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Optics of the corneal epithelium
1Department of Ophthalmology, Bascom Palmer Eye Institute, University of Miami School of Medicine, Fla.
Background:
The refractive effect of the corneal epithelium in relation to the cornea as a whole is relatively unknown, yet it may affect the efficacy of keratorefractive surgical procedures, most notably photorefractive keratectomy and epikeratoplasty. This article investigates the refractive effect of the epithelium on the cornea.
Methods:
We measured the corneal keratometry in 10 fresh human eye-bank eyes with and without epithelium. All readings were performed at 2.0- and 3.6-millimeter diameter zones with an automated keratometer.
Results:
When the epithelium was removed, there was an increase in corneal refractive power in every case. The mean change was +1.03 D (range, 0.55 to 1.85 D) at the central 2.0-millimeter diameter zone and +0.85 D (range, 0.29 to 1.60 D) at the 3.6-millimeter diameter zone. We also found a change in both the power and axis of astigmatism in most cases.
Conclusions:
The corneal epithelium plays an active role in determining the final surface power of the cornea. It accounts for an average of 1.03 D of the power of the eye at the central 2-millimeter diameter zone. The difference in astigmatism suggests that the epithelium does not form a layer of uniform thickness over Bowman's layer. It appears that the epithelium attempts to minimize abrupt changes in surface contour. These findings suggest that in refractive procedures such as photorefractive keratectomy the refractive contribution of the epithelium must be taken into account to improve predictability.