Lasers in Dermatology

In dermatology, thermal effects of laser radiation are commonly used, especially coagulation and vaporization. Since the optical parameters of skin, i.e. absorption and scattering, are strongly wavelength-dependent, various kinds of tissue reaction can be evoked by different laser systems. In clinical practice, mainly five types of lasers are currently being used: argon ion lasers, dye lasers, CO2 lasers, Nd:YAG lasers, and ruby lasers.

A schematic cross-section of the human skin is given in Fig. 4.61. The skin grossly consists of three layers: epidermis, dermis, and subcutis. The outer two layers - epidermis and dermis - together form the cutis. The epidermis contains so-called keratocytes and melanocytes which produce keratin and melanin, respectively. Both keratin and melanin are important protective proteins of the skin. Most of the dermis is a semi-solid mixture of collagen fibers, water, and a highly viscous gel called ground substance. The complex nature of the skin creates a remarkable tissue with a very high tensile strength which can resist external compression but remains pliable at the same time. Blood vessels, nerves, and receptors are primarily located within the subcutis and the dermis.

Image Skin Cross Section Colour
Fig. 4.61. Cross-section of human skin

On a microscopic scale, the air-skin interface is quite rough and therefore scatters incident radiation. Absorption of light by chromophores such as hemoglobin or melanin give skin its unique color. Optical scattering by collagen fibers in the ground substance largely determines which fraction of incident light penetrates into connective tissues. Detailed studies regarding the optical parameters of skin were performed by Graaff et al. (1993b).

Radiation from the argon ion laser is strongly absorbed by hemoglobin and melanin as already illustrated in Fig. 2.4. This laser is thus predestined for superficial treatments of highly vascularized skin. Apfelberg et al. (1978) and Apfelberg et al. (1979a) investigated laser-induced effects on various abnormalities of the skin. The most frequent indications for the application of argon ion lasers are given by port wine stains (naevi flammei). Earlier methods of treating these malformations - e.g. cryotherapy, X-ray, or chemical treatment - had failed, and patients were advised to accept their misery. The idea of removing port wine stains with argon ion lasers has led to the most significant progress of lasers in dermatology so far. The treatment itself requires a lot of patience, since several sessions are necessary over a period of up to a few years. The faster the treatment is to come to an end, the higher is the probability for the occurrence of scarring. However, "patient" patients are usually rewarded with an acceptable outcome. In Figs. 4.62a-b, two photographs of the pre- and postoperative states of a laser-treated port wine stain are shown.

Treatment of port wine stains with argon ion lasers is usually performed in several sessions. First, a small test area of approximately 4 mm2 is irradiated. During this test, a suitable laser power is determined by gradually increasing it until the skin visibly pales. According to Dixon and Gilbertson (1986) and Philipp et al. (1992), laser powers of 2-5 W are applied during an exposure time of 0.02-0.1s. Immediately after laser exposure, inflammation of the skin frequently occurs. After four weeks, the test area is checked for recanalization and scarring. And after another four weeks, a second test area is treated. If both tests lead to acceptable results, the whole stain is exposed. Multiple exposures of the same area should be avoided in any case. Laser treatment may be repeated after a few years, but it is advisable to choose pulsed dye lasers for the second time. Haina et al. (1988) did not recommend treatment of patients up to 16 years of age, since otherwise severe scarring might occur. Laser radiation is usually applied by means of a flexible handpiece. In the treatment of facial stains, the eyes of both patient and surgeon must be properly protected. One disadvantage of treating port wine stains with argon ion lasers is that it is rather painful to the patient. Depending on the location and spatial extent of the stain, treatment is performed during either local or complete anesthetization.

Less painful and probably even more efficient is the treatment of port wine stains with dye lasers. Although quite expensive, these machines have recently gained increasing significance in dermatology, especially in the treatment of port wine stains and capillary hemangiomas. Detailed studies were reported by Morelli et al. (1986), Garden et al. (1988), and Tan et al. (1989). Frequently, Rhodamine dye lasers are used which emit radiation at wavelengths in the range 570-590 nm. Typical pulse durations of 0.5 ms and energy densities of 4-10 J/cm2 have been recommended. About 20-60 s after laser exposure, the color of the treated skin turns red, and after another few

Argon Lasers Dermatology
Fig. 4.62. (a) Preoperative state of a port wine stain. (b) Postoperative state of the same stain after several treatments with an argon ion laser (pulse duration: 0.3 s, power: 2.5 W, focal spot size: 2 mm). Photographs kindly provided by Dr. Seipp (Darmstadt)

minutes livid blue. Although pain is less pronounced as with argon ion lasers, patients frequently talk of triple pain perception: mechanical impact during the light flash, stabbing pain shortly afterwards, and finally a longer lasting heat wave within the skin. The irradiated area itself might be irritating for several days. One major advantage of treating port wine stains with dye lasers is that this procedure can be successfully performed among children as reported by Tan et al. (1989).

The basic mechanism by which pulsed laser radiation can cause selective damage to pigmented structures in vivo has been termed selective photother-molysis and was thoroughly described by Anderson and Parrish (1983). It requires the presence of highly absorbing particles, e.g. pigments of the skin. Extensive experimental and theoretical studies were recently performed by Kimel et al. (1994) and van Gemert et al. (1995). With their results, treatment of port wine stains will be further improved in the near future.

In dermatology, the CO2 laser is used for tissue vaporization. Compared to the conventional scalpel, it offers the possibility of precise tissue removal without touching the tissue. Thus, feeling of pain is significantly reduced. External ulcers and refractory warts are common indications. In warts, however, deep lesions should be performed to reduce the probability of recurrence.

Recently, argon ion and CO2 lasers have also gained attention in efficiently removing tattoos. Clinical studies were reported by Apfelberg et al. (1979b) and Reid and Muller (1980). Today, ruby lasers are commonly used for tattoo removal as stated by Scheibner et al. (1990) and Taylor et al. (1990). Indeed, good results can be obtained, although they do depend on the dyes used in the tattoo. It is extremely important that all dye particles are removed during the same session. In Figs. 4.63a-b, two photographs are shown which prove the efficiency of laser-induced tattoo removal.

Radiation from the Nd:YAG laser is significantly less scattered and absorbed in skin than radiation from the argon ion laser. The optical penetration depth of Nd:YAG laser radiation is thus much larger. According to Seipp et al. (1989), major indications for Nd:YAG laser treatments in dermatology are given by deeply located hemangiomas or semimalignant skin tumors. However, argon ion and CO2 lasers should never be replaced by Nd:YAG lasers when treating skin surfaces.

Dermatology is one of the few medical disciplines where biostimulative effects of laser radiation have been reported. Positive stimulation on wound healing is one of the current topics of controversy as discussed in Sect. 3.1. A considerable number of papers has been published, but most of the results could not be reproduced, and initial claims could thus not be verified. Moreover, the principal mechanisms of biostimulation have not yet been understood. In general, one should be very careful when using laser radiation for such purposes, especially when applying so-called "soft lasers" with extremely low output powers which most probably do not evoke any effect at all other than additional expenses according to Alora and Anderson (2000).

Large Intestine Ruminant
Fig. 4.63. (a) Preoperative state of a tattoo. (b) Postoperative state of the same tattoo after six complete treatments with an argon ion laser (pulse duration: 0.3 s, power: 3W, focal spot size: 0.5 mm). Photographs kindly provided by Dr. Seipp (Darmstadt)
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