A hot topic at the cutting edge of orthokeratology (ortho-k) research and practice is altering lens design to influence myopia control efficacy. Ortho-k has the greatest volume of evidence for slowing axial eye growth in myopic children; data combined from several studies shows a consistent effect of 0.27mm less growth over two years (Sun et al, 2015).

Ortho-k also holds the special distinction of being the only intervention with evidence for slowing anisomyopic eye growth (Tsai et al, 2021)—in the presence of astigmatism up to 3.50DC (Chen et al, 2013) and for partial treatment of high myopes from –6D to –8D (Charm and Cho, 2013). Can there be more to expect from this workhorse of myopia management?

Perhaps so.

Clinical reports observe a type of myopia stability achieved with ortho-k that doesn’t seem reflected in research studies. Back in 2010, this author presented clinical data, utilizing an algorithm of refraction and topographical outcomes, to indicate that in 62 children aged 7 to 18 with a mean age of 12.7 years, only 21% demonstrated any myopia progression over two years (Johnson, 2010). A 2013 paper using similar clinical data for 26 ortho-k wearers with a mean age of 12 years found that a subset (64%) showed “apparent total arrest of manifest myopic refractive change” (Downie and Lowe, 2013).

Part of the reason for this observation is the careful use of refraction and topography to indicate myopia stability, when detecting myopic change by axial length measurement can be several times more discriminating (Wolffsohn et al, 2019). Moreover, these clinical data sets have a mean age of 12 to 13 years. Younger wearers will show faster progression and less “stability” of myopia, which can lead a clinician to think that “ortho-k is not working,” although research data indicates that children aged 6 to 8 years get the greatest benefit from the reduction in rapid axial elongation provided by ortho-k (Cho and Cheung, 2017).

Clinicians may observe apparent stability of myopia in ortho-k wearers who are older, and when not measuring axial length. By comparison, meta-analysis research data has included participants typically aged 6 to 12 years and shown axial growth reduced by around 50% over two years (Sun et al, 2015). New ortho-k research, though, is seeking to do better by “customizing” ortho-k.

All ortho-k lenses are “customized” to the patient’s topography and refraction, but in this case, reducing the back optic zone diameter (BOZD) is the key factor under investigation. The first randomized controlled trial on this compared 5mm to 6mm BOZD lenses and found a significantly better myopia control effect for the 5mm lens in the first six months (Guo et al, 2021). The second and fourth six-month periods showed a similar progression trajectory in both groups, while the third six-month period (from 12 to 18 months) showed a slightly better effect of the 5mm group (Guo et al, 2022).

Over two years, the 5mm group’s eyes grew 0.15mm ± 0.21mm compared to the 6mm group’s 0.35mm ± 0.23mm (Guo et al, 2022). More data is to be reported, but it would appear that the better result with the 5mm lens was not predicted by factors such as age, pupil size, or baseline refraction (Guo et al, 2022).

This leaves practitioners with the promise of “better” ortho-k, but without the clear clinical guidance to put it into practice. Before fitting all young patients into 5mm BOZD lenses, beware that reduced contrast vision has been noted in some wearers (Carracedo et al, 2019). Muddying the waters, one study found that treatment zone decentration was a better predictor of axial length control in ortho-k than smaller treatment zone size (Lin et al, 2021). One 2020 study by Chen and colleagues confirmed—while another 2022 study by Sun and colleagues refuted—this finding of more decentration being correlated with less eye growth.

Despite the many “knowns” about ortho-k for myopia control, there is much to learn. This fascinating space is worth watching. CLS

REFERENCES

1. Sun Y, Xu F, Zhang T, et al. Orthokeratology to control myopia progression: a meta-analysis. PLoS One. 2015 Apr 9;10:e0124535.
2. Tsai HR, Wang JH, Chiu CJ. Effect of orthokeratology on anisometropia control: A meta-analysis. J Formos Med Assoc. 2021 Dec;120:2120-2127.
3. Chen C, Cheung SW, Cho P. Myopia control using toric orthokeratology (TO-SEE study). Invest Ophthalmol Vis Sci. 2013 Oct 3;54:6510-6517.
4. Charm J, Cho P. High myopia-partial reduction orthokeratology (HM-PRO): study design. Cont Lens Anterior Eye. 2013 Aug;36:164-170.
5. Johnson KL. Refractive indicators for stability of myopia in paediatric overnight orthokeratology. Abstracts of the 34th BCLA Annual Clinical Conference, Birmingham, 2010 Abstracts. Cont Lens Anterior Eye. 2010;33:279-280.
6. Downie LE, Lowe R. Corneal reshaping influences myopic prescription stability (CRIMPS): an analysis of the effect of orthokeratology on childhood myopic refractive stability. Eye Contact Lens. 2013 Jul;39:303-310.
7. Wolffsohn JS, Kollbaum PS, Berntsen DA, et al. IMI - Clinical Myopia Control Trials and Instrumentation Report. Invest Ophthalmol Vis Sci. 2019 Feb 28;60:M132-M160.
8. Cho P, Cheung SW. Protective Role of Orthokeratology in Reducing Risk of Rapid Axial Elongation: A Reanalysis of Data From the ROMIO and TO-SEE Studies. Invest Ophthalmol Vis Sci. 2017 Mar 1;58:1411-1416.
9. Guo B, Cheung SW, Kojima R, Cho P. One-year results of the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: a prospective randomised clinical trial. Ophthalmic Physiol Opt. 2021 Jul;41:702-714.
10. Guo B, Cheung SW, Kojima R, Cho P. Myopia control effect of small back optic zone diameter orthokeratology lenses over two years. Invest Ophthalmol Vis Sci. 2022;63:1464.
11. Carracedo G, Espinosa-Vidal TM, Martínez-Alberquilla I, Batres L. The Topographical Effect of Optical Zone Diameter in Orthokeratology Contact Lenses in High Myopes. J Ophthalmol. 2019 Jan 2;2019:1082472
12. Lin W, Li N, Gu T, Tang C, Liu G, Du B, Wei R. The treatment zone size and its decentration influence axial elongation in children with orthokeratology treatment. BMC Ophthalmol. 2021 Oct 12;21:362.
13. Chen R, Chen Y, Lipson M, et al. The Effect of Treatment Zone Decentration on Myopic Progression during Or-thokeratology. Curr Eye Res. 2020 May;45:645-651.
14. Sun L, Li ZX, Chen Y, He ZQ, Song HX. The effect of orthokeratology treatment zone decentration on myopia progression. BMC Ophthalmol. 2022 Feb 15;22:76.