Transient viscous response of the human cornea probed with the Surface Force Apparatus (original) (raw)

Corneal biomechanical properties : Measurement, modification and simulation

Bibliography xx effect of the corneal hydration state and different medical preservation solutions on their effect on the corneal biomechanical response. This is especially important in order to improve in vitro experiments on eyes obtained from an eye bank. This chapter is based on the publication by S. Kling and S. Marcos, entitled "Effect of Hydration State and Storage Media on Corneal Biomechanical Response From in vitro Inflation Tests" in Journal of Refractive Surgery. This work was also presented in a oral contribution at the 2010 Euromech-518 Workshop "Biomechanics of the Eye" in London (UK). The author of this thesis (i) did the sample preparation, (ii) performed the measurements, (iii) did the data analysis and (iv) prepared the manuscript in collaboration with S. Marcos.

Review of in-vivo characterisation of corneal biomechanics

Medicine in Novel Technology and Devices, 2021

The study of corneal biomechanics in vivo has been evolving fast in recent years. While an organised corneal structure is necessary for its transparency, resistance to occasional external insults and bearing the intraocular pressure (IOP), which several clinically relevant events can disturb. This review focuses on three techniques that are available for clinical use, namely the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY, USA), the Corvis ST (Oculus Optikger€ ate GmbH, Wetzlar, Germany) and the Brillouin Optical Scattering System (Intelon Optics Inc., Lexington, MA, USA). The principles and the main parameters of each device are discussed along with their strategies to improve accuracy in the IOP measurement, corneal ectasia diagnosis, evaluation of corneal cross-linking procedures, and planning of corneal refractive surgeries.

Corneal Biomechanics: A Paradigm shift in Studying Corneal Pathology

The Official Scientific Journal of Delhi Ophthalmological Society, 2017

Biomechanics links corneal structure to its function. An area of intense research, corneal biomechanics can distinguish normal from ectatic corneas and has applications in various fields including cataract surgery, intraocular pressure measurement and glaucoma. It is also likely to serve as an important tool in the selection of suitable candidates for refractive surgery. This article reviews various methods of assessment of biomechanical properties of the cornea and its applications along with case examples to highlight the role it plays in clinical practice.

Biomechanical model of human cornea based on stromal microstructure

Journal of Biomechanics, 2010

The optical characteristics of the human cornea depends on the mechanical balance between the intraocular pressure and intrinsic tissue stiffness. A wide range of ophthalmic surgical procedures alter corneal biomechanics to induce local or global curvature changes for the correction of visual acuity. Due to the large number of surgical interventions performed every day, a deeper understanding of corneal biomechanics is needed to improve the safety of these procedures and medical devices. The aim of this study is to propose a biomechanical model of the human cornea, based on stromal microstructure. The constitutive mechanical law includes collagen fiber distribution based on X-ray scattering analysis, collagen cross-linking, and fiber uncrimping. Our results showed that the proposed model reproduced inflation and extensiometry experimental data [Elsheikh et al., Curr. Eye Res., 2007; Elsheikh et al., Exp. Eye Res., 2008] successfully. The mechanical properties obtained for different age groups demonstrated an increase in collagen cross-linking for older specimens. In future work such a model could be used to simulate non-symmetric interventions, and provide better surgical planning.

A viscoelastic anisotropic hyperelastic constitutive model of the human cornea

Biomechanics and modeling in mechanobiology, 2017

A constitutive model based on the continuum mechanics theory has been developed which represents interlamellar cohesion, regional variation of collagen fibril density, 3D anisotropy and both age-related viscoelastic and hyperelastic stiffening behaviour of the human cornea. Experimental data gathered from a number of previous studies on 48 ex vivo human cornea (inflation and shear tests) enabled calibration of the constitutive model by numerical analysis. Wide-angle X-ray scattering and electron microscopy provided measured data which quantify microstructural arrangements associated with stiffness. The present study measures stiffness parallel to the lamellae of the cornea which approximately doubles with an increase in strain rate from 0.5 to 5%/min, while the underlying stromal matrix provides a stiffness 2-3 orders of magnitude lower than the lamellae. The model has been simultaneously calibrated to within 3% error across three age groups ranging from 50 to 95 years and three str...

Corneal Viscoelastic Properties from Finite-Element Analysis of In Vivo Air-Puff Deformation

PLoS ONE, 2014

Biomechanical properties are an excellent health marker of biological tissues, however they are challenging to be measured in-vivo. Non-invasive approaches to assess tissue biomechanics have been suggested, but there is a clear need for more accurate techniques for diagnosis, surgical guidance and treatment evaluation. Recently air-puff systems have been developed to study the dynamic tissue response, nevertheless the experimental geometrical observations lack from an analysis that addresses specifically the inherent dynamic properties. In this study a viscoelastic finite element model was built that predicts the experimental corneal deformation response to an air-puff for different conditions. A sensitivity analysis reveals significant contributions to corneal deformation of intraocular pressure and corneal thickness, besides corneal biomechanical properties. The results show the capability of dynamic imaging to reveal inherent biomechanical properties in vivo. Estimates of corneal biomechanical parameters will contribute to the basic understanding of corneal structure, shape and integrity and increase the predictability of corneal surgery.

Experimental Determination of Corneal Elastic Constants and Their Use in Biomechanical Modeling

Applied Sciences, 2021

Corneal biomechanics aims to establish the physico-mathematical bases that allow for predicting the corneal response to physiological and pathological situations by creating models of tissue behavior. Determining the characteristic parameters of these models is a formidable challenge in the biomechanical modeling process. To contribute to corneal tissue characterization, an experimental set-up was designed, built and tested to study corneal behavior by applying changes in pressure. The elastic constants of porcine corneas were determined, and a Young’s modulus of 0.188 MPa and 26.22% hysteresis were obtained. A computational cornea model was developed to analyze the influence of different factors. Minor variations in the applied conditions were found for apical displacement and pachymetry, and the corneal behavior was reproduced. However, the optical power behavior was affected by variations in the applied conditions, and the experimentally obtained data could not be reproduced. Des...

Biomechanics of the Anterior Human Corneal Tissue Investigated with Atomic Force Microscopy

Investigative Ophthalmology & Visual Science, 2012

To investigate the biomechanics of the anterior human corneal stroma using atomic force microscopy (AFM). METHODS. AFM measurements were performed in liquid on the anterior stroma of human corneas, after gently removing the epithelium, using an atomic force microscope in the force spectroscopy mode. Rectangular silicon cantilevers with tip radius of 10 nm and spring elastic constants of 25-and 33-N/m were used. Each specimen was subjected to increasing loads up to a maximum of 2.7 N with scan speeds ranging between 3-and 95-m/s. The anterior stromal hysteresis during the extension-retraction cycle was quantified as a function of the application load and scan rate. The elastic modulus of the anterior stroma was determined by fitting force curve data to the Sneddon model. RESULTS. The anterior stroma exhibited significant viscoelasticity at micrometric level: asymmetry in the curve loadingunloading response with considerable hysteresis dependent both on the application load and scan rate (P Ͻ 0.01). The mean elastic modulus ranged between 1.14 and 2.63 MPa and was constant over the range of indentation depths between 1.0 and 2.7 m in the stroma. CONCLUSIONS. At microscale level, the mechanical response of the most anterior stroma is complex and nonlinear. The microstructure (fibers' packing, number of cross-links, water content) and the combination of elastic (collagen fibers) and viscous (matrix) components of the tissue influence the type of viscoelastic response. Efforts in modeling the biomechanics of human corneal tissue at micrometric level are needed. (Invest

Dynamics Modeling of Corneal Deformation under Air Puff with Linear and Nonlinear Viscoelastic Model

2020

Background: The aim of the study is to model the corneal dynamic deformation under an air puff excitation. The deformation response of the cornea was modeled by using linear and nonlinear viscoelastic models. The corneal deformation responses generated from the linear and nonlinear viscoelastic model were correlated with the clinical results, which were obtained from Corneal Visualization Scheimpflug Tonometer (Corvis ST) to evaluate the comparable biomechanical parameters of the cornea. Methods: A prompt deformation occurs when the external force applied to the cornea. Then a continuous deformation follows. A simple mass, spring and dashpot system were used to model human eyeball. Results: In linear viscoelastic model, the corneal elastic stiffness commanded behavior of the corneal deformation and its maximum, when the viscous component affected for its lateral shifting and marginally alter the magnitude.Whereas, in the nonlinear viscoelastic model, the corneal material nonlinearity co...