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International Journal
of Ophthalmology
2017; 10(9): 1474-1476
·Brief Report·
Effect
of open ultraviolet germicidal irradiation lamps on functionality of excimer
lasers used in cornea surgery
Jaroslavas Belovickis1, Aliaksei Kurylenka2, Vadim Murashko2
1Vilnius University, Sauletekio 3, Vilnius 10222, Lithuania
2Novoe Zrenie, Klumova 5a, Minsk 220009, Belarus
Correspondence to: Jaroslavas Belovickis. Vilnius University, Sauletekio 3, Vilnius 10222,
Lithuania. Jaroslavas. Belovickis
@stud.tu-darmstadt.de
Received: 2016-12-23
Accepted: 2017-05-18
Abstract
We
report on the impact of direct ultraviolet germicidal irradiation (UVGI) on
reflective optics, used in the excimer laser system
Allegretto Eye-Q. The aim of our work was to confirm our hypothesis based on
long-rate observations of obtained anomalies in post-operative results that are
attributed to degradation of reflective optics upon ultraviolet radiation. The
presence of direct UVGI coupled with humidity in the operating environment
caused merging anomalies and unwanted post-operative correction values.
Ultraviolet-A radiation caused a similar effect on the reflective cover of the
mirrors.
KEYWORDS: ultraviolet radiation; excimer lasers; post-operative anomalies; laser-assisted
in situ keratomileusis; mirror degradation
Citation: Belovickis J, Kurylenka A, Murashko V. Effect
of open ultraviolet germicidal irradiation lamps on functionality of excimer
lasers used in cornea surgery.
Int J Ophthalmol 2017;10(9):1474-1476
INTRODUCTION
Lmps emitting ultraviolet germicidal
irradiation (UVGI) have been used as an effective method for air disinfection
in hospital operating rooms since 1936[1]. Such
lamps produce short-wavelength ultraviolet (UV) radiation, also known as
ultraviolet-C (UV-C)[2]. Producing
UV radiation (190-290 nm) of sufficient intensity demonstrates
strong disinfection effect and prevents spread of certain infectious diseases
by inactivating microbes via damaging their deoxyribonucleic acid (DNA)[3-5]. Therefore,
low-pressure mercury (Hg) discharge lamps (254 nm) are commonly used as the
most effective ones in sterilization of work facilities[6].
To increase the efficiency of air purification, shielded UV lamps using forced
circulation of air by a fan, have been developed[7].
In this case the probability of unwanted chemical effects in surgical
environments is reduced[1]. Nevertheless, lamps of
open UV-C are still being used in air
disinfection and such long-term exposure to UV can cause various negative effects such as UV
degradation.
In polymers the long-term exposure to UV
light activates oxidation of bonds forming carbonyl groups in the polymer
chains. Such radiation breaks molecular bonds and leads to cracking, bleaching
in dark opaque materials or yellowing of the surface[8].
In first surface mirrors (front-coated mirrors) very similar effects may occur[9]. In
comparison with ordinary mirrors these types of mirrors have the reflecting
surface placed on the front surface of the glass being more vulnerable and less
protected from corrosion. The reflective coating is usually made of silver or
aluminum[9]. Both metals are subjected to
corrosion, also known as oxidation, which can be accelerated by UV light[10]. To prolong the specular reflectance of the mirrors,
various adhesives and protective layers are deposited on the metallic layer and
such technique also eliminates needless reflection from glass surface in back
mirrors. Furthermore, it is used when working with various instruments
requiring precision such as excimer lasers. The increasing use of Argon
fluoride (ArF) excimer lasers (193 nm) in
surgical procedures has produced various optical components and thin film
coatings for this type of radiation[11-13].
For example, elements being responsible for tracking of a pupil’s movement
during refractive eye surgery by reflecting the infrared (IR) illumination to
an IR camera, or mirrors deflecting the UV laser beam from a beam path to the
surgical area. Since these elements are more sensitive to environmental changes
due to its location, it may merge in an improper functionality of excimer laser
systems. So far,
little has been published on it and, therefore, the aim of this study was to
confirm our hypothesis based on long-rate observations of obtained anomalies in
post-operative results that are attributed to degradation of reflective optics
upon UV radiation.
METHODS
We performed a post-operative analysis of
patients treated by laser-assisted in situ keratomileusis (LASIK). Subjects were identified
through an electronic WaveLight Oculyzer Patient Database. Informed consent was
obtained from all subjects and the study was conducted in accordance to the
Declaration of Helsinki.
For this study two WaveLight Allegretto Eye-Q
excimer laser systems with ablation repetition frequency of 400 Hz were used to
perform LASIK procedures[14].
For pre-operative and post-operative
non-contact measurements and analysis of the anterior eye surface the WaveLight
Oculyzer II diagnostic device was used.
Each laser system was located in a separate
operating-room with different models of UVGI disinfecting lamps. For direct and
shielded UV-C sources ОБН-150 (OBN-150) and UltraViol
NBVE 110 NL lamp models (254 nm) were used, respectively. Both laser systems
contained new identical and recently replaced non-defective reflective IR
mirrors. These mirrors reflect the IR radiation from the cornea to the IR
camera, thus allowing to track the position of the pupil.
A layer of sodium chloride solution (9 g/L of NaCl) was partially deposited on both IR mirrors of two systems
and exposed to UV-C radiation. To compare effects of longer wavelengths on the
mirror surface a black light source LAMP08TBL (315-400 nm) was used in the experiment.
A time dependent degradation of the mirror
surface within the period of half a year was investigated upon both UV-C and
ultraviolet-A (UV-A) radiation sources.
RESULTS AND DISCUSSION
Initially, surgeons using laser systems,
pre-exposed to open UVGI sources, observed anomalies in measured curvatures of
the post-operative anterior surface of the cornea. To explain such discrepancy,
we assumed that the reflective mirrors could be affected by the open UVGI
lamps. Indeed, both checked IR mirrors had clearly visible degradation signs (Figure 1).
Figure 1 Degraded mirrors for reflection of
IR illumination (eye tracking).
For comparison, reflective mirrors in the
laser system located in the operating room with shielded UVGI lamps had no
visually observed degradation in its reflective surface. Moreover, after both
mirrors used for reflection of IR illumination were replaced with new ones, no
abnormal results after LASIK procedures were obtained throughout the period of
observation.
To confirm our hypothesis, the previously
explained experiment was conducted. Both IR mirrors were placed at its original
positioning within the laser system and the left side of the mirrors was
continuously poured with NaCl solution. Afterwards, the degree of mirror
degradation (the corroded part of the mirror surface) was investigated and was shown in Figure 2.
Figure 2 IR mirror degradation upon UV-C
radiation From left to right,
the new and the washed out after degradation surface. The dashed line
corresponds to the boundary between the mirror area upon (left) and without
(right) the NaCl solution. Below, IR mirror degradation upon UV-A radiation: a
droplet of NaCl solution, a dried surface and a degraded mirror cover after
being washed out.
Since continuous exposure to UV is more
harmful to the optics than intermittent exposure[10],
even mirrors applied to reflection of pulsed laser beam may degrade upon UV-C
irradiation. This type of UV degradation upon impact of humidity, also known as
oxidation, was previously investigated and published by Hernandez et al[9]. Meanwhile, similar consequences concerning the surface
degradation were observed upon exposure to longer UV wavelengths (Figure 2). The mirror surface is significantly corroded in the area the
droplet was left and dried, suggesting that the covering surface also can be
effected by UV radiation of longer wavelengths[5,9,11,13].
The findings of this investigation showed
that the reflectivity of the mirror surface is effected by the presence of
humidity due to modification of the protective coating, regardless to other
mechanisms as a simple contamination or sputtering erosion (enhanced radiation
effects)[5,11,13,15]. The presence of humidity on both mirrors may be
explained by its location in the vicinity of the ablation area. In this case,
small droplets or spurts of NaCl solution may pass to the mirror surface
occasionally during the LASIK procedures. It is also possible, that the mirror
degradation is induced by the treatment beam (193 nm, <2 W, pulse energy <1.5 mJ). We assume, that the UV
radiation is reflected from medical instruments used during the surgery
process. Various pincers etc, made of metal alloys, can easily reflect
the radiation to the eye-tracking mirror as they are merely used in the
vicinity of the pupil during the ablation procedure. Our assumption seems to be
valid due to a round shape (the shape of the treatment beam is also round) of
the degradation area seen in Figure 1.
In conclusion, this study has shown, that the
presence of direct UV germicidal irradiation coupled with humidity in the
operating environment may cause merging anomalies and unwanted post-operative
correction values. UVGI lamps and humidity may therefore serve as a key factor
of enhanced mirror corrosion and disruption of the protective coating. Findings
of this study may be useful to engineers and medical staff for preventing laser
optics from being damaged.
ACKNOWLEDGEMENTS
Belovickis J is grateful to Andrej Kungurov
and Jevgenij Mackevich for consulting in writing this article.
Conflicts of Interest: Belovickis J, None; Kurylenka A, None; Murashko V, None.
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