Many lasers have been proposed and clinically tested for use in the removal of excess or unwanted hair. When choosing the appropriate wavelength and pulse characteristics, tissue interactions should be thoroughly analyzed. The long pulse Nd:YAG laser may provide advantages over other wavelengths currently used. This can be especially true for darker skin types because of the reduced melanin absorption of this wavelength. Additionally, the right fluence and spot size paired with the long pulse Nd:YAG laser should provide an ideal combination of proper treatment dosimetry, minimum treatment time and clinical efficacy.

Technical Factors Relating to the Use of Lasers for Hair Removal

Achieving adequate results when using a laser to treat excess hair is a function of many aspects. Success will be determined, in part, by tissue physics, instruments, and physiology, as well as patient expectations, physician experience, and the pre- and post treatment care used. The following is a discussion of the first two aspects.

Experiments by Oliver, and later by Kim and Choi, indicate that successful destruction of unwanted hair may be achieved by injuring the middle third of the hair follicle—in particular, the bulb, the bulge and the outer root sheath.[i,ii,vii] Therefore, this region becomes the target tissue for any instrument used to create the required injury.

The concept of using a laser to produce the required injury and thus prevent the growth of unwanted hair has existed since the early days of laser use in medicine. Many different wavelengths of laser light as well as modalities have been investigated, and various commercial systems have been developed. Each individual system has had some limited degree of success in treating certain patients with certain skin types. No one system has proven to be a panacea. In order to achieve clinical success, the ideal laser system will need to produce significant injury to the follicle without producing other post treatment sequelae.

Wavelength considerations

Skin is an optically complex medium. The photon transport in the skin depends on its absorption and scattering properties. Additionally, the optical properties of human skin depend on person, anatomical location or even temperature. Photons of the wavelengths between 600 nm and 1200 nm penetrate deeper into skin, travelling a few millimeters before being absorbed in the dermis.[4]

Commercial lasers differ by wavelength, pulse duration, and fluence, all of which have some effect on the eventual outcome of the treatment. The wavelengths currently available include 694 nm (ruby lasers), 755 nm (alexandrite lasers), 800 nm (diode lasers), and 1064 nm (Nd:YAG lasers). These wavelengths were chosen because of their relatively high absorption in melanin (the primary target) and low absorption in other skin components. As skin color darkens with increased amounts of melanin, the depth of penetration decreases due to competition between melanin in the skin and hair. The absorption coefficient for melanin over the range of wavelengths drops by an order of magnitude moving from the shortest wavelength to the longest.

Since the concentration of melanin in a hair follicle always exceeds that in the skin DEJ (dermal-epidermal junction), most of the energy will get absorbed in the follicle. However, there is always a risk of some of the energy absorbed in the melanin of the skin. Thus, wavelengths less absorbed in melanin would be considered safer for darker skin, where the concentration of melanin is quite high (see Figure 1).[3]

Pulse duration considerations

Since it is clinically desirable to selectively treat the hair follicle without producing any injury to the surrounding tissue, laser energy is always delivered in single, short pulses. The target tissue approaches its maximum temperature once the laser light has been absorbed. It then cools by transferring the heat to the adjacent tissue. The time required for the target tissue temperature to decrease by 50% is called the thermal relaxation time (TRT). In order to destroy the target tissue, the laser pulse duration should approximate its TRT. Consequently, in order to avoid the destruction of tissue, the laser pulse duration should be much longer than the tissue TRT.

Because the epidermis is the first tissue exposed to the incoming beam of light, it is most likely to be injured if the energy is delivered for too short a period of time, i.e., the laser duration on tissue should exceed the TRT of the epidermis.[iii]

Likewise, in order to injure the hair follicle, the laser energy must be delivered in a pulse whose duration is shorter than or equal to the follicle’s TRT. If the pulse is too long, the follicle will be able to dissipate the energy before achieving destructive temperatures.

The TRT of the epidermis is estimated to be between 1 and 10 ms, depending on the thickness. The TRT for a hair follicle is estimated to be between 40 and 100 ms, depending on its diameter.[iv,v] Thus, the laser used for the removal of hair must be capable of delivering a pulse whose duration is longer than about 10 ms, but shorter than about 100 ms. To properly treat all hair thicknesses, the pulse duration should be close to 50 ms. Some commercial alexandrite, ruby and diode lasers are capable of producing pulse durations that fall within these specifications, but many are not.

Nd:YAG lasers can be either continuous wave, long pulse, or Q-switched. Pulses from a Q-switched Nd:YAG laser are much too short to assure long-term hair removal since their durations rarely exceed tens of nanoseconds. Pulsed-arclamp-pumped Nd:YAG lasers (sometimes referred to as "long pulse" Nd:YAG lasers) can produce a long enough pulse (up to 50 ms) at a sufficiently high energy level. Continuous wave Nd:YAG lasers could be "shuttered" to produce short pulses, but their output power would be too low to provide sufficient treatment energy.

Care should be taken to ensure that a laser system with the appropriate pulse duration is being used to achieve clinical efficacy. Many first generation hair removal lasers produced pulses that were significantly shorter in duration than suggested by the evaluation of the target tissue thermal relaxation time, as described above.[6]

Epidermal cooling

Besides tailoring the pulse duration to avoid the destruction of the epidermis, other methods can be employed to prevent thermal injury to the epidermis, such as cooling. Cooling the epidermis also allows higher amounts of energy to be used without adverse effects. An optimum cooling method will cool and protect the epidermis, while not cooling the hair shaft.

One cooling method consists of spraying the epidermis with a cryogenic spray. This spray instantly evaporates drawing heat from the epidermis. This temporarily freezes the epidermis. If applied correctly, the cooling will last for the duration of the local exposure to the laser energy. A new application is required for each epidermal area being lased. If applied too heavily, or for too long of a period, the spray can cause damage to the epidermis. When the epidermis freezes, its optical characteristics change. This can change the reaction of the epidermis to the incoming laser beam. If moisture is present in the air, then small ice crystals (frost) will form on the surface being sprayed. This white frost layer can reflect a substantial part of the laser light. Additionally, this method only cools a very shallow layer of skin leaving the layers below, unprotected.

A second cooling method involves applying a water-based gel to the epidermis. The gel is previously cooled in a refrigerator. This method is cumbersome to use and also changes the optical properties of the epidermis. Because the gel is absorbing heat with each pulse, as well as normal heat from the tissue, it must be wiped off and replaced with fresh, cool gel constantly during the treatment.

Another method to cool the skin is to drag a constantly chilled metal paddle along the treatment area immediately before the laser exposure. The temperature of the skin will depend on how long this metal piece will stay in the same area before this area is exposed to the laser energy. Unfortunately, the visibility of the treatment area is obscured during the treatment.

A preferred method of epidermal cooling is by means of an actively chilled contact window. The window is made out of a material that is optically transparent and has a high thermal conductivity—typically synthetic sapphire is chosen. The window is pressed against the skin and the laser is fired through it. The window is continuously cooled with circulating fluid chilled close to 0ºC. Active cooling in this manner allows the physician to maintain a constant temperature at the window and thus effectively cool the tissue below. For shorter wavelengths, an added benefit may be achieved by pressing the window against the skin, which helps to shorten the distance to the area where the target follicle resides.[vi] Most of the systems allow for a good visual of the treatment area. Some of these cooling devices, however, are attached to the handpiece in such a way that they block the view of the treatment area.

Spot size and delivery considerations

The size of the area on the epidermis exposed to each laser pulse varies greatly from laser to laser. With a large diameter beam (>2 mm), any of these wavelengths can produce sufficiently deep injury since over 50% of the output energy can reach tissue greater than 2 mm below the surface. Generally, however, a large spot size is desired for two reasons: a) to reduce the number of exposures required for a given treatment area; b) for laser energy to reach the required depth in tissue.

Depending on the laser wavelength, the scatter of the light in the skin before reaching the target chromophore varies. With equivalent fluences and absorption, greater scattering wavelengths require larger spot sizes to penetrate as deep as a longer wavelength with less scatter, e.g, a ruby laser would require a larger spot size than an alexandrite to reach the same depth in tissue, the alexandrite requires a larger spot size than an 800 nm diode laser and the diode would require a larger spot size than the Nd:YAG laser.

Another important factor is placing the spots in a precise manner. Since ideally during laser hair removal, visual tissue effect is minimal, it is quite difficult to distinguish treated areas from non-treated ones. Delivering the laser energy using a computer-controlled scanner greatly facilitates operation versus applying individual spots manually. The area covered with a scanner is larger than the area covered with individual spots, which simplifies the alignment and time issues. It additionally reduces the chance of overlap and thus decreasing excess thermal spread.

Figure 2*. This graph compares the percentage of Alexandrite, Ruby and Nd:YAG light that penetrates 3.3mm in white skin as a function of spot size. E.g., at 5mm spot size about 13% of Nd:YAG energy penetrates down to 3.3mm, while about 10% of Ruby and Alexandrite energy penetrates to the same depth.

Figure 3*. This graph compares the percentage of Alexandrite, Ruby and Nd:YAG light that penetrates 3.3mm in black skin as a function of spot size. E.g., at 5mm spot size about 13% of Nd:YAG energy penetrates down to 3.3mm, while about 5% of Ruby and 8% of Alexandrite energy penetrate to the same depth.

For large treatment areas, a scanner can decrease the treatment time, and increase the precision of energy placement on the tissue, as well as decrease operator fatigue. For instance, even though a 10 mm diameter beam in a 'free hand' manner will cover a 100 cm2 area slightly faster than a scanner using a 5 mm beam, the scanner only has to be repositioned 21 times versus 114 times for the 10 mm hand piece.

* Figures 2 and 3 are derived from Z. Zhao, P. Fairchild, "Dependence of Light Transmission Through Human Skin on Incident Beam Diameter at Different Wavelengths", SPIE Proceddings, Vol. 3254, 01.98

Conclusion

When evaluating laser systems from a strictly technical viewpoint, it is necessary to consider a multitude of factors: wavelength, pulse duration, spot size, and delivery device. In addition, clear clinical evidence of device efficacy should become part of the evaluation.

Footnotes:

i. Oliver RF. The experimental induction of whisker growth in the hooded rat by implantation of dermal papillae. J Embryol Exp Morphol 1967;18:43- 51.

ii. Kim J-C, Choi Y-C. Hair follicle regeneration after horizontal resectioning: implications for hair transplantation. In: Stough DB, Haber RS, eds. Hair replacement: surgical and medical. St Louis: Mosby-Year Book; 1995. p. 358-63.

iii. Olsen EA. Methods of hair removal. J Am Acad Dematol 1999; 40:143-55.

iv. Wheeland RG. Laser-assisted hair removal. Dermatol Clin 1997;15:459-77.

v. Grossman MC, Dierickx CC, Farinelli W, Flotte T, Anderson RR. Damage to hair follicles by normal-mode ruby laser pulses. J Am Acad Dematol 1996;35:889-94.

vi. Anderson RR, Grossman MC, Farinelli W. Hair removal using optical pulses. US Patent #5,735,844: 1998.

vii. E. Victor Ross, Zvi Ladin, M. Kriendel, C. Dierikx. Theoretical considerations in laser hair removal. Derm. Clinics, V 17, #2-99

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