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      Determination of Corneal Biomechanical Behavior in-vivo for Healthy Eyes Using CorVis ST Tonometry: Stress-Strain Index

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          Abstract

          Purpose: 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.

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          Most cited references29

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          Determining in vivo biomechanical properties of the cornea with an ocular response analyzer.

          David Luce (2005)
          To study the results of an ocular response analyzer (ORA) to determine the biomechanical properties of the cornea and their relationship to intraocular pressure (IOP). Reichert Inc., Depew, New York, USA. The ORA (Reichert) makes 2 essentially instantaneous applanation measurements that permit determination of corneal and IOP effects. Measurements of several populations indicate that corneal hysteresis, a biomechanical measure, varied over a dynamic range of 1.8 to 14.6 mm Hg and was only weakly correlated with corneal thickness (r(2)=0.12); this is related to the observation that some subjects with relatively thick corneas have less-than-average corneal hysteresis. Corneal hysteresis changes diurnally, presumably as a result of hydration changes. Keratoconus, Fuchs' dystrophy, and post-LASIK patients demonstrated low corneal hysteresis. The corneal hysteresis biomechanical measure may prove valuable for qualification and predictions of outcomes of refractive surgery and in other cases in which corneal biomechanics are important.
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            Introduction of Two Novel Stiffness Parameters and Interpretation of Air Puff-Induced Biomechanical Deformation Parameters With a Dynamic Scheimpflug Analyzer.

            To investigate two new stiffness parameters and their relationships with the dynamic corneal response (DCR) parameters and compare normal and keratoconic eyes.
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              Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes.

              To compare the biomechanical properties of normal, post-laser in situ keratomileusis (LASIK), and keratoconic corneas evaluated by corneal hysteresis and the corneal resistance factor measured with the Reichert Ocular Response Analyzer (ORA). Instituto Oftalmológico de Alicante, Vissum, Alicante, Spain. Two hundred fifty eyes were divided into 3 groups: normal (control group), post-LASIK, and keratoconus. The corneal biomechanical properties were measured with the ORA, which uses a dynamic bidirectional applanation process. The main outcome measures were intraocular pressure, corneal hysteresis, and the corneal resistance factor. The control group had 165 eyes; the LASIK group, 65 eyes; and the keratoconus group, 21 eyes. In the control group, the mean corneal hysteresis value was 10.8 mm Hg +/- 1.5 (SD) and the mean corneal resistance factor, 11.0 +/- 1.6 mm Hg. The corneal hysteresis value was lower in older eyes, and the difference between the youngest age group (9 to 14 years) and oldest age group (60 to 80 years) was statistically significant (P = .01, t test). One month after LASIK, corneal hysteresis and the corneal resistance factor decreased significantly, from 10.44 to 9.3 mm Hg and from 10.07 to 8.13 mm Hg, respectively. In the keratoconus group, the mean corneal hysteresis was 7.5 +/- 1.2 mm Hg and the mean corneal resistance factor, 6.2 +/- 1.9 mm Hg. There were statistically significant differences in both biomechanical parameters between keratoconic eyes and post-LASIK eyes (P<.001, t test). The corneal hysteresis and corneal resistance factor values were significantly lower in keratoconic eyes than in post-LASIK eyes. Future work is needed to determine whether these differences are useful in detecting keratoconus when other diagnostic tests are equivocal.
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                Author and article information

                Contributors
                Journal
                Front Bioeng Biotechnol
                Front Bioeng Biotechnol
                Front. Bioeng. Biotechnol.
                Frontiers in Bioengineering and Biotechnology
                Frontiers Media S.A.
                2296-4185
                16 May 2019
                2019
                : 7
                : 105
                Affiliations
                [1] 1School of Engineering, University of Liverpool , Liverpool, United Kingdom
                [2] 2St Paul's Eye Unit, Royal Liverpool and Broadgreen University Hospital , Liverpool, United Kingdom
                [3] 3Department of Biomedical Science, Humanitas University , Rozzano, Italy
                [4] 4Eye Center, Humanitas Clinical and Research Center , Rozzano, Italy
                [5] 5Rio de Janeiro Corneal Tomography and Biomechanics Study Group , Rio de Janeiro, Brazil
                [6] 6Department of Ophthalmology, Federal University of the State of Rio de Janeiro , Rio de Janeiro, Brazil
                [7] 7Department of Ophthalmology and Visual Science, Department of Biomedical Engineering, The Ohio State University , Columbus, OH, United States
                [8] 8NIHR Biomedical Research Centre for Ophthalmology, Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology , London, United Kingdom
                [9] 9School of Biological Science and Biomedical Engineering, Beihang University , Beijing, China
                Author notes

                Edited by: Jonathan Vande Geest, University of Pittsburgh, United States

                Reviewed by: Uriel Zapata, EAFIT University, Colombia; Henryk Teodor Kasprzak, Wrocław University of Technology, Poland

                *Correspondence: Ashkan Eliasy eliasy.ashkan@ 123456gmail.com

                This article was submitted to Biomechanics, a section of the journal Frontiers in Bioengineering and Biotechnology

                Article
                10.3389/fbioe.2019.00105
                6532432
                31157217
                bb2a54d1-80a8-491b-a50a-d6f59bfde0f1
                Copyright © 2019 Eliasy, Chen, Vinciguerra, Lopes, Abass, Vinciguerra, Ambrósio, Roberts and Elsheikh.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 09 October 2018
                : 24 April 2019
                Page count
                Figures: 7, Tables: 1, Equations: 8, References: 36, Pages: 10, Words: 6220
                Categories
                Bioengineering and Biotechnology
                Original Research

                cornea,biomechanics,material properties,numerical modeling,finite element modeling

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