How Lasers Work in Dermatology
Laser Skin Rejuvenation, Skin Blemish and Hair Removal Principles.
Aug 12, 2009
Different types of lasers, depending upon the medium they contain, emit specific wavelengths of laser light. These may either be in the visible spectrum of light, 400 nm (violet) to 700 nm (red) or in the infrared spectrum, above 700 nm.
What are the Chromophores in the Skin that are Targeted by Lasers?
Target cells and substances within the skin, called chromophores, depending upon their wavelengths, absorb the laser energy, and get heated up. The common chromophores in the skin that absorb the laser light are water, melanin, oxyhemoglobin and desoxyhemoglobin.
What is Meant by Selective Photothermolysis of Lasers in the Skin?
The heat dissipated by the laser light produces destructive and/or regenerative changes within the chromophore containing tissue. This is known as selective photothermolysis. Selective photothermolysis means a tissue is specifically targeted with laser light energy without affecting the surrounding structures.
When the laser light strikes the skin, it could be absorbed, transmitted, reflected or scattered. Only absorption produces any noticeable change within the tissue. The effects could be chemical, thermal or mechanical changes caused by the laser light energy. The more the energy absorbed, the greater will be the effect on the tissue.
What is Thermal Relaxation Time while Using Lasers in Dermatology?
The latest lasers with short pulses give time for the tissue to cool a little bit in between the laser beams, a phenomenon known as thermal relaxation. This reduces chances of thermal injury to the surrounding skin structures and minimizes chances of adverse effects like blistering, burning and scar formation. The thermal relaxation time is defined as the time taken by the target tissue to dissipate approximately 63% of the heat absorbed from the laser energy. In most laser systems, this thermal relaxation time is enhanced and assisted through use of in-built cooling system or external cooling with ice packs.
How Lasers Work in Vascular and Pigmented Skin Blemishes
In skin lesions where there is pooling of blood, as in hemangiomas, the chromophore is the oxyhemoglobin. Oxyhemoglobin absorbs laser light in the range of 418, 542 and 577 nm, the visible light range. In pigmented skin blemishes, the chromophore is the melanin, which has a broad range of absorption of both visible and infrared wavelengths. The heat absorbed destroys and disintegrates the target tissue which is then gradually absorbed or scavenged by the blood and lymphatic system in the skin. Thus the blemishes are cleared.
How Laser Skin Photo-Rejuvenation and Resurfacing Work
Skin cells contain 70-90% water and this absorbs the light energy produced in the range of the infrared rays. Many of the lasers like Erbium YAG act on the water content of the cells producing visible effects in the dermal and epidermal structures of the skin. These are particularly useful in laser assisted photo rejuvenation and laser resurfacing of the skin.
The Principle behind Laser Hair Removal
As the laser energy targets the melanin pigment in the hair follicles, the melanin pigment in the skin compete with the hair melanin and absorb the heat. As the laser energy settings for the hair removal is not tolerated by the skin, in darker skin types, damage to the skin resulting in hyper or hypo pigmentation, blistering or scarring may take place with normal lasers.
Some lasers like the NdYAG laser overcomes this difficulty by a process called homogenous photothermolysis, where the laser energy is absorbed by the hemoglobin in the blood supplied to actively multiplying cells in the hair germinal layer. The absorption by the melanin pigment is low in this case and collateral skin damage and complications are minimized even in dark skin types.
The destruction of the actively multiplying hair follicles through the destruction of the blood supply results in permanent destruction of the hair follicle. This is the principle behind the laser hair removal.
Thus the lasers work in dermatology through their unique action on the target tissues by a special characteristic known as selective photothermolysis.
Related Articles
Source
- Eubanks SW. Lasers in Dermatology. In Dermatology Secrets. 1996: 332-340, Eds. JE Fitzpatrick & JL Aeling; Hanley & Belfus, Inc.
- Focus on Fractional Laser Resurfacing
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