Biomedical Engineering Reference
In-Depth Information
applications, including acne scarring, striae, and hyper-
pigmentation (Fig. 11.23).
mechanisms of action
The mechanisms by which ablative fractional lasers affect the
skin are unclear. Both microscopic and biochemical studies sug-
gest that the effi cacy of ablative fractional treatments is related
to molecular level effects. Several investigations have examined
the wound healing process. Clinical observations suggest that
fractional lasers increase collagen and/or elastin. Initial work by
Orringer et al. examined the molecular effects on the skin of a
fully ablative device. They found that laser treatment created a
proinfl ammatory cascade that induced the expression of matrix
metalloproteinases (MMPs). This wound healing sequence
allowed for the breakdown of damaged collagen with replace-
ment by new, well-structured collagen bundles (16).
More recent work has explored the underlying processes
specifi c to fractional treatments. Xu et al. (17) demonstrated
the expression of heat shock protein 70 within hours after laser
treatment with persistence of these proteins for 3 months
following the procedure. HSP 47 was detected 3 months fol-
lowing the procedure and persisted for an additional 3 months,
suggesting the underlying mechanism for continuous
improvement by serving as a “procollagen chaperone promot-
ing neocollagenesis.”
Reilly et al. studied ablative fractional lasers through assays
of proteins in treated and normal skin. In their study, the levels
of expression of MMPs were evaluated and compared with
untreated skin at various times. RNA production for MMPs
was specifi cally evaluated. It was discovered that interleukin-1
Fractional
1.3 mm
beam
Confluent
beam
Mix of 1.3 mm
and 0.15 mm
beams
(Fusion mode_CORE laser)
Figure 7.6
Tongue depressor showing various beam patterns with carbon
dioxide lasers.
transition between treated and untreated skin (15). In the case
of lasers that scan a fi xed pattern and where the density per
pass can be set by the user (i.e., between 5% and 25% per pass),
greater pass numbers at smaller densities will allow for a more
random distribution of microbeams (better case) versus fewer
passes with greater density per pass. For example, if one applies
25% density with some overlap (OL), the density in the OL
area will be 50% versus only increasing from 5% to 10% when
overlapping in the lower density case. The drawback to multi-
ple passes is the extra time requirement to complete the proce-
dure. More recent developments include handpieces that use a
“dynamic” mode in which the distribution of individual
microbeams is randomized partly by the deliberate motion of
the handpiece during the scan completion. Some companies
incorporate both a dynamic and a “static” mode for their CO
2
scanners (Lutronic, Korea or Active FX, Lumenis), the former
deployed by the physician in a constantly moving pattern, the
latter in a more conventional “let the scanner complete the
scan” and then move the handpiece fashion.
With equivalent fl uences at 10.6
β
and tumor necrosis factor are induced quickly after laser
treatment. Subsequently, MMPs in various concentrations
were produced. MMP-1 and MMP-3, both collagenases,
appeared in higher proportions along with MMP-9 (gelatin-
ase). MMP-13 was also elevated posttreatment. MMP-13
seems to be a key protein in the reorganization of collagen and
may play a primary role in the structuring of newly formed
collagen after laser resurfacing (18). The role of coagulation as
an independent factor in neocollagenesis and elastogenesis is
unclear, that is, if one creates the same volumetric injury with
a higher ratio of coagulation to ablation in individual craters,
is there a tendency toward a more robust response with
increasing degrees of coagulation?
Orringer et al. (19) has studied confl uent and ablative frac-
tional wounds and found that fractional wounds created less
collagen than their confl uent counterparts. This observation
might explain the superior results achieved with confl uent
resurfacing versus fractional resurfacing in perioral rhytides,
a region where fractional systems have “underperformed”
(Fig. 7.7). They have also estimated that for ablative and nonab-
lative
equivalent
wounds (i.e., when the same total cellular vol-
ume has been damaged, Fig. 7.5), the ablative fractional wound
should create about 3
m, longer pulse durations
deliver more heat to tissue. Histologically, longer pulse dura-
tions (and therefore lower power densities) transform a cylin-
der of ablation and coagulation into a more triangular defect.
This prolonged heating can result in subepidermal clefting at
the edge of the crater and nonspecifi c thermal damage, which
may be responsible for increased rates of postinfl ammatory
erythema and hyperpigmentation (15). Microspot size is gen-
erally fi xed for a particular device—an exception is the
Lumenis Encore system, where two handpieces allow for 120-
and 1300-
μ
the collagen of the nonablative wound
some 3 months after irradiation (personal communication).
Another study showed that for nonablative wounds (frac-
tional), the total wound volume was more likely to determine
the degree of neocollagenesis versus individual parameters
of depth or density, that is, a low-density high-depth pattern
created about the same degree of new collagen as one with
high density and small depths (20).
×
m spot sizes, respectively.
Alma Lasers (Buffalo, Grove IL, USA) has created a roller
type scanner microspot Er:YAG and CO
2
system. The Er laser
is driven by its base intense pulsed light (IPL) platform,
whereas the CO
2
laser is a stand-alone product.
In addition, a radiofrequency (RF) pixel device (Pixel RF,
Alma Lasers) is available and is being evaluated for several
μ
