Increasing interest in orthokeratology (ortho-K) has seen the major contact lens manufacturers battle it out for control. But what is it about ortho-K that is creating such a stir and how does it control myopia?
The theme of the conference dinner at the Orthokeratology Society of Oceania’s (OSO) 13th congress, held at the Gold Coast in early October, was Survivor. Delegates were encouraged to use whatever means to ensure superiority for their table, and so the evening ensued, quickly slipping into a good humoured rort where nothing could be considered safe unless firmly bolted to the floor.
Whether intended or not, the theme was well chosen, as a glance across the landscape reveals that orthokeratology (ortho-K) is itself at the centre of a Survivor battle, this time between the major contact lens manufacturers who are caught in an arms race buying up ortho-K labs.
It’s a bit like the way CDs… pushed aside vinyl records
CooperVision recently acquired Paragon (manufacturer of the CRT design) and ProCornea, while Essilor has acquired Euclid (manufacturer of the Emerald design). This creates an interesting paradox. The soft contact lens companies, whose more technologically advanced product engulfed market share from the previously dominant conventional lens designs, are now putting themselves into the ortho-K game. It’s a bit like the way CDs (compact discs) pushed aside vinyl records. The reason for this change in heart? The growing hotbed of myopia control.
What is it about ortho-K that is making these soft contact lens manufacturing Survivor contestants battle it out for superiority? Currently accepted theories on how contact lenses slow progression of myopia are based around altering peripheral refraction and improving accommodation lag. The former rendering the peripheral image shell to fall inside the eye,1 and the latter anteriorly shifting the whole image shell.2 It turns out that the optical profiles required to deliver these outcomes can be achieved from ortho-K. But they can also be achieved from centre distance multifocal soft lens designs – so what is it about ortho-K that is sufficiently different for these major soft lens companies for them to want to buy them up?
THE REFRACTIVE EFFECT
Modern ortho-K lens designs create a myopia correcting refractive effect by temporarily molding the anterior corneal shape into a flatter profile during overnight wear, thereby reducing the refractive power of the cornea. When done correctly, corneal power is flattened by just the right amount to render the eye slightly hyperopic to account for the natural reduction in effect that occurs during the waking day as the anterior corneal profile slowly returns towards its pre-ortho-K wearing shape. Therein already lies two distinct differences: The soft lens correction effect will largely remain static throughout the working day but has the disadvantage that its myopia control profile will only be present while the lenses are worn.
Change to corneal power can be easily assessed using a corneal topographer, by subtracting post wear from pre-ortho-K lens wear maps. When viewed as a heat map, flattened areas that indicate reduction in power and hence correction of myopia appear blue and steepened areas that indicate increase in power and hence an increase to myopia appear red. Figure 1 shows the ideal ‘bullseye’ correction outcome, where a uniformly flattened central zone is surrounded by a distinct anulus of red steepening. The central blue zone corrects the myopia to restore foveal focus and the red ring, through adding plus power, bends the peripheral image shell forwards to create the peripheral myopic defocus effect shown to be beneficial for slowing progression of myopia in animal studies.3
It doesn’t all end there though, because coaxial light rays are also affected – these are rays of light that enter the eye parallel to the visual axis and once refracted by the various refractive surfaces of the eye will intersect with the visual axis. The displacement from where more peripheral coaxial rays intersect to form a focus along the central axis, relative to coaxial rays that are closer to the central axis, is termed spherical aberration. Higher values of spherical aberration describe a greater spread of focus.4 Myopic ortho-K induces positive spherical aberration, where coaxial rays further from the central axis are refracted closer to the front of the eye than central rays.5 Centre distance multifocal designs provide a similar effect, except this time they are intended as a way to spread the range of focus to compensate for the loss of focus caused by presbyopia.
All of this creates uncertainty over how ortho-K and centre distance multifocals create their myopia control effect, but before we dig deeper into this, we should understand how ortho-K compares to soft lens designs for actual reduction in myopia progression, as published in peer reviewed journals. Two recently published metaanalysis papers, which both took a highly critical approach to selecting which published material to include, concurred in reporting an overall 45 per cent reduction in axial eye elongation compared to either single vision contact lens or spectacle lens controls.6,7 In comparison, similarly rigorous meta-analysis of soft lens options reveals a broader 30 to 50 per cent range of myopia control effect,8 with the study reporting the best soft lens myopia control effect following a protocol that dispensed the add power required to neutralise near esophoria.9 On this basis, ortho-K appears to provide a more consistent myopia controlling effect without need for a tailored approach to prescribing.
So, what is it about ortho-K that results in more consistent outcomes – when it comes down to it, don’t they provide similar optical profiles as we’ve already discussed?
Well, actually they don’t. When we drill down into the optical profile produced from ortho-K, it has been shown that there is a one to one relationship between the amount of peripheral plus power produced and the degree of refractive change.10 A -1.00D change to refraction results in a +1.00D of peripheral plus power, -2.00D refractive change providing +2.00D peripheral plus etc. The peripheral plus power provided by a centre distance multifocal design isn’t directly comparable to the labelled ‘add’ power, and in fact appears to be significantly less than that provided by ortho-K .11 Could this explain why ortho-K is more effective? Well maybe, however…
A recent study presented at the 2018 Association for Research in Vision and Ophthalmology (ARVO) meeting in Hawaii, reported no difference in axial eye elongation in a rigorously conducted study comparing a simulated ortho-K profile applied to a soft lens against single vision soft lenses.12 Except, that it wasn’t a fully accurate representation as they had applied a consistent add power instead of respecting the one to one ratio between add and targeted correction found from ortho-K lens fits.10 The authors concluded that their results indicate the myopia control effect provided by ortho-K may not be optical in nature. This being the case, it follows that the physical characteristic of an ortho-K lens must therefore be the defining attribute.
If keeping an open mind, a physical effect is not out of the question. Ortho-K lenses, being rigid in nature and thicker than soft lenses, do provide more bulk pushed by the eyelids against the surface of the eye during sleep. This is what ultimately creates the refractive effect, but similarly, the eye and orbit are a closed system so conceivably the lens is adding bulk to the front of the eye, which is trapped by the eyelid during sleep. Could an ortho-K lens effectively push the globe in a posterior direction, which is limited by the depth of the orbit, and thereby hinder growth if there is no space to grow into? OK, I’m perhaps starting to push the limits, but it needs to be considered, given the outcomes from this recent soft lens ortho-K simulation study.
Cast the net wider, and there is considerable interest right now in the effect of pupil diameter, treatment zone diameter, and effect on myopia control. A recently conducted Chinese study revealed that participants with pupil diameters larger than the cohort average demonstrated less axial eye elongation over two years of ortho-K lens wear.13 The authors speculated that the effect was likely due to enhancement of the myopic shift in the peripheral retina. However, pupil diameter also influences spherical aberration and those with larger pupil diameters are likely to exhibit increased amounts of ortho-K induced positive spherical aberration and thereby a larger depth of focus. Meaning once again we are stuck trying to understand whether the resultant effect of habitual pupil diameter is provided through effect on peripheral refraction profile or accommodation demand through altering optical focus.
This is all great for developing myopes with large pupils that are interested in ortho-K and suggests that ortho-K, when used for myopia control, should perhaps be avoided in patients with smaller pupil diameters.
TREATMENT ZONE DIAMETER
An alternative approach is to reduce the ortho-K induced treatment zone diameter (TZD), which is the zone of central flattening, so that the zone of steepening responsible for creating peripheral myopic defocus is moved closer towards the central axis. This being the case, those with smaller pupil diameter should get similar amounts of myopic defocus to those with larger pupil diameters wearing standard ortho-K lens designs. Recently, through modelling different ortho-K TZDs on to the front surface of a scleral lens and measuring peripheral refraction profiles while worn, it was shown that this kind of effect was achieved from reducing TZD.14 The challenge this raises is whether it is possible to reliably and consistently control the ortho-K induced treatment zone diameter by altering ortho-K lens design parameters.
A 2013 published study I was involved in revealed no difference from altering back optic zone diameter, or peripheral lens curvature in isolation, leading us to conclude that, “Attempting to customise refraction and topography changes through manipulation of ortho-K lens parameters appears to be a difficult task”.15
This was subsequently challenged by Marcotte-Collard et al in their 2018 paper reporting differences in treatment zone diameter between two different ortho-K lens designs.16 While not a specific attempt to manipulate the TZD, they had shown that a difference in the TZD effect was possible through a difference in lens design. Hot off the press, I can report from repeating our earlier study design (but this time manipulating different ortho-K lens components), we were able to make a consistent 1mm reduction to treatment zone diameter (unpublished data). We are still evaluating the effect on measured peripheral refraction. All of this aside, it is important to understand that longitudinal studies investigating differential effect on axial eye elongation are needed before reducing TZD in ortho-K can be deemed beneficial towards providing a myopia control effect.
GET BACK TO THE ROOTS
In wrapping up, I read reports that vinyl records are largely re-emerging because they are bringing listeners back to the roots of music. Music presented on compact discs offers a consistently good result with minimal need for understanding, which is arguably the aim of disposable soft contact lens manufacturers, and something they have achieved with considerable success. Vinyl records, on the other hand, will still sound mostly good, regardless of equipment or experience… however, once you immerse yourself into the discipline, the whole experience becomes much deeper and more fulfilling.
With the knowledge we have to hand, and continuing research developments, ortho-K and its use in myopia control offers a similar path back to the roots of contact lens fitting.
Dr. Paul Gifford is a research scientist and industry innovator who graduated as an optometrist from City University, London in 1995, then worked in clinical practice for a decade before being awarded his PhD in hyperopic orthokeratology and contact lens optics in 2009 from the University of New South Wales (UNSW), Sydney. Paul’s experience includes every facet of the optometry profession, from clinical practice to academia, research and industry. He holds over 40 peer reviewed and professional publications, and has presented more than 40 conference lectures in Australia and internationally. Dr. Gifford holds three professional fellowships; and has been conferred two prestigious research awards from the British Contact Lens Association, along with seven other research awards during his PhD, and two post-doctorate research grants. He holds an adjunct academic position at UNSW and consults to the contact lens industry on projects relating to product and systems design and software solutions including machine learning.
Available at www.mivision.com.au