Dr Paul Gifford consults with the contact lens industry on development of contact lens designs and their implementation into clinical practice. He is a partner in private practice conducting clinical research on use of contact lenses and orthokeratology to control progression of myopia, and holds an Adjunct Senior Lecturer position at the School of Optometry and Vision Science, University of New South Wales, Sydney.
Orthokeratology (OK) has been shown to be effective in slowing progression of myopia by around 45% compared to single vision correction, with similar efficacy independent of the lens design used.1,2 The changes OK makes to the corneal optical profile have the effect of altering the shape of the retinal image profile, causing off axis images to focus anteriorly to the retina. Peripheral myopic defocus has been shown to slow axial eye growth, and with it myopia progression, in various animal models.3 It is this same mechanism that has been proposed to underly OK’s propensity for slowing progression of myopia, though to date there is no supporting scientific evidence to suggest that this is actually the case.
The closest evidence towards a peripheral refraction mechanism to date was published by Chen et al, who, when analysing results from their two-year longitudinal study, divided the measured data from their participant cohort either side of the mean measured pupil diameter.4 The authors reported greater myopia controlling effect in those that had larger than average pupils, and negative effect (myopia progressed faster than the controls) in those that had smaller than average pupils. Their suggested explanation was the larger pupils allowed more of the corneal plus powered change that forms around the OK induced treatment zone (TZ) to fall inside the pupil. This, in turn, resulted in a larger area of the peripheral retina experiencing myopia defocus and consequently greater myopia control effect (Fig 1).
Following this argument, the logical next step to improve myopia control efficacy would be to cause more OK induced corneal plus power to fall inside the pupil. To test this scientifically the following questions needed to be tested and answered:
- Can OK lens design be reliably altered to create additional peripheral corneal plus power?
- Will this result in more myopic peripheral defocus?
The remainder of this article will review the fledgling research work to date that has been conducted around these two questions.
Working with Pauline Kang and Helen Swarbrick we investigated this topic back in 2013.5 In a prospective repeated measures study design we altered the back optic zone diameter and the tightness of peripheral lens fit to investigate whether either of these changes would alter measured peripheral refraction profile relative to a standard OK lens design. Measurements conducted after 14 nights of OK lens wear showed that neither modification altered measured peripheral refraction, leading to the conclusion that attempting to customize refraction and topography changes through manipulation of OK lens parameters appears to be a difficult task. In further work, Kang and Swarbrick also compared three different commercial OK lens designs, again following a prospective repeated measures study design.6 After 14 nights of lens wear, they reported minimal difference to relative peripheral refraction profile between the three designs. They did find differences between the designs in terms of their effect on corneal keratometry measures, but did not explore any change to corneal shape in more detail.
Marcotte-Collard et al filled this gap in knowledge by investigating differences in changes to corneal power profile resulting from wear of two commercial OK lens designs: the four-curve CRT (Paragon) and five-curve Dreamlens (Bausch + Lomb).7 The Dreamlens lens was found to create a smaller TZ diameter, but despite this there was no difference in the plus power profile surrounding the TZ between the two lenses. The authors speculated that this difference could influence OK efficacy for myopia management as well as lens selection when used for myopia correction. Their argument based on the smaller TZ diameter bringing more of the surrounding plus power within the pupil margin, and fitting in with the previously discussed myopia control mechanism suggested by Chen et al.
In summary, specifically altering OK lens design (Kang et al) or commercial lens type (Kang & Swarbrick) did not alter peripheral refraction profile, but adding an extra back surface zone, from four to five (Marcotte-Collard), led to a reduction in TZ diameter, and their suggestion that this could alter peripheral refraction profile. It was not known if Kang and Swarbrick found a difference in TZ diameter, so the relationship between TZ diameter and peripheral refraction in their study could not be established.
I was involved in a study which was designed to explore this area further. Following a prospective repeated measures design, our team assigned participants to wear lenses (standard design and modified design) overnight for seven nights, in random order. The test lens had been modified to reduce the back optic zone, alter back optic zone eccentricity, and alter parameters of the mid-peripheral lens curves. After seven nights, lens wear was stopped for at least one week and the alternate lens design then worn for seven nights. The advantage of this study design was that the same participants were involved in all conditions, so any differences in effect were likely due to differences between the lens designs rather than patient differences. We measured changes to TZ diameter and changes to peripheral refraction, finding the test design significantly reduced TZ diameter. While this appeared to affect the peripheral refraction profile, this observable difference did not reach statistical significance. A poster of this work, presented at the ARVO conference in Vancouver, Canada, 2019, can be downloaded from the following link.
Whilst our recent work provided more information to help understand the effect of customised lens design, we still do not have all the answers. It appears changing TZ diameter might have a beneficial impact on peripheral refraction profile, but we cannot say that for sure, because the observed difference did not reach significance. We continue to look at the data generated in this study, trying to understand the trends amongst the variable peripheral refraction results that occurred. A presentation on this analysis was recently given at the International Myopia Conference in Tokyo, 2019.
What does this all mean in the real world? My take is that for now there is no reliable evidence to support the need for customizing OK lens designs for myopia control, and in fact we are not going to receive this evidence until customized OK designs have been shown to improve efficacy for myopia control in a longitudinal study. We are also still somewhat in the dark about how OK slows progression of myopia. This however, should not stop you prescribing any of the currently available commercial designs with the intent to slow progression of myopia because good evidence does exist to show efficacy in terms of reduced axial length elongation with these standard OK designs.1,2
- Si, J.-K. et al. Orthokeratology for myopia control: a meta-analysis. Optom Vis Sci.2015;92:252–257.
- Sun, Y. et al. Orthokeratology to control myopia progression: A meta-analysis. PLoS One. 2015;10:e0124535.
- Smith, E. L. Charles F. Prentice Award Lecture 2010: A case for peripheral optical treatment strategies for myopia. Optom Vis Sci. 2011;88:1029-44.
- Chen, Z. et al. Impact of pupil diameter on axial growth in orthokeratology. Optom Vis Sci. 2012;89:1636–40.
- Kang, P., Gifford, P. & Swarbrick, H. Can manipulation of orthokeratology lens parameters modify peripheral refraction? Optom Vis Sci. 2013;90:1237–1248.
- Kang, P. & Swarbrick, H. The influence of different OK lens designs on peripheral refraction. Optom Vis Sci. 2016;93:1112–1119.
- Marcotte-Collard, R., Simard, P. & Michaud, L. Analysis of two orthokeratology lens designs and comparison of their optical effects on the cornea. Eye Contact Lens Sci. Clin. Pract. 2018;44:322-329