Introduction

The non-specific damage of human hair follicles with a laser was noted over 50 years ago. However, it was not until the theory of selective photothermolysis was proposed by two Harvard dermatologists, Rox Anderson and John Parrish, that the concept of selectively targeting a particular chromophore based on its absorption spectra and size was realized. In 1996, this group also reported the first successful use of a normal-mode ruby laser for long-term and permanent hair removal.

Removing unwanted body hair is today a worldwide trend, and hair removal using laser or other light-based technology is one of the most highly requested cosmetic procedures. Prior to the advent of laser hair removal (LHR), only temporary methods for removing unwanted hair were available such as bleaching, plucking, shaving, waxing, and chemical depilatories. Threading, a form of epilation using a cotton thread, is a common practice in some cultures. In addition to not providing permanent hair removal, these methods are also inconvenient and tedious. Electrolysis is a technique in which a fine 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. However, this technique is impractical for treating large areas, extremely tedious, operator dependent, and with variable efficacy in achieving permanent hair removal. Eflornithine (α-difluoromethylornithine or DFMO) is a topical inhibitor of ornithine decarboxylase that slows the rate of hair growth and is currently FDA cleared for the removal of unwanted facial hair in women. In this chapter, we provide a detailed overview on LHR including discussion of hair follicle biology, the science behind LHR, key factors in optimizing treatment, and future trends.

Basic hair biology

The hair follicle is a hormonally active structure (Fig. 4.1) that is anatomically divided into an infundibulum (hair follicle orifice to insertion of the sebaceous gland), isthmus (insertion of the sebaceous gland to the insertion of the arrector (erector) pili muscle), and inferior (insertion of the arrector pili to the base of the hair follicle) segments. The dermal papilla provides neurovascular support to the base of the follicle and helps form the hair shaft.

Figure 4.1 Hair follicle anatomy.

Reproduced from Tsao SS, Hruza GJ 2005 Laser hair removal. In: Robinson JK, Hanke CW, Sengelmann RD, Siegel DM (eds) Surgery of the Skin. Elsevier Mosby, Philadelphia, p 575-588.

Every hair follicle is controlled by a programmed cycle that is dependent on the anatomical location. The hair cycle consists of anagen, catagen, and telogen phases. Anagen is characterized by a period of active growth where the hair shaft lengthens. A catagen transition period follows in which the lower part of the hair follicle undergoes apoptosis. A resting period, telogen, then ensues, and regrowth occurs when anagen resumes. Hair regrowth (entry into another anagen cycle) is dependent on stem cells within or near the hair bulb matrix. Slow-cycling stem cells have also been found in the follicular bulge arising off the outer root sheath at the site of the arrector pili muscle attachment.

The main types of hair include lanugo, vellus, and terminal 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. Terminal hair shafts range from 150 to 300 µm in 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).

The amount and type of pigment in the hair shaft determine hair color. 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.

Definitions of what constitutes excessive or unwanted body hair depends on cultural mores, but can usually be classified as either hypertrichosis or hirsutism. Hirsutism is the abnormal growth of terminal hair in women in male-pattern (androgen-dependent) sites such as the face and chest. Hypertrichosis is excess hair growth at any body site that is not androgen dependent. Additionally, the use of grafts and flaps in skin surgery can often introduce hair to an area that causes a displeasing appearance or functional impairment.

Mechanism of LHR

The theory of selective photothermolysis enables precise targeting of pigmented hair follicles by using the melanin of the hair shaft as a chromophore. Melanin has an absorbance spectrum that matches wavelengths in the red and near-infrared (IR) portion of the electromagnetic spectrum. To achieve permanent hair removal, the biological ‘target’ is the follicular stem cells located in the bulge region and / or dermal papilla. Due to the slight spatial separation of the chromophore and desired target, an extended theory of selective photothermolysis was proposed that requires diffusion of heat from the chromophore to the desired target for destruction. This requires a laser pulse duration that is longer in duration than if the actual chromophore and desired target are identical. Temporary LHR can result when the follicular stem cells are not completely destroyed, primarily through induction of a catagen-like state in pigmented hair follicles. Temporary LHR is much easier to achieve than permanent removal when using lower fluences. Long-term hair removal depends on hair color, skin color, and tolerated fluence. Roughly 15–30% long-term hair loss may be observed with each treatment when optimal treatment parameters are used (Fig. 4.2). A list of laser and light devices that are currently commercially available for hair removal is given in Table 4.1.

Figure 4.2 Laser hair removal is safe and effective. (A) The upper cutaneous lip of a hirsute female. (B) Appearance of same subject following only three treatments with a long-pulsed 755 nm alexandrite laser used with a 12 mm spot size, 16 J/cm², 3 ms pulse duration and DCD setting of 30/30/0. (C) Axilla of an adult female. (D) Following four treatments with a long-pulsed diode laser with a large spot size and vacuum-assisted suction. The fluence used was 12 J/cm² and the pulse duration was 60 ms. Both of the above subjects achieved excellent hair reduction and further benefit would likely be attained with additional treatments.

Table 4.1 Commercially available lasers and light sources for hair removal^*

Key factors in optimizing treatment

LHR has revolutionized the ability to eliminate unwanted hair temporarily and permanently in many individuals of all skin colors. Proper patient selection, preoperative preparation, informed consent, understanding of the principles of laser safety, and laser and light source selection are key to the success of laser treatment. An understanding of hair anatomy, growth and physiology, together with a thorough understanding of laser–tissue interaction, in particular within the context of choosing optimal laser parameters for effective LHR, should be acquired before using lasers for hair removal.

Patient selection

A focused medical history, physical examination, and informed consent, including setting realistic expectations and potentials risks, should be performed prior to any laser treatment (Box 4.1). Patients with evidence of endocrine or menstrual dysfunction should be appropriately worked up. Similarly, patients with an explosive onset of hypertrichosis should be evaluated for paraneoplastic etiologies. Treatment of a pregnant woman for non-urgent conditions is discouraged by the authors, although there is no evidence suggesting a potential risk to pregnant women undergoing LHR. The past medical history should be reviewed to identify patients with photosensitive conditions, such as autoimmune connective tissue disorders, or disorders prone to the Koebner phenomenon. A history of recurrent cutaneous infections at or in the vicinity of the treatment area might warrant the use of prophylactic medications. Any past history of keloid or hypertrophic scar formation should be elicited as well. Previous hair removal methods, including past laser treatments, should be reviewed. Any methods of hair shaft epilation (e.g. waxing or tweezing) that entirely remove the target chromophore render LHR less effective for at least 2 weeks. Although there is little evidence for the time frame a patient must wait after complete epilation of the hair shaft and laser treatment, we recommend a minimum of 6 weeks. Shaving and depilatory creams can be used up to the day of laser treatment as they do not remove the entire hair shaft.

Box 4.1

Pertinent medical history for laser / pulsed light hair removal