LASER HAIR REMOVAL:SCIENTIFIC PRINCIPLES AND PRACTICALASPECT

Keywords:medical fiber optics, surgical fibers,  Time:02-11-2015

ABSTRACT

The use of lasers for hair removal has been studied for a number of years.  In this procedure, laser light is absorbed by melanin in the hair shaft, damaging the follicular epithelium.  Aclinical study evaluated the use of the LightSheer™ Diode Laser for hair removal. Of 92 patients, all had temporary hair loss and 89% had permanent hair reduction.  Regrowing hairs were shown to be thinner and lighter than previously.  Extensive clinical use of this high-power, pulsed diode laser has resulted in recommendations for patient selection and proper use of the laser.  Appropriate fluence settings have been shown to cause permanent hair reduction without damaging the epidermis, regardless of skin type.（Medfibers provides high quality Medical Fiber,as Disposable Bare Fiber,Diffuser Fiber,Holmium Fiber,Polyimide Fiber,Side Fire Fiber）

BACKGROUND

Laser hair removal focuses on the endogenous chromophore melanin, which is mainly found in the hair shaft, with a small amount present in the upper third of the follicular epithelium (Figure 1).  When an appropriate energy source (such as a laser) is directed at the skin, light is primarily absorbed in the hair shaft melanin.  Heat is generated and diffuses to the surrounding follicular epithelium.  Asimilar principle applies to laser treatment of vascular lesions, where the heat generated after absorption by hemoglobin is transferred from the blood to the vascular endothelial cells.

Laser hair removal is based on the principles of selective photothermolysis: a combination of the appropriate laser wavelength, pulse duration, and fluence.

Wavelengths between approximately 700 and 1000 nanometers (nm) are selectively absorbed by melanin; the competing chromophores (oxyhemoglobin and water) absorb less energy at these wavelengths.  Figure 2 shows the absorption of different chromophores in the skin.  Therefore, any light source that operates between 700 and 1000 nm is appropriate for targeting melanin in the hair shaft.

Figure 2.  The absorption of various chromophores as a function of wavelength.  Ruby lasers operate at 694 nm, alexandrite lasers at 755 nm, diode lasers at 800 nm and Nd:YAG lasers at 1064 nm.  (Adapted from Boulnois JL.  Photophysical processes in recent medical laser developments:  a review.

Pulse duration (or pulse width) must be equal to or shorter than the thermal relaxation time of the target to confine thermal damage.  The thermal relaxation time of the whole follicular structure depends on its diameter and is on the order of tens of milliseconds.  Consequently, the laser source must have a range of pulse widths to selectively damage different size follicles.

Pulse width must be matched with the appropriate amount of fluence (energy per unit area) necessary to cause follicular damage.

Hair removal devices available today include 694 nm ruby lasers, 755 nm alexandrite lasers, 800 nm diode lasers, 1064 nm Nd:YAG lasers, and filtered xenon flashlamps.  This paper focuses on an 800 nm diode laser (LightSheer Diode Laser, Lumenis Inc., Santa Clara, CA). This wavelength effectively targets the melanin while deeply penetrating the dermis.

HAIR LOSS AND REGROWTH

One hundred patients were treated in a clinical study with the high-power, pulsed diode laser. The study evaluated different combinations of fluence and pulse width in eight test sites.  The patients were followed-up at one, three, six, nine, and 12 months following the last treatment. Ninety-two patients completed the study.  Hair loss was assessed from hair counts using digital photographs before treatment and at each follow-up visit.  Tattoos identified the location of each test site.

The study showed that the high-power diode laser induces two separate effects: temporary hair loss and permanent hair reduction.

Temporary hair loss occurs in all patients, for all hair colors and at all laser fluences.  It usually lasts from one to three months.

Permanent hair reduction is defined as a significant reduction in the number of terminal hairs at a given body site that is stable for a period of time longer than the follicles’complete growth cycle (Figure 3, Table I).  Test sites were mainly given on the back and thighs, where complete hair growth cycles vary between six months and a year.  Aone year follow-up allowed time for one to two complete growth cycles at these anatomic sites.

There is a difference between permanent hair reduction and complete hair loss.  Complete hair loss implies that there are no regrowing hairs.  This can be a temporary or permanent phenomenon.  The LightSheer Diode Laser  usually produces complete but temporary hair loss, followed by a partial but permanent hair reduction.  This is an important distinction to make when setting patient expectations.

With this laser, 100% of the patients experienced temporary hair loss, while 89% of the patients had permanent hair reduction at one year follow-up.  Of the 11% of patients who did not have long-term hair loss, most had blond hair.  Because blond hair contains less melanin than darker hair, there is less chromophore for the laser to target, and the response is less. However, these patients experienced temporary hair loss.

Numbers cited for hair loss only take into account the absolute number of hairs. They do not reflect the fact that the regrowing hairs are lighter and thinner than before, which also adds to apparent clinical hair loss. Hair color was measured by calculating the absorption coefficient from the hairs’transmission of 700 nm light.  Hair diameter was measured from digital photographs.  The study showed that the regrowing hairs appeared lighter (with a transmission coefficient 1.41 times higher than the value before treatment) and were thinner (with a decrease in the mean hair diameter by 19.9%) than the original hairs.

Histologic observations support two mechanisms for permanent hair reduction: miniaturization of coarse hair follicles to vellus-like hair follicles, and destruction of the hair follicle with granulomatous degeneration, leaving a fibrotic remnant. Clinically, both of these mechanisms produced reduction in hair.

The study design used a fixed set of fluence-pulse-width combinations in each patient, regardless of skin type. If skin type and color had been matched to appropriate fluences, the incidence of side effects could have been reduced. Epidermal damage was seen in 6% of cases. Textural change occurred in 3% of cases, where triple pulsing was used at the highest fluence. These changes disappeared after three months.  Transient pigment changes were seen in about 10% of cases and usually occurred in the darker skin types or in patients who had tans at the time of treatment.