Introduction

Surface ablation of the cornea was the very first procedure in which an ophthalmic excimer laser was applied to correct refractive errors of the human eye. By reshaping the surface of the cornea, myopia, hyperopia, and astigmatism were reduced or fully corrected. The new method eventually led to the establishment of a new subspecialty within ophthalmology – refractive surgery.

Epidemiological considerations and terminology

The prevalence of myopia in Western populations is estimated at about 25%1. In some Asian populations it is as high as 70%–90%. According to some epidemiological evidence, the prevalence of myopia is increasing especially in Asia. Although the etiology of myopia is not quite clear, there is substantial evidence that both genetic and environmental factors play a role.

A crude estimate of the prevalence of hyperopia (≥3.0 diopters [D]) in Western populations provides a range of 5.8%–11.6%.

Photorefractive keratectomy (PRK) came into clinical practice at about 19902. In PRK the epithelium is abraded prior to excimer treatment. Toward the end of the decade, laser epithelial keratomileusis (LASEK) began to establish itself as an alternative surface procedure3. In LASEK, an epithelial flap is prepared manually, rolled up before the laser is applied, and then rolled back over the bare stroma after ablation. The most recent variant of surface ablation, epipolis (Greek for surface) LASIK (Epi-LASIK) was introduced in 20034. Epi-LASIK also involves the use of an epithelial flap, but the flap is prepared with a specialized microkeratome. In the course of the evolution of surface ablation techniques, the term advanced surface ablation (ASA), as distinct from the original PRK, has also been used.

Fundamental principles and goals of surgery

In myopia, the refractive power of the eye is greater than that required to focus a distant object on the fovea. Generally, this occurs because the corneal curvature is too steep or the eye too long. As a result, distant objects are focused in front of the retina and appear blurred. In hyperopia, the optical and/or anatomical conditions are reversed, with the refractive power of the eye too low, causing objects to focus behind the retina.

Treating the cornea with an excimer laser provides a practical, extraocular means of correcting refractive errors. In myopia, the cornea is ablated centrally, causing it to become flatter. Flattening the cornea reduces its optical power, thereby decreasing or eliminating myopia. In hyperopia, ablation is performed in the mid-periphery. This steepens the central cornea, thus increasing its power. In this way, a hyperopic refractive error can be corrected. By virtue of the biomechanical properties of the cornea, and because of different post-ablation epithelial healing patterns, it is easier and more predictable to flatten the cornea than to steepen it.

The goal of surgery is to enable the patient to function normally in daily life without spectacle or contact lens correction. Ideally, the entire refractive error is eliminated by the excimer procedure, but often a minor residual error remains, which generally is readily accepted by the patient. If, due to regression of effect or other complications, a significant residual error remains, a retreatment usually succeeds in alleviating the problem. In special circumstances, such as monovision correction in presbyopic myopes, the aim of surgery may be to correct the refractive error of one eye fully (distance vision), while under-correcting the fellow eye on purpose (near vision).

Indications for surgery

Surface ablation has its limitations, and cannot be used to correct very high levels of refractive error. The set limits for correction differ between different countries, and are sometimes even strictly defined by national societies. Also, recommended limits for myopia are not the same as those for hyperopia. For myopia, there is a general international consensus of an upper limit between –6.0 D and –8.0 D, although some surgeons, using special software, have presented results of higher corrections. For hyperopia, the generally accepted limit is from +3.0 D to +4.0 D, even through the FDA in the United States has approved treatment for up to +6.0 D. Practical experience has shown that a correction of even +3.50 D has a greater chance of producing disturbing optical complications and regression than say a +2.0 D treatment. Astigmatism correction can be attempted up to 5.0 D–6.0 D.

Surface ablation is indicated in normal eyes with refractive errors that meet the limits of treatment. A special indication applies to eyes for which stromal (i.e. non-surface) excimer ablation is contraindicated. Such eyes include those with thin corneas (<500 µm), epithelial basement membrane dystrophies and risk for trauma in professional life.

Operation techniques