Biomedical Engineering Reference
In-Depth Information
7
Ablative fractional lasers
Douglas A. Winstanley and E. Victor Ross
background
The popularity of laser treatment for curing photoaging has
increased within the past decade. Although the results of fully
ablative (confl uent) resurfacing are predictable and substan-
tive, there are signifi cant risks and an obligatory period of
downtime. This limited the procedure to a select group of
patients.
Ablative fractional technology has revolutionized the fi eld of
laser dermatology. Since its introduction by Manstein et al. in
2004 (1), a variety of nonablative and ablative fractional lasers
have become available. Decreased healing times and a lower
incidence of adverse outcomes make the use of fractional tech-
nology appealing to physicians and patients. Fractionated lasers
are being used in a wider variety of applications, including
treatment of photoaging, acne scars, and surgical scars. More
recent applications include the treatment of pigmentation dis-
orders, removal of tattoos, and destruction of premalignancies.
Traditional ablative devices operate in the infrared spec-
trum, using water as the primary chromophore. These lasers
have been used for many years to address rhytides and photo-
aging. However, the downtime and associated risks of scarring
and dyspigmentation limited their application to those
patients who could accommodate 1-2 weeks of minimal social
interaction. The distinction of ablative fractional devices from
fully ablative lasers is pixilation of the laser beam. These
devices achieve epidermal and dermal injury without damag-
ing the entire skin surface. The islands of undamaged skin
serve as reservoirs for rapid healing. Coupled with a shortened
healing time is a decrease in the overall risks of the procedure,
including infection, scarring, and dyspigmentation (2). Together,
these properties make ablative fractional treatments safer
while still achieving signifi cant improvement in tone, texture,
and pigmentation of the skin. However, the lack of confl uent
laser ablation usually results in the lack of permanent resolu-
tion of dyspigmentation, cutaneous carcinoma, or premalig-
nant skin disease. This chapter addresses the current state
of ablative fractional laser technology. A brief review of the
technology, molecular effects, applications, and complications
is discussed.
The primary characteristic among ablative lasers that distin-
guishes one wavelength from another is the degree of water
absorption. The absorption of water is relatively low through-
out the visible wavelengths and near-infrared spectrum. Beyond
approximately 1200 nm, the absorption of water increases, with
the peak absorption at 2935 nm. There are currently three
available wavelengths among the ablative fractional lasers
(with respective tissue absorption coeffi cients, assuming 70%
water content): 2790-nm erbium-doped:yttrium-scandium-
gallium-garnet laser (Er:YSGG), with an absorption coeffi cient
of 4000 cm
−1
; 2940-nm erbium-doped:yttrium-aluminum-
garnet (Er:YAG) laser, with an absorption coeffi cient of
10,000 cm
−1
, nearly 16 times that of CO
2
; and 10,600-nm CO
2
laser, with an absorption coeffi cient over 800 cm
−1
(Fig. 7.1).
With water serving as the chromophore for these devices, sev-
eral skin components are targeted. In general, the affected struc-
tures are keratinocytes, collagen, and blood vessels (3). A recent
article examined the defi nition of the terms “ablation” (4) and
“fractional.” In this article, the authors confi ned “nonablative”
treatments to those where the epidermis is not “structurally
breached,” and where erosions and other changes associated
with ablation do not occur. Also they confi ne “fractional”
treatments to those where less than 50% of the surface area is
damaged and where microinjuries are no greater than 500
m
in diameter. We respectfully disagree on this point and would
prefer that fractional injuries be divided into
macrospot
(wounds >500
μ
μ
m in diameter), small-depth (<200-
μ
m deep)
lesions, and
microspot
(<500
μ
m in diameter), higher-depth
(>200-
m depth) lesions. Although the “spirit” of the descrip-
tions in the article by Alam et al. (4) is correct, formally “abla-
tion” should be defi ned in terms of “removal” or vaporization
(5). Where the term “ablation” becomes more ambiguous is in
cases where water is “vaporized” within a skin structure, but
the gross structural (skin framework) change is not immedi-
ately observed. An example is conventional pulsed CO
2
laser
with fl uences in the range of 3-5 J/cm
2
(applications with
larger spots). In this case, the epidermis, although irreparably
damaged and denatured, remains on this skin as a skeleton
(the water having been vaporized with the keratin skeleton
remaining) (Fig. 7.2). Within days this layer “sloughs” off and
the dermis is exposed. In this case, although the epidermis is
not grossly removed in the immediate lasing session, the vapor-
ization of water satisfi es the criteria of ablation. Visible light
technologies and wavelengths with small water absorption
coeffi cients, on the other hand, do not result in vaporization
and therefore are termed nonablative.
Because the chromophore is water, fractional lasers from
2.79 to 10.6
μ
characteristics of ablative
fractional lasers
Although the specifi cs of laser-tissue interactions are dis-
cussed elsewhere in this text, a review of some of the specifi c
components of ablative fractional injury is imperative to
understanding the differences in these devices and how set-
tings can be optimized for maximum cosmetic enhancement
and minimum downtime.
μ
m with larger absorption coeffi cients will ablate
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