Acne vulgaris is the primary skin condition affecting teenagers, with an overall prevalence of 93.3% in Australia [1] and 91.3% in Hong Kong [2]. Despite various treatments, the post-acne eruption scarring rate is up to 11%–14% [3]. The severity of scarring is associated with the degree and duration of the initial inflammatory nodulocystic acne and the time delay of effective treatment, irrespective of sex and location [4]. It may also be caused by self-manipulation of the lesions and comedones, such as acne excorie. Acne scars can lead to multiple problems, including psychosocial stress, loss of self-esteem, performance impairment, unemployment, and negative financial consequences. It is well known that there is no single treatment that can best tackle such atrophic scars. Therefore, a combined multimodal treatment approach is a rule. Typical treatment options include a combination of approaches such as resurfacing, collagen regeneration, tissue augmentation, adhesion breaking, and surgical methods.

Resurfacing and collagen regeneration treatments for acne scars include cryotherapy, chemical peeling, lasers and lights, and radiofrequency. Tissue augmentation includes the use of fillers and fat grafting. Adhesion breaking and other surgical methods include dermabrasion, subcision, and excision. However, cryotherapy treatment of hypertrophic scars requires several weeks for recovery and bears an increased risk of permanent hypopigmentation and scarring [5]. Although medium-depth chemical peeling and microdermabrasion are helpful for smaller depressed scars, they are ineffective for ice-pick scars or deep fibrotic boxcar scars; these require aggressive cross-peel and may not be suitable for all patients. Moreover, dermabrasion is associated with significant pain and downtime.

With increasing understanding and evidence in the field, various lasers with and without fractional technology of multiple wavelengths and intensities provide more options to improve acne scars and rejuvenation. Through effective early treatment of inflammatory lesions, the aim is to reduce scar severity in its extent, duration, and tissue remodeling at the cellular and chemical response levels.

Acne scars are classified as either atrophic (90%) or hypertrophic. The inflammatory process in acne lesions, leading to the destruction of dermal collagen, results in irregular depressions of atrophic scars. Instead, hypertrophic and keloidal scars exhibit excessive inflammatory activities, collagen deposition, and decreased collagenase activity. This classification allows physicians to determine the best approach before treatment (Table 1).

Table 1 . Acne scarring subtypes.

Subtype	Feature
Atrophic
Boxcar scar	Shallow (smaller than 0.5 mm), deep (bigger than 0.5 mm)
	Width is usually about 1.5 to 4 mm
	Sharp vertical edges
	Depressions with oval or round shape
Rolling scar	Width is usually about 4 to 5 mm
	Formulated from tethering of dermis of skin with normal appearing
	The appearance is due to anchoring abnormally by dermal fibrous tissue to the subcutaneous tissue
	Shallower but broader
Ice-pick scar	Narrow, usually deep (smaller than 2 mm)
	Apex is narrower than the opening
	Territorialize vertically to the subcutaneous tissue or deep tissue
Hypertrophic (usually come across on the jaw, sternum, shoulder, and the back regions)
Keloidal	Territorialize beyond primary border of wound
	Erythematous, purple color nodules or papules
Hypertrophic	Restrained to the previous damaged area
	Firm, raised and pinkish papule
	Deposition of excess collagen

Scars’ improvement may be related to the direct effects of the laser and cellular chemical response. Wilmink et al. [6] demonstrated the expression of the heat shock protein HSP70 when the skin was heated to 45°C–55°C using laser pre-conditions for wound repair enhancement. After laser irradiation, Leclère and Mordon [7] detected the production of this protein near hair follicles, their surrounding vasculature, and sebaceous glands. Resurfacing lasers allow for larger areas or full-face treatments with the promotion of collagen neogenesis and scarred skin contraction [8]. Lasers can transmit heat energy to a more precise targeted area than other conventional resurfacing techniques such as chemical peel. The choice of the optimal laser system and settings depends on the scarring characteristics. A list and the key characteristics of the lasers employed for acne scarring treatment are provided in Table 2.

Table 2 . Acne scarring laser (non-fractionated vs. fractionated).

Laser type	Non-fractionated	Fractionated
Non-ablative laser	532 nm KTP	Fractional 1,540 nm Er-glass and 1,927 nm Thulium fiber
	585 nm PDL	Fractional 1,550 nm Er-doped
	595 nm PDL
	755 nm picosecond pulse duration
	1,450 nm diode
	1,320 nm Nd:YAG
	1,064 nm Nd:YAG
Ablative laser	Combined CO2 Er:YAG: 10,600 nm/2,940 nm	Fractional 2,790n m YSGG
	2,940 nm Er:YAG	Fractional 2,940 nm Er:YAG
	10,600 nm CO2	Fractional 10,600 nm CO2

KTP, potassium-titanyl-phosphate; PDL, pulse dye laser; Nd:YAG, neodymium-doped yttrium-aluminum-garnet; Er:YAG, erbium-doped yttrium-aluminum-garnet; YSGG, yttrium-scandium-gallium-garnet..

Non-ablative lasers impound thermal energy to the dermis for collagen remodeling, sparing the epidermis compared with ablative lasers which vaporize the tissue directly from the skin; this leads to a long, complicated recovery process and downtime. Non-ablative lasers have the visible advantages of fewer side effects and a shorter recovery time. Nevertheless, more treatment sessions are needed for non-ablative lasers to achieve clinical results that are comparable to those of ablative lasers (Table 3).

Table 3 . Comparison of effectiveness and side effects of non-ablative fractional resurfacing (NAFR) laser vs. ablative fractional resurfacing (AFR).

Side effects and mean improvement	AFR	NAFR
PIH maximum lasting period	6 months	7.5 days
Patients with PIH	0%–92.3%	0%–13%
Length of erythema	3–14 days	1–3 days
Pain score (grade 1–10)	5.9–8.1	3.9–5.66
Mean improvement	26%–83%	26%–50%

Values are presented as mean or range..

PIH, post inflammatory hyperpigmentation..

Mid- or near-infrared laser application decreases the risk of burns, which is higher in Fitzpatrick skin phenotype (FST) III–IV patients as their melanin absorbs less energy in this wavelength range [9]. These treatments always require the concomitant use of surface cooling, which reduces epidermal damage because the water content within the targeted tissue absorbs thermal energy, followed by neocollagenesis.

The 1,064-nm neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser disperses heat energy at a deeper level, which is mainly absorbed by melanin and oxyhemoglobin. Atrophic scarring provides a 20%–30% improvement, with favorable recovery and minimal side effects [10]. Lipper and Perez [11] enrolled ten FST I-V acne scarring patients; after eight treatment sessions, a 29.3% improvement was observed with no significant side effects. Maluki and Mohammad [12] conducted a study on 10 Iraqi patients with FST III. After five treatment sessions, no significant effects were observed. Friedman et al. [13] showed that the studied group reported a 39.2% improvement after five treatments over a six-month period, with petechiae and erythema scores reported as 1.1 and 1.8 (grading 0–3), respectively. Yaghmai et al. [14] performed a split-face study with 12 FST I-III patients with the long-pulsed 1,064-nm and 1,320-nm lasers. An average improvement of 28% was observed with the 1,064-nm laser, and 22% with the 1,320-nm laser, with short-lasting erythema and mild procedural pain with the 1,064-nm laser therapy. Evidence suggests that a 532-nm potassium-titanyl-phosphate laser prevents sequelae scarring better than its effect in acne scar treatment [15].

A 585-nm pulsed dye laser (PDL) to treat keloid, hypertrophic, and erythematous scarring is widely used in the laser market. Its mechanism involves the emission of light energy that targets oxyhemoglobin to induce vascular changes. After several treatments, decreases in lesion size, pruritus, pain, and scarring erythema were observed [15].

In Alster and McMeekin’s study [16] of twenty-two FST II-III patients with hypertrophic and erythematous facial acne scarring treated with this laser, improvements of 67.5% and 72.5% were observed after one and two treatments, respectively. The mechanism involves the elimination of the dilated vasculatures trapped inside the sclerotic collagen and the thermal damage to the microvasculature, leading to the release of cytokines with fibroblasts activation and consequent collagen remodeling; the purpuric reaction is not uncommon.

Patel and Clement [17] performed a study of ten patients with atrophic scars in a single session using the 585-nm PDL. The authors found a mean overall improvement (physician assessment) score of 2.6/4, and a patient self-reported satisfaction score of 2.9/4. They also emphasized the dissociation of efficacy and side effects with skin type.

The 595-nm PDL targets oxyhemoglobin in the treatment of acne and redness scars, which works on the same principle as the 585-nm PDL. In a study of 20 FST III-IV patients with redness in acne scars, they were treated with 595-nm PDL for two sessions at one-month intervals [18]. There were improvements of 24.9% and 57.6% after the first and second treatments, respectively. The side effects were limited to temporary redness and edema at the treated sites, with well-tolerated intraoperative pain.

The 755-nm Alexander Picosecond Pulse-Duration Laser was introduced in 2013 for tattoo removal. Brauer et al. [19] have conducted six treatments at 1–2 months intervals in 17 FST I-V patients with atrophic acne scars. They reported a mean scar volume reduction of 24.3%, and all patients eventually scored good-to-excellent. The reported side effects included transient erythema lasting less than two days, and an average pain score of 2.83/10.

A 1,450-nm diode laser was studied by Chua et al. [20] with fifty-seven FST IV-V atrophic acne scar patients for four to six treatments. Modest improvements were observed (15%–20%). However, the treatment carries a high risk of post-inflammatory hyperpigmentation, which increases with darker skin types; from 18% in FST I–III to 39% in FST VI patients. Furthermore, the procedure was less effective on patients with darker skin.

The 1,320-nm Nd:YAG laser is a mid-infrared laser that requires a cooling system to protect the epidermis from procedural damage [21]. Rogachefsky et al. [22] found significant improvements in mixed and atrophic acne scars in 12 patients treated with this laser for three sessions and no significant postoperative complications. Other studies have also revealed similar, modest results without notable side effects [21,23].

Ablative lasers achieve excellent results in treating acne scars; however, their use is limited by significant side effects, complications, and prolonged downtime. Non-ablative lasers preserve the epidermis using additional cooling methods to minimize side effects [15].

Fractional lasers for skin resurfacing are a recent development that produces faster recovery, decreased risks, and less treatment downtime [24].

The laser beam irradiation produces discrete columns of heat-damaged microthermal-treatment zones from the skin surface to the dermis, surrounded by undamaged tissue with rapid repair of the epidermis through the migration of adjacent non-injured keratinocytes. They have been successfully used to treat acne scars [24]. Table 3 provides an overview and comparison of the effectiveness and side effects of both non-ablative and ablative fractional resurfacing [25].

Alster et al. [24] used a non-ablative laser (1,550-nm erbium-doped fractional laser) to treat 53 FST I-V patients with acne scarring for three months. They showed that 90% of the patients experienced a 51%–75% improvement at six months and did no report adverse effects, such as further scarring or dyspigmentation. Furthermore, skin phenotype, sex, and age did not affect the clinical response efficacy. Evidence has revealed that, although higher fluences yield better results than higher densities, they still lead to more microthermal zone damage with more side effects [25].

The 1,540-nm erbium-glass fractional laser is an alternative to the abovementioned laser with relatively similar results and side effects. Improvements in the appearance of box scars are better than rolling and ice-pick scars; 52.9%, 43.1%, and 25.9%, respectively [15].

In a prospective trial of 20 FST IV patients treated with a 10,600 nm CO₂ fractional laser, half of them achieved a 51%–100% clinical improvement, seven of them showed a 26%–50% improvement, while only three of them showed a 0%–25% improvement [26]. The authors reported side effects such as crusting, petechiae lasting for one week, and mild-to-moderate erythema persisting for two months. Interestingly, another split-face study that enrolled 13 acne scarring patients with FST I–III showed that a treatment scheme, either monthly or 3-monthly, yielded no significantly different results [27]. Another study with more than two thousand 10,600 nm CO₂ fractional laser treatments reported no scarring or permanent depigmentation [28].

In a six-month prospective study using a 2,940 erbium-doped yttrium-aluminum-garnet (Er:YAG) fractional laser for five treatments, an 80% acne scarring improvement was observed, associated with minimal side effects reported, only mild erythema persisting for three days, and no infection, scarring, or hyperpigmentation [29].

The 2,790-nm yttrium-scandium-gallium-garnet (YSGG) fractional laser showed 50%–90% improvement of atrophic acne scars in 70% of patients after only two laser treatments, with no pigment changes or scarring, and only a mild degree of reported erythema persisting 4 to 7 days. The YSGG fractional laser provides better control of intraoperative bleeding than the Er:YAG laser. It produces better ablation, coagulation, and side effect profile than the 2,940 Er:YAG fractional laser and 2,790-nm YSGG fractional laser [30]. Table 4 compares the disadvantages and advantages of Er:YAG and CO₂ lasers [31].

Table 4 . Comparison between Er:YAG and CO₂ laser.

Laser	Disadvantage	Advantage
Er:YAG	- Collagen remodeling is limited (important for scar alleviation). - Operative time longer for multiple laser passes required for better results. - Little to no tissue contraction. - Intra-operative bleeding.	- Fewer complications than CO2 laser potentially. - Shorter postoperative recovery period.
CO2	- Greater risk of development of hypopigmentation and scarring. - Prolonged post-operative course.	- More tissue contraction. - Excellent hemostasis. - Prolonged results with continuation of collagen remodeling.

Er:YAG, erbium-doped yttrium-aluminum-garnet..

The picosecond fractional laser has emerged as a more promising alternative. With ultrashort pulse durations, it creates significant photomechanical (rather than photothermal) effects compared to conventional Q-switch lasers. Fractionated tiny spots in an array arrangement increase the peak power of the laser beam, resulting in more laser-induced optical breakdown (LIOB) of the dermal tissue. This vacuole effect is followed by wound healing and a remodeling response with neocollagenesis. Adverse effects include pain and transient redness due to the breakdown of superficial capillaries. Nevertheless, the side effects are still better tolerated than with ablative lasers. Even in a relatively aggressive setting with high fluence, it is rare to have post inflammatory hyperpigmentation and a lower number of treatments required than a non-ablative laser [19,32]. Moreover, fractional plasma formation at the skin’s surface leads to secondary skin resurfacing, achieving further skin improvement benefits (Fig. 1). These are the practical treatment effects of the atrophic scars from acne vulgaris.

Figure 1. The laser-induced optical breakdown leads to the formation of vacuoles that promote neocollagenesis and remodeling while the fractional plasma ablation effect leads to the resurfacing effect. LIOB, laser-induced optical breakdown.

Three factors affect the efficacy of acne scarring improvement: treatment scheme, appropriate laser selection, and patient selection. The patient must be aware of the limitations of the selected laser treatment, and physicians should also care for the patient’s psychological and physical health. Moreover, the patient’s FST is crucial when choosing a laser type as varying concentrations of melanin appear in the stratum basale of FST types I–IV.

Type III and IV patients are at a higher risk of scarring, textural changes in the skin, and dyschromia following laser treatment. A more conservative laser setting, including lower fluence and density, and smaller spot sizes with adequate skin-surface cooling, are mandatory when treating these patients.

Er:YAG and ablative CO₂ lasers provide the best curative effects for treating acne scarring in all skin types [31]. Fractional and non-fractional lasers effectively improve atrophic acne scars, whereas PDLs provide marginal improvements in keloid scarring and variable effectiveness in hypertrophic scarring [15].

Fractionated technologies have gained popularity in the laser market for both non-ablative and ablative lasers. They provide precise lasers with well-ordered tissue injury columns for a significant skin resurfacing capability, with minimal side effects and downtime, but similar efficacy compared to non-fractionated technologies. Pain scores, hypo- and hyperpigmentation, and erythema tend to last for less and to a lesser degree with non-ablative lasers than with ablative lasers. Nonetheless, both lasers are associated with scarring, infection, and acne flares post-treatment in a small number of cases. The application of fractionated technologies in picosecond lasers allows revolutionary beneficial effects, including LIOB and plasma ablation resurfacing, which are effective in treating atrophic scars with a faster recovery owing to the skip layer property.

Non-ablative lasers play an essential role in mild atrophic acne scarring treatment. Studies have compared different laser systems (1,540 nm, 1,450 nm, and 1,320 nm mid-infrared lasers) in acne scar treatment; others have compared different skin conditions with the same laser type [15]. However, good epidermal protection is necessary, as diffuse dermal injury may occurs secondary to heat caused by laser irradiation. Collagen remodeling is triggered when cells are heated above 50°C. Skin resurfacing in patients with acne scars using these lasers remains unexplored and repeated treatments are often needed [32].

Non-ablative laser resurfacing technology is relatively harmless, although excess energy dissipation to the skin leads to blistering or post-inflammatory hyperpigmentation. The results with non-ablative acne scars were neither apparent in histology nor in objective assessments by other investigators [15].

In general, patients felt that the treatment procedures had good efficacy and satisfaction. Furthermore, other lasers that successfully treated acne scarring were the 1,064 nm long-pulsed Q-switch laser, and 595 nm PDL. The main advantage of skin remodeling using a non-ablative laser over skin resurfacing using an ablative laser is that it has minimal to no downtime after therapy.

The authors have nothing to disclose.