Biomedical Engineering Reference
In-Depth Information
(
A
)
(
B
)
Figure 6.35
(
A
) Treatment of the neck in the same manner as the face may result in severe scarring. (
B
) Treatment goals must be much more conservative, but even
a single pass with the same laser parameters used for the face may result in scarring of the lower neck.
have undergone resurfacing after a lower-lid blepharoplasty
without stabilizing the lateral canthal tendon. It may also
occur if laser resurfacing is performed too aggressively in this
region without attention to the laser-tissue interaction.
To minimize the occurrence of ectropion, we recommend
that the patient's skin elastic recoil be tested, the so-called snap
test, with close observation for loose tissue folds and the effect
on the lid margin when tightened. If the lid margin moves eas-
ily, close observation during the procedure is essential to avoid
excessive tightening of the lid. In addition, laser density should
not exceed 20-30% in this region to limit nonspecifi c thermal
damage of dermal tissue, and only one or two passes should be
performed with careful attention to the tightening effect. The
cheeks should be treated before the periorbital area so that the
additive tightening effects of this area are known before peri-
orbital resurfacing. Scleral show was found in 3% of patients
less than 4 months postoperatively and in 2% more than
4 months postoperatively in a report of 1000 procedures (231).
Ectropion occurred in 0.3% of patients in this series.
resurfacing typically have a bell-shaped Gaussian laser beam
profi le (232).
The Er:YAG laser produces light in the near-infrared (NIR)
portion of the electromagnetic spectrum at 2.94 mm. The
broad water-absorption band extends from just under 2 mm
to beyond 10 mm, ensuring superfi cial absorption of NIR
light. The Q-switched Er:YAG laser ablates approximately
15-20
μ
m of skin and leaves such a thin layer of thermal dam-
age (5
m) that it is not hemostatic. The Er:YAG laser has been
investigated using a Q-switched pulse of 90
μ
s, but pulse-to-
pulse instability and a low-intensity tail of a Gaussian beam
have made this beam profi le undesirable. Instead, the laser is
used in its normal-spiking mode, emitting a macropulse of
approximately 250
μ
s, made up by a train of 1-ms micropulses
with pulse-to-pulse stability of ±2% (233). The Er:YAG laser
was fi rst studied in this mode to determine whether it could be
an effective resurfacing device.
When the coeffi cient of absorption for water is compared
directly, that for the CO
2
laser (10.6 mm) is approximately
790
μ
m
−1
, whereas the erbium laser peak at 2.94 mm is approxi-
mately 13,000
μ
Synechiae
Synechiae are adhesions that occur when two adjacent areas of
de-epithelialized skin are in contact with each other in a fold
and a bridge of epithelium develops over the top of the fold.
This occurs primarily on the lower eyelid and has the appear-
ance of an unusual crease or a faint white line 1-2 weeks post-
operatively. Treatment consists of cutting the epidermal bridge
with a fi ne-tipped scissors, lancet, or scalpel. The patient then
must carefully roll a moist cotton-tipped applicator over the
area frequently to avoid recurrence. Synechiae almost always
resolve without problems (231).
m
−1
, more than 16 times greater than that of the
CO
2
laser (Fig. 6.36). This results in its energy being absorbed
much more readily in a thinner layer of tissue than with the CO
2
laser. In fact, calculations of the absorption coeffi cient, assum-
ing tissue to be 70% water, show this energy to be absorbed in
about 1
μ
m of tissue (11,24,233-239). This results in effi cient
tissue ablation with very little scattering of the beam and mini-
mal residual thermal damage. However, actual clinical and
experimental data reveal deeper tissue penetration than this cal-
culated optical penetration (239).
The ablation threshold for the Er:YAG laser is about
1.5 J/cm
2
(233,235,240), and the ablation effi ciency about
2-3
μ
e
:yag laser
Er:YAG Laser-Tissue Interaction
The short-pulsed Er:YAG laser is a fl ashlamp-pumped YAG crys-
tal laser system doped with atoms of the element erbium. Laser
energy is generated within a cavity containing the fl ashlamp-
excited YAG crystal rod, mirrors at each end, and a cooling
system. On exiting the cavity, the laser light is focused into a
beam delivery system that typically incorporates an articulated
arm, which allows the use of handpieces capable of producing
highly collimated beams. Er:YAG lasers used in cutaneous
r
m/pulse/J/cm
2
up to about 10 J/cm
2
(24,233,235,237).
However, ablation rates per pulse of 16 (235), 30 (238), and
400
μ
m (at 80 J/cm
2
) (233) have been reported, which seem
inconsistent with the reported optical penetration depth of the
Er:YAG laser of about 1
μ
m.
This deeper ablation process occurs because ablation at
2.94
μ
m is an explosive process caused by rapid heating, vapor-
ization, and consequent high-pressure expansion of irradiated
tissue (233,239). Explosive particle ejection occurs when a
gradient exists between the atmospheric pressure of the
μ
