Corneal biomechanical parameters in keratoconus eyes with abnormal elevation on the back corneal surface only versus both back and front surfaces

AbstractCorneal biomechanical parameters were compared in 100 keratoconus eyes with abnormal elevation on the back corneal surface only (group 1), versus both the back and front surfaces (group 2). Scheimpflug tomography with Pentacam HR, corneal biomechanical assessments using Corvis ST and Ocular Response Analyzer (ORA) and corneal epithelium thickness maps using anterior segment optical coherence tomography were assessed. There were no significant differences in the IOP measured using Corvis ST and ORA, age or sex between the two groups. Statistically significant differences were found in all corneal shape parameters and all new parameters of Corvis ST: corneal stiffness parameter at first applanation (SP-A1), integrated inverse radius (IR) and deformation amplitude ratio (DAR)) between groups (p < 0.001). The classic parameters of ORA including corneal hysteresis (CH) and corneal resistance factor (CRF) were about 1.00 mmHg higher in group 1 (p < 0.001). In conclusion, keratoconus eyes with abnormal elevation limited to the back corneal surface have lower grade, stiffer corneal biomechanical parameters and less asymmetric shape. This is consistent with progressive biomechanical weakening from the first detectable back surface elevation to manifestation on the front surface as the severity overwhelms the ability of the epithelium to compensate.

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Effect of Surface Roughness on Ultrasonic Testing of Back-Surface Micro-Cracks

Applied Sciences ◽

10.3390/app8081233 ◽

2018 ◽

Vol 8 (8) ◽

pp. 1233 ◽

Cited By ~ 1

Author(s):

Zhe Wang ◽

Ximing Cui ◽

Hongbao Ma ◽

Yihua Kang ◽

Zhiyang Deng

Keyword(s):

Surface Roughness ◽

Ultrasonic Testing ◽

Amplitude Ratio ◽

Ultrasonic Inspection ◽

Front Surface ◽

Signal Amplitude ◽

Back Surface ◽

Element Analysis ◽

Arithmetic Average ◽

Micro Cracks

Surface roughness is one of the main factors that affect the ultrasonic testing of micro-cracks. This article theoretically analyzes the relationship between the changes in the energy intensity of crack echo waves and roughness-modified transmission coefficients. A series of simulations are carried out using two-dimensional sinusoidal curves as rough surface. Then, parallel experiments are performed on sample surfaces with different arithmetic average heights (Ra). The signal amplitude ratio factor (SARF) is defined to assess the ultrasonic detection capacity for micro-cracks. Both finite element analysis and experimental results show that signal amplitude decreases with an increase in Ra, resulting in signal-to-noise ratio loss. Amplitude attenuation caused by the rough back surface is less than that caused by the rough front surface. It is difficult to identify the signal of micro-cracks with a depth less than 400 μm when the Ra of the front surface is larger than 15 μm. Cracks with depths of more than 200 μm can be distinguished when the back-surface roughness is less than 24 μm. Furthermore, the amplitude of the micro-crack signal increases slightly with variation in the horizontal parameter (Rsm). This study provides a valuable reference for the precision evaluation of micro-cracks using ultrasonic inspection.

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Relationship between corneal biomechanical parameters and corneal sublayer thickness measured by Corvis ST and UHR-OCT in keratoconus and normal eyes

Eye and Vision ◽

10.1186/s40662-020-00225-z ◽

2021 ◽

Vol 8 (1) ◽

Author(s):

Yong Li ◽

Zhiqiang Xu ◽

Qiaoli Liu ◽

Yuzhou Wang ◽

Kan Lin ◽

...

Keyword(s):

Layer Thickness ◽

Amplitude Ratio ◽

Normal Subjects ◽

Stiffness Parameter ◽

Biomechanical Stability ◽

Cross Sectional ◽

Corvis St ◽

Sublayer Thickness ◽

Biomechanical Parameters ◽

The Relationship

Abstract Background To explore the relationship between corneal biomechanical parameters and corneal sublayer thickness using corneal visualization Scheimpflug technology (Corvis ST) and ultrahigh-resolution optical coherence tomography (UHR-OCT) in clinical and suspected keratoconus and normal eyes. Methods Cross-sectional prospective study. A total of 94 eyes of 70 participants were recruited. Twenty five eyes of 19 keratoconus patients, 52 eyes of 34 patients showing high risk of developing keratoconus according to the Belin/Ambrosio Enhanced Ectasia Display, and each eye of 17 normal subjects were enrolled. All participants underwent Corvis ST, Pentacam, and UHR-OCT examinations at the same time. Stiffness parameter A1 (SP-A1), deformation amplitude ratio (DA ratio), and other biomechanical parameters were recorded using Corvis ST. The vertical and horizontal thickness profiles of central 3 mm corneal epithelium, Bowman’s layer, and stroma as measured by the perpendicular distance between the neighboring interfaces were generated using UHR-OCT. The flat keratometry and steep keratometry were obtained using Pentacam. Analysis of correlation was applied to explore the association between variables. Results Most of the biomechanical parameters and corneal sublayer thickness profiles showed statistical differences among three groups. A statistically significant linear relationship was noted between SP-A1 and DA ratio in all three groups. SP-A1 was found to be positively correlated with epithelial and Bowman’s layer thickness in the keratoconus (KC) group, and with stromal thickness in all three groups. In the normal and suspected keratoconus (SKC) groups, only stromal thickness was included in the stepwise linear regression to predict SP-A1, whereas in the KC group, steep keratometry and Bowman’s layer thickness were included. Conclusions Significant and different correlations were noted between corneal stiffness and corneal sublayer thickness in different groups, indicating that corneal sublayers may play different roles in maintaining corneal biomechanical stability between keratoconus and normal eyes.

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Accuracy of the indices of MS-39 anterior segment optical coherence tomography in the diagnosis of keratoconic corneas

European Journal of Ophthalmology ◽

10.1177/11206721211063720 ◽

2021 ◽

pp. 112067212110637

Author(s):

Rania S Elkitkat ◽

Yasser Rifay ◽

Hesham M Gharieb ◽

Hossam Eldin A Ziada

Keyword(s):

Optical Coherence Tomography ◽

Anterior Segment ◽

Front Surface ◽

Receiver Operating Curve ◽

P Value ◽

Optical Coherence ◽

Corneal Surface ◽

Cross Sectional ◽

The Mean ◽

Group 2

Purpose To determine the normative and the cut-off values of various indices available in the MS 39 Anterior Segment Optical Coherence Tomography (MS-39 AS-OCT) for keratoconus (KC) diagnosis, and to detect the accuracy of the variable available parameters. Methods This cross sectional observational study was conducted at Dr Rifay Ophthalmology Center, Rabat, Morocco, on 172 eyes with KC (group 1) and 248 eyes of healthy controls (group 2). Participants were screened using MS-39 AS-OCT (CSO, Firenze, Italy). The investigated indices included: keratometric indices, pachymetric indices, elevation indices, Keratoconus Summary Indices (KSI), aberration indices, and epithelial mapping evaluation. Results Most of the studied indices were significantly different between both groups (P value <0.001). Twenty two indices had Area Under the Receiver Operating Curve (AUROC) values >0.950, eight of which were from the KSI. The indices with the highest AUROC values were: Root Mean Square (RMS) for the front corneal surface at 6 mm circle (AUROC = 0.996, and best cut-off >1.69 with sensitivity of 97.67% and specificity of 97.98%) and RMS per unit of area for the front surface at 6 mm circle (AUROC = 0.996, and best cut-off >0.06 with sensitivity of 97.67% and specificity of 97.98%). Epithelial thickness-derived metrics were not among the parameters with the highest accuracy. Conclusions This study presented the mean and the cut-off values for a plethora of parameters available in the MS-39 AS-OCT. The results of this study show that MS-39 AS-OCT is a valuable equipment for diagnosing keratoconic corneas, with a high accuracy detected for many parameters.

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Corneal biomechanical properties in myopic eyes evaluated via Scheimpflug imaging

10.21203/rs.3.rs-18474/v2 ◽

2020 ◽

Author(s):

A-Yong Yu ◽

Hui Shao ◽

Anpeng Pan ◽

Qinmei Wang ◽

Zixu Huang ◽

...

Keyword(s):

Axial Length ◽

High Myopia ◽

Anterior Segment ◽

Biomechanical Properties ◽

Corvis St ◽

Negatively Associated ◽

Scheimpflug Imaging ◽

Biomechanical Parameters ◽

Corneal Biomechanical Properties ◽

Myopia Control

Abstract Background: To investigate the biomechanical properties of the cornea in myopic eyes using corneal visualization scheimpflug technology (Corvis ST). The relationships between the biomechanical properties of the cornea and the degree of myopia were also investigated.Methods: 265 eyes of 265 subjects were included. Based on spherical equivalent (SE) in diopters (D), participants were divided into four groups: low myopia/control (SE: -0.50 to -3.00D), moderate myopia (SE: -3.00 to -6.00D), high myopia (SE: -6.00 to -10.00D) and severe myopia (SE greater than -10.00D). Axial length (AL), anterior segment parameters, and corneal biomechanical properties were obtained with the Lenstar LS900, Pentacam HR and Corvis ST, respectively.Results: Mean (±SD) SE was -7.29±4.31D (range: -0.63 to -25.75D). Mean AL was 26.31±1.82mm (range: 21.87 to 31.94mm). Significant differences were detected within the four groups in terms of six corneal biomechanical parameters: deformation amplitude (DA), time from start until second applanation (A2-time), length of flattened cornea at the second applanation (A2-length), corneal velocity during the first and second applanation (A2-velocity), time from start to highest concavity (HC-time), and central curvature at highest concavity (HC radius). AL was positively associated with DA whereas negatively associated with A1-velocity and A2-length. SE was positively associated with A2-time, HC-time and A2-velocity, whereas negatively associated with DA. IOP was positively associated with four corneal biomechanical parameters and negatively associated with three parameters.Conclusions: Eyes with severe myopia showed greater DA, lesser A2 time, HC time, and faster A2-velocity compared to low to high myopia. This suggests the cornea becomes weaker and more deformable with elongation of axial length with corresponding increases in myopia. DA, A2-time and A2-velocity could be useful corneal biomechanical indicators in patients with myopia.

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Corneal tomographic changes during corneal rigid gas-permeable contact lens wear in keratoconic eyes

British Journal of Ophthalmology ◽

10.1136/bjophthalmol-2020-317057 ◽

2020 ◽

pp. bjophthalmol-2020-317057

Author(s):

Shizuka Koh ◽

Ryota Inoue ◽

Naoyuki Maeda ◽

Yoshinori Oie ◽

Vishal Jhanji ◽

...

Keyword(s):

Contact Lenses ◽

Amplitude Ratio ◽

Anterior Segment ◽

Three Dimensional ◽

Biomechanical Properties ◽

Corneal Surface ◽

Surface Flattening ◽

Contact Lens Wear ◽

Fourier Harmonic ◽

The Difference

Background/AimsWe aimed to investigate the refractive changes in the posterior corneal surface in keratoconus (KC) associated with wearing spherical corneal rigid gas-permeable contact lenses (corneal GPs) with apical touch or three-point touch fitting and the effect of spherical corneal GPs on corneal biomechanics.MethodsPatients with KC wearing corneal GPs every day without facing complications were enrolled as a single group. Corneal tomographic data were obtained using a three-dimensional anterior segment optical coherence tomography from the same eye with and without corneal GPs. Dioptric data from the central 3-mm zone of the posterior corneal surface were decomposed into spherical, regular astigmatism, asymmetry and higher-order irregularity components using Fourier harmonic analysis. The corneal biomechanical indices were deformation amplitude ratio within 2 mm, integrated radius, stiffness parameter at first applanation and linear Corvis Biomechanical Index. Correlations between the difference in Fourier indices with and without corneal GPs and the corneal biomechanical parameters were analysed.ResultsThirty-two eyes of 32 patients with KC were enrolled. Spherical, regular astigmatism and asymmetry components were significantly smaller with corneal GP wear than without the wear (all p<0.001). All biomechanical indices were significantly correlated with the difference in the spherical components with and without corneal GPs.ConclusionCorneal biomechanical properties of KC were correlated with posterior corneal surface flattening induced by wearing corneal GPs on the spherical components. This effect is greater in biomechanically weaker corneas.

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Estimation of final cylindrical spectacle correction by Pentacam

Function and Disability Journal ◽

10.32598/fdj.3.7 ◽

2020 ◽

Vol 3 (1) ◽

pp. 61-68

Author(s):

Raziye Dayyani ◽

◽

Ebrahim Jafarzadehpur ◽

Ramin Salouti ◽

Ali Mirzajani ◽

...

Keyword(s):

Regression Analysis ◽

Front Surface ◽

Back Surface ◽

P Value ◽

Corneal Surface ◽

Imaging Results ◽

The Mean ◽

Irregular Astigmatism ◽

Eye Clinic ◽

Spectacle Correction

Background and Objectives: Keratoconus is a non-inflammatory asymmetric corneal degenerative disease characterized by the steepening and distortion of the cornea. In this disease, irregular astigmatism makes the subjective refraction difficult. Using the Pentacam, we aimed to determine the corrective astigmatism of the spectacle. Methods: The subjective refraction and Pentacam imaging were performed for 317 keratoconic patients who referred to the Salouti Eye Clinic, Shiraz, Iran. Then, the astigmatism values obtained with subjective refraction were compared with the values of anterior and posterior corneal surface astigmatism obtained with the Pentacam imaging. Results: The mean age of 317 keratoconic patients was 29.81 years (range: 15 to 45 years). The minimum, maximum, and average astigmatism of the corneal front surface were 0.3, 16.8, and 3.21 diopters, respectively. Also, the minimum, maximum, and average astigmatism of the corneal back surface were 0, 2.8, and 0.67 diopters, respectively. Moreover, the P value was lower than 0.001 in the regression analysis of the subjective refraction for both the power and axis of the cylinder. Conclusion: The regression formulas obtained in this study can accurately (with a probability of 99%) predict refractive astigmatism, using corneal astigmatism.

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Corneal biomechanical properties in myopic eyes evaluated via Scheimpflug imaging

10.21203/rs.3.rs-18474/v1 ◽

2020 ◽

Author(s):

A-Yong Yu ◽

Hui Shao ◽

Anpeng Pan ◽

Qinmei Wang ◽

Zixu Huang ◽

...

Keyword(s):

Anterior Segment ◽

Biomechanical Properties ◽

Control Group ◽

Spherical Equivalent ◽

Corvis St ◽

Scheimpflug Imaging ◽

Biomechanical Parameters ◽

Anterior Segment Parameters ◽

Corneal Biomechanical Properties ◽

Myopia Control

Abstract Background To investigate the biomechanical properties of the cornea in myopic eyes using corneal visualization scheimpflug technology (Corvis ST). The relationships between the biomechanical properties of the cornea and the degree of myopia were also investigated.Methods 265 eyes of 265 subjects were included. According to spherical equivalent (SE) in diopters (D), participants were divided into four groups: low myopia/control group (SE: -0.50 to -3.00D), moderate myopia group (SE: -3.00 to -6.00D), high myopia group (SE: -6.00 to -10.00D) and severe myopia group (SE greater than -10.00D). Axial length (AL), anterior segment parameters, and corneal biomechanical properties were obtained with the Lenstar LS900, Pentacam HR and Corvis ST, respectively. Results Mean (±SD) SE was -7.29±4.31D (range: -0.63 to -25.75D). Mean AL was 26.31±1.82mm (range: 21.87 to 31.94mm). Significant differences were detected within the four groups in terms of six corneal biomechanical parameters: deformation amplitude (DA), time from start until second applanation (A2-time), length of flattened cornea at the second applanation (A2-length), corneal velocity during the first and second applanation (A2-velocity), time from start to highest concavity (HC-time), and central curvature at highest concavity (HC radius). DA was positively correlated with AL ( r =0.16, P =0.009) and negatively correlated with SE ( r =-0.20, P =0.001). A2-time, A2-velocity, A2-length and HC-time were positively correlated with SE and negatively correlated with AL.Conclusions The alterations in corneal biomechanics may be associated with the degree of myopia. DA and A2-velocity may be useful corneal biomechanical indicators in patients with myopia.

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The effect of PRK on corneal epithelium thickness and pachmetry in treatment of myopia.

International Journal of Research in Pharmaceutical Sciences ◽

10.26452/ijrps.v9ispl1.1285 ◽

2018 ◽

Vol 9 (SPL1) ◽

Author(s):

Furkaan Majied Hamied ◽

Deyaa Neama Kadhim ◽

Sohaib A Mahmood

Keyword(s):

Refractive Error ◽

Anterior Segment ◽

Thickness Change ◽

Corneal Epithelial ◽

Epithelial Thickness ◽

Refractive Outcomes ◽

Epithelium Thickness ◽

Profile Changes ◽

Corneal Epithelial Thickness ◽

Myopic Astigmatism

In order to facilitate the corneal stromal ablation in photorefractive keratectomy the epithelium is removed so corneal repair associated with changes in epithelium and stroma. To study the corneal epithelial thickness and pachymetry profile changes after photorefractive keratectomy (PRK) for myopia. Retrospective analysis of the postoperative corneal epithelial thickness and pachymetry profile changes in 22 eyes of 12 patients treated with PRK for myopia or myopic astigmatism. Corneal and epithelial thickness maps within the central 6 mm were obtained by anterior segment spectral-domain optical coherence tomography (SD-OCT) preoperatively and at 3 months postoperatively. Correlations between pachymetry,epithelial thickness changes and the amount of correction,were analyzed.Compared to preoperative values,the central 2 mm and the paracentral 2 to 5 mm zone epithelium was 1 ± 2.85 and 1 ± 3.11 μm thicker,respectively,at 3 months postoperatively (P <.05). The spherical equivalent (SE) changed from-2.80 ± 2.028 diopters (D) preoperatively to -0.40 ± 0.42 D at 3 months postoperatively. Females show greater postoperative epithelial thickening, 2.6 ± 3.77 μm,than males,0.34 ± 1.98 μm. There was a trend toward greater epithelial thickening with a larger amount of programmed SE correction, and thinner preoperative epithelium. No correlation between epithelial thickness change and postoperative change in refraction was detected.Negative correlation between between age, refractive error,with the pre and post-operative pachymetry. In general female pachymetry reading is higher than it in male.The corneal epithelial thickness increased after PRK up to 3 months postoperatively. It was affected by the amount of myopia treated, gender, and preoperative epithelial thickness. The refractive outcomes did not affected by the postoperative epithelial thickening. Negative correlation between between age, refractive error, with the pre and post-operative pachymetry. In general female pachymetry reading is higher than it in male.

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