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Excimer Lasers in Refractive Surgery

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Excimer Lasers in Refractive Surgery Excimer Lasers in Refractive Surgery REVIEW Excimer Lasers in Refractive Surgery Ajla Pidro1, Alma Biscevic1,2, ABSTRACT Melisa Ahmedbegovic Pjano1, Introduction: In the field of ophthalmology, laser technology is used in many basic and clinical Ivana Mravicic2, Nita Bejdic1, disciplines and specialities. It has played an important role in promoting the development of oph- Maja Bohac2 thalmology. 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 1 Eye Polyclinic Svjetlost, Sarajevo, Bosnia corneal refractive surgery. Methods: This article was performed based on a literature review and and Herzegovina Internet search through scientific databases such as PubMed, Scopus, Web of Science and Google 2University Eye Hospital Svjetlost, School Scholar. Result: The literature on excimer laser technology addresses the technical and physical of Medicine University of Rijeka, Zagreb, aspects of excimer lasers including types, characteristics and commercially available lasers on the Croatia 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. Correspoding author: Alma Biscevic, dr. Mustafe Pintola 23, Sarajevo. Tel: +387 60 31 94 712. Keywords: Excimer laser, corneal refractive surgery, ablation profile, laser pulse. Fax: +387 33 762 771. E-mail: alma@svjetlost- sarajevo.ba. ORCID ID:http//www.orcid.org/ 1. INTRODUCTION tochemist with 21 US patents to his 0000-0002-6496-2853.. In the field of ophthalmology, name; and Wynne was a physicist, laser technology is used in many who was the manager of the group doi: 10.5455/aim.2019.27.278-283 basic and clinical disciplines and at IBM. The excimer laser uses re- ACTA INFORM MED. 2019 DEC 27(4): 278-283 specialities. It has played an im- active gases, such as chlorine and Received: Oct 04, 2019 • Accepted: Dec 11, 2019 portant role in promoting the de- fluorine, mixed with inert gases, velopment of ophthalmology. Ad- such as argon, krypton and xenon. vancements in technology have When electrically excited, the gas allowed improvements in the sur- mixture emits energetic pulses of gical safety, efficacy, speed and ultraviolet light, which can make versatility of the laser, especially very precise, minute changes to ir- in corneal refractive surgery. Be- radiated material, such as poly- cause of the increasing numbers of mers. applications in ophthalmology and In the year 1987, Stephen Trokel, their successful implementations, MD first used the Excimer Laser on ophthalmic use of laser technology the cornea. Dr. Steven Trokel intro- is expected to continue flourishing. duced Photorefractive Keratectomy (1) (PRK). He also patented the Excimer laser for vision correction and 2. HISTORICAL FACTS performed the first laser surgery Invention of the Excimer Laser on a patient’s eyes in 1987. Upon Three researchers at the IBM ® learning of the IBM work, ophthal- Thomas J. Watson Research Center mologist Stephen Trokel, affiliated in Yorktown, New York (Samuel with Columbia Presbyterian Med- © 2019 Ajla Pidro, Alma Biscevic, Melisa Blum, Rangaswamy Srinivasan and ical Center in New York City, came Ahmedbegovic Pjano, Ivana Mravicic, Nita James J. Wynne) had been exploring to the Watson Research Center in Bejdic, Maja Bohac new ways to use the excimer laser the summer of 1983 to collabo- This is an Open Access article distributed under the that had been recently acquired by rate on experiments with Srini- terms of the Creative Commons Attribution Non- Commercial License (http://creativecommons.org/ their laser physics and chemistry vasan and researcher Bodil Braren. licenses/by-nc/4.0/) which permits unrestricted non- group. Blum was an expert in mate- Trokel, Srinivasan, and Braren commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. rials science; Srinivasan was a pho- wrote a paper introducing the idea 278 REVIEW / ACTA INFORM MED. 2019 DEC 27(4): 278-283 Excimer Lasers in Refractive Surgery of using the laser to reshape or sculpt the cornea (the genic (10, 11). This may be in part a result of shielding clear covering on the front of the eye) in order to correct of the nucleus by the cell’s cytoplasm. Several attri- refractive errors, such as myopia or hyperopia. Their butes of the argon-fluoride excimer laser ablation paper, published in a major ophthalmology journal in make it particularly appropriate for corneal sculpting. December 1983, launched a worldwide program of re- The laser energy is well absorbed near the corneal sur- search to develop excimer laser-based refractive sur- face and, thus, should have few deep direct or sec- gery. New York City ophthalmologist, Steven Trokel ondary mechanical (shock-wave) effects on the corneal made the connection to the cornea and performed the tissue. The ablation process is rapid, and excess energy first laser surgery on a patient’s eyes in 1987. The next is ejected with the effluent plume (11). There is minimal ten years were spent perfecting the equipment and the thermal damage to the surrounding tissue. Because techniques used in laser eye surgery. In 1996, the first of these qualities, as a cold laser, the 193 nm excimer Excimer laser for ophthalmic refractive use was ap- laser can be used to meticulously reshape large areas proved in the United States (2). of the corneal surface while minimizing damage to re- Excimer laser principle of work maining tissue (12). The excimer laser is based on the combination of two The excimer laser technique is qualitatively dif- gases: a noble gas and halogen. Both of these are gen- ferent from refractive surgical techniques such as ra- erally stable in their normal low-energy state. When a dial or astigmatic keratotomy, which achieves corneal high-voltage electrical discharge is delivered into the reshaping through biomechanical changes mediated laser cavity containing these gases, the gases combine through thin knife incisions. to form a higher energy excited-gas state compound. The term “excimer” is derived from a contraction of 3. AIM “excited dimer”. On the dissociation of this high-en- This article is designed to review the evolution of ergy compound, a photon of energy is released that laser technology in refractive surgeries in ophthal- corresponds to the bond energy of the noble gas-hal- mology, mainly focusing on the characteristics of the ogen molecule (3, 4). This wavelength of light energy is excimer laser applied in corneal refractive surgery. amplified in the laser system, resulting in the produc- tion of a discrete high energy pulse of laser energy. The 4. METHODS specific wavelength of an excimer laser depends on the The relevant articles were searched from online data composition of the gases used in the laser system. Ex- sources including PubMed, Scopus, Web of Science and cimer laser systems in current clinical use rely on argon Google Scholar. and fluorine gases. The argon-fluorine excimer lasers emit energy at a wavelength of 193 nm. This wave- 5. RESULTS length falls in the UV-C range of the light spectrum. The retrieved articles were reviewed by the authors In contrast, the krypton-fluoride excimer laser used and the results are presented along with the relevant in early laboratory studies emits a wavelength of 248 discussion. nm (5, 6). Laser energy at 193 nm is very well absorbed 5.1. TYPES OF EXCIMER LASERS by the proteins, glycosaminoglycans and nucleic acids The first generation excimer lasers were “broad beam comprising the cornea because of its sufficient photon lasers” or “full beam lasers“ that created less uni- energy (6.4 eV) and precision (only penetrating the su- form surface profiles than the newer generations. Full perficial layer; 0.3 µm). The tissue-ablation depth is beam enables faster treatment (for given frequency) positively correlated with the logarithm of laser den- and is less sensitive for decentration, but homogenizes sity; 1-J/cm 2 energy can ablate approximately 1-µm of slower, gives irregular treatment of the surface and corneal tissue. Since 193 nm photon is of higher energy has more expressed thermal effect. It is needed to use than the molecular bond strength of these compounds, masks for achievement of desired treatment form, and absorption of the laser energy results in breaking of the it is not possible to perform custom treatments (12). bounds. The resulting molecular fragments are ejected Newer-generation of excimer lasers use scanning from the surface of the cornea at supersonic speeds (7- beams or flying spots, with smaller spot sizes and more 9). efficient eye trackers. Systems for scanning slit delivery The goal is to reshape the cornea so that rays of light act like flying spot systems and it exceed some limita- that enter the eye are focused clearly onto the retina. tions of full beam systems, but maintained the speed of It is important to understand that the excimer laser the treatment and low decentration sensitivity. System does not cut tissue like a scalpel; rather it ablates or re- uses additional diaphragm between full beam and the moves tissue from the corneal surface. The ablated ma- eye, which flows through hexagonal beam of a smaller terial appears as an effluent plume that upon analysis diameter (10 mm x 1 mm) to the eye, and improves ho- has been shown to consist of a variety of high-molec- mogeneity of the beam. Ablation masks rotate, en- ular-weight hydrocarbons (9). There is a concern about abling performance in different directions (12). the potential for mutagenesis or carcinogenesis with Flying spot systems convert laser beam in small any laser radiation, especially in the ultraviolet light round spot (between 0.6 and 2.0 mm). System uses only spectrum. Studies have been done showing that the 193 central most homogenic part of the beam, and beam di- nm excimer laser is neither mutagenic nor carcino- rection is controlled by a mirror with rotation function.
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