Interview: Dr. Hoon Hur of Choice Dermatology Clinic in Pyeong-chon
Selective photothermolysis vs. general photothermolysis
Photoacoustic effect is associated with less side effects compared to the photothermal effect. However, both are not completely free from the risk of side effects. Simultaneous actions by both the photothermal and photoacoustic effects can cause side effects from thermal damage as well as physical impact. Moreover, selective photothermolysis is not possible with the picosecond laser and the light energy can go beyond the target tissue and cause general photothermolysis which can lead to post-inflammatory hyperpigmentation (PIH) and scarring.
For example, in tattoo treatment, one may think that the laser is absorbed only into the ink particles, however, this is not true. The laser acts not only on pigment particles but on melanosomes of keratinocytes and epidermal melanocytes. This can damage the epidermis and leave scars. Although in a much smaller range than with the nanosecond laser, the picosecond laser can still cause scarring. Therefore, it can be said that the picosecond laser and nanosecond laser have the same mechanism of causing side effects although the severity of side effects can differ. If the severity of side effects with the nanosecond laser is said to be 100, that of the picosecond laser would be 60-70.
This is also seen in melasma treatments. Using the picosecond laser on melasma often causes PIH, which means that the laser photons are not limited to the target tissue but also impact surrounding tissues. This process can be explained as following; keratinocytes makes up 95% of the epidermis, whereas melanocytes only takes up less than 1%. Even when melanocytes are targeted, laser beams are first absorbed into keratinocytes and cause damage. Keratinocytes then release cytokines such as endothelin-1, ɑ-MSH, basic fibroblast growth factor, and prostaglandin, etc. to stimulate melanocytes. This can lead to worsened melasma or PIH.
HELIOSⅡ/LOTUSⅡ/HYPERION – Manufacturer: LASEROPTEK(www.laseroptek.com)
Many doctors incorrectly believe that selective photothermolysis is possible with picosecond lasers. In theory, selective photothermolysis seems possible as the energy is limited to the chromophore, however, it is not applicable to live cells containing melanin chromophores which only go through general photothermolysis.
This is also the case in laser toning. The initial treatment given at 1.4J may bring favorable outcomes at first. However, when repeated, it can cause PIH and worsening of melasma. At the fluence level of 1.4J, selective photothermolysis fails to take place in the sub-cellular level, instead general photothermolysis takes place which damages keratinocytes and leads to PIH-causing cytokines. Therefore, it is advisable to lower the fluence in treatments following the initial improvement.
-To be continued-
HELIOSⅡ/LOTUSⅡ/HYPERION – Manufacturer: LASEROPTEK(www.laseroptek.com)
