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Review Refractive Surgery Surface Ablation Techniques for Myopia – A Review of the Advances Over the Past 25 Years Jesper Hjortdal Department of Ophthalmology, Aarhus University Hospital NBG, Aarhus, Denmark P hotorefractive keratectomy for correction of myopia was the first excimer laser-based technique to be developed. During the last 25 years, excimer lasers have improved technologically, several variations of the technique have been developed and pre- and postoperative pharmaceutical therapies have been investigated. This review article summarises these developments and the published meta-analyses on comparison of surface ablation techniques and laser in situ keratomileusis. The main conclusion is that there seem to be no differences between the clinical results obtained with the different variations of surface ablation techniques. Keywords Photorefractive keratectomy, laser in situ keratomileusis (LASIK), refractive surgical procedures, myopia, excimer laser Disclosure: Jesper Hjortdal discloses that Aarhus University Hospital has signed a research support agreement with Carl Zeiss Meditec, Jena Germany. No funding was received in the publication of this article. This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors. Authorship: All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval to the version to be published. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Received: 21 April 2017 Accepted: 15 June 2017 Citation: European Ophthalmic Review, 2017;11(1):31–4 Corresponding Author: Jesper Hjortdal, Department of Ophthalmology, Building 10, 2nd floor, Aarhus University Hospital NBG, 8000 Aarhus C, Denmark. E: jesphjor@rm.dk Photorefractive keratectomy (PRK) is the first laser-based technique that aimed to change the refractive power of the cornea. The principle of excimer laser ablation of the cornea is based on controlled emission of 193 nm pulses generated from excited dimers of argon fluoride (ArF) gases. When such a gas is put under electrical stimulation and high pressure, the excited compound can give up its excess energy by undergoing spontaneous emission, which very quickly (on the order of a picosecond) dissociates back into two unbound atoms. Directing the emitted light towards, for example, biological materials will result in ablation of surface molecules. In the early 1980s it was shown that using this approach biological materials, including the cornea, could be incised using the excimer laser. 1–3 After short time, it was realised that actual surface ablation of the corneal stroma was possible after removal of the corneal epithelium 4 and that the resulting surface was very smooth. 5 In 1987, excimer laser-based keratotomy was performed in a human eye, 6 but it was the surface ablation application that pushed development of excimer lasers. 7,8 The first preliminary results in human eyes were published the late 1980s 9–12 and excimer lasers became commercially available around 1990. In the early 1990s larger follow-up series of patients undergoing PRK for myopia were published. 13–15 Since then, many modifications of PRK have been introduced, and a common uniting name for all these techniques is ‘surface ablation’. This review will focus on surface ablation for myopia. Development of excimer lasers Directing the location of the excimer laser rays is essential to change the surface shape of the cornea in a controlled way. The first lasers were based on a scanning slit of excimer laser pulses, which was modified by a mechanical aperture built into a suction device. The resulting ablation of the cornea resulted in a stepwise change in the surface shape of the corneal stroma, but these steps were smoothed by the healed corneal epithelium, making a smooth surface and tear film. Simultaneously, wide-field excimer lasers were introduced. These lasers ablated the full diameter of the ablation zone simultaneously, and the change in shape was based on controlling a differential efficiency between central and peripheral rays. After a few years, most excimer lasers became based on using a flying spot of small size, which in a controlled way could reshape the corneal surface into almost any shape. Thus, myopia, hyperopia, astigmatism and even irregular astigmatism could in principle be corrected, if the flying laser spot was controlled properly. Today, all excimer lasers on the market are based on the flying spot principle, but the repetition rate of the lasers vary. The original surface ablation technique Originally, PRK was performed as follows. After topical anaesthesia, the patient was placed on the excimer laser bed, the eye to be treated was draped and a speculum inserted. The corneal epithelium was removed centrally using a spatula. The patient was then asked to look at a central fixation light in the excimer laser, and the eye was manually kept in a steady position using a suction device, and laser ablation was initiated. The diameter of the ablation zone was typically small, often less than 5 mm. 12 After ablation, the eye was treated with antibiotic drops and steroids. The epithelium typically healed within a few days. Over time, development of superficial haze in the TOU CH MED ICA L MEDIA 31