Principles for Photoepilation with Lasers

Light energy emitted from a monochromatic laser (a single wavelength of light) or an IPL source (produces a broad spectrum of light using filters to block unwanted wavelengths) is absorbed by a pigmented object (in this case, hair) and is converted into heat energy. This heat energy destroys the hair follicle, causing a long-term reduction in hair growth. This process of transforming light to heat energy and destroying targeted pigmented tissue is called selective photothermolysis.

The concept of selective photothermolysis was first presented by Anderson and Parrish in 1983. This is the process by which thermal damage is confined to the particular target tissue. It is based on two important concepts. The first is that chromophores (e.g., hair, blood vessels, melanosomes/pigment) are objects that preferentially absorb light of specific wavelengths (Fig. 48-1). The second is thermal relaxation time (TRT), which is defined as the time required for an object to cool to 50% of the temperature resulting from laser exposure. When the target tissue absorbs the laser light, energy is changed to heat, which causes thermal tissue damage and heat transference to the surrounding tissues. For lasers, pulse width is the duration (in milliseconds [msec]) that the light energy is applied. A laser with a pulse width less than the TRT of the target conducts very little heat to the surrounding tissues. Consequently, it is possible to confine the laser’s destructive effect to a specific area of tissue based on the chromophore content and the rapidity at which the light energy is applied (Fig. 48-2).

Figure 48-1 Relative absorption of light by biologic tissues. By selecting specific wavelengths of light, a selective effect on biologic tissue is achieved. Whenever light hits tissue, it can be transmitted, scattered, reflected, or absorbed, depending on the type of tissue and the selected wavelength(s) (color) of the light. However, light absorption and subsequent tissue heating must take place to achieve any biologic effect, and a given wavelength of light may be strongly absorbed by one type of tissue and be transmitted or scattered by another. Different tissues have different absorption characteristics depending on their specific components (i.e., skin is composed of cells, hair follicles, pigment, blood vessels, sweat glands). The main absorbing targets, or chromophores, of tissues are (1) hemoglobin in blood; (2) melanin in skin, hair, and moles; and (3) water (present in all biologic tissue).

Figure 48-2 Three principles of selective photothermolysis: (1) Penetrating wavelength of light should be absorbed selectively by target tissue; (2) pulse duration should match thermal relaxation time of target tissue; and (3) sufficient fluence (J/cm²) should be applied to damage target tissue.

(Redrawn from a presentation by Brian Zelicksen, MD, American Society for Laser Medicine and Surgery Meeting, April 2002.)

Lasers are monochromatic, which means the light is of a single wavelength or color. Each type of laser has a different wavelength and each chromophore absorbs the specific light energy in that laser wavelength, converting it to thermal energy. Melanin is the target chromophore for photoepilation. Hemoglobin, another chromophore, is targeted to destroy vascular lesions. Water and collagen are also chromophores. Each chromophore has a spectral absorption pattern that determines the wavelengths of light that are most absorbed and converted to thermal energy. It is important to match the wavelength of the laser with the specific chromophore for targeted destruction and to avoid damage to adjacent tissues. The amount of energy needed to destroy the hair follicle is usually close to the energy level at which skin damage occurs. Determining the treatment energy level for a particular patient without causing excessive surrounding tissue damage is the most clinically demanding task. Certain types of lasers are then chosen because they are more effective at destroying hair follicles without causing excessive surrounding damage, which could lead to burns or scarring.

The lasers used for epilation fall into four categories: alexandrite (755 nm), neodymium-doped yttrium aluminum garnet (Nd:YAG; 1064 nm), ruby (694 nm), and diode (810 nm).

Treatment tip or spot sizes vary from 2 to 15 mm, and pulse widths vary from 10 to 100 msec. In addition, the repetition rate can vary from 1 to 10 or more pulses per second (hertz). Faster repetition rates improve efficiency in treating larger areas such as the back or legs. The size of the area being affected by each burst of energy (spot size) is variable from machine to machine and from laser tip to laser tip. In general, the larger the spot size, the greater the depth of light penetration at the same energy level. Lasers and IPL machines most often measure the energy level delivered to the skin in joules per square centimeter (J/cm²), also known as fluence (Fig. 48-3A).

Figure 48-3 Intense pulsed-light and laser system. A, Lumenis One. B, LightSheer handpiece. C, Multi-Spot Nd:YAG handpiece.

(Courtesy of Lumenis, Santa Clara, Calif.)

Intense pulsed-light (Fig. 48-3B and C) devices are flashlamp devices that emit light over the entire visual spectrum and hence are not monochromatic like lasers. Specificity for different hair colors and skin types may be achieved with various cut-off filters and fluence settings. Recently, IPL devices have increased in popularity because of their versatility and cost effectiveness. With one device and multiple handpieces/filters, physicians can treat most conditions. With regard to photoepilation, IPL devices are now equally as efficacious as lasers. In addition, a hybrid device (Syneron) uses a combination of IPL followed by radiofrequency. It is based on the principle of electrical impedance, whereby electrical energy flows preferentially to a warmer target. This device reportedly has some beneficial effects for gray and white hairs.

The ideal patient for photoepilation is someone with light skin and dark hair, so that all the generated thermal energy is focused on the melanin chromophore of the hair. Light hair is very difficult to treat because it is not differentiated from the surrounding light skin. Studies are ongoing using a melanin dye (Melamax; Creative Technologies, Chesapeake, VA) that is driven into the hair root by an ultrasound device. Preliminary data showed modest improvement in the effect of hair removal lasers on white or blonde hair. The effect was greater on vellus than terminal hairs. Similarly, it is very difficult in most instances to treat black hair on black skin. When patients are tanned, care must be taken lest the skin itself be burned during the treatments.

Cooling during treatment is important. IPL devices contact the skin and use a combination of a chilled gel and a chilled sapphire tip. The latter is a vast improvement over glass tips, which cracked frequently from the stress of temperature changes. Some devices (e.g., LightSheer Diode Laser; Lumenis, Santa Clara, Calif) use a cold-water chamber to cool tissue. Dynamic cooling devices use a chlorofluorocarbon, U.S. Environmental Protection Agency–approved refrigerant spray. This is available only on lasers produced by Candela (Wayland, Mass), which has proprietary rights to the technology. A final alternative chilling method is using cold air blown on the site being treated (Zimmer MedizinSystems Corp., Irvine, Calif).

The FDA has recently approved a home-use laser (TRIA Beauty, Dublin, Calif) for hair removal. This is a diode laser and costs approximately $600. Preliminary data, although limited, show beneficial results.

Hair Growth Phases and Treatment Implications

Hair follicles have been difficult to treat with lasers. There is a wide variation in depth at different anatomic sites, with the deepest follicular bulbs at 5 mm. This is beyond the range of penetration of most lasers (Figs. 48-4 and 48-5). Whereas upper lip hair follicles range from 1 to 2.5 mm in depth, pubic and axillary hairs may lay as deep as 5 mm. In addition, most authorities agree that only the anagen phase of follicular growth is responsive to laser-induced thermal energy damage. The percentage of follicles in the anagen phase at any one anatomic site varies from 30% on the trunk to as high as 80% on the scalp. Therefore, to understand laser hair removal more fully, a knowledge of hair anatomy and development is necessary.

Figure 48-4 Depth of penetration of various cutaneous lasers. Alex, alexandrite; Er, erbium; KTP, potassium titanyl phosphate; PD, pulsed dye.

Figure 48-5 Depth of hair follicles in different body locations. Dotted line indicates penetration depth of light frequency.

Hair is composed of keratinous fibers that grow from follicles over the entire body surface except the palms and soles. The number of follicles is finite at birth. Growth involves three stages (Fig. 48-6). Anagen is the active growth phase of the hair follicle, during which the hair contains abundant melanin. Catagen is a period of regression when cell division terminates in the long part of the follicle and the lower part of the follicle begins to involute. The final, resting phase is called telogen, during which the old hair is emitted and shed before the development of a new hair begins. During telogen there is very little or no melanin in the follicle and hence laser treatments will have very little to no effect. The length of these three individual phases of hair growth varies widely with anatomic site (Table 48-1). Because of this, patients must be advised that 100% hair reduction may be impossible because of the relative unresponsiveness of the telogen follicle to laser photoepilation. Hairs, particularly on the trunk, may remain in telogen for longer than 3 months. Therefore, patients need to be advised that follow-up treatments may be needed up to 1 year after initiation of therapy to allow for conversion of telogen hairs to anagen.

Figure 48-6 The different phases of the hair cycle: anagen, catagen, and telogen. Labeled structures include arrector pili muscle (APM), bulge (B), cortex (C), dermal papilla (DP), epidermis (E), inner root sheath (IRS), matrix (M), medulla (Md), outer root sheath (ORS), and sebaceous gland (S). B and B* denote quiescent and activated bulge cells, respectively. Follicular structures above the dotted line form the permanent portion of the follicle; keratinocytes below the bulge degenerate during catagen and telogen.

(Redrawn from Cotsarelis G, Sun TT, Lavker RM: Label-retaining cells reside in the bulge area of pilosebaceous unit: Implications for follicular stem cells, hair cycle, and skin carcinogenesis. Cell 61:1329–1337, 1990.)

TABLE 48-1 Hair Depth and Hair Cycle

Hairs are of two types: terminal hairs are thick, long, and pigmented with melanin and found throughout the body surface, whereas vellus hairs are thin, short, and depigmented.

Figure 48-6 demonstrates the structure and life cycle of a typical hair. The hair itself grows from the bulb, which consists of the hair matrix and the dermal papilla. The papilla is an area of highly vascularized connective tissue that provides the nutrients for the rapidly dividing cells of the matrix. During periods of active growth, matrix cells divide every 24 to 72 hours and migrate upward to become keratinized and packed into layers that compose the hair shaft.

The “bulge,” which is a protrusion near the attachment of the arrector pili muscle, has recently been determined to consist of stem cells important in hair regeneration. The bulge is generally located 1 to 1.5 mm below the cutaneous surface. As mentioned previously, hairs grow in recurrent cycles (see Table 48-1). Therefore, the target of laser thermolysis is twofold: the bulge and the papilla.