ABSTRACT
The approach to the treatment of pigmented lesions with lasers depends on the anatomic location of pigment (epidermal, dermal or mixed), the nature of pigment (endogenous or exogenous), and its tissue distribution (extracellular, intracellular). In most pigmented lesions, the chromophore is melanin, although other exogenous and endogenous pigments can be targeted. In order to achieve specificity, it is necessary to use wavelengths that avoid absorption by other skin chromophores and penetrate to the desired depth. A selective window for targeting melanin lies between 630 nm and 1100 nm, where there is good skin penetration and preferential absorption of melanin over oxyhemoglobin. Short wavelengths (<600 nm) can damage only superficial pigmented lesions leaving deeper structures intact, while longer wavelengths (>600 nm) can target pigmented lesions in the dermis, such as nevi of Ota, and most tattoos. Pigment specificity of lasers is not only wavelength dependent, but also pulse width dependent. The determination of the appropriate pulse duration is primarily based on the size of the target and its thermal relaxation time. The primary site of laser-induced damage is most likely the melanosome, the intracellular organelle in which melanin is synthesized and stored.1 With an estimated thermal relaxation time that ranges from 250 to 1000 ns, depending on their size, melanosomes require nanosecond laser pulses (<1 μs) for their selective disruption. Recent evidence, however, suggests that longer pulse durations, in the millisecond range, may effectively remove epidermal pigmentation, especially in individuals with darker skin complexion.2
