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Contact Lens Spectrum

Article

GP INSIGHTS

OPTIC ZONE SIZES AND ORTHO-K LENSES

By: JOHN MARK JACKSON, OD, MS, FAAO
Contact Lens Spectrum May 1, 2022 Vol 37, Issue May 2022 Page(s): 12

Interest in orthokeratology (ortho-k) has increased dramatically in the last decade, primarily due to its ability to slow myopia progression (Cho et al, 2005). Ortho-k has long been used to give visual freedom to adults by reducing their dependence on daytime correction.

In the last few years, more information has emerged regarding how designs for adults and children need to differ to achieve the best therapy outcomes.

Optical Effects

The main difference is related to optics. Myopic ortho-k flattens the central cornea and produces a ring of midperipheral steepening (the “power ring”). This corrects for the myopia but also increases positive spherical aberration (PSA).

Increasing the PSA can increase visual effects like halos and starbursts, potentially degrading visual acuity (Takahiro et al, 2007). The impact of the PSA depends partly on the width of the power ring and its relation to the patient’s pupil size (a smaller ring within the pupil increases the effect).

Designs for Adults

Ortho-k in adults is geared toward providing the best possible visual acuity. Traditionally, most ortho-k lenses have an optic zone size of about 6mm, which yields a relatively wide ring of steepening. This typically puts the power ring outside or close to the edge of the pupil (Figure 1), minimizing the effect of the PSA on visual acuity.

Figure 1. An orthokeratology power ring that is mostly outside of the pupil.
Figure 1. An orthokeratology power ring that is mostly outside of the pupil.

Depending on the amount of customization available with the lens design, the optic zone size could be slightly increased if needed to push the power ring farther out.

Designs for Myopia Control

The midperipheral power ring is thought to be the mediator of myopia control (Gifford et al, 2020). The steeper this area is and the smaller it is within the patient’s pupil, the more the PSA increases (Figure 2). This is similar to increasing the add power in a distance-center soft multifocal (Walline et al, 2020).

Figure 2. A power ring that is inside the pupil.
Figure 2. A power ring that is inside the pupil.

Studies have shown that using a smaller ortho-k optic zone size in children enhances the myopia control effect (Chen et al, 2012; Pauné et al, 2021; Guo et al, 2021), likely from the increased PSA (Lau et al, 2020). Recent posters at the 2022 Global Specialty Lens Symposium showed increased levels of PSA with smaller optic zones (Kojima et al, 2022; Fujimoto et al, 2022).

Exactly how much of a reduction in optic zone size is needed is still under investigation, but studies have used down to a 5.5mm or even a 5.0mm zone (Guo et al, 2021). Aspheric base curves are typically used with the decreased optic zones to maintain a consistent amount of plus in the power ring. As with adults, increasing the amount of PSA can also decrease visual quality, but it wasn’t a significant decrease for most patients in the cited studies.

In Conclusion

Tailoring ortho-k lens designs to the patients using them makes sense. As we learn more about ortho-k and its effects on myopia control, we can continue to refine our lens designs to benefit our patients more. Ortho-k does not appear to be a “one-optic-zone-size-fits-all” proposition.

References

  1. Cho P, Cheung SW, Edwards M. The longitudinal orthokeratology research in children (LORIC) in Hong Kong: a pilot study on refractive changes and myopic control. Curr Eye Res. 2005 Jan;30:71-80.
  2. Takahiro H, Chikako O, Yuko I, Tetsuhiko K, Tetsuro O. Contrast sensitivity function and ocular higher-order aberrations following overnight orthokeratology. Invest Ophthalmol Vis Sci. 2007 Feb;48:550-556.
  3. Gifford P, Tran M, Priestley C, Maseedupally V, Kang P. Reducing treatment zone diameter in orthokeratology and its effect on peripheral ocular refraction. Cont Lens Anterior Eye. 2020 Feb;43:54-59.
  4. Walline J, Walker M, Mutti D, et al. Effect of High Add Power, Medium Add Power, or Single-Vision Contact Lenses on Myopia Progression in Children. JAMA. 2020 Aug 11;324:571-580.
  5. Chen Z, Niu L, Xue F, et al. Impact of Pupil Diameter on Axial Growth in Orthokeratology. Optom Vis Sci. 2012 Nov;89:1636-1640.
  6. Pauné J, Fonts S, Rodríguez L, Queirós A. The Role of Back Optic Zone Diameter in Myopia Control with Orthokeratology Lenses. J Clin Med. 2021 Jan 18;10:336.
  7. Guo B, Cheung SW, Kojima R, Cho P. One-year Results of the Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study. Ophthalmic and Physiol Opt. 2021 Jul;41:702-714.
  8. Lau J, Vincent SJ, Cheung SW, Cho P. Higher-order Aberrations and Axial Elongation in Myopic Children Treated With Orthokeratology. Invest Ophthalmol Vis Sci. 2020 Feb 7:61:22.
  9. Kojima R, Caroline P, Lampa M, et al. How Does Optical Zone Size Affect Ortho-k Treatment? Poster presented at the 2022 Global Specialty Lens Symposium, Jan. 2022, Las Vegas.
  10. Fujimoto M, Invergo A, Kojima R, Caroline P, Lampa M, Andre M. Spherical Aberration with Small vs. Large Orthok Optical Zones. Poster presented at the 2022 Global Specialty Lens Symposium, Jan. 2022, Las Vegas.

Dr. Jackson is a professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. He has received travel funding from Paragon Vision Sciences. You can reach him at jjackson@sco.edu.

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