Biomedical Engineering Reference
In-Depth Information
Previous studies investigating the use of NAFR (225,228)
and AFR (229) indicated that treatment density, rather than
energy, was a stronger determining factor in the development
of PIH in predisposed patients. Therefore, to reduce the risk of
PIH associated with NAFR and AFR when treating Asian
patients, the total treatment density (MTZ/cm
2
) should be
reduced. However, on the other hand, the total number of
treatment sessions should be increased in order to maintain
the clinical effi cacy.
intervals (168). We note a similar response rate when treating
Asian and black patients. Studies have demonstrated that in
the ideal patients with fair skin and dark hair, the possibility
for long-term hair reduction after a single treatment is approx-
imately 80% (242,243).
A critical threshold fl uence is needed to obtain effi cacy. This
fl uence is determined as the lowest fl uence that can produce
perifollicular swelling and erythema appearing a few minutes
after laser irradiation. When treating individuals with skin
types IV-VI, we carefully select the endpoint that produces
slight perifollicular swelling and erythema. If there is a sign of
acute epidermal injury, including whitening, blistering, or
Nikolsky's sign (forced epidermal separation), the fl uence
should be reduced. Generally, the treatment fl uence should be
at 75% of the Nikolsky's threshold fl uence (244). A 2-year
follow-up study on the effi cacy of normal-mode ruby laser
hair removal notes that sites treated with highest fl uence
(60 J/cm
2
) obtained the greatest hair reduction (64.3%) (245).
Epidermal injury is an anticipated consequence when treat-
ing a darker complexioned patient. Whereas the goal of light-
assisted hair removal is permanent follicular destruction, there
is also a risk of epidermal damage during hair removal, espe-
cially in nonwhite individuals whose epidermis contains larger
amounts of melanin. When treating this group of patients, in
order to be able to use the highest tolerable fl uence to achieve
better hair reduction while minimizing epidermal damage,
three important considerations, including wavelength and
pulse duration of the light sources, and epidermal cooling
methods, should be taken into account.
The fi rst important determinant is wavelength of the light
source. The ideal laser wavelength for hair removal is a wave-
length that is preferentially absorbed by melanin but not by
surrounding tissue. Lasers emitting longer wavelengths have
the advantage of being able to penetrate deeper into the der-
mis, minimizing the possibility of absorptive interference by
epidermal melanin (242,244). Color contrast between epider-
mis and the hair shaft (and bulb) are crucial in determining
the optimal wavelength. For high contrast (dark hair, light
skin), the low range of the wavelength (650-700 nm) can be
employed without risking serious injury to the epidermis (and
subsequent hypo- and hyperpigmentation). For lighter hair
and darker skin, the longer wavelength (800 nm and greater)
should be applied (246).
Studies in patients with skin phototypes III-V have shown
that temporary hair reduction after treatment with normal-
mode ruby (694 nm) laser (237,247), LP ruby (694 nm) laser
(248,249), LP alexandrite (755 nm) laser (233,250), diode
(810 and 940 nm) laser (236,251,252), long-pulsed Nd:YAG
(1064 nm) laser (231,252), and IPL (253,254) hair-removal sys-
tems was comparable in skin phototypes I-II. However, the inci-
dence of undesirable side effects occurred more often compared
with those noted in fair-skinned patients. Although most of
these side effects, including treatment pain, erythema, edema,
blistering, crusting, erosion, purpura, folliculitis, and pigmen-
tary changes (hypopigmentation and hyperpigmentation), were
transient and self-limited, pigmentary alterations usually
required longer to resolve (more than 3 months). In addition,
these side effects often occurred on tanned skin, in patients with
skin phototype III and higher, or on sites treated with excessively
high-energy fl uences.
laser hair removal
The use of laser and pulsed light sources for hair removal is
increasing because the treatment is relatively safe and effective
in removing large areas of unwanted hair. Although it has been
well documented that the ideal patients for this procedure are
individuals with dark hair and fair skin, unwanted or excessive
hair is also a cosmetic concern for the Asian population.
Most of the available laser and light source systems with
wavelengths in the red and near-infrared regions have been
designed to cause selective photothermal damage to pig-
mented hair follicles. Improved effi cacy is accomplished by the
high specifi city and selectivity contributed by the accurate
selection of an appropriate wavelength and pulse duration to
maximize follicular damage and minimize unwanted injury to
the epidermis. To target the follicle, these lasers and light
sources either count on endogenous melanin within the fol-
licular epithelium or hair shaft.
When using light to target these endogenous chromophores,
there is also risk of epidermal injury when laser light pene-
trates the target. Melanin-containing structures, including
melanocytes, melanosome-containing keratinocytes, or nevus
cells may also be thermally injured when irradiated by red and
near-infrared lasers. Consequently, epidermal injury due to
the absorption of laser energy by epidermal melanin may
occur to a certain degree during the laser impacts.
Most clinical studies on the effi cacy of laser and IPL systems
in removing unwanted hair have been performed in fair-
skinned individuals because it was theoretically postulated
that a larger amount of epidermal melanin in Asian and non-
white individuals may result in a decrease in clinical effi cacy
and cause an increase in the incidence of adverse effects, espe-
cially pigmentary and textural changes, and scarring. Cur-
rently, several reports on these light-assisted hair removal
systems performed on darker-skinned persons have shown
comparable results to those of fair-skinned persons with a
slightly increased risk of complications (230-238). In addi-
tion, laser and pulsed light hair-removal systems have been
proven to be safe and effective in treating some follicular-
related disorders in dark-skinned patients, including pseudo-
folliculitis barbae (239) and trichostasis spinulosa (240,241).
It is diffi cult to compare the effi cacy and side effects of light-
assisted hair removal systems because of the variability in the
treated anatomic sites and in the types, pulse widths, spot
sizes, and repetition rates of the light sources used, as well as
differences in the treatment regimens. All laser and IPL sys-
tems have been shown to temporarily remove hair. Reduction
of hair growth occurs for all hair colors and at all fl uences.
Blonde, red-, or white-haired individuals are unlikely to expe-
rience permanent hair reduction, but hair loss in these persons
can be sustained by treatment at approximately 3-month
