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

PRK

The technician enters the patient data and the treatment protocol into the laser computer and numbers are double-checked by the surgeon. The use of preoperative oral sedatives varies among surgeons and different cultures. Topical anesthetic drops can be started as early as 20 minutes prior to surgery and, unlike in stromal ablation, preservatives are not contraindicated since they may aid in epithelial abrasion by breaking up the hemidesmosomes. With the patient on the operating table and the lid speculum in place, a marker with the desired diameter is used to indicate the area of the planned procedure (usually the central 7–9 mm). Thereafter, abrasion of the epithelium is performed5. This can be done with a blade, with a rotating brush, with 20% ethanol (applied for 20–30 seconds), or with the excimer laser itself (not widely used). Once the epithelium has been removed and Bowman’s membrane displays a smooth aspect, the patient is instructed to focus on a fixation light, the surgeon centers the microscope on the treatment area and initiates the laser treatment (Fig. 25.1). A tracker is used to guard against excessive eye movements. At the end of the procedure an antibiotic ointment and a patch are applied. Some surgeons prefer preservative-free antibiotic drops combined with a soft contact lens.

Fig. 25.1 View of a cornea after having undergone photorefractive keratectomy (PRK) for myopia. The ablation with the excimer laser creates a concave curvature (exaggerated by the artist for didactic purposes), which is filled by epithelium during the healing process.

LASEK

While the initial steps are identical with the PRK procedure, the epithelium is handled differently. The preparation of the epithelial flap is initiated with a dull trephine combined with an alcohol well. The trephine serves to make an approximately 70 µm deep circular epithelial incision, leaving an uncut sector at 12 o’clock serving as a hinge. 20% ethanol is then instilled into the well and allowed to act on the epithelium for 20–30 seconds. The ethanol loosens the epithelium and makes it easier to prepare an intact flap. A special instrument (often termed a ‘hoe’) is then inserted into the circular epithelial incision at 6 o’clock and used to separate the epithelium from Bowman’s layer. The separation is continued until the flap is complete and bunched up at the hinge. The laser procedure is the same as for PRK (Fig. 25.2). At the end of the excimer ablation, a contact lens is applied to prevent flap dislocation. Preservative-free antibiotic drops are given, and the contact lens is usually removed after 3–4 days.

Fig. 25.2 A cornea ready for excimer surgery using the laser epithelial keratomileusis (LASEK) strategy. The epithelial flap, having been prepared manually, is seen on the left. Bowman’s membrane is now exposed, and the laser beam is visible above.

Epi-LASIK

The initial steps are identical to PRK, but preservative-free anesthetic drops are preferred to prevent epithelial damage. The cornea is marked with a dye to make sure the flap can be properly aligned in case it inadvertently separates completely. In contrast to LASEK, the epithelial flap is prepared with a microkeratome that acts as a separator, using a blunt PMMA or metal blade (Fig. 25.3). A suction ring is placed on the eye to achieve an adequate intraocular pressure (IOP) for the creation of the flap. After the suction is applied and the IOP registered, the keratome is activated to create a flap with a nasal hinge. The flap is folded over and protected with a sponge. The ensuing laser procedure is the same as for PRK. Once ablation is completed, the flap is flipped back into place and a contact lens applied. The postoperative regimen is similar to that of LASEK. Some surgeons prefer to discard the flap altogether, choosing to use the keratome for the smooth surface (Bowman’s layer) and the sharp epithelial edges it creates around the ablation zone, believing that healing is faster and less symptomatic in the absence of a flap.

Fig. 25.3 An example of a microkeratome (epikeratome), used for the mechanical creation of an epithelial flap in the Epi-LASIK procedure.

Intraoperative complications

Intraoperative adverse events are rare in all three variants of surface ablation, but reliable numbers about the incidence of such events are hard to come by in the peer-reviewed literature.

Abrupt eye movements during laser ablation used to cause decentrations in the early days of the excimer era, but modern eye trackers have made decentration an unusual complication with current laser units. Input of data on refraction and cylinder axis is subject to human error, but the feeding of the laser with the correct numbers should be done in the relative calm that reigns before the patient enters the operating suite, and the data should be double-checked with an assistant before the laser is deemed ready for use. Malfunction of the laser during the actual ablation procedure is extremely unusual.

Beginners tend to underestimate the importance of the hydration both of Bowman’s layer before initiating the ablation and of the stroma during the surgery. If Bowman’s layer and the stroma are too dry (often because abrasion of the epithelium takes too long), an over-correction is likely to result, because more stroma is ablated per laser pulse, deviating from the algorithm in which normal hydration of the cornea is postulated. If the cornea is too wet, the opposite can happen, with some of the laser energy being absorbed by the excess water, resulting in an under-correction. Too slow a preparation of the cornea for ablation increases the risk of dryness; a too aggressive rinsing of the corneal surface is the most frequent cause of over-hydration.

Unique to LASEK and Epi-LASIK are potential flap-associated complications. In LASEK, the creation of the epithelial flap may fail in the sense that, once preparation is complete, one or more holes are visible in the flap itself. Experience has shown that, if flaps with holes are repositioned over the ablated surface, corneal haze can develop in areas corresponding to the location of the holes. Accordingly, such damaged flaps should be amputated, and the planned LASEK then converted to PRK. In Epi-LASIK, an inadvertent free flap or an incomplete flap also leads to a conversion to PRK. If the keratome blade causes a stromal incursion during the creation of the flap, surgery usually has to be abandoned and the flap replaced. This complication is rare with modern keratomes.

Postoperative care

After surface ablation, several issues need to be dealt with: pain6, epithelial healing, prophylactic antibiotics, and anti-inflammatory agents.

In PRK, a bandage contact lens is placed on the bare stroma to aid in epithelial healing and reduce ocular irritation and pain. In LASEK and Epi-LASIK, the contact lens also serves to secure the epithelial flap in place. The lens is usually removed after 3–4 days. Unpreserved antibiotics are used in all three procedures to guard against infection. They can be discontinued once the epithelium has healed.

Postoperative pain has always been considered a major disadvantage of surface ablation (compared with LASIK). In the 20-year history of surface procedures, pain treatment has been improved and refined to such an extent that many patients nowadays instead complain only of discomfort and soreness. The prophylaxis of pain includes oral non-steroidal anti-inflammatory drugs (NSAIDs) and other painkillers, the immediate postoperative cooling of the cornea with chilled or frozen balanced salt solution, topical NSAID drops, and preservative-free tetracaine for a day or two. Once the epithelial defect has closed, topical steroids are given to suppress stromal inflammation.

Postoperative complications

Infectious keratitis is likely the most serious complication after surface ablation. The reported risk of bacterial keratitis is between 0.01% and 0.8%, and the infection typically occurs within 1 week of the procedure7. Since surface ablation patients tend to have discomfort and tearing in the immediate postoperative period, an early diagnosis of bacterial keratitis may be missed if no counseling is given on the signs and symptoms of infection.

Late-onset corneal haze was a major concern in the early days of PRK. The problem has been substantially reduced for a number of reasons. Extremely deep ablations, an important factor in the development of haze, are no longer being performed. Exposure to ultraviolet radiation has also been identified as a risk factor, so that ultraviolet-protective eyewear is now recommended during the first year after surgery. Also, mitomycin C is increasingly used prophylactically during the surface ablation procedure, having been shown to prevent the formation of haze.

While not a genuine complication, delayed visual recovery is a reality that distinguishes surface ablation from LASIK. Re-epithelialization and stromal remodeling make for a slower visual rehabilitation. Most patients achieve functional visual acuity (20/40 or better uncorrected visual acuity [UCVA]) within the first week, but a return to 20/20 or better can take a month or so8.

Induced higher order aberrations (HOAs), such as spherical aberration and coma, can be documented in a variety of refractive procedures, but surface ablation causes less HOAs than conventional LASIK. However, there is emerging evidence that wavefront-guided and other customized ablations may decrease the gap or even equalize the two procedures with respect to induced HOAs.

Assessment of surgery

Long-term studies of PRK have reinforced the excellent safety profile and stability of surface ablation. Even 12-year data from one of the earliest clinical trials showed no significant change in mean spherical equivalent refraction between 1, 6, and 12 years9. In the mean time, lasers and ablation profiles have improved substantially, indications have become more rigorous, and customized treatments have increased the quality of vision.

By the nature of things, surface ablation is usually compared with LASIK. Comparative studies have generally shown no differences in refractive or visual results between the two methods, when conventional ablation patterns are used. However, wavefront-guided surface ablation has been shown to achieve visual outcomes superior to those of wavefront-guided LASIK performed with a mechanical mikrokeratome. Also, when both procedures were compared after the use of wavefront-optimized technology, better outcomes were reported for surface ablation, with more eyes achieving a postoperative UCVA that matched or surpassed preoperative best spectacle corrected visual acuity.

When LASEK evolved from PRK, and when epi-LASIK developed out of the LASEK experience, each new procedure at first appeared to offer well-defined advantages over the previous method. With time, however, the differences between modern PRK, LASEK, and Epi-LASIK appear to fade away10. Evidence is mounting that the visual and refractive results as well as the problems specifically associated with surface ablation (pain, discomfort, haze, delayed visual recovery) are quite similar for the three procedures. While some studies have shown specific advantages for one method over the other, it seems safe to state that, on the whole, all three deliver excellent outcomes.