Abstract
Abstract: :
Purpose:Ultraviolet lasers are routinely used in keratorefractive surgery. Application of this technology to intraocular surgery is limited by difficulty in transmitting short wavelengths through an optic fibre and the fluid filled environment of the eye. We describe in vitro histological findings of treating the retinal pigment epithelium (RPE) and choroid with a device adaptable for intraocular surgery. Methods:The choroid and RPE were dissected from freshly enucleated porcine eyes. The tissue was clamped flat, RPE uppermost, in balanced salt solution. Half the specimens had the RPE gently debrided. We used a neodymium–YAG laser (SLII–10, Continuum, Santa Clara, CA), with a custom harmonic generator, producing a wavelength of 266 nm and pulse duration of 6 ns. The output was launched using a wave guide into an optical fibre taper, tip diameter of 200 µm, mounted vertically above the specimen, in a streotactic frame. Parameters studied were selected combinations of energy density (0.08, 0.12, 0.16, 0.2, 0.3 and 0.4 J/cm2), pulse count (1, 2, 5 and 10) and fibre to tissue distance (30, 50, 100 and 150 µm). After treatment the tissue was fixed in glutaraldehyde, epoxy embedded, stained with toludine blue and examined by light microscopy. A total of 164 lesions were studied on 10 specimens. Results:In specimens with the RPE intact, even single pulses of 0.08 J/cm2 produced clearly defined zones of RPE cell injury, consisting of cell swelling, pale staining and disruption of the basal aspects of the cells. At higher energies the RPE layer was absent from the treated areas. At all energies tested tissue injury was evident in the superficial choroid, up to 50 µm in depth. In the treated areas both the cells and extra–cellular matrix stained less intensely. Some melanocytes were ruptured with melanosomes scattered through the extra–cellular matrix. There was vacuolation in the stroma, most pronounced 20 µm deep to Bruch’s membrane. The intensity of these changes increased with the energy. Multiple pulses produced small increments in damage, until a threshold was reached at 5 pulses of 0.3 J/cm2, where coalescence of vacuolation produced loss of Bruch's membrane, choriocapillaris and superficial stroma. Varying the distance between the fibre and the tissue form 30 to 150 µm made little difference. Conclusions:We have demonstrated the possibility of intraocular use of a short wavelength laser. To determine optimal laser parameters for selective ablation in the complicated intraocular environment, the mechanism of laser–tissue interaction requires further study.
Keywords: laser • retinal pigment epithelium • choroid