Smaller Lens Zone Slows Childhood Myopia by Reshaping Corneal Mechanics

A one-year randomized clinical trial has found that orthokeratology lenses with a smaller back optic zone diameter slowed axial elongation of the eye more effectively than the traditional 6-mm design in myopic children, and that the difference may be tied to how each lens alters the cornea’s mechanical behavior. The study, conducted at Tianjin Eye Hospital and published in Eye and Vision, enrolled 36 children aged 9 to 12 years with low myopia and fitted each child with a 5-mm optic zone lens in one eye and a 6-mm optic zone lens in the fellow eye, allowing a rare within-child comparison of two lens designs under identical conditions.

Orthokeratology, commonly called ortho-k, uses rigid gas-permeable lenses worn overnight to reshape the cornea. The lenses flatten the central cornea and steepen the mid-periphery, temporarily correcting refractive error while also slowing the eye’s excessive growth that underlies progressing myopia. Previous research has established that ortho-k reduces axial length elongation by roughly 43 to 63 percent compared with no intervention, but the efficacy varies with lens design, and smaller back optic zone diameters have repeatedly shown an advantage. What remained unclear was the mechanism, and the new study set out to test whether corneal biomechanics, measured in vivo, could explain part of the difference.

The trial was prospective, randomized, and double-blind, with randomization assignments masked from screening investigators, outcome assessors, technicians, and participants. Each child wore the lenses for 8 to 10 hours nightly, with compliance tracked through daily wear diaries. The researchers used the Corvis ST tonometer, a Scheimpflug imaging device that records corneal deformation at 4,330 frames per second in response to an air puff, to measure 14 biomechanical parameters at baseline and at 1 day, 7 days, 1 month, 6 months, and 12 months after lens wear began. Axial length was measured with optical biometry at baseline, 6 months, and 12 months. Of the 36 children recruited, 30 completed the full year, contributing 60 eyes to the final analysis, and no complications such as corneal abrasion, staining, or infiltrative events were observed.

The headline result concerned eye growth. At 6 months, eyes wearing the 5-mm optic zone lens elongated by an average of 0.103 millimeters, while fellow eyes with the 6-mm lens elongated by 0.158 millimeters, a statistically significant difference. At 12 months the gap widened: mean elongation was 0.191 millimeters in the 5-mm group versus 0.258 millimeters in the 6-mm group. Because each child served as their own control, with baseline spherical equivalent refraction, keratometry, axial length, central corneal thickness, and intraocular pressure matched between the two eyes, the comparison isolates the effect of the optic zone diameter itself.

The biomechanical measurements told a subtler story. After adjusting for central corneal thickness and intraocular pressure, two confounders the authors note were often overlooked in earlier work, the stiffness parameter at first applanation, or SP-A1, rose significantly above baseline from the first day through 6 months in both lens groups. At 12 months, the first applanation time and the integrated radius had significantly increased, while the first applanation velocity and the stress-strain index, or SSI, had significantly decreased in both groups. These shifts, the authors argue, reflect enhanced corneal resistance to deformation together with increased elasticity and plasticity, changes that may help the cornea hold its remodeled shape after the lenses are removed each morning.

Notably, direct comparisons between the two lens designs found almost no significant biomechanical differences. The only exception was the stress-strain index at 6 months, which changed significantly more in the 6-mm group. At every other time point and for every other parameter, the two designs produced statistically indistinguishable effects. The authors suggest this apparent inconsistency with earlier finite element modeling, which predicted that smaller zones generate greater contact pressure over a smaller area, may stem from the limits of Scheimpflug-based measurement, which captures overall corneal behavior but may miss localized, spatially varying mechanical changes.

The most intriguing finding emerged from correlation analysis. In the 6-mm lens group, changes in axial length were significantly and positively correlated with changes in first applanation time at both 6 and 12 months, with correlation coefficients of 0.385 and 0.424 respectively. Since a longer first applanation time indicates greater corneal elasticity, this suggests that eyes whose corneas became more elastic also experienced more axial growth. No such correlations appeared in the 5-mm group. The authors propose that the traditional 6-mm lens induces relatively greater intrinsic changes in corneal elasticity, which may partly undermine its myopia control efficacy, while the smaller 5-mm zone may offset these biomechanical shifts, potentially through stronger optical signals such as peripheral myopic defocus and higher-order aberrations generated closer to the central cornea.

The interpretation comes with important caveats that the authors themselves emphasize. The correlations were described as weak to moderate, the sample was small, and the study included only Chinese children, raising the possibility of selection bias. Corvis ST parameters represent global corneal biomechanics and may not capture the subtle regional variations that different lens designs produce; the authors call for future work using regional tools such as Brillouin microscopy or optical coherence elastography. Ethical considerations also shaped the design: because stopping ortho-k in children could cause axial length rebound, the study examined only continuous wear, and after the 12-month visit all participants received 5-mm lenses in both eyes.

Within those limits, the study offers the first randomized, double-blind, self-controlled evidence linking sequential corneal biomechanical changes to myopia control outcomes across two optic zone designs over a full year. The authors frame their findings as support for an optic-mechano-biological coupling model, in which lens geometry, corneal mechanics, and retinal optical signals interact to determine treatment efficacy. For clinical practice, the results support individualized lens selection, with the suggestion that children with rapidly progressing myopia may benefit more from a smaller back optic zone diameter. The trial was registered with the Chinese Clinical Trial Registry under ChiCTR2200061048, and the authors note that further histological and in vivo studies are needed to clarify the microstructural basis of the biomechanical changes, since prior animal work in rabbits and monkeys has not shown significant alterations in collagen fibril arrangement after ortho-k.

For a condition affecting a growing share of the world’s children, and one whose high-myopia complications, including myopic maculopathy, retinal detachment, and glaucoma, account for an estimated 10 to 30 percent of global visual impairment and blindness, even modest refinements in lens design carry public health weight. This trial does not claim that biomechanics is the sole mechanism, but it provides measurable, longitudinal evidence that the cornea’s mechanical response to overnight lens wear differs in ways that track with how well each design slows the eye’s growth, a perspective its authors hope will guide the next generation of personalized orthokeratology fitting.

Subject of Research: Corneal biomechanics and myopia control with orthokeratology lenses of different back optic zone diameters in children

Article Title: Effects of orthokeratology with different back optic zone diameters on corneal biomechanics and myopia control: a 1-year randomized, double-blind, self-controlled study

Article References: Xie, R., Huo, Y., Chen, X., Liu, X., Guo, Y., Qu, Z., Li, Y., Zhang, X., Zou, H., Zhao, G., & Wang, Y. (2026). Effects of orthokeratology with different back optic zone diameters on corneal biomechanics and myopia control: a 1-year randomized, double-blind, self-controlled study. Eye and Vision, 13(1), Article 31. https://doi.org/10.1186/s40662-026-00503-2

Image Credits: AI Generated

DOI: 10.1186/s40662-026-00503-2

Keywords: orthokeratology, myopia control, corneal biomechanics, axial length, back optic zone diameter, children, randomized trial, Corvis ST, corneal stiffness, lens design, Effects, different