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      Corneal Biomechanical Properties in Varying Severities of Myopia

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          Abstract

          Purpose: To investigate corneal biomechanical response parameters in varying degrees of myopia and their correlation with corneal geometrical parameters and axial length.

          Methods: In this prospective cross-sectional study, 172 eyes of 172 subjects, the severity degree of myopia was categorized into mild, moderate, severe, and extreme myopia. Cycloplegic refraction, corneal tomography using Pentacam HR, corneal biomechanical assessment using Corvis ST and Ocular Response Analyser (ORA), and ocular biometry using IOLMaster 700 were performed for all subjects. A general linear model was used to compare biomechanical parameters in various degrees of myopia, while central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered as covariates. Multiple linear regression was used to investigate the relationship between corneal biomechanical parameters with spherical equivalent (SE), axial length (AXL), bIOP, mean keratometry (Mean KR), and CCT.

          Results: Corneal biomechanical parameters assessed by Corvis ST that showed significant differences among the groups were second applanation length (AL2, p = 0.035), highest concavity radius (HCR, p < 0.001), deformation amplitude (DA, p < 0.001), peak distance (PD, p = 0.022), integrated inverse radius (IR, p < 0.001) and DA ratio (DAR, p = 0.004), while there were no significant differences in the means of pressure-derived parameters of ORA between groups. Multiple regression analysis showed all parameters of Corvis ST have significant relationships with level of myopia (SE, AXL, Mean KR), except AL1 and AL2. Significant biomechanical parameters showed progressive reduction in corneal stiffness with increasing myopia (either with greater negative SE or greater AXL), independent of IOP and CCT. Also, corneal hysteresis (CH) or ability to dissipate energy from the ORA decreased with increasing level of myopia.

          Conclusions: Dynamic corneal response assessed by Corvis ST shows evidence of biomechanical changes consistent with decreasing stiffness with increasing levels of myopia in multiple parameters. The strongest correlations were with highest concavity parameters where the sclera influence is maximal.

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          Role of the sclera in the development and pathological complications of myopia.

          N McBrien (2003)
          Myopia is one of the most prevalent ocular conditions and is the result of a mismatch between the power of the eye and axial length of the eye. As a result images of distant objects are brought to a focus in front of the retina resulting in blurred vision. In the vast majority of cases the structural cause of myopia is an excessive axial length of the eye, or more specifically the vitreous chamber depth. In about 2% of the general population, the degree of myopia is above 6 dioptres (D) and is termed high myopia. The prevalence of sight-threatening ocular pathology is markedly increased in eyes with high degrees of myopia ( > -6 D). This results from the excessive axial elongation of the eye which, by necessity, must involve the outer coat of the eye, the sclera. Consequently, high myopia is reported as a leading cause of registered blindness and partial sight. Current theories of refractive development acknowledge the pivotal role of the sclera in the control of eye size and the development of myopia. This review considers the major biochemical mechanisms that underlie the normal development of the mammalian sclera and how the scleral structure influences the rate of eye growth during development. The review will characterise the aberrant mechanisms of scleral remodelling which underlie the development of myopia. In describing these mechanisms we highlight how certain critical events in both the early and later stages of myopia development lead to scleral thinning, the loss of scleral tissue, the weakening of the scleral mechanical properties and, ultimately, to the development of posterior staphyloma. This review aims to build on existing models to illustrate that the prevention of aberrant scleral remodelling must be the goal of any long-term therapy for the amelioration of the permanent vision loss associated with high myopia.
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            Detection of Keratoconus With a New Biomechanical Index.

            Riccardo Vinciguerra, Renato Ambrósio, Ahmed Elsheikh … (2016)
            To evaluate the ability of a new combined biomechanical index called the Corvis Biomechanical Index (CBI) based on corneal thickness profile and deformation parameters to separate normal from keratoconic patients.
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              Structural and ultrastructural changes to the sclera in a mammalian model of high myopia.

              N McBrien, L Cornell, A Gentle (2001)
              The development of high myopia is associated with scleral thinning and changes in the diameter of scleral collagen fibrils in humans. In the present study, the association between these scleral changes and the losses in scleral tissue that have previously been reported in animal models were investigated to determine the relationship between changes in collagen fibril architecture and thinning of the sclera in high myopia. Myopia was induced in young tree shrews by monocular deprivation of pattern vision for short-term (12 days) or long-term (3-20 months) periods. Scleral tissue from normal animals over a wide age range (birth to 21 months) was also collected to provide data on the normal development of the sclera. Light and electron microscopy were used to measure scleral thickness and to determine the frequency distribution of collagen fibril diameters in the sclera. Tissue loss was monitored through measures of scleral dry weight. Significant scleral thinning and tissue loss, particularly at the posterior pole of the eye, were associated with ocular enlargement and myopia development after both short- and long-term treatments. However, collagen fibril diameter distribution was not significantly altered after short-term myopia treatment, whereas, from 3 months of monocular deprivation onward, significant reductions in the median collagen fibril diameter were noted, particularly at the posterior pole. The results of this study demonstrated that loss of scleral tissue and subsequent scleral thinning occurred rapidly during development of axial myopia. However, this initial tissue loss progressed in a way that did not result in significant alterations to the collagen fibril diameter distribution. In the longer term, there was an increased number of small diameter collagen fibrils in the sclera of highly myopic eyes, which is consistent with findings in humans and is likely to contribute to the weakened biomechanical properties of the sclera that have previously been reported.
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                Author and article information

                Contributors
                Journal
                Journal ID (nlm-ta): Front Bioeng Biotechnol
                Journal ID (iso-abbrev): Front Bioeng Biotechnol
                Journal ID (publisher-id): Front. Bioeng. Biotechnol.
                Title: Frontiers in Bioengineering and Biotechnology
                Publisher: Frontiers Media S.A.
                ISSN (Electronic): 2296-4185
                Publication date (Electronic): 21 January 2021
                Publication date Collection: 2020
                Volume: 8
                Electronic Location Identifier: 595330
                Affiliations
                [1] 1Eye Research Center, Mashhad University of Medical Sciences , Mashhad, Iran
                [2] 2Health Promotion Research Center, Zahedan University of Medical Sciences , Zahedan, Iran
                [3] 3Department of Optometry, School of Paramedical Sciences, Mashhad University of Medical Sciences , Mashhad, Iran
                [4] 4Refractive Errors Research Center, Mashhad University of Medical Sciences , Mashhad, Iran
                [5] 5Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland , Auckland, New Zealand
                [6] 6Ophthalmology & Visual Science, Biomedical Engineering, The Ohio State University , Columbus, OH, United States
                [7] 7Department of Optometry, School of Rehabilitation Sciences, Iran University of Medical Sciences , Tehran, Iran
                Author notes

                Edited by: Bernardo Innocenti, Université Libre de Bruxelles, Belgium

                Reviewed by: Yoshitaka Nakao, Kimura Eye and Internal Medicine Hospital, Japan; FangJun Bao, Affiliated Eye Hospital of Wenzhou Medical College, China

                *Correspondence: Hamed Momeni-Moghaddam hmomeni_opt@ 123456yahoo.com

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

                Article
                DOI: 10.3389/fbioe.2020.595330
                PMC ID: 7859342
                PubMed ID: 33553113
                SO-VID: a7b9b095-e34f-4f52-8789-fe3d6005e22d
                Copyright © Copyright © 2021 Sedaghat, Momeni-Moghaddam, Azimi, Fakhimi, Ziaei, Danesh, Roberts, Monfared and Jamali.
                License:

                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
                Date received : 16 August 2020
                Date accepted : 21 December 2020
                Page count
                Figures: 1, Tables: 3, Equations: 0, References: 47, Pages: 9, Words: 6673
                Categories
                Subject: Bioengineering and Biotechnology
                Subject: Original Research

                Keywords: myopia,cornea,corneal biomechanics,corvis st,ora,axial length
                Data availability:
                Keywords: myopia, cornea, corneal biomechanics, corvis st, ora, axial length

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