Over the years, numerous laser and light-based devices have been developed to effectively remove unwanted hair. These innovations have been based on applying principles of laser physics to selectively targeted hair follicles. As our understanding of hair biology has grown and laser technology has advanced, various different light-based sources have become increasingly effective and efficient in removing undesirable hair. This review will focus on the most popular laser devices commonly used today, which include the alexandrite, diode, and 1064 Nd:YAG lasers. In addition, intense pulsed light (IPL) devices for hair removal are now growing in popularity, and will also be reviewed.
Gurpreet S. Ahluwalia (ed.), Cosmetic Applications of Laser and Light-Based Systems, 145-156,
© 2009 William Andrew Inc.
Successful light-based hair removal is predicated on an understanding of hair biology. All human hair show various stages of hair growth [1]. The anagen or growth phase is variable in duration (up to six years) and leads to the catagen or regression phase, which is usually constant at around three weeks. The telogen or resting phase follows just prior to the resumption of the anagen phase and lasts approximately three months. At any given time, the majority of hair follicles (80-85%) are in anagen and the remaining follicles are either in the catagen phase (2%) or the telogen phase (10-15%).
The anagen duration varies greatly depending on age, season, anatomic region, sex, hormonal levels, and certain genetic predisposition. For example, scalp hair are in the anagen cycle from 48-72 months, while thigh and leg hairs are in anagen from 1-6 months. It is these variations that lead to the tremendous disparity in hair cycles reported by various investigators [1-5].
Long-term hair removal requires a laser or light source impact on one or more growth centers of hair. Anecdotal approaches have suggested that the pluripotential stem cells of the bulge, dermal papilla, and hair matrix must be treated in the anagen cycle for effective hair removal [2]. If the damage is not permanent during this cycle, it has been suggested that follicles will move into the telogen stage as they fall out. Thus, all the follicles may become synchronized after the first laser treatment, providing the patient a temporary reduction of approximately three months. The hair follicle will then return to anagen based on the natural hair cycle. However, this time-honored theory of optimal anagen treatment times has been challenged by Dierickx et al., whose widely accepted findings suggest that anagen/telogen cycling does not have the significant impact on laser induced response as was earlier thought [7]. In accordance with this view, most clinicians have found that attempts to correlate hair removal efficacy with growth cycles to be fruitless.
Another view has emerged since a recent study by Orringer et al. examined the effects of laser hair removal on the immunohistochemical properties of hair follicles using both a 1064 nm Nd:YAG laser and an 800 nm diode laser [8]. After a single laser treatment, they found that the immunostaining properties of the follicle, including the bulge region, remained mostly unchanged. They concluded that laser hair removal may occur by a functional alteration of follicular stem cells, rather than the commonly viewed theory of light – source-based hair destruction.
Currently, there is no agreement on a definition for treatment-induced “permanent” hair loss. In addition, there are no studies evaluating the long-term durability of laser hair removal. Permanence, defined as an absolute lack of hair in a treated area for the lifetime of the patient, may be an unrealistic goal. Most researchers agree with Dierickx et al., who have proposed to define “permanent” hair loss as a significant reduction in the number of terminal hair after a given treatment, that is stable for a period longer than the complete growth cycle of hair follicles at any given body site [6]. If no hair regrows after this time period, it can be assumed that the growth centers have no capacity to recover from injury, and are not simply in telogen.
I t is now accepted that almost any laser can induce at least a temporary hair loss. Fluences as low as 5 J/cm2 can induce this effect, which tends to last 1-3 months. The mechanism of action appears to be an induction of catagen and telogen. Permanent hair reduction, occurring at higher fluences is seen in 80% of individuals, seen with a variety of light-based systems, and is fluence-dependent.