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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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