Biomedical Engineering Reference
In-Depth Information
(
A
) Micro ablative column (MAC)
(
B
) Micro necrotic column (MNC)
Figure 7.5
(
A
) Ablative and (
B
) nonablative fractional wounds with same “wounded” volumes and dimensions.
greater cosmetic outcomes than 100% coagulative injuries
(nonablative fractional) with the same geometries (Fig. 7.5),
the counterpoint would be that microwounds that are about
70:30 ratios of ablation:coagulation seem to create longer heal-
ing times but possibly better cosmetic outcomes than those
where either (
i
) the ratio of ablation to coagulation might be
90:10 (i.e., fractional Er:YAG) or (
ii
) only coagulative microin-
juries (also known as nonablative fractional wounds) are cre-
ated. There is considerable variation in parameters between
different devices, making comparisons diffi cult (8).
change in the pulse duration alters the residual thermal
damage (RTD)], changes in fractional Er:YAG pulse duration
over a range of 0.25-60 ms, at least for deeper ablation
(>500
m), did not make a great impact on RTD (or immedi-
ate postoperative hemorrhage) (13,14). On the other hand,
another study by Dierickx et al. showed an increase in RTD
from about 15 to 25
μ
m when increasing pulse duration from
0.25 to 5 ms when the same 5 mJ/mb were applied over a
180-
μ
m-diameter microspot. Ablation, on the other hand, was
25% deeper with the shorter pulse.
μ
fractionated er:ysgg lasers
nm)
The fractionated 2790-nm device achieves more thermal dam-
age than the 2940-nm laser without the depth and extent of
thermal injury of the 10,600-nm laser (9,10). Because of
dynamic changes in water absorption in this wavelength range,
the ablation dynamics are more “Er:YAG-like” than “CO
2
-like.”
The currently available platform (Pearl and Pearl Fractional,
Cutera, Brisbane, California, USA) has both confl uent and
ablative fractional settings, which can be used in the same
treatment session (11). The depth of ablation is controlled by
adjusting the microbeam pulse energy of the fractional laser
and the macrofl uence (1-3.5 J/cm
2
) of the confl uent hand-
piece. The laser assembly in this device fi ts inside the hand-
piece, and although this design adds some weight to the
handpiece, an advantage is that beam alignment issues are
mitigated and reliability increases.
(
2790
fractionated co
2
lasers
nm)
There are a number of commercially available fractional
CO
2
devices. The various designs cover a range of deploy-
ment types, microspot sizes, densities, and pulse durations
(Table 7.1 and Fig. 7.6).
Scanning handpieces are commonly used. There are two
broad types of scanners. In one case (Fraxel Re:Pair, Solta, Santa
Clara, California, USA), the microbeam is emitted via a rolling
tip and multiple passes are made (usually four) at a certain
percent coverage per pass. The multipass approach tends to
create a random distribution of microbeams that prevents
unwanted patterning or hot or cold spots within the scanned
areas. The primary advantage to using a random scanning
delivery system is to avoid the potential issues of bulk heating
as well as avoid patterning of the microbeams. One popular
CO
2
fractional laser (Deep FX, Lumenis, San Jose, California,
USA) scans a “pattern” of microbeams over a certain area after
which the operator moves the handpiece to an adjacent spot
and repeats the process. These fi xed scanners can deliver a
range of patterns, including sequential, linear, hexagonal, and
square footprints. With these devices, there is a sharp demarca-
tion between the areas that are treated and those that remain
untouched. The advantage of this particular fi xed scanner laser
is very short pulses (on the order of 20-200
(
10,600
fractionated e
nm)
The characteristic that distinguishes the Er:YAG laser most
from CO
2
laser is the stronger water absorption at the 2940-nm
wavelength (12,000 cm
−1
for Er:YAG vs. 800 cm
−1
for CO
2
).
Because the laser beam is so highly absorbed by water, the
effect is almost pure ablation with very little heating of the sur-
rounding skin. The lack of coagulation is evident by the post-
operative pinpoint bleeding observed usually after exceeding a
200-
:yag lasers
(
2940
R
s vs. >1 ms for
most competitor fractional CO
2
lasers). The shorter pulses
result in faster healing times and less posttreatment erythema.
A potential advantage of random distributions of micro-
beams is to provide a more “feathered” margin between treated
and untreated areas, resulting in a more natural appearing
μ
m depth of ablation (12). Some fractional Er:YAG lasers
scan a raster pattern, whereas others emit a larger beam that is
broken up into smaller beamlets by a microlens array. A recent
study showed that unlike the CO
2
fractional lasers [where
μ
