Intraocular Photodisruption With Picosecond and Nanosecond Laser

Tissue Effects of Picosecond and Nanosecond Photodisruption 3039
b
FIGURE 7. Semithin sections through picosecond laser effects
within the corneal stroma. The pulse energy was 80 jtj.
The distance between laser focus and corneal endothelium
was 130 nn\ in (a) and 60 fim in (b). Endothelial damage is
marked by arrowheads. The scales correspond to a length of
100/ini.
was located at the level of the retinal pigment epithelium.
Focusing above a retinal vessel led to a hemorrhage
when the distance between the vessel and the
laser focus was less than 300 fj.m. When the pulses were
focused onto the retinal pigment epithelium or slightly
behind it, no macroscopic vessel damage could be observed,
even when the laser focus was located directly
below a vessel (arrow in Fig. 10). Figure 11 shows histo-
logic sections through retinal lesions arising when the
distance Ax between the laser focus in the vitreous and
the vitreoretinal interface was 200 ^m (Fig. 11a) and
400 fim (Fig. lib), respectively. In both cases, damage
to the neuroretina and the outer nuclear layer, probably
caused by the cavitation bubble dynamics, is visible.
For Ax > 500 /mi, no retinal damage was observed.
This means that the damage range is about half a millimeter
at a laser pulse energy of 200 /*J.
DISCUSSION
Thresholds for Plasma Formation
Table 1 shows that the threshold energy for plasma
formation in clear ocular media is, on average, 12
times lower for ps pulses than for ns pulses. This
agrees quite well with the factor of 13 reported by us
for distilled water. 6 Docchio et al 9 presented similar
values: a factor of 10 for calf vitreous and of 14 for
distilled water. The variation in the absolute values of
the breakdown thresholds for different tissues observed
at constant pulse duration can partly be attributed
to different tissue properties, and to some extent
to the different experimental configurations used for
the investigation of the various tissues (see Fig. 2).
The breakdown thresholds in cataractous lens nuclei
(Fig. 3) are always independent of the pulse dura-
FIGURE 8. Intrastromal cavities produced by a series of ps
pulses. The laser light was incident from the right. The pulse
energy was 80 /xj in (a) and 300 n] in (b). The pulses were
applied with 10 Hz repetition rate when the corneal specimen
was moved in a vertical direction. The scale represents
0.5 mm. The photographs were taken immediately (