Intraocular Photodisruption With Picosecond and Nanosecond Laser

Tissue Effects of Picosecond and Nanosecond Photodisruption 3035
from a slaughterhouse. A central portion of the cornea
was excised using a 10 mm trephine. Each specimen
was mounted on a teflon holder and immersed in
a cuvette with physiological saline. The specimen
could be precisely positioned with an xyz-micrometer
stage. To produce incisions in Descemet's membrane,
the laser pulses were applied from the endothelial side
with a repetition rate of 10 Hz, when the specimen was
moved laterally with the translation stage (Fig. 2a). We
compared the effects of ps pulses with 50 n] pulse
energy to those of ns pulses with a pulse energy of 1
mj. Both energies are close to the threshold for plasma
formation at the respective pulse duration.
Intrastromal corneal laser effects were investigated to
analyze the feasibility of intrastromal refractive surgery
with ps pulses. Enucleated bovine eyes (n = 6)
were mounted into an eye holder fixed on a translation
stage. Initially, the optical axis of the eye was coaxial to
the laser beam axis, and the laser pulses were focused
through the cornea onto the endothelium (see Fig. 2b,
which shows an enlarged view of the central cornea).
The eye was then moved laterally while ps pulses with
80 ^J pulse energy and 10 Hz repetition rate were
applied. The laser effects produced this way were located
at an increasing distance from the corneal endothelium
because of the curvature of the cornea. This
allows an accurate determination of the range for endothelial
damage from serial sections through the
laser effects.
To compare the suitability of ps and ns pulses for
cataract fragmentation, laser pulses were focused into
a)
FIGURE 2. Irradiation geometries for the investigation of tissue
effects produced by picosecond and nanosecond
Nd:YAG laser pulses, (a) Incision of Descemet's membrane
in a corneal specimen, (b) Corneal instrastromal tissue evaporation
in an enucleated eye (enlarged view), (c) Cataract
emulsification. (d) Focusing into the vitreous body close to
the retina.
the clear lenses of whole enucleated bovine eyes and
into human lens nuclei obtained by extracapsular cataract
extraction. One hundred microjoule ps pulses
and 1.1 mj ns pulses were focused into the bovine
lenses (n = 11) at a distance of about 4 mm behind the
anterior lens capsule (Fig. 2c), generating a series of
parallel lines. The laser effects were documented by
slit lamp photography. The human lens nuclei were
stored in saline at 4°C for a maximum of 24 hours
until the experiment was performed. The lenses (n =
23) had varying degrees and types of cataract. After
removing any cortical remnants, the lens nuclei were
gently clamped in a holder and immersed into a cuvette
filled with saline, and ps and ns laser pulses were
focused into the posterior third of the nuclei. The
threshold for plasma formation was determined as described
above in the clearest and in the most turbid
parts of the lens nuclei.
To measure the range for retinal damage during vitreous
surgery with ps pulses, we used the irradiation
geometry shown in Figure 2d. Bovine eyes (n = 7) were
hemidissected and mounted on an xyz-translation
stage, whereby traction on the retina was avoided as
much as possible. Series of ps laser pulses with a pulse
energy of 200 /xj each were then focused into the vitreous
at various well-defined distances from the internal
limiting membrane, where the retinal vessels are located.
The distance between the retinal vessels and the
laser focus was controlled with the micrometer translation
stage. The precise control of this distance was
facilitated by the eyecup preparation used, allowing
optimal illumination and observation of the retina.
Histology. After laser exposure, all corneal specimens
were fixed in 4% glutaraldehyde, postfixed in
buffered 2% osmium tetroxide, and dehydrated in alcohol.
The specimens with intrastromal laser effects
were put into propyleneoxide and then embedded in
epon 812. Series of semithin sections (1.5 ixm) were
stained with toluidine blue for light microscopy. The
specimens with incisions in Descemet's membrane
were critical point dried in CO2 and sputter coated
with gold. The surface morphology of the lesions was
studied using a scanning electron microscope (JSM
35, JEOL, Tokyo, Japan). After scanning electron microscopy,
the specimens were put into 100% ethanol,
then in propyleneoxide, and later they were embedded
in epon 812 for the preparation of semithin sections.
In this way, the surface morphology and histologic
sections of the same corneal lesions could be investigated.
For analysis of the laser effects in the retina, the
whole hemidissected globe was fixed in a 4% glutaraldehyde
solution for at least 24 hours. Afterward, most
of the vitreous was removed, and retinal specimens
containing the lesions were prepared. They were further
processed for histology as described above for the
corneal specimens.