## 4 Clinical Indications
### Skin Aging
The study included 18 patients (Fitzpatrick I-III), with 10 completing the full treatment protocol. Each participant underwent three laser sessions, with 4-week intervals. Assessment of the skin was performed by analyzing photographs taken at baseline and at 1, 3, and 6 months of follow-up after treatment completion. During each visit, patient satisfaction and treatment-related discomfort were also documented for both wavelengths (532 nm and 1064 nm). The treatment utilized only fractional optics, with the handpiece configured to deliver 100 microbeams per hexagonal area with a 10-mm diameter. The micro-spot sizes were 700 \(\upmu\)m for the 532 nm wavelength and 400 \(\upmu\)m for the 1064 nm wavelength. A maximum of six passes were performed for each wavelength, first for 1064 nm, followed by 532 nm. These results indicate that the laser used in that study was well tolerated by the patients (minimal pain, mild side effects), and resulted in mild-to-moderate improvement in wrinkle reduction and pigmentation correction, with pigmentary improvements being more pronounced than wrinkle reduction. This effect was most likely attributable to the use of the 532 nm wavelength. Additionally, the 532 nm wavelength was also linked with more intense erythema and greater discomfort immediately after the treatment [58]. [PERSON] et al. [59] investigated the possibility of skin rejuvenation using a fractional PS Nd:YAG laser (1064 nm). They conducted an in vivo study on the skin of old and young female hairless mice, performing topographical assessments, histological analyses, and electron microscopy of the treated area. The first part of the study was supposed to confirm the efficacy of the PS Nd:YAG laser, while the second attempted to elucidate its mechanism of action. The study included sixty-week-old and twenty-week-old mice. Older mice underwent three treatments, with a 10-day interval between the sessions, while the younger mice received only a single treatment. Each treatment involved five passes of the laser beam using the following parameters: spot size--10 \(\times\) 10 mm, fluence--0.24 J/cm\({}^{2}\), pulse frequency--3 Hz. Histological and molecular analyses were conducted at 1 and 6 h after the laser treatment. A significant improvement in skin topography was observed in the older mice on the surfaces which were treated with the laser. The skin surface became noticeably smoother. Histological images of the skin samples from the treated areas in older mice revealed higher dermal thickness, with a higher content of collagen, elastin, and key extracellular matrix components. Additionally, an increase in epidermal thickness was observed in the laser-treated area. In contrast, histological images of the skin of the younger mice showed no distinct microthermal damage but localized epidermal erosion and focal condensation of the collagen, accompanied by extravasation of red blood cells, one-hour post-treatment. No significant perivascular inflammatory cell infiltration was detected, either 1 or 6 h post-treatment. These authors also found elevated levels of heat shock proteins, growth factors, and platelet-activating factors in the laser-treated area as compared to the control group. These findings suggest that the mechanism of action of the fractional PS Nd:YAG laser (1064 nm) is associated with partial disruption of the collagen fibrils and capillaries through photomechanical and photothermal effects, which induces a mild inflammatory response, leakage of plasma and red blood cells, and increased expression of the growth factors involved in dermal regeneration [59]. Studies showing the effects of using picosecond lasers to reduce signs of skin aging are presented in Table 5.