Recent developments in cutaneous lasers.

Laser surgery is one of the most rapidly advancing fields in medicine. In dermatology and ophthalmology, lasers now routinely provide the most precise form of surgery ever practiced, often for cosmetic goals. Consumer demand has generated intense interest from physicians and industry. This has led to the development of safer and more effective lasers with a wide range of applications. The theory of selective photothermolysis, introduced by Anderson and Parrish in 1981, is the basis for many advancements in dermatological lasers [1]. It allows for highly localized destruction of light-absorbing “targets” in skin, with minimal damage to the surrounding tissue. To achieve selective photothermolysis, appropriate wavelength, exposure duration, and sufficient fluence are necessary. Various targets absorb at different wavelengths, and the wavelength of the laser should be absorbed more by the target structure than by surrounding structures. Light absorbed in the target structure is converted to heat, which immediately begins to diffuse away. In general, exposure duration should be shorter than or about equal to the thermal relaxation time of the target. Thermal relaxation time is the time it takes the target to cool significantly and is proportional to the square of the diameter of the target. Therefore, the thermal relaxation time of a small object is much shorter than that of large objects. A useful approximation is that the thermal relaxation time in seconds of a target is equal to the square of its diameter in millimeters. Thus, a 0.1-mm blood vessel cools significantly in about 0.01 sec (10 msec). Finally, sufficient energy must be delivered to cause the desired effect, destruction of the target. Fluence (energy/area) necessary for treatment is inversely proportional to the fraction of light absorbed by the targets. Thus, higher fluence is necessary when using weakly absorbed wavelengths, treating targets that contain less chromophore (the light absorbing substance), or targets that are deep within the skin. Clinically, selective photothermolysis involves ensuring that a maximum tissue-damaging temperature occurs only in the desired tissue targets. When treating dermal targets (blood vessels, tattoos, hair, etc.), light must pass through the epidermis. Epidermal injury is the most frequent cause of side effects in these settings. A classic example of selective photothermolysis is the use of pulsed lasers in the treatment of port-wine stains (PWS). PWS consist of dilated venules in which the principal chromophore is oxyhemoglobin. The principal absorption peaks of oxyhemoglobin are in the blue–green– yellow portion of the visible range (418, 542, and 577 nm). In general, longer wavelengths penetrate more deeply into skin because of less scattering by dermal collagen, and the wavelengths 577–595 nm, within one absorption band of oxyhemoglobin, are well suited to target vessels in PWS. The first lasers designed for selective photothermolysis of PWS were pulsed dye lasers emitting pulses of about 1 msec duration. This pulse width corresponds to thermal relaxation time of vessels down to about 30 mm in diameter, typical for pediatric PWS. Laser light absorbed by hemoglobin is converted into heat, which damages the endothelium and surrounding vessel wall, followed by thrombosis, a vasculitis, and the removal of the abnormal venules [2]. The ideal pulse duration for PWS treatment is probably approximately 10 msec [3], and several laser systems are emerging that emit longer pulses.