How does the profile of corneal shape change in response to OrthoK lens wear?

Early successes in correcting myopia with OrthoK lenses were thought by these practitioners to be due to the cornea being bent into a different shape in response to the flat PMMA lenses that they were fitting at that time.

Fortunately the pioneering research of Prof Helen Swarbrick and her team in 1998 revealed that this was not the case when they were instead able to show that the central cornea thins in response to OrthoK lens wear and the mid-peripheral cornea thickens. Because the back (posterior) surface curvature of the cornea remains unchanged, the induced change in thickness profile has the effect of causing the front (anterior) surface of the cornea to flatten. I say fortunately because I'm not sure if OrthoK would be around today if it turned out that the cornea was actually being physically bent in shape!

In the myopic eye the cornea is too powerful causing the refracted image of distant objects to fall in front of the retina. Flattening the anterior surface of the cornea has the effect of reducing the overall refractive power of the eye, which if done by just the right amount will correct any myopia.

Subsequent research has revealed that most of the central corneal thinning in response to OrthoK lens wear occurs within the anterior epithelium layer. Earlier studies revealed that thickening of the paracentral cornea was primarily stromal in origin, but more recent studies using more advanced imaging techniques have shown thickening of the epithelium layer to be largely responsible (see image below). The change in corneal thickness profile occurring within the epithelium layer is important to understand as it explains some of the limits of refraction correction that can be achieved from OrthoK.

Cornea + OK lens

The mechanisms that cause the corneal epithelial profile to alter in this way in response to OrthoK lens wear is not really understood. In his text book John Mountford proposed a squeeze film force theory where the lens on eye creates a suction force in the trapped tear film in the mid periphery and compression force in the central trapped tear film. An alternative hypothesis is that the lens acts like a mold with the softer anterior surface of the corneal epithelium adopting the profile of the more rigid overlying lens. Which mechanism it turns out to be doesn’t really matter when it comes to fitting OrthoK lenses, the important message is that the cornea is not being bent from OrthoK lens wear - it is just the profile of the anterior surface that is being altered.

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About Paul

Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.