Nonsurgical Lasers for Treating Skin

One of the first clinical applications of selective photothermoly-sis was the pulsed dye laser. The first model was developed at the Candela Corporation of Massachusetts in 1983. This laser was engineered to selectively treat abnormal blood vessels in the skin. The primary objective of the first pulsed dye laser was to treat port wine stains, a type of birthmark composed of a patch of skin with a greatly increased density of capillaries (tiny blood vessels), enough to impart a permanent, intense red color. (Port wine stains arise in childhood and can be emotionally devastating if present in cosmetically sensitive areas such as the face or distal [exposed] extremities. In these lesions, the skin generally has a normal texture and in fact is normal except for the dense concentration of capillaries.) These excessive capillaries serve no physiologic function and can be removed completely from the skin without causing any harm.

Prior to the development of the pulsed dye laser, the argon laser was the best option for treating blood vessels. Introduced in the 1970s, the argon laser produces blue-green light with a wavelength of 514 nm. This color is well absorbed by the hemoglobin molecule in red blood cells and is near a peak in the absorption spectrum of hemoglobin (fig. 4.1), and thus has a selective effect on vascular tissue. This laser was used primarily by ophthalmologists to destroy abnormal blood vessels in the retina that occur in diseases such as diabetes and can lead to blindness if untreated. The argon laser was used with some success to treat cutaneous blood vessels. Most responsive were large facial vessels (telangiectases), which are common in people with the acne-like skin disease rosacea and can also occur in people who have had excessive chronic sun exposure. The physical and optical properties of port wine stains are different from those of telangiectases such that treating them with the argon laser was quite difficult. The laser characteristics effective in destroying the capillaries would also very likely damage the skin enough to cause a burn and a scar. The problems with the argon laser included a wavelength that was too short to enable adequate depth of penetration into the skin (thus not reaching the deeper capillaries of a port wine stain) and a pulse duration that was too long to result in selective photothermolysis.

The engineers at Candela Corporation tried to improve on the argon laser with a new laser design that enabled a very short pulse (less than half a millisecond). This laser was powered by a flash lamp: a bright electric lamp that flashed on for a brief time. The laser cavity contained a dye dissolved in alcohol. The dye was an organic compound and could be altered in such a way that the laser wavelength could be changed. The laser was thus tunable and could generate different wavelengths. It was called a "flash lamp-pumped tunable dye laser" or a "pulsed dye laser."

Consulting with dermatologists, these engineers tried different laser wavelengths to treat port wine stains. They found significant differences in treatment response with changes in wavelength as little as a few nanometers. Ultimately, the most effective wavelength for the majority of port wine stains was 585 nm. This wavelength is close to the absorption peak of hemoglobin (fig. 4.1) but is long enough to penetrate into the dermis, the skin level in which the blood vessels are located. This deeper penetration, combined with short pulse duration, gave the pulsed dye laser significant advantages in both efficacy and safety compared to the older argon laser.

Because telangiectases are relatively superficial, they can be effectively treated with shorter laser wavelengths than those required for port wine stains. Even continuous lasers such as the argon, the krypton, and newer green light (532 nm) diode lasers can be used with mechanically switched pulses (0.05 to 0.10 seconds) to safely remove these vessels. These continuous lasers require careful use because too long a pulse duration and/or too high a power level could damage the skin. When used properly, these lasers produce a practical selective photothermolysis because the unwanted vessels can be removed without damaging other skin components. Two additional lasers that are somewhat less common than the argon and krypton laser and that produce similar clinical results are the copper vapor and copper bromide lasers.

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