Contribution of the corneal epithelium to anterior corneal topography in patients having myopic photorefractive keratectomy

Purpose: To assess the variations in corneal topographic characteristics after removal of the epithelium in patients having myopic photorefractive keratectomy (PRK). Setting: Rothschild Foundation, Paris, France. Methods: Forty-four eyes of 25 patients with myopia had corneal topography examination with the Orbscan II device (Bausch & Lomb) before removal of the corneal epithelium preoperatively and after removal of the corneal epithelium during PRK. On each examination, elevation, curvature, and pachymetry parameters were recorded and analyzed (paired 2-sided Student t test). Results: The mean difference in central pachymetry between preoperative and epithelial removal was 37.84 mu m +/- 9.82 (SD) (range 19 to 58 mu m). The mean best-fit spherical surface radius was 7.75 +/- 0.28 mm (range 7.25 to 8.42 mm) before removal of the epithelium and 7.92 +/- 0.29 mm (range 7.39 to 9.16 mm) after removal of the epithelium (P <.0001). The mean simulated K-value difference increased from 0.75 +/- 0.55 diopter (D) (range 0.1 to 4.7 D) before removal to 1.21 +/- 0.66 D (range 0.2 to 4.7 D) after removal (P <.0001). The mean simulated value decreased from 43.77 +/- 1.83 D (range 40.25 to 47.00 D) to 42.44 +/- 1.73 D (range 37.05 to 45.50 D), respectively. The mean power (3.0 mm) decreased from 44.42 +/- 1.59 D (range 40.4 to 47.2 D) before removal to 43.46 +/- 1.37 D (range 39.7 to 46.9 D) after removal. The mean irregularity index increased from 1.07 +/- 0.35 D (range 0.5 to 2.5 D) to 2.03 +/- 0.38 D (range 1.3 to 3.3 D), respectively (P <.0001). The mean asphericity value (Q) changed from -0.44 +/- 0.14 (range -0.72 to -0.20) to -0.65 +/- 0.46 (range -1.04 to 0.14), respectively (P=.003). Conclusions: The epithelium affected the topographic properties of the cornea by significantly reducing corneal topographic astigmatism and irregularity. This might prove to be important in the assessment of patient candidacy for and treatment planning in refractive surgery.