Efficacy of different optical zone design with different myopia control for corneal refractive therapy (CRT): a one-year prospective cohort study

Abstract

Purpose

This study aims to compare the one-year myopia control effects of orthokeratology (OK) lenses with different optical zone designs in children with varying degrees of myopia.

Methods

A total of 200 children with myopia aged 7–12 years with spherical equivalent refraction (SER) ranging from − 0.50 D to − 6.00 D were enrolled. They were non-randomly divided into four groups based on the baseline refractive power and the final choice on OK lenses type: Low myopia 6.0 group, Mod myopia 6.0 group, Low myopia 5.0 group, and Mod myopia 5.0 group, with 50 children in each group. Data from the right eye were used for analysis. We comparatively analysed the one-year follow-up axial length (AL) and AL elongation between each group. We also analyzed the interaction effect between different optical center designs and different initial myopic refractive powers on AL elongation over 1 year by two-way ANOVA, and compared the proportions of non-progressive myopia among the four groups.

Results

At the 1-year follow-up, AL differed significantly across different myopia subgroups, with Mod myopia groups showing longer AL than low myopia groups, but no significant difference was noted between lens design subgroups. For one-year AL elongation, subgroups with small optical zone designs and those with initial moderate myopia had lower values. A slight interaction effect was observed between lens optical center design and initial myopia severity (F = 6.700, P = 0.010, partial η²=0.033). The one-year AL elongation was 0.280 ± 0.170, 0.148 ± 0.167, 0.154 ± 0.121, and 0.141 ± 0.185 in Low Myopia 6.0 group, Mod Myopia 6.0 group, Low Myopia 5.0 group, and Mod Myopia 5.0 group, respectively; the corresponding non-progressive myopia rates were 62%, 36%, 58%, and 64%. The Mod Myopia 5.0 group presented the slowest AL elongation and the highest non-progressive myopia rate, whereas Mod myopia 6.0 group and Low myopia 5.0 group also showed slower AL elongation and higher non-progressive myopia rates than those of the Low myopia 6.0 group.

Conclusion

CRT lenses with a small optical zone design exhibited better myopia control compared to those with a larger optical zone design, especially in children with low myopia.

Introduction

The high incidence of myopia among children has become a significant public health concern, seriously affecting visual health. Consequently, the need for effective myopia prevention and control is urgent [1–4]. Various approaches exist to manage the occurrence and development of myopia, among which orthokeratology (OK) lenses, with an inverse geometric design, have gained widespread acceptance [5–8]. By wearing OK lenses overnight, children can achieve clear unaided vision the next day while also slowing the progression of myopia. These lenses are widely accepted by both parents and adolescents and are frequently used in clinical practice. The effects of different designs of OK lenses in delaying the progression of myopia vary, and the extent of their influence also differs depending on the severity of myopia [9, 10]. Different optical zone designs represent a core strategy for the individualized fitting of OK lenses, enhancing myopia control efficacy and balancing visual quality. Relevant studies have confirmed that small optical zones (5 mm, 5.5 mm) exhibit superior advantages in suppressing axial length(AL) elongation, while selection should be tailored to corneal morphology and visual demands of patients. In clinical practice, comprehensive evaluation should be performed based on factors such as the patient’s age, initial myopic refractive error, and corneal topography to achieve optimal therapeutic outcomes and wearing comfort [11]. In this study, we comparatively analyzed the one-year myopia control effects of corneal refractive therapy (CRT) lenses with different optical zone designs in children with different degrees of baseline myopia to identify more effective myopia control measures and provide clinical data to support myopia control and potentially prevent blindness.

Materials and methods

General information

This study enrolled 200 children with myopia who visited the optometry centres of the First Affiliated Hospital of Guangxi University of Traditional Chinese Medicine and Jingliang Eye Hospital of Guangxi Medical University and opted to wear CRT lenses between May 2023 and May 2024. The children were non-randomly grouped based on intial myopic refractive power and the legal guardians’ final decision on OK lenses type. The inclusion criteria were: (1) age 7–12 years; (2) myopia diagnosis in both eyes confirmed by mydriasis optometry, with spherical equivalent refraction (SER) ranging from − 0.50 D to − 6.00 D, and an interocular difference of less than 1.50 D; and (3) best corrected visual acuity of ≥ 0.8 in both eyes. The exclusion criteria were: (1) prior use of myopia control measures, such as low-intensity red light, visual function training, or 0.01% low-concentration atropine eye drops, within the past six months; (2) presence of other eye diseases besides myopia or unsuitability for OK lenses; and (3) discontinuation of lens use or inability to adhere to the prescribed wearing schedule. All participants and their families provided signed informed consent. This study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee (2025-E0348).

Subgroup intervention

A total of 200 children with myopia aged 7–12 years with SER ranging from − 0.50 D to − 6.00 D were enrolled. They were non-randomly divided into four groups based on intial myopic refractive power and the final choice on OK lenses type: Low myopia 6.0 group, Mod myopia 6.0 group, Low myopia 5.0 group, and Mod myopia 5.0 group, with 50 children in each group.Each child was required to wear CRT lenses for at least 8 h every night and was expected to adhere strictly to the prescribed wearing schedule. All participants were fitted with OK lenses according to standardised fitting guidelines and attended follow-up visits at one day, one week, and one month after starting to wear the lenses, and every three months thereafter. To assess and record the myopia control effects of OK lenses, the children were comprehensively evaluated by measuring their naked vision, assessing their ocular surface using a slit lamp, evaluating corneal reshaping using corneal topography, and measuring ALduring the follow-up visits.

Detailed informations of CRT OK lenses

CRT OK lenses, manufactured by Paragon Vision Science Inc, USA, are rigid gas-permeable contact lenses featuring a three-zone design, which consists of the Base Curve (BC), Return Zone Depth (RZD), and Landing Zone Angle (LZA). The central thickness of the lens is 0.16 mm, with an overall diameter ranging from 9.5 to 12 mm and an Optical Zone Diameter (OZD) spanning 5.0 to 7.0 mm. The BC diameter ranges from 6.5 to 10.5 mm, and the RZD has a fixed width of 1 mm. The LZA adopts a tangential contact design, with its width varying according to the total lens diameter. The oxygen permeability coefficient of the lens is 100 × 10⁻¹¹ cm²/s (mL O₂/mL mmHg). Specifically, CRT 5.0 has an OZD of 5.0 mm, while CRT 6.0 has an OZD of 6.0 mm; all other parameters are essentially consistent between the two OK lens types. Lens fitting was performed in accordance with the clinical guidelines for OK lens fitting.

Specific indicator acquisition

Age was recorded as a whole number; for example, children aged 9.1–9.9 years were categorised as 9 years old. To measure SER (spherical lens + 1/2 cylindrical lens), compound tropicamide eye drops (0.4 mL/vial) (Shenyang XINGQI Pharmaceutical Co., Ltd.) were used to paralyse the ciliary muscle. Specifically, the drops were administered three times at 5-minute intervals, and after 30 min, optometry was performed using a computerised optometry instrument (AR-310 A, Nidek, Japan). The measurement was performed three times under automatic mode, and the average value was recorded. AL, K1, K2, anterior chamber depth (ACD), and white-to-white distance (WTW) were measured using an IOLMaster 500 (Zeiss, Germany). Intraocular pressure was measured using a non-contact tonometer (NT-510, Nidek, Japan), with three readings averaged for analysis. All procedures were performed by the same trained examiner, and only data from the right eye of each participant were collected and analysed. One-year AL elongation was calculated by subtracting the baseline AL from the AL measured after one year. The annual AL elongation cutoff was set at 0.2 mm; children with an AL elongation of ≤ 0.2 mm were classified as having non-progressive myopia [12]. The percentage of non-progressive myopia was calculated for each group.

Sample size calculation

This study employed the sample size calculation formula for Two-way ANOVA, with the specific formula presented below. Specifically, the two-tailed significance level (α) was set at 0.05 (Z 1 − α/2 = 1.96), and the power (1 − β) was 80% (Z(1 − β) = 0.84). The variance of the dependent variable (σ²=0.06 D²) was obtained from previous OK lens studies [13]. The number of subgroups for baseline myopia degree was k₁=2 (low myopia and mod myopia), and the number of OK lens design types was k₂=2 (CRT5.0 group and CRT6.0 group). The minimum clinically meaningful effect size was 0.25 D, and the intraclass correlation coefficient (ICC) was set at 0.5 [14]. Accounting for a 15% attrition rate, the calculated minimum required sample size was 74 participants. This study included 200 participants, which meets the statistical minimum sample size requirement.

Statistical methods

Data were analysed using SPSS 27.0 (IBM Corporation, Armonk, NY). Measurement data were expressed as mean ± standard deviation, and all variables were tested for normality. The age, baseline AL, baseline SER, AL, K1, K2, ACD, WTW, and AL elongation between groups were analysed using one-way ANOVA, followed by simple effect analyses with Bonferroni correction to interpret the interaction. A comparative analysis of gender distribution and the percentage of non-progressive myopia between the two groups was performed using a four-grid chi-square test. Two-wayANOVA was conducted to examine the effects of optical center design (large vs. small) and initial myopia degree (low vs. moderate) on one-year AL elongation, with subsequent simple effect analyses (Bonferroni correction) to clarify the interaction effect .The level was set at 0.05 and adjusted to 0.025 for multiple comparisons.

Results

Comparison of basic characteristics between all groups

As shown in Table 1, there was no significant difference between the all groups in terms of basic characteristics (e.g. gender, corneal KS, corneal KF, ACD, WTW, and NCT).There were significant differences among age, initial SER, and baseline AL. However, no statistically significant differences were observed between the Low myopia 6.0 group and the Low myopia 5.0 group, nor between the Mod myopia 6.0 group and the Mod myopia 5.0 group (all P-values > 0.05).

Table 1.

Comparison of parameters between all groups

Baseline parameters	Low myopia 6.0 group (n = 50)	Mod myopia 6.0 group (n = 50)	Low myopia 5.0 group (n = 50)	Mod myopia 5.0 group (n = 50)	P
Sex, male (n, %)	26(52.0)	27(46.0)	28(44.0)	20(60)	0.376
Age/year	9.34 ± 1.10a	10.56 ± 1.25b	9.74 ± 1.19	10.68 ± 1.66	<0.001
Initial SER/D	-1.72 ± 0.52a	-4.18 ± 0.81b	-1.83 ± 0.74	-4.45 ± 0.86	<0.001
Baseline AL/mm	24.32 ± 0.56a	25.44 ± 0.51b	24.46 ± 0.64	25.54 ± 0.84	<0.001
Corneal Ks	43.84 ± 1.19	43.54 ± 1.15	43.68 ± 1.52	43.77 ± 1.21	0.675
Corneal Kf	42.63 ± 1.10	42.29 ± 1.19	42.29 ± 2.21	42.40 ± 1.27	0.650
ACD	3.19 ± 0.21	3.06 ± 0.34	3.06 ± 0.33	3.06 ± 0.30	0.080
WTW	12.23 ± 0.63	12.17 ± 0.54	12.35 ± 0.41	12.14 ± 0.55	0.210
NCT	14.38 ± 2.08	14.92 ± 2.81	14.92 ± 2.28	15.02 ± 2.29	0.526

Note: All data are presented as mean ± SD. SER: spherical equivalent refractive error, D: dioptre, AL: axial length, Ks: steep keratometry, Kf: flat keratometry, ACD: anterior chamber depth, WTW: white-to-white distance, NCT: non-contact tonometer

^aLow myopia 6.0 group compare with Low myopia 5.0 group, P>0.05

^bMod myopia 6.0 group compare with Mod myopia 5.0 group, P>0.05

Comparison of one-year AL elongation between different optical zone designs and different myopia degrees

The results of two-way analysis of variance (ANOVA) for 1 year AL elongation are presented in Table 2. Significant main effects were detected for both independent variables: (1) optical center design (F = 14.879, P < 0.001, partial η²=0.071), with the small optical center group exhibiting significantly smaller AL elongation (CRT5.0); and (2) initial myopia degree (F = 16.437, P < 0.001, partial η²=0.077), where the Mod myopia group showed significantly smaller AL elongation. Additionally, a small statistically significant interaction was observed between optical center design and initial myopia degree (F = 6.700, P = 0.010, partial η²=0.033).

Table 2.

Results of two-way analysis of variance (ANOVA) for 1 year AL elongation

Source of Variation	Degrees of freedom (df)	Sum of squares (SS)	Mean Square (MS)	F	P	Partial η²	95% Confidence Interval
Main effect of optical center design(Large vs. Small)	1	0.393	0.393	14.879	< 0.001	0.071	0.023–0.135
Main effect of initial myopia degree(Low vs. Mod)	1	0.434	0.434	16.437	< 0.001	0.077	0.028–0.142
Interaction (Design ×Myopia)	1	0.177	0.177	6.700	0.010	0.033	0.004–0.085

*Note: Partial η² quantified effect size (small < 0.06, medium 0.06–0.14, large > 0.14)

Figure 1 presents an interaction plot that visually illustrates the interaction pattern between optical center design and initial myopia degree. Specifically, in low myopia, the optical center design exerts a more pronounced effect on AL elongation; whereas in moderate myopia, the difference between the two optical center design groups is relatively small.

Fig. 1.

Interaction plot of optical center design and initial myopia degree

Table 3 presents the results of simple effects analyses (Bonferroni correction for multiple comparisons, adjusted α = 0.025). Specifically, in patients with low myopia, the Low myopia 5.0 group (small optical center design) exhibited significantly smaller 1-year AL elongation (0.154 ± 0.121 mm) than the Low myopia 6.0 group (large optical center design) (0.280 ± 0.170 mm; t = 4.27, adjusted P < 0.001, Cohen’s d = 0.85, 95% CI: 0.087–0.165), with a large effect size. Additionally, in the large optical center design (Low myopia 6.0 vs. Mod myopia 6.0 groups), patients with moderate myopia showed significantly smaller AL elongation (0.148 ± 0.167 mm) compared with those with low myopia (0.280 ± 0.170 mm; t = 3.92, adjusted P < 0.001, Cohen’s d = 0.78, 95% CI: 0.075–0.185), indicating a medium-to-large effect size. No significant difference in AL elongation was detected between the Mod myopia 6.0 and 5.0 groups (0.148 ± 0.167 mm vs. 0.141 ± 0.185 mm; t = 0.20, adjusted P = 0.842, Cohen’s d = 0.04, 95% CI: -0.059–0.073) or between the Low myopia 5.0 and Mod myopia 5.0 groups (0.154 ± 0.121 mm vs. 0.141 ± 0.185 mm; t = 0.42, adjusted P = 0.678, Cohen’s d = 0.08, 95% CI: -0.063–0.090), with negligible effect sizes (Cohen’s d < 0.2) for both non-significant comparisons. Figure 2 is a bar chart of the four groups, indicating that the 1-year AL elongation was the highest in the Low myopia 6.0 group and the lowest in the Mod myopia 5.0 group.

Table 3.

Results of simple effect analyses (Bonferroni correction)

Comparison	Sample Size (n1/n2)	1 year AL elongation (mm, Mean ± SD)	t	Adjusted P	Cohen’s d	95% Confidence Interval
Low myopia 6.0 group vs. Low myopia 5.0 group	50/50	0.280 ± 0.170 vs. 0.154 ± 0.121	4.27	< 0.001	0.85	0.087–0.165
Mod myopia 6.0 group vs. Mod myopia 5.0 group	50/50	0.148 ± 0.167 vs. 0.141 ± 0.185	0.2	0.842	0.04	-0.15
Low myopia 6.0 group vs. Mod myopia 6.0 group	50/50	0.280 ± 0.170 vs. 0.148 ± 0.167	3.92	< 0.001	0.78	0.075–0.185
Low myopia 5.0 group vs. Mod myopia 5.0 group	50/50	0.154 ± 0.121 vs. 0.141 ± 0.185	0.42	0.678	0.08	-0.136

*Note: Bonferroni correction was used for multiple comparisons (adjusted α = 0.025); Cohen’s d quantified effect size (small < 0.2, medium 0.2–0.8, large > 0.8)

Fig. 2.

Bar chart of 1year AL elongation among four groups

Comparison of non-progressive myopia rates between all groups

Children with myopia with one-year AL elongation of ≤ 0.2 mm were classified as having non-progressive myopia, and the non-progressive myopia rates were subsequently calculated and compared across groups.

Figure 3 presents the bar chart of non-progressive myopia rates among the four groups. Statistical analysis revealed a significant difference in the non-progressive myopia rate among the four study groups (P < 0.001). Specifically, the rate peaked at 64% in the Mod myopia 6.0 group, followed by the Low myopia 5.0 group (62%) and Mod myopia 5.0 group (58%), whereas the Low myopia 6.0 group showed the lowest rate (32%).The rates in other three groups were significantly higher than that in the Low myopia 6.0 group (all P < 0.05), whereas no statistically significant difference was observed among these three groups (all P > 0.05). These findings may indicate that CRT 5.0 has a superior effect over CRT 6.0 in inhibiting myopia progression in children with low myopia.

Fig. 3.

Bar chart of non-progressive myopia rates among four groups

Discussion

Orthokeratology (OK) lenses are widely utilized for myopia control in children and adolescents, with their safety and efficacy well-established. Technological advancements and clinical feedback have driven the development of increasingly personalized OK lens designs. This study aimed to identify the optimal OK lens choice for children with varying degrees of myopia by comparing the one-year myopia control effects of OK lenses with different optical zone designs. The goal is to provide valuable clinical data to support evidence-based practice and enhance doctor-patient communication during lens fitting.

Key findings: superior efficacy of small optical zone design, especially in low myopia

The study revealed a significant difference in axial length (AL) elongation between different optical zone designs, particularly stratified by baseline myopia severity. The one-year AL elongation in the Low myopia CRT5.0 group was much lower than that in the Low myopia CRT6.0 group (0.154 ± 0.121 mm vs. 0.280 ± 0.170 mm). In contrast, no statistically significant difference was observed between the Mod myopia 5.0 group and the Mod myopia CRT6.0 group (0.141 ± 0.185 mm vs. 0.148 ± 0.167 mm). These findings indicate that CRT5.0 lenses with a small optical zone design are more effective in myopia control, especially for children with low myopia. Two-way ANOVA further demonstrated that subgroups with small optical zone designs and those with initial moderate myopia exhibited lower AL elongation, with a slight interaction effect detected between lens optical zone design and baseline myopia severity.

Consistency with and divergence from existing literature

Benchmarking studies have evaluated the clinical effects of OK lenses with different optical zone designs. For instance, in a prospective cohort study, Li et al. found that the one-year AL elongation in the 5-mm small optical zone group was lower than that in the 6-mm large optical zone group (0.13 ± 0.18 mm vs. 0.27 ± 0.15 mm) [15]. Notably, the AL elongation in the small optical zone group of that study was similar to ours, whereas the value in their large optical zone group was slightly lower than that in our Low myopia CRT6.0 group yet higher than that in our Mod myopia CRT6.0 group. A retrospective study by Ding et al. also showed that OK lenses with a small optical zone design were more effective, reporting a one-year AL elongation of 0.15 ± 0.18 mm [16]. Furthermore, Guo et al. in a randomized controlled trial observed that the one-year AL elongation in the small and large optical zone groups was 0.04 ± 0.15 mm and 0.17 ± 0.13 mm, respectively, both lower than values in the present study [17]. These discrepancies may be attributed to differences in research design, population characteristics, and sample size. Meta-analyses have concluded that OK lenses with a small optical zone design are more effective in controlling AL elongation than those with a large optical zone design [18, 19], which supports our primary conclusion.

Clinical implications for personalized lens fitting

The question of whether all children with myopia should be fitted with small optical zone OK lenses is clinically relevant, given that such designs may influence visual quality, have lower tolerance for positional deviations, require a longer fitting period, and are relatively expensive. Our subgroup analysis provides guidance: the one-year AL elongation in the Low myopia 5.0 group was significantly lower than that in the Low myopia 6.0 group (P < 0.001), while no significant difference was found between the Mod myopia 5.0 and 6.0 groups (adjusted P = 0.842). Similarly, the percentage of non-progressive myopia was higher in the Low myopia 5.0 group (62%) than in the Low myopia 6.0 group (36%), but no significant difference existed between the moderate myopia groups (58% vs. 64%, P = 0.539). These results indicate that children and adolescents with low myopia are more likely to benefit from OK lenses with a small optical zone design, offering valuable guidance for personalized fitting.

Potential mechanisms underlying the observed effects

Clinical studies confirm that OK lenses with an optical zone of 5.0 ~ 5.5 mm exhibit superior efficacy and adaptability in adolescents with low myopia without increasing adverse reaction risks [20].Small optical zone design of OK lenses exerts superior myopia control efficacy through multiple underlying optical mechanisms. Reducing the back optical zone diameter enables the reverse curve to be positioned closer to the visual axis, which steepens the mid-peripheral corneal refractive gradient and generates a stronger and more centrally located peripheral myopic defocus ring. Such design optimizes the distribution of defocus signals within the pupillary area, substantially increasing the magnitude and coverage of retinal peripheral myopic defocus while maintaining satisfactory central visual quality. Meanwhile, a smaller optical zone moderately induces physiological elevation of higher-order aberrations dominated by positive spherical aberration and coma, which further expands the effective coverage of anti-myopia defocus across the mid-peripheral retina. In children and adolescents with physiological pupil diameter ranging from 4.0 mm to 5.5 mm under natural light conditions, small optical zone design can precisely align the peripheral defocus ring with the pupillary margin, avoiding the waste of defocus signals outside the pupil encountered in conventional large optical zone designs. This pupillary matching characteristic significantly improves the utilization efficiency of effective myopic defocus, thereby producing a stronger inhibitory effect on axial eye growth and delaying the progression of pediatric myopia [21–24]. So, the potential mechanisms may include: strengthening peripheral myopic defocus and increasing physiological higher-order aberrations through a small optical zone design, which synergistically inhibits AL elongation.

Study innovations and limitations

Most prior studies report a negative correlation between baseline refractive error and AL elongation [25], or slower AL elongation in children with longer baseline AL [26], but did not classify participants by myopia degree for lens design comparison. This study introduces a novel grouping approach (“optical zone design × baseline myopia severity”), providing new insights for clinical practice. However, limitations exist. The sample size (n = 200) is relatively small, which may affect the reliability and generalizability of the findings.In addition, this study adopted a non-randomized design, and participants were enrolled based on parental choice, which may introduce selection bias and limit the robustness of the results. Larger sample sizes and more clinical trials, particularly randomized controlled designs, are needed to validate these results and strengthen their clinical applicability.Recent evidence has indicated that pupil size is an important factor affecting myopia control outcomes in children wearing orthokeratology or optical defocus lenses. Therefore, incorporating pupil size into the analysis would have made the present study more comprehensive and convincing.

Conclusion

In summary, this study found that the CRT5.0 group was more effective in controlling AL elongation than the CRT6.0 group, especially in children and adolescents with low myopia. This finding provides theoretical and clinical evidence to support the selection of OK lenses in practice, helping clinicians and patients make informed decisions based on individual myopia severity.

Abbreviations

OK

Orthokeratology

SER

Spherical equivalent refraction

AL

Axial length

CRT

Corneal refractive therapy

BC

Base curve

RZD

Return zone depth

LZA

Landing zone angle

OZD

Optical zone diameter

ACD

Anterior chamber depth

WTW

White-to-white distance

ICC

Intraclass correlation coefficient

Author contributions

Wei-Xia Lai and Meng-Tong Wang contributed equally to this work. All co-authors contributed significantly to data interpretation, critical revision of the intellectual content, and final approval of the manuscript. Each author agrees to be accountable for all aspects of the work.

Funding

This study was supported by the Science and Technology Project of Guangxi Center for Disease Prevention and Control (Grant No. GXJKKJ2025YB004).

Data availability

The datasets analyzed during the current study are not publicly available due to ethical restrictions but are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the The Ethics Committee of Guangxi Medical University (Approval No.: 2025-K0141). The research adhered to the tenets of the Declaration of Helsinki. All participants provided written informed consent prior to enrollment.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.