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Jay W. McLaren, William M. Bourne, Sanjay V. Patel; Stromal Reflectance after Photorefractive Keratectomy: A Paired Comparison between Epithelial Removal by Rotary Brush and Excimer Laser-Scrape. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1469.
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In a randomized paired-eye study, we compared the brightness of confocal images of corneas, as a measure of reflectance and backscatter, after photorefractive keratectomy (PRK) with the epithelium removed by a rotary brush to image brightness from corneas with the epithelium removed by a laser-scrape technique.
Nineteen patients received PRK for myopia or myopic astigmatism. One eye of each patient was randomized by ocular dominance to epithelial removal by rotary brush (Amoils) and the fellow eye to epithelial removal by laser-scrape (excimer ablation to a depth of 43 μm followed by scrape of the basal epithelium). Preoperative spheroequivalent refractive error was -3.6 ± 1.6 D (mean ± SD) and -3.4 ± 1.5 D for eyes to be treated by rotary brush and laser-scrape respectively (p=0.32). Corneas were examined by confocal microscopy (ConfoScan 4) before and at 1, 3, 6 and 12 months after PRK. The mean confocal image brightness was determined through the stroma in each of 20 anteroposterior layers of 5% stromal thickness, and was normalized to scatter units (SU), the concentration of Amco Clear (GFS Chemicals) in nephelometric turbidity units that produced the same image brightness. Image brightnesses were compared between treatments at all visits and at each visit after PRK to brightnesses before PRK. Differences were assessed by using paired t-tests or signed-rank tests, with Bonferroni-adjustment for multiple comparisons. The average minimum detectable difference (MDD) was determined for non-significant comparisons (α=0.05/4 or 0.05/5, β=0.20).
Mean image brightness at all depths in the stroma was not significantly different between treatments at any time (p>0.12, MDD=116 SU). Before treatment, brightness at 20% of stromal depth was 925 ± 188 SU and 877 ± 170 SU for corneas destined to rotary brush and laser-scrape methods respectively (p=0.99). At 1 month, brightness from the same region was 790 ± 125 SU and 840 ± 139 SU after rotary brush and laser-scrape respectively (p=0.79). Image brightness was lower at 15% to 25% depth at 1 month (p<0.045) and 20% depth at 3 months (p=0.044) after rotary brush treatment than it was before surgery, but was not lower at later visits (p>0.57) or in other stromal regions at any visit (p>0.10, MDD=173 SU). Brightness after the laser-scrape treatment was not different from brightness before PRK at any time (p>0.22, MDD=188 SU).
Reflectivity of the corneal stroma, a measure of haze, after PRK with the rotary brush method of removing corneal epithelium is not different from reflectivity after PRK with the laser-scrape method. Reflectivity of the anterior corneal stroma does not increase after PRK.
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