INTRODUCTION
The removal of unwanted hair, an accepted practice in
many cultures, has been performed since ancient times. It has evolved from temporary modes of depilation (removing hair visible above the skin) and epilation (removal of the
entire hair) such as abrasion, chemical depilatories, shaving, waxing, tweezing, "sugaring," and threading, to procedures which provide more permanent hair
reduction. Although permanent epilation became possible with the advent of electrolysis, this procedure proved to be, like some of the traditional methods noted above, tedious
and time consuming; additionally, in those with curlier hair and darker skin phototypes (IV–VI), electrolysis was a difficult procedure, prone to result in exacerbation
of inflammation, pseudo-folliculitis barbae, post-inflammatory hyperpigmentation, and scarring. Laser hair removal (LHR) revolutionized the field of epilation, providing a
more cosmetically scrupulous, time-saving alternative to the hair-by-hair approach of electrolysis and the traditional methods of hair removal. Initially contraindicated in
patients with ethnically dark or sun-tanned skin, LHR was recommended only for non-tanned, phototype I–III skin because of the risk of untoward side effects and
worsening of pre-existing conditions more often seen in phototype IV–VI skin.
Mechanisms of Epilation in LHR in Skin of Color
The theory of selective photothermolysis forms the foundation of cosmetic, light-based laser therapy. This theory states that thermal injury can be limited to the absorbing chromophore by using the appropriate wavelength, pulse duration, and fluence (energy density expressed as J/cm²), thereby protecting the surrounding tissue.1 In LHR, the target chromophore is the melanin-rich hair shaft and bulb, but to ensure effective results, thermal injury must also include the surrounding hair follicle. Finally, destruction of progenitor stem cells is required to permanently inhibit hair growth. In order to achieve effective results with LHR and to assure confinement of thermal damage to the hair follicle, the pulse duration should be less than or equal to the thermal relaxation time of the hair shaft, estimated to be in the range of 10–100 milliseconds.2
A "unifying" hypothesis has identified two stem cell reservoirs in the human anagen hair follicle: a distal outer root sheath reservoir and a proximal outer root
sheath reservoir containing amelanotic stem cells, which may act as a melanocytic reservoir.3 In the mouse model, these stem cells are identified within the bulge,4 in the
human hair follicle, the bulge is thought to reside in the lower third of the follicle of the outer root sheath.
Therefore, a modified theory of selective photothermolysis is used to describe the mechanism of LHR because the melanocytic chromophore target as well as the amelanotic (hair
follicle and stem cell) target must both be destroyed to accomplish permanent hair reduction. To achieve this goal, longer pulse durations are used, allowing heat propagation
to the entire hair follicle.
