Intraocular Photodisruption With Picosecond and Nanosecond Laser

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Tissue Effects of Picosecond and Nanosecond Photodisruption 3041 800jinr 200|j,nr Ax 400|anr 600[iri RPE location of laser focus FIGURE 10. Macrophotograph of laser effects within the retina caused by ps pulses with a pulse energy of 200 n]. Ax is the distance between the laser focus in the vitreous and the retinal vessels as adjusted in the cenLer of each line of laser exposures. At the sides of each line, the laser focus was located within the retina or at the pigment epithelium. Focusing directly onto a vessel always caused a hemorrhage. The arrow indicates a vessel showing no macroscopic damage when the laser pulses had been focused on the pigment epithelium, that is, at a level about-300 fim below the vessel. by the bubble expansion and partly by the jet formation during bubble collapse, which is described elsewhere. 714 Although the extent of tissue disruption by cavitation varies from case to case and decreases when the pulse energy is reduced, it is apparently the largest obstacle to a spatial confinement of the effects of intraocular photodisruption. Potential Applications of Picosecond Pulses Picosecond laser pulses can, in principle, be applied for all indications for which nanosecond pulses are presently used, whereby they offer the advantage of a reduced damage range. When very fine tissue effects are intended or when the application site is very close to sensitive ocular structures, such as the corneal endothelium or the retina, picosecond pulses are likely to be the only means to achieve the surgical aim. Possible new applications of this kind are corneal intrastromal refractive surgery, cataract fragmentation, and vitreous surgery close to the retina. Other uses could be trabeculopuncture for treatment of glaucoma and, as Figure 5 shows, the selective removal of a cell layer on a tissue surface, for example, polishing a lens capsule after cataract surgery. Intrastromal refractive surgery. Several authors have put forward the idea that refractive changes of the cornea may be achieved by removal of an intrastromal tissue layer through plasma-mediated evaporation. 1115 " 17 They have expressed the hope that the corneal curvature can be changed without damaging the corneal epithelium, Bowman's membrane, or the endothelium, and that thereby the haze and regression arising during the healing process of the cornea can be diminished. This aim cannot be accomplished using conventional photodisruptors delivering nanosecond laser pulses because their tissue effects are too coarse. 715 If, as an alternative, the ps laser is used for
3042 Investigative Ophthalmology &: Visual Science, June 1994, Vol. 35, No. 7 FIGURE 11. Histologic sections through retinal lesions arising from 200 ^tj ps pulses focused into the vitreous at a distance of 200 pm (a) and 400 /im (b) from the retina. The sections have been obtained from the specimen shown in Figure 11. refractive surgery, a key factor is to avoid endothelial damage. Our experiments show that endothelial damage occurs when 80 n] ps pulses are focused up to 130 fxm from the endothelium (Fig. 7). Damage was avoided when the distance between endothelium and laser focus was larger than 150 /xm. Fortunately, intrastromal lamellar keratectomy will give the largest change in corneal curvature if performed within the anterior part of the cornea. 16 There are nevertheless difficulties with this technique. Figures 6 to 8 demonstrate that the volume of the evaporated tissue (which approximately equals the plasma volume) is small compared to the tissue displacement caused by the expansion of the laser plasma. The laser-induced cavities disturb the initial optical properties of the cornea, making accurate focusing difficult. The bubbles from earlier pulses reflect and refract the light of the subsequent laser pulses. This can lead to the formation of multiple plasmas and, thus, to multiple cavities, especially when the laser pulse energy is relatively high. The cavities are often distributed in an irregular manner (Fig. 8), and it is not always possible to evaporate a homogenous tissue layer of definite thickness. This re- duces the likelihood of obtaining a predictable change in corneal curvature. Cataract fragmentation. The picosecond Nd:YAG laser has been suggested as a method to fragment and then aspirate a cataract through a small capsular opening while retaining the lens capsule. 16 This allows filling of the capsular bag with a clear material to restore lenticular function. 18 " 20 Ultrasonic phacoemulsification is difficult to use with hard nuclear cataracts and requires a 2.5 to 3 mm incision. 21 Both the ns laser and the mode-locked ps pulse trains need up to 10 mj to fragment dense cataracts, 22 " 25 and even at these high energies the probability of breakdown is still only 40%. 24 We observed that ns pulses up to 21 mj pulse energy were required to produce a plasma in the posterior third of cataractous human lenses (Fig. 3). At such high pulse energies, there is a substantial risk of capsule rupture from cavitation bubble expansion, even when the laser pulses are focused well away from the capsule. The use of single ps pulses of much lower energy offers a solution to this problem. By ps pulses of only 3 mj, plasmas were produced in any part of all nuclei tested, and less than 1 mj was needed for 48% of the 23 lens nuclei investigated. Because the fragmentation of lens material requires a large number of laser exposures, a high repetition rate of about 1 kHz, together with a scanning system, would be necessary to allow application of the laser exposures in a reasonable time. Vitreoretinal surgery close to the retina. Various clinicians have reported on vitreoretinal surgery with ns Nd:YAG laser pulses 426 " 32 and with mode-locked ps pulse trains. 31 They have observed that when applying pulse energies of 3 to 5 mj, there has to be a distance of about 3 mm between the application site and the retina to avoid retinal and choroidal hemorrhage. 4 ' 2G - 28 ' 31 This result agrees well with experimental studies on rabbit eyes. 33 " 33 By employing ps pulses, it will be possible to treat disorders much closer to the retina because the retinal damage range for ps pulses with 200 /uj pulse energy was found to be only 0.5 mm (Figs. 10, 11). Near the optical axis of the eye, the damage range may be even further reduced by using pulse energies closer to the breakdown threshold of about 35 /xj. A minimal damage range of approximately 200 juj seems to be achievable, considering that the range is proportional to the cube root of the laser pulse energy. 7 ' 8 This may render epiretinal membranotomy possible, as well as the dissection of vitreous strands in the vicinity of the retina. In the periphery, the focal spot is degraded by aberrations caused by the oblique passage of the laser light through the optical system of the eye. This leads to an increase of the threshold energy for plasma formation and establishes a need to use higher pulse energies. This problem is,
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Intraocular Photodisruption With Picosecond and Nanosecond Laser

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