Biomedical Engineering Reference
In-Depth Information
and found that 88% of these patients felt their appearance had
improved, 75% of them would recommend the procedure,
and 71% of them would undergo laser skin resurfacing again
(57). However, despite these favorable numbers, many laser
surgeons felt the rate of complications remained too high for
comfort.
Subsequently, there have been attempts to mimic the results
of ablative resurfacing without as long a recovery period or as
high a risk of complications. The erbium:yttrium-scandium-
gallium-garnet (Er:YSGG) that emits energy at a wavelength
of 2790 nm has gathered interest for skin resurfacing (58-62).
Technically speaking, the Er:YSGG laser is minimally ablative
in the sense that it does not result in signifi cant tissue removal
but results in detachment at the epidermal-dermal junction
and a varying amount of thermal damage in the papillary
dermis. The water absorption coeffi cient of the Er:YSGG
laser is 5000 cm
2
, which is roughly midway between Er:YAG
(12,500 cm
2
) and CO
2
(1000/cm
2
) (58). Interest in the Er:YSGG
laser, much like the “modulated” Er:YAG, stems from its
potential to impart a balance of depth and thermal impact not
achievable with either of the other ablative wavelengths. The
recovery time for Er:YSGG laser resurfacing based on limited
current literature is about 3-5 days and is considerably shorter
than traditional CO
2
resurfacing.
In 2004, Dieter Manstein and Rox Anderson reported a novel
concept of fractionated photothermolysis (63), which has revo-
lutionized the fi eld of laser resurfacing. Fractional treatment uses
laser beams to damage or remove an array of thousands of
microscopic columns of skin. There are currently both ablative
(see chap. 7) and nonablative fractional resurfacing devices (see
chap. 8). Ablative fractional lasers include CO
2
(10,600-nm wave-
length) and erbium (Er:YAG, 2940-nm wavelength; Er:YSGG,
2790-nm wavelength), and nonablative fractional lasers include
erbium (1410, 1440, 1540, and 1550 nm) and thulium (1927
nm), which are thought to be extremely safe but not devoid of
complications. While there is a current shift toward fractionated
approaches today for resurfacing, understanding the literature
and uses of ablative nonfractionated resurfacing is critical to a
global understanding of laser resurfacing. Additionally, tradi-
tional ablative resurfacing remains the gold standard in terms of
effi cacy. This chapter summarizes our 30+ years of experience
with laser resurfacing.
and Schliftman (70) in 1987 and by David et al. (71) in 1989 for
the treatment of wrinkling.
Advantages of the Pulsed CO
2
Laser
The potential advantages of pulsed-laser cutaneous resurfac-
ing relate to the precise control of tissue vaporization, mini-
mization of residual thermal damage, and achievement of
hemostasis. These advantages are achieved by single-pulse
vaporization of tissue using a high-energy, submillisecond,
pulsed CO
2
laser. The CO
2
wavelength (10,600 nm) is effi -
ciently absorbed in water, resulting in an optical penetration
depth of 20-30
μ
a
) of water
at 10.6 mm is approximately 790 cm
−1
and in tissue 553 cm
−1
(790 cm
−1
× 70%, assuming water content of tissue to be
70%) (72). Each pulse exposure should remove about one
optical penetration depth (20-30
μ
m. The absorption coeffi cient (
μ
m) of tissue and leave two
to four times (40-120
m) that of residual thermal damage.
In cutaneous applications, intracellular water is the laser's
target, and on absorption, heat transfer results in instanta-
neous tissue heating to greater than 100°C boiling of water
and its vaporization and cellular ablation. If this energy is
delivered with a continuous beam, the tissue becomes pro-
gressively desiccated, and because of loss of its water target,
heat accumulates in tissue, reaching temperatures of up to
600°C. This results in diffusion of heat to several hundred
micrometers below the level of vaporization.
A layer of thermal necrosis greater than 100
μ
m interferes
with wound healing and may result in signifi cant scarring when
it extends deeper into the dermis (73-75). To avoid this, the
laser energy must be delivered rapidly, in less time than it takes
the tissue to cool (estimated to be 1 ms or less) (73,76-96).
If the individual pulses do not deliver adequate energy to
vaporize tissue, multiple pulses become necessary and compli-
cate the situation because of heat diffusion between pulses
(73). Ideally, the tissue should be vaporized in a single pulse
because this dissipates heat away from the treated tissue and
does not allow signifi cant heat diffusion to occur below the
level of vaporization.
μ
Delivery Systems and CO
2
Laser-Tissue Interactions
A study regarding the effects of pulse duration on the ablation
threshold and residual thermal damage (84) revealed a number
of signifi cant fi ndings regarding the laser-tissue interaction.
These fi ndings included the following: (
i
) that when tissue abla-
tion occurs, the pulse duration is not critical because a rapid
temperature drop occurs just below the ablation front (85,88);
(
ii
) that residual thermal damage in this situation approaches a
theoretic minimum of 50
the co
2
laser
Although physicians showed considerable interest in the
development of resurfacing parameters with the CO
2
laser in
the early to middle 1980s, the safety profi le for use of the CW
CO
2
laser prohibited its use. Successful low-fl uence CW CO
2
laser removal of the epidermis was reported (64,65) as well as
revision of skin grafts (66) and treatment of acne scars (67) in
1985-1991. However, this technique was found to be too risky
to use over large surface areas because it has too high a risk of
scarring. These treatment techniques can be traced back to
Leon Goldman's report of treatment of actinic cheilitis with
the CO
2
laser in 1968 (68). This treatment technique, however,
was not widely used for this condition until popularized by
David's report of a series of patients in 1985 (69).
The more limited use of the CO
2
laser in combination with
trichloroacetic acid (TCA) peeling was reported fi rst by Brauner
m (85,87-90); and (
iii
) that increases
in pulse width cause increased thermal damage at the edges of
craters resulting from a Gaussian beam. Also, an increase in the
ablation threshold at longer pulse durations is related to ther-
mal diffusion during the pulse. To confi ne thermal damage, the
velocity of ablation is critical and requires a value of 0.65 cm or
greater to achieve minimal thermal damage.
The ablation depth per pulse at 7.5 J/cm
2
was found to be
35
μ
m, too small to account for the clinical improvement seen
in two or three passes of the CO
2
laser in treating wrinkles
measuring 300
μ
μ
m in depth. This aspect is discussed in greater
detail later.
