Laser, Intense Pulsed Light, and Radiofrequency for the Treatment of Burn Scarring: A Systematic Review and Meta-Analysis

: Burns and scarring are considered some of the greatest problems in public health because of their frequent occurrence. Today, photo-electric technology shows promising results in the treatment of burn scars. Over the years, more clinical trials and more technologies for scarring have emerged. The aim of this study was to determine better timing and methods of photo-electric therapy for burn scars. This study was registered in PROSPERO (CRD42023397244), following the PRISMA statement


Introduction
Burn scars are considered one of the greatest problems in public health [1,2]. Hypertrophic scarring occurs in 30 to 90 percent of patients following burns [3][4][5]. Most burn patients have to suffer physical pain and pruritus in the first stage. As long-term effects, dysfunction and aesthetic deformations in some severe cases also negatively impact patients' self-confidence, making them feel inferior. Today, first-line therapy for scarring includes surgery, pression therapy, silicone sheets and gel formulations, intralesional pharmacologic treatments, and many others [6][7][8]. However, the recurrence rate of scars after surgery is high at up to 45-100% [9][10][11]. Intralesional pharmacologic treatments are also commonly used, including triamcinolone acetonide (TAC) and fluorouracil (FU) [12,13]. Overall, burn scar characteristics can certainly be improved [14][15][16][17] but also cause many side-effects. For example, TAC can cause atrophy, hypopigmentation, hypertension, hirsutism, and even Cushing's syndrome [18], and FU may also have myelosuppression activities, causing leukopenia, infection, anemia, and other side effects. Additionally, some treatments cannot obtain satisfactory results. Other methods, including radiotherapy, cryotherapy, and massage therapy, are not commonly used clinically for a variety of reasons [9,19,20]. Meanwhile, photo-electric technology, which shows promising results in the treatment of burn scars [21], produces photophysical (such as thermal, mechanical, and electromagnetic) and photobiological effects (such as photochemistry and photobiological regulation) by skin exposure.
Today, the increasing interest in photo-electric therapy is making a great difference in the treatment of scarring. Photo-electric therapy in scarring includes pulsed dye lasers (PDLs), neodymium-doped yttrium aluminum garnet (Nd: YAG) lasers, low-level lasers (LLLT), intense pulsed light (IPL), ablative fractional carbon dioxide lasers (CO 2 AFL), and radiofrequency (RF). On Pubmed.gov, over two thousand results since 1967 can be found for the photo-electric treatment of scars. However, nearly 1800 of these results were published after the year 2000, and over a thousand were published in the last decade. Thus, in recent years, more clinical trials have been conducted and more technologies for scarring have emerged. These technologies include narrow-spectrum intense pulsed light (DPL) and Q-switched frequency-doubled Nd: YAG lasers. Due to the abovementioned factors, photo-electric treatment has become an efficient modality of therapy for burn scars with few side effects. Some systematic reviews have noted that CO 2 AFL is a safe, costeffective, and efficacious procedure for burn scars [22] that offers objective improvements specifically for chronic burn scars [23]. However, systematic reviews of other treatments are still scarce. We located a systematic review about the effectiveness of laser therapy for hypertrophic burn scars; this review noted that the evidence is not adequate to reach a conclusion [24]. Additionally, we found a systematic review and meta-analysis about surgical scars that showed that laser therapy may be a useful modality to minimize surgical scars when applied earlier on [25]. However, this study only used four articles to perform the meta-analysis, so more research should be carried out to support this result. We also found a systematic review of early laser intervention in scarring [26]. The results were uncertain as to whether early laser treatment can reduce scar formation, and more highquality research is needed for a definitive conclusion. There were also some reviews on this topic [27,28]. Globally, there are still many deficits in photo-electric therapy, and no detailed protocol is available. We still have no agreed-upon methods or parameters for the treatment of burn scars, which may cause many side-effects. This study focuses on photo-electric therapy, which can be used in the first period of scarring to prevent progression in a worse direction. We also explore when and how to use these treatments to achieve the most effective outcomes.

Search Strategy
Firstly, this study was registered in PROSPERO (CRD42023397244), following the PRISMA statement, and was carried out in concordance with the PRISMA checklist, which is included in the Supplementary Materials (File S1).
We employed the following search strategy to identify the clinical evidence reported in the biomedical literature: In October 2022, we searched PubMed.gov, Embase, and the Cochrane library (1980-October 2022) for published case reports, clinical studies, clinical trials, controlled clinical trials, and randomized controlled trials related to the photo-electric treatment of burns. We included no restriction for language. The mesh terms we utilized were 'burn' AND ('laser' OR 'light' OR 'radiofrequency') AND 'therapy*' AND 'cicatrix'. The details of our search strategy are provided in Table 1.

Selection Inclusion
To be included in the analysis, an original article had to meet the following inclusion criteria: (1) subject: patients who had clinically obvious scars, with more than 50% of the sample featuring scarring due to burns; (2) intervention: treatment of scars needed to involve photo-electric therapy; (3) outcome: Vancouver Scar Scale (VSS) score, Patient and Observer Scar Assessment Scale (POSAS), Visual Analogue Score (VAS), and scar thickness (mm) measured with ultrasonography; (4) control: pretreatment in individuals or other treatments or an untreated area control; (5) study design: randomized controlled trial (RCT), non-randomized control trial, pre-post study of the same person, cohort study, case-control study, and/or comparative study; (6) a mention of scar duration.
The exclusion criteria were as follows: (1) subject: more than 50% of the sample due to etiologies other than burns; (2) intervention: treatment of scars did not involve photoelectric therapy; (3) outcomes: measurement methods did not include the Vancouver Scar Scale (VSS), the Patient and Observer Scar Assessment Scale (POSAS), the Visual Analogue Score (VAS), or scar thickness (mm) measured with ultrasonography; (4) control: no control; (5) study design: case report or case series; (6) no mention of scar duration ( Figure 1).

Data Extraction
Two independent investigators browsed all included studies and recorded the features and outcomes of the trials using a data extraction form. The following variables were summarized in a standard Excel file: first author's name, year of publication, study design, control, duration of follow-up, sample size, country, patients' baseline characteristics, the type of treatment, the parameters used, the requirements, whether or not any other scar treatments were used concurrently, and the main outcomes (VSS, total POSAS, POSASpatient, POSAS-observer, VAS, and thickness). If the study used multiple evaluation data, then our selection order was as follows: (1) total POSAS, (2) POSAS-observer, (3) POSASpatient, (4) VSS, and (5) VAS and thickness. These instruments are the most widely used and objective assessment criteria for burn scars. We also contacted the corresponding authors for more detailed information when the necessary data were not presented in the original study. Discrepancies between investigators were resolved by discussion and consensus.

Methodological Quality Assessment of Included Studies
The quality assessment for all studies was performed using the study quality assessment of the National Heart, Lung, and Blood Institute (NIH) [29]. This assessment has different scales for each type of study, with ratings of good, fair, and poor.

Statistical Analysis
Consensus in China indicates that the length of immature scarring varies greatly between individuals and is dependent on a number of factors. Most scars reach maturity in 6-12 months, but the average immature period for hyperplastic scars can be 22-46 months [30]. As a result, to determine the best time to start treatment and the best method for scar treatment within one year, we analyzed relevant data by dividing the samples into the following groups: scarring for less than six months, scarring for six months to one year, and scarring for longer than one year. We used Review Manager 5.4 to calculate the std. mean difference (SMD) or weighted mean difference (WMD) with a 95% confidence interval (95% CI) for continuous outcomes. As substantial heterogeneity was identified, we used only the random-effects model. A p-value less than 0.05 was judged to be statistically significant.

Time of Intervention
Photo-electric therapy offered significant improvement for each period of burn scarring ( Figure 2) (Chi 2 = 6.05, df = 2 (p = 0.05), I 2 = 67.0%). Furthermore, for the group with scarring for over one year and the group with scarring between six months and one year, there was a significant difference in improvement (Chi 2 = 5.43, df = 1 (p = 0.02), I 2 = 81.6%) ( Figure S1). However, there was no significant difference between the group with less than 6 months of scarring compared to the other two groups. For deeper insight, we also analyzed the improvement using only thickness and VSS. Interestingly, we found that in terms of thickness, the photo-electric therapy presented a significant difference in improvement of the scar over one year (Figure 3) (Chi 2 = 6.40, df = 2 (p = 0.04), I 2 = 68.8%). However, in VSS, although there was no significant difference between the two groups, scarring less than one year presented a higher effect size than scarring over one year (Figure 4) (Chi 2 = 1.37, df = 2 (p = 0.50), I 2 = 0%).  . Each trial is represented by a green point, and the size of the point is proportional to the information in that trial. The ends of the horizontal bars denote 95% confidence intervals (Cis). Black diamonds indicate the overall results of all trials. * In order to distinguish between the two Li's articles published in 2021, we have marked this one with an asterisk [43].  [35,43,45,48,53,58,63]. Each trial is represented by a green point, and the size of the point is proportional to the information in that trial. The ends of the horizontal bars denote 95% confidence intervals (Cis). Black diamonds give the overall results of all trials. * In order to distinguish between the two Li's articles published in 2021, we have marked this one with an asterisk [43].

Method for Burn Scarring
We also analyzed the effects of different photo-electric therapies for all periods of burn scars. The results showed significant differences between the methods ( Figure 5) (Chi 2 = 20.38, df = 3 (p = 0.0001), I 2 = 85.3%). Overall, therapies that included IPL were found to work best.

Publication Bias
Our assessment showed no evidence of significant publication bias based on formal statistical tests (Egger's test, p = 0.056 > 0.05).

Discussion
In this study, we found that treatments for scarring over six months and one year have significant improvement differences in general presentation compared to other periods of scarring. The formation of scarring can be divided into three stages: inflammation, proliferation, and remodeling. During the first few days after an injury, corresponding to the inflammation stage, a variety of chemokines and vessel active mediators are produced at the site of the injury [70]. Then, in the proliferation stage, vessel active mediators, such as vascular endothelial growth factor (VEGF), from the previous stage induce microvascular scar tissue, which leads to scar proliferation [71][72][73]. The degree of microvascular density and scar hyperplasia are positively correlated [74]. Therefore, theoretically, intervention in this period can reduce angiogenesis, which can help relieve pruritus, contracture scars, and prevent scar growth and dysfunction. Histological analysis showed that the density of blood vessels in scar tissue increases significantly starting at one month after wound healing [74]. Then, hypertrophic scars generally develop in 2~6 months [27]. However, the results vary greatly between individuals due to different factors. Notably, burn scarring, the time of healing, and remodeling can be prolonged [30], so the best time for intervention in burn scarring is within one year, as shown by our results; interventions may also need to be personalized. Poetschke, J et al. [28] published a similar review on the treatment of immature scarring and concluded that a treatment algorithm should be formulated according to each patient's needs. In conflict with our results, Brewin, M. P et al. [27] proposed that the treatment of PDL should begin before six months, when the scarring remains immature. This difference depends on the definition of immature scarring. As we mentioned, the duration of immature scarring varies greatly between individuals, making it difficult to clearly determine the ideal intervention time. Thus, the best way to deliver treatment is to follow-up with the patient as early as possible and avoid starting treatment too late. Treatment within one year is a good choice based on our results. Lastly, in the remodeling phase, the scar no longer presents redness, and for a hypertrophic or keloid scar, the scar may continue to thicken. This agrees with our outcome that in terms of thickness, photo-electric therapy corresponds to significant differences in improvement of the scar over one year.
In our study, treatment with the addition of IPL offered better improvements than other devices, especially for burn scars treated within one year. IPL therapy is non-invasive, non-surgical, and preliminarily filtered, forming an intense light with a wavelength of 400 to 1200 nm. IPL is not a laser but has similar characteristics to a laser [72]. Through the function of selective photothermolysis, light energy is absorbed by chromophore oxyhemoglobin, which is abundant in the blood vessels, causing photocoagulation of the vascular endothelium. This chromophore can also be absorbed by melanin in the epidermis. Thus, after the application of IPL, melanosomes in the epidermal melasma quickly move to the surface of the skin, undergo desquamation, and take the form of tiny crusts. Li. N et al. [75] used IPL to treat 35 Chinese patients who had a history of skin burns within the past year. The results showed that IPL is effective and safe in Chinese patients with postburn hyperpigmentation and telangiectasia. Meanwhile, as the maximum absorption by collagen occurs in the visible and near-infrared spectra [76], the light can also be absorbed by collagen. It was further confirmed that, with IPL, the activity of fibroblasts is increased, causing upregulation of type-I and type-III collagens at the mRNA and protein levels and rearranging elastin fibers both in vitro and in vivo [77,78]. However, we did not find a convincing systematic review that evaluated the effectiveness of IPL. In the systematic review of Vrijman, C et al. [76], the authors did not find any evidence for the efficacy of IPL therapy, as no study met the inclusion criteria. Zuccaro, Jv [24] found only one study about IPL, which reported mild-to-significant improvement in scarring.
With the development of technology, filtering narrow-spectrum intense pulsed light (DPL) of 500-600 nm through the spectrum at both ends can make treatment energy more concentrated; when the spot is large and uniform, the energy is lower and can more effectively protect normal skin tissue around the scar. This method offers the precision of a laser and the safety of strong pulsed light, greatly improving the curative effects. DPL still contains the absorption peak of hemoglobin, reduces the absorption of light by other tissue, and can use higher energy to block blood vessels; DPL can also inhibit angiogenesis, is more specific than IPL, and leads to less pain than PDL [72,79]. Zhang et al. [79] used DPL to treat 90 patients with scars after 3 weeks but within 1 year. After treatment for 3 months, the pruritus of scars was obviously alleviated. The degree of microvascular regeneration was related to the formation of erythema in the scar, which, in turn, became taller and harder, as well as the level of the hypertrophy of the scar [80]. As a result, we suggest that once the scar heals and appears red, photo-electric therapy that targets neovascularization should be started.
At the same time, the effectiveness of other methods cannot be ignored. Our study still showed a great effect of CO 2 AFL, PDL, LLLT, and RF on scar appearance. The CO 2 AFL can create 3D microthermal damage zones, thereby decreasing wound repair time and adverse reactions. Additionally, the use of fractional carbon dioxide laser treatment for hypertrophic scars can promote a decrease in type I collagen in scar tissue and an increase in type III collagen, which is closer to the collagen structure of normal skin tissue [81]. This treatment can also cause damage to blood vessels, producing scar ischemia and releasing collagenase to break down collagen, while the thermal effect of lasers can also stimulate collagen synthesis and remodeling, which helps to promote collagen remodeling as well as improve the appearance of scars [50]. A recent study compared the effects of starting CO 2 AFL treatment at multiple times after injury. One month after the last treatment, the results showed that CO 2 AFL was more effective for scars after more than 12 months in terms of height and pliability. However, for hardness and redness, scars at 1-3 months presented better results than other groups. The authors suggested that the ideal time point for the initiation of early fractional laser treatment could be within 1 month after injury [38]. However, there are some common side effects of CO 2 AFL, including erythema, seepage, bleeding, swelling, pigmentation, and deterioration of scarring [38,82]. Lower density with moderate laser energy for treating scars was proposed to avoid such problems.
PDL (pulsed dye laser) is the most widely used and effective type of laser for preventing early scarring [83]. According to the principle of selective photothermolysis, hemoglobin has two absorption peaks at 542 and 578 nm. Therefore, a laser at 585 nm will have a noticeable effect on eliminating blood vessels. Meanwhile, through the function of photothermolysis, collagen fibers are heated, and the disulfide bonds are broken, enabling them to be catabolized. In this way, collagen over-deposition can be prevented, stimulating collagen remodeling and allowing for the structure of scar epithelial tissue to be reconstructed [46,84]. However, the efficacy of PDL is limited by the thickness of the lesion. PDL penetrates to a depth of approximately 1.2 mm [85]. The most common side effect of PDL is postdelivery purpura, which persist for up to 7-10 days [41]. When the PDL energy is too high, pigment loss can easily occur [27]. Thus, to treat deeper lesions and reduce the side effects at the same time, more methods for combining other treatments with PDL need to be developed. Recently, Naoaki Rikihisa et al. [86] found that intravenous preadministration of carbonyl hemoglobin vesicles (CO-HbVs) followed by the application of vascular selective laser irradiation to the blood vessels of rabbit pinna reduced thermal damage to the perivascular tissue and partially enhanced vascular damage. In combination with Nd:YAG, PDL first changes hemoglobin into methemoglobin, which can absorb more energy from the Nd:YAG laser and thus penetrate more deeply [84].
LLLT treats scars in a different way by acting on the skin through photobiomodulation (PBM), which is an efficient and safe therapeutic modality for postburn scars. LLLT was found to suppress the viability of fibroblasts, inhibit the proliferation and formation of collagen in skin, and increase apoptosis of fibroblasts through mitochondria [87,88]. LLLT can also improve macrophage migration and phagocytosis independently of TGFβ signaling [89]. At present, LLLT is known to cause no side effects, which is a great advantage in early scar treatment [48]. It is known with certainty that LLLT promotes beneficial effects in the early stages of burn injury. However, we still need more basic and clinical studies to understand the relevant mechanisms and direct the best parameters for each type of burn, each type of skin, and each stage of scarring.
Finally, the mechanism underlying the RF stimulation of collagen fiber remodeling is likely protein denaturation caused by the effects of heat followed by the stimulation of collagen synthesis due to the increased expression of heat shock proteins [90]. With the development of fractional technology, in 2010, fractional microplasma radiofrequency technology (FMRT) was developed and initially used for the treatment of facial scars and rhytids [91]. Pinheiro et al. [92] compared the histological examination of postburn hypertrophic scar tissue treated with RF. The results showed that the treated area featured collagen fiber density in the papillary and reticular dermis similar to that of normal skin. This density was significantly greater in the area with no RF treatment [93]. However, the thermal effects of radiofrequency occur in the deeper layers of the skin, so short-term irritation, edema, and even burns can occur [94]. Additionally, numbness and sensory dullness can occur when thermal coagulation leads to demyelination of sensory nerves [95], so more experienced operators are needed for treatment.

Conclusions
In conclusion, this study indicated that treatment starting between six months and one year after injury had better outcomes in terms of the general presentation of scarring. Meanwhile, using IPL for burn scarring treatment seems to have better effects than other methods, especially for scarring within one year. We suggest using IPL and, especially, DPL for the treatment of early burn scarring. Notably, the scientific evidence in this area remains insufficient. We need more clinical trials of higher quality and less heterogeneity to confirm our results.

Limitations
There are still some limitations to this study. First, the majority of studies we included used pretreatment controls. Most burn scars improve over time, but concurrent control experiments in this area are extremely scarce. There is also a lack of higher-quality clinical trials, such as RCTs, and an inability to apply a double-blinded method for laser therapy. Meanwhile, some studies featured very short follow-ups. Significant heterogeneity was also observed among the studies included in this systematic review, including in the parameters, the application of different treatments, and the lack of general assessments evaluating burn scars. As a result, more well-performed, larger RCTs need to be carried out. The various evaluation criteria (POSAS for the patient and observer, VSS, etc.) also need to be standardized to further verify our results.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ebj4020013/s1, Figure S1: Forest plot showing the effects of photo-electric therapy for burn scars according to VSS, POSAS, thickness, and VAS for the group with scarring for over one year and the group with scarring between six months and one year; File S1: the PRISMA-2020-Checklist [96][97][98].