2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 5: Photoablation: Techniques and Outcomes
This chapter includes related videos, which can be accessed by scanning the QR codes provided in the text or going to www.aao.org/bcscvideo_section13.
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. Excimer laser ablation algorithms can be classified generally as conventional, wavefront-optimized, wavefront-guided, and topography-guided.
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.
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.