Biomedical Engineering Reference
In-Depth Information
Box 6.1
Collagen Shrinkage with CO
2
Laser
Collagen Molecule
• Three polypeptide chains in helical conformation result from repetitive positioning of glycine at every third amino acid locus
• Intermolecular cross-links provide tensile strength and are inhibited by heat
• Heat dissociates interpeptide bond; molecules remain intact
• Hydrothermal shrinkage occurs through molecular structure transition from a triple helix to a random coil
• Immediate contraction of fibers occurs to about one-third their original length
• Collagen is progressively more cross-linked with time, so new collagen is less stable and contracts at lower temperature
Collagen-Heat Reaction
• Polypeptide strands of heated tissue have capacity to resume the original intrachain characteristics
• Collagen readily self-assembles into fibrils, which cross-link in a spontaneous, progressive manner
• A myriad of molecular conformations are possible
• Denaturation process is both time and temperature dependent; in general, a 2°C rise in temperature reduces shrinkage time by 50%
• Very narrow temperature range exists for shrinkage without destroying fibrils
• Instantaneous shrinkage temperature of human collagen was found to be 62.9°C, with a range of 57-65°C
• As temperature rises, more and more cross-link bonds are broken until denaturation and loss of structure occur
• Percent shrinkage is directly proportional to heat applied while not exceeding threshold for denaturation
(
A
)
(
B
)
Figure 6.18
Skin tightening and collagen shrinkage result in clinical improvement in nasolabial folds and cheeks. (
A
) Before treatment; (
B
) 1 year after full-face
CO
2
laser resurfacing.
effects (Fig. 6.18). Precise tightening of folds of loose eyelid
tissue (Fig. 6.19) or atrophic scars (Fig. 6.20) can dramatically
improve clinical results without signifi cant risk if careful clini-
cal observation is used.
By targeting high points of glabellar and forehead lines, bet-
ter smoothing of these areas may be achieved. This type of tis-
sue change has been shown to persist 4 years or more and is
thought to be a permanent structural change in the skin once
the new collagen formation has occurred.
thermal injury are seen, that is, yellow-brown discoloration of
the skin. Often, when treatment is discontinued because of vis-
ible thermal effects, visible wrinkle lines are still present, par-
ticularly on the glabella, forehead, and upper lip (Fig. 6.21A,B).
These lines may gradually improve during the healing process
and may gradually subside over 6-12 weeks (Fig. 6.21C,D).
This occurs as a consequence of new collagen formation and
collagen remodeling.
A multicenter study demonstrated that UltraPulse laser
resurfacing results in up to a 600% increase in the Grenz zone
layer. A new layer of healthy, fi ne collagen fi brils is laid down
parallel to the epidermis and measures 100-400
New Collagen Formation and Collagen Remodeling
To avoid excessive thermal injury, it is recommended that the
laser resurfacing procedure be discontinued when signs of
m in depth,
replacing the layer of solar elastosis vaporized by the laser.
μ
