• [Issue] Understanding picosecond laser III

    Interview: Dr. Hoon Hur of Choice Dermatology Clinic in Pyeong-chon


    General photothermolysis takes place at the cellular level, therefore, the focus should be epidermal keratinocytes, not melanocytes. In other words, maximum damage should be incurred in melanocytes, while minimizing damage to keratinocytes that make up 95% of the epidermis. I recommend using parameters slightly lower than the pre-set levels to reduce side effects. This may require a larger number of treatments, however, that is trouble worth taking to lower the risk of side effects.

    A single treatment may be sufficient depending on the condition. For example, as the target for treating lentigo senilis is keratinocytes, a single treatment is sufficient to clear away the lesions. It goes without saying that accurate diagnosis should precede the treatment for the best outcome.



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    The impact of wavelength and pulse duration on efficacy

    The very first picosecond laser to hit the market was Picosure, the 755nm alexandrite picosecond laser. The 755nm wavelength has a high absorption rate in melanin which can compromise efficacy in pigmentary conditions such as melasma as this could cause PIH. It could, however, be effective in pore reduction. Perifollicular fibrosis should be first removed to improve pores. The 755nm wavelength is better absorbed in follicular melanocytes in the outer hair root sheath compared to 1064nm. Therefore, the primary target of follicular melanocytes is pulverized in heat which leads to microsubcision of the secondary target, the fibrosis tissue (The mechanism of action in which this occurs is explained in detail in my study The Treatment of a CMN Using a New Combination Therapy: IPL Therapy and Dr. Hoon Hur's Golden Parameter Therapy published in Journal of Dermatology and Therapies.) However, the 1064nm wavelength which has superior absorption in melanin than the 755nm, is more effective in clearing melasma.

    The efficacy of tattoo removal can also vary based on the wavelength. The wavelength impacts how finely tattoo ink particles are pulverized. Although the absorption rate is similar, shorter wavelengths pulverize tattoo ink particles better than longer wavelengths. The 450ps picosecond laser removes tattoo more effectively than the 755ps picosecond laser. When tattoo ink particles are more finely pulverized, they are more effectively metabolized by macrophages. As tattoo ink is heavy metal, it is not easily digested by macrophages, however, when it is reduced to fine particles, macrophages can excrete them easily through the lymphatic ducts.

    Moreover, the 375ps pulse duration is less likely to cause epidermal side effects compared to 750ps. A shorter pulse duration means less side effects. However, when the pulse duration is too short, this can create gas plasma and lead to side effects. Plasma can form even at 375ps but not at the level that will damage the epidermis.


    The Gaussian beam profile vs. the top-hat beam profile

    I have been told that the picosecond laser produces a top-hat beam profile and not the Gaussian beam profile, however, I am not fully convinced. The center of the Gaussian beam has high energy but the energy level falls 30% toward the periphery. In the top-hat mode, the beam produces an even level of energy. In my experience, I saw that the pigment was removed in a donut shape following picosecond irradiation. I think there are two causes to this phenomenon. One reason can be that melanocyte activity is lower at the center of the lesion, whereas it is stronger toward the peripheries. Another reason can be that as the laser energy dwindles toward the periphery of the beam and it created a ring-shaped darker region. For these reasons, I am not fully convinced that the picosecond laser produces the top-hat mode. To compensate for this inconsistency of energy, some doctors differ the energy levels from the center to the edges of the lesion. I find this rather complicating and prefer overlapping irradiation only on the peripheries of the lesion. 

    -To be continued-

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