Biomedical Engineering Reference
In-Depth Information
aggregate into immature soluble collagen fi brils, which are
then cross-linked by the action of lysyl oxidase to form mature,
stronger collagen fi bers (178).
be particularly avoided during this phase, because the incidence
of contact irritant and allergic dermatitis is enhanced by the
absence of epithelium (187). We have found the incidence to
increase from about 3% to 4% when topical agents are used in
non-laser-treated skin to 20% or higher when they are used
after laser skin resurfacing. Bacitracin is a frequent sensitizer,
with a fi vefold increase in dermatitis reported from 1990 to
1994 (188). In addition, a compound such as petrolatum, which
usually does not cause contact sensitivity, has a greater potential
to sensitize in this situation (100,189).
Maturation Phase
The total amount of collagen in a wound reaches a maximum
at 2-3 weeks, but collagen remodeling continues over months
to years (179). This characterizes the third phase of wound
healing, the maturation phase. The earliest collagen fi bers are
thin and unorganized, becoming thicker, cross-linked, and
parallel to skin tension lines with time. As new collagen is
formed, abnormal or damaged collagen is broken down by
collagenases and proteases produced by fi broblasts, macro-
phages, and infl ammatory cells. Proteoglycans, responsible for
water storage in the healing wound, decrease, and water is
reabsorbed as the wound heals (180,181).
Remodeling of the collagen matrix ends up in gradual shrink-
age, thinning, and paling of the scar. Once the collagen bed is
established as a stable matrix, collagen production, and resorp-
tion continue in a steady, balanced state in a normally healed scar.
Tensile strength increases from 5% of original strength at 2 weeks
to 80% in a mature scar (182). Contraction of a scar, which
begins 1 week after wounding, is not caused by excessive deposi-
tion of collagen, but rather by the effects of transformation of
fi broblasts to myofi broblasts (183,184). These cells produce con-
tractile proteins with characteristics of smooth muscle cells.
Granulation tissue has been shown to contain as much actomyo-
sin as an equivalent weight of smooth muscle (185). Further-
more, it has been demonstrated that contraction of myofi broblasts
can be inhibited by smooth muscle relaxants (159,183). It has
been proposed that migrating fi broblasts interact with their sur-
rounding matrix components to reorganize connective tissue
fi bers to induce shrinkage (186). These factors can result in as
much as a 45% reduction in wound surface area (159).
wound care
The benefi ts of wound occlusion have been well documented
(52,190,191). Wound occlusion creates a moist environment
that protects the wound from exogenous bacteria, inhibits
crust formation, enhances reepithelialization, and reduces
patient discomfort. Occlusion can be achieved by either open
or closed wound care techniques.
Open wound care techniques facilitate surveillance of the
laser wound for clinical signs of infections or other complica-
tions. Additionally, they are relatively inexpensive and do not
elicit the feeling of claustrophobia that some patients can
experience with occlusive dressings. However, to achieve the
healing benefi ts of occlusion, open wound care techniques
demand around-the-clock wound care by the patient and thus
are largely dependent on patient compliance.
Open-wound care generally consists of frequent soaking
with cool distilled water containing a small amount of white
vinegar. One teaspoon per cup is generally the starting con-
centration (0.25% acetic acid). The mildly acidic pH has an
antibacterial effect, which is especially important in suppress-
ing
Pseudomonas
species. The skin should be soaked for
20 minutes every 1-2 hours. The objective is to soak away the
serous exudate and any necrotic tissue on the skin surface. Just
patting the skin with a moist gauze pad is not adequate.
Between soakings, the skin should be liberally and continu-
ously coated with petrolatum (Vaseline®), Aquaphor® (Beiers-
dorf, Charlotte, North Carolina, USA), or Theraplex®. This
coating may be gently removed with gauze or facial tissue
before the next soaking. Ice packs or frozen peas may be
applied during this period for symptomatic pain relief.
The application of petrolatum to the wound surface results
in a semiocclusive, moist environment. Studies have shown
that wounds treated with petrolatum compared with those
treated with a topical antibiotic have no increased incidence of
infection and heal at the same rate (182). In contrast, in a study
of accidental burns treated with either petrolatum mesh gauze
or nitrofurazone, 75% of the petrolatum group had bacteria
reported as “too numerous to count” compared with only 10%
of the nitrofurazone group (192).
The composition of a bland emollient apparently may have an
effect on wound healing. A U.S.P. petrolatum ointment decreased
wound healing by 17%, a white petrolatum cream increased
wound healing by 24%, and a lotion with propylene glycol with-
out petrolatum increased wound healing by 15% (193).
Wound-healing studies have shown that vehicles with the
proper combination of fatty acids result in faster healing than
do petrolatum or Aquaphor (194). Fatty acids that refl ect the
composition of fatty acids in human membranes are required
for the proliferation of epithelial cells (195,196).
postoperative care
If the patient develops noticeable intraoperative swelling, usually
detected in the periorbital area, dexamethasone (Decadron®,
10-12 mg by IV push) may be administered. Although some
physicians routinely use a Decadron Dospak, oral prednisone
(20 mg every day for 7 days), or triamcinolone (Kenalog®,
40 mg IM) postoperatively, the effects on suppression of fi bro-
blast activity and infection surveillance must be considered.
Single-dose IV Decadron is rapidly cleared and thus should
not interfere with wound healing. Because postoperative swell-
ing generally clears rapidly (2-5 days), the choice of the
shorter-acting corticosteroids is preferable.
Minimal thermal damage that results in poor coagulation of
dermal blood vessels and poor hemostasis characterizes treat-
ment with the short-pulsed Er:YAG laser. Application of wet
gauze compression is useful to deal with the bleeding that is
typically seen after resurfacing with the short-pulsed Er:YAG
laser (12). Patients are told to expect a mild burning sensation
for the fi rst 1-3 days and signifi cant swelling for the fi rst
2-5 days postoperatively. The edema that develops in the fi rst
48 hours postoperatively can be controlled with icepacks, head
elevation at night, and in severe cases, with oral corticosteroids.
During the fi rst 3-5 days, signifi cant serous exudate is pres-
ent as well, related to the swelling and absence of an epithelial
barrier. Use of topical antibiotics and other topical drugs should
