Research on distribution characteristics of corneal asphericity of myopic eyes
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Abstract:
Objective
To evaluate the distribution characteristics of the asphericity (Q-value) of the corneal surfaces in a Chinese population who were candidates for excimer laser surgery; to provide some theoretical bases for clinical diagnosis and treatment.
Methods
This was a perspective study. Using the Pentacam HR measuring and analyzing system based on Scheimpflug photography, the right eyes of 75 Chinese myopes were evaluated for excimer laser surgery. Patients were divided into a low corneal astigmatism (<+1.00 D) group and a medium-high corneal astigmatism (≥+1.00 D) group according to the anterior corneal surface astigmatism. The analysis was based on the distribution characteristics of the mean Q-values of both the anterior and posterior corneal surfaces and the distribution characteristics of the Q-value at different angles (20°, 25° and 30°) and along different semi-meridians (superior, inferior, nasal and temporal). Single factor analysis of variance was used to determine if there was a difference among Q-values within a particular group. The differences between the two groups were analyzed by an independent samplest-test.
Results
The mean Q-values of the 30° angle in Chinese myopes evaluated for excimer laser surgery were-0.17±0.09 and-0.02±0.16 on the anterior and posterior corneal surfaces, respectively. The mean Q-values of the anterior corneal surface at different angles were negative and there was no statistically significant difference between the low corneal astigmatism group and medium-high corneal astigmatism group. There was a significant difference between the Q-values of the 2 groups at different angles (F=6.340, 9.963,P 0.05). In the medium-high corneal astigmatism group, QT30 was significantly bigger than QN30(P QN30 and QI30>QS30, but there was no statistically significant difference between QT30 and QS30.
Conclusion
The curvature along the anterior corneal surface becomes flatter from the center to the periphery, while it becomes steeper from the center to the periphery at the posterior corneal surface. The rate of the flattening of the anterior corneal surface increased as the range of the angle increased, while the rate decreased as the posterior corneal surface decreased. All the shapes on the anterior surface along different semi-meridians were long ovals. Corneal astigmatism does not affect corneal shape. The shapes of the posterior surface along the superior and inferior semi-meridians were short ovals, but were long ovals along the nasal and temporal semi-meridians.
Key words:
Cornea; Asphericity; Scheimpflug PhotographyKeywords:
Scheimpflug principle
Astigmatism
Chinese population
To evaluate the correlation of the mean curvature and shape factors between both corneal surfaces for different corneal diameters measured with a Scheimpflug photography-based system.Vissum Corporation, Alicante, Spain.Case series.Randomly selected healthy normal eyes had a comprehensive ophthalmologic examination including anterior segment analysis with the Sirius system as follows: anterior and posterior mean corneal radius for 3.0 mm, 5.0 mm, and 7.0 mm; anterior and posterior mean shape factor for 4.5 mm and 8.0 mm; central (CCT) and minimum corneal thickness; and anterior chamber depth (ACD).The study enrolled 117 eyes (117 subjects; aged 7 to 80 years). The mean anterior mean corneal radius:posterior mean corneal radius ratio was 1.19 (range 1.12 to 1.27) for all corneal diameters (P = .86). The correlation coefficient between the anterior and posterior mean corneal radius was 0.85 or more for all corneal diameters. The anterior mean shape factor:posterior mean shape factor ratio for the 2 corneal diameters analyzed was approximately 1 (range 0.45 to 4.03). The correlations between anterior and posterior mean shape factors were extremely poor and not significant. Multiple regression analysis showed that the central posterior mean corneal radius was significantly correlated with the anterior mean corneal radius, CCT, and spherical equivalent (R(2) = 0.77, P<.01).Central posterior corneal curvature could be predicted from the anterior corneal curvature, pachymetry, and the refractive status of the eye but not from the corneal shape factor.No author has a financial or proprietary interest in any material or method mentioned.
Scheimpflug principle
Radius of curvature
Anterior Eye Segment
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To compare the anterior and posterior corneal curvature in eyes with previous radial keratotomy (RK) to normal unoperated eyes.In this retrospective observational case series, 29 eyes from 29 consecutive patients were analyzed and compared to a control group of 71 unoperated eyes. Corneal imaging was obtained by a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH). Anterior and posterior corneal curvature radii were measured at the 3-mm zone.The mean anterior and posterior corneal radii were 9.54 ± 0.89 and 8.54 ± 1.01 mm, respectively, both values being significantly higher than in the control group (7.81 ± 0.28 and 6.40 ± 0.24 mm, respectively, P<.0001). The mean anterior-to-posterior corneal curvature ratio was 1.12 ± 0.07, a value significantly lower than in the control group (1.22 ± 0.03, P<.0001). Mean corneal flattening was more evident in the posterior (33.44%) than in the anterior (22.15%) corneal curvature. The mean keratometric index, as calculated with the Gullstrand equation for thick lenses, was 1.3319 ± 0.0026, a value significantly higher than in the control group (1.3281 ± 0.0011, P<.0001). Linear regression detected a significant and directly proportional relationship between the number of radial incisions and flattening of both corneal surfaces (P<.0001).After RK, both corneal surfaces flatten but do not deform in parallel as commonly accepted, as shown by the fact that the anterior-to-posterior corneal curvature ratio decreases. This finding invalidates the standard keratometric index and thus has relevant implications for intraocular lens power calculation in RK eyes.
Scheimpflug principle
Radial keratotomy
Flattening
Anterior Eye Segment
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Citations (30)
Objective: To investigate the effect of myopia on human anterior corneal surface asphericity index(Q value).Methods: 300 myopes were chosen from LASIK candidates at randome in our center,and the datum of bilateral anterior corneal surface asphericity index(Q value) were harvested by using Orbscan-Ⅱz corneal topography system made by BauschLomb corp.Then all cases were analyzed by dividing into two groups according to right and left eyes,or into three groups according to different degrees and courses of myopia respectively,in order to explicit the characteristic in myopes.Results: Q value between the right and left myopic eyes showed significant differences(P=0.000).But differences among the groups of which neither different degrees or courses of myopia were observed(all P0.05).Conclusions: Q value in myopes were different between bilateral eyes.And it was relative constant in spite of different refractions and courses of myopia.
LASIK
Anterior surface
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Objective: To compare the difference and agreement of corneal refractive power and astigmatism measured by the new swept-source anterior segment optical coherence tomography (SS-AS-OCT) device (CASIA2) and Scheimpflug imaging device (Pentacam) in age-related cataract patients. Methods: In this cross-sectional study, 112 eyes of 112 patients with age-related cataract were examined before phacoemulsification in the Tianjin Medical University Eye Hospital from April to May 2020. The steep keratometry (Ks), flat keratometry (Kf), mean keratometry (Km), degree and axis of astigmatism of the corneal anterior and posterior surfaces and the total cornea were recorded. The difference in astigmatism was analyzed by the arithmetic method and the vector method. The difference of data was evaluated using Paired t test or Wilcoxon test. The agreement of data was evaluated using Bland-Altman plots. Results: The patients were 44 males and 68 females with an average age of (67±10) years. There were no statistically differences in the Ks and Km values of the corneal anterior surface between the Pentacam and the CASIA2 (both P>0.05). There was significant difference in the Kf values of the corneal anterior surface between the Pentacam and the CASIA2 [(44.24±1.73) D vs. (44.14±1.64) D; t=2.278; P 0.05). Vector operation results showed that the astigmatism difference vector (DV) of the Pentacam and the CASIA2 on the anterior surface of the cornea was 0.06 D@57°±0.64 D,>0.50 D in 47 eyes (41.96%). The astigmatism DV on the posterior surface of the cornea was 0.07 D@174°±0.21 D, >0.50 D in 2 eyes (1.79%). The astigmatism DV on the total cornea was 0.13 D@3°±0.69 D,>0.50 D in 59 eyes (52.68%). The results of the two devices were positively correlated (r values of the Ks, Kf, Km, and astigmatism degree on the anterior surface of the cornea were 0.970, 0.968, 0.976, and 0.697, respectively, on the posterior surface of the cornea were 0.918, 0.875, 0.925, and 0.517, respectively, and on the total cornea were 0.951, 0.955, 0.959, and 0.622, respectively; all P<0.01). Bland-Altman analysis showed that the Ks, Kf, Km, and astigmatism degree of the corneal anterior and posterior surfaces measured by the two devices were consistent, with 2.68% to 8.04% of the measured values outside 95% limits of agreement (95%LoA). The 95%LoA of the total corneal Ks, Kf, and Km between the two devices was 0.01 to 2.28 D, 0.06 to 2.27 D, and 0.10 to 2.20 D, respectively, with a wide 95%LoA range and poor consistency. Conclusions: The corneal refractive power and astigmatism measured by the CASIA2 and the Pentacam shows little difference on the anterior surface of the cornea, with good agreement. However, the refractive power results of the posterior surface of the cornea and the total cornea show great difference, suggesting that these two instruments cannot be used interchangeably in clinical practice. (Chin J Ophthalmol, 2021, 57: 48-55).
Scheimpflug principle
Astigmatism
Anterior Eye Segment
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Citations (6)
Objective To investigate the distribution of corneal thickness in myopic patients and the correlation of corneal thickness,refractive power and corneal curvature in myopia.Methods In a cross-sectional study,a total of 264 people (458 eyes) with myopia or myopic astigmatism were examined.One hundred twenty-two males and 142 females participated in the study.Corneal thickness was measured with a Pentacam system.Measurements were taken at the corneal apex,center point,thinnest point and the average value of all corneal thicknesses measured along 2 mm,4 mm,6 mm and 8 mm diameter rings with the apex location at its center.The average values for corneal thickness and curvature were recorded in an increasingly clockwise direction for the right eyes,and in a counterclockwise direction for the left eyes with the binocular temporal side set to 0°.The subjects were divided into three groups according to refractive error: the low myopia group (refractive error ＜-3.00 D),the moderate myopia group (refractive error ＞-3.00 D and ＜-6.00 D) and the high myopia group (refractive error ＞-6.00 D).ANOVA was used to analyze the differences for the three groups.The different points for corneal thickness between two groups were compared using an independent samples t test.Pearson correlation analysis was used to study the correlation of corneal thickness at different points and the relationship between corneal thickness and other factors.Results The mean values of corneal thickness at the corneal apex、center point and thinnest point were 529.58±22.38 μm,529.75±22.51 μm,and 527.78±22.7 μm.The greatest mean corneal thickness measured along the different diameter rings with the apex location at its center was always at 90 degrees and the mean values were 544.14±22.07 μm,583.15±24.72 μm,646.27±27.34 μm,and 716.52±27.70 μm.The lowest mean corneal thickness measured along the different diameter rings was at 315 degrees,315 degrees,337.5 degrees,and 342 degrees,and the values were 532.48±23.51 μm,549.24±26.97 μm,590.02±27.06 μm,and 657.37±32.03 μm.The mean values of corneal thickness at the corneal apex,center point and thinnest point were found to have a positive correlation between the left and tight eyes (r=0.768,0.548,0.671,P＜0.05).No significant correlation was found between corneal thickness and refractive error or tangential curvature (P＞O.05).Conclusion For the different diameter rings of corneal thickness in myopia,the peripheral corneal thickness values were lowest in the temporal and inferotemporal areas and greatest in the superior area.There was no relationship between corneal thickness and refractive error and corneal curvature.
Key words:
Myopia; Corneal thickness; Diopter; Curvature
Astigmatism
Apex (geometry)
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Objective To evaluate the distribution of asphericity of the anterior and posterior corneal surfaces of young myopic patients,and to study the relationship between the Q-value and other factors.Methods Two hundred and three patients(406 eyes) with a spherical equivalence(SE) from -1.00 D to -11.38 D and age from 18 to 36 years were examined with a Pentacam measurement system and an evaluation system for the anterior segment of the eye.Q-values,corneal aberrations,and corneal astigmatisms were assessed.Results The Q20,Q25,Q30,Q35,and Q40 of the anterior corneal surface were -0.14±0.15,-0.20±0.11,-0.24±0.09,-0.30± 0.23,and -0.31±0.23,respectively.The Q20,Q25,Q30,Q35,and Q40 of the posterior corneal surface were 0.48 ±0.32,0.27 ±0.25,0.09±0.20,-0.05±0.18,and-0.20±0.17,respectively.Positive relationships were found between the anterior corneal surface Q30 and the sphere(r=0.11),cylinder(r=0.19),spherical equivalent(r= 0.15),corneal astigmatism(r=0.31),and the values of Z40(r=0.64) and RMSh(r=0.56) of the anterior corneal surface.Positive relation-ships were also found between the posterior corneal surface Q30 and the values of Z40(r=0.82),RMSh(r=0.59),and RMSg(r=0.24) of the posterior corneal surface.Variables such as gender,age,and the history of corneal contact lens wear had no significant effect on the Q30.The difference in the Q-value between the right and left eyes showed no statistical significance.Conclusion The anterior corneal surface of most young myopic patients is prolate,the Q-value of the anterior and posterior corneal surface becomes smaller as the measurement area is extended.Many factors affect the Q-value.
Anterior surface
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AIM: To explore the effect of the posterior astigmatism on total corneal astigmatism and evaluate the error caused by substituting the corneal astigmatism of the simulated keratometriy (simulated K) for the total corneal astigmatism in age-related cataract patients. METHODS: A total of 211 eyes with age-related cataract from 164 patients (mean age: 66.8±9.0y, range: 45-83y) were examined using a multi-colored spot reflection topographer, and the total corneal astigmatism was measured. The power vector components J0 and J45 were analyzed. Correlations between the magnitude difference of the simulated K and total cornea astigmatism (magnitude differenceSimK-Tca), anterior J0, and absolute meridian difference (AMD) between the anterior and posterior astigmatisms were calculated. To compare the astigmatism of the simulated K and total cornea both in magnitude and axial orientation, we drew double-angle plots and calculated the vector difference between the two measures using vector analysis. A corrective regression formula was used to adjust the magnitude of the simulated K astigmatism to approach that of the total cornea. RESULTS: The magnitude differenceSimK-Tca was positively correlated with the anterior corneal J0 (Spearman’s rho= 0.539; P<0.001) and negatively correlated with the AMDR (Spearman’s rho=-0.875, P<0.001). When the anterior J0 value was larger than 1.3 D or smaller than -0.8 D, the errors caused by determining the total corneal astigmatism with the karatometric calculation tended to be greater than 0.25 D. An underestimation by 16% was observed for against the rule (ATR) astigmatism and an overestimation by 9% was observed for with the rule (WTR) astigmatism when ignoring the posterior measurements. CONCLUSION: Posterior corneal astigmatism should be valued for more precise corneal astigmatism management, especially for higher ATR astigmatism of the anterior corneal surface. We suggest a 9% reduction in the magnitude of the simulated K in eyes with WTR astigmatism, and a 16% addition of the magnitude of the simulated K in eyes with ATR astigmatism.
Meridian (astronomy)
Astigmatism
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Meridian (astronomy)
Scheimpflug principle
Astigmatism
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Objective To evaluate the asphericity of the anterior and posterior comeal surfaces in myopic eyes with different corneal diameters.Methods It was a cross-sectional study.One hundred and sixteen eyes of 116 myopic or myopic astigmatism patients were enrolled.The preo perative spherical refraction was -1.25～-10.75 D,and the cyclindrical refraction was 0.00～-3.50 D.The Q-value and spherical aberration of the anterior and posterior corneal surfaces for different corneal diameters (6,7,8,9 mm) were measured with a Pentacam.The correlations between Q-value and spherical aberration,corneal refraction and central corneal thickness were investigated usiug a Pearson correlation,and a linear regression was used to establish the regressione quation of Q-value and spherical abberration.Results The Q6 mm,Q7 mm,Q8 mm,Q9 mm of the anterior corneal surfaces of the 116 eyes were -0.24±0.09,-0.29±0.09,-0.33±0.09,-0.39±0.10,respectively; posterior corneal surfaces were -0.12±0.12,-0.21±0.11,-0.30±0.10,-0.38±0.10,respectively.The Q-values of the anterior corneal surfaces were more negative than the posterior corneal surfaces,the differences were significant except the 9 mm (t=-10.35,-6.68,-3.46,P＜0.01).Positive relationships were found between the Q-value of anterior and posterior corneal surfaces for 6-9 mm corneal diameters (r=0.30,0.37,0.40,0.42,P＜0.01).For a 6 mm corneal diameter,the regression equation of the anterior corneal surface was SA=0.663Q+0.415,and the posterior corneal surface was SA=0.159Q-0.177.Positive relationships were also found between the anterior Q-values and cyclinder refractions for 6-9 mm corneal diameters (r=0.28、0.27、0.25、0.22,P＜0.05).Negetive relationship were found between the posterior Q value and comeal thickness for 7-9 mm corneal diameter (r=-0.18,-0.21,-0.24,P＜0.05).ConcIusion Both the anterior and the posterior corneal surfaces in myopic eyes were prolate.As the corneal diameter increased,the Q-values became more negative.The relationship between Q-value and corneal spherical aberration can provide more valuable instructions for refractive surgeons when designing aspheric ablation.
Key words:
Cornea; Myopia; Q-value; Spherical aberration
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Purpose : The purpose of this study was to investigate the distribution of corneal asphericity of all ages in Koreans and changes in corneal asphericity according to age and gender. Methods : A total of 317 people(317 eyes) were analyzed retrospectively. The refractive error was analyzed by the autorefractometer. The corneal asphericity and curvature were measured in the central 6 mm zone of the cornea and analyzed by a rotating scheimplug camera. Results : The anterior and posterior corneal Q-values were -0.17±0.18(ranged -0.84 to 0.72) and -0.11±0.29(ranged -0.89 to 1.27), respectively. As the age group increased, the anterior corneal Q-value did not show a significant difference(p=0.359), while the posterior corneal Q-value showed statistically significant differences of 0.01±0.18, 0.10±0.29, -0.12±0.22, -0.20± 0.25, -0.27± 0.20 and –0.33±0.31 in the 30s, 40s, 50s, 60s, 70s and over 80’s, respectively(p＜0.001). The posterior corneal Q-value was negatively correlated with the age(r=-0.480, p＜0.001). Conclusion : The anterior corneal Q-value did not show a significant difference in gender and age while the posterior corneal Q-value changed from oblate to prolate shape as the age increased.
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