Relationship between corneal astigmatism and the distribution of corneal thickness along different principal meridians

doi:10.21203/rs.3.rs-3228754/v1

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Relationship between corneal astigmatism and the distribution of corneal thickness along different principal meridians

https://doi.org/10.21203/rs.3.rs-3228754/v1

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Aim: To analyze the relationship between corneal astigmatism (CA) and the distribution of corneal thickness along different principal meridians.

Methods: A total of 362 patients with 684 eyes who underwent preoperative examination for refractive surgery at Ineye Hospital of Chengdu University of TCM from April 2021 to August 2022 were included in the study. The automatic refractor was used to measure the degree and axis of CA. The Sim-K values and the mean corneal thickness of the principal meridian of maximum refractive power and the mean corneal thickness of the principal meridian of minimum refractive power were obtained using Sirius for the 3mm, 4.5mm, and 6mm range. The correlation between the difference in mean corneal thickness (d_ct) of the two principal meridians and CA measured by a computer refractometer was analyzed. Then, the participants were divided into three groups based on the degree of CA: low CA group: -1.00D~0.00D (210 eyes), moderate CA group: -2.00D~-1.25D (229 eyes), high CA group: ≤-2.25D (245 eyes). The range of d_ct values was analyzed for different degrees of CA.

Results: There was a negative correlation between d_ct and CA in the 3mm, 4.5mm, and 6mm ranges (r=-0.220, P<0.001; r=-0.214, P<0.001; r=-0.199, P<0.001). There was no difference in d_ct between the low and moderate astigmatism groups within a 3mm range (P>0.05). However, there were significant differences in d_ct between the low and high, moderate and high astigmatism groups within a 3mm range, as well as among the low, moderate, and high astigmatism groups within a 4.5mm and 6mm range (P<0.05).

Conclusion: The average corneal thickness of the principal meridian with maximum refractive power is thicker than that of the principal meridian with minimum refractive power. Additionally, as the degree of CA increases, the d_ct also increases. Corneal astigmatism is closely related to the distribution of corneal thickness in different principal meridians. In order to improve the accuracy of corneal astigmatism measurement, the factors of corneal thickness distribution in different principal meridians should be considered.

corneal astigmatism

corneal thickness

relevance

With significant advancements in corneal refractive surgery and the widespread adoption of intraocular lens (IOL), as well as the increasing demand for refractive cataract surgery in recent years, the objective of surgery has evolved beyond restoring vision to achieving improved postoperative visual quality as the primary clinical goal. It is important to note that the accurate measurement of total corneal astigmatism (TCA) directly affects the quantification of refractive surgery, the selection of IOL type, and the recovery of postoperative visual function. The conventional approach to measuring corneal astigmatism is no longer appropriate.

At present, with the continuous advancement of inspection equipment, the ability to directly or indirectly collect and analyze information about the posterior corneal surface is improving. By improving the repeatability and reliability of measurements of astigmatism on the posterior corneal surface, it greatly enhances the accuracy of TCA[1]. However, considering that posterior surface astigmatism has little effect on the accuracy of TCA, we believe that corneal thickness may also be a factor affecting TCA. Therefore, this study aims to analyze the relationship between corneal astigmatism and the distribution of corneal thickness in different meridians, in order to explore whether considering corneal thickness can help obtain more accurate and realistic results of corneal astigmatism.

1.1 Objects Collect patient information for preoperative examination of refractive surgery from April 2021 to August 2022 at Ineye Hospital of Chengdu University of TCM. Inclusion criteria: Complete medical records, including the degree and axis of corneal astigmatism measured by automatic refractor; and measurement results before surgery of the Sirius analysis system showing "OK" quality, including Sim-K and principal meridian mean corneal thickness. This study was approved by the Medical Ethics Committee of Ineye Hospital of Chengdu University of TCM (No. 2022yh-021), and all patients provided signed informed consent.

1.2 Classify according to the degree of astigmatism Corneal astigmatism degrees measured by automatic refractor ranging from − 1.00D to 0.00D are classified as the low CA group, degrees ranging from − 2.00D to -1.25D are classified as the moderate CA group, and degrees less than or equal to -2.25D are classified as the high CA group.

1.3 Sirius Inspection Method To avoid interference from external light sources, the examination should be conducted in a dimly lit room. The examinee's lower jaw was placed on the mandibular tray, the forehead was pressed against the forehead tray, the patient's eye height was adjusted, the examinee was instructed to open both eyes to fully expose the cornea, and to gaze at the indicator directly in front of the examiner, the examiner adjusted the focus according to the instrument's on-screen prompts, and then the built-in rotating Scheimpflug camera and the Placido ring completed the scanning and shooting of 25 Scheimpflug images and 1 Placido ring image within 2s, 3 consecutive measurements, the instrument was selected to display the results of the measured Scheimpflug image area ≥ 90%, center positioning ≥ 90%, and Placido disk coverage area ≥ 80%. Its Sim-K was recorded.

The corneal thickness map is selected, and the cursor is moved from the center of the thickness map to the sides of the principal meridians at 1.5 mm, 2.25 mm, and 3 mm, respectively. The thickness at each point is recorded, and the average value is calculated as the mean corneal thickness within the 3mm, 4.5mm, and 6mm range of the principal meridians.

1.4 Statistical Methods Retrospective Case Study. SPSS 26.0 software was used for statistical analysis. The measurement data conforms to a normal distribution and is expressed as mean ± standard deviation. Using Cronbach's Alpha coefficient(CoA), intraclass correlation coefficient(ICC) and Bland-Altman analysis was conducted to assess the consistency between automatic refractor and Sirius measurements of corneal astigmatism and axis. Cronbach's Alpha coefficient and intraclass correlation coefficient are both indicators used to evaluate reliability. It is generally believed that an ICC value of ≥ 0.75 indicates strong consistency. CoA > 0.7, indicating a high level of consistency. Spearman correlation was used to analyze the correlation between d_ct and degrees of CA at three different ranges: 3mm, 4.5mm, and 6mm, respectively. one-way analysis of variance was used to compare the differences in d_ct within the range of 3mm, 4.5mm, and 6mm for different degrees of corneal astigmatism. P < 0.05 was considered statistically significant.

2.1 Clinical Features A total of 362 patients (684 eyes) were included in the study, with an age range of 18 to 48 years (mean age of 25.08 ± 5.92 years). According to the results of the automatic refractor, there were 210 eyes with a low degree of astigmatism, 229 eyes with a moderate degree of astigmatism, and 245 eyes with a high degree of astigmatism. Among them, there were 663 eyes with with-the-rule astigmatism, 6 eyes with against-the-rule astigmatism, and 15 eyes with oblique astigmatism, accounting for 96.93%, 0.88%, and 2.19% respectively.

2.2 Consistency Analysis of Corneal Astigmatism Measured by automatic refractor and Sirius The results showed that the ICC and CoA of the degree of corneal astigmatism were both greater than 0.90. Additionally, the ICC and CoA of the axis of corneal astigmatism were greater than 0.75 and 0.8, respectively. In addition, the 95% agreement limit of the degree and axis of the corneal astigmatism measured by the two instruments is shown in Table 1. There were 4.09% (28/684) and 2.92% (20/684) points outside the 95% limits of agreement, respectively, as shown in Figs. 1 and 2.

Table 1

Consistency of degree and axis of corneal astigmatism measured by automatic refractor and Sirius
Parameter	ICC	CoV	Bland-Altman
			Mean difference	95%LoV
			Mean difference	Lower LoA	Upper LoA
ΔK	0.933	0.967	0.10	-0.59	0.78
Axis	0.781	0.877	0.3	-15	15.6
Note: ΔK = corneal astigmatism, Axis = the axis of corneal astigmatism, ICC = intra-group correlation coefficient, CoA = Cronbach's Alpha coefficient, 95%LoA = 95%limits of agreement.

2.3 Correlation analysis between d _ct values in different ranges and CA Correlation analysis showed that dct values in the ranges of 3mm, 4.5mm, and 6mm were negatively correlated with CA (Table 2).

Table 2

Correlation analysis between d_ct values in different ranges and CA
	Mean ± standard deviation	P-value^* (Correlation coefficient)
in the ranges of 3mm	5.9 ± 2.7	<0.0001 (r=-0.219)
in the ranges of 4.5mm	13.2 ± 5.1	<0.0001 (r=-0.214)
in the ranges of 6mm	21.09 ± 7.6	<0.0001 (r=-0.198)
^*Spearman correlation coefficient was used to analyze the correlation between d_ct values in the ranges of 3mm, 4.5mm, and 6mm and corneal astigmatism

2.4 The differences in d _ct within different ranges for different degrees of corneal astigmatism The results of a one-way ANOVA showed that there was no statistical significance in d_ct values between low and moderate degrees of CA in the 3mm range (P > 0.05). Additionally, there was statistical significance in d_ct values between low and high degrees, as well as moderate and high degrees of CA in the 3mm range. The d_ct values of different degrees of CA in the range of 4.5mm and 6mm showed statistical significance (P < 0.05), as shown in Table 3.In addition,as the degree of CA increased, the d_ct values also increased in the range of 3 mm, 4.5 mm, and 6 mm, as shown in Fig. 3.

Table 3

Comparison of d_ct values of different degrees of CA in different ranges
group	3mmd_ct(µm)	4.5mmd_ct(µm)	6mmd_ct(µm)
low CA(D)	5.23 ± 2.62	11.81 ± 5.33	19.04 ± 7.31
moderate CA(D)	5.68 ± 2.47	13.26 ± 4.84	21.16 ± 7.30
high CA(D)	6.60 ± 2.87	14.32 ± 4.95	22.61 ± 7.68
F	15.768	14.120	13.034
P	<0.001	<0.001	<0.001
Note: low CA: -1.00D ~ 0.00D, moderate CA: -2.00D ~ 1.25D, high CA: ≤-2.25D

Corneal tissue is highly transparent and has a smooth surface. It is shaped like a convex-concave lens. Before the slit scanning technique was applied to ophthalmology in the 1990s, it was not possible to directly measure the curvature radius of the posterior corneal surface. Thus, the curvature radius of the anterior corneal surface was measured using various principles and methods. Then, the simulated corneal refractive power (SimK) was calculated by assuming a refractive index (usually 1.3375).

SimK = (1.3375–1.000) / R_anterior

* The refractive index of air is 1.000, and the radius of curvature within a specific range of the central anterior surface of the cornea is denoted as R_anterior (in meters).

Both keratometer and corneal topographer are commonly used to obtain corneal refractive power by this method.

Corneal astigmatism is the main component contributing to total astigmatism. It is also the most significant cause of residual astigmatism following cataract surgery, intraocular collamer lens implantation, and corneal refractive surgery. Even today, the measurement of corneal astigmatism is based on SimK. However, by analyzing the ratio of posterior and anterior surface curvature radius of the cornea, many scholars[2],[3],[4] have found that the corrected corneal refractive index is different from the traditional corneal refractive index.Therefore, traditional methods of measuring corneal astigmatism do not meet the requirements for accurate corneal astigmatism, and the factors that influence this need further research.

In recent years, with the continuous advancement of technical equipment for examination, it has become possible to directly or indirectly collect and analyze information on the posterior corneal surface. A growing number of studies have also confirmed the clinical value of posterior corneal astigmatism (PCA) in the evaluation of TCA[5^],[6], which should not be overlooked. Ignoring or simply estimating PCA by assuming refractive index may lead to clinically significant deviations in TCA magnitude and axis[7]. Therefore, more and more scholars study the reliability and repeatability of PCA and TCA measured by devices based on different principles[8],[9],[10], which has important clinical significance for the accuracy of corneal astigmatism measurements.

However, TCA should not be considered simply as a vector superposition of anterior corneal astigmatism and PCA on the same plane. Sepehr Feizi et al.[11] used Pentacam to assess the astigmatism of the posterior corneal surface in patients who underwent phacoemulsification with implantation of a non-toric monofocal intraocular lens. They subsequently measured the manifest refractive astigmatism after the patients wore a spherical hard contact lens. After comparing the manifest refractive astigmatism with the astigmatism of the posterior corneal surface, they found only a weak correlation. There may be other factors that affect the accurate measurement of corneal astigmatism, one of which we believe is the thickness of the cornea. At present, various studies ignore the corneal thickness of the principal meridians when measuring corneal astigmatism, and treat it only as a thin lens. Most studies on corneal thickness primarily focus on corneal refractive surgery[12], intraocular pressure[13], etc.,rather than its measurement in corneal diopters. Based on this, we explored the relationship between corneal astigmatism and corneal thickness by correlating the difference between the mean thicknesses of the two principal meridians of the cornea and corneal astigmatism, with the aim of providing a basis for further accurate astigmatism measurements.

The automatic refractor is widely used in clinics as a routine inspection device for assessing the corneal refractive state. The corneal astigmatism is obtained by calculating the maximum and minimum refractive curvatures of the cornea through image analysis. Sirius, using the rotating Scheimpflug camera principle in combination with the Placido disk, enables the entire corneal surface to be covered, and then utilizes a specific patented technology to combine the Scheimpflug image data with the anterior surface of the cornea as measured by the Placido disk, and calculates parameters related to the anterior surface of the cornea, the posterior surface of the cornea, as well as the entire corneal thickness. Our analysis showed good agreement between the two instruments for corneal astigmatism and axis, and we therefore concluded that we could use the corneal astigmatism axial direction measured by Sirius to obtain the corresponding corneal thickness.

Sirius shows the entire corneal thickness in the form of a topographic map, so that the corneal thickness in any area of the cornea can be determined easily and with good reproducibility[14]. We analyzed the correlation between corneal astigmatism measured by automatic refractor and d_ct in the 3mm, 4.5mm, and 6mm ranges, respectively, and the results showed that dct in the 3mm, 4.5mm, and 6mm ranges were negatively correlated with CA(r=-0.220, P < 0.001;r=-0.214, P < 0.001༛r=-0.199, P < 0.001). It is shown that both in the conventional 3-mm corneal curvature measurement range and in the larger 4.5-mm and 6-mm measurement ranges, both for conformal astigmatism, retro-conformal astigmatism, and oblique-axis astigmatism, the mean corneal thickness of the principal meridian of maximum refractive power was demonstrated to be thicker than that of the principal meridian of minimum refractive power. The difference between the mean thicknesses of the two principal meridians was also greater as the degree of corneal astigmatism increased. Therefore, it can be assumed that there is a relationship between corneal astigmatism and corneal thickness.

The radii of curvature of the two principal meridians of the cornea were not collected during the data collection process in this study, but the astigmatism values were obtained directly. In the following studies, the corresponding data can be continued to be collected and the refractive power of the two principal meridians of the cornea can be calculated by the Gaussian thick lens formula to obtain the corneal astigmatism.

Gaussian Thick Lens Formula:

KGoF = (n1 - n0) / Ranterior + (n2 - n1) / Rposterior - (CT / n1) × [(n1 - n0) / Ranterior] × [(n2 - n1) / Rposterior]

* Set the corneal refractive index of air to n0 = 1.000; The actual corneal refractive index was n1 = 1.376; The refractive index of the aqueous humor was n2 = 1.336

On this basis, further comparisons were made to see if there was a difference between the corneal astigmatism values considered for corneal thickness and those measured by conventional methods, and whether the difference was clinically significant.

d_ct

the difference in mean corneal thickness of the two principal meridians

corneal astigmatism

TCA

total corneal astigmatism

PCA

posterior corneal astigmatism

CoA

Cronbach’s Alpha coefficient

ICC

intraclass correlation coefficient

Ethics approval and consent to participate

The study was approved by the Medical Ethics Committee of Ineye Hospital of Chengdu University of TCM (No. 2022yh-021) and then conducted in compliance with the relevant ethics guidelines. All the study procedures described herein were performed in accordance with the Declaration of Helsinki. All participants provided written informed consent.

Consent for publication

Not applicable.

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflict of interest

The authors declare no competing interests.

Funding

Not applicable.

Authors’ Contributions

Liting Zhu：Formal analysis and Writing

Zheng Yuan：Data curation and Investigation

Shuangfeng Liang：Investigation

Dian Zhao：Investigation

ChunYang Zhou：Conceptualization

Acknowledgements

Not applicable.

Authors and Affiliations

Eye College of Chengdu University of TCM, Chengdu, Sichuan, China

Liting Zhu, Zheng Yuan, Shuangfeng Liang, Dian Zhao

Eye College of Chengdu University of TCM, Ineye Hospital of Chengdu University of TCM, Chengdu, Sichuan, China

Chunyang Zhou

Corresponding author

Correspondence to ChunYang Zhou

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No competing interests reported.

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Relationship between corneal astigmatism and the distribution of corneal thickness along different principal meridians

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Abstract

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Ethics approval and consent to participate

Consent for publication

Availability of data and materials

Conflict of interest

Funding

Authors’ Contributions

Acknowledgements

Authors and Affiliations

Corresponding author

References

Additional Declarations

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