## 4 Clinical Indications
### Skin Aging  
Skin aging is a physiological process which occurs with progressing time. The primary clinical manifestations of skin aging include pigmentary changes, telangiectasia, loss of firmness, and formation of wrinkles, which is associated with cellular aging, reduced synthesis of the collagen, and increased cell degradation. Skin aging is influenced by two types of factors: intrinsic and extrinsic. Intrinsic factors primarily include the cumulative effects of time and the effect of the related physiological characteristics, whereas extrinsic factors include ultraviolet radiation, smoking, exposure to wind and to harmful chemicals. These stimuli, especially the extrinsic factors, cause macromolecular structural damage and the related functional changes in skin cells, leading to accelerated aging of the skin [52; 53]. Non-invasive aesthetic procedures, e.g., laser therapy, are employed to improve the appearance of the aging skin. Laser technology provides a variety of skin rejuvenation options, including ablative, non-ablative, and fractional laser treatments, and the choice of the method depends on the condition of the skin [54; 55]. Fractional ablative lasers, such as the CO \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  
* * mMASI—modified Melasma Area and Severity Index, ** VAS—Numerical Visual Analog Scale, *** PHIL—Post-inflammatory hyperpigmentation.  
\end{table} 
Table 4: Studies showing the effects of using picosecond lasers to reduce hyperpigmentation. laser (10,600 nm) and the Er:YAG laser (2940 nm), are considered to be the gold standard for skin rejuvenation. They offer the chance of significant clinical improvement but they are associated with lengthy recovery periods and marked adverse effects, including edema and persistent crusting. Non-ablative lasers, which stimulate regenerative processes within the skin while preserving the integrity of the epidermis, offer an interesting alternative [56]. Among others, picosecond lasers with fractional modes belong to that category. The high fluence pulses emitted by these lasers induce the synthesis of collagen and elastin through photomechanical effects, without surface ablation [9]. Due to their favorable safety profile, particularly in individuals with higher Fitzpatrick skin phototypes, non-ablative PS lasers have become a popular method of achieving skin rejuvenation among Asian patients [57]. [PERSON] et al. [57], used a 755 nm PS alexandrite laser with a diffractive lens array to assess its efficacy and safety in facial skin rejuvenation among the Indonesian population. The study included 20 participants, aged 36-55 years, with clinical signs of photoaging (Fitzpatrick III-V). Each patient underwent two treatment sessions, with a four-week interval. Identical treatment parameters were applied to all patients (6 mm, 0.71 J/cm\({}^{2}\), 10 Hz), with a minimum of 3-4 passes until mild erythema was achieved. The condition of the skin was evaluated at baseline and four weeks after treatment completion, based on the standardized photographs of the affected area. Wrinkle count, pigmentation, and skin texture were taken into consideration. One month after treatment completion, moderate improvement (25-50% reduction in skin changes) was observed in 40% of the participants, while good improvement (50-75% reduction in skin changes) was confirmed in 60% of the patients. Mild erythema and transient swelling, which are typical after skin procedures, were present in all participants. Additionally, five patients developed solar urticaria, and two (Fitzpatrick III) developed petechiae. All adverse effects resolved within a few days and no cases of post-inflammatory hyperpigmentation or hypopigmentation were observed. These findings suggest that the 755 nm PS laser is an effective and safe modality for facial skin rejuvenation in Indonesian patients [57]. [PERSON] et al. [58] investigated the efficacy of a fractional PS laser (1064/532 nm) in reducing pigmentation irregularities and fine wrinkles on the face, which are typical signs of photoaging.