Biomechanical behaviour – Anisotropy of eye cornea through experimental strip tests (original) (raw)
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Frontiers in Bioengineering and Biotechnology
This study aims to introduce and clinically validate a new algorithm that can determine the biomechanical properties of the human cornea in vivo. Methods: A parametric study was conducted involving representative finite element models of human ocular globes with wide ranges of geometries and material biomechanical behavior. The models were subjected to different levels of intraocular pressure (IOP) and the action of external air puff produced by a non-contact tonometer. Predictions of dynamic corneal response under air pressure were analyzed to develop an algorithm that can predict the cornea's material behavior. The algorithm was assessed using clinical data obtained from 480 healthy participants where its predictions of material behavior were tested against variations in central corneal thickness (CCT), IOP and age, and compared against those obtained in earlier studies on ex-vivo human ocular tissue. Results: The algorithm produced a material stiffness parameter (Stress-Strain Index or SSI) that showed no significant correlation with both CCT (p > 0.05) and IOP (p > 0.05), but was significantly correlated with age (p < 0.01). The stiffness estimates and their variation with age were also significantly correlated (p < 0.01) with stiffness estimates obtained earlier in studies on ex-vivo human tissue. Conclusions: The study introduced and validated a new method for estimating the in vivo biomechanical behavior of healthy corneal tissue. The method can aid optimization of procedures that interfere mechanically with the cornea such as refractive surgeries and introduction of corneal implants.
Assessment of Corneal Biomechanical Behavior Under Posterior and Anterior Pressure
Journal of Refractive Surgery, 2013
he transparent cornea provides a tough protective envelope for the ocular contents and helps the eye maintain its general shape. 1 Corneal mechanical stiffness, which is a function of corneal thickness, curvature, and material properties, is important for maintaining equilibrium under intraocular pressure (IOP) and sustaining an anterior shape that is suitable for clear vision. Although corneal thickness and curvature can be measured using ultrasound and keratometry techniques, the material properties can only be estimated ex vivo using infl ation test methods. 2,3 Infl ation tests subject the cornea to simulations of the physiologic conditions in the form of posterior hydrostatic pressure representing IOP and edge supports approximately representing the connection to the sclera. 4 The pressure-deformation behavior of the cornea, as monitored experimentally, is then converted through either inverse fi nite element analysis 5 or shell analogy 6 into constitutive models-or stress-strain relationships-of corneal tissue.
Determination of the modulus of elasticity of the human cornea
Journal of refractive surgery (Thorofare, N.J. : 1995), 2007
To determine the material behavior of the human cornea in the form of simple relationships between the modulus of elasticity and intraocular pressure (IOP) and to establish the effect of age on the material behavior. Human corneal specimens with age between 50 and 95 years were tested under inflation conditions to determine their behavior. The corneas were subjected to two extreme load rates to represent dynamic and static loading conditions. The pressure-deformation results were analyzed using shell theory to derive the relationship between the modulus of elasticity and IOP. The corneas demonstrated a nonlinear hyperelastic behavior pattern with an initial low stiffness stage and a final high stiffness stage. Despite the nonlinearity of the pressure deformation results, the relationship between the modulus of elasticity and the applied pressure was almost linear. A considerable increase was noted in the values of the modulus of elasticity associated with both age and load rate. Gen...
Determination of the true intraocular pressure and modulus of elasticity of the human cornea in vivo
Bulletin of mathematical biology, 1999
The purpose of this study was to determine the true intraocular pressure and modulus of elasticity of the human cornea in vivo. The cornea was modeled as a shell, and the equations for the deformations of a shell due to applanating and intraocular pressures were combined to model the behavior of the cornea during applanation tonometry. At certain corneal dimensions called the calibration dimensions, the applanating and intraocular pressures are considered to be equal. This relationship was used to determine the modulus of elasticity of the cornea and the relationship between the applanating and intraocular pressures. The true intraocular pressure (IOPT) was found to be related to Goldmann's applanating pressure (IOPG) as IOPT = IOPG/K , where K is a correction factor. For the calibration corneal thickness of 0.52 mm, the modulus of elasticity E in MPa of the human cornea was found to be related to the true intraocular pressure IOPT in mmHg as E = 0.0229IOPT. The generalization of the Imbert-Fick law that takes into account the effect of corneal dimensions and stiffness was found to be given by IOPT = 73.5W/(K A), where W is the applanating weight in gf (gram force) and A is the applanated area in mm 2. The calculated true intraocular pressure and modulus of elasticity were found to agree with published experimental results. The mathematical model developed may therefore be used to improve results from applanation tonometry and to estimate the mechanical property of the cornea in vivo.
Diagnostics in Ocular Imaging, 2020
A healthy cornea generates about 70% of the total eye refractive power of about 60 diopters [1]. Consequently, variations in biomechanical and geometrical properties of cornea can intensely affect corneal refractive power and may interrupt the eye vision. Evaluation of corneal biomechanical properties is essential for different ophthalmological operations such as refractive surgeries [2] and for accurate measurement of intraocular pressure (IOP) [3]. Changes in mechanical properties of the cornea result in corneal diseases, such as corneal ectasia, as well as cornea refractive problems [4]. So, evaluation of corneal material properties can be used as a beneficial tool for recognizing the corneal diseases such as keratoconus [5]. Moreover, accurate estimation of IOP makes detection of pathological diseases, such as glaucoma, more feasible [6]. Ocular Response Analyzer ® (ORA) is an older biomechanical evaluation device which evaluated intraocular pressure as well as corneal hysteresis (CH) and corneal resistance factor (CRF) as corneal biomechanical properties. Luce studied the results of ORA tonometry test to estimate biomechanical properties of the cornea and their relationship to IOP [7]. He expressed that corneal hysteresis measured by ORA provides valuable data for qualification of refractive surgery outcomes.
Biomechanical properties of human and porcine corneas
Experimental Eye Research, 2008
The suitability of porcine corneas as approximate models for human corneas in mechanical property characterisation studies is experimentally assessed. Thirty seven human donor corneas and thirty four ex-vivo porcine corneas were tested under inflation conditions to determine their short-term stressestrain behaviour and long-term creep behaviour up to 2.8 h (10,000 s). Vertical strips extracted from further 12 human corneas and 10 porcine corneas were subjected to stresserelaxation tests for up to 20 min at different stress levels. Human and porcine corneas were observed to have almost the same form of behaviour under short and long-term loading. They both exhibited non-linear stressestrain behaviour and reacted to sustained loading in a similar fashion. However, human corneas were significantly stiffer than porcine corneas. They also crept less under long-term loading and could sustain their stress state for longer compared to porcine corneas. These differences, in addition to others identified earlier in relation to corneal mechanical anisotropy, cast doubt on the suitability of porcine corneas as models for human corneas in mechanical studies.
Effect of cornea material stiffness on measured intraocular pressure
Journal of Biomechanics, 2008
Intraocular pressure (IOP) in the human eye as measured by a Goldmann applanation tonometer (GAT) is known to be affected by individual differences in central corneal thickness (CCT). However, data from clinical studies also show considerable scatter in the correlation between measured IOP and CCT. One possible implication of the large observed scatter is that the true IOP (IOPT) also depends significantly on individual variations in the material stiffness properties of the cornea. This hypothesis is explored and evaluated herein using computational simulation of applanation tonometry. A simplified 2D finite element model of the eye, which employs a calibrated nonlinear transversely isotropic material model for the cornea, is developed, and a series of GAT simulations is carried out to study the effect of geometry and material properties of the cornea on the IOP readings obtained via GAT. The results of this parametric study provide a simple correction equation, which quantifies the effect on measured IOP of variations in CCT and corneal material stiffness. In addition, several previously proposed IOP correction equations are compared with the one proposed here. Published by Elsevier Ltd.
Coupled Biomechanical Response of the Cornea Assessed by Non-Contact Tonometry. A Simulation Study
PLOS ONE, 2015
The mechanical response of the cornea subjected to a non-contact air-jet tonometry diagnostic test represents an interplay between its geometry, the corneal material behavior and the loading. The objective is to study this interplay to better understand and interpret the results obtained with a non-contact tonometry test. A patient-specific finite element model of a healthy eye, accounting for the load free configuration, was used. The corneal tissue was modeled as an anisotropic hyperelastic material with two preferential directions. Three different sets of parameters within the human experimental range obtained from inflation tests were considered. The influence of the IOP was studied by considering four pressure levels (10-28 mmHg) whereas the influence of corneal thickness was studied by inducing a uniform variation (300-600 microns). A Computer Fluid Dynamics (CFD) air-jet simulation determined pressure loading exerted on the anterior corneal surface. The maximum apex displacement showed a linear variation with IOP for all materials examined. On the contrary, the maximum apex displacement followed a cubic relation with corneal thickness. In addition, a significant sensitivity of the apical displacement to the corneal stiffness was also obtained. Explanation to this behavior was found in the fact that the cornea experiences bending when subjected to an air-puff loading, causing the anterior surface to work in compression whereas the posterior surface works in tension. Hence, collagen fibers located at the anterior surface do not contribute to load bearing. Non-contact tonometry devices give useful information that could be misleading since the corneal deformation is the result of the interaction between the mechanical properties, IOP, and geometry. Therefore, a noncontact tonometry test is not sufficient to evaluate their individual contribution and a complete in-vivo characterization would require more than one test to independently determine the membrane and bending corneal behavior.
Mechanics of Time-Dependent Materials, 2018
Non-contact tonometers, including ORA and Corvis ST, are not only used to estimate intraocular pressure (IOP) in clinical surveys but are also utilized to evaluate biomechanical properties of the cornea or anterior eye. However, for the cornea a realistic material model is still a controversial issue, and the main goal of the present study is to make this clearer. To this aim, the corneal biomechanical response is modeled by using a four-element linear viscoelastic model, which is characterized by in-vivo clinical data from Corvis ST tonometer. IOP tonometry tests on 5 normal and 5 keratoconic cases are accomplished by Corvis ST tonometer. Images from cornea deformation due to applied air jet are acquired from Corvis ST and are converted to the corneal deformation profiles by image processing techniques. By excluding the eye globe rigid body motion (retraction) from the total eye displacement, pure deformation of the cornea is obtained and used to calculate the required material properties. By calculating retardation time, contribution of the material viscosity during the test is estimated. The results show that viscosity effects do not substantially contribute to the cornea response during dynamic tests for both normal and keratoconic corneas. Indeed, the viscous effect comes from the eye globe rigid body motion.
Application of structural analysis to the mechanical behaviour of the cornea
Journal of The Royal Society Interface, 2004
Structural engineering analysis tools have been used to improve the understanding of the biomechanical behaviour of the cornea. The research is a multi-disciplinary collaboration between structural engineers, mathematical and numerical analysts, ophthalmologists and clinicians. Mathematical shell analysis and nonlinear finite-element modelling have been used in conjunction with laboratory experiments to study the behaviour of the cornea under different loading states and to provide improved predictions of the mechanical response to disease and injury. The initial study involved laboratory tests and mathematical back analysis to determine the corneal material properties and topography. These data were then used to facilitate the construction of accurate finite-element models that are able to reliably trace the performance of cornea upon exposure to disease, injury or elevated intra-ocular pressure. The models are being adapted to study the response to keratoconus (a disease causing l...