INTRODUCTION
Picosecond devices were commercially introduced a number of years ago to better treat tattoos and pigmented lesions. The picosecond Alexandrite laser effectively treats most tattoos. Difficult to treat green ink shows the most dramatic rapid clearing with this device. The absorption of 755 nm light by melanin also makes this laser an excellent treatment for Nevus of Ota and other pigmented lesions.The fractional optic was introduced for use in facial rejuvenation, abnormal pigmentation, and acne scars. Brauer and colleagues reported improvement in acne scars associated with new collagen, tissue and melanin after a series of treatments.1 Our center was the first to characterize the histological changes induced by this fractional optic. We observed intra-epidermal vacuoles located in the stratum granulosum of the epidermis measuring approximately 60 microns in diameter.2, 3, 4This localized zone of intra-epidermal injury was observed in individuals who had a melanin index (MI) of greater than 15, as measured by skin pigmentation meter (Skintel™) and who were skin type II or greater. This correlates with an individual who has some degree of epidermal pigmentation. We postulated that this injury is the result of laser induced optical breakdown (LIOB).4 This fractional delivered laser energy light appears to be absorbed by melanin creating a situation where one or more free electrons are generated (Figure 1). The number of free electrons grows in an avalanche process. A plasma region is created which absorbs energy from the laser pulse. The localized area of plasma generates heat to create a localized steam bubble. Histology taken within minutes after treatment reveals a vacuole created by the damage (Figure 2). There is very little damage noted by microscopy to the surrounding keratinocytes. At 24 hours, this void is filled with rehydrated (Figure 3) cellular debris which stains positively with Fontana Mason indicating the presence of melanin (Figure 4) Over the next three weeks the vacuole contracts and forms a zone of microscopic epidermal necrotic debris (MENDS), which is exfoliated (Figure 2).In low melanin individuals with skin type I and II, there is a different clinical picture and histology. Scattered hemorrhagic papules are sometimes seen with the highest energy settings. Histology reveals scattered areas of dermal hemorrhage often associated with vacuoles filled with red blood cells at the dermal/epidermal junction suggesting absorption of this high energy light by hemoglobin.4This device has been safely and effectively used for facial rejuvenation and acne scars in darker skin types. These epidermal vacuoles are seen with all commercially available spots 6mm (.71J/cm2), 8mm (.40J/ cm2), and 10mm (.25J cm2). The size of these vacuoles decreases as the energy decreases with the use
