The average thickness of the central cornea is approximately 550 microns (mcns), slightly more than 1/2 a millimeter. (A credit card is 700 mcns thick.) The center of the cornea is the thinnest point, with increasing thickness toward the periphery. Corneas which are too thin or too thick, or have an unusual shape, may be an indication of a corneal thinning disease. Rigid contact wear can thin the cornea, and toric contact lenses for astigmatism can cause thinning inferiorly, as the weighted bottom of a toric contact lens rocks back and forth, leaving a skirt pattern of thinning.
Refractive surgery (Lasik, PRK, etc.) can only be done on the front half of the cornea, and a minimum of a 300 mcn bed should be preserved. Considering a 100 mcn flap in Lasik, or a 50 mcn epithelium removal in PRK (which is only done on thin corneas), refractive surgery leaves very little margin for error—as little as 1-1.5/100th of a millimeter of tissue, on which to perform surgery. The equipment itself has standard deviations from intended settings, for both the flap cut and ablation of tissue, which can adversely affect the refractive outcome and long term health of the cornea.
No refractive surgery has ever withstood the test of time! Refractive surgery removes corneal tissue that cannot be replaced, and in an amount that is a “best guess”. The chance of getting a “perfect” refractive result in both eyes, which will last a lifetime without needing glasses or contact lenses, and without subsequent problems with eye health, is approximately zero!
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