Correlation between the dioptric power, astigmatism and surface shape of the anterior and posterior corneal surfaces (original) (raw)
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Posterior corneal curvature and its influence on corneal dioptric power
Acta Ophthalmologica, 2009
An algorithm for estimation of the posterior corneal curvature is presented and applied on data from normal and keratoconic eyes. Radius of central posterior corneal curvature are demonstrated to be (mean f SD) 6.71 f 0.23 mm and 5.58 f 0.78 mm in normals and keratoconic eyes, respectively. This corresponds to a ratio between posterior and anterior corneal curvature at 0.85 and 0.83 in the groups mentioned. Both these ratios are significantly smaller than the corresponding ratio at 0.88 in Gullstrand's schematic eye which on corneal dioptric power results in offset errors at 0.20D and 0.46D in normal and keratoconic eyes. It is further demonstrated that the ratio is not constant over the corneal surface, resulting in central peripheral dioptric offset errors between 0.2D and-0.31D in normals and between 0.46D and-0.38D in keratoconic eyes. On corneal dioptric power it is finally shown that a variation in the refractive index of aqueous humor has a 30 times larger influence than a similar variation in corneal refractive index. Key words: posterior corneal curvaturecorneal thickness profilephotokeratoscopykeratometrycorneal dioptric powerkeratoconusintraocular lens implant.
European journal of ophthalmology
To evaluate agreement in measurements of astigmatic axis power and location between keratometry and computer assisted videokeratography (corneal topography) on normal corneas with less than 1.50 D of idiopathic astigmatism. Keratometric readings with the 10 SL/O Zeiss ophthalmometer and corneal topographic maps with the TMS-1 were obtained by two independent examiners on 32 normal corneas. Measurement agreement between the two instruments was evaluated in regard to steep and flat meridian power and location, and in astigmatism magnitude (D). The limits of agreement (d-2 SD to d+2 SD) between the two instruments were found to be broad for clinical purposes in measuring the steep meridian power (-0.16 to -1.20 D), flat meridian power (0.43 to -1.25 D), and astigmatism (0.60 to -1.12 D). A constant bias of the TMS-1 towards the 10 SL/O Zeiss ophthalmometer was found, in measuring steeper both principal meridians and higher amount of astigmatism. Mean location difference was 19 degrees ...
The central-peripheral radius of the normal corneal curvature
Acta Ophthalmologica, 2009
Both eyes of 40 normal persons were examined with a photokeratoscope. A method for analyzing the photokeratoscopic data based on the principle of least square fitting is presented. It is demonstrated that the corneal flattening expressed as the relative change in the radius of the corneal curvature is proportional with the square of the chord distances from apex. The factor of proportionality RV is defined as the coefficient of radius variation, which, together with the radius of the apex curvature K, are characteristic constants for a given meridian in a given eye. Expressed as mean and SD, the study demonstrates for the horizontal meridian K = 7.86 nim (0.25) and RV = 0.71 X for the vertical meridian the corresponding dimension are K = 7.65 nim (0.24) and RV = 0.70 X nini-2 (0.38 X 1 V). These results iniply a change in the radius of the corneal curvature 1 nim and 5 mm from apex at, respectively, 0.7% and 17.7%. The precisions expressed as SD, by which the 2 parameters may be determined, are for K 0.03 nini and RV 0.12 X ninr2 (0.26 X mni-z. Kq words: Corneaphotokeratoscopyradius of curvatureniateniatical model.
Astigmatic Vector Analysis of Posterior Corneal Surface; Healthy versus Keratoconic Corneas
Egyptian Journal of Ophthalmology, (Mansoura Ophthalmic Center), 2022
To define an unconventional diagnostic factor for keratoconus. Design: Observational descriptive comparative cross sectional study Method: This study included two hundred and forty-four eyes of 244 patients divided into groups; normal corneas, or controls (C, n [100]), fruste (FFKc, n [28]) and manifest keratoconus (Kc, n [116]). Full Ophthalmic examination was performed. All candidates were examined using a rotating Scheimpflug corneal tomographer (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) to obtain corneal measurements. Astigmatic vector analyses were carried out according to the method proposed by Thibos. Results: The area under receiver operating characteristic curve (AUC) for posterior corneal APV between normal and manifest keratoconus was 0.73 (95% confidence interval): 0.66-0.80. By using ROC curve Sensitivity, Specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy at cutoff 0.30 were (65.0%, 80.0%, 78.9%, 66.1% and 73.1% respectively). As regard posterior corneal Blur; the AUC between normal and manifest keratoconus was 0.92 (95% confidence interval): 0.88-0.96. By using ROC curve Sensitivity, Specificity, PPV, NPV and accuracy at cutoff 6.65 were (85.3%, 89.0%, 90.0%, 84.0% and 86.1%) respectively. Conclusion: Vector analysis of posterior corneal astigmatism; APV and Blur, is a simple, unbiased and complementary way in the differentiation of normal from manifest keratoconus.
Effect of Corneal Tilt on the Determination of Asphericity
Sensors, 2021
Purpose: 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. Methods: 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. Results: In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of th...
A review of corneal diameter, curvature and thickness values and influencing factors*
African Vision and Eye Health
The cornea is an important ocular structure involved in the mediation of visual perception. It is the principal refractive surface of the eye and vision can be significantly affected by relatively small changes in its structure and parameters. Measurement of corneal parameters is important in the diagnosis and management of ocular diseasessuch as keratoconus and glaucoma, and also in the fitting of contact lenses or with refractive surgery such as Laser-Assisted in situ Keratomileusis(LASIK) and photorefractive keratectomy (PRK). The human corneal diameter, anterior curvature and centre thickness as well as factors influencing them are reviewed in this article. This review will be useful to eye care professionals who routinely measure these parameters when fitting contact lenses and assessing, diagnosing as well as managing corneal and other ocular conditions. (S Afr Optom 2013 72(4) 185-194)
Journal of the Optical Society of America A, 2006
Generally, the analysis of corneal topography involves fitting the raw data to a parametric geometric model that includes a regular basis surface, plus some sort of polynomial expansion to adjust the more irregular residual component. So far, these parametric models have been used in their canonical form, ignoring that the observation (keratometric) coordinate system is different from corneal axes of symmetry. Here we propose, instead, to use the canonical form when the topography is referenced to the intrinsic corneal system of coordinates, defined by its principal axes of symmetry. This idea is implemented using the general expression of an ellipsoid to fit the raw data given by the instrument. Then, the position and orientation of the three orthogonal semiaxes of the ellipsoid, which define the intrinsic Cartesian system of coordinates for normal corneas, can be identified by passing to the canonical form, by standard linear algebra. This model has been first validated experimentally obtaining significantly lower values for rms fitting error as compared with previous standard models: spherical, conical, and biconical. The fitting residual was then adjusted by a Zernike polynomial expansion. The topographies of 123 corneas were analyzed obtaining their radii of curvature, conic constants, Zernike coefficients, and the direction and position of the optical axis of the ellipsoid. The results were compared with those obtained using the standard models. The general ellipsoid model provides more negative values for the conic constants and lower apex radii (more prolate shapes) than the standard models applied to the same data. If the data are analyzed using standard models, the resulting mean shape of the cornea is consistent with previous studies, but when using the ellipsoid model we find new interesting features: The mean cornea is a more prolate ellipsoid (apical power 50 D), the direction of the optical axis is about 2.3°nasal, and the residual term shows three Zernike coefficients significantly higher than zero (third-order trefoil and fourthand sixth-order spherical). These three nonzero Zernike coefficients are responsible for most of the higherorder aberrations of the average cornea. Finally, we propose and implement a simple method for threedimensional registration of corneal topographies, passing from the general to the canonical form of the ellipsoid.
Scleral Shape and Its Correlations With Corneal Astigmatism
Cornea, 2018
Purpose: To assess the correlation between scleral shape and corneal astigmatism. Methods: Twenty-two participants (11 non-astigmatic and 11 astigmatic) aged from 19 to 36 years and with no previous ocular surgeries were included in this study. Three-dimensional (3D) corneo-scleral maps from both eyes (44 eyes) were acquired using a corneo-scleral topographer (Eye Surface Profiler). Each 3D map was split up in 13 concentric annuli, each 0.5 mm-wide, staring at 1.0 mm radius from the corneal apex to the scleral periphery at 7.5 mm from the apex. Each ring was fitted to a quadratic function of the radial distance to the apex, to calculate the elevation difference between the raw data and the fitting surface ring. For each ring the resulting elevation difference between original and fit data profile was fit to a sum of sines function. Decentration and astigmatic terms obtained from the sinusoidal fit were analyzed and compared between non-astigmatic and astigmatic groups. Results: In astigmatic eyes corneal and scleral asymmetry are highly correlated, while both appear independent from each other in non-astigmatic eyes. No significant difference between astigmatic and non-astigmatic eyes was found for decentration term (p > 0.05/[N (Bonferroni)), while for the astigmatic component the differences were statistically significant (p < 0.05/N (Bonferroni)). Conclusion: Corneal and scleral shape are correlated in astigmatic eyes, suggesting that astigmatism is not restricted to the cornea, but should rather be considered a property of the entire eye globe.
Journal of Cataract & Refractive Surgery, 2012
To compare the corneal astigmatism (magnitude and axis location) derived by total corneal power (TCP), automated keratometry, and simulated keratometry. Siriraj Hospital, Mahidol University, Bangkok, Thailand. Prospective comparative study. Eyes with previous ocular surgery or abnormalities were excluded. All patients were examined with the ARK 730A autokeratometer and the Galilei analyzer. The steepest and flattest corneal power along with the steepest axis of the TCP, automated keratometry, and simulated keratometry were recorded. Vector analysis (J0 and J45) was calculated. Analysis of variance with Bonferroni correction was performed for multiple comparisons. Outcome measures were the magnitude and axis location of astigmatism. One hundred eyes of 100 cataract patients were randomly selected. There was no statistically significant difference in the mean steepest axis between TCP (93.31 ± 68.75 [SD]), automated keratometry (94.24 ± 64.78), and simulated keratometry (92.42 ± 64.30). However, the mean magnitude of astigmatism measured by TCP (1.23 ± 0.75) was significantly higher than that measured by automated keratometry (0.93 ± 0.68) (P=.01) but not than that measured by simulated keratometry (1.08 ± 0.68) (P=.43); there was no statistically significant difference in J0 or J45. Twenty two (40%) of 54 eyes with more than 1.00 diopter of TCP astigmatism had more than 10 degrees of axis difference from automated keratometry. The magnitude of TCP astigmatism was higher than that of automated keratometry. The axis location was similar. However, there was more than 10 degrees of axis difference between automated keratometry and TCP in patients with high astigmatism. No author has a financial or proprietary interest in any material or method mentioned.