Not all ortho-k lenses are equal: does a smaller treatment zone slow eye growth more?

By Jagrut Lallu — BOptom (Hons), MSc Specialty Lenses (Hons), FIAOMC. Therapeutic Optometrist & Contact Lens Specialist. Founder, New Zealand Eye Research Centre; Partner, Rose Optometry; WCO Asia-Pacific Myopia Ambassador; International Myopia Institute NZ Ambassador; Clinical Senior Lecturer, Deakin School of Optometry; Honorary Teaching Fellow, University of Auckland.

Orthokeratology (ortho-k) lenses are worn overnight to gently reshape the front of the eye, giving clear daytime vision and, on average, slower eye growth in children with myopia. One idea for improving the effect is to make the central flattened "treatment zone" smaller, bringing the ring of peripheral myopic defocus closer to the centre of the pupil. A randomised trial from Shanghai tested this directly by comparing two ortho-k lenses that differed in a single parameter.

What the researchers did

The trial ran at a single centre, the Shanghai Eye Disease Prevention and Treatment Centre, between August 2022 and March 2025. Eighty children aged 8–14 years with low myopia (−1.00 to −3.00 D) and less than 1.50 D of astigmatism, none of whom had worn contact lenses, were randomised 1:1. Both groups wore ortho-k lenses from the same manufacturer (Essence, part of Lepu Medical), nightly for at least eight hours:

• Test lens: base curve 5.4 mm (designed to produce a smaller treatment zone)
• Standard lens: base curve 6.0 mm

All other lens parameters were the same. Children, evaluators and the technicians measuring axial length (IOLMaster 700) were masked, though the authors acknowledge the fluorescein fitting pattern could have revealed the lens type. Six children withdrew (5 test, 1 standard), leaving 74 children (139 eyes) in the analysis. The primary outcome was change in axial length over 12 months.

What they found

The test lens did produce a smaller treatment zone (8.73 versus 11.21 mm², estimated from topography maps). After 12 months, eyes in the test group had grown 0.077 ± 0.14 mm, compared with 0.168 ± 0.20 mm in the standard group (p = 0.004). The groups began to separate at 6 months.

In the authors' adjusted mixed-effects model, which accounted for sex, age, weight, baseline refraction and axial length, corneal thickness and eye pressure, eyes wearing the standard lens grew 0.077 mm more over 12 months (95% CI 0.023 to 0.131; p = 0.006). A 95% confidence interval is the range in which the true difference plausibly lies; because it does not cross zero, the result is unlikely to be chance alone. A p-value of 0.006 means a difference this large would be unlikely if the two lens designs truly performed the same.

Statistical versus clinical significance

Two points are easy to miss. First, both groups wore ortho-k, so 0.077 mm is the additional slowing over a standard ortho-k lens, not the effect compared with glasses. Second, the lower end of the interval (0.023 mm) is very small. Whether a difference of this size accumulates into something clinically meaningful over several years is unknown.

There is also a mechanistic puzzle. When the authors looked eye by eye, the size of the treatment zone itself did not predict eye growth (p = 0.53), whereas younger age, thinner corneas and lower eye pressure did. So while the smaller-zone lens was associated with less growth, the trial does not show clearly that the smaller zone is the reason.

Safety

No severe adverse events were reported and no microbial keratitis was described. The most common events were corneal staining, conjunctivitis and photophobia, but the paper does not report how many children in each group were affected, which limits any comparison of safety between designs. Corneal shape, thickness and tear stability were similar between groups at 12 months; measured eye pressure was slightly higher in the test group (16.4 versus 15.2 mmHg; p = 0.01). Good lens hygiene remains central to ortho-k safety, as covered in our ortho-k lens care guide.

Limitations of this evidence

• No single-vision control: the trial compares two ortho-k designs only.
• Retrospective registration: the trial was registered in December 2025, after it had finished, so we cannot confirm the outcomes and analysis were set in advance.
• Manufacturer: both lenses came from one manufacturer; the paper does not say whether lenses were donated or whether the company had any role. Funding was a national clinical specialty grant, and no competing interests were declared.
• Short and small: 12 months, single centre, 40 children per group, more withdrawals from the test group, and a completers-only analysis.
• Missing outcome: change in refraction was listed as a secondary outcome but not reported.
• Baseline imbalances in refraction (−1.80 versus −2.08 D), sex, weight and corneal curvature, acknowledged by the authors.
• Reporting inconsistencies: different p-values are given in different places for treatment-zone size and tear break-up time, and the main 12-month comparison relied on eye-level t-tests.
• Adherence (actual wear time) was not reported, and results come from children in Shanghai.

What this means in practice

This trial adds to growing interest in how ortho-k lens design might influence myopia control, and its findings apply to the specific lens studied. It is an early signal rather than a reason to assume all smaller-zone designs perform better. Lens choice depends on corneal shape, prescription, pupil size, comfort and follow-up.

The optometry team at Rose Optometry in Hamilton, including Jagrut Lallu and Jacqueline Rowe, fits and monitors ortho-k as part of our myopia control care, with regular axial length measurement. If you would like to talk through whether ortho-k suits your child, you can book a free Myopia Chat with Jagrut.

References

• Zhang Q, Peng C, Wang Y, Chu C, Yu T, Zhu J. Efficacy and Safety of Specialty-Designed Orthokeratology Lenses with a Reduced Corneal Central Zone Moulding Area in Children with Low Myopia: A Prospective, Randomised, Controlled Trial. Ophthalmic Physiol Opt. 2026. PMID: 42839112. https://doi.org/10.1007/s44402-026-00194-1