Biomedical Engineering Reference
In-Depth Information
6
Skin resurfacing with ablative lasers
Omar A. Ibrahimi, Richard E. Fitzpatrick, Mitchel P. Goldman,
and Suzanne L. Kilmer
introduction
Interest has surrounded facial rejuvenation for centuries. The use
of topical agents such as soured milk, vegetable extracts, and mud
packs has been documented in ancient civilizations. However,
only in the past few decades has rejuvenation been approached
from a scientifi c basis. With a gradual increase in the median age
of the American population, particularly the 74 million individ-
uals over the age of 55, much attention has been focused on
“antiaging” therapies. In particular, the use of retinoic acid, alfa-
hydroxy acids, and antioxidants in conjunction with sunscreens
and sunblocks has been investigated to prevent or reverse the
photoaging process. Although these agents may have signifi cant
benefi ts, they generally fall short of the clinical expectations of
those using them as a primary rejuvenation treatment. Other
treatment modalities, such as botulinum toxin or fi llers, are capa-
ble of counteracting specifi c aspects of the aging process but do
little to modify the overall appearance of the skin.
To achieve a more dramatic clinical result, a variety of chem-
ical agents have been used to induce a layer of necrosis of vari-
able depths, effectively peeling away the outermost layer of
sun-damaged skin, with subsequent dermal healing and reepi-
thelialization resulting in a more youthful appearance. Similar
results have been achieved by mechanically removing these
outer layers with dermabrasion.
Laser resurfacing of photodamaged skin has rapidly become
the gold standard for rejuvenation of photoaging facial skin.
The trend began in the early 1990s with the arrival of a new
generation of carbon dioxide (CO
2
) lasers, and in the past sev-
eral years has shifted toward fractional resurfacing, as described
by Manstein and Anderson (see chaps. 7 and 8), due to its
shorter downtime. Despite this shift, fully ablative laser resur-
facing remains the gold standard. Here, we discuss in detail the
use of the CO
2
, the erbium:yttrium-aluminum-garnet (Er:YAG)
lasers to rejuvenate the skin.
resulted in the generation of excessive nonspecifi c thermal
damage, which clinically translated into high rates of scarring
and pigmentary disturbances (1). The disappointing experience
with the CW CO
2
laser in the 1980s resulted in a subsequent loss
of interest in laser technology for cutaneous resurfacing.
However, important research over the following 10 years led to
increased knowledge of the laser-tissue interaction and the
mechanisms of rhytid ablation culminating in the development
of a new generation of improved resurfacing lasers. The early
1990s marked the arrival of a new generation of CO
2
lasers that
more closely implemented the principles of selective photother-
molysis (2) by delivering peak fl uences above the ablation
threshold of cutaneous tissue (5 J/cm
2
) with tissue-dwell times
that were shorter than the 1-ms thermal relaxation time of the
epidermis. This iteration of CO
2
lasers limited tissue-dwell time
either by shortening the pulse duration, for example, UltraPulse
(Coherent—now Lumenis, Inc.,Santa Clara, California, USA),
or by using scanning technology to rapidly sweep a CW CO
2
laser beam over the tissue such that the laser beam did not
remain in contact with any particular spot on the tissue for lon-
ger than 1 ms, for example, SilkTouch (Sharplan Lasers Inc., now
Lumenis Inc.). These high peak power, short-pulsed, and rapidly
scanned CO
2
laser systems made it possible for laser surgeons to
precisely and effectively ablate 20-30
m of skin per pass, leaving
behind a considerably smaller residual zone of thermal damage
(up to 150
μ
m) than left behind by the previous generation of
CW CO
2
lasers (up to 600
μ
m) (2). Clinically, this technological
advancement translated into superior clinical results and a much
more favorable safety profi le than seen previously with the
CW CO
2
lasers. Relatively predictable depths of injury, ranging
from intraepidermal to 350
μ
m, could be produced by varying
the pulse energy and the number of laser passes (Fig. 6.1).
Given the impressive clinical results achieved with the high
peak power, short-pulsed, and rapidly scanned focused-beam
CO
2
lasers, they quickly replaced chemical peels and derm-
abrasion as the treatment of choice for cutaneous resurfacing.
Improvement in pretreatment photodamage by approximately
50% or more was possible (Fig. 6.2). However, the impressive
results achieved with these new-generation CO
2
lasers were
not without drawbacks. Adverse sequelae such as delayed
hypopigmentation and prolonged erythema occurred (3-5).
These side effects, as well as the signifi cant “downtime” typi-
cally associated with CO
2
laser resurfacing, were not accept-
able to many patients and signifi cantly dampened the initial
enthusiasm associated with their use.
As the CO
2
laser typically operates near its tissue ablation
threshold (5 J/cm
2
) in most resurfacing applications, a large
μ
the evolution of skin resurfacing
with lasers
Laser resurfacing has been a fi eld that has evolved over the past
30 years. The CO
2
laser, which produces infrared light with a
10,600-nm wavelength, was the fi rst resurfacing laser to be uti-
lized for cutaneous resurfacing. Given that water represents
70% of the total volume of the skin and the CO
2
laser's wave-
length is strongly absorbed by water, this laser was initially
considered an ideal tool for superfi cial ablation of the skin.
However, early on, the CO
2
laser was used in a continuous-wave
(CW) mode with a tissue-dwell time that was well above the
thermal relaxation time of the superfi cial skin (1 ms). This
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