Lasers and Light Devices in the Treatment of Cosmetic Pigmentary Disorders in Asian Skin

Goh Chee Leok

Disclosures

Dermatology Nursing 

In This Article

Mechanisms of Action of Lasers and Light Devices on Pigmentary Skin Disorders

Principle of Selective Photothermolysis

Selective photothermolysis is the fundamental principle underlying the low-risk laser treatment of vascular lesions, pigmented lesions, and tattoos. Melanin, the main chromophore in most epidermal and dermal pigmented lesions, has a broad absorption spectrum extending from the UV range through the visible and infrared spectra (see Figure 1). The selective range of wavelengths for targeting melanin lies between 630–1,100 nm, where there is good skin penetration and preferential absorption by melanin over other skin chromophores such as oxyhaemoglobin and water.

Pigment specificity of lasers and light devices are dependent on both the wavelength and the pulse width.

Pulse Width and Wavelength Considerations

Melanosomes are the primary target of pigment-specific lasers. Electron microscopy studies have demonstrated that melanosomes targeted by short-pulsed lasers exhibit frank membrane disruption and disorganization of their internal content (Anderson & Parrish, 1983; Parrish, Anderson, Harrist, Paul, & Murphy, 1983; Watanabe et al., 1991).

With an estimated thermal relaxation time ranging from 250–1,000 nanoseconds, melanosomes require sub-microsecond laser pulses for their selective destruction. Pulse durations from 40–750 nanoseconds are able to disrupt melanosomes, but longer pulse durations, in the millisecond domain, do not appear to cause specific melanosome damage (Stratigo, Dover, & Ardnt, 2000). The use of pulse widths in the subnanosecond domain (femtoseconds [10±15] and picoseconds [10−12]) has been recently introduced in an effort to confine mechanoacoustic damage and minimize disruption of the surrounding tissue (Herd, Alora, Smoller, Arndt, & Dover, 1999).

Shorter wavelengths (<600 nm) damage pigmented cells with lower energy fluences, while longer wavelengths (>600 nm) penetrate deeper into the skin but need more energy to induce melanosome disruption. In clinical application, shorter wavelengths are selected to treat superficial pigmented lesions such as freckles and lentigines while longer wavelengths are chosen to treat pigmented lesions in the dermis, such as naevus of Ota, Hori's naevus, and most tattoos.

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