Epithelial Thickness After Hyperopic LASIK: Three-Dimensional Display with Artemis Very High-Frequency Digital Ultrasound

Abstract

PURPOSE To characterize the epithelial thickness profile in a population of eyes after LASIK for hyperopia or hyperopic astigmatism. METHODS The epithelial thickness profile was measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan Inc) across the central 10-mm diameter of the cornea on 65 eyes at least 3 months after hyperopic LASIK using a 7-mm ablation zone with the MEL 80 excimer laser (Carl Zeiss Meditec). Maps of the average, standard deviation, minimum, maximum, and range of epithelial thickness were plotted. The cross-sectional hemi-meridional epithelial thickness profile was calculated using annular averaging. Linear regression analysis was performed to evaluate correlations between epithelial thickness, spherical equivalent refraction treated, and maximum simulated keratometry. RESULTS The mean thinnest epithelial thickness was 39.7±5.6 μm and the mean thickest epithelial thickness was 89.3±14.6 μm. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning within the 4-mm diameter zone surrounded by an annulus of thick epithelium, with the thickest epithelium at the 3.4-mm radius. The epithelium was on average 10-μm thicker temporally than nasally at the 3.4-mm radius. Central epithelium was thinner and paracentral epithelium was thicker for higher hyperopic corrections and steeper maximum simulated keratometry. CONCLUSIONS Three-dimensional high-resolution ultrasound mapping of epithelial thickness profile after LASIK for hyperopia demonstrated thinner epithelium centrally and thicker epithelium paracentrally. Presumably, the paracentral epithelial thickening compensated in part for the stromal tissue removed by the hyperopic ablation, whereas the central epithelial thinning compensated for the localized increase in corneal curvature.

Structural Changes Induced by a Corneal Shape-Changing Inlay, Deduced From Optical Coherence Tomography and Wavefront Measurements

“…However, what stops the remodeling, leading to the equilibrium state (e.g., the central thickening in myopic LASIK 17 and the central thinning in hyperopic LASIK 18 or over the keratoconic cone)? 19 One proposal is that the local slope drops below some threshold, resulting in a diminished response to the force moving epithelial cells.…”

“…In hyperopic LASIK, the untreated central zone thins by approximately 8 lm in response to the peripheral ablation and midperipheral thickening. 18 Comparing the mean of keratoconic (KC) subjects to normals, the epithelium above the cone thins on the order of 9 lm, in response to the stromal thinning and anterior buldging. 19 Keratoconic thinning is a function of the KC grade, thinning between approximately 14 and 19 lm at the highest level.…”

Artemis very high-frequency digital ultrasound guided femtosecond laser recut after flap complication

“…Measurement of the epithelial thickness is particularly important in these cases because the epithelium thickens centrally after myopic ablation and paracentrally after hyperopic ablation. 11,12 While rare, partial flaps are one of the most common complications related to mechanical microkeratome flap creation in LASIK. [13][14][15] For this reason it is important to have a good strategy for managing partial flaps that minimizes the increased incidence of complications related to recutting a second flap.…”

Inaccuracies in Reporting the Accuracy of Flap Creating Devices

“…2 We wish to point out that determining the original fl ap thickness profi le generated cannot be accurately performed by using the full fl ap thickness measurement postoperatively due to changes in the epithelium. [3][4][5][6][7][8] The study by Ju et al 1 used the Visante OCT (Carl Zeiss Meditec) and measured the fl ap thickness at 10 points over the cornea (central, 1.5-mm, and 3.0-mm radii) 1 week and 1 month after LASIK for myopia up to Ϫ8.38 diopters (D) spherical equivalent refraction. The fl ap thickness profi le was reported to be of uniform thickness because no statistically signifi cant differences were noted in mean fl ap thickness among any of the 10 measurement locations.…”

“…The changes in the epithelial thickness profi le after LASIK can be up to 18 µm for myopia 3 and 24 µm for hyperopia. 4 Therefore, the only way of usefully and accurately determining the actual original fl ap thickness created by a microkeratome or a femtosecond laser is to employ temporally displaced measurements of the epithelial thickness preoperatively and the stromal component of the fl ap postoperatively (once all edema has subsided). This method has been described previously using Artemis very high-frequency digital ultrasound [5][6][7][8] with an epithelial thickness repeatability of 0.58 µm and a fl ap thickness repeatability of 1.68 µm.…”