Excimer Lasers in Refractive Surgery

Correspoding author: Alma Biscevic, dr. Mustafe Pintola 23, Sarajevo. Tel: +387 60 31 94 712. Fax: +387 33 762 771. ab.ovejaras-tsoltejvs@amla. ORCID ID: http//www.orcid.org/0000-0002-6496-2853.

Received 2019 Oct 4; Accepted 2019 Dec 11.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Introduction:

In the field of ophthalmology, laser technology is used in many basic and clinical disciplines and specialities. It has played an important role in promoting the development of ophthalmology.

Aim:

This article is designed to review the evolution of laser technology in refractive surgeries in ophthalmology, mainly focusing on the characteristics of the excimer laser applied in corneal refractive surgery.

Methods:

This article was performed based on a literature review and Internet search through scientific databases such as PubMed, Scopus, Web of Science and Google Scholar.

Result:

The literature on excimer laser technology addresses the technical and physical aspects of excimer lasers including types, characteristics and commercially available lasers on the market.

Conclusion:

The conclusion on this forum aims to help understand the benefits of excimer laser use in ophthalmology, with focus on correction of refractive errors.

Keywords: Excimer laser, corneal refractive surgery, ablation profile, laser pulse

1. INTRODUCTION

In the field of ophthalmology, laser technology is used in many basic and clinical disciplines and specialities. It has played an important role in promoting the development of ophthalmology. Advancements in technology have allowed improvements in the surgical safety, efficacy, speed and versatility of the laser, especially in corneal refractive surgery. Because of the increasing numbers of applications in ophthalmology and their successful implementations, ophthalmic use of laser technology is expected to continue flourishing. (1)

2. HISTORICAL FACTS

Invention of the Excimer Laser

Three researchers at the IBM ® Thomas J. Watson Research Center in Yorktown, New York (Samuel Blum, Rangaswamy Srinivasan and James J. Wynne) had been exploring new ways to use the excimer laser that had been recently acquired by their laser physics and chemistry group. Blum was an expert in materials science; Srinivasan was a photochemist with 21 US patents to his name; and Wynne was a physicist, who was the manager of the group at IBM. The excimer laser uses reactive gases, such as chlorine and fluorine, mixed with inert gases, such as argon, krypton and xenon. When electrically excited, the gas mixture emits energetic pulses of ultraviolet light, which can make very precise, minute changes to irradiated material, such as polymers.

In the year 1987, Stephen Trokel, MD first used the Excimer Laser on the cornea. Dr. Steven Trokel introduced Photorefractive Keratectomy (PRK). He also patented the Excimer laser for vision correction and performed the first laser surgery on a patient’s eyes in 1987. Upon learning of the IBM work, ophthalmologist Stephen Trokel, affiliated with Columbia Presbyterian Medical Center in New York City, came to the Watson Research Center in the summer of 1983 to collaborate on experiments with Srinivasan and researcher Bodil Braren. Trokel, Srinivasan, and Braren wrote a paper introducing the idea of using the laser to reshape or sculpt the cornea (the clear covering on the front of the eye) in order to correct refractive errors, such as myopia or hyperopia. Their paper, published in a major ophthalmology journal in December 1983, launched a worldwide program of research to develop excimer laser-based refractive surgery. New York City ophthalmologist, Steven Trokel made the connection to the cornea and performed the first laser surgery on a patient’s eyes in 1987. The next ten years were spent perfecting the equipment and the techniques used in laser eye surgery. In 1996, the first Excimer laser for ophthalmic refractive use was approved in the United States (2).

Excimer laser principle of work

The excimer laser is based on the combination of two gases: a noble gas and halogen. Both of these are generally stable in their normal low-energy state. When a high-voltage electrical discharge is delivered into the laser cavity containing these gases, the gases combine to form a higher energy excited-gas state compound. The term “excimer” is derived from a contraction of “excited dimer”. On the dissociation of this high-energy compound, a photon of energy is released that corresponds to the bond energy of the noble gas-halogen molecule (3, 4). This wavelength of light energy is amplified in the laser system, resulting in the production of a discrete high energy pulse of laser energy. The specific wavelength of an excimer laser depends on the composition of the gases used in the laser system. Excimer laser systems in current clinical use rely on argon and fluorine gases. The argon-fluorine excimer lasers emit energy at a wavelength of 193 nm. This wavelength falls in the UV-C range of the light spectrum. In contrast, the krypton-fluoride excimer laser used in early laboratory studies emits a wavelength of 248 nm (5, 6). Laser energy at 193 nm is very well absorbed by the proteins, glycosaminoglycans and nucleic acids comprising the cornea because of its sufficient photon energy (6.4 eV) and precision (only penetrating the superficial layer; 0.3 μm). The tissue-ablation depth is positively correlated with the logarithm of laser density; 1-J/cm 2 energy can ablate approximately 1-μm of corneal tissue. Since 193 nm photon is of higher energy than the molecular bond strength of these compounds, absorption of the laser energy results in breaking of the bounds. The resulting molecular fragments are ejected from the surface of the cornea at supersonic speeds (7-9).

The goal is to reshape the cornea so that rays of light that enter the eye are focused clearly onto the retina. It is important to understand that the excimer laser does not cut tissue like a scalpel; rather it ablates or removes tissue from the corneal surface. The ablated material appears as an effluent plume that upon analysis has been shown to consist of a variety of high-molecular-weight hydrocarbons (9). There is a concern about the potential for mutagenesis or carcinogenesis with any laser radiation, especially in the ultraviolet light spectrum. Studies have been done showing that the 193 nm excimer laser is neither mutagenic nor carcinogenic (10, 11). This may be in part a result of shielding of the nucleus by the cell’s cytoplasm. Several attributes of the argon-fluoride excimer laser ablation make it particularly appropriate for corneal sculpting. The laser energy is well absorbed near the corneal surface and, thus, should have few deep direct or secondary mechanical (shock-wave) effects on the corneal tissue. The ablation process is rapid, and excess energy is ejected with the effluent plume (11). There is minimal thermal damage to the surrounding tissue. Because of these qualities, as a cold laser, the 193 nm excimer laser can be used to meticulously reshape large areas of the corneal surface while minimizing damage to remaining tissue (12).

The excimer laser technique is qualitatively different from refractive surgical techniques such as radial or astigmatic keratotomy, which achieves corneal reshaping through biomechanical changes mediated through thin knife incisions.

3. AIM

4. METHODS

The relevant articles were searched from online data sources including PubMed, Scopus, Web of Science and Google Scholar.