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Excimer Lasertreatment of Corneal Surface Pathology: a Laboratory And 258 BritishJournalofOphthalmology, 1991,75,258-269 ORIGINAL ARTICLES Br J Ophthalmol: first published as 10.1136/bjo.75.5.258 on 1 May 1991. Downloaded from Excimer laser treatment ofcorneal surface pathology: a laboratory and clinical study David Gartry, Malcolm Kerr Muir, John Marshall Abstract to bond breakdown.'3 Ultraviolet radiation in The argon fluoride excimer laser emits this spectral domain does not propagate well in radiation in the far ultraviolet part of the air, and at any biological interface the photons electromagnetic spectrum (193 nm). Each are virtually all absorbed within a few microns of photon has high individual energy. Exposure of the surface. Thus, with each laser pulse a layer of materials or tissues with peak absorption tissue only a few molecules thick will be ablated around 193 nm results in removal of surface fromthesurface. Tissuedamageinduced by other layers (photoablation) with extremely high clinical lasers is achieved by concentrating laser precision and minimal damage to non- energy into a focused point. However, the irradiated areas. This precision is confirmed in excimerlaserbeamhasalargecross sectional area, a series ofexperiments on cadaver eyes and the and since every photon in the beam has the poten- treatment of 25 eyes with anterior corneal tial to produce tissue change the entire cross sec- disease (follow-up 6 to 30 months). Multiple tion can be utilised. The 1 cm by 2 cm rectangular zone excimer laser superficial keratectomy is profile is adjusted by cylindrical quartz lenses, considered the treatment of choice for rough, and the resultant square beam profile becomes painful corneal surfaces. Ali patients in this circularby passing the emergent beam through an group were pain-free postoperatively. Where aperture.9 This facility to photoablate large areas good visual potential exists, ablation ofa single of cornea permits selective removal of tissue to axial zone is recommended and results in induce a refractive change (photorefractive kera- improved visual acuity and reduction of glare. tectomy - PRK).9 I'l8 A hyperopic shift was noted in this group. For PRK and superficial keratectomy to become accepted in clinical practice an under- http://bjo.bmj.com/ standing of the nature and quality of wound The potential of excimer lasers in ophthalmic healing is essential. Complete epithelial wound surgery was first suggested in 1983' following healing is necessary to re-establish the outer their successful industrial application in etching osmotic barrier of the cornea and optical bril- highly precise patterns into plastics.' A distinct liance. Studies have been undertaken to assess advantage of argon fluoride excimer laser radia- latency of wound healing, epithelial migration tion (193 nm) is that there is no significant and adhesion properties, and the presence or on September 24, 2021 by guest. Protected copyright. damage beyond exposed areas. Ultrastructural absence of hyperplasia. 9 Stromal wound healing examination of corneal excisions in animal has been examined in relation to loss or distur- models confirmed that these could be produced bance of transparency, keratocyte infiltration, with a very high degree of accuracy and control. and scar formation.'4"620 Endothelium has been Damage to adjacent unexposed tissue was assessed in relation to potential cell loss or long- limited to 300 nm beyond the boundary of the term population changes. 19 21 Finally the putative zone of ablationi--9 In non-perforating excisions mutagenic effects of ultraviolet radiation on the of varying depths endothelial damage was noted cornea have been assessed by both unscheduled only when the base of the excision was within DNA repair and tissue culture, coupled with 40 iim ofDescemet's membrane, which is similar enzyme poisoning techniques.22 23 None of these to but less extensive than diamond knife inci- studies has highlighted areas of concern that sions. 8'I Furthermore, excisions can be con- would preclude the beginning of clinical trials trolled precisely, in terms of uniformity along with the excimer laser at 193 nm (Sliney D, the length ofa single cut, reproducibility between personal communication). St Thomas's Hospital, cuts, and in anticipated cut depth ± London SEI 7EH (accuracy 3%, In February 1988, on the basis of laboratory D Gartry diamond knife incisions ± 12%). The first data, ethical committee approval was given to M Kerr Muir clinical application was therefore the correction treat overt corneal lesions with the excimer laser. ofastigmatism with T-cut excisions. 12 The aim of this Institute of study was to determine whether Ophthalmology, Judd The nature of corneal damage induced by the laser could be used for wide area ablations to Street, London excimer radiation at 193 rum is unique, because it remove opacities and produce an optically WC1H 9QS results from an extremely high photon energy smooth surface. Since photoablation occurs J Marshall and high peak power. At this wavelength each across the entire area exposed, with repeated Correspondence to: Mr D Gartry. photon has an energy of 6-4 electron volts, and, pulses the original surface contour will be repro- Accepted for publication as this exceeds the binding voltage of carbon- duced in the base of the ablated disc. It was 22 October 1990 carbon bonds, a single absorbed photon may lead necessary therefore to devise a means of smooth- Excimer laser treatment ofcorneal surfacepathology: a laboratory andclinical study 259 ing an uneven surface. The simplest method of stroma had been ablated to the same depth as the achieving this was to apply a liquid with approxi- original cylinder. In practice this process was mately the same ablation properties as the monitored by changes in tissue fluorescence to be Br J Ophthalmol: first published as 10.1136/bjo.75.5.258 on 1 May 1991. Downloaded from cornea. The surface tension of the liquid would described in the results. ensure that any surface irregularities would be negated. If irregularities in the corneal surface had a different ablation rate from that of the EXCIMER LASER SMOOTHING OF SURGICAL surrounding tissue - for example, the calcium LAMELLAR KERATECTOMY BEDS deposits in band keratopathy - it would be This experiment was designed to assess the necessary to shield underlying tissue with a potential of the excimer laser in combination liquid while the crystalline peaks underwent with masking agents to smooth irregular corneal ablation. surfaces. A series of surgical lamellar keratec- This paper describes the use of the excimer tomies was performed on donor eyes. The initial laser for the production of large-area, optically incision was made with a disposable 6 mm smooth corneal surfaces in the laboratory and in trephine, and the lamellar dissection was done the clinic. with a steel blade and Paufique's knives, begin- ning at the base of the trephine cut. HPMC was applied to the keratectomy base and its amount Materials and methods and distribution adjusted, while multiple over- After ethical committee approval all clinical lapping ablation zones were produced until the trials were carried out in the Department of base was smooth. Ophthalmology at St Thomas's Hospital, London. A Summit Technology UV200 excimer laser was used with spectral emission at 193 nm. HISTOPATHOLOGY The pulse energy resulted in a radiant exposure After laser exposure a 5 mm penetrating incision of 180 mJ/cm2; the pulse frequency was fixed at into the posterior globe was made and the eyes 10 Hz and tissue removed per pulse was 0-24 [tm. were immersed in fixative. The initial fixative The beam configuration was circular in cross used was 2 5% glutaraldehyde buffered in 1 M section, and a number of fixed beam diameters sodium cacodylate with 10 g/l calcium chloride were used between 1 and 4 mm. at a final pH of 7-4. After 24 hours the corneas were prepared for light microscopy (LM) and electron microscopy (EM). Those for scanning EXCIMER LASER PATTERN ABLATION IN CADAVER EM were postfixed overnight in 2% osmium EYES tetroxide buffered in 0.1 M sodium cacodylate, (1) The aim of this experiment was to create an dehydrated through a series of ascending con- irregular surface in cadaver corneas and then to centrations of acetone, and critical-point dried. determine whether the excimer laser could Dried samples were sputter-coated with a 30 nm smooth these irregular surfaces. Donor eyes layer ofgold before examination in a Hitachi 520 http://bjo.bmj.com/ were used within 16 hours of death and secured scanning electron microscope (SEM). For trans- in a Tudor Thomas stand. Ocular rigidity was mission electron microscopy (TEM) specimens maintained by injecting normal saline into the were postfixed for only one hour in the osmium vitreous through the pars plana. After removal of tetroxide solution described above, dehydrated the epithelium with a hockey end blade the globe in alcohol, and then embedded in Araldite was placed directly beneath the laser aperture (CY212) via epoxypropane. Sections were cut at and the helium-neon aiming beams focused on 1 ptm on glass knives for LM and on diamond on September 24, 2021 by guest. Protected copyright. the centre of the cornea. An area of stroma was knives for EM and mounted on 200 mesh copper masked by placing a piece of bent wire on the grids, stained with uranyl acetate and lead citrate corneal surface in the path of the beam. Fifty before examination in an AEI 801 transmission pulses of laser energy were then directed on to electron microscope. the partially occluded corneal surface with a 3 mm diameter beam. This resulted in excavation of the stromal surface in areas not shielded by the TREATMENT OF BAND KERATOPATHY AND ALLIED wire. In areas beneath the wire, ridges oforiginal SUPERFICIAL PATHOLOGIES tissue remained. Twenty five patients underwent excimer laser The next objective was to remove the induced superficial keratectomy to remove band kerato- pattern with the laser to restore a smooth surface pathy or to smooth roughened corneal surfaces across the entire ablated zone.
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