Biomedical Engineering Reference
In-Depth Information
5
Hair removal
Omar A. Ibrahimi and Suzanne L. Kilmer
introduction
The ability of lasers to nonspecifi cally damage hair follicles
was noted nearly 50 years ago in the fi rst reports on the use
of lasers on human skin (1,2). However, it was not until the
theory of selective photothermolysis was proposed by
Rox Anderson and John Parrish at the Wellman Center for
Photomedicine at Harvard Medical School that the concept of
selectively targeting a particular chromophore based on its
absorption spectra and size was realized (3). Several years later,
this group also reported one of the fi rst successful uses of a
normal-mode ruby laser for long-term and permanent hair
removal (4,5).
Today, removing unwanted body hair is a worldwide trend,
and photoepilation by laser or other light-based technology
is one of the most highly requested procedures in cosmetic
dermatology (6). Alternative methods for removing unwanted
hair include bleaching, plucking, shaving, waxing, and chem-
ical depilatories. Threading is a common practice in some
cultures. Unfortunately, these methods do not provide a per-
manent solution to unwanted hair, and can be inconvenient
and tedious (7,8). Electrolysis is a method for hair removal in
which a fi ne needle is inserted deep into the hair follicle and
uses electrical current, thereby destroying the hair follicle and
allowing for permanent hair removal of all types of hair
(9,10). However, this technique is extremely operator depen-
dent and effi cacy in achieving permanent hair removal is
variable among patients (9,10). It is also impractical in terms
of treating large areas. Efl ornithine is a topical inhibitor of
ornithine decarboxylase that slows the rate of hair growth
and is effective for decreasing unwanted facial hair (8), and is
currently indicated for the removal of unwanted facial hair in
women. Efl ornithine can be combined with lasers and intense
pulsed light (IPL) for hair removal (11,12). In this chapter,
we provide a detailed overview on laser hair removal (LHR),
including discussion of hair follicle biology, the science
behind LHR, key factors in optimizing treatment, and future
directions.
supplies the cells of the proliferating matrix at the base of the
follicle, helps form the hair shaft.
Each hair follicle consists of a permanent (upper) and non-
permanent (lower) part, with the follicular bulge forming
the lowermost aspect of the permanent part. In periods of
active growth (anagen) the rapidly developing bulbar matrix
cells differentiate into the hair shaft and the hair lengthens.
A transition period follows in which the bulbar part of the
hair follicle undergoes degradation through apoptosis (cata-
gen). A resting period (telogen) phase ensues, and regrowth is
started once again in early anagen. Stem cells within the hair
follicle regenerate the follicle within or near the hair bulb
matrix. Slow-cycling stem cells have also been found in the fol-
licular bulge arising off the outer root sheath at the site of
arrector pili muscle attachment. The duration of each growth
phase is body site dependent.
There are three main types of hair: lanugo, vellus, and termi-
nal hairs. Lanugo hairs are fine hairs that cover a fetus and are
shed in the neonatal period. Vellus hairs are usually non-
pigmented, and have a diameter of roughly 30-50 µm. Termi-
nal hair shafts range from 150 to 300 µm in cross-sectional
diameter. The type of hair produced by an individual follicle is
capable of change (e.g., vellus to terminal hair at puberty or
terminal to vellus hair in androgenic alopecia).
Hair color is determined by the amount of pigment in the
hair shaft. Melanocytes produce two types of melanin—
eumelanin, a brown-black pigment, and pheomelanin, a red
pigment. Melanocytes are located in the upper portion of the
hair bulb and outer root sheath of the infundibulum.
Excessive and unwanted body hair ranges in severity, depend-
ing on cultural mores, and can usually be classifi ed as either
hypertrichosis or hirsutism (13). Hirsutism is defi ned as the
abnormal growth of terminal hair in women in male-pattern
(androgen-dependent) sites, such as the face and chest. Hyper-
trichosis refers to excess hair growth at any body site that is not
androgen dependent (13). Additionally, the use of grafts and
fl aps in dermatologic and reconstructive surgery can often
introduce hair to an area that causes a displeasing appearance
or functional impairment.
the hair follicle
The hair follicle is a complex, hormonally active structure with
a programed growth pattern (Fig. 5.1). It is anatomically
divided into the infundibulum (hair follicle orifi ce to insertion
of the sebaceous gland), isthmus (insertion of the sebaceous
gland to the insertion of the arrector pili muscle), and inferior
(insertion of the arrector pili to the base of the hair follicle)
segments. The dermal papilla, a neurovascular structure that
mechanism of lhr
The theory of selective photothermolysis enables one to selec-
tively target pigmented hair follicles by using the melanin of
the hair shaft as a chromophore (3). Melanin is capable of func-
tioning as a chromophore for wavelengths in the red and near-
infrared (NIR) portion of the electromagnetic spectrum (14).
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