього епітелію і дистрофічні процеси у власній речовині рогівки, що проявляються її набряком, потовщенням і дезорганізацією колагенових волокон. На 14-21-у доби у зоні опіку спостерігаються регенераторні процеси з відновленням структурної організації власної речовини рогівки та з частковим відновленням поверхневого епітелію. У тварин з опіком рогівки лугом на тлі попередньо змодельованого гіпотирозу спостерігаються аналогічні за характером зміни, однак вони мають менш реактивний перебіг, розтягнуті у часі, з нижчим ступенем регенераторного відновлення. Ключові слова : хімічний опік рогівки, гіпотироз, патоморфологічні зміни. Summary. The article presents the results of pathomorphological changes and morphometrical abnormalities of the corneal structures of rabbits with its chemical burns on the background of previously simulated hypothyroidism. It was established that when the cornea is burnt with alkali in the early period (up to 7 days) there are destructive changes of the anterior epithelium and degenerative processes in the substance of the cornea, manifested by its swelling, thickening and disorganization of collagen fibers. On the 14-21 days in the area of burn regenerative processes with the recovery of the structural organization of corneal material and with the partial restoration of the surface epithelium was observed. In animals with the burn of cornea with alkali at the background of previously simulated hypothyroidism similar in nature changes are observed, but they have less reactive course, lengthy, with the lower degree of regenerative recovery. Keywords: chemical burn of the cornea, hypothyroidism, pathomorphological changes.

Numerical model of optical coherence tomographic vibrography imaging to estimate corneal biomechanical properties.

Most techniques measuring corneal biomechanics in vivo are biased by side factors. We demonstrate the ability of optical coherence tomographic (OCT) vibrography to determine corneal material parameters, while reducing current prevalent restrictions of other techniques (such as intraocular pressure (IOP) and thickness dependency). Modal analysis was performed in a finite-element (FE) model to study the oscillation response in isolated thin corneal flaps/eye globes and to analyse the dependency of the frequency response function on: corneal elasticity, viscoelasticity, geometry (thickness and curvature), IOP and density. The model was verified experimentally in flaps from three bovine corneas and in two enucleated porcine eyes using sound excitation (100–110 dB) together with a phase-sensitive OCT to measure the frequency response function (range 50–510 Hz). Simulations showed that corneal vibration in flaps is sensitive to both, geometrical and biomechanical parameters, whereas in whole globes it is primarily sensitive to corneal biomechanical parameters only. Calculations based on the natural frequency shift revealed that flaps of the posterior cornea were 0.8 times less stiff than flaps from the anterior cornea and cross-linked corneas were 1.6 times stiffer than virgin corneas. Sensitivity analysis showed that natural vibration frequencies of whole globes were nearly independent from corneal thickness and IOP within the physiological range. OCT vibrography is a promising non-invasive technique to measure corneal elasticity without biases from corneal thickness and IOP.

Application of 3-dimensional printing technology to construct an eye model for fundus viewing study.

ObjectiveTo construct a life-sized eye model using the three-dimensional (3D) printing technology for fundus viewing study of the viewing system.MethodsWe devised our schematic model eye based on Navarro's eye and redesigned some parameters because of the change of the corneal material and the implantation of intraocular lenses (IOLs). Optical performance of our schematic model eye was compared with Navarro's schematic eye and other two reported physical model eyes using the ZEMAX optical design software. With computer aided design (CAD) software, we designed the 3D digital model of the main structure of the physical model eye, which was used for three-dimensional (3D) printing. Together with the main printed structure, polymethyl methacrylate(PMMA) aspherical cornea, variable iris, and IOLs were assembled to a physical eye model. Angle scale bars were glued from posterior to periphery of the retina. Then we fabricated other three physical models with different states of ammetropia. Optical parameters of these physical eye models were measured to verify the 3D printing accuracy.ResultsIn on-axis calculations, our schematic model eye possessed similar size of spot diagram compared with Navarro's and Bakaraju's model eye, much smaller than Arianpour's model eye. Moreover, the spherical aberration of our schematic eye was much less than other three model eyes. While in off- axis simulation, it possessed a bit higher coma and similar astigmatism, field curvature and distortion. The MTF curves showed that all the model eyes diminished in resolution with increasing field of view, and the diminished tendency of resolution of our physical eye model was similar to the Navarro's eye. The measured parameters of our eye models with different status of ametropia were in line with the theoretical value.ConclusionsThe schematic eye model we designed can well simulate the optical performance of the human eye, and the fabricated physical one can be used as a tool in fundus range viewing research.

Ten most progression of corneal and ocular surface disease research in China

Ten researches that may represent the most advanced cornea and ocular surface disease (OSD) related studies in China were voted by specialists from Chinese Cornea Group of Ophthalmology Society.These researches were concentrated in the following fields:Clinical and basic research in infectious keratitis ; clinical application of modern corneal surgeries and technology; tissue engineering corneal material development ; prevention and treatment of immune rejection after corneal transplantation,et al.These studies represented the level of our national cornea and ocular surface specialists in the international cornea and OSD field and have become the landmark study of our specialists. Key words: Cornea; Ocular surface disease; Surgical innovation ; Tissue engineering; Limbal stem cells; Immune rejection

Calibrating corneal material model parameters using only inflation data: an ill-posed problem.

Goldmann applanation tonometry (GAT) is a method used to estimate the intraocular pressure by measuring the indentation resistance of the cornea. A popular approach to investigate the sensitivity of GAT results to material and geometry variations is to perform numerical modelling using the finite element method, for which a calibrated material model is required. These material models are typically calibrated using experimental inflation data by solving an inverse problem. In the inverse problem, the underlying material constitutive behaviour is inferred from the measured macroscopic response (chamber pressure versus apical displacement). In this study, a biomechanically motivated elastic fibre-reinforced corneal material model is chosen. The inverse problem of calibrating the corneal material model parameters using only experimental inflation data is demonstrated to be ill-posed, with small variations in the experimental data leading to large differences in the calibrated model parameters. This can result in different groups of researchers, calibrating their material model with the same inflation test data, drawing vastly different conclusions about the effect of material parameters on GAT results. It is further demonstrated that multiple loading scenarios, such as inflation as well as bending, would be required to reliably calibrate such a corneal material model.

Treatment of bullous keratopathy with corneal intralamellar anterior lens capsule implantation combined with transplantation of amniotic membrane

Objective To investigate the clinical efficacy of corneal intralamellar anterior lens capsule implantation combined with transplantation of amniotic membrane for bullous keratopathy.Methods Eleven eyes of 11 cases suffering from bullous keratopathy received corneal intralamellar anterior lens capsule implantation combined with tramsplantation of amniotic membrane on the cornea.The clinical efficacy and complication were observed during 12 months follow-up.Results In all cases,the pain was alleviated in 9 cases,the vision acuity increased in 6 cases and the corneal defect healed in 8 cases within 3 weeks.Conclusion In the primary hospital where corneal material is lacking,the application of corneal intralamellar anterior lens capsule implantation combined with transplantation of amniotic membrane on the cornea can be a choice for bullous keratopathy.The surgery can relieve pain and improve visual acuity partly. Key words: Keraopathy, bullous; Implantation, intralamellar, corneal, anterior lenscapsule; Transplantation, amniotic membrane

Influence of glutaraldehyde on the biomechanical properties of the rabbit cornea using inflation testing and inverse modeling

Objective To determine the effect of glutaraldehyde on corneal biomechanical properties using inflation testing and inverse modeling.Methods Ten Japanese white rabbits were included in this experimental study.One eye was randomly selected from each rabbit and subjected to a PBS solution with 4％ glutaraldehyde treatment for 30 minutes (experimental group); the contralateral eye was suspended with a PBS solution without glutaraldehyde for the same amount of time (control group).Specimens in both groups were loaded by cycles of pressure from 2.1 to 45.0 mmHg.And the experimentally obtained pressure-deformation data were used to derive the stress-strain behavior of each eye using an inverse modeling procedure.Corneal thickness and diameter were measured before the inflation test.The paired t test and paired Hotelling's T square test were used to analyze and compare the differences in corneal thickness,diameter,and tangent modulus between the two groups.Results Differences in the average corneal thickness and diameter in the experimental group before treatment with glutaraldehyde were not statistically significant from the control group.Glutaraldehyde significantly reduced corneal thickness.Corneal specimens treated with glutaraldehyde displayed significantly greater stiffness than those in the control group.With increments of stress,the growth ratio of the tangent modulus became increasingly lower (from 199.5％ to 59.7％).Conclusion Glutaraldehyde treatment causes an increase in corneal material stiffness,while the material stiffening ratio decreased with an increase in stress. Key words: Biomechanics; Cornea ; Inflation test ; Inverse modeling; Tangent modulus ; Glutaraldehyde

Effects of bathing solution on tensile properties of the cornea

The cornea is a transparent tissue with the major functions of protecting the inner contents of the eye and refracting incoming light. The biomechanical properties of the cornea strongly depend on the microstructure and composition of the stromal layer, a hydrated bio-gel. The uniaxial strip testing is a convenient and well-accepted experimental technique for characterizing corneal material parameters. It is known that the water content of specimens in this method depends on the osmolality of the bathing solution. The present study was designed to investigate the effects of different bathing solutions on uniaxial tensile material properties of the cornea. The tensile behavior of bovine corneal samples was measured in six different bathing solutions, i.e. hypertonic solution (12% NaCl solution), common preserving isotonic solutions (e.g. phosphate buffer saline, ophthalmic balanced salt solution, and 0.9% NaCl solution), hypotonic solution (distilled water), and neutral solution (mineral oil). It was observed that the bathing solution had significant influence on the tensile behavior of the corneal samples. In particular, the specimens tested in bathing solutions causing less swelling had significantly stiffer tensile properties. Furthermore, a simple mathematical model based on Voigt composite material model was developed to represent the measured solution-dependent tensile properties. The present study suggests that extra attention should be paid to corneal thickness (hydration) in uniaxial tensile experiments. It also provides important data on tensile properties of the cornea; such information could significantly contribute to improving the accuracy of numerical predictions of corneal biomechanics.

Patient-Specific Finite Element Simulations of Standard Incisional Astigmatism Surgery and a Novel Patterned Collagen Crosslinking Approach to Astigmatism Treatment.

Finite element analysis has been explored as a tool for estimating the refractive outcomes of corneal interventions such as laser in situ keratomileusis and photorefractive keratectomy [1]. Corneal astigmatism is a common geometrical abnormality that can be quantified topographically as the difference in curvature or dioptric refractive power of the steepest and flattest meridians, often represented as simulated keratometry values. Astigmatic keratotomy (AK) is an incisional method for correcting astigmatism by placement of paired, peripheral corneal arcuate incisions along the steep axis of astigmatism that act to flatten the steep axis. AK is a convenient method of astigmatism correction, but it is less predictable than other surgical options, has a limited range of effect, can induce higher-order corneal aberrations that reduce visual acuity, and can destabilize the cornea due to its dependence on disruption of collagen lamellar continuity [2]. Corneal collagen crosslinking (CXL) is an alternative method for minimally invasive corneal refractive alterations through augmentation of corneal material strength. The current clinical implementation of collagen crosslinking involves broad treatment of the corneal surface with UVA irradiation after imbibitions of photosensitizing riboflavin solution and is directed primarily toward the stabilization of corneal ectasia. Previous finite element analyses from our group have explored the relationship between the stiffening effect of CXL and changes in corneal curvature [3] and have studied candidate patterns for a novel method of astigmatism treatment using CXL. The goal of this study was to compare the simulated corneal topographic effects to a standard incisional approach to astigmatism treatment to patterned CXL [4].

Hydration dependent biomechanical properties of the corneal stroma

The cornea is responsible for about seventy percent of refractive power of the eye and transmits more than ninety percent of the incident light. The refractive power and transparency of the cornea are largely contingent upon active maintenance of its precise curvature and highly regular microstructure. The biomechanical properties of the cornea are mainly derived from the stromal layer. Over past decades, many computational models have been proposed to predict the corneal behavior in normal and/or diseased state. The predictions of these numerical methods strongly depend on accurate description of corneal mechanical properties. The present study used unconfined compression technique to characterize the dependence of corneal material parameters on thickness variations due to hydration/dehydration. A series of unconfined compression tests was performed on porcine corneal buttons and stress relaxation response of the samples was obtained. A transversely isotropic biphasic model was used to analyze experimental measurements at each ramp-and-hold step. The in-plane Young's modulus, out-of-plane (transverse) Young's modulus, and permeability coefficient of the samples were determined as a function of average thickness. The average thickness variation due to swelling (or dehydration) was between 0.69 mm and 1.27 mm. It was found that corneal material parameters depend strongly on the tissue thickness (hydration). In particular, the in-plane elastic modulus increased with decreasing the thickness (p < 0.05) and the permeability coefficient decreased with decreasing the thickness (p < 0.05). Furthermore, it was observed that although the out-of-plane modulus was almost constant as the average thickness varied between 1.07 mm and 1.27 mm, it increased with tissue thickness outside this range (p < 0.05). The findings of this study provide important information on biomechanical properties of corneal stroma and are useful in computational simulations of the cornea.