This article was originally published in a sponsored newsletter.

Science has underpinned orthokeratology’s effectiveness on controlling myopia progression. Thankfully, research continues on the methods practitioners can use to alter the design of a contact lens to customize the result for each patient.

When a young myope presents with excessive axial elongation, the clinician has choices to improve management. One choice is adjunct therapy, usually low-dose atropine. Another choice is modifying the lens back optic zone diameter (BOZD) to change the treatment zone size.

The BOZD of an orthokeratology contact lens correlates with the diameter of the treated area on a patient’s cornea. Many lens designs start with a BOZD around 5.5 mm to 6.0 mm, though this can be increased or decreased to meet the patient’s needs and treatment goals.

When it comes to improving the management of a patient’s progressing myopia, clinical trials have revealed that reducing the BOZD can improve control over axial length.^1,2 Specifically, the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) study concluded that a 5 mm BOZD did not significantly decrease lens performance or ocular integrity, yet improved control of axial lengthening by 0.13 mm compared to a 6 mm BOZD over one year.¹

Another recently published study corroborates the VOLTZ results, in that an orthokeratology lens creating a smaller treatment zone yields improved control over axial length compared with designs that have larger treatment zones at both 12- and 18-month timepoints.² Interestingly, this study used a 6 mm BOZD in both groups, but increased the sagittal depth of the lens to decrease the treatment zone size in one group.²

The takeaways from these studies: First, BOZD of the orthokeratology lens is not synonymous with treatment zone. A BOZD is a parameter of the lens itself, while treatment zone is the area of the anterior corneal surface being manipulated by the lens.

Second, the treatment zone diameter on the cornea must also decrease if the goal of the reduction in the BOZD is to improve control over axial elongation.

Clinicians cannot assume that the cornea will change enough to achieve a desired effect; they must verify this with topographic imaging over time.

How do smaller treatment zones improve axial length control? The suggested mechanisms are an increase in spherical aberration and decreased contrast sensitivity negatively impact image quality on the retina, which, in turn, slows axial length changes.^2,3

Much is left to be explained on this subject, however the ability to customize orthokeratology lens parameters to maximize myopia management remains an exciting topic.

REFERENCES

1. 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.
2. Gong G, Zhang BN, Guo T, et al. Efficacy of orthokeratology lens with the modified small treatment zone on myopia progression and visual quality: a randomized clinical trial. Eye Vis (Lond). 2024 Sep 2;11:35.
3. Carracedo G, EspinosaVidal 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.