Intraocular Photodisruption With Picosecond and Nanosecond Laser

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Tissue Effects of Picosecond and Nanosecond Photodisruption FIGURE 4. (a, b) Scanning electron micrographs of cuts through Descemet's membrane. The cuts were produced with 50 jxj ps pulses in (a) and with 1 mj ns pulses in (b). In each case, about 100 pulses were applied at a repetition rate of 10 Hz. (c, d) Semithin sections through the lesions shown in (a, b). The length of the scales corresponds to 100 ftm. the pulse energy could be reduced by a factor of 7 6 to 8. Figure 6 shows plasma formation and cavitation when ps pulses were used instead of ns pulses. in corneal tissue in comparison to the maximal bubble Tissue Effects on Descemet's Membrane size reached in water. Both the effects in cornea and in water were produced with 300 /uj pulse energy. The Figure 4 shows scanning electron micrographs and se- bubbles in water expand rapidly in about 40 to 45 /us mithin sections of cuts in Descemet's membrane producedand reach the collapsed stage after 80 to 90 us. The with ps and ns Nd:YAG laser pulses. Using ps pulses bubble expansion in the cornea is strongly decelerated with an energy of 50 fx] per pulse, it was possible to because of the high viscosity of the corneal tissue. The achieve a dissection of Descemet's membrane with a photograph in Figure 6, taken 3 /is after breakdown, width of 30 to 40 /im and a range of endothelial dam- shows the maximally expanded stage of the bubble in age of about 130 /xm (Figs. 4a, 4c). The width of the dissection is only slightly larger than the plasma diame- the cornea. It is much smaller than the bubble in water (it has less than ter. Nanosecond pulses with a pulse energy of 1 mj, however, caused gross ruptures in Descemet's membrane and a large collateral damage zone of about 400 /um (Figs. 4b, 4d). A dislocation of the stromal lamellae below the cut is observed in both cases, but it is stronger for the ns pulses. Figure 5 demonstrates that it is possible to produce laser effects that are almost selective to the corneal endothelium. To achieve this high degree of spatial confinement, ps pulses with an energy of 35 /ij (i.e., close to the breakdown threshold) were used. Cavitation and Tissue Effects in the Corneal Stroma Intrastromal corneal tissue effects produced by ps pulses and their origin via cavitation are presented in Figures ] /3 of its diameter and about ] /3o part of its volume). Although the bubble expansion ceases in less than 3 n& after plasma formation, the bubbles reach a size much larger than the volume of the laser plasma (see Fig. 6). Even 2 minutes after breakdown, the volume of the cavity is still more than 10 times larger than the plasma volume. This demonstrates that the effect of the laser pulses is mainly displacement but only to a small extent evaporation of tissue. The intrastromal cavities collapse slowly and disappear only about 1 hour after their generation. The lobular form of the bubbles immediately after their generation is probably related to the arrangement of the collagen lamellae within the stroma. Figure 7 presents the histologic appearance of intrastromal laser effects created with 80 ti] ps pulses. The corneal specimens were fixed about 5 minutes 3037
3038 Investigative Ophthalmology 8c Visual Science, June 1994, Vol. 35, No. 7 FIGURE 5. (a) Scanning electron micrograph of laser effects selectively produced in the corneal endothelium using a series of ps pulses with a pulse energy of 35 juj each, (b) Seinithin section through the same lesion. The scales correspond to a length of 100 nm. after laser exposure. At this time, the cavities had a diameter of about 60 /mi, and the cavity volume exceeded the volume of evaporated tissue, which is approximately equal to the plasma volume by a factor of more than 10. Correspondingly, a strong deformation of the collagen lamellae around the cavity is observed. In Figures 7a and 7b, some endothelial cells are missing below the laser effects. Damage detectable by light microscopy was observed up to a distance of 150 pm between the laser focus and the corneal endothelium. Figure 8 shows intrastromal effects created when the laser pulses were applied with 10 Hz repetition rate while the specimen was slowly moved in a lateral direction. Using a pulse energy of 80 ^J, a homogeneous cavity could be produced that has a width of 160 to 230 ^m and a length of 2 mm (Fig. 8a). The corneal epithelium and Descemet's membrane are apparently not damaged. An increase of the laser pulse energy to 300 fx] leads to a spongelike agglomeration of bubbles rather than to one homogeneous cavity (Fig. 8b). Series of individual cavities were, however, observed as well with 80 ^J pulse energy. Tissue Effects in the Lens Figure 9 shows slit lamp photographs of ps and ns laser effects in bovine lenses produced with 100 fij and 1.1 mj pulse energy, respectively. The effects look similar to the cavities produced in corneal tissue, and, like them, they disappear after about 1 hour. The application of 1.1 mj ns pulses sometimes resulted in a splitting of the lens fibers and a creation of one or several large bubbles reaching up to the anterior lens capsule (Fig. 9b). These bubbles block the laser light path and the view toward the posterior part of the lens. The ps laser effects are much finer (the diameter of the cavities is reduced approximately by a factor of 3), and a splitting of the lens fibers was not observed. The laser effects in human lens nuclei varied with the optical and mechanical properties of the cataract. Soft lenses developed vacuoles and bubbles where an optical breakdown occurred, similar to the picture in Figure 9. Dense nuclei developed more discrete pits, cracks, and lamellar clefts independent of the pulse length. The breakdown thresholds for both pulse durations are given in Figure 3. Tissue Effects at the Retina Figure 10 is a macrophotograph of laser effects that were produced with a series of 200 n] ps pulses to determine the range for retinal damage during vitreous surgery close to the vitreoretinal interface. In the center of each line of laser exposures, the laser focus was located in the vitreous body. The distance Ax from the retinal vessels varied between 200 jum and 800 ftm. At a certain distance from the center, the laser focus was located within the retina and further laterally it 2 min SE f » 0.5 mm FIGURE 6. Comparison between the cavitation bubble dynamics in corneal stroma {left) and in water (right). The bubbles in the cornea were photographed 3 fis, 10 seconds, and 2 minutes after the laser pulse was released, and the picture of the bubble in water was taken 40 ^s after the laser pulse. Both pictures are printed with the same magnification. All bubbles were produced with 300 n] ps pulses.
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Intraocular Photodisruption With Picosecond and Nanosecond Laser

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