CHAPTER 5

Photoablation: Techniques and Outcomes

The 193-nm argon-fluoride (ArF) excimer laser treats refractive error by ablating the anterior corneal stroma to create a new radius of curvature. Two major refractive surgical techniques use excimer laser ablation. In surface ablation techniques, including photorefractive keratectomy (PRK), laser subepithelial keratomileusis (LASEK), and epipolis laser in situ keratomileusis (epi-LASIK), the Bowman layer is exposed either by debriding the epithelium through various methods or by loosening and moving, but attempting to preserve, the epithelium. In LASIK, the excimer laser ablation is performed under a lamellar flap that is created with either a mechanical microkeratome or a femtosecond laser. Currently available excimer laser ablation algorithms can be classified generally as conventional, wavefront-optimized, or wavefront-guided.

Excimer Laser

Background

The excimer laser uses a high-voltage electrical charge to transiently combine atoms of excited argon and fluorine; when the molecule, or dimer, reverts to its separate atoms, a charged photon is emitted. The word excimer comes from “excited dimer.” Srinivasan, an IBM engineer, was studying the far-ultraviolet (UV; 193-nm) ArF excimer laser for photoetching of computer chips. He and Trokel, an ophthalmologist, not only showed that the excimer laser could remove corneal tissue precisely with minimal adjacent corneal damage—photoablation—but they also recognized its potential use for refractive and therapeutic corneal surgery.

Photoablation, the removal of corneal tissue with minimal adjacent corneal damage, occurs because the cornea has an extremely high absorption coefficient at 193 nm. A single 193-nm photon has sufficient energy to directly break carbon–carbon and carbon–nitrogen bonds that form the peptide backbone of the corneal collagen molecules. Excimer laser radiation ruptures the collagen polymer into small fragments, expelling a discrete volume and depth of corneal tissue from the surface with each pulse of the laser (Fig 5-1) without significantly damaging adjacent tissue.

Figure 5-1 Schematic representations of corneal recontouring by the excimer laser. 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 removals 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.)

Surface Ablation

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 and radial keratotomy; 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. 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.

Majmudar PA, Forstot SL, Dennis RF, et al. Topical mitomycin-C for subepithelial fibrosis after refractive corneal surgery. Ophthalmology. 2000;107(1):89–94. Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science. 1986; 234(4776):559–565.

Trokel SL, Srinivasan R, Braren B. Excimer laser surgery of the cornea. Am J Ophthalmol. 1983;96(6):710–715.

LASIK

The term keratomileusis comes from the Greek words for “cornea” (kerato) and “to carve” (mileusis). Laser in situ keratomileusis, which combines keratomileusis with excimer laser stromal ablation, is currently the most frequently performed keratorefractive procedure because of its safety, efficacy, quick recovery of vision, and minimal patient discomfort. LASIK combines 2 refractive technologies: excimer laser stromal ablation and creation of a stromal flap.

Wavefront-Optimized and Wavefront-Guided Ablations

Conventional excimer laser ablation treats lower-order, or spherocylindrical, aberrations such as myopia, hyperopia, and astigmatism. These lower-order aberrations constitute approximately 90% of all aberrations. Higher-order aberrations make up the remainder; such aberrations cannot be treated with spectacles. Ophthalmologists are still learning about the visual impact of higher-order aberrations in the normal population. In fact, the small amounts of higher-order aberrations found in this population may not adversely affect vision. Higher-order aberrations are also a by-product of excimer laser ablation. Some higher-order aberrations can cause symptoms—such as loss of contrast sensitivity and nighttime halos and glare—that decrease the quality of vision. The aberrations most commonly associated with these visual complaints are spherical aberration and coma. See Chapter 1