In an earlier blog I described how OrthoK remodels the thickness profile of corneal epithelium – so what can this tell us about limits to refractive change that can be created?
The average thickness of a person’s anterior corneal epithelium is 50µm, so if we are going to squish it we can’t squish it by more than 50µm (1µm = 1/1000mm). In fact, the research suggests that the most squishing that we can achieve is around 2/3 of the original thickness, so around 35µm. The capacity for thickening the epithelium is not so limited, but instead the problem we face here is what to fill the epithelium with, after all we can’t just syringe liquid into it.
The simple model of change is that the OrthoK lens when worn squishes the contents of the central corneal epithelium towards the outer parts of the cornea. The centre thins while the displaced contents cause the mid peripheral epithelium to thicken but can only really thicken by the amount of material displaced from the central cornea. It is of course much more complicated than that, and in fact we don’t fully understand why or how the corneal epithelium changes its thickness profile in response to OrthoK lens wear. However, just following these thoughts, if no additional substrate is added to the epithelium, then the mid peripheral layer can’t really become any thicker than the amount of thinning that has occurred centrally. All up, and please only consider these as approximate terms, this means that change to the corneal epithelium thickness profile is limited to around 35µm of thinning centrally, and around 35µm of thickening paracentrally.
With this big explanation over, “so what” you might well say. Well the so what is that this limits the amount of flattening that can occur, which when everything is calculated out creates refractive change limits of around -4.50D, with some variation around this depending on corneal shape. “But some OrthoK manufacturers tell me that they can treat higher myopia” you reply, and this is where things get even more complicated, as what these manufacturers essentially try to do with their designs is squish material from the outer peripheral parts of the corneal epithelium towards the para-central regions, so in effect thickening the para-central region by more than the central corneal material displacement, leading to a higher degree of relative flattening when comparing the central to para-central regions. This creates more depth of focus and consequently higher myopic correction. While I don’t necessarily dispute the success of this approach, it is clearly going to make for more complicated lens fitting regardless of manufacturers claims.
If you are an experienced OrthoK lens fitter then by all means attempt higher corrections, just be aware that as you attempt ever higher corrections your chance of succeeding will diminish. I also encourage you to read my follow up blog on this topic. For those of you that are starting out or are early in your OrthoK fitting career, for now I suggest you stick to fitting patients with lower levels of myopia that are easier to manage. Once you have developed a solid base I will be the first to encourage you to branch into more difficult territories.