August 17, 2026
By Kevin Chan, OD, MS, FAAO, FBCLA, IACMM
Study aim
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Li et al. studied whether two orthokeratology lens designs with smaller back optical zone diameters (BOZD) differed in their ability to slow childhood myopia progression. It also aimed to examine the corneal power patterns they produced. The comparison mainly focused on 5.0 mm-BOZD corneal refractive therapy (CRT) lenses and 5.6 mm-BOZD vision shaping treatment (VST) lenses. In particular, the authors examined not only axial length growth but also detailed corneal topography-derived relative corneal refractive power (RCRP) as a potential surrogate measure.
Study design
- The study was retrospective and included 136 eyes from children aged 8–11 years with low myopia ranging from −0.50D to −3.00D.
- Children were fitted with either 5.0 mm-BOZD CRT lenses or 5.6 mm-BOZD VST lenses.
- Axial length was measured at baseline, six months, and 12 months.
- Corneal topography maps were also analyzed at baseline and after one year to quantify treatment zone size (TZS), treatment zone decentration (TZD) and RCRP characteristics.
Measuring parameter analyses
Axial length outcomes
- The 5.6 mm-BOZD VST group showed significantly slower axial elongation than the 5.0 mm-BOZD CRT group at both follow-up points.
- At six months: axial length increased by 0.09 ± 0.08 mm in the 5.6 mm-BOZD VST group compared with 0.15 ± 0.10 mm in the 5.0 mm-BOZD CRT group.
- At 12 months: the difference became larger: axial length increased by 0.16 ± 0.14 mm in the VST group versus 0.31 ± 0.18 mm in the CRT group. Both between-group differences were statistically significant (P<0.001)
Corneal power and treatment-zone outcomes
- The VST lenses induced a steeper RCRP profile than the CRT lenses, as reflected by a smaller 3/4X value, which indicates that the RCRP curve reached three-quarters of its peak value closer to the corneal center.
- The VST group also had a higher summed RCRP value within the pupillary region, suggesting greater cumulative relative corneal refractive power across the area most relevant to retinal image formation.
- In contrast, maximum RCRP, treatment zone size and treatment zone decentration were similar between the two groups, suggesting that the difference in myopia control was more closely related to the distribution and steepness of corneal power change than to the size or centration of the treatment zone alone.
Clinical Interpretations & Conundrums
While the 5.0 mm-BOZD CRT lens had the smaller nominal BOZD, the 5.6 mm-BOZD VST lens produced the more favorable RCRP profile in this study. This indicates that BOZD alone does not fully determine the optical effect; in fact, overall lens geometry, fitting characteristics, sagittal depth, reverse-curve behavior and how the lens interacts with the individual cornea may all affect the final corneal power distribution.
An important, yet also overlooked, premise worth noting is that peripheral corneal power is NOT the same as peripheral refraction. This clinical nuance further extends the clinical debate–What does myopia control “dose” really consist f? Size of the Bull’s eye pattern? Or, corneal curvature, corneal refractive power, pupil-dependent optics or retinal defocus in combination as well? This is not a minor technicality; it is central to how orthokeratology should be interpreted and customized in the future.
The implication is that smaller treatment zones, greater paracentral corneal power or a more prominent annular “plus” ring should not inherently or absentmindedly be interpreted as greater myopia control efficacy. These topographic features potentially alter retinal image quality and may plausibly influence peripheral or simultaneous myopic defocus. Collectively, the assumption that smaller optic zone size alone yields better outcomes could be oversimplified, which may result in our blind spots.
Editor’s Perspective
The simplistic claim that optic zone size alone predicates treatment success remains inconclusive and controversial. While it seemingly provides a sound rationale for myopia research, there are also much more clinical and behavioral factors in heterogeneity that could play into the resulting retinal image quality (or lack thereof), thus affecting treatment efficacy. The field should keep questioning the hypothesis, but retire the overconfidence.
Abstract
Comparing Myopia Control Efficacy and Corneal Power Changes in Children Wearing Corneal Refractive Therapy (CRT) and Vision Shaping Treatment (VST) Orthokeratology Lenses with Smaller Back Optic Zone Diameters
Na Li, Wenting Jiang, Na Wang, Zhaoying Cai, Bei Du, Weiping Lin, Ruihua Wei
Purpose
To compare axial length (AL) elongation and relative corneal refractive power (RCRP) changes in children wearing corneal refractive therapy (CRT) and vision shaping treatment (VST) lenses with smaller back optical zone diameters (BOZD).
Methods
Results
Conclusions
