Biomedical Engineering Reference
In-Depth Information
sites on the upper lip of 20 patients to receive either two resur-
facing passes with the UltraPulse CO
2
laser alone or a combi-
nation of two passes with the UltraPulse CO
2
laser (300 mJ,
95 W, CPG density of 6, 7.5 J/cm
2
, and a spot diameter of
2.25 mm) followed by three passes with the Er:YAG laser
(7-mm spot, 5.2 J/cm
2
). Reduced duration of postoperative
crusting (6.5 vs. 7.4 days), edema (6.0 vs. 6.3 days), and pruri-
tus (4.8 vs. 5.5 days) was found in areas receiving the com-
bined CO
2
-Er:YAG laser treatment. Medium-to-deep rhytides
were improved 70% with both treatments, and there were no
occurrences of permanent adverse side effects (32).
distance between the tissue and the handpiece will then deter-
mine the fl uence delivered, and the handpiece may be moved
closer to or farther from the tissue depending on the amount
of tissue ablation desired. This is best done holding the hand-
piece at an acute angle to tissue in a manner such that tissue
elevations above the desired plane are preferentially irradiated,
thereby decreasing the risk of cutting holes or troughs in the
tissue (Fig. 6.40).
Treatment using these techniques will result in maximal
smoothing of tissue with minimal residual thermal damage.
However, a second consequence of minimal thermal damage
is that blood vessels are not cauterized, and therefore bleeding
is encountered. Generally, the fi rst dermal pass may result in
scattered pinpoint bleeding at most as small dermal capillaries
are encountered in the papillary dermis (Fig. 6.41). After an
additional one to three passes, larger vessels are reached,
which results in more signifi cant bleeding requiring applica-
tion of pressure. Bleeding that is brisk enough to interfere
with treatment is usually encountered only with rhinophyma.
In contrast to the smooth, glistening appearance of the papil-
lary dermis, ablation into the reticular dermis typically pro-
duces an uneven surface as sebaceous gland lobules become
visible (272).
Although many physicians using the Er:YAG laser do not
wipe away the fi ne tissue debris on the surface of treated skin
between passes, using dry to damp gauze for this purpose is
benefi cial. By wiping this fi ne layer away, the laser has direct
access to tissue to be treated, and the physician can more easily
and more accurately visualize the anatomic location of the
plane being treated. Wiping is particularly useful when per-
forming epidermal passes, because it allows better identifi ca-
tion of epidermis and dermis. Fragments of residual epidermis
or lesional tissue that remains on the surface may be wiped
cleanly away, avoiding the need to ablate these spots of tissue
and providing a more even plane of tissue to be treated on the
next pass.
Surgical Technique with the Er:YAG Laser
The depth of ablation is a function of the pulse energy and spot
size or fl uence. With the same pulse energy, a smaller spot size
results in a higher fl uence and therefore a greater depth of vapor-
ization. However, a smaller spot size also results in a tendency to
create holes and an uneven texture. Also, the repetition rate and
the pulse energy are inversely related, so high pulse energy limits
the repetition rate, at least within the limits of a 20-W laser. Vir-
tually, all systems that are currently available have an upper-limit
repetition rate of 10-20 Hz, which limits the value of a pulse-
scanning system, because these repetition rates can be manipu-
lated by hand accurately by experienced laser surgeons.
Approximately, 2-5
m of tissue per J/cm
2
are ablated per
pass with currently available Er:YAG laser systems. In contrast
to CO
2
laser resurfacing, ablation depth does not diminish
with each subsequent pass because the zone of residual
thermal damage left behind by the Er:YAG laser is minimal
(20-50
μ
m) (2).
Most often a spot size of 3-5 mm with pulse energy 1-2 J
(fl uences of 5.1-10.2 J/cm
2
) is used. If a collimated beam is
available, it is generally preferable at this stage. A repetition
rate that can be easily handled is chosen (5-10 Hz), and spots
are placed with 10-30% overlap. Therefore, with pulse ener-
gies of 1-2 J and a 5-mm spot size (fl uences of 5.1-10.2 J/cm
2
),
two to three passes with a short-pulsed Er:YAG laser are gener-
ally required to completely vaporize the epidermis (Fig. 6.39).
It is much more diffi cult to ascertain when the epidermis is
completely removed with the erbium laser compared with the
CO
2
laser, because the epidermis does not separate with a sub-
epidermal vesicle as it does with the CO
2
laser. To determine
the depth of vaporization into the skin, it is best to cover the
skin in an orderly manner with relatively uniform passes
rather than to treat with a random airbrush-type technique.
This type of technique must be used cautiously because it
leads to uneven results with a much greater potential of inad-
vertent, deeper vaporization. Working in repetitive single
passes allows better control and knowledge of depth.
It is best to remove the epidermis completely over the whole
area to be treated and then to assess the dermal irregularities.
Almost any patient being treated for photodamage will require
some degree of dermal ablation, in addition to removal of the
epidermis, because the textural irregularities and wrinkling
originate in this layer. One to three passes are done in an even
manner, just as was done with epidermal removal, with each
pass ablating an additional 15-30
μ
Complications of Cutaneous Resurfacing
with the Short-Pulsed Er:YAG Laser
Overall, the side effect profi le of the short-pulsed Er:YAG laser
has been found to be considerably more favorable than that of
the CO
2
laser. As with CO
2
laser resurfacing, potential compli-
cations of resurfacing with the short-pulsed Er:YAG laser
include erythema, dyspigmentation (hyperpigmentation and
hypopigmentation), scarring, and infection.
Prolonged Erythema
Prolonged erythema (greater than 4 months' duration), reported
in up to 20% of patients after CO
2
laser resurfacing, has a much
lower incidence after resurfacing with the short-pulsed Er:YAG
laser. The intensity of the erythema correlates well with the
depth of ablation and degree of residual thermal damage, hence
its lower incidence when the short-pulsed Er:YAG laser is used.
Khatri et al. performed a split-face study comparing multipass
(three to fi ve passes) resurfacing with a short-pulsed (300
s)
Er:YAG laser versus multipass (three passes) CO
2
laser resurfac-
ing. At the 2-month follow-up, they reported a 24% incidence of
erythema in the Er:YAG treated sites versus 62% for the CO
2
laser-treated sites. At the 6-month follow-up, 10% of the CO
2
laser-treated sites and none of the Er:YAG-treated sites had
μ
m of tissue.
At this point, dermal irregularities may be removed in a
tissue-sculpting mode. This is most easily accomplished with a
focused handpiece having a focal spot size of 1-2 mm. The
μ
