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Current Eye Research

ISSN: 0271-3683 (Print) 1460-2202 (Online) Journal homepage: http://www.tandfonline.com/loi/icey20

Predictive role of paracentral corneal toricity usingelevation data for treatment zone decentrationduring orthokeratology

Zhouyue Li, Dongmei Cui, Wen Long, Yin Hu, Liying He & Xiao Yang

To cite this article: Zhouyue Li, Dongmei Cui, Wen Long, Yin Hu, Liying He & Xiao Yang (2018):Predictive role of paracentral corneal toricity using elevation data for treatment zone decentrationduring orthokeratology, Current Eye Research, DOI: 10.1080/02713683.2018.1481516

To link to this article: https://doi.org/10.1080/02713683.2018.1481516

Accepted author version posted online: 26May 2018.

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Predictive role of paracentral corneal toricity using elevation

data for treatment zone decentration during

orthokeratology

Running title: Influencing factors of Ortho-k lens fitting

Author: Zhouyue Li , Dongmei Cui , Wen Long , Yin Hu , Liying He , Xiao Yang *

Institutional affiliation: State Key Laboratory of Ophthalmology, Zhongshan

Ophthalmic Center, Sun Yat-sen University, Guangzhou, China.

*Correspondence:

*Xiao Yang, Zhongshan Ophthalmic Center, 54 S. Xianlie Road, Guangzhou 510060,

China. Email: Yangx_zoc@163.com. Telephone: +8602087330412; Fax numbers:

+862087333271

Abstract

Purpose: To investigate the influence of paracentral corneal toricity using elevation

data on the treatment zone decentration of spherical and toric orthokeratology

(Ortho-k) lens.

Methods：Corneal elevation difference (CED) was defined as the difference of

corneal elevation between the two principle meridians at 8-mm chord, representing

the paracentral corneal toricity. Seventy-five subjects included in this prospective

study were divided into low CED group (LCED<30μm, n=25) and high CED group

(HCED≥30μm, n=50). All subjects in the LCED group and 25 subjects in the HCED

group (HCED I) were fitted with spherical Ortho-k, the other 25 subjects in the

HCED group (HCED II) were fitted with toric Ortho-k. Corneal topography data from

the right eyes were obtained at baseline and after 1 month of lens wear. The amount

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and direction of treatment zone decentration among the three groups were compared,

and its relationship with corneal shape parameters, including central and paracentral

corneal toricity, corneal asymmetry, flat-k, and eccentricity, and lens diameter were

analyzed using univariable and multivariate linear regression models.

Results: The magnitude of treatment zone decentration was greatest in the HCED I

group ((LCED vs. HCED I vs. HCED II: 0.47±0.15mm vs. 0.73±0.15mm vs.

0.47±0.19mm, respectively; ANOVA, P < 0.01). Among participants fitted with

spherical Ortho-k, the magnitude of treatment zone decentration was significantly

correlated to paracentral CED after adjusting for the other corneal parameters and lens

diameter (standard β=0.599, P < 0.01). No significant correlation between these

parameters was found among those fitted with toric Ortho-k.

Conclusions: Eyes with greater paracentral CED tend to have increased decentration

of spherical Ortho-k lens, whereas toric Ortho-k appears to reduce the amount of lens

decentration in eyes with CED at 8-mm chord above 30 μm.

Keywords: orthokeratology; decentration; paracentral corneal toricity; corneal

elevation difference; treatment zone

Introduction

Orthokeratology (Ortho-k), with a reverse-geometry design, has been shown effective

in improving unaided visual acuity during daytime after removal of the lens [1, 2].

Recent studies have demonstrated the efficacy of Ortho-k in slowing the axial

elongation in myopic children by approximately 50% compared with single vision

spectacles or soft contact lens [3-5]. However, despite successful lens-fitting,

decentration of the Ortho-k lens treatment zone is common [9-11], potentially causing

visual disturbance due to induction of astigmatism [6], higher-order aberrations, and

reduction in contrast sensitivity [7, 8].

The exact mechanism of treatment zone decentration is still unclear, but appears to be

multifactorial. Greater baseline corneal toricity was recently reported to cause higher

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amount of treatment zone decentration after single overnight use of spherical Ortho-k

lens in eyes with minimal (≤1.50 diopters cylinder [DC]) and moderate (1.50 to 3.50

DC) corneal toricity [9]. Lens diameter appears to be another influencing factor [10, 11].

Thus, a smaller lens diameter is related to greater amounts of treatment zone

decentration. Data from paracentral corneal region where the first alignment curve of

an Ortho-k lens mostly likely falls (between the chords of 7 to 9mm) [12], may also

play a critical role in treatment zone decentration. The relationship between lens

decentration and corneal elevation asymmetry at 8-mm chord was confirmed by a

recent study showing that both direction and magnitude of lens decentration were

influenced by paracentral corneal asymmetry [13]. Compared to corneal curvature data

[14], paracentral corneal elevation difference (CED) between the principle meridians

reflects paracentral corneal toricity in a more straightforward way. The relation

between central corneal toricity and treatment zone decentration suggests that

difference between two principle meridians may be an important predictor for

treatment zone decentration. However, to our knowledge, no previous study has

investigated the relationship between the paracentral corneal toricity using elevation

data with treatment zone decentration. Furthermore it is important to identify the main

contributor to treatment zone decentration, because that can help us provide better

Ortho-k lens fitting in clinical practice. Therefore, the aim of this study was to

investigate the influence of paracentral corneal toricity using elevation data at 8-mm

chord on the treatment zone decentration of spherical and toric Ortho-k lens. In

addition, multiple linear regression models were used to evaluate the relative

contribution of potential factors in determining the treatment zone decentration of

spherical and toric Ortho-k lens.

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Subjects and methods

Participants

This study was conducted at Zhongshan Ophthalmic Center (Guangzhou, China)

between December 2016 and March 2017. The study adhered to the tenets of the

Declaration of Helsinki and was approved by the ethical committee of Zhongshan

Ophthalmic Center, Sun Yat-sen University. Written consent was obtained from

guardians of all children before enrollment.

The inclusion criteria included: age between 8 and 18 years, mean sphere between

-1.00 and -4.00 D, refractive astigmatism no greater than -1.50D, and visual acuity

correctable to 20/20 or better. The exclusion criteria included: ocular surface diseases,

fundus diseases, or a history of Ortho-k treatment. Seventy-five right eyes of 75

subjects were enrolled. Twenty-five subjects fitted with spherical Ortho-k, of which

the CED at 8-mm chord was less than 30μm, were categorized as low CED group

(LCED). Fifty subjects with CED at 8-mm chord more than 30μm were categorized as

high CED group (HCED). Twenty-five subjects in HCED group were fitted with

spherical Ortho-k lens (served as HCED I group); the other 25 subjects were fitted

with toric Ortho-k lens (served as HCED II group).

Lens fitting

Lenses used in this study were the spherical and toric four-zone reverse-geometry

gas-germeable rigid contact lenses (Emerald series; Euclid,Herndon, VA). For both

lens types, the back optic zone diameter was 6.2 mm, the width of reverse curve 0.5

mm, and the width of peripheral curve 0.5 mm. The total lens diameter for a typical

trial lens was 10.6mm. All these lenses were made by BOSTON EQUALENS II

(oprifocona) and had a nominal Dk of 127×10-11 (cm2/s) (ml O2/ ml_mmHg)

(ISO/Fatt) according to fitting guidelines supplied by the manufacturer.

For spherical lens fitting, subtraction of the spherical refractive error and a Jessen

factor of 0.75 from the baseline corneal flat-k were used to determine the back optical

zone radius. The baseline corneal flat-k and eccentricity over 8-mm chord of the

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corneal topography were used to determine the alignment curve radius of the first lens.

Lens diameter (about 90% HVID) was determined according to the horizontal visible

iris diameter (HVID) data. The maximum lens diameter that could be ordered was

11.4mm. For subjects in HCED II group, toric lens design was chosen according to

the CED at 8-mm chord, i.e., 1.00D toricity for 30μm difference, and adding 0.25D

for every 7~8μm of increasing CED.

Lens fitting evaluation was performed using fluorescein. The alignment curve was

determined until a classic bull’s eye was shown, with a central touch surrounded by a

narrow and deep annulus of tears trapped in the reverse curve area. Ideal lens fitting

was defined as Ortho-k lens with good centration (lens edge should not go beyond the

limbus) and appropriate movement (approximately 1mm on a blink). If the corneal

response after the first overnight was poor, showing for example displacement, smiley

and frowny face topographic pattern, a new lens with adjusted parameters would be

used. Subjects who could not show satisfactory fits despite repeated modifications (3

pairs of lenses) were excluded from the study. In the current study, only two subjects

in the HCED I group required a second pair of lenses due to poor lens decentration at

the overnight visit. All the participants were instructed to return for a follow-up visit

one day, one week, and one month after the primary lens fitting. Ortho-k lens fitting,

visual acuity, ocular health and corneal topography (Medmont E-300, Australia) were

evaluated at each visit. After lens dispensing, all subjects were requested to wear their

Ortho-K lenses every night for at least 7 consecutive hours.

Determination of CED and corneal asymmetry vector

Corneal topography was conducted using the Placido ring-based Medmont E300

(Australia, Medmont Company) before and after lens delivery at each visit. The

baseline CED at 8-mm chord was obtained from at least 4 topography readings.

Firstly, the mean of average height from the flat and steep meridians at 8-mm chord

were recorded respectively. By this way, the flat and steep meridians determined for

CED calculation was the same as that from K readings. CED values were then created

by subtraction of average height at the flat meridian from the average height at the

steep meridian. The asymmetry between different corneal quadrants was calculated

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according to the method Reported by Chen et al. [13]. The elevation difference of the

two principle meridians of corneal astigmatism at the 8-mm chord was recorded. Then

vector analyses based on these differences were conducted to determine the

magnitude and direction of a vector (corneal asymmetry vector). The angle acquired

by counterclockwise rotating around the keratometery centre from 3 o’clock of cornea

to the vector was defined as the direction of the corneal asymmetry vector.

Determination of the decentration of treatment zone

The magnitude and direction of treatment zone decentration were determined using

Image-Pro Plus software (IPP; produced by Media Cybernetics Corporation, USA).

Our previous study [15] has demonstrated that both the magnitude and direction of

treatment zone decentration could be determined using IPP software with good

repeatability and reproducibility. After one month of Ortho-k lens treatment, a

difference map was obtained by subtracting the pre-orthokeratology tangential

curvature map from the post-orthokeratology tangential curvature map. Figure 1

illustrates how the decentration of treatment zone was determined. The step diopter of

the tangential subtractive map was set to 0.01D in custom settings and whole image

was captured with a format setting at a maximum resolution of 1366*768 pixels.

Various points surrounding the border of the treatment zone area on which the powers

are all zero were indicated manually and then connected to be a closed zone using IPP

software. The distance from the center of the depicted circle (defined as the center of

treatment zone) to the corneal vertex normal was defined as the treatment zone

decentration. Angle (0~359°) acquired by counterclockwise rotating around vertex

normal from 3 o’clock was defined as the angle of treatment zone decentration. In the

current study, one experienced observer who was masked to the study groups assessed

treatment zone centration.

Data analysis

Only data from the right eye were used for statistical calculation. Statistical analysis

was performed using SPSS Version 16.0 (SPSS 16.0, Inc., Chicago, IL). P<0.05 at

two tails was considered to be statistically significant. The baseline corneal shape and

lens parameters in this study included baseline corneal asymmetry vector and CED at

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8-mm chord, corneal flat-k, toricity based on the simulated keratometry or Sim K

calculated approximately at a 3-mm chord diameter, eccentricity and lens diameters.

Data were firstly tested for normality using Sample K-S test. Chi-squared test or one

way ANOVA was used to test difference in demographic data, baseline corneal shape

and lens parameters among three groups where appropriate. Paired t-test was used to

analyze the change in corneal flat-k, toricity, and eccentricity after Ortho-k.

Difference in the magnitude and direction of treatment zone decentration, change of

corneal flat-k, toricity and eccentricity after Ortho-k among the three groups was also

analyzed using one way ANOVA, with Bonferroni post-hoc testing on indication and

correction for multiple comparisons.

The association between corneal shape and lens parameters and treatment zone

parameters was tested using univariable linear regression analyses. High correlations

among explanatory variables are likely to cause multicollinearity, which artificially

inflates the variance of the estimated regression coefficients and thus makes the

estimators less trustworthy for prediction. The multiple linear regression models

included only independent factors based on the result of univariable linear regression

analyses, which were performed to test the association among all the baseline corneal

shape and lens variables. In this study, CED was significantly correlated to corneal

toricity in both spherical Ortho-k (LCED and HCED I group Combined) (adjusted

R²=0.525, ANOVA, F=52.972, P < 0.01) and toric Ortho-k groups (HCED II group)

(adjusted R²=0.134, ANOVA, F=4.707, P=0.041). Two multiple linear regression

models were established for participants fitted with spherical (LCED and HCED I

group Combined) and toric Ortho-k. Model 1 explored association of treatment zone

decentration with magnitude of baseline CED at 8-mm chord, adjusting for corneal

asymmetry vector at 8-mm chord, flat-k, eccentricity, and lens diameter against the

magnitude of treatment zone decentration. Model 2 explored association of treatment

zone decentration with the magnitude of baseline corneal toricity, adjusting for

corneal asymmetry vector at 8-mm chord, flat-k, eccentricity, and lens diameter

against the magnitude of treatment zone decentration.

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Results

All the 75 subjects completed this one-month study. Baseline information with respect

to the demographic data, corneal shape and lens parameters are summarized in Table

1. At baseline, there was no statistically significant difference of gender (Chi-squared

test, P=0.913), age, refractive sphere, flat-k, corneal eccentricity along the flattest

meridian, amount and angle of corneal asymmetry vector at 8-mm chord, HVID and

lens diameter in the three groups (ANOVA, all P > 0.05). There was no significant

difference in baseline refractive cylinder (post-hoc multiple comparisons, P=0.846),

corneal toricity (post-hoc multiple comparisons, P=0.162) and CED at 8-mm chord

(post-hoc multiple comparisons, P=0.303) between the HCED I and HCED II groups.

Baseline refractive cylinder, corneal toricity and CED at 8-mm chord in the LCED

group were significant smaller than that in the HCED I and HCED II groups (post-hoc

multiple comparisons, all P < 0.01).

After one month of Ortho-k treatment, both of the flat-k and corneal eccentricity

along the flattest meridian significantly decreased from baseline in the three groups

(Paired t-test, all P < 0.01). The magnitude of change in the flat-k (ANOVA, P=0.170)

and corneal eccentricity along the flattest meridian (ANOVA, P=0.505) were similar

among the three groups (Table 2). Corneal toricity did not significantly change from

baseline in the LCED group (paired t-test, P=0.204), but significantly decreased from

baseline in the HCED I and HCED II group (paired t-test, both P < 0.01) (Table 2).

The magnitude of corneal toricity change in HCED I group was significantly smaller

than that in HCED II group (post-hoc multiple comparisons, P=0.045).

As shown in Figure 2A, there was no significant difference in the magnitude of

treatment zone decentration between LCED and HCED II groups (post-hoc multiple

comparisons, 0.47±0.15mm vs. 0.47±0.18mm, respectively; P=0.965). The magnitude

of treatment zone decentration in HCED I group (0.73±0.15mm) was significantly

greater than the other two groups (ANOVA, P < 0.01). As shown in Figure 2B, the

mean angle of treatment zone decentration was similar among the three groups

(LCED vs. HCED I vs. HCED II: 206.01±52.91° vs. 210.11±45.50° vs.

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202.02±58.55°, respectively; ANOVA, P=0.863). For overall displacement,

inferotemporal decentration was the most common (LCED vs. HCED I vs. HCED II:

68% vs. 76% vs. 56%, respectively) while the superionasal decentration was the rarest

(LCED vs. HCED I vs. HCED II: 4% vs. 4% vs. 8%, respectively) for the three

groups (Figure 2B). The angle of treatment zone decentration and the baseline angle

of corneal asymmetry vector were significantly correlated in spherical group (LCED

and HCED I group Combined) (Adjusted R²=0.074, ANOVA, F=4.889, P = 0.032),

but not in the toric group (HCED II) (Adjusted R²=0.018, ANOVA, F=1.439,

P=0.243).

Results of univariable and multiple linear regression analyses evaluating the

relationship between the amount of treatment zone decentration and corneal shape and

lens parameters in spherical Ortho-k group (LCED and HCED I group Combined) are

summarized in Table 3. In spherical Ortho-k group, the magnitude of lens

decentration was significantly associated with baseline CED at 8-mm chord, corneal

toricity and corneal asymmetry vector at 8-mm chord according to the results of

univariable analyses (all P < 0.01). In multiple linear regression model 1, both of the

CED (standard β (95% CI) ：0.737 (0.563, 0.910), P < 0.01) and corneal asymmetry

vector at 8-mm (standard β (95% CI): 0.245 (0.071, 0.418), P=0.007) significantly

contributed to the magnitude of treatment zone decentration (Adjusted R²=0.645,

ANOVA, F=45.458, P < 0.01), whereas the other factors, including corneal flat-k,

eccentricity and lens diameter, did not influence the treatment zone decentration (all

P>0.05). In multiple linear regression model 2, both of the corneal toricity (standard β

(95% CI) ：0.599 (0.387, 0.811), P < 0.01) and corneal asymmetry vector at 8-mm

(standard β (95% CI): 0.298 (0.086, 0.510), P=0.007) significantly contributed to the

magnitude of treatment zone decentration (Adjusted R²=0.463, ANOVA, F=22.105, P

< 0.01), whereas the other factors, including corneal flat-k, eccentricity and lens

diameter, did not influence the treatment zone decentration (all P > 0.05). In toric

Ortho-k group (HCED II group), both univariable and multiple linear regression

analyses showed no significant association between the magnitude of treatment zone

decentration and any corneal shape and lens parameters (all P>0.05).

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Discussion

Despite of the efficacy of Ortho-k lens in reducing refractive error, visual disturbance

due to treatment zone decentration [6-8] remains a problem in clinical practice. Corneal

toricity [9], corneal asymmetry [13] and eccentricity [16] are some of the factors that may

influence lens centration. To our knowledge, this is the first study that investigates the

potential effect of paracentral corneal toricity using elevation data on the treatment

zone decentration with different spherical Ortho-k lens designs. In addition, the effect

of toric Ortho-k lens on the treatment zone decentration in eyes with greater

paracentral corneal toricity was also investigated.

It is reasonable to assume that good alignment of the lens back surface with the

corneal surface will enhance lens fitting stability. Maseedupally et al. [14] found that

the difference of corneal curvature between temporal versus nasal sectors or inferior

versus superior sectors in the central 5-mm zone were significantly smaller than that

in paracentral corneal zone between the chords of 5 to 8mm. In addition, the

alignment curve, which supports most of the weight of the Ortho-k lens, plays an

important role for the stabilization of an Ortho-k lens on the cornea. Thus, data from

the peripheral corneal region between the chords of 7 and 9mm, where the first

alignment curve of Ortho-k lens mostly likely falls on [12], are critical for lens

centration and needs to be examined carefully. Instead of using the curvature data we

used a simplified method based on paracentral corneal toricity elevation [14]. The

magnitude of treatment zone decentration in eyes with CED at 8-mm chord above 30

μm was greater than that in eyes with minimal CED at 8-mm chord with spherical

Ortho-k. This finding is further supported by the positive relationship when all the

data from eyes fitted with spherical Ortho-k were combined between baseline CED at

8-mm chord and the magnitude of treatment zone decentration after adjustment for

baseline corneal asymmetry, flat-k, eccentricity and lens diameter.

Most myopic children are also astigmatic [17, 18]. Conventional fitting of spherical

Ortho-k lens is mainly based on the matching of lens sag height with the corneal sag

height along the flattest corneal meridian. When the amount of corneal toricity

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increases, this method of selecting lens parameters is thought to influence lens fitting

stability due to the unequal alignment of Ortho-k lens along the principle meridians.

Swarbrick et al. [9] found that eyes with higher amount of central corneal toricity

display greater amounts of treatment zone decentration during spherical Ortho-k. In

consistency with Swarbrick’s result, we found a significant association between

central corneal toricity and treatment zone decentration. However, paracentral corneal

toricity using elevation data may be a better predictor for decentration than the central

corneal toricity, as the standard coefficients of univariable and multiple linear

regression analyses for paracentral CED were greater than that for central corneal

toricity. The relative contribution of central and paracentral corneal toricity to

spherical lens decentration could be further investigated by their significant

correlation. However, as also found by Maseedupally et al, these parameters did not

correspondent one to one [14]. Thus, it appears that the curvature in the central corneal

zone is significant different from that in the paracentral region. During Ortho-k lens

fitting it is therefore important to assess paracentral CED in eyes with high central

corneal toricity or in cases where bad lens centration occurs despite low central

corneal toricity.

Another important finding in the current study was that when fitted with toric Ortho-k,

the magnitude of treatment zone decentration was significant smaller than when fitted

with spherical Ortho-k in eyes with similar great CED at 8-mm chord (above 30 μm).

This indicates that toric fitting technique improves lens fitting stability in eyes with

greater paracentral CED. Interestingly, the association between treatment zone

decentration and other potential factors showed that all factors including CED,

corneal asymmetry and central corneal toricity etc. were independent of lens

decentration in toric Ortho-k subjects. The absence of any association between the

amount of treatment zone decentration and the magnitude of corneal shape parameters

in spherical Ortho-k subjects versus toric Ortho-k subjects suggests that toric fitting

technique may help improve lens centration by weakening the influence of these

corneal shape parameters, especially for CED at 8-mm more than 30μm. It should be

noticed here that the sample size in the toric Ortho-k group was relative small and

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further studies with larger sample size therefore are warrented.

In a previous study [10] we found that the amount of lens decentration with a lens

diameter of 11.0mm was significant smaller than that with lens diameters between

10.2 and 10.6mm (0.41mm vs. 0.77mm, respectively). Hiraoka et al. [11] found a

relative higher amount of lens decentration of 0.85mm when all the eyes were fitted

with the same lens diameter of 10.0mm. This suggests that a larger lens diameter may

help limit lens decentration. But uniform lens size is not feasible in clinical practice

because the corneal size varies with different subjects. Lens diameter in the current

study was determined according to the HVID result. In contrast, both of our current

data and Chen et al.’s [16] showed that the HVID and lens diameter were independent

of lens decentration during spherical Ortho-k when lens diameter was tailored to the

subject’s corneal size.

As regards direction of decentration for overall displacement, inferotemporal

decentration was the most common while the superionasal decentration was the rarest

for all the three groups. This is consistent with results from previous studies [9-11, 16].

Baseline corneal asymmetry vector has been suggested as a possible explanation [16].

In the current study, the mean directions of the corneal asymmetry vector among the

three groups were all inferotemporal, and both the amount and direction significant

contributed to the magnitude and angle of treatment zone decentration.

Eyelid force is one potential factor influencing the alignment between lens and

corneal surface that was not investigated in the current study. During Ortho-k lens

fitting many factors should be taken into account, including corneal parameters, lens

design and eyelid forces. Further studies are needed to investigate how a combination

of these factors may influence Ortho-k lens fitting.

In conclusion, the amount of paracentral corneal toricity using elevation data at 8-mm

chord appears to be a better predictor of lens decentration than central corneal toricity

and corneal asymmetry. When the CED at 8-mm chord was above 30 μm, a toric

Ortho-k fitting appears to improve lens centration and should therefore be

recommended.

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Acknowledgments

This work was supported by grants from National Natural Science Foundation of

China, China (grant no.: 81200716).

Disclosure

All the authors declare that they have no competing interests.

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Table 1. Comparison of demographic data, baseline corneal shape parameters

and orthokeratology lens diameter among the three groups (mean ± SD).

Parameters Spherical Toric P-value LCED, n=25 HCED I, n=25 HCED II, n=25 Gender (F/M) 12/13 11/14 13/12 0.913 Age (years) 11.68±2.01 12.72±2.01 11.88±1.94 0.154 Sphere (D) -2.67±0.80 -2.94±0.91 -2.57±0.82 0.278 Cylinder (D) -0.16±0.29 -0.53±0.38 -0.55±0.41 <0.001

Flat-K (D) 42.90±1.38 42.52±1.18 42.87±1.25 0.504 Corneal toricity (D) 0.71±0.36 1.38±0.43 1.54±0.36 <0.001

Corneal eccentricity 0.66±0.07 0.66±0.08 0.64±0.07 0.612 CED at 8-mm chord (μm) 15.92±7.00 37.96±6.18 40.07±10.36 <0.001

Corneal asymmetry vector (μm) 29.17±12.69 31.05±11.52 29.02±12.70 0.507 Angle of asymmetry vector (°) 192.22±43.50 193.68±54.15 194.21±49.49 0.989 HVID (mm) 11.61±0.25 11.56±0.36 11.54±0.33 0.728 Lens diameter (mm) 10.49±0.22 10.47±0.28 10.42±0.28 0.667

Corneal eccentricity: corneal eccentricity along the flattest meridian; CED at 8-mm

chord: corneal elevation difference at 8-mm chord; HVID: horizontal visible iris

diameter. Difference in gender was tested using Chi-squared test, and difference in the

other variables was tested using ANOVA analysis with post-hoc multiple comparisons.

P<0.05 at two tails was considered to be statistically significant. Statistically

significant numbers are in bold face.

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Table 2. Change of corneal flat-k, toricity and corneal eccentricity after one

month of Ortho-k lens wear among the three groups (mean ± SD).

Parameters Spherical Toric P-value LCED, n=25 HCED I, n=25 HCED II, n=25 (ANOVA) △Flat-K (D) -2.10±0.52* -2.31±0.64* -1.96±0.74* 0.170 △Corneal toricity (D) -0.10±0.40 -0.32±0.46* -0.57±0.41* 0.001 △Corneal eccentricity -0.35±0.11* -0.33±0.15* -0.30±0.16* 0.505

△Corneal eccentricity: change of corneal eccentricity along the flattest meridian. *:

paired t-test P < 0.01.

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Table 3. Results of univariable and multiple linear regression analyses evaluating

the relationship between treatment zone decentration with corneal shape and

lens parameters in spherical Ortho-k group.

Treatment zone decentration

Univariable Multiple*

standard β (95% CI) P standard β (95% CI) P Model 1

CED at 8-mm chord 0.775 (0.592, 0.958) <0.001 0.737 (0.563, 0.910) <0.001 Corneal asymmetry vector 0.360 (0.089, 0.631) 0.010 0.245 (0.071, 0.418) 0.007

Model 2 Corneal toricity 0.630 (0.405, 0.855) <0.001 0.599 (0.387, 0.811) <0.001 Corneal asymmetry vector 0.360 (0.089, 0.631) 0.010 0.298 (0.086, 0.510) 0.007

*Adjusted for corneal flat-k, eccentricity and lens diameter using multiple linear

regression analyses.

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Figure 1. Determination of the decentration of treatment zone using Image-Pro

Plus software. Various points surrounding the border of treatment zone area on which

the powers are all zero were indicated manually. Then all the points were connected to

form a yellow closed zone which represented the border of treatment zone. The red

cross symbol was the center of treatment zone. Distance of the solid yellow line

represented the amount of decentration, and the angle α represented the direction of

decentration.

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Figure 2. Distribution of treatment zone decentration. A: comparison of the

magnitude of treatment zone decentration among three groups. B: Overview of

treatment zone lens decentration. 0 to 360 degrees represent the meridian degree set

on the corneal topography map. Dashed and solid circles outline the distance of

0.5mm and 1mm to the corneal vertex, respectively. **: p-value was less than 0.01.

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