It has long been observed that select patients who had sustained relatively superficial burns to the face frequently noted improvement of the texture and pigmentation of their face after the burn had healed—sometimes with a dramatic decrease in superficial wrinkling. This has been seen in flash burn injuries from gas stoves and mine gas explosions. For a number of years, attempts have been made to replicate these results in a reliable and safe way. Given that therapeutic measures affecting only the epidermis did not effectively address the stubborn problems of superficial wrinkling and acne scarring, it was recognized early on that this would require the development of a means of applying heat in a very specific fashion so as to maximize the therapeutic effect with penetration of the papillary, and even the superficial reticular, dermis. At the same time, in order to avoid the risks of pigmentary changes and scarring, it was necessary to minimize damage to the skin microcirculation, the adnexal organs, dermal melanin granules and melanocytes, and the deeper reticular dermis. This was a tall order. Direct sources of heat energy are terribly nonspecific, indiscriminately damaging anything in their paths.

With the development in the mid-1960s of a unique light source with unusual characteristics called the light amplification by stimulated emission of radiation (laser), it was immediately evident that this device held promise of being the selective source of heat which could accomplish safe and effective resurfacing. This is because the physics of lasers enabled the release of energy in a form of light that could theoretically penetrate a specific structure before changing into another form of energy: heat. Because light absorption is specific in pigmented structures and the organic world is full of pigmented structures, one could then select a laser that emits a particular "color" of light that would be maximally absorbed by a particular "colored" material in the skin (technically "wavelength" is more accurate than "color," as color refers only to the visible wavelengths). If the heat could be applied very selectively, the laser would enable specific cellular damage, permitting safe and effective resurfacing.

The first task was to choose the correct laser wavelength. For the sake of simplicity, one can assume that there are three light-absorbing structures or chromophores in the skin: melanin, hemoglobin, and water. Clearly, resurfacing attempts to apply energy superficially, avoiding melanin and hemoglobin. Since tissue water is abundant in the skin, laser wavelengths that are absorbed maximally in water are potentially those that may be useful in resurfacing. The three wavelengths that are maximally absorbed in water are CO2, erbium-YAG, and excimer. The first two are infrared (IR); the latter ultraviolet (UV). UV light, with its propensity for breaking bonds in DNA molecules, is con-traindicated in rapidly dividing skin, whereas it is very useful in the relatively acellular hydrophilic environment of the cornea.

Of the remaining two wavelengths, the erbium-YAG has a greater affinity for water; therefore, its effect is more superficial than that of the CO2. In addition, less heat is generated with the erbium, which is advantageous in that it produces less post-inflammatory erythema, but disadvantageous in that it does not produce sufficient heat to stimulate neocollagen formation and reversal of elastosis, as occurs with the CO2 laser. Each laser has found a different role in the resurfacing armamentarium, but the CO2 laser remains the gold standard for the treatment of surface wrinkles and, many believe, acne scarring.

The continuous-wave CO2 laser was one of the first available in medicine. When resurfacing was first attempted with this laser, results were decidedly mixed. The problem was that the continuous wave inevitably pumped excessive heat into the skin. Therefore, the complications of scar formation and pigmentary changes were common, and the procedure was considered too risky for adoption.

The next major advance in the field of resurfacing was the recognition that the skin model was able to divest itself of excess heat if the energy was applied very rapidly and in large amounts. The observed value for the threshold at which heat was no longer efficiently divested, and instead was conducted to adjacent tissue (leading to dermal scarring), is known as the thermal relaxation time; in human skin it is less than 1 ms in duration. CO2 lasers capable of delivering the heat in a duration of less than 1 ms were found to be effective and safe to use for resurfacing. The laser manufacturers chose one of two methods to achieve this: one was to use collimated pulsed devices that put out discrete packets of light, each high in energy and less than 1 ms in duration. This type of laser is epitomized by the Coherent UltraPulse laser (Coherent Medical Group, Santa Clara, CA). The other method was to use a scanning device attached to a continuous-wave laser, which moved the beam very rapidly so that any one unit area of skin never received energy for more than the thermal relaxation time. The Sharplans devices (ESC/Sharplan Medical Systems, Needham, MA) took this path. Both devices are quite capable of producing excellent and safe resurfacing, but it is the author's personal opinion that the collimated, pulsed devices are inherently more user-friendly and safer. Other improvements in these devices involve ease and rapidity of use. The actual delivery of dose is a mature engineering task and has not changed dramatically during the past 5 years.

During the past 5 years, there has been a trend toward the erbium lasers and away from CO2. This preference has been shaped by a number of factors. First, there is no doubt that the postoperative course is easier on the erbium laser patient (and surgeon) than the CO2 laser patient. Patients treated with the erbium laser heal faster (4 to 7 days vs 8 to 10 days) and have significantly less erythema afterward. However, in my opinion, erbium laser treatment is simply unable to deliver the level of improvement seen with the CO2 laser. This is probably because the erbium laser does not generate the 55 to 70°C necessary to stimulate neocollagen formation. Although some laser manufacturers claim that they can replicate CO2 treatment with their erbium laser by repeated overtreatment, the reality is that the CO2 remains the most effective treatment for deep wrinkles, acne scarring and muscle action lines. Having said that, if you wish to treat the neck or dorsal hand, it is far safer to do so with the erbium laser than the CO2 laser.

More recently, newer devices claim to combine the two wavelengths to take advantage of both. Generally these are low-power lasers (usually on the CO2 side). No credible published data confirm these sales claims. Likewise, the efficacy of the 1380-nm Nd-YAG laser, which purports to treat the collagen of the dermis without in any way damaging the epidermis, awaits clinical evidence.

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