2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 5: Photoablation: Techniques and Outcomes
Surface ablation procedures were initially performed as PRK, the sculpting of the de-epithelialized corneal stroma to alter refractive power, and they underwent extensive preclinical investigation before being applied to sighted human eyes. Results of early animal studies provided evidence of relatively normal wound healing in laser-ablated corneas.
The popularity of PRK decreased in the late 1990s when LASIK began to be performed because of LASIK’s faster recovery of vision and decreased postoperative discomfort. Although more LASIK than surface ablation procedures are still performed, the number of surface ablations has increased in recent years. PRK remains an especially attractive alternative for specific indications, including irregular or thin corneas; epithelial basement membrane disease (often called map-dot-fingerprint dystrophy); previous corneal surgery, such as penetrating keratoplasty (PKP) and radial keratotomy (RK); and treatment of some LASIK flap complications, such as incomplete or buttonholed flaps. Surface ablation eliminates the potential for stromal flap–related complications and may have a decreased incidence of postoperative dry eye as compared to LASIK. Corneal haze, the major risk of PRK, decreased markedly with the use of adjunctive mitomycin C; subsequently, the use of PRK for higher levels of myopia has increased.
Figure 5-1 Schematic representations of corneal recontouring by the excimer laser for a myopic ablation. A, Correction of myopia by flattening the central cornea. B, Correction of hyperopia by steepening the central corneal optical zone and blending the periphery. C, Correction of astigmatism by differential tissue removal 90° apart. Note that in correction of myopic astigmatism, the steeper meridian with more tissue removal corresponds to the smaller dimension of the ellipse. D, In LASIK, a flap is reflected back, the excimer laser ablation is performed on the exposed stromal bed, and the flap is then replaced. The altered corneal contour of the bed causes the same alteration in the anterior surface of the flap.
(Illustrations by Jeanne Koelling.)
Majmudar PA, Forstot SL, Dennis RF, et al. Topical mitomycin-C for subepithelial fibrosis after refractive corneal surgery. Ophthalmology. 2000;107(1):89–94.
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Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol. 1983; 96(6):710–715.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.