Estudios de validación.

Although much of the biological threshold data for laser interaction with ocular tissues can be understood as

following the prediction of either a photochemical or

thermal model at threshold, a thermally initiated acoustic transient is needed to explain suprathreshold effects for

short pulse widths. When one considers the final outcome of

the interaction, more complex models — incorporating the aspects of biological repair or biological amplification of the initial effect may be needed.

From a clinical standpoint, there remain questions

related to the sequelae of most types of laser tissue

interaction. Largely unexplored are the possible

interaction mechanisms of laser ablation of the cornea and

l e n s . Indeed the details of optical breakdown in ocular

tissues are not adequately understood. It is not clear if

tiieelectrical conductivity of tissue or the preionization of a tissue subjects it to different effects from high

irradiance focal laser irradiation. It is not at all clear

what physical factors influence the smoothness of an ablated

surface. Could optical plasma formation be necessary for a

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clean ablation of corneal or lenticular tissue, or could it

r- "" * ^ " " 1

be a deterrent? Must a thermal component be necessary for laser ablation (in order to remove tissue)? How does

wavelength influence the interaction mechanisms?

Other areas of research required include the further understanding of picosecond and femtosecond-pulsed injury of

the retina. Currently, there are conflicting results from

ultrashort pulsed injury studies. Some studies of

sub-microsecond laser injury suggest a threshold of nearly constant energy (Birngruber, et al., 1987), whereas other studies suggest that the threshold depends on the peak power in the pulse— at least for pulse durations between 5 ps and 100 ps (Goldman, et al., 1977; Taboada and Gibbons, 1978). There are also apparently conflicting results regarding the wavelength dependence of picosecond laser retinal injury thresholds (Goldman, 1977; Bruckner and Taboada, 1982). Competing damage processes result from multi-photon

absorption and electron avalanche effects as occur in solids

higher threshold for 532 run when compared to 1064 nm at the same pulse duration (30 ps) if the data of Goldman are

correct (Goldman, 1977). Regardless of the interaction

mechanisms in the sub-nanosecond regime, one must ask whether any of the know effects have potential value for clinical application.

Still further research is also necessary to understand the effects of speckle upon retinal function at very low, chronic exposure durations as reported by Zwick (Zwick and

Jenkins, 1982). It may well be that if we force ourselves

to stare at a large area diffuse laser reflection, the speckle which produces a myriad of areas of very high contrast borders at the retina may affect those neural mechanisms that detect borders in our normal world.

Fortunately, most of us find it uncomfortable to stare at speckle for long periods, and this effect is hopefully just

a laboratory curiosity. In any case, it is unlikely that

this reported phenomenon of speckle will have any relevance to the application of pulsed lasers in ophthalmology.

Refractive surgery of the cornea poses many interesting

research questions. Over the past few years two spectral

regions have been explored for this potential laser

applications the UV-C (i.e., 100-280 nm) and the IR-C (3 -

1000 pm) where corneal absorption is highest. Pulsed

argon-fluoride (ArF) excimer (193 nm)UV-C lasers and HF (2.9 pm) and CO-2 (10.6nm) infrared lasers are the most obvious

candidates for this type of laser surgery. Wolbarsht has

also argued for using laser radiation at 2.94 pm (e.g., ErsYAG) because of water's extremely shallow penetration

depth of the order of 1.0 pm— the peak of water absorption. While this choice of wavelength means that water molecules will certainly absorb, one should remember that the

objective is to cut collagen fibres cleanly. At 2.94 pm

essentially all of the incident laser energy is coupled into water molecules which then change to the gaseous phase.

This change of state will create local volumetric changes which may blow off sections of collagen fibres and other

biological molecules. Energy must be transferred to the

collagen matrix by heat transfer which is not at all

selective. Perhaps an infrared wavelength exists where

water absorbs, but where collagen molecules also absorb

substantially. A resonant absorption by the collagen

aggregate molecules could be far more effective in

fragmenting collagen fibres and achieving a smooth cut. While a single IR photon would normally not have sufficient energy to break a bond, the high irradiances available from a q-switched laser might produce sufficient multi-photon

resonant absorptions to achieve cleavage. By contrast to

infrared laser interactions which are thermal in nature, ultraviolet laser interactions appear to be dominated by photochemical interactions, and studies at the molecular level may be necessary to understand what effects are really pos s i b l e .

Although selective IR photochemistry has been

demonstrated in the gaseous phase [37-38], it is not clear whether it could be effective in the condensed phase since

absorptions are greatly broadened. Nevertheless, it can

investigating.