Biomedical Engineering Reference
In-Depth Information
(
A
)
(
B
)
(
C
)
(
D
)
Figure 6.21
Clinical improvement often occurs in a delayed manner secondary to new collagen formation and collagen remodeling. (
A
) Deep upper-lip lines before
treatment. (
B
) Immediately after treatment, these lines remain very visible. (
C
) 3 days after treatment, swelling intensifi es visibility of some lines. (
D
) 6 months
later, lines are gone.
However, these same tissue studies have shown that with
stacking pulses on top of one another, the zone of thermal
necrosis gradually increases and may increase to greater than
300
m, although the depth of vaporization remains basically
unchanged. The procedure of vaporization appears to level off
because the tissue target (i.e., water) changes as the procedure
progresses. In the fi rst pass, the laser is interacting with intra-
cellular water. Once the epidermis has been stripped away,
however, the laser is acting primarily with extracellular water
in the dermis. In addition, the water content of the epidermis
is closer to 80%, whereas in the dermis it is closer to 60%. The
dermis is made up primarily of protein fi brils of collagen and
elastic tissue. These fi bers have a much higher threshold of
vaporization than water. Therefore, the amount of tissue
vaporized with each pass becomes less and less.
Complicating the situation even further is the layer of ther-
mal necrosis, which measures generally between 50 and 100
μ
m,
is desiccated, and has very little water. Therefore, it is diffi cult to
vaporize deeper than 200-300
μ
m using single-pulse vaporiza-
tion with a collimated beam. However, stacking pulses result in
cumulative thermal effects and may allow deeper vaporization
because of the higher fl uence that results from rapid pulse
succession.
This is a hazardous procedure because the residual thermal
damage is greater with pulse stacking and may progress to the
point of interfering with wound healing, which may cause
scarring. This occurs without obvious clinical signs of poten-
tial problems, but subtle signs may be distinguished by experi-
enced observation. When stacking pulses on top of each
other, a yellow-brown discoloration of the tissue is observed
clinically. This is initially subtle and clears with saline wiping.
μ
Figure 6.22
Failure to adhere carefully to single-pulse vaporization may result
in inadvertently stacking pulses and extending thermal necrosis deeply into
dermis. This may result in delayed wound healing and scarring.
As further passes are performed, the yellow color gradually
deepens to brown and will not get wiped away. When this
yellow-brown discoloration is seen, the physician can be cer-
tain that this area will be the last to heal and that this tissue will
slough. Although this pulse-stacking technique can be used to
resurface deeper because of the resultant tissue slough, it is a
potentially dangerous technique that can result in poor wound
healing and scarring (Fig. 6.22).
