In general, corneas with greater eccentricity offer greater potential for OK induced refractive change than more spherical corneas, which was shown quite nicely by John Mountford in his landmark paper published in 1997.1 John established a regression formula to associate induced refractive power change to change in corneal eccentricity that had a high correlation. He also stated that the endpoint of OK induced myopia change was a spherical cornea. As a spherical cornea has 0 eccentricity, it follows that the more eccentricity the cornea has at baseline, the greater the potential for refractive change. It has since been shown that the anterior cornea profile can be turned slightly oblate from OrthoK wear, which gives a little more scope for change, however even taking this into consideration, higher degrees of corneal eccentricity at baseline would still indicate a likelihood of achieving a greater amount of refractive change.
If we delve a little deeper into Mountford’s paper, an association between change to refraction and change in corneal sagittal height can be established. John didn’t publish this association, but he did publish his dataset within the paper from which, assuming a common chord of 9mm for his apical curvature and eccentricity measurements, the change in sagittal height can be calculated. This reveals that across this common 9mm chord sag height increases in line with induced refractive change as expected to find a cubic relationship shown in the figure below.
Other research has shown that corneal posterior surface curvature is not altered by OrthoK, so it is safe to infer that change to sag height corresponds to an equivalent change in corneal thickness differential across the measured 9mm chord. E.g. a -100µm change to corneal sag could be -100µm change to central corneal thickness alone, -50µm change to the central corneal thickness and +50µm change to the paracentral corneal thickness at the 9mm chord measurement extremes, or any combination that adds up to an overall thickness change of -100µm.
What makes this cubic fit interesting is that in a previous post I described how refractive change in OrthoK is largely due to induced changes in the corneal epithelium thickness profile. The cubic fit shows that the effect of refraction change on corneal thickness change increases for higher refractive errors and thereby imposing even greater limits. It needs to be kept in mind though that this is an extrapolation that requires extra real data points to validate. If this relationship is followed, however, it can be seen that we are getting well beyond epithelial thickness limits to generate the -235µm change to corneal thickness that would be required to create -10.00D refractive change. These physical constraints can be met by reducing the size of the measured chord thereby resulting in smaller treatment zone sizes for larger refractive corrections, which is what is presented in journals and at conferences. The only alternative is that once the central epithelial thinning limits are reached the mid-peripheral cornea needs to be increasingly thickened alongside increasing refractive error change.
The lens designs that claim to correct higher refractive error adopt this approach and try to induce greater pressure to the peripheral cornea using a lens back surface profile that is also designed to push the displaced peripheral epithelial tissue into the para-central relief zone. Whether this happens however to the best of my knowledge this has not been independently validated. If you have some experience under your belt, by all means, give this designs a try, but keep in mind that regardless of design higher refraction targets are always going to be more difficult to achieve. If starting out I suggest you keep to safe ground, and that is corneas with reasonably high degrees of eccentricity. If we consider the -2.50 limit I suggest for your first few patients, using Mountford’s analysis you're likely to achieve good success if you stick to a corneal eccentricity of 0.5 or more in your baseline maps.
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.
