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© Borgis - New Medicine 4/2016, s. 126-129 | DOI: 10.5604/14270994.1228144
*Csaba Szekrenyesi1, Kata Rez1, Zoltán Z. Nagy1, 2
Surface temperature change of PMMA plates in refractive surgery performed with two types of modern excimer lasers
1Faculty of Health Sciences, Semmelweis University, Budapest, Hungary
Head of Faculty: Professor Zoltán Zsolt Nagy, MD, PhD
2Department of Ophthalmology, Semmelweis University, Budapest, Hungary
Head of Department: Professor Zoltán Zsolt Nagy, MD, PhD
Summary
Introduction. The conditions in the operating room can have an influence on the postoperative results of the refractive surgery performed with excimer lasers.
Aim. The aim of this study was to examine the relationship of the temperature of PMMA plates (polymethylmethacrylate), environmental conditions and the energy load caused by the laser impulses during refractive surgery procedure.
Material and methods. Wavelight Allegretto and Schwind Amaris excimer lasers were used for the study. Polymethylmethacrylate (PMMA) plates with parameters: -10 Dpt myopic ablation and 6.5 mm optical zone were used as a target for photoablation during the calibration of excimer lasers. Energy load and temperature rise were measured with high accuracy infrared thermometer.
Results. Logarithmically rising temperature was measured for both lasers. Maximal temperature measuredwas 48°C for the Allegretto laser and 43°C for the Amaris laser.
Conclusions. Higher energy load with smaller repetition rate causes higher thermal load. Thermal load of the PMMA plate during the ablation is dependent on the environmental conditions, such as temperature, humidity, air flow, smoke plume evacuation, etc.
Introduction
The accuracy of the excimer lasers used in refractive surgery is critical, considering the short- and long-term refractive results and wound healing. There is great demand to make the procedure faster and causing less discomfort for the patient, and, consequently, for the technical development of instruments used. On the other hand, the long-term results and the possible side-effect make it essential to precisely control the physical parameters (energy profile, repetition rate, airflow) of excimer laser during the whole refractive procedure (1, 2). The corneal temperature remains one of the most important parameters to control (3-6).
Before the refractive procedure, a speculum is applied onto the eye of the patient in to keep the eye open as well as to ensure constant temperature of the cornea. The removal of the superficial part of the corneal epithelium or isolation of the flap may change temperature in the eyeball (7).
During the procedure, laser impulses cause the ablation of corneal surface. According our preliminary study, part of the energy is transformed into heat and its amount is dependent on the frequency of the laser, its diameter and energy profile, as well as ablation strategy of the modern flying-spot devices (8). The cornea undergoes dynamic thermal changes during the procedure and exchanges heat with the surrounding tissues, as well as with the environment (9). Aqueous humor circulation causes thermal convection deeper into the eye, and the air flow enabled by laser smoke plume evacuators causes convection to the environment10. The cornea also radiates the heat (10).
In vivo studies examine the maximal temperature of the cornea during the treatment in clinical circumstances, the factors influencing corneal temperature, as well as the effect of corneal temperature on long-term results and wound healing, and the function of the heat-shock proteins (11-13).
In vitro studies frequently use PMMA (polymethylmethacrylate) plates and measure their surface temperature. The plates are also widely used for preoperative testing of the lasers – they serve for the calibration, the comparison of different lasers and their ablation profile, and the investigation of the relationship between the energy and temperature load (14-17).
This study examines the time factor of the temperature rise during excimer laser treatment equal to -10 D myopic refractive treatment on PMMA test plates on two different excimer lasers. Laser smoke plume evacuators were installed and the room conditions were equal to those used in the operating room.
Aim
The aim of this study was to examine the relationship of the temperature of PMMA plates, environmental conditions and the energy load caused by the laser impulses during refractive surgery procedure.
Material and Methods
Two models of excimer lasers were used for this study – Schwind Amaris 500E, operating on 500 Hz frequency, and Allegretto Wavelight, operating on 400 Hz frequency. Both of them were used on factory settings.
Environmental conditions of the room were stable. The temperature was set to 22 ± 1°C and humidity was 35 ± 5%.
Laboratory-calibrated, high accuracy infrared thermometer (EBRO TLC 730, WTW GmbH, Germany) was used to the measure the corneal surface temperature. Accuracy and resolution of the thermometer was 0.1°C. All measurements were obtained from 8 cm distance. The distance (D) and the measured spot size (S) were proportional, with D/S = 8/1. The spot size applied by 8 cm distance was 1 cm, therefore, equal to the size of the photoablation area. The thermometer was stored in the operating room in order to adjust it to the temperature of the environment.

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Piśmiennictwo
1. Müller B, Boeck T, Hartmann C: Effect of excimer laser beam delivery and beam shaping on corneal sphericity in photorefractive keratectomy. J Cataract Refract Surg 2004 Feb; 30(2): 464-470. 2. Khoramnia R, Lohmann CP, Wuellner C et al.: Effect of 3 excimer laser ablation frequencies (200 Hz, 500 Hz, 1000 Hz) on the cornea using a 1000 Hz scanning-spot excimer laser. J Cataract Refract Surg 2010 Aug; 36(8): 1385-1391. 3. Arba-Mosquera S, Shraiki M: Analysis of the PMMA and cornea temperature rise during excimer laser ablation. J Mod Opt 2010; 57(5): 400-407. 4. Ishihara M, Arai T, Sato S et al.: Measurement of the surface temperature of the cornea during ArF excimer laser ablation by thermal radiometry with a 15-nanosecond time response. Lasers Surg Med 2002; 30(1): 54-59. 5. Langenbucher A, Seitz B, Kus MM, Naumann GOH: Thermal effects in excimer laser trephination of the cornea. Graefes Arch for Clin Exp Ophthalmol 1996; 234 (suppl.): 142-148. 6. Maldonado-Codina C, Morgan PB, Efron N: Thermal consequences of photorefractive keratoectomy. Cornea 2001; 20: 509-515. 7. Kim MJ, Kim JC, Park WC et al.: Effect of thermal preconditioning before excimer laser photoablation. J Korean Med Sci 2004; 19: 437-446. 8. Szekrenyesi Cs, Sandor G, Gyenes A et al.: Change of corneal surface temperature using different excimer lasers with different repetitive frequency in surface refractive procedures. New Med 2016; 3: 79-83. 9. Dantas PE, Martins CL, de Souza LB, Dantas MC: Do environmental factors influence excimer laser pulse fluence and efficacy? J Refract Surg 2007 Mar; 23(3): 307-309. 10. Bende T, Seiler T, Wollansack J: Corneal thermal gradients. Graefes Arch for Clin Exp Ophthalmol 1988; 226: 277-280. 11. de Ortueta D, Magnago T, Triefenbach N et al.: In vivo measurements of thermal load during ablation in high-speed laser corneal refractive surgery. J Refract Surg 2012 Jan; 28(1): 53-58. 12. Tsubota K, Toda I, Itoh S: Reduction of subepithelial haze after photorefractive keratectomy by cooling the cornea. Am J Ophthalmol 1993; 115: 820-821. 13. Nagy ZZ, Toth J, Nagymihaly A, Suveges I: The role of ultraviolet-B in corneal healing following excimer laser in situ keratomileusis. Pathol Oncol Res 2002; 8(1): 41-46. 14. Gottsch JD, Rencs EV, Cambier JL et al.: Excimer laser calibration system. J Refract Surg 1996 Mar-Apr; 12(3): 401-411. 15. Dorronsoro C, Siegel J, Remon L, Marcos S: Suitability of Filofocon A and PMMA for experimental models in excimer laser ablation refractive surgery. Opt Express 2008 Dec 8; 16(25): 20955-20967. 16. Naroo SA, Charman WN: Surface roughness after excimer laser ablation using a PMMA model: profilometry and effects on vision. J Refract Surg 2005 May-Jun; 21(3): 260-268. 17. O’Neill W, Kapadia P, Thomas T: A study of laser-based removal of polymethylmethacrylate bone cement. J Laser Appl 1996 Jun; 8(3): 149-154. 18. Kasagi Y, Yamashita H: HSP47 expression in cornea after excimer laser photoablation. Jpn J Ophthalmol 2002 Mar-Apr; 46(2): 123-129. 19. Canals M, Elies D, Costa-Vila J, Coret A: Comparative study of ablation profiles of six different excimer lasers. J Refract Surg 2004 Mar-Apr; 20(2): 106-109. 20. Doga AV, Shpak AA, Sugrobov VA: Smoothness of ablation on polymethylmethacrylate plates with four scanning excimer lasers. J Refract Surg 2004 Sep-Oct; 20 (5 suppl.): 730-733. 21. Wernli J, Schumacher S, Wuellner C et al.: Initial surface temperature of PMMA plates used for daily laser calibration to control the predictability of corneal refractive surgery. J Refract Surg 2012 Sep; 28(9): 639-644.
otrzymano: 2016-09-02
zaakceptowano do druku: 2016-11-30

Adres do korespondencji:
*Csaba Szekrenyesi
Faculty of Health Sciences Semmelweis University
17 Vas Str., 1088 Budapest, Hungary
tel.: +36-1-2486-5965
e-mail: szekrenyesics@se-etk.hu

New Medicine 4/2016
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