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Argon-fluoride excimer laser ablation of stratum comeum causes deeper tissue damage than expected for thermal or
photochemical mechanisms, suggesting thatphotoacoustic waves have arole in tissue damage. Laserirradiation (193 nm, 14
ns pulses, 1-2 Hz) attworadiantexposures, 60 and 160 mJ/cm2perpulse was usedto ablate the stratumcomeumofskin. Light
and electron microscopy ofimmediate biopsies demonstrated damage to fibroblasts as deep as 88 and 220 jun, respectively,
below the ablation site. Ablation throughwaterwas usedtoinertially confine the ablation zone. Partial ablationofs.c. through
airproducedno damage, whereas partial ablation through water damaged skin to amean depth of 1 14.5 8.8( Full
thickness ablation of s.c. through air and water produced damage zones measuring 192.2 16.2 and 293.0 71.6 rim,
respectively (p <0.05). The increased depth ofdamage in the presence ofinertial confinementprovided by the layer of water
strongly supports a photoacoustic mechanism ofdamage. The depths ofdamage for thelarge spot, line, and small spots were
43 1 164 urn, 269 96xni, andno damage. The spot size dependence ofthedepthofdamage is consistentwiththe geometric
attenuation one would expect to be present from a pressure wave related phenomena. Sequential biopsies were taken over a
7 day period for light and transmission electron microscopy. At 24 hours, there was necrosis of the epidermis and papillary
dermis subjacent to the ablation site, with neutrophils surrounding and demarcating the affected area. The necrotic zone
sloughedby48 hours. Thereepithelializationwas completeby7 days. The sequenceofrepairis similartoknife wound healing
which we have previously studied, and is analogous to other wound healing processes.
We have used an experimental model of ArF excimer laser ablation of stratum corneum to investigate laser-induced
photoacoustic damage. The evidence for the injury being due to pressure transients is indirectbutcompelling. Whether these
pressuretransients are acoustic transients orshockwaves has notbeendetermined, although itis ourprejudicethatshockwaves
are the predominant force under these conditions. It is important to consider the possible effects of pressure transients in
evaluating laser-tissue interactions, particularly when using short pulse, high peak power lasers.
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