Correlations of anterior and posterior corneal parameters in Chinese myopic patients: a retrospective multicenter study

Introduction

It is commonly believed that the shapes of the anterior and posterior corneal surfaces are highly correlated with each other. Previous studies have demonstrated moderate to strong anterior-posterior corneal correlations (APCC) in parameters of the two corneal surfaces (e.g., corneal astigmatism, corneal curvature, and corneal asphericity) in normal eyes (1-5). In fact, traditional methods of calculating total corneal astigmatism, known as keratometric astigmatism (6), are mainly based on measurement of the anterior corneal astigmatism (ACA), assuming the cornea is a single dioptric surface with a fixed anterior-to-posterior curvature ratio (7). However, the anterior and posterior corneal surfaces are not always correlated according to some fixed ratios (8). In fact, APCC may vary according to different conditions, such as refractive status, corneal size, and even depth of the anterior chamber. However, to date there no study has been conducted to verify this hypothesis.

APCC may be changed in eyes with keratoconus (5,9,10), even in eyes with subclinical or early keratoconus (4,5), where changes at the posterior cornea precede those at the anterior cornea (11). Therefore, APCC may be used in assisting keratoconus diagnosis. In this scenario, it is essential to identify the affecting factors of APCC. We hypothesize that APCC may be affected by some ocular factors, but no previous studies have addressed this issue.

Pentacam is a Scheimpflug-based imaging device for anterior segment structures including the cornea, anterior chamber, and lens. Using a rotating Scheimpflug camera, the device is capable of capturing 50 rotational images of the cross-sections of the anterior segment. Based on the measurements, parameters of the anterior and posterior corneas and the anterior chamber are generated by a built-in software. Pentacam has been commonly used to measure both the anterior and posterior corneas due to the good repeatability and reproducibility (7). Simultaneous measurement of both the anterior and posterior corneal surfaces makes Pentacam an ideal device to study APCC.

In this study, we aimed to investigate APCC of corneal curvature, astigmatism, eccentricity, and asphericity, according to different refractive status and anterior segment dimension in a large number of Chinese myopic patients from multiple ophthalmic centers. Our results may shed light on the understanding of correlations between the anterior and posterior corneal surfaces. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-24-333/rc).

Methods

Participants

In this retrospective multicenter study including Guangzhou Aier Eye Hospital (GZ), Shenyang Aier Eye Hospital (SY), Chengdu Aier Eye Hospital (CD), Wuhan Aier Eye Hospital (WH), and Hankou Aier Eye Hospital (HK), myopic eyes undergoing refractive surgery during 2017–2019 were recruited (12,13). The centers were chosen to cover cities from the south (GZ), northeast (SY), central (WH and HK), and southwest (CD) of China (12). The inclusion criteria were as follows: myopic eyes with a manifest spherical equivalent (SE) ≤−0.50 diopter (D) and good quality Scheimpflug scans. Only the right eye of each patient was included for analysis. The exclusion criteria were as follows: coexisting corneal diseases, keratoconus, forme fruste keratoconus, severe dry eye, previous ocular trauma or surgery, uveitis, glaucoma, wearing soft contact lenses within the previous two weeks or rigid gas-permeable lenses within one month, and age younger than 18 years (unstable refraction) or older than 40 years (to reduce changes of the anterior chamber parameters caused by the crystal lens) (12,13). The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Institutional Review Boards (IRBs) of GZ (No. GZAIER2019IRB20), SY (No. 2021-001-01), CD (No. IRB20190005), WH (No. 2019IRBKY05), and HK (No. HKAIER-2019IRB-006-01). Since we only reviewed medical records of the patients, and no individual patient could be identified from the data, the requirement for informed consent was waived by the IRBs.

Examinations

All the eyes underwent routine preoperative examinations, including best-corrected visual acuity (BCVA), intraocular pressure (IOP), cycloplegic and manifest refraction, anterior segment examination by slit-lamp, corneal topography, and Pentacam measurements. The eyes were divided into four groups according to the manifest SE: low myopia (−3.00 D< SE ≤−0.50 D, LM), moderate myopia (−6.00 D< SE ≤−3.00 D, MM), high myopia (−10.00 D< SE ≤−6.00 D, HM), and extremely high myopia (SE ≤−10.00 D, EHM) (14).

Pentacam examination was performed by experienced technicians as previously described (12,13). The Pentacam instrument (Pentacam; Oculus GmbH, Wetzlar, Germany) was used to obtain the ocular biometrics according to a standard measurement protocol in every center (12,13). Only images that covered at least the central 8.0 mm of corneal surface and had the image quality labelled as ‘OK’ on the display were used (12,13). Anterior and posterior corneal radius in the flat and steep meridians in the 3.0 mm central zone, eccentricity and asphericity of the anterior and posterior cornea, pachy apex (PA), corneal volume (CV) in the 3.0 mm diameter area, white-to-white (WTW) corneal diameter, anterior chamber depth (ACD), and anterior chamber volume (ACV) were measured by the Pentacam instrument (12,13). The Sim K, ACA, and posterior corneal astigmatism (PCA) in the 3.0 mm central zone were calculated as previously described (12-14). The Pentacam machine has excellent repeatability with small inter- and intra-observer variability (15-17).

Statistical analysis

Pooled data of the five ophthalmic centers were used for analysis. A Kolmogorov-Smirnov (KS) test was used to evaluate normality of all variables. Data of patient demographics were presented as mean ± standard deviation (SD). For APCC analysis, the pooled data were divided into four groups according to different myopia severity and quartile levels (Q1–Q4) of the corneal and anterior chamber parameters [mean simulated corneal curvature (SimKm), PA, CV, WTW, ACD, and ACV]. Since mean corneal curvature (Km), corneal astigmatism, corneal eccentricity, and corneal asphericity did not follow normal distribution in our samples, Spearman’s correlation test was used for anterior-posterior correlation analysis of corneal curvature, astigmatism, eccentricity, and corneal asphericity in each myopia group and quartile level. Multivariate linear regression was used for anterior-posterior correlation analysis of corneal curvature, astigmatism, eccentricity and corneal asphericity adjusted for myopia severity, SimKm, PA, CV, WTW, ACD, and ACV. Statistical significance was considered when P<0.05.

Results

Demographics

Demographics and corneal parameters of the eyes in the five ophthalmic centers are shown in Table 1. A total of 8,365 patients (8,365 eyes) were included in the study (2,460 eyes from GZ, 2,435 eyes from SY, 1,586 eyes from CD, 1,556 eyes from WH, and 328 eyes from HK). The mean age of the patients was 25.1±5.4 years. Mean manifest SE of the eyes was −5.13±2.05 D, with a mean manifest spherical error of −4.78±1.98 D and a mean manifest astigmatism of −0.70±0.63 D. There were 993 eyes in the LM group, 4,796 eyes in the MM group, 2,416 eyes in the HM group, and 160 eyes in the EHM group. There were significant differences in age, gender, manifest SE, spherical error, and astigmatism among patients from different ophthalmic centers (all P<0.001).

Coefficients of anterior-posterior correlations of the corneal curvature, astigmatism, eccentricity, and asphericity in different myopia and quartile groups are shown in Table 2 and Figure 1-6. Each number in Table 2 is the anterior-posterior correlation coefficient in one quartile group.

Figure 1 Scattergrams showing APCC in different myopic groups. (A) APCC of mean corneal curvature. (B) APCC of corneal astigmatism. (C) APCC of corneal eccentricity. (D) APCC of corneal asphericity. Km, mean corneal curvature; EHM, extremely high myopia; HM, high myopia; MM, moderate myopia; LM, low myopia; APCC, anterior-posterior corneal correlations.

Figure 2 Spearman correlation coefficients of APCC in four quartile groups of SimKm, pachy apex, CV, ACD, ACV, and WTW. (A) APCC of mean corneal curvature. (B) APCC of corneal astigmatism. (C) APCC of corneal eccentricity. (D) APCC of corneal asphericity. Km, mean corneal curvature; SimKm, mean simulated corneal curvature; CV, corneal volume; ACD, anterior chamber depth; ACV, anterior chamber volume; WTW, white-to-white; APCC, anterior-posterior corneal correlations.

Figure 3 Scattergrams showing anterior-posterior correlations of mean corneal curvature. (A) In different quartile groups of SimKm. (B) In different quartile groups of pachy apex. (C) In different quartile groups of CV. (D) In different quartile groups of ACD. (E) In different quartile groups of ACV. (F) In different quartile groups of WTW. SimKm, mean simulated corneal curvature; Km, mean corneal curvature; CV, corneal volume; ACD, anterior chamber depth; ACV, anterior chamber volume; WTW, white-to-white.

Figure 4 Scattergrams showing anterior-posterior correlations of corneal astigmatism. (A) In different quartile groups of SimKm. (B) In different quartile groups of pachy apex. (C) In different quartile groups of CV. (D) In different quartile groups of ACD. (E) In different quartile groups of ACV. (F) In different quartile groups of WTW. SimKm, mean simulated corneal curvature; CV, corneal volume; ACD, anterior chamber depth; ACV, anterior chamber volume; WTW, white-to-white.

Figure 5 Scattergrams showing anterior-posterior correlations of corneal eccentricity. (A) In different quartile groups of SimKm. (B) In different quartile groups of pachy apex. (C) In different quartile groups of CV. (D) In different quartile groups of ACD. (E) In different quartile groups of ACV. (F) In different quartile groups of WTW. SimKm, mean simulated corneal curvature; CV, corneal volume; ACD, anterior chamber depth; ACV, anterior chamber volume; WTW, white-to-white.

Figure 6 Scattergrams showing anterior-posterior correlations of corneal asphericity. (A) In different quartile groups of SimKm. (B) In different quartile groups of pachy apex. (C) In different quartile groups of CV. (D) In different quartile groups of ACD. (E) In different quartile groups of ACV. (F) In different quartile groups of WTW. SimKm, mean simulated corneal curvature; CV, corneal volume; ACD, anterior chamber depth; ACV, anterior chamber volume; WTW, white-to-white.

APCC in different myopia groups

Scatter plots of the correlations between anterior-posterior corneal curvature, astigmatism, eccentricity, and asphericity in different myopia groups are shown in Figure 1. Correlations of the anterior and posterior Km were significantly negative with similar R values (−0.85 to −0.88) in all the myopic groups (Figure 1A, all P<0.001). On the contrary, correlations between the ACA and PCA were significantly positive with slightly increased R values (0.65 to 0.75) from the LM group to the EHM group (Figure 1B, all P<0.001). The eccentricity of anterior and posterior corneal surfaces was also correlated positively in all the myopic groups, whereas the LM, MM, and EM groups presented slightly greater R values (0.36 to 0.38) than the EHM group (0.27) (Figure 1C, all P<0.001), and similar findings were observed in the asphericity of the anterior and posterior corneal surfaces (Figure 1D, all P<0.001).

Anterior-posterior correlations of corneal curvature in different quartile groups

The coefficients of anterior-posterior correlations of the corneal curvature in different quartile groups are shown in Table 2, Figure 2A, and Figure 3. Correlations between the anterior and posterior Km were significantly negative in every quartile group, with better R values observed in group Q1 (−0.63) and Q4 (−0.64) than in group of Q2 (−0.36) and Q3 (−0.37) SimKm (Figure 2A, Figure 3A, and Table 2, all P<0.001), and with similar R values in quartile groups of PA (−0.90 to −0.91), CV (−0.90 to −0.91), ACD (−0.88 to −0.89), ACV (−0.87 to −0.88), and WTW (−0.84 to −0.85) (Figure 2A, Figure 3B-3F, and Table 2, all P<0.001).

Anterior-posterior correlations of corneal astigmatism in different quartile groups

The coefficients of anterior-posterior correlations of the corneal astigmatism in different quartile groups are shown in Table 2, Figure 2B, and Figure 4. Significant positive correlations between the ACA and PCA were observed in all the quartile groups, with the R values slightly increased with quartile levels of SimKm (0.66 to 0.74) (Figure 2B, Figure 4A, and Table 2, all P<0.001), and with similar R values in different quartile groups of PA (0.69 to 0.71), CV (0.68 to 0.72), ACD (0.70 to 0.72), ACV (0.69 to 0.72), and WTW (0.69 to 0.72) (Figure 2B, Figure 4B-4F, and Table 2, all P<0.001).

Anterior-posterior correlations of corneal eccentricity in different quartile groups

The coefficients of anterior-posterior correlations of the corneal eccentricity in different quartile groups are shown in Table 2, Figure 2C, and Figure 5. Significant positive correlations were observed between the anterior and posterior corneal eccentricity in all the quartile groups, with the R values slightly increased with quartile levels of SimKm (0.31 to 0.43), but slightly increased with quartile levels of PA (0.32 to 0.44) and CV (0.30 to 0.43) (Figure 2C, Figure 5A-5C, and Table 2, all P<0.001). The R values were similar across the quartile levels of ACD (0.31 to 0.38), ACV (0.32 to 0.36), and WTW (0.34 to 0.37) (Figure 2C, Figure 5D-5F, and Table 2, all P<0.001).

Anterior-posterior correlations of corneal asphericity in different quartile groups

The coefficients of anterior-posterior correlations of the corneal asphericity in different quartile groups are shown in Table 2, Figure 2D, and Figure 6. Positive correlations were also observed between the anterior and posterior corneal asphericity in all the quartile groups. The R values were slightly increased with quartile levels of SimKm (0.34 to 0.45), but slightly increased with quartile levels of PA (0.36 to 0.45) and CV (0.34 to 0.44) (Figure 2D, Figure 6A-6C, and Table 2, all P<0.001). Similar with the correlations of eccentricity, the R values for asphericity correlation were similar across the quartile levels of ACD (0.33 to 0.40), ACV (0.33 to 0.39), and WTW (0.36 to 0.40) (Figure 2D, Figure 6D-6F, and Table 2, all P<0.001).

Factors influencing APCC

The results of multivariate linear regression for factors influencing anterior-posterior correlations of corneal curvature, astigmatism, eccentricity, and asphericity are shown in Table 3. The results indicated multiple factors affecting the anterior-posterior correlations of these corneal parameters.

Discussion

In the present multicenter study, we demonstrated the correlations between the anterior and posterior corneal surfaces according to different manifest SE and anterior segment dimension in a large number of Chinese myopic patients from different parts of mainland China. Using the pooled data of the five cohorts with diversity in age, sex, and manifest SE, negative correlations were observed between the anterior and posterior Km, whereas positive correlations were observed between the ACA and PCA, between anterior and posterior eccentricity, as well as between anterior and posterior asphericity. We also found that R values of the APCC varied in different myopia severity and quartile groups of some corneal parameters (i.e., SimKm, PA, and CV). Our results suggest that the correlations between anterior and posterior corneal morphologies are not universally constant, but are affected by multiple ocular factors. These findings may help to improve our understandings of the APCC in different conditions.

Assessment of corneal curvature plays an important role in refractive surgery planning, intraocular lens power calculation, and early diagnosis of keratoconus (9,18-20). In traditional keratometry, the cornea is assumed to have a fixed ratio between the anterior and poster corneal curvature, and the anterior corneal curvature is used to calculate the total corneal curvature. With recognition of the changes of anterior and posterior corneal curvature correlations in pathological conditions, the ratio between the anterior and posterior curvature radii were adopted to evaluate the risk of hyperopia shift after Descemet membrane endothelial keratoplasty (21). In the present study, the R values for APCC of Km were similar in different myopic groups and quartile groups of PA, CV, ACD, ACV, and WTW (−0.84 to −0.91). The results were consistent with a previous study in which the R value for APCC of Km was −0.94 in normal eyes (5). The APCC of Km in every myopia group was strong in our study, with an R value of at least −0.85. This finding indicates that myopia severity does not affect the APCC of Km and the accuracy of total corneal curvature measurement using traditional keratometry. However, APCC of Km was much weaker in groups Q2 (R: −0.36) and Q3 (R: −0.37) than in groups Q1 (R: −0.63) and Q4 (R: −0.64) of SimKm. This finding may be due to weaker corneal biomechanics in eyes with high corneal curvature. Our previous study has shown that higher corneal curvature is associated with thinner corneal thickness (22). It was also shown that the corneal stress was determined by corneal thickness, with greatest stress at thinnest corneal thickness and greatest curvature (23). Thus, the steepest corneas are more subject to the changes in both of the anterior and posterior Km. Furthermore, this finding (APCC of Km was much weaker in group Q2 and Q3 of SimKm) also suggests that the APCC of Km is much worse when the SimKm is around the median of distribution, and measurement of both the anterior and posterior corneal curvatures is needed to obtain an accurate total corneal curvature in these patients to achieve better outcomes after refractive surgery. The changes in APCC of Km regarding to SimKm may be also important in pathological conditions such as keratoconus. In the early stage of keratoconus, protrusion of the cornea starts from the posterior surface, and the APCC of Km may be altered accordingly. It was shown that the R value for APCC of Km decreased to −0.34 and −0.56 in eyes with keratoconus (5). Thus, APCC of Km may be used as an indicator of keratoconus. According to our results, APCC of Km is similar across different myopic groups, but decreased in Q2 and Q3 quartiles of SimKm. Thus, evaluation of changes in APCC of Km also needs to consider the quartile of SimKm. Since SimKm is also an important parameter in diagnosis and monitoring of keratoconus, further studies are needed to investigate the association between changes in APCC of Km and progression of keratoconus.

Management of PCA and ocular residual astigmatism during astigmatism correction has become a hot topic in cataract and refractive surgery (24), and many efforts have been made to predict the PCA using ACA (2,25). However, PCA prediction based on ACA alone is not accurate enough (8). In the present study, the R values for the correlation between ACA and PCA were increased from 0.65 in the LM group to 0.75 in the EHM group, indicating a stronger ACA-PCA correlation in eyes with higher severity of myopia. This may be due to different corneal biomechanics in higher severity of myopia. It was shown that corneal biomechanical parameter stress-strain index (SSI) was decreased with increasing myopia and astigmatism in the Chinese participants (26). Thus, the corneas in patients with higher degree of myopia are more elastic and subject to the deformation of the anterior and posterior surfaces. The R values of ACA-PCA correlation were also slightly increased with quartile levels of SimKm (0.66 to 0.74). These findings indicate that the ACA-PCA correlation is affected by multiple factors and the integration of various anterior segment parameters is needed to make a more accurate PCA prediction (1). Moreover, the moderate ACA-PCA correlations may be due to the greater hereditability of the posterior corneal surface compared to the anterior corneal surface (27,28). Surprisingly, the strength of ACA-PCA correlation seems to be higher in keratoconus eyes compared to normal eyes, although controversy still remains (5,9,10). This may be due to the unsynchronized changes of the ACA and PCA during the development of keratoconus, and the tendency towards a better alignment of the PCA to the ACA in keratoconus eyes (27-29).

In the present study, there were weak correlations (R: 0.29 to 0.45) between the anterior and posterior corneal asphericity in all the quartile groups. These findings were different from previous studies where poor APCC of corneal asphericity in normal eyes (1,5) and good APCC in keratoconus eyes were observed (1,2). It was shown that the R values for the APCC of corneal asphericity in normal eyes were 0.03–0.04 and 0.17 in the previous studies (1,5). On the contrary, the R values of the correlations were 0.62–0.89 and 0.86–0.94 in keratoconus eyes (1,2). These findings together suggest that in terms of APCC of corneal asphericity, myopic eyes are in between the normal eyes and keratoconus eyes. The roles of APCC of corneal asphericity in corneal ectasia warrant further investigation. In similarity with the ACA-PCA correlations, the APCC of corneal asphericity is also affected by multiple factors such as myopia severity, SimKm, PA, and CV, suggesting complex interactions between the anterior-posterior corneal shapes and other anterior segment parameters.

In the present study, the R values for APCC of corneal astigmatism, eccentricity, and asphericity were increased with higher quartile levels of SimKm, but the R values for APCC of corneal eccentricity and asphericity were decreased with higher quartile levels of PA and CV. These findings suggest that different corneal parameters have different effects on the APCC. Further studies are required to investigate the mechanisms of each corneal parameter affecting the APCC of corneal shapes.

Our study has some limitations. Firstly, the study is limited by the retrospective nature and its inherent biases. Secondly, the Pentacam measurements were performed by different technicians in the five centers, and were subject to inter-observer variations. We did not compare the APCC using data obtained from different imaging devices due to lack of other devices that can image the posterior cornea. Thus, we cannot infer whether our results can be used interchangeably with those obtained from other devices. Thirdly, for the generalizability of our findings to other populations, conclusions of our study can only be applied to Chinese myopic patients from the same age group. In older patients, hyperopic or emmetropic patients, or people of different ethnic groups, the APCC may be different. Fourthly, the pooling of data from multiple centers might introduce variability, although using the same inclusion criteria may help reduce such variability. Fifthly, the exact mechanisms underlying the APCC remain a mystery to us, although some affecting factors are identified in the present study. Due to the cross-sectional design of the study, we could not investigate the changes of APCC after corneal refractive surgery. Further studies are needed to address this issue. Lastly, we could not compare the APCC in different axial length groups, because the axial length record was only available in GZ, but not in other centers. Nevertheless, the results may be similar with those in different myopia groups.

For the APCC, future studies are needed to reveal the exact mechanisms, such as corneal biomechanics and genetics affecting corneal morphology. Comparison of APCC between different corneal imaging devices is also required. Clinical studies are needed to evaluate the usefulness of APCC in diagnosis and progress monitoring of keratoconus. Longitudinal studies should be conducted to investigate the impact of APCC on long-term outcomes of refractive surgery.

Conclusions

The present study has demonstrated that the correlations between anterior and posterior corneal morphologies are not universally constant. The APCC of corneal curvature, astigmatism, eccentricity, and asphericity can be affected by the severity of myopia and some other corneal parameters. Our results may shed light on the understanding of correlations between the anterior and posterior corneal surfaces.

Acknowledgments

Funding: This work was supported by the National Key R&D Program of China (No. 2022YFF1202603) and the Research Fund Project of Clinical Research Institute of Aier Eye Hospital Group (Nos. AR2009D5 and AR2209D2).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-24-333/rc

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-24-333/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Institutional Review Boards (IRBs) of GZ (No. GZAIER2019IRB20), SY (No. 2021-001-01), CD (No. IRB20190005), WH (No. 2019IRBKY05), and HK (No. HKAIER-2019IRB-006-01). Since we only reviewed medical records of the patients, and no individual patient could be identified from the data, the requirement for informed consent was waived by the IRBs.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Montalbán R, Piñero DP, Javaloy J, Alió JL. Scheimpflug photography-based clinical characterization of the correlation of the corneal shape between the anterior and posterior corneal surfaces in the normal human eye. J Cataract Refract Surg 2012;38:1925-33. [Crossref] [PubMed]
2. Montalbán R, Piñero DP, Javaloy J, Alio JL. Correlation of the corneal toricity between anterior and posterior corneal surfaces in the normal human eye. Cornea 2013;32:791-8. [Crossref] [PubMed]
3. Dubbelman M, Sicam VA, Van der Heijde GL. The shape of the anterior and posterior surface of the aging human cornea. Vision Res 2006;46:993-1001. [Crossref] [PubMed]
4. Schlegel Z, Hoang-Xuan T, Gatinel D. Comparison of and correlation between anterior and posterior corneal elevation maps in normal eyes and keratoconus-suspect eyes. J Cataract Refract Surg 2008;34:789-95. [Crossref] [PubMed]
5. Piñero DP, Alió JL, Alesón A, Escaf Vergara M, Miranda M. Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus. J Cataract Refract Surg 2010;36:814-25. [Crossref] [PubMed]
6. Teus MA, Arruabarrena C, Hernández-Verdejo JL, Sales-Sanz A, Sales-Sanz M. Correlation between keratometric and refractive astigmatism in pseudophakic eyes. J Cataract Refract Surg 2010;36:1671-5. [Crossref] [PubMed]
7. Camps VJ, Pinero Llorens DP, de Fez D, Coloma P, Caballero MT, Garcia C, Miret JJ. Algorithm for correcting the keratometric estimation error in normal eyes. Optom Vis Sci 2012;89:221-8. [Crossref] [PubMed]
8. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg 2012;38:2080-7. [Crossref] [PubMed]
9. Tomidokoro A, Oshika T, Amano S, Higaki S, Maeda N, Miyata K. Changes in anterior and posterior corneal curvatures in keratoconus. Ophthalmology 2000;107:1328-32. [Crossref] [PubMed]
10. Montalbán R, Alio JL, Javaloy J, Piñero DP. Correlation of anterior and posterior corneal shape in keratoconus. Cornea 2013;32:916-21. [Crossref] [PubMed]
11. Golan O, Hwang ES, Lang P, Santhiago MR, Abulafia A, Touboul D, Krauthammer M, Smadja D. Differences in Posterior Corneal Features Between Normal Corneas and Subclinical Keratoconus. J Refract Surg 2018;34:664-70. [Crossref] [PubMed]
12. Hu Y, Zhu S, Xiong L, Fang X, Liu J, Zhou J, Li F, Zhang Q, Huang N, Lei X, Jiang L, Wang Z. A multicenter study of the distribution pattern of posterior corneal astigmatism in Chinese myopic patients having corneal refractive surgery. Sci Rep 2020;10:16151. [Crossref] [PubMed]
13. Xu G, Hu Y, Zhu S, Guo Y, Xiong L, Fang X, Liu J, Zhang Q, Huang N, Zhou J, Li F, Lei X, Jiang L, Wang Z. A multicenter study of interocular symmetry of corneal biometrics in Chinese myopic patients. Sci Rep 2021;11:5536. [Crossref] [PubMed]
14. Tang C, Wu Q, Liu B, Wu G, Fan J, Hu Y, Yu H. A Multicenter Study of the Distribution Pattern of Posterior-To-Anterior Corneal Curvature Radii Ratio in Chinese Myopic Patients. Front Med (Lausanne) 2021;8:724674. [Crossref] [PubMed]
15. Piñero DP, Saenz González C, Alió JL. Intraobserver and interobserver repeatability of curvature and aberrometric measurements of the posterior corneal surface in normal eyes using Scheimpflug photography. J Cataract Refract Surg 2009;35:113-20. [Crossref] [PubMed]
16. Shankar H, Taranath D, Santhirathelagan CT, Pesudovs K. Anterior segment biometry with the Pentacam: comprehensive assessment of repeatability of automated measurements. J Cataract Refract Surg 2008;34:103-13. [Crossref] [PubMed]
17. Luo YH, Zhong Q, Ouyang PB, Guo XJ, Duan XC. Repeatability and agreement of CCT measurement in myopia using entacam and ultrasound pachymetry. Int J Ophthalmol 2012;5:329-33. [Crossref] [PubMed]
18. Seitz B, Langenbucher A, Nguyen NX, Kus MM, Küchle M. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999;106:693-702. [Crossref] [PubMed]
19. Kim M, Eom Y, Lee H, Suh YW, Song JS, Kim HM. Use of the Posterior/Anterior Corneal Curvature Radii Ratio to Improve the Accuracy of Intraocular Lens Power Calculation: Eom's Adjustment Method. Invest Ophthalmol Vis Sci 2018;59:1016-24. [Crossref] [PubMed]
20. Morishige N, Magome K, Ueno A, Matsui TA, Nishida T. Relations Among Corneal Curvature, Thickness, and Volume in Keratoconus as Evaluated by Anterior Segment-Optical Coherence Tomography. Invest Ophthalmol Vis Sci 2019;60:3794-802. [Crossref] [PubMed]
21. Diener R, Eter N, Alnawaiseh M. Using the posterior to anterior corneal curvature radii ratio to minimize the risk of a postoperative hyperopic shift after Descemet membrane endothelial keratoplasty. Graefes Arch Clin Exp Ophthalmol 2020;258:1065-71. [Crossref] [PubMed]
22. Jiang L, Du Z, Sun W, Zhu S, Xiong L, Fang X, Zhou J, Zhang Q, Lei X, Zeng Q, Wang Z, Hu Y. Associations between corneal curvature and other anterior segment biometrics in young myopic adults. Sci Rep 2024;14:8305. [Crossref] [PubMed]
23. Roberts CJ, Knoll KM, Mahmoud AM, Hendershot AJ, Yuhas PT. Corneal Stress Distribution Evolves from Thickness-Driven in Normal Corneas to Curvature-Driven with Progression in Keratoconus. Ophthalmol Sci 2024;4:100373. [Crossref] [PubMed]
24. Teus MA, Arruabarrena C, Hernández-Verdejo JL, Cañones R, Mikropoulos DG. Ocular residual astigmatism's effect on high myopic astigmatism LASIK surgery. Eye (Lond) 2014;28:1014-9. [Crossref] [PubMed]
25. Miyake T, Shimizu K, Kamiya K. Distribution of posterior corneal astigmatism according to axis orientation of anterior corneal astigmatism. PLoS One 2015;10:e0117194. [Crossref] [PubMed]
26. Liu Y, Pang C, Ming S, Fan Q. Effect of myopia and astigmatism deepening on the corneal biomechanical parameter stress-strain index in individuals of Chinese ethnicity. Front Bioeng Biotechnol 2022;10:1018653. [Crossref] [PubMed]
27. Mahroo OA, Oomerjee M, Williams KM, O'Brart DP, Hammond CJ. High heritability of posterior corneal tomography, as measured by Scheimpflug imaging, in a twin study. Invest Ophthalmol Vis Sci 2014;55:8359-64. [Crossref] [PubMed]
28. Heydarian S, Hashemi H, Yekta A, Ostadimoghaddam H, Derakhshan A, Aghamirsalim M, Khabazkhoob M. Heritability of Corneal Curvature and Pentacam Topometric Indices: A Population-Based Study. Eye Contact Lens 2019;45:365-71. [Crossref] [PubMed]
29. Savini G, Næser K, Schiano-Lomoriello D, Mularoni A. Influence of Posterior Corneal Astigmatism on Total Corneal Astigmatism in Eyes With Keratoconus. Cornea 2016;35:1427-33. [Crossref] [PubMed]