## 4 Clinical Indications
### Post-Acne Scarring  
Acne vulgaris is a chronic skin condition, which is characterized by the presence of different types of skin lesions, including open and closed comedones, pustules, papules, and cysts, depending on the course of the disease. Inadequate treatment or severe forms of acne may lead to scarring [17], which is an undesirable and long-term consequence of the disease, negatively impacting the body image and self-esteem [18; 19]. Atrophic scarring, the pathophysiology of which is associated with an excessive and prolonged inflammatory response, loss of collagen in the skin, and damage to the sebaceous glands, represents the most common type of acne scars. Several treatment methods which effectively address the issue of acne scarring are available, and these include ablative lasers, radiofrequency, and microneedling. However, they often require a prolonged period of recovery, are associated with significant pain, and may carry the risk of post-inflammatory hyperpigmentation (PIH). Contemporary dermatology and cosmetology offer various procedures which are linked with minimal risk of side effects, allowing the affected patients to return to daily activities shortly after the treatment [11; 17]. Among others, the fractional PS laser, whose mechanism of action is based on fractional photothermolysis, is an example of such modern modality. In 2014, fractional photothermolysis was approved by the Food and Drug Administration (FDA) for the treatment of atrophic acne scars [6; 20]. Compared to the nanosecond lasers, picosecond lasers deliver energy fluence at a shorter pulse duration, allowing for an adequate photoacoustic effect while lowering nonspecific photothermal damage. As a result, the safety of the treatment has considerably increased, with reduced risk for PIH. Recently, numerous studies have demonstrated the efficacy of fractional PS lasers for treating acne scarring, with minimal side effects [21]. The results of using diffractive lens array mode of a picosecond pulsed alexandrite laser (755 nm) for the reduction of scarring are particularly promising [3]. [PERSON] et al. [22], were the first to confirm the potential of a 755-nm alexandrite PS pulse duration laser with diffractive lens array in their cohort of 20 patients (15 women, 5 men) with acne scarring and I-V skin types. Their patients underwent six treatment sessions with a picosecond laser every 4-8 weeks. Three months after the final session, scar imaging and histopathological evaluation were performed. Mean reduction of the scar volume was 27%, and microscopic analysis revealed an increase in the number of elastic and collagen fibers in the dermis. The patients reported to be'satisfied' or 'highly satisfied' with the overall appearance and texture of their skin during the final treatment session and at 1 and 3 months of follow-up. They also reported transient erythema and edema, both of which resolved within a few hours (max. two days) after the procedure [22]. [PERSON] et al. [23] conducted a comparative study to assess the efficacy of a non-ablative fractional laser (1550 nm) and a picosecond laser (755 nm) with DLA for the treatment of atrophic facial acne scars among Asian patients. The non-ablative laser demonstrated superior efficacy, although the picosecond laser treatment proved to be less painful, induced only transient erythema, and did not result in PIH or exacerbation of the acne lesions [23]. [PERSON] et al. [24], reported excellent, long-lasting efficacy of the 755 nm PS laser with a diffractive lens in treating atrophic acne scars. These authors also emphasized the suitability of this laser for Asian skin types, without the risk of PIH. Their study included three cases with the longest clinical follow-up after treatment with the 755 nm PS laser and diffractivelens. The effectiveness of the treatment was evaluated using photographic documentation. Each patient underwent 4-6 treatment sessions.