Ablation of Vitreous Tissue with Erbium:YAG Laser
Matthias Krause,1 Diana Steeb,2 Hans-Jochen Foth,2 Josef Weindler,1
and Klaus Wilhelm Ruprecht1
PURPOSE. Using a noncontact erbium (Er):yttrium—aluminium—garnet (YAG) laser, ablation of
vitreous was compared to distilled water in vitro.
The porcine vitreous body and distilled water were ablated in vitro at different pulse
lengths and pulse energies. Selected pulse energies were 25, 35, 45, 75, and 100 mj (pulse rate: 1
Hz; laser beam diameter at the surface of the sample: 2 mm). Pulse lengths were at 140 ± 3 /xsec,
190 ± 4 /xsec, and 240 ± 5 /xsec. The loss of weight in vitreous tissue and distilled water was
measured using precision scales and corrected for evaporation, respectively. The Mann-Whitney U
test was used to assess the significance of differences in ablation rates of water and vitreous. P <
0.05 was considered statistically significant.
METHODS.
Reproducible and constant ablation rates were found in both vitreous and distilled water
in each of 10 consecutive series of 50 laser pulses at constant laser parameters. Ablation rates per
pulse (jag/jasec) of vitreous tissue were as follows: 30 jag to 45.8 /xg (140 /xsec), 10.4 jag to 53.8
/xg (190 /xsec), and 17.9 /xg to 24.2 jag (240 jasec). The ablation rates exhibited a linear correlation
with increasing pulse energies and also with decreasing pulse lengths. Considering the pulse
lengths of 190 jasec and 240 jasec with all pulse energies tested, the ablation rates of distilled water
were significantly higher (P < 0.05) than ablation of vitreous tissue. The ablation rates at a pulse
length of 140 jasec were not significantly different. The differences per pulse were as follows: 0.5
jag to 2.1 jag (140 /xsec), 1.9 jixg to 6.0 jag (190 jasec), and 35 jLig to 8.7 /xg (240 /xsec).
RESULTS.
CONCLUSIONS. Vitreous ablation is possible using Er:YAG laser. The ablation characteristics of
vitreous have proved to be similar but not equal to that of water. (Invest Ophthalmol Vis Sci. 1999;
40:1025-1032)
I
nfrared lasers, such as neodymium (Nd):yttrium—aluminium— garnet (YAG), carbon dioxide (CO2), holmium
(Ho):YAG, and, more recently, erbium (Er):YAG, have
characteristics that suggest a valuable role in surgery. These
lasers transect and ablate tissue based on the absorption of
energy by tissue water with consecutive vaporization.1"4 Er:
YAG laser radiation of 2.94-jam has an extremely short penetration depth in water (~1 /xm) and, hence, allows tissue
ablation with micrometer precision. Er:YAG lasers have been
successfully used in dentistry,5'6 orthopedics,7 and neurosurgery.8'9 In ophthalmic surgery, Er:YAG lasers have been investigated in cornea],10"13 cataract,1114"17 and glaucoma14 surgery. In retinal and vitreous surgery, Er:YAG laser radiation
delivered by different devices to the intraocular space offers
the possibility of tractionless cutting of retina and membranes
that are difficult to cut mechanically.141819 Wolbarsht20 is
among the first to point out the advantages and possibilities of
laser-induced removal of the vitreous body. He discussed this
proposal based on the CO2 laser and the hydrogen fluoride
laser and stated that the smallest thermal damage is observed
by choosing a pulse duration of 1 jasec. Based on this work,
further research was performed.21 In the technical field, the
common Er:YAG pulse duration is within the range of several
hundred microseconds, which has proved to be suitable in
ophthalmic surgery for phacoemulsification and vitrectomy.22
Theoretical models of laser-induced vaporization also were
developed. The first models were mainly based on thermal
confinement (i.e., heat should not diffuse out from the irradiated volume of tissue), whereas more recent models included
dynamic effects like the formation of cavitation bubbles.23
With respect to laser vitrectomy, exact data of vitreous ablation
by Er:YAG laser energy are essential to evaluate optimal laser
parameters, to determine the ablation enthalpy, and to compare vitreous ablation with that of distilled water.
MATERIALS AND METHODS
From the 'Department of Ophthalmology and Eye Hospital Homburg, University of Saarland, Homburg, Germany; and the d e p a r t m e n t
of Physics, University of Kaiserslautern, Germany.
Submitted for publication May 5, 1998; revised October 22, 1998;
accepted November 19, 1998.
Proprietary interest category: N.
Reprint requests: Matthias Krause, 4 Longfellow Place, Unit 2906,
Boston, MA 02114.
Investigative Ophthalmology & Visual Science, May 1999, Vol. 40, No. 6
Copyright © Association for Research in Vision and Ophthalmology
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Er:YAG Laser
The flash lamp-pumped Er:YAG laser used for these experiments was developed at the Department of Physics of the
University of Kaiserslautern (Kaiserslautern, Germany).24 The
system was used in free-running mode with an emission wavelength of 2.94 jam. The repetition rate of the pulses was 1 Hz
and remained unchanged during the measurements. An elec1025
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Krause et al.
IOVS, May 1999, Vol. 40, No. 6
200
180
160
140
05
long pulse
T = 240 |JS
tensi
120
100
short pulse
c
T = 140US
80
middle pulse T = 1 9 0 U S
60
40
250
500
1000
750
1250
1500
1750
2000
Time ([is )
FIGURE 1. Time course of laser-light intensity of three single laser pulses measured at different pulse lengths and different pulse energies. The
shape of the curves could be reproduced at constant laser parameters. For each laser parameter varied (pulse length, pulse energy), the pulse
lengths of 140 ± 3 p.sec, 190 ± 4 /xsec, and 240 ± 5 ;u.sec were calculated from 50 consecutive laser pulses before ablation, a.u., arbitrary units;
T, laser pulse duration (full width half maximum).
tronic trigger was used to maintain constant repetition rates.
Pulse energies of 25, 35, 45, 75, and 100 mj were selected.
Because of the limited maximal voltage in the electric system,
the pulse energy of short laser pulses (140 /xsec) was restricted
to 45 mj at maximum. To control fluctuation of the pulse
energies, the laser output was monitored by a pyroelectric
detector (Coherent Components Auburn, CA) before starting
and immediately after finishing each series of measurements at
fixed laser parameters.
The length of laser pulses was selected by combining
them with the capacities of the discharge unit: 100 JU,F, 200 piF,
and 400 /xF to 140 ± 3 jixsec, 190 ± 4 jasec, and 240 ± 5 /Asec,
respectively. The pulse length was measured by an indiumarsenide photodiode (type J12D-M204-R01M; EG&G Company, London, UK). The configuration of the laser pulse was
detected by a digital multichannel storage oscilloscope (TDS
540 A; Tektronix, Wilsonville, OR). Figure 1 shows three examples of single laser pulses at different pulse lengths used for
the experiments. The shape of the curves could be reproduced
at constant laser parameters. For each laser parameter varied
(pulse length, pulse energy), the pulse lengths of 140 ± 3 /u-sec,
190 ± 4 jasec, and 240 ± 5 /xsec were calculated before
ablation from 50 consecutive laser pulses. Also, the pulse
lengths were measured after finishing the ablation measurements to ensure that there was no drift in the pulse lengths.
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Experimental Setup
A highly reflective mirror set at a 45° angle to the laser beam
was used to direct the laser beam perpendicularly into the
center of the surface of the water and vitreous (Fig. 2). The
distance between the output mirror and the surface of the fluid
in the test tube was 60 cm. The divergence of the laser beam
was 0.0018 rad. Before starting and immediately after finishing
each series of laser pulses, the diameter of the laser beam was
adjusted to 2 mm at the distance of the tissue sample.
To avoid diffraction, as may happen with an aperture,
adjustment was done by producing bleaching effects on blackened photopaper with maximum sensitivity in the infrared
range of the spectrum (Agfa classic paper; Agfa Gevaert N.V.,
Mortsel, Belgium) and measuring the diameter of the spots.
This method has been tested for accuracy by adjustment of a
round aperture of 2-mm diameter concentric to the EnYAG
laser beam. Then, the photopaper was fixed close to the back
plane of the aperture. For all laser energies and pulse lengths
tested, a round bleaching effect of 2-mm diameter with a
distinct margin was detected on the photopaper.
Preparation and Ablation of Vitreous Tissue
Fresh porcine eyes were obtained from an abbatoir and immediately stored in physiological electrolyte solution at a temper-
Vitreous Ablation with Erbium:YAG Laser
IOVS, May 1999, Vol. 40, No. 6
1027
laser resonator
D
n
\ \ mirror
tissue sample
FIGURE 2. A highly reflective mirror set at a 45° angle to die laser beam was used to direct the laser beam
peq^endicularly into the center of the surface of water and vitreous. The distance between the output mirror
and the surface of the fluid in the test tube was 60 cm. The divergence of the laser beam was 0.0018 rad.
ature of +6°C. The time interval between removing the eyes
and starting the experiments was less than 48 hours. Preparation was done by creating a 360° incision 4 mm posterior and
parallel to the limbus, using surgical knife and scissors. The
anterior part of the eye, including the ciliary body, was removed. The center of the vitreous was carefully lifted for 5 cm
with an anatomic forceps and separated by scissors from the
rest of the vitreous remaining in the posterior segment of the
porcine eye. The vitreous grasped with the forceps was immediately transferred into a test tube. In all maneuvers done,
vitreous gel was removed. No liquid drops were observed on
the surface of the vitreous gel. Considering one sample, the
procedure of vitreous removal was not repeated either in the
same porcine eye or in another eye. The procedure of vitreous
removal was performed only once to assure that vitreous gel
was removed from the same anatomic location in the core part
of the vitreous body. Two more identical test tubes were filled
with distilled water, which was kept at room temperature.
The weights of vitreous and distilled water were measured
before and after, respectively, laser treatment using precision
scales (BOSCH SAE 80/200; Jungingen, Germany; range, 0-80
g; precision, 0.01 mg). This value reflects both the net laser
ablation rate and evaporation of water. Simultaneous weight
measurements were done in a third sample containing distilled
water. No laser treatment was performed in the third sample,
and, thus, loss of weight was merely attributed to evaporation.
Distilled water was used for measuring evaporation and calculating the net ablation rates of vitreous because prior pilot
experiments comparing water and vitreous did not reveal a
significant difference regarding the loss of weight by evaporation. In both vitreous tissue and distilled water net ablation
rates were considered as the difference in the amounts of
weight loss measured with and without laser treatment, respectively. Room temperature in the laboratory was 20°C ±
2°C. The water content in the air was permanently controlled
with a hygrometer and maintained at 50% ± 5%. The variation
of water content in the air within a series of measurements at
constant laser parameters was kept at ±1%.
Weight measurements (3 single measurements) were performed immediately before and after the application of 50
consecutive laser pulses at constant laser parameters. A series
of pulses was applied instead of a single pulse to reduce
fluctuation of weight measurements. To minimize random error, each series of 50 laser pulses was performed 10 times each
in water and vitreous with experimental conditions being unchanged for the whole procedure. In particular, the samples
were not changed. Hence, each laser parameter selected was
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investigated in a cumulative series of 500 consecutive laser
pulses at constant experimental conditions. New samples were
provided immediately before starting each new series of ablation measurements at different laser parameters.
Statistics
The initial hypothesis (Ho) to be determined was that there
were no significant differences between ablation of water and
vitreous. Data were expressed as mean ± SD. The MannWhitney U test was used to assess the significance of differences in ablation rates of water and vitreous. P < 0.05 was
considered statistically significant.
RESULTS
Reproducible and constant ablation rates were found in both
vitreous and distilled water in each of 10 consecutive series of
50 laser pulses at constant laser parameters (Fig. 3). In most
measurements, SD was within 15% of the total amount of
ablated weight (Tables 1, 2). Ablation rates per pulse (/xg//xsec)
ranged from 30 fig to 53.8 jag in vitreous and from 11.7 jug to
55.7 /xg in distilled water (Tables 1, 2). Considering the pulse
lengths of 190 /xsec and 240 ptsec of all pulse energies tested,
the ablation rates of distilled water were significantly higher
(P < 0.05) than the ablation rates of vitreous tissue (Fig. 4,
Tables 1, 2). The ablation rates at a pulse length of 140 jixsec
were not significantly different. Considering a single laser
pulse, the differences of weight loss between ablation of water
and vitreous were as follows: 0.5 /xg to 2.1 /xg (140 jasec), 1.9
/xg to 6.0 /xg (190 /xsec), and 3.5 fxg to 8.7 /xg (240 /xsec). The
amount of tissue ablation both in vitreous and distilled water at
a single laser pulse exhibited a linear increase with increasing
pulse energies (Fig. 4) and also with decreasing pulse lengths
(Fig- 5).
DISCUSSION
Thermal models are commonly used to describe tissue ablation
by application of infrared laser light.2526 Investigations on the
EnYAG laser used for vitreoretinal surgery aimed at development of a model for laser-generated bubble expansion3 and a
model for predicting intravitreal transient thermal phenomena
based on intravitreal temperature measurements.'' Heat accumulation in vitreous tissue and water with application of consecutive laser pulses was theoretically avoided in the present
investigation by using a low repetition rate of pulses (1 Hz) and
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Krause et al.
IOVS, May 1999, Vol. 40, No. 6
0,0
i
50
100
150
200
•
250
i
300
•
i
•
350
i
•
400
i
450
•
r
500 550
Number of pulses
FIGURE 3- Vitreous tissue ablation in dependence on number of laser pulses applied on the surface of
tissue. For all pulse energies used, the ablation rates of vitreous exhibited a linear increase with growing
number of pulses. The data given in this figure were measured at a laser pulse duration of 240 /xsec. The
following pulse energies were used • 100 mj, T 75 mj, • 45 mj, • 35 mj, • 25 mj.
pulse lengths far exceeding the thermal relaxation time of
water Tthcrm of approximately 1 jasec at the absorption peak
near 3 jLim.27 Hence, heat diffusion away from the area of tissue
exposed to laser radiation may explain constant ablation rates
detected when laser parameters are kept unchanged (Fig. 3).
Using a CO2 laser and an Er:YAG laser, Zweig et al.28'2y measured a linear increase of ablation crater depth in gelatin
(density, 0.66 g/cm3 and 0.84 g/cm3, respectively) with number of pulses. Gijsbers et al.30 ablating colored polyacrylamide
gel sticks with an Ar+ continuous laser and Hibst31 investigating ErYAG laser ablation of hydrated agarose, porcine skin,
and porcine corneal tissue reported similar results. In the
present study, the range of ablation measured in vitreous tissue
was 3.0 jag to 53-8 /u,g per pulse for energies ranging from 25
mj to 100 mj (Table 1). The data show that significant amounts
of vitreous tissue can be ablated by Er:YAG laser. Ablation rates
increased linearly with increasing pulse energies (Fig. 4) and
also with decreasing pulse lengths (Fig. 5). A variety of methodological differences exist between studies measuring tissue
ablation (e.g., the profile of laser light emission and the type of
material ablated). However, after adjustment for different diameters of the laser beam, the amount of ablated material
calculated by simple approximation from the data of these
studies is, with the exception of the study of Ren et al.,13
within the magnitude of our results. In all studies a linear
correlation with increasing energy of laser pulses was reported
for corresponding depths of the ablation crater. For ablation of
skin tissue, Hibst hypothesized that the linear correlation
found may be based on the effect that laser radiation is not
significantly attenuated by ablation material.31 Hibst ablated
porcine corneal tissue with a normal spiking EnYAG laser
(fluence: 1.27-3.25 J/cm2; pulse length: 130 jusec). The crater
TABLE 1. Net Ablation Rates of Vitreovis Measured at Different Pulse Lengths and Pulse Energies
Pulse Length (/u,s)
140
240
190
Pulse Energy (inj)
50 Pulses
1 Pulse
25
35
45
75
100
896 ± 101
1090 ±75
1211 ± 8 5
17.9 ± 2. 0
21.8 ± 1. 5
24.2 ± 1. 7
50 Pulses
522 ±
768 ±
1033 ±
1789 ±
2690 ±
60
23
131
115
135
1 Pulse
10.4 ±
15.4 ±
20.7 ±
35.8 ±
53.8 ±
1.2
4.7
2.6
2.3
2.7
50 Pulses
150
528
821
1557
2289
±
±
±
±
±
40
30
359
125
187
1 Pulse
3-0 ± 0.8
10.6 ± 0.6
16.4 ± 7.2
311 ±2.5
45.8 ± 3.7
The amount of weight ablated is given as average ± SD, expressed as micrograms, for 50 and 1 laser pulses, calculated from 10 series of 50
laser pulses applied at constant laser parameters.
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Vitreous Ablation with Erbium:YAG Laser
IOVS, May 1999, Vol. 40, No. 6
1029
TABLE 2. Net Ablation Rates of Distilled Water Measured at Different Pulse Lengths and Pulse Energies
Pulse Length (ms)
140
240
190
Pulse Energy (mj)
50 Pulses
1 Pulse
50 Pulses
1 Pulse
50 Pulses
1 Pulse
25
35
45
75
100
998 ± 81
1117 ± 276
1266 ± 37
20.0 ± 1.6
22.3 ± 5.5
25.3 ± 5.1
722 ± 49
1032 ± 39
1335 ± 161
2021 ± 121
2786 ± 106
14.4 ± 1.0
20.6 ± 0.8
26.7 ± 3.2
40.4 ± 2.4
55.7 ±2.1
586 ± 35
856 ± 29
1120 ± 59
1808 ± 94
2463 ± 31
11.7 ± 0.7
17.1 ±0.5
22.4 ± 1.2
36.2 ± 1.9
49.3 ± 0.6
The amount of weight ablated is given as average ± SD, expressed as micrograms, for 50 and 1 laser pulses, calculated from 10 series of 50
laser pulses applied at constant laser parameters.
depth was between 7 jam and 30 /xm.31 Zweig et al.29 used an
Er:YAG laser for drilling holes in gelatin targets (density: 0.84
g/cm3; calculated fluence: 128-442 J/cm2). Gailitis et al.15
drilled holes into cataractous human crystalline lenses (fluence,
4.5-195 J/cm2; pulse length, 250 ptsec). Ren et al.13 ablated
corneal tissue with a normal spiking EnYAG laser in 10 calf
eyes. Approximation of ablated weight of tissue was not possible based on the data given in the text. A survey of further
studies evaluating laser ablation of different tissues is given by
Hibst.31 Data of vitreous ablation are to the best of our knowledge not yet available in the literature.
20
30
Figure 6 shows the laser energy calculated for ablation of
1 g of vitreous tissue at different pulse energies and pulse
lengths in comparison to evaporation enthalpy of water (energy needed for heating water up to 100°C and consecutive
evaporation). Increasing the energy per laser pulse and shortening the pulse length reduces the overall energy required for
ablation of 1 g of vitreous tissue. Hibst31 found the energy
needed for Er:YAG laser ablation of skin being lower than the
evaporation enthalpy of water and hypothesized that besides
vapor tissue particles containing fluid may be expelled from
the target. On the basis of this hypothesis, one can assume
40
100
110
Pulse energy ( mJ )
FIGURE 4. Tissue ablation in dependence on energy of EnYAG laser pulses. For all pulse lengths
investigated, ablation rates of vitreous and distilled water exhibited linear increases with increasing pulse
energies. Considering the pulse lengths of 190 p,sec and 240 i±sec with all pulse energies tested, the
ablation rates of distilled water were significantly higher (P < 0.05) than ablation of vitreous tissue. The
ablation rates at a pulse length of 140 jusec were not significantly different, T, laser pulse duration (full
width half maximum), • long pulses (vitreous; T = 240 ^isec), • middle pulses (vitreous; T = 190 /xsec),
• short pulses (vitreous; T = 140 /xsec), • long pulses (vitreous; T = 240 jusec), O middle pulses (vitreous;
T = 190 jasec), A short pulses (vitreous; T = 140 /xsec).
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Krause et al.
IOVS, May 1999, Vol. 40, No. 6
O)
E
c
o
+3
iS
.Q
i
130
140
150
160
•
i
170
•
i
•
180
i
190
•
r
200
210
220
230
240 250
Pulselength ([is )
FIGURE 5. Vitreous tissue ablation in dependence on duration of laser pulses. For all pulse lengths used,
the ablation rates in the vitreous exhibited a linear decrease with increasing pulse length. • 100 mj, • 75
mj, • 45 mj, • 35 mj, • 25 mj.
from our investigations that rapid localized heat accumulation
during ablation with short pulses (e.g., 140 jusec) and high
energies per pulse may induce disruptive effects (Fig. 7). In
contrast, more energy is required for ablation of a certain
amount of vitreous by using pulses of longer duration (e.g., 240
or lower energies per pulse. Heat accumulation may be
2800Evaporation enthalpy
pure water
2600
->
2400-
"O
-g 2200
O
Q)
2000G)
1800LU
1600-
1400
20
30
40
80
90
100
110
Energy per pulse ( mJ )
FIGURE 6. Overall laser energy calculated for ablation of 1 g of vitreous tissue at different pulse energies and
pulse lengths. Licreasing the energy per laser pulse and reducuig the pulse length reduced the overall energy
required for ablation of 1 g of vitreous tissue. For comparison, die evaporation enthalpy of water is illustrated
by the horizontal line, T, laser pulse duration (full width half maximum), • long pulses (vitreous; T =
• middle pulses (vitreous; T = 190 jLisec), • short pulses (vitreous; T = 140 /isec).
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IOVS, May 1999, Vol. 40, No. 6
Vitreous Ablation with Erbium:YAG Laser
1031
B
FIGURE 7. Schematic illustration of a hypothesis explaining why different amounts of tissue are ablated
at different pulse energies and different pulse lengths. 25 (A) When using pulses of longer duration (e.g.,
240 jxsec) and low pulse energies for ablation of vitreous tissue, heat accumulation is less rapid and the
thermal transition zone (gray plane) is larger than pulses of shorter duration. Tissue is mainly ablated by
vaporization. (B) When using short pulse lengths (e.g., 140 jusec) or high pulse energies, rapid localized
heat accumulation may induce disruptive effects. Besides vapor, vitreous particles containing fluid are
expelled from the target. Less energy is required for ablation of a certain amount of vitreous than ablation
with longer pulses or low energies per pulse.
less rapid and the thermal transition zone larger than pulses of
shorter duration or higher energy. Tissue is mainly ablated by
vaporization. Disruption of tissue particles may occvir less
frequently.
Comparative measurement of ablation of distilled water
has been performed to prove consistency of data. The water
content of human vitreous tissue lies between 98.5% and
99.7%.32 Hence, results of ablation of distilled water were
expected to be similar to those of vitreous tissue ablation.
Actually, ablation rates of distilled water exhibited a linear
increase with increasing pulse energies and also with decreasing pulse lengths. Furthermore, as found in vitreous tissue, the
ablation rates of water were constant in consecutive series of
10 measurements (50 laser pulses each series). High power
densities and disruptive effects similar to those of vitreous
tissue may have increased the range of ablation rates measured
at 140 p,sec (pulse energy, 35 mj and 45 mj) and, hence, may
have affected the calculated SD and statistical analysis performed in comparison with vitreous tissue. Considering the
pulse lengths of 190 jixsec and 240 /xsec with all pulse energies
tested, the ablation rates of distilled water were significantly
higher than ablation rates of vitreous tissue (Fig. 4, Tables 1, 2).
Evaporation enthalpy of soluble vitreous proteins (5 kj/g33)
exceeds the amount necessary for evaporation of water (2.6
kj/g). Besides, the ultrastructural organization of the vitreous
may affect parameters of thermal laser tissue interaction (e.g.,
absorption coefficient, heat conductivity, and heat capacity).
The water of the vitreous is stabilized by an ordered meshwork
of very fine collagen fibrils that are tied together in loosely
parallel bundles or sprays by bridges of sulfated glycosaminoglycan.34"36 The glycosaminoglycans of the vitreous (hyaluronan and chondroitin sulfate) aggregate with themselves and
with each other in solution. The protein cores of the proteoglycans are attached to collagen fibrils.34'35
Pulse energies lower than 25 mj were not used because
prior pilot measurements did not reveal a loss of weight for a
pulse energy of 15 mj (pulse length, 240 jasec). A significant
loss of weight by ablation may exist that is, however, within
the SD of measurements. Another possible explanation is a
steep decrease of ablation with pulses below a certain threshold energy. Investigation of laser pulse energies lower than 25
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mj may require a different methodological approach and, thus,
was not part of this study. Pulse energies were also limited to
100 mj because considerable visual and acoustic signs of shock
waves were observed for higher energy levels (e.g., 125 mj),
and, thus, such energies are inappropriate for use in intraocular
surgery because they may lead to severe injury of ocular tissue.
However, future quantitative measurements of shock waves
and in vivo investigation, including histology, may provide
detailed data on this topic.
To the best of our knowledge this is the first time laser
ablation of vitreous tissue has been quantified. Noncontact
ablation was chosen instead of placing a laser fiber tip beneath
the surface of material to avoid recondensation interfering with
measurement of ablation. Ablation characteristics of the vitreous have proven to be similar but not equal to those of water.
The distinct physical and chemical properties may explain the
significantly lower ablation rate of vitreous tissue than distilled
water. Further investigation is required to measure ablation
using fibers in contact with tissue.
Acknowledgment
The authors thank Stephan Gotzinger for scientific support concerning
temperature-related effects of Er:YAG laser radiation.
References
1. Walsh JT, Deutsch TF. EnYAG laser ablation of tissue: measurement of ablation rates. Lasers Surg Med. 1989;9:327-337.
2. Walsh JT, Flotte TJ, Deutsch TF. EnYAG laser ablation of tissue:
effect of pulse duration and tissue type on thermal damage. Lasers
SurgMed. 1989;9:3l4-326.
3. Berger JW, D'Amico DJ. Modeling of erbium:YAG laser-mediated
explosive photovaporisation: implications for vitreoretinal surgery. Ophthalmic Surg Lasers. 1997;28:133-139.
4. Berger JW, Bochow TW, Talamo JH, D'Amico DJ. Measurement
and modelling of thermal transients during EnYAG laser irradiation
of vitreous. Lasers Surg Med, 1996;19:388-396.
5. Kermany O, Lubatschowski H, Asshauer T, et al. Q-Switched CTE:
YAG (2,69 M-m) laser ablation: basic investigations on soft (comeal)
and hard (dental) tissues. Lasers Surg Med. 1993; 13:537-542.
6. Visuri SR, Walsh JT, Wigdor HA. Erbium laser ablation of dental
hard tissue: effect of water cooling. Lasers Surg Med, 1996; 18:
294-300.
1032
Krause et al.
7. Zahn H, Jungnickel V, Ertl T, Schmid S, Miiller G. Knochenchirurgie mit clem ErYAGLaser. Lasermedizin. 1997;13:31-36.
8. Cubeddu R, Sozzi C, Taroni P, Valentini G, Bottiroli G, Croce AC.
Ablation of brain by Erbium laser: study on dynamic behavior and
tissue damage. Proc SPIE. 1994;2077:13-20.
9. Fischer JP, Dams J, Gotz MH, et al. Plasma-mediated ablation of
brain tissue with picosecond laser pulses. Appl Physiol. 1994;58:
493-499.
10. Peyman GA, Badaro RM, Khoobehi B. Corneal ablation in rabbits
using an infrared (2.9-M-m) Erbium: YAG laser. Ophthalmology.
1989;96:1160-1170.
11. Bende T, Seller T, Wollensak J. Photoablation mit dem ErYAG Laser
an okularen Geweben. Fortschr Ophthalmol. 1991;88:12-l6.
12. Belgorod BM, Ediger MN, Weiblinger RP, Erlandson RA. Tangential
corneal surface ablation with 193- and 308-nm Excimer and
2936-nm Erbium-YAG laser irradiation. Arch Ophthalmol. 1992;
110:533-536.
13. Ren Q, Venugopalan V, Schomacker K, et al. Mid- infrared laser
ablation of the cornea. Lasers Surg Med. 1992;12:274-281.
14. Peyman GA, Katoh N. Effects of an erbium: YAG laser on ocular
structures. Int Ophthalmol. 1987;10:245-253.
15. Gailitis RP, Patterson SW, Samuels MA, Hagen K, Ren Q, Waring
GO III. Comparison of laser phacovaporization using the ErYAG and the Er-YSGG laser. Arch Ophthalmol. 1993; 111:697700.
16. Ross BJ, Puliafito CA. ErbiunrYAG and holmium: YAG laser ablation of the lens. Lasers Surg Med. 1994;15:74-82.
17. Wetzel W, Brinkmann R, Koop N, Schroer F, Birngruber R. Photofragmentation of lens nuclei using the EnYAG laser: preliminary
report of an in vitro study. GerJ Ophthalmol. 1996;5:281-284.
18. Brazitikos PD, D'Amico DJ, Bernal MT, Walsh AW. Erbium. YAG
laser surgery of the vitreous and retina. Ophthalmology. 1995; 102:
278-290.
19. D'Amico DJ, Brazitikos PD, Marcellino GR, Finn SM, Hobart JL.
Initial clinical experience with an ErbiunrYAG laser for vitreoretinal surgery. Am J Ophthalmol. 1996;121:4l4-425.
20. Wolbarsht ML. Laser surgery: CO 2 or HF'. / Quant Electron. 1984;
20:1427-1432.
21. Wolbarsht ML. New thoughts on vitreous surgery. Lasers Ophthalmol. 1986; 1:73-81.
Downloaded From: http://iovs.arvojournals.org/ on 06/17/2017
IOVS, May 1999, Vol. 40, No. 6
22. Mrochen M, Petersen H, Wiillner C, Seiler T. Experimentelle Ergebnisse zur Erbium:YAG Laservitrektomie. Klin Monatsbl Augenheilkd. 1998; 212:50 -54.
23. Frenz M, Pratisto H, Ith M, et al. Transient photoacoustic effects
induced in liquids by pulsed erbium lasers. Proc SPIE. 1994;2134A:
402-411.
24. Steeb D. IR-Laserablation: Abtrag von Glaskorpergewebe durch
Erbium: YAG Laserstrahlung [diploma thesis), Kaiserslautern,
Germany: Universitat Kaiserslautern; 1997.
25. McKenzie AL. How far does thermal damage extend beneath the
surface of CO 2 laser incisions? Phys Med Biol. 1983;28:905-912.
26. Partovi F, Izatt JA, Cothren RM, et al. A model for thermal ablation
of biological tissue using laser radiation. Lasers Surg Med. 1987;
7:141-154.
27. Niemz MH. Laser-Tissue Interactions. New York: Springer; 1996:
68-76.
28. Zweig AD, Weber HP. Mechanical and thermal parameters in
pulsed laser cutting of tissue. IEEE J Quantum Electron QE.
1987;23:1787-1792.
29. Zweig AD, Frenz M, Romano V, Weber HP. A comparative study of
laser tissue interaction at 2.94 /xm and 10.6 /im. Appl Phys B.
1988;47:259-265.
30. Gijsbers HM, Selten FM, van Gemert MJC. CW laser ablation velocities as a function of absorption in an experimental one-dimensional tissue model. Lasers Surg Med. 1991;11:287-296.
31. Hibst R. Technik, Wirkungsweise und medizinische Anwendungen von Holmium- und Erbium-Lasern. Landsberg: EcomedVerlag; 1996:21-76.
32. Eisner G. Clinical anatomy of the vitreous. Duane's Ophthalmology
on CD-ROM. [Monograph on CD-ROM]. Corporate Technology
Ventures, Producers. Philadelphia: JB Lippincott; 1997.
33. Valderrama GL, Menefee RF, Krenek BD, Berry MJ. Chemical laser
interaction with human corneal tissue. Proc SPIE. 1989;1064:135-l45.
34. Scott JE. Extracellular matrix, supramolecular organisation and
shape.JAnat. 1995;187:259-269.
35. Scott JE. The chemical morphology of the vitreous. Eye. 1992;6:
553-555.
36. Funk RHW, Apple DJ, Naumann GOH. Embryologie, Anatomie und
Untersuchungstechnik. In: Naumann GOH, ed. Pathologie des
Auges. New York: Springer; 1997:44-46.