Epithelial Thickness in the Normal Cornea: Three-dimensional Display With Very High Frequency Ultrasound

PURPOSE—To characterize the in vivo epithelial thickness profile in a population of normal eyes. METHODS—An epithelial thickness profile was measured by Artemis 1 (Ultralink LLC) very highfrequency (VHF) digital ultrasound scanning across the central 10-mm diameter of the cornea of 110 eyes of 56 patients who presented for refractive surgery assessment. The average, standard deviation, minimum, maximum, and range of epithelial thickness were calculated for each point in the 10×10mm Cartesian matrix and plotted. Differences between the epithelial thickness at the corneal vertex and peripheral locations at the 3-mm radius were calculated. The location of the thinnest epithelium was found for each eye and averaged. Correlations of corneal vertex epithelial thickness with age, spherical equivalent refraction, and average keratometry were calculated. RESULTS—The mean epithelial thickness at the corneal vertex was 53.4±4.6 μm, with no statistically significant difference between right and left eyes, and no significant differences in age, spherical equivalent refraction, or keratometry. The average epithelial thickness map showed that the corneal epithelium was thicker inferiorly than superiorly (5.9 μm at the 3-mm radius, P<.001) and thicker nasally than temporally (1.3 μm at the 3-mm radius, P< 001). The location of the thinnest epithelium was displaced on average 0.33 mm temporally and 0.90 mm superiorly with reference to the corneal vertex. CONCLUSIONS—Three-dimensional thickness mapping of the corneal epithelium demonstrated that the epithelial thickness is not evenly distributed across the cornea; the epithelium was significantly thicker inferiorly than superiorly and significantly thicker nasally than temporally with a larger inferosuperior difference than nasotemporal difference. The human corneal epithelium has five to seven cell layers and an accepted central thickness of approximately 50 to 52 μm. 1 Bowman’s layer, a dense collagenous layer approximately 8 to 10 μm thickness lies between the epithelium and stroma. The anterior margin of Bowman’s

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