Effect of Corneal Tilt on the Determination of Asphericity

<b>Purpose:</b> To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. <b>Methods:</b> This cross-sectional study starts with a...

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Autores principales:	Alejandra Consejo, Arwa Fathy, Bernardo T. Lopes, Renato Ambrósio, Ahmed Abass
Formato:	article
Lenguaje:	EN
Publicado:	MDPI AG 2021
Materias:	eye cornea tilt asphericity parametric eye models Chemical technology TP1-1185
Acceso en línea:	https://doaj.org/article/383de9b821464ea4a6eea1f5855e42ff
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spelling	oai:doaj.org-article:383de9b821464ea4a6eea1f5855e42ff2021-11-25T18:58:07ZEffect of Corneal Tilt on the Determination of Asphericity10.3390/s212276361424-8220https://doaj.org/article/383de9b821464ea4a6eea1f5855e42ff2021-11-01T00:00:00Zhttps://www.mdpi.com/1424-8220/21/22/7636https://doaj.org/toc/1424-8220<b>Purpose:</b> To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. <b>Methods:</b> This cross-sectional study starts with a mathematical simulation proving the concept of the effect that the eye’s tilt has on the corneal asphericity calculation. Spherical, conic and biconic models are considered and compared. Further, corneal asphericity is analysed in the eyes of 177 healthy participants aged 35.4 ± 15.2. The optical axis was determined using an optimization procedure via the Levenberg–Marquardt nonlinear least-squares algorithm, before fitting the corneal surface to spherical, conic and biconic models. The influence of pupil size (aperture radii of 1.5, 3.0, 4.0 and 5.0 mm) on corneal radius and asphericity was also analysed. <b>Results:</b> In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of the tilt angle in both positive and negative directions and aperture radii in all models. Fitting the cornea to spherical models did not show a significant difference between the raw-measured corneal surfaces and the levelled surfaces for right and left eyes. When the conic models were fitted to the cornea, changes in the radii of the cornea among the raw-measured corneal surfaces’ data and levelled data were not significant; however, significant differences were recorded in the asphericity of the anterior surfaces at radii of aperture 1.5 mm (<i>p</i> < 0.01). With the biconic model, the posterior surfaces recorded significant asphericity differences at aperture radii of 1.5 mm, 3 mm, 4 mm and 5 mm (<i>p</i> = 0.01, <i>p</i> < 0.01, <i>p</i> < 0.01 & <i>p</i> < 0.01, respectively) in the nasal temporal direction of right eyes and left eyes (<i>p</i> < 0.01, <i>p</i> < 0.01, <i>p</i> < 0.01 & <i>p</i> < 0.01, respectively). In the superior–inferior direction, significant changes were only noticed at aperture radii of 1.5 mm for both right and left eyes (<i>p</i> = 0.05, <i>p</i> < 0.01). <b>Conclusions:</b> Estimation of human corneal asphericity from topography or tomography data using conic and biconic models of corneas are affected by eyes’ natural tilt. In contrast, the apical radii of the cornea are less affected. Using corneal asphericity in certain applications such as fitting contact lenses, corneal implant design, planning for refractive surgery and mathematical modelling when a geometrical centre of the eye is needed should be implemented with caution.Alejandra ConsejoArwa FathyBernardo T. LopesRenato AmbrósioAhmed AbassMDPI AGarticleeyecorneatiltasphericityparametriceye modelsChemical technologyTP1-1185ENSensors, Vol 21, Iss 7636, p 7636 (2021)
institution	DOAJ
collection	DOAJ
language	EN
topic	eye cornea tilt asphericity parametric eye models Chemical technology TP1-1185
spellingShingle	eye cornea tilt asphericity parametric eye models Chemical technology TP1-1185 Alejandra Consejo Arwa Fathy Bernardo T. Lopes Renato Ambrósio Ahmed Abass Effect of Corneal Tilt on the Determination of Asphericity
description	<b>Purpose:</b> To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. <b>Methods:</b> This cross-sectional study starts with a mathematical simulation proving the concept of the effect that the eye’s tilt has on the corneal asphericity calculation. Spherical, conic and biconic models are considered and compared. Further, corneal asphericity is analysed in the eyes of 177 healthy participants aged 35.4 ± 15.2. The optical axis was determined using an optimization procedure via the Levenberg–Marquardt nonlinear least-squares algorithm, before fitting the corneal surface to spherical, conic and biconic models. The influence of pupil size (aperture radii of 1.5, 3.0, 4.0 and 5.0 mm) on corneal radius and asphericity was also analysed. <b>Results:</b> In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of the tilt angle in both positive and negative directions and aperture radii in all models. Fitting the cornea to spherical models did not show a significant difference between the raw-measured corneal surfaces and the levelled surfaces for right and left eyes. When the conic models were fitted to the cornea, changes in the radii of the cornea among the raw-measured corneal surfaces’ data and levelled data were not significant; however, significant differences were recorded in the asphericity of the anterior surfaces at radii of aperture 1.5 mm (<i>p</i> < 0.01). With the biconic model, the posterior surfaces recorded significant asphericity differences at aperture radii of 1.5 mm, 3 mm, 4 mm and 5 mm (<i>p</i> = 0.01, <i>p</i> < 0.01, <i>p</i> < 0.01 & <i>p</i> < 0.01, respectively) in the nasal temporal direction of right eyes and left eyes (<i>p</i> < 0.01, <i>p</i> < 0.01, <i>p</i> < 0.01 & <i>p</i> < 0.01, respectively). In the superior–inferior direction, significant changes were only noticed at aperture radii of 1.5 mm for both right and left eyes (<i>p</i> = 0.05, <i>p</i> < 0.01). <b>Conclusions:</b> Estimation of human corneal asphericity from topography or tomography data using conic and biconic models of corneas are affected by eyes’ natural tilt. In contrast, the apical radii of the cornea are less affected. Using corneal asphericity in certain applications such as fitting contact lenses, corneal implant design, planning for refractive surgery and mathematical modelling when a geometrical centre of the eye is needed should be implemented with caution.
format	article
author	Alejandra Consejo Arwa Fathy Bernardo T. Lopes Renato Ambrósio Ahmed Abass
author_facet	Alejandra Consejo Arwa Fathy Bernardo T. Lopes Renato Ambrósio Ahmed Abass
author_sort	Alejandra Consejo
title	Effect of Corneal Tilt on the Determination of Asphericity
title_short	Effect of Corneal Tilt on the Determination of Asphericity
title_full	Effect of Corneal Tilt on the Determination of Asphericity
title_fullStr	Effect of Corneal Tilt on the Determination of Asphericity
title_full_unstemmed	Effect of Corneal Tilt on the Determination of Asphericity
title_sort	effect of corneal tilt on the determination of asphericity
publisher	MDPI AG
publishDate	2021
url	https://doaj.org/article/383de9b821464ea4a6eea1f5855e42ff
work_keys_str_mv	AT alejandraconsejo effectofcornealtiltonthedeterminationofasphericity AT arwafathy effectofcornealtiltonthedeterminationofasphericity AT bernardotlopes effectofcornealtiltonthedeterminationofasphericity AT renatoambrosio effectofcornealtiltonthedeterminationofasphericity AT ahmedabass effectofcornealtiltonthedeterminationofasphericity
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Effect of Corneal Tilt on the Determination of Asphericity

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