Energy‐based devices in the treatment of acne scars in skin of color

Acne scarring is disfiguring and psychologically taxing on patients. Many energy‐based modalities have emerged and been studied for the treatment of acne scarring; however, there is a paucity of these studies in skin phototypes IV–VI.

topical agents, antibiotics, and isotretinoin, acne is a chronic disease and early intervention is critical in reducing the risk of scarring and hyperpigmentation. 1 Lasers have become increasingly popular in the management of acne and even more so in treating the residual acne scarring and hyperpigmentation. 3 Non-ablative lasers are resurfacing lasers that work through fractional photothermolysis in which thousands of microscopic thermal zones are created in the skin while sparing the epidermis. 3 The areas targeted by non-ablative lasers are thermally injured to deliver energy deep within the dermis to stimulate collagenolysis and neocollagenesis. 3 On the contrary, ablative lasers work by removing the epidermis and delivering energy deep into the dermis which stimulates neocollagenesis. This has traditionally been considered a more aggressive form of treatment with prolonged downtime and high rate of adverse events. 4 Similarly, it has been regarded as high risk in patients of color. 4 Both ablative and non-ablative lasers have been well studied and proven to significantly improve the appearance of acne scarring and hyperpigmentation. 4 Acne and acne scarring affect all races; however, there is a disparity of literature between treatments for acne scarring in skin of color patients. More specifically, many practitioners are reluctant to treat acne scarring in patients of color with laser energy devices due to higher risk of hyperpigmentation and worsening scarring compared to lighter skinned patients. 4 This article will serve as a recent update on the efficacy of both non-ablative and ablative lasers in the treatment of acne scarring in skin of color patients, as well as radiofrequency and microneedling devices. By increasing awareness of the efficacy and safety of treating acne scars in skin of color, we hope to increase the armamentarium of treatment options for this patient population.

| ME THODS
In order to study both the efficacy and potential risks associated with treating acne scarring in patients of color with both nonablative and ablative lasers, we conducted a PubMed search using keywords (individually and in combination) including "ablative lasers," "non ablative laser," "acne scar laser," "radiofrequency microneedling", "ethnic skin," "skin of color," and "darker skin." Of the 700+ papers that were reviewed, 29 articles were selected from the time frame 2011 to 2021, 19 of which are summarized in Tables 1-3. Articles were selected depending on their relevance to acne scarring management with lasers on specifically patients of Fitzpatrick skin types IV-VI. The studies excluded from this review were any studies with combination treatments other than ablative lasers, non-ablative lasers, and radiofrequency microneedling including topicals, PRP, microneedling and light-based energy devices, studies not including Fitzpatrick skin types IV-VI or limiting their patient population to only skin types I-III, literature review articles, case reports, posters, letters, studies not written in English, and studies treating anything other than acne scarring.

| AB L ATIVE L A S ER S
Ablative resurfacing lasers, such as CO 2 and Er:YAG, are considered to be the most effective treatment modalities for improvement of acne scars; however, risk of unwanted side effects limit their use in ethnic skin. Table 1 depicts relevant details of recent literature covering outcomes and safety of ablative lasers in skin phototypes (SPTs) IV-VI. During this literature review, most of the studies obtained covered SPTs IV in Asian skin; therefore, more studies including SPTs V-VI are needed.
Traditional Er:YAG lasers (2940 nm) are highly selective for water, which results in superficial ablation with less underlying thermal damage. In contrast, CO 2 lasers have a lower selectivity for water and penetrate deeper, resulting in additional thermal damage and coagulation of the surrounding tissue. Although the more aggressive properties of CO 2 lasers allow for better collagen remodeling and tissue contraction, it also has higher risk of adverse effects, prompting caution of its use in patients prone to scarring and hyperpigmentation. 5 Fractional CO 2 reduces the density of ablation which allows for improvement in skin rejuvenation and scarring with shorter recovery periods and a reduced side effect profile compared to traditional laser resurfacing. This is due to residual islands of normal skin which serve as a stem cell reservoir for healing. While safer than purely ablative treatment, fractional treatment continues to ablate the dermal-epidermal junction maintaining the risk of pigmentary abnormalities. Data regarding the effectiveness and safety of fractional CO 2 laser treatment in Fitzpatrick skin types IV-VI are lacking. Ochi et al. 6 conducted a retrospective study looking at the efficacy and complication rates of facial acne scars treated with fractional CO 2 laser in the Asian patients. Fifteen percent of patients had adverse effects with the most common being hyperpigmentation (6.4%) followed by blistering (4.0%). Fractional laser treatment settings, most notably both fluence and treatment density, can impact the extent of dermal-epidermal junction disruption which can influence the occurrence of PIH. Earlier reports have shown that a higher treatment density was associated with a greater degree of PIH. 6,7 In Alajlan et al.'s 8 retrospective analysis, it is mentioned that higher rates of adverse reactions were observed with higher fluences in SPTs IV and V with fractional CO 2 laser. Therefore in these skin types, it is recommended to reserve higher fluences of 125-175 mJ/cm 2 with 30%-40% coverage for moderate to severe acne scars in SPTs IV-VI and mid fluence 125 mJ/cm 2 with 20%-30% coverage in mild acne scars in this patient population. If the patient has a previous history of melasma or other facial pigmentations, lower fluences of 60 mJ/cm 2 with 10% coverage for 2-6 sessions are recommended depending on acne scar severity. 8 Majid and Imran. 9 found that with fractional CO 2 laser monotherapy (fluence 15-25 J/ cm 2 , density 100-150 MTZ/cm 2 , and energy 40-45 mJ), rolling and superficial boxcar scars responded the best while ice pick pitted scars responded the least. This finding has therapeutic importance as patients with predominantly pitted or ice-pick scarring will likely require additional modalities of treatment.
There are some studies in Asian skin types III and IV that show fractional CO 2 lasers are a safe and effective treatment for acne scarring. 6 To our knowledge, there is no published literature on fractional CO 2 lasers in SPT VI. Therefore, this device should be used with caution in this patient population, and a high risk for hyperpigmentation should be considered. In our personal experience, low density settings should always be a starting point for patients with pigmented skin with a test spot on lateral preauricular skin for assessment.
The Er:YAG laser, like CO 2 , has ablative and fractional ablative properties. Because depth can be controlled by most devices, it is often more precise without the intrinsic thermal ablation. The effects of three different modes of Er:YAG laser (microlaser peel [ablative] vs. fractional ablative vs. combined mode) in the treatment of facial acne scars in SPT III-IV were studied by Chen et al. 10 In this randomized control study, they divided patients into three groups. One group received the microlaser peeling mode with a depth of 60 mm and a repetition rate of 20%. The fractional ablative mode was performed with a depth of 300 mm and a fractional density of 8%. The combined mode group received a fractional depth of 200 mm, density of 8%, peeling depth of 30 mm, and repetition rate of 20%. The improvement of acne scarring in all three modes was found to be statistically significant. Combined mode was found to have the greatest efficacy; however, it was also associated with more severe complications such as PIH and pain (p < 0.05). Fractional ablative mode was the second most effective and associated with the lowest rate of adverse effects, making it the safest choice for patients of color.
As previously mentioned, although Er:YAG laser has more precise depth and less aggressive thermal effects it is considered to be less effective in stimulating collagen contraction, intraoperative hemostasis, and coagulation. Newer lasers have been created to combine the accuracy of short-pulsed Er:YAG with the controlled thermal effects of CO 2 in attempts to address the limitations of conventional

| NON -AB L ATIVE L A S ER S
Non-ablative lasers are popularly used to treat acne scars and have become the preferred treatment option due to reduced downtime and greater tolerability. 3 Similar to ablative systems, this modality induces collagen remodeling in the dermis via thermal induction by targeting water as its chromophore. 3 In contrast to ablative, nonablative lasers do not vaporize the overlying epidermis; therefore, they have less downtime and adverse reactions. 8 However, due to its TA B L E 2 Non-ablative lasers for acne scar resurfacing in SPT IV-VI  Non-ablative fractional resurfacing with an erbium-doped glass 1550 nm laser has been studied in patients with SPT IV-VI and has shown to be efficacious and low-risk in treating acne scars. 12,13 Alexis et al. 12   junction is not compromised, we are categorizing these lasers as non-ablative in this paper.

| R ADIOFREQUEN C Y MICRONEEDLING
Radiofrequency microneedling (RF) is a form of controlled skin injury that passes a current through the dermis at a predetermined needle length creating small thermal wounds to stimulate dermal remodeling and the production of new collagen fibers. 19 This technology causes neocollagenogenesis by stimulating the release of growth factors with minimal disruption to the epidermis. 20 While RF microneedling has been used to cauterize tissues and vessels, destroy tumors, and tighten pharyngeal tissues in sleep apnea, recent research has focused on the use of RF as an alternative to conventional methods of acne scar treatment. 21 RF targets acne scarring by mechanically disturbing the dermal fibrotic strands that contribute to skin retraction of atrophic scars and by stimulating collagen synthesis to replace fibrotic scar tissue with healthy, pliable collagen. 19 Over the course of multiple treatments, RF microneedling will soften scar defects and make acne scarring appear less noticeable.
Radiofrequency treatments offer a great advantage over lasers for darker skin types as they offer deeper treatment levels without the risk of hyperpigmentation and have minimal downtime. 22 This is because they allow for adjustment of needle depth which create uniform punctures at controlled depths with less epidermal damage and risk of dyspigmentation. Another advantage of RF is its ability to create specific thermal zones where channels are created for RF energy to reach the deeper skin layers with minimal energy dispersed in the epidermis. A study of 14 men and 5 women with skin types III-V were treated with a series of 3 sessions of non-ablative RF microneedling to treat facial acne scarring. 23 Researchers found significant improvement in 12%, moderate in 58%, and minimal in 29% with PIH occurring in only 1 patient that resolved in 4 weeks. 23 Chandrashekar et al. 20  Despite the differences between RF microneedling and more conventional lasers, RF can be used as monotherapy or as an adjuvant therapy with lasers when treating acne scarring. Simmons et al. 19 showed that pretreatment with a non-ablative laser lowers the skin's resistance to current flow and allows deeper penetration of the radiofrequencies, decreased level of discomfort, and reduced amount of RF energy needed to reach the optimal thermal dose. 19 Ultimately, the use of a non-ablative laser prior to RF microneedling can decrease the collateral effects to the surrounding dermis and decrease the chance of blistering, burns or inflammatory nodules. 19 Radiofrequency microneedling has opened the door to a more lasers. 24 They work by targeting the sebaceous gland and its associated hair follicle, causing thermal coagulation and decreased sebum production. 25 The absorption maxima of sebum has been shown to be around 1728 nm. 26,27 Due to the fact that, in addition to sebum, these infrared lasers also target water, hemoglobin, and melanin, they are somewhat limited in their potential for achieving maximal results in acne treatment. Most recently, an infrared Indium Phosphide (InP) diode laser with a wavelength of 1726 nm has been developed to selectively target sebaceous glands and decrease sebum production in patients with inflammatory acne. By selectively targeting sebum, this InP diode laser causes preferential thermal coagulation of sebaceous glands without damaging the overlying epidermis. Clinical trials showed that inflammatory lesion counts were reduced by a mean of 49.5% ± 26.5% from baseline. Of in acne scarring with no serious adverse events. 28 Laser devices in the treatment of acne vulgaris and scarring offer some advantages over conventional treatment such as absence of compliance issues, medication interactions, and adverse effects associated with systemic medications. 29 Although further studies are necessary, the development of a device with a wavelength specific to the absorption peak of sebum is a promising addition to the treatment armamentarium for active acne vulgaris and acne scarring.

| CON CLUS ION
Ablative and non-ablative lasers have proven to be effective for treatment of acne scars in ethnic skin. The risk of developing adverse effects such as post-inflammatory hyperpigmentation is contingent upon several factors including SPT, laser device, fluence, and moreso density settings. Non-ablative fractional lasers have been considered first line for the treatment of acne scars in skin of color due to their better safety profile; however, they are less efficacious and require more treatments compared to ablative lasers. Newer studies looking at P-DOE technology and variable-pulsed Er:YAG provide promising results to broaden the variety of limited treatment options available for acne scarring in darker skin types. In our experience, in a vacuum of laser technology, non-ablative and picosecond MLA/ DOE are safe for all skin types with low fluence and low density. We reserve fractional ablative treatment in light to medium skin toned patients with no history of pigmentary anomalies with low density settings, regardless of fluence/depth. The biggest limitation of our review and literature is the lack of head to head or side by side studies in skin of color patients. However, new technology in the form of fractionated radiofrequency and radiofrequency with microneedling is slowly replacing laser treatment for an array of conditions in skin of color patients due to minimal risk of PIH.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available in PubMed at https://pubmed.ncbi.nlm.nih.gov/.

E TH I C A L A PPROVA L
Authors declare human ethics approval was not needed for this study.