Br J Ophthalmol: first published as 10.1136/bjo.70.7.482 on 1 July 1986. Downloaded from British Journal of Ophthalmology, 1986, 70,482-501 A comparative study of corneal incisions induced by diamond and steel knives and two ultraviolet radiations from an excimer laser JOHN MARSHALL,' STEPHEN TROKEL,2 STEPHEN ROTHERY,' AND RONALD R KRUEGER2 From the 'Institute of Ophthalmology, Judd Street, London WCJH 9QS, and the 2Edward S Harkness Eye Institute, Columbia- Presbyterian Medical Center, 635 West 165th Street, New York, NY 10032, USA SUMMARY This paper reviews the potential role of excimer lasers in corneal surgery. The morphology of incisions induced by two wavelengths of excimer laser radiation, 193 nm and 248 nm, are compared with the morphology of incisions produced by diamond and steel knives. Analysis suggests that ablation induced by excimer laser results from highly localised photo- chemical reactions and that 193 nm is the optimal wavelength for surgery. The only significant complication of laser surgery is loss of endothelial cells when incisions are within 40 ,um of Descemet's membrane. Excimer lasers may herald a new epoch in surgery of The discrete focal localisation of the effects the cornea. The term excimer is derived from the first induced in target materials is currently under investi- two and last syllables of the words that describe the gation. Two theories have been proposed. The first of physical state of the lasing media used in such these proposes that beam tissue interactions are lasers-excited dimers of inert gases. Excited dimers predominantly the result of ultrafast thermal events, http://bjo.bmj.com/ are two atoms of an inert gas bound in a highly or so called photon-phonon interactions, and that the excited state with atoms of halogen to form a discrete nature of the induced change arises because temporary association as a diatomic rare gas halide. of the very short penetration or adsorption depth of The decay of these unstable molecules is accom- the ultraviolet radiation.' The second theory asserts panied by the emission of a highly energetic photon that the target reaction is not thermal but probably of ultraviolet light. Excimer lasers are systems whose photoablation, or photon-induced molecular emission wavelengths are variable and determined by decompensation.7 The protagonists of photoabla- on October 2, 2021 by guest. Protected copyright. a specific energy transition, characteristic of the tion suggest that molecules in the target area are particular gases used to fill the lasing cavity. For released as a direct result of rupture of inter- example the emission for argon fluoride occurs at molecular bonds whose uncoupling is induced by 193 nm, and that for krypton fluoride at 248 nm.' absorption of high energy photons. Such a They emit their radiation as a train of individual mechanism is possible, because in the ultraviolet pulses whose duration is typically about 10 nano- spectrum individual photon energies are in excess of seconds, and with a pulse repetition frequency of 6 electron volts and are thus capable of disrupting between 5 and 15 Hz. valency electrons. Thus only those molecules that Interest in their surgical potential was initiated by absorb ultraviolet radiation directly are affected, and reports from scientists in both the materials and adjacent areas would show no conductive effects. It is electronic industries that the intense ultraviolet suggested that, once molecules have been disrupted, radiation emitted by such lasers could be used to etch the small, energetic, and volatile fragments rapidly submicron patterns into the surfaces of plastics and ablate. This mode of action would produce the other polymers2 with a remarkable degree of sharply defined boundaries of irradiated areas. It accuracy and with no degradation processes being could also account for the smooth surface of the conducted to unirradiated areas." substrata exposed by surface erosion during excimer Correspondence to Professor J Marshall. irradiation, with the total depth of the erosion being 482 Br J Ophthalmol: first published as 10.1136/bjo.70.7.482 on 1 July 1986. Downloaded from A comparativestudy ofcorneal incisions induced by knives and two ultraviolet radiations 483 a function of the absorption depth at any given 5, 8, 10, 15, 30, or 45 seconds. The higher frequencies wavelength an the duration of the exposure. and longer exposures were generally used at 193 nm, The concept of photoablation with its implicit giving fluence or dose values of 150, 300, and 450 precision has provoked the initiation of a number of mJ/cm'. These exposure parameters were derived pilot studies involving irradiation of biological from previous experiments where it was calculated tissues.' Early results with high resolution troughs or that 1 J/cm' at 193 nm ablates corneal tissue to a trenches being eroded in such tissues as human hairs depth of 1 [tm.9 It was hoped that this range of inspired Trokel to collaborate with Srinivasan and doses would provide specimens with a variety of others to examine the potential of excimer lasers for depths of incisions, including full penetration of the corneal surgery.9 cornea. The present paper examines the erosions induced Fourteen albino rabbits each weighing about 2 kg by two different wavelengths of ultraviolet light and were anaesthetised with intravenous phenobarbital. attempts to consider the optimal factors for potential The animals were placed on a positioning board with clinical use. In addition ultrastructural changes the target eye held open with a lid speculum and the induced by lasers are compared with those generated cornea centred at the focus of the laser delivery by conventional instruments. system. A vertical or horizontal incision was made on the cornea of each eye. The laser was imaged in such Methods and materials a way that the resultant incision extended over the limbus. Six of the animals were exposed to radiation LASER STUDIES at 193 nm and eight to 248 nm. All animals were The laser used throughout the present study was a killed between 5 and 30 minutes after exposure with Model 102E Excimer laser produced by Lambda an intravenous overdose of phenobarbital. Physik (Gottingen, West Germany). The cavity was filled, to the manufacturers' recommended levels, CONVENTIONAL INCISIONS with either argon and fluorine or krypton and fluorine Four rabbits ofsimilar size and weight were anaesthe- gases in order to provide emissions at 193 nm or 248 tised in an identical manner. Each eye was prepared nm respectively. Given both the practical problems and draped and one eye of each animal was incised by and the expense of changing gases, experiments at a steel blade and the other by a diamond knife. Two the two different wavelengths were performed as a of the animals received incisions in the vertical plane sequential series rather than consecutive individual and two received them in the horizontal meridian. exposures of alternating wavelengths. The laser was All incisions were made by a single pass of the cutting http://bjo.bmj.com/ modified so that the cavity was configured as an instrument across the central cornea, and pentrating unstable resonator, and this produced a marked to between one-half and one-third of the corneal decrease in the far field divergence of the emergent thickness. A new Sharpoint microsurgical blade was beam. used for each of the steel knife incisions, and those of The laser output was shaped with a 15 mm quartz diamond were performed with a new trifaceted Micra cylindrical lens and passed through a 15x3 mm diamond knife. aperture placed adjacent to the cylindrical lens. The on October 2, 2021 by guest. Protected copyright. convergent beam was focused directly on to the HISTOLOGY AND ULTRASTRUCTURE specimen target. At the focus the laser produced a Immediately after death the eyes were removed, and sharp rectilinear image some 21 mm long. At 193 nm a 5 mm penetrating incision was made at the ora the total output of this laser was 80 mJ, but this was serrata before the globes were placed in 3% glutaral- reduced to a maximum of20 mJ after passing through dehyde buffered in Earl's buffer at 4°C. After 24 the collimator and aperture. The output at 248 nm hours in this initial fixative the eyes were examined was significantly higher with values of up to 100 mJ under a dissecting microscope, and corneal samples and 50 mJ in comparable locations. The duration of were isolated and divided such that each site of individual pulses within the pulse train also varied irradiation or conventional incision could be pro- with wavelength, being about 14 nanoseconds with cessed for examination by both scanning (SEM) and argon fluoride and 9 ns in the case of krypton transmission (TEM) electron microscopy. fluoride. One further variable is the rate at which the Specimens for SEM were postfixed overnight in peak power is reached in any given pulse at the two 2% osmium tetroxide buffered in 0.1 M sodium different wavelengths.' There is a marked sub- cacodylate. They were dehydrated through a series of structure in excimer laser pulses, and this may be of ascending concentrations of acetone in water before importance in analysing induced biological effects. being critical-point dried (Emscope CPD 750). Dried The laser was run at pulse repetition frequencies of samples were coated with a 30 nm layer of gold in a either 3, 5, or 10 Hz and exposure durations were of sputter coater (Emscope SC 500) prior to being Br J Ophthalmol: first published as 10.1136/bjo.70.7.482 on 1 July 1986. Downloaded from 484 John Marshall, Stephen Trokel, Stephen Rothery, and Ronald R Krueger examined in either a Hitachi 510 or 520 scanning ablation. In appearance the smooth surfaced zone electron microscope.