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Orthokeratology (Ortho K) is a better choice than regular glasses for children with myopia, but why is it so?
Myopia is the eye condition that make objects far away blurry, while objects up-close are clear. This is because the eye is too long for its focusing power, resulting in the image of a distant object being focused in front of the retina, rather than on it. People with myopia are also commonly called short-sighted, or near-sighted — often, people find these terms confusing.
Both glasses and Ortho K correct myopia by refocusing this out-of-focus image backwards onto the surface of the retina, so the image becomes clear. Glasses do this by bending light with a concave lens (a minus-powered lens). Ortho K restores clear vision by flattening the front surface of the eye (the cornea — the clear window where light enters the eye) by the use of a mould worn at night-time, reducing the curvature of the cornea so that light is focused further back, onto the retina.
So if glasses and Ortho K do the much same thing in restoring clear vision, why is Ortho K effective in slowing myopia progression in children but glasses are not?
The answer lies in how peripheral vision is corrected, not just central vision. When we see, the sharpest part of our vision comes from the central area of our retina, known as the macula. While our peripheral vision is also vital, objects that fall away from the centre of our vision isn't in sharp focus. We don't tend to notice this as our eyes are constantly moving to where we want to see.
Glasses are made to give us sharp vision in the centre of our vision, in our macula area. The optics of single-vision spectacle lenses, originally developed centuries ago, assume that the eye is a round sphere. In a short-sighted, or myopic, eye the shape of the eyeball is actually an oblong shape. Oval, not round.
The higher the degree of myopia, the more oblong the eye shape becomes, due to the eye being stretched and elongated length-wise. It is also this elongation that leads to significantly increased risks of eye disease development — glaucoma, retinal detachment, cataract and myopic macular degeneration are all associated with myopia.
What happens with glasses for correction of myopia is that while the central part of vision is in clear focus, peripheral vision is well out of focus, with the peripheral image being focused beyond the surface of the retina. The technical term for this phenomenon is called 'peripheral hyperopic defocus'. Myopia research in recent times has found that this image blur in the peripheral retina is part of what drives the progression of myopia in children.
When the eye senses the blurred peripheral image focused behind the retina it tries to grow to 'catch-up' to the position of this image — this is a stimulus for abnormal eye growth in myopia. And the greater the myopia, due to an increase in eye elongation, or a more oblong eye shape, the greater the mismatch between central focus and peripheral focus, and thus the greater the stimulus for the eye to continue growing longer. This is why regular glasses do not help slow myopia progression; in fact, the optics of glasses can accelerate progression by inducing further peripheral hyperopic defocus.
Glasses only correct central vision. Ortho K corrects both central and peripheral vision. Optimised focusing of peripheral vision is why Ortho K is better than glasses for children with progressive myopia.
With Ortho K, we can achieve different points of focus for central vision and for peripheral vision. We restore clear central vision by placing the central image onto the retina, and in addition we focus peripheral light on or in front of the retina, by means of a graded treatment effect that naturally occurs with Orthokeratology. Instead of having just one corrective lens power, as occurs with ordinary glasses, with Ortho K treatment there is a change of corrective power on the surface of the eye from the centre to the edge, more closely aligned to the oblong shape of the eye.
By reducing the peripheral image hyperopic defocus we reduce or eliminate the signals for the eye to grow longer. This more sophisticated way of restoring vision from the centre to the periphery, giving better focusing of light across the back of the eye, is why Ortho K can slow the progression of myopia when regular glasses cannot. For short-sighted children whose eyes are continually getting worse, Ortho K is a far better option than glasses in being able to correct their eyesight as well as reducing the rate of vision deterioration.