Biomedical Engineering Reference
In-Depth Information
In contrast to the CO
2
laser, the Er:YAG laser has 16 times
greater affi nity for water and a signifi cantly lower tissue abla-
tion threshold (1.6 J/cm
2
), which allows the Er:YAG to be
operated at 8-10 times above its ablation threshold in most
resurfacing applications (6). Therefore, most of the energy
delivered with the Er:YAG laser is utilized to ablate rather than
heat the tissue, and residual thermal damage zones not exceed-
ing 30-50
CO
2
Excimer
Argon KTP
Nd
Ho
Er
0.0
CO
2
1.0
Er
0.00
Pigmented
tissue
0.01
0.02
2.0
3.0
Comparison
of
Physical
Parameters
for
Er:YAG
and
CO
2
4.0
100.000
10.000
m have been confi rmed both in vitro and in vivo
with the Er:YAG laser (23,24,235,238-240).
Given the narrow zone of residual thermal damage produced
by the Er:YAG laser, tissue desiccation does not signifi cantly
increase with each subsequent pass and the ablation plateau
characteristic of CO
2
laser resurfacing is not reached. In con-
trast to the coagulative, desiccating effect of the CO
2
laser on
dermal blood vessels and dermal tissue, the Er:YAG laser causes
vasodilation of dermal blood vessels and transudation of fl uid
that maintains a high water content in the target tissue and
allows the Er:YAG laser to continue with effi cient ablation (10).
The absence of tissue coagulation, however, results in bleeding
as the vessels of the superfi cial dermal plexus are severed. This
may limit the depth of ablation that is achievable (23,24,240).
Clinical use of high-fl uence, small-diameter beams is unde-
sirable, because it may result in an irregular surface because of
the deep instantaneous ablation.
This energy is delivered with a pulse duration that is far
below the 1-ms thermal relaxation time calculated for that
layer of human skin heated by the pulsed CO
2
laser, with the
pulse generally in the range of 250-350
μ
Melanin
1.000
100
Water
Hemoglobin
10
1.0
Oxyhemoglobin
0.1
0.01
0.001
0.0001
0
.
2
1
.
0
1
0
2
0
Wavelength (Microns)
Figure 6.36
Er:YAG laser wavelength of 2.94
μ
m is absorbed 16 times more
readily than that of CO
2
laser.
environment and pressure within tissue. When tissue water is
evaporated and prevented from escaping from tissue fast
enough, tissue pressures may exceed atmospheric pressures
when temperatures exceed 100°C. Because the tissue surface is
cooled by expansion of the vapor, temperature and pressure
may then be higher in a subsurface layer. The steam pressure of
water increases rapidly with increasing temperature. Indeed,
these high-temperature, high-pressure gases forming at the
absorption site are thought to result in explosive tissue removal
(233,239). High-speed photography has shown ablated mate-
rial to leave the surface at supersonic velocities (103 m/s) (241).
The ablation of tissue during the pulse of the Er:YAG laser is
a dynamic process during which material heated at the begin-
ning of the pulse is removed during the pulse, clearing a path
for radiation to be deposited deeper in tissue during the same
pulse (233). High-speed photography has shown that tissue
removal begins within 200
s. However, because
of the short penetration depth of the 2.94-mm wavelength, the
laser-heated layer of tissue is only 1-
μ
m thick and this layer has
a thermal relaxation time of approximately 1
μ
μ
s (89,239,242).
Single pulses of less than 1-
s duration are needed to mini-
mize thermal diffusion during the laser pulse. In the normal-
spiking mode, the Er:YAG laser emits approximately twenty
1-
μ
s after the beginning of the pulse
(242). The explosive ejection of this ablated material removes
it from the beam pathway. Furthermore, water vaporized by
the pulse has a much lower coeffi cient of absorption than does
liquid water and therefore does not interfere with the beam,
because it is essentially transparent (233,239). The explosive
ejection of ablated material from the tissue surface causes the
characteristic “popping” sound heard during Er:YAG laser skin
resurfacing.
Interestingly, lower fl uences (less than 10 J/cm
2
) of a thermal
energy analyzer CO
2
laser using 2-
μ
μ
s micropulses in a macropulse burst of approximately
200
s. Because of effi cient tissue vaporization and interpulse
cooling time, minimal thermal damage occurs. Each micro-
pulse can ablate tissue and act independently (232,243), but in
general the thermal damage from normal-spiking-mode irra-
diation is more extensive than that of Q-switched irradiation.
Most erbium lasers now available can deliver 1-3 J per pulse,
at repetition rates of 1-10 Hz, so very high irradiances may be
achieved, particularly with a focused beam. Although physi-
cians and manufacturers typically report results as pulse
energy and spot size, it would be much more meaningful and
allow easier comparison if treatment parameters were dis-
cussed in fl uence (J/cm
2
) (Table 6.8).
Thermally induced dermal collagen contraction is thought to
underlie the superior clinical results achieved with the CO
2
laser.
In contrast to the CO
2
laser (15-40% reported intraoperative
collagen contraction), fi rst-generation low-power short-pulsed
Er:YAG lasers did not produce signifi cant tissue contraction.
The development of a second generation of more powerful
Er:YAG lasers allowed investigators to demonstrate small but
measurable degrees of tissue contraction after Er:YAG laser skin
resurfacing. In 1998, Hughes et al. demonstrated an immediate
4% linear tightening of the skin immediately after two or three
passes with a short-pulsed (250
μ
s pulses result in the same
ablation effi ciency, supporting this instantaneous vaporiza-
tion model (233). This in-depth absorption explains the tear-
ing of tissue seen in vascular tissue, the large ablation depths
per pulse, and the zones of residual thermal damage that
increase greatly at high fl uences (233,239).
The CO
2
laser is operated near its tissue ablation threshold
(5 J/cm
2
) in most resurfacing applications. This means that a
large fraction of its energy is invested to heat, rather than
ablate the target tissue. Therefore, the CO
2
laser produces rela-
tively large residual thermal damage zones of up to 150
μ
m
and causes signifi cant desiccation of the target tissue after only
a few passes. With each subsequent pass of the CO
2
laser, the
amount of vaporized tissue diminishes while the extent of
thermal necrosis increases and a “plateau” of ablation is typi-
cally reached after the fourth pass.
μ
s) Er:YAG laser (5-mm spot
diameter, fl uences of up to 4.6 J/cm
2
), which increased to a max-
imum of 14% 16 weeks postoperatively (244).
μ
