THE SIMULATION STUDY ON THE DEFORMATION OF RABBIT CORNEA AFTER REFRACTIVE SURGERY WITH DIFFERENT CUTTING THICKNESS

Based on the inflation tests data of rabbit cornea, finite element analysis has been applied to determine the material parameters, simulate corneal refractive surgery and study postoperative corneal deformations. The corneal profile and apical displacement data were recorded during the inflation experiment of five rabbit corneas. Inverse finite element method was applied to determine the material parameters from the corneal apical displacements. Based on the determined material parameters and the corneal profile information, we established five corneal geometry models that simulate refractive surgery with different cutting amounts. We analyzed displacements at corneal apex and cutting edge, corneal surface curvatures under different pressures. Both Ogden model ([Formula: see text]) and Yeoh model ([Formula: see text]) gave good fits to the experiment data. The maximum of error square sum between the calculated value and the experimental value of the displacements per point at the corneal profile was less than 0.06[Formula: see text]mm. For each model with the increase of pressure, the displacement at cutting edge was larger than that at corneal apex, both of them increased, and curvature radius of anterior and posterior corneal surface increased slowly, but the refractive power decreased slowly and tended to be a stable value. Under the same pressure, the larger the cutting amount, the larger the displacements at corneal apex and cutting edge with a cutting edge displacement of about 1.10 (less ablation model) and 1.02 (larger ablation model) times the corneal vertex displacement. Both Ogden model and Yeoh model can be used to describe corneal mechanical responses of inflation experiment. After refractive surgery, the displacement at cutting edge is larger than that at corneal apex, the curvature radius of anterior (posterior) corneal surface increases (decreases), and the refractive power decreases.

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Evaluation of the B/F Ratio Distribution of Cataract Patients Without Corneal Abnormalities and Its Effect on Ocular Parameters

Clinical & Investigative Medicine ◽

10.25011/cim.v44i4.37197 ◽

2021 ◽

Vol 44 (4) ◽

pp. E46-54

Author(s):

Michael Deng ◽

Lina Lan ◽

Tianhui Chen ◽

Min Zhang ◽

Jiahui Chen ◽

...

Keyword(s):

Axial Length ◽

Central Corneal Thickness ◽

Spherical Aberration ◽

Linear Regression Analysis ◽

Corneal Thickness ◽

Curvature Radius ◽

Multivariate Linear Regression Analysis ◽

Corneal Surface ◽

Ratio Distribution ◽

Corneal Radius

Purpose: To evaluate the distribution of the posterior-anterior corneal radius ratio (B/F ratio; posterior corneal radius/anterior corneal radius) in patients without corneal abnormalities, and to investigate which parameters affect this ratio. Methods: Five thousand eyes from 5,000 patients who underwent cataract surgery were recruited to this study. We explored the linear relationship between B/F ratio and 13 variables using Principal Component-Multivariate Linear Regression Analysis. Results: The B/F ratio was negatively correlated with the difference between simulated keratometry (SimK) and true net power (TNP), central corneal thickness, spherical aberration (SA), and posterior corneal astigmatism and positively correlated with posterior corneal radius, corneal posterior surface, axial length (AL) and anterior corneal radius. Several variables (central corneal thickness, difference between SimK and TNP and asphericity coefficient (Q-value) of the posterior corneal surface) had the highest loading on the final score. B/F ratio reflects the refractive state and anatomical structure of the cornea: thus, higher B/F ratios were associated with larger posterior corneal surface curvature radius, longer axial length, thinner central corneal thickness, lower high order aberrations of the cornea and SA, and the numerical difference between simK and TNP gradually reduced. In clinical practice, for patients with lower B/F ratio, special care should be taken in the choice of system used for intraocular lens (IOL) measurements.

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The impact of irregular corneal shape parameters on visual acuity and contrast sensitivity

BMC Ophthalmology ◽

10.1186/s12886-020-01737-x ◽

2020 ◽

Vol 20 (1) ◽

Author(s):

Sanita Liduma ◽

Artis Luguzis ◽

Gunta Krumina

Keyword(s):

Visual Acuity ◽

Contrast Sensitivity ◽

Cross Sectional Study ◽

Shape Parameters ◽

Corneal Surface ◽

Cross Sectional ◽

Corneal Shape ◽

Different Diameters ◽

The Impact ◽

Corneal Apex

Abstract Background To understand which irregular corneal parameters determine the visual quality in keratoconus subjects. Methods The cross-sectional study examined the eyes of 44 subjects, graded from the first to third keratoconus stages by Amsler-Krumeich classification. We obtained measurements in two ways: (a) by projecting two perpendicular axes onto a cornea (first, through the central point of the cornea and keratoconus apex; second, as the perpendicular axis) to read the elevation values at points on these axes as parameters characterising the corneal surface; (b) by projecting circles with different diameters around the central part of the cornea (1, 2, and 3 mm) and reading elevation values at points equally displaced on these circles as parameters characterising an anterior surface slope. Irregular corneal shape parameters’ correlations with visual acuity and contrast sensitivity were determined in order to understand which corneal slope parameter has the strongest correlation with visual acuity and contrast sensitivity. Results Parameters characterising the corneal surface’s correlations with contrast sensitivity were from r = 0.25 (p = 0.03) at 3 cpd to r = 0.47 (p < 0.01) at 9 cpd for the highest elevation and from r = 0.33 (p = 0.09) at 5 cpd to r = 0.40 (p < 0.01) at 11 cpd for the lowest elevation in all subjects together, while for visual acuity the parameters were r = 0.30 (p < 0.01) for the highest elevation and r = 0.21 (p = 0.06) for the lowest elevation in all subjects together. The correlation between contrast sensitivity and the highest and lowest corneal point in all measured cornea was stronger for subjects with a peripheral corneal apex than for those with a central apex. In keratoconus subjects, contrast sensitivity displayed a strong correlation with slope in the central part of the cornea (with a radius of 1 mm) ranging from 0.48 (p < 0.01) at 3 cpd to 0.61 (p < 0.01) at 9 cpd. Conclusion Contrast sensitivity has a higher correlation with corneal shape parameters than with visual acuity. Subjects with a peripheral corneal apex had stronger correlations with visual acuity and contrast sensitivity than did subjects with a central apex. In keratoconus subjects, the strongest correlation was for contrast sensitivity and elevation (slope) in the region within a 1 mm radius of the corneal centre in the opposite direction of the keratoconus apex (direction (ax) CB).

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UNDERSTANDING THE VISCOELASTIC PROPERTIES OF RABBIT CORNEA BASED ON STRESS RELAXATION TESTS AND CYCLIC UNIAXIAL TESTS

Journal of Mechanics in Medicine and Biology ◽

10.1142/s0219519417400358 ◽

2017 ◽

Vol 17 (07) ◽

pp. 1740035 ◽

Cited By ~ 2

Author(s):

HAIXIA ZHANG ◽

XIUQING QIAN ◽

LIN LI ◽

ZHICHENG LIU

Keyword(s):

Stress Relaxation ◽

Test Data ◽

Viscoelastic Properties ◽

Linear Optimization ◽

Constitutive Models ◽

Optimization Techniques ◽

Rabbit Cornea ◽

Uniaxial Test ◽

Material Parameters ◽

Biomechanical Behavior

Background: Determining the viscoelastic properties of cornea is important in the fields of understanding of the tissue’s response to mechanical actions and the accurate numerical simulation of corneal biomechanical behavior under the effects of keratoconus and refractive surgery. To address this need, we present an approach to model the viscoelastic response of rabbit cornea from uniaxial test data. Methods: The corneal strip samples from six rabbits were obtained to perform cyclic uniaxial tension tests and stress relaxation tests. We investigated the suitability of six constitutive models, including empirical models and hyperelastic models, by a quasi-linear viscoelastic law. Applying non-linear optimization techniques, we found material parameters for each different strip sample. Results and conclusions: The model gave a better fit to loading data with [Formula: see text], and predicted the unloading data in the cyclic uniaxial tests with errors-of-fit ranging from 0.03 to 0.06. The results indicate that the best model is the power of the first invariant of strain with Prony form relaxation model, and that the method to identify the material parameters are valid for modeling the visoelastic response of cornea from uniaxial test data.

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Corneal Surface Ablation Laser Refractive Surgery for the Correction of Myopia: A Network Meta-analysis

Journal of Refractive Surgery ◽

10.3928/1081597x-20180905-01 ◽

2018 ◽

Vol 34 (11) ◽

pp. 726-735 ◽

Cited By ~ 5

Author(s):

Daizong Wen ◽

Ruixue Tu ◽

Ian Flitcroft ◽

Qinmei Wang ◽

Yingying Huang ◽

...

Keyword(s):

Refractive Surgery ◽

Meta Analysis ◽

Corneal Surface ◽

Surface Ablation ◽

Ablation Laser ◽

Laser Refractive Surgery

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Treating intrasurgical complications of corneal refractive operation with customized ablation. A clinical case

Russian Ophthalmological Journal ◽

10.21516/2072-0076-2021-14-3-102-105 ◽

2021 ◽

Vol 14 (3) ◽

pp. 102-105

Author(s):

N. V. Khodzhabekyan ◽

A. T. Khandzhyan ◽

A. V. Ivanova ◽

K. B. Letnikova ◽

A. S. Sklyarova

Keyword(s):

Visual Acuity ◽

Excimer Laser ◽

Refractive Surgery ◽

Clinical Case ◽

Photorefractive Keratectomy ◽

Corneal Surface ◽

Surface Errors ◽

Spherical Component ◽

Uncorrected Visual Acuity ◽

Corneal Refractive Surgery

Purpose: to evaluate the prospects of customized Finalfit based ablation for optical rehabilitation of a patient with corneal surface errors caused by intrasurgical microtome-induced damage of a corneal flap during a LASIK excimer laser correction of vision.Material and methods. In Helmholtz Center, a patient with such damage was given a phototherapeutic keratectomy (PTK) combined with topographyguided photorefractive keratectomy.Results. In two months after the surgery, the patient’s uncorrected visual acuity rose from 0.15 to 1.0; the spherical component of refraction changed from -3.5 to +1.0 D, the cylindrical component of refraction fell from 1.75 to 0.25 D. Aberrometry demonstrated a decrease of RMS from 2.19 to 0.61 D, Total HOAs from 2.281 to 0.829; Tilt from 0.406 to 0.313; HOA from 1.152 to 0.247, Coma from 0.298 to 0.124, Trefoil from 1.088 to 0.094, SA from 0,127 to 0,021.Conclusion. The clinical case demonstrates the expediency and efficiency of corneal remodeling for the rehabilitation of induced errors of the corneal surface by excimer laser correction of vision based on PTK (Flex scan) + FinalFit after corneal refractive surgery.

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Progress in Modern Cataract Surgery – New Steps and Algorithms for Precise Measuring and Intraocular-lens Calculations

European Ophthalmic Review ◽

10.17925/eor.2017.11.02.95 ◽

2017 ◽

Vol 11 (02) ◽

pp. 95

Author(s):

Fritz H Hengerer ◽

Gerd U Auffarth ◽

Ina Conrad-Hengerer ◽

◽

...

Keyword(s):

Cataract Surgery ◽

Anterior Chamber ◽

Intraocular Lens ◽

Axial Length ◽

Refractive Surgery ◽

Anterior Chamber Depth ◽

Corneal Surface ◽

Intraocular Lens Calculations ◽

Target Refraction ◽

Intraocular Lens Calculation

In standard cataract surgery, one of the major goals is to reach target refraction. Based on keratometry measurements, axial length and anterior chamber depth, most of the intraocular lens calculation formulae are suitable to achieve this aim. Further evaluation of corneal refractive parameters like anterior and posterior corneal surface by Scheimpflug devices led to a significant enhancement of precision in astigmatic and post-refractive surgery cases.

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Posterior corneal surface changes after refractive surgery

Ophthalmology ◽

10.1016/s0161-6420(01)00634-0 ◽

2001 ◽

Vol 108 (8) ◽

pp. 1415-1422 ◽

Cited By ~ 30

Author(s):

Everardo Hernández-Quintela ◽

Sopit Samapunphong ◽

Bilal F Khan ◽

Beatriz Gonzalez ◽

Paul Chung-Shien Lu ◽

...

Keyword(s):

Refractive Surgery ◽

Corneal Surface ◽

Surface Changes

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Where Do We Stand with The Ever Evolving Technology?

Pakistan Journal of Ophthalmology ◽

10.36351/pjo.v34i2.945 ◽

2019 ◽

Vol 34 (2) ◽

Author(s):

Yehia Salah Eldin Mostafa

Keyword(s):

Refractive Surgery ◽

New Technology ◽

Surgical Techniques ◽

Albert Einstein ◽

Well Being ◽

Technological Innovations ◽

Corneal Surface ◽

The Road ◽

Surgical Field ◽

Fast Evolution

"Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution." Albert Einstein. “Once a new technology rolls over you, if you're not part of the steamroller, you're part of the road”. Stewart Brand. The developments in the field of ophthalmology have gone through tremendous and fast evolution over the last 40 years. The purpose of this evolution is the well being of our patients. In the surgical field we aim at efficiency, safety and predictability. Technological innovations are always aimed to improve a surgical need or a patient outcome. For example with the development of modern phacoemulsification machines and techniques it became possible to aspirate the hardest of nuclei through a tiny incision down to 1.8 mm, which was not possible manually. So innovations in surgical techniques have evolved rapidly to improve the use of the technology. Another example is the introduction of excimer laser technology in refractive surgery which gave us the opportunity of precision in carving the corneal surface to attain the desired refractive result which was not possible with any previous technique.

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The Impact of Irregular Corneal Shape Parameters on Visual Acuity and Contrast Sensitivity

10.21203/rs.2.23099/v5 ◽

2020 ◽

Author(s):

Sanita Liduma ◽

Artis Luguzis ◽

Gunta Krumina

Keyword(s):

Visual Acuity ◽

Contrast Sensitivity ◽

Cross Sectional Study ◽

Shape Parameters ◽

Corneal Surface ◽

Cross Sectional ◽

Corneal Shape ◽

Different Diameters ◽

The Impact ◽

Corneal Apex

Abstract Background: To understand which irregular corneal parameters determine the visual quality in keratoconus subjects.Methods: The cross-sectional study examined the eyes of 44 subjects, graded from the first to third keratoconus stages by Amsler-Krumeich classification. We obtained measurements in two ways: (a) by projecting two perpendicular axes onto a cornea (first, through the central point of the cornea and keratoconus apex; second, as the perpendicular axis) to read the elevation values at points on these axes as parameters characterising the corneal surface; (b) by projecting circles with different diameters around the central part of the cornea (1, 2, and 3 mm) and reading elevation values at points equally displaced on these circles as parameters characterising an anterior surface slope. Irregular corneal shape parameters’ correlations with visual acuity and contrast sensitivity were determined in order to understand which corneal slope parameter has the strongest correlation with visual acuity and contrast sensitivity.Results: Parameters characterising the corneal surface’s correlations with contrast sensitivity were from r=0.25 (p=0.03) at 3 cpd to r=0.47 (p<0.01) at 9 cpd for the highest elevation and from r=0.33 (p=0.09) at 5 cpd to r=0.40 (p<0.01) at 11 cpd for the lowest elevation in all subjects together, while for visual acuity the parameters were r=0.30 (p<0.01) for the highest elevation and r=0.21 (p=0.06) for the lowest elevation in all subjects together. The correlation between contrast sensitivity and the highest and lowest corneal point in all measured cornea was stronger for subjects with a peripheral corneal apex than for those with a central apex. In keratoconus subjects, contrast sensitivity displayed a strong correlation with slope in the central part of the cornea (with a radius of 1 mm) ranging from 0.48 (p<0.01) at 3 cpd to 0.61 (p<0.01) at 9 cpd.Conclusion: Contrast sensitivity has a higher correlation with corneal shape parameters than with visual acuity. Subjects with a peripheral corneal apex had stronger correlations with visual acuity and contrast sensitivity than did subjects with a central apex. In keratoconus subjects, the strongest correlation was for contrast sensitivity and elevation (slope) in the region within a 1 mm radius of the corneal centre in the opposite direction of the keratoconus apex (direction (ax) CB).

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