Biomedical Engineering Reference
In-Depth Information
treatment (9,10). Because scarring may result from thermal
spread, care should be taken to minimize thermal damage to
the surrounding collagen. Scarring is a particular concern in
treating large, deeply pigmented lesions such as large congenital
nevi, where the bulk of the dermis is taken up by nested mela-
nocytes and an extensive collateral thermal damage can occur
to the minimal normal dermis that remains (14,15).
that lack a signifi cant melanin content [(i.e., dermal, com-
pound, or congenital melanocytic nevi (CMN)].
The success of the QS lasers in the realm of pigmented
lesions is based on the ability of these lasers to selectively target
melanosomes situated within melanocytes and keratinocytes.
The melanosome-specifi c damage is due to the absorption of
high-energy, nanosecond laser pulses (6,7). Long
-
pulsed lasers
in the millisecond domain were developed to target pigmented
hair. These lasers can also be used to target epidermal and der-
mal pigment found in larger clumps such as those in nested
melanocytes or confl uent melanin in the epidermis (8-11,14).
Intense Pulsed Light
The noncoherent, broadband, intense pulsed light (IPL) source
has also shown effi cacy in the treatment of pigmented lesions.
IPL is particularly effective in the treatment of epidermal
pigmented lesions, such as lentigines. This modality can also
be safely used on darker skin types when used in a double
or triple pulsed mode that allows the epidermis to cool
between light pulses as well as its low risk of postinfl ammatory
hyperpigmentation.
Pigmented Lesion History
A principle that must be strictly adhered to by the laser sur-
geon is that a pigmented lesion with atypical features should
never be treated with laser. Atypical features can be summa-
rized by the acronym ABCDE:
A = asymmetry
B = border
C = color
D = diameter
E = evolving
Fractional Photothermolysis
Fractional photothermolysis is based on the use of laser
microbeams to damage or remove an array of thousands of
microscopic columns of skin. There are both ablative and
non-ablative fractional resurfacing devices. Ablative frac-
tional lasers include CO
2
(10600 nm wavelength) and erbium
(Er:YAG, 2940-nm wavelength; Er:YSGG, 2790-nm wave-
length), which vaporize an array of small channels through
the skin, up to approximately 1.0-1.5 mm deep. Non-ablative
fractional resurfacing devices are based on erbium (1540
and 1550 nm) and Nd:YAG lasers (1440 nm), and more
recently the thulium laser (1927 nm). Because fractional
photothermolysis removes or damages a portion of the
treated area, it has the potential to improve certain types of
pigmentary aberrations. Advances using this technology for
pigmentary disorders are forthcoming.
Lesions with atypical features should not be treated with laser
and should be evaluated by a dermatologist. A patient with a
personal or family history of melanoma or dysplastic nevi
should be advised against laser treatment of nevi.
The treatment of CMN remains controversial (16,17),
and should not be undertaken without an expertise in laser
surgery. It remains unclear whether laser treatment of con-
genital nevi decreases the risk of malignant transformation,
although some argue that decreasing the melanocytic bur-
den reduces the overall risk of the lesion becoming malig-
nant. However, large congenital nevi can also be cosmetically
and psychologically challenging and improvement in the
appearance of these lesions may greatly benefit the well
being of patients with large sized congenital nevi.
lesion
lassification
To effectively treat pigmented lesions, one must be profi -
cient at diagnostic classifi cation and also be familiar with
the histopathological characteristics of the lesion. With this
information, the lesion can effectively be categorized accord-
ing to the depth of the target pigment distribution: epider-
mal, dermal, or a combination of both. The most suitable
device can then be decided upon (Table 3.1).
C
laser treatment for epidermal lesions
Epidermal lesions are amenable to treatment with multiple
modalities because of their superfi cial location; virtually
all injuries confi ned to the epidermis heal without scarring.
The 532-nm (FD Nd:YAG) and 694-nm (ruby) wavelengths
are the most appropriate for epidermal lesions, followed by
the 755-nm (alexandrite) and, least effective, the 1064-nm
wavelength (long pulse Nd:YAG). Even though less penetrat-
ing, the shorter wavelengths are particularly useful because
their greater melanin absorption best targets the superfi cial
melanin in keratinocytes and melanocytes. In addition,
lesions with less melanin such as lighter lentigines, freckles,
or CALMs can still be effectively targeted. Longer pulsed
pigment-specifi c lasers and ablative and non-ablative frac-
tional photothermolysis lasers are also capable of treating
epidermal pigmentation, although fractional photothermoly-
sis would be used more appropriately in the context of a dif-
fuse epidermal pigmentary aberration.
Pigment Location
Epidermal pigmented lesions include lentigo, café-au-lait
macule (CALM), ephelis, junctional nevus, nevus spilus, and
seborrheic keratosis (SK). Dermal pigmented lesions include
blue nevus and nevus of Ota or Ito. Some pigmented lesions,
such as melasma, Becker's nevus, compound nevus, and con-
genital nevus have both an epidermal and a dermal compo-
nent. For some pigmented lesions, the target is melanosomes
in keratinocytes, whereas in most cases it is melanosomes in
melanocytes or the whole melanocyte. In some cases, the
spread of thermal damage from pigment-containing melano-
cytes may be advantageous in targeting adjacent melanocytes
