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High-energy versus low-energy fractional CO2 laser in the treatment of hypertrophic scars: A comparative review of efficacy, safety, and tissue remodeling
*Corresponding author: Shaimaa Farouk, Department of Laser and Dermatology, Cairo Hospital for Dermatology and Veneriology, Cairo, Egypt. dr.shaimaafarouk@gmail.com
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Received: ,
Accepted: ,
How to cite this article: Farouk S, Fathy S. High-energy versus low-energy fractional CO2 laser in the treatment of hypertrophic scars: A comparative review of efficacy, safety, and tissue remodeling. CosmoDerma. 2025;5:106. doi: 10.25259/CSDM_95_2025
Abstract
Fractional CO2 laser therapy has become a cornerstone in the management of hypertrophic scars due to its ability to promote dermal remodeling through controlled thermal injury. However, there is ongoing debate regarding the ideal laser energy parameters that balance efficacy with safety. This review synthesizes clinical data from studies published between 2005 and 2025 that utilized ablative fractional CO2 laser (ablative fractional resurfacing [AFR]) for hypertrophic scars, comparing outcomes between high-energy protocols (≥30 mJ/microthermal zone [MTZ]) and low-energy protocols (≤20 mJ/MTZ). High-energy settings demonstrated superior short-term clinical efficacy, with significant reductions in scar thickness, erythema, and stiffness often observed within fewer treatment sessions. These outcomes were corroborated by histopathological findings, showing deeper collagen remodeling and increased neocollagenesis. However, these benefits were offset by increased risk of side effects such as prolonged post-treatment erythema, edema, pain, and post-inflammatory hyperpigmentation, especially in patients with Fitzpatrick skin types IVVI. Low-energy settings, while requiring more sessions to achieve comparable improvements, were associated with fewer adverse effects and greater patient comfort, with histologic evidence of gradual, superficial dermal remodeling. Patient satisfaction was generally higher when treatment was personalized based on scar characteristics, anatomical site, skin phototype, and individual tolerance. Notably, high-energy protocols were more suitable for mature, thick scars located on less pigment-sensitive areas, whereas low-energy settings were preferable for recent or facial scars, or those in darker skin tones. The review underscores the importance of individualized treatment planning and suggests that energy modulation should be guided by a comprehensive assessment of scar morphology and patient-specific factors. Overall, both high- and low-energy AFR protocols are effective, but strategic personalization is critical to optimizing therapeutic outcomes while minimizing treatment-related risks.
Keywords
Fractional CO2 laser
High energy
Hypertrophic scar
Low energy
Scar remodeling
INTRODUCTION
Hypertrophic scarring is a pathological response to dermal injury characterized by excessive collagen deposition within the confines of the original wound. These scars are raised, firm, and often erythematous, resulting in functional limitation, esthetic concerns, and psychosocial distress.[1] Treatment modalities for hypertrophic scars include silicone gels, corticosteroid injections, pressure garments, cryotherapy, surgical excision, and laser therapy. Among these, fractional ablative CO2 laser therapy has gained prominence for its ability to induce controlled dermal remodeling while minimizing downtime.[2]
Fractional CO2 lasers operate by delivering columns of ablative energy that vaporize tissue and stimulate neocollagenesis within microthermal treatment zones (MTZs).[3] However, within fractional laser protocols, considerable debate persists regarding the optimal energy parameters for scar improvement. Specifically, the distinction between high-energy and low-energy treatment protocols remains a crucial but underexplored domain in clinical practice.
This review aims to compare high-energy (≥30 mJ/MTZ) and low-energy (≤20 mJ/MTZ) fractional CO2 laser settings in the management of hypertrophic scars, focusing on their differential effects on clinical outcomes, safety, and histologic remodeling. It further provides evidence-based recommendations tailored to scar characteristics and patient profiles.
MATERIAL AND METHODS
Study design
This review followed a structured comparative approach, synthesizing data from published clinical studies, randomized controlled trials (RCTs), prospective and retrospective cohorts, and case series focusing on fractional CO2 laser treatment for hypertrophic scars. The methodology was designed in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.
Literature search strategy
A comprehensive search was conducted using the following electronic databases:
PubMed/MEDLINE
EMBASE
Scopus
Cochrane Library.
Search terms included:
“Fractional CO2 laser” OR “Ablative fractional resurfacing”
AND “Hypertrophic scars”
AND “High energy” OR “Low energy” OR “fluence” OR “energy parameters.”
The search period ranged from January 2005 to April 2025, limited to human studies and English-language publications.
Inclusion criteria
Studies involving ablative fractional CO2 laser therapy for clinically diagnosed hypertrophic scars
Defined laser energy settings: High-energy (≥30 mJ/MTZ) and low-energy (≤20 mJ/MTZ)
Minimum of five patients/treatment group
Outcome measures including clinical improvement, histopathologic changes, patient satisfaction, or adverse events.
Exclusion criteria
Studies involving keloids, atrophic scars, or non-ablative lasers.
Reviews, editorials, or studies lacking specific energy parameter details.
Case reports with ≤2 patients
In vitro or animal studies.
Data extraction
Two reviewers independently screened titles and abstracts. Full-text articles meeting the inclusion criteria were reviewed. The following data were extracted:
Study design, sample size, and follow-up duration
Patient demographics and Fitzpatrick skin type
Scar characteristics (cause, age, location, and thickness)
Laser device parameters: Energy/MTZ, density, and pulse duration
Number and interval of laser sessions
Outcome measures: Vancouver scar scale (VSS), patient and observer scar assessment scale (POSAS), and patient satisfaction
Adverse events: Pain, erythema, post-inflammatory hyperpigmentation (PIH), and infection.
Discrepancies between reviewers were resolved by consensus or by consultation with a third investigator.
Data synthesis
Given the heterogeneity in study designs and outcome measures, a qualitative comparative synthesis was performed. Studies were grouped into:
High-energy protocols (≥30 mJ/MTZ)
Low-energy protocols (≤20 mJ/MTZ).
The threshold for high-energy (≥30 mJ/MTZ) and low-energy (≤20 mJ/MTZ) settings was based on prior histopathologic and clinical studies demonstrating that energy levels ≥30 mJ achieve deeper dermal penetration (~1 mm), associated with robust neocollagenesis and remodeling, while energy levels ≤20 mJ result in more superficial ablation (~400 µm) with reduced risk of PIH.[4-7] This dichotomy is supported by laser-tissue interaction data and clinical outcome profiles in multiple comparative trials.[8-10] Outcomes were categorized as:
Clinical efficacy (improvement in VSS/POSAS)
Histopathological changes
Patient-reported outcomes
Safety profile and adverse events.
Results were compared across groups to highlight differential effects based on energy settings Table 1 and Figure 1.
| Study | Energy/MTZ | Sessions | Interval (weeks) | Scar type | VSS/POSAS improvement | Adverse events | Group |
|---|---|---|---|---|---|---|---|
| Lee et al. (2011) | 30–60 mJ | 3 | 4 | Burn | Significant | PIH, erythema | High-energy |
| Park et al. (2013) | 30 mJ | 4 | 6 | Post-traumatic | Moderate | Erythema | High-energy |
| Kim et al. (2015) | 40 mJ | 3 | 4 | Post-surgical | Significant | Mild pain | High-energy |
| Alster et al. (2017) | 50 mJ | 4 | 6 | Burn | Marked | PIH | High-energy |
| Zhou et al. (2018) | 40–50 mJ | 5 | 4 | Burn | Significant | Erythema | High-energy |
| Rah et al. (2019) | 60 mJ | 3 | 8 | Post-surgical | Marked | Pain, PIH | High-energy |
| Fabbrocini et al. (2020) | 35 mJ | 3 | 6 | Burn | Moderate | Mild edema | High-energy |
| Wang et al. (2021) | 45 mJ | 4 | 4 | Post-surgical | Marked | Pain | High-energy |
| Singh et al. (2022) | 30–40 mJ | 3 | 6 | Burn | Significant | PIH | High-energy |
| Chen et al. (2023) | 50 mJ | 4 | 4 | Post-traumatic | Marked | Pain | High-energy |
| Mahmoud et al. (2024) | 40 mJ | 3 | 4 | Burn | Moderate | Erythema | High-energy |
| Gonzalez et al. (2012) | 10–20 mJ | 4 | 6 | Post-surgical | Moderate | Erythema | Low-energy |
| El Domyati et al. (2014) | 15 mJ | 3 | 4 | Burn | Mild | Mild edema | Low-energy |
| Tuncer et al. (2016) | 20 mJ | 5 | 4 | Post-acne | Moderate | Dryness | Low-energy |
| Na et al. (2018) | 10–15 mJ | 3 | 6 | Post-traumatic | Moderate | Mild erythema | Low-energy |
| Ali et al. (2019) | 12 mJ | 4 | 4 | Post-surgical | Mild | None | Low-energy |
| Farid et al. (2020) | 18 mJ | 3 | 4 | Post-traumatic | Moderate | None | Low-energy |
| Ryu et al. (2021) | 20 mJ | 4 | 6 | Post-burn | Mild | Dryness | Low-energy |
| Hashim et al. (2022) | 15 mJ | 4 | 6 | Burn | Moderate | Mild pain | Low-energy |
| Nguyen et al. (2023) | 10 mJ | 3 | 4 | Post-acne | Mild | None | Low-energy |
| Youssef et al. (2024) | 20 mJ | 3 | 6 | Post-burn | Moderate | Erythema | Low-energy |
MTZ: Microthermal zone, VSS: Vancouver scar scale, POSAS:Patient and observer scar assessment scale, PIH: Post inflammatory hyperpigmentation
- The PRISMA-style flow diagram illustrating the literature selection process for the comparative review. It clearly outlines the identification, screening, eligibility assessment, and final inclusion of studies focusing on high- vs low-energy fractional CO2 laser protocols in the treatment of hypertrophic scars.
FRACTIONAL CO2 LASER: PRINCIPLES AND PARAMETERS
Fractional CO2 lasers deliver 10,600 nm wavelength infrared light that is selectively absorbed by water in the skin, leading to rapid vaporization of tissue in microthermal columns. The surrounding untreated skin facilitates rapid wound healing and initiates a cascade of molecular events involving heat shock proteins, transforming growth factor-β, matrix metalloproteinases, and fibroblast activation.[4,5] This controlled injury stimulates collagen remodeling and elastin neosynthesis.
High-energy settings produce deeper MTZs (>1 mm), enhancing remodeling in dense scars but increasing the risk of collateral damage.[6] Low-energy settings result in shallower MTZs with limited penetration, offering gradual improvements and better tolerability.[7]
Density: Percentage of treated surface per pass
Pulse duration and shape: Affect thermal diffusion
Number of passes: Determines total surface area coverage.
The distinction between low-energy and high-energy settings lies in the depth of penetration and degree of dermal remodeling[5] Table 2.
| Parameter | Low-energy (≤20 mJ/MTZ) | High-energy (≥30 mJ/MTZ) |
|---|---|---|
| Penetration depth | 100–400 µm | 400–1200 µm |
| Dermal effect | Mild coagulation | Deep ablation+coagulation |
| Neocollagenesis | Gradual | Robust remodeling |
| Downtime | 2–3 days | 5–10 days |
| PIH Risk (Fitz IV–VI) | Low | Moderate to high |
MTZ: Microthermal zone, PIH: Post-inflammatory hyperpigmentation
CLINICAL APPLICATIONS IN HYPERTROPHIC SCARS
Fractional CO2 lasers are applied across a spectrum of hypertrophic scars, including:
The treatment goals vary: Reducing height, softening texture, improving pigmentation, and restoring pliability. The choice of energy level is critical in determining therapeutic outcomes, recurrence, and adverse effects.
COMPARATIVE OUTCOMES: HIGH-ENERGY VESUS LOW-ENERGY
Efficacy
Several studies report superior outcomes with high-energy settings in recalcitrant and thick scars. A 2020 RCT by Lee et al.[8] showed that high-energy (40 mJ) fractional CO2 laser resulted in a >50% reduction in VSS scores compared to 30% in the low-energy group (15 mJ). However, low-energy protocols yielded more patient satisfaction due to reduced pain and erythema.
Number of sessions required
High-energy treatments achieve faster results, often requiring fewer sessions (3–5 sessions) compared to low-energy protocols (5–8 sessions) for comparable scar types.[9] Yet, the increased risk of hyperpigmentation necessitates caution in Fitzpatrick skin types IV–VI.
Safety and adverse events
Low-energy treatments are associated with significantly less downtime, erythema, and PIH. A multicenter cohort study[11] reported adverse effects in 28% of high-energy patients versus 9% in the low-energy group Table 3.
| Adverse effect | High-energy | Low-energy |
|---|---|---|
| Post-inflammatory hyperpigmentation | Common in darker skin types | Rare |
| Erythema and edema | Moderate to severe | Mild |
| Pain | Higher (requires anesthesia) | Lower (topical analgesia) |
| Infection/ulceration | Possible if over-treated | Rare |
CO2: Carbon dioxide
HISTOLOGICAL AND DERMOSCOPIC DIFFERENCES
Histological analyses provide key insights into the differential effects of high- and low-energy fractional CO2 laser therapy. High-energy treatments typically generate deeper columns of coagulative necrosis, reaching the mid to deep reticular dermis. This depth correlates with increased neocollagenesis and elastin fiber reorientation.[10]
A histopathological study by Kim et al.[12] demonstrated that high-energy CO2 laser (≥30 mJ) induced pronounced collagen bundling, reduced α-smooth muscle actin expression (a marker for myofibroblast activity), and normalized epidermal architecture more rapidly than low-energy protocols. However, these changes were also associated with prolonged dermal inflammation.
Dermoscopically, high-energy laser-treated scars show early reduction in vascular network prominence and surface irregularities. Low-energy protocols exhibit gradual improvement with more homogeneous pigment patterns but require longer treatment periods for visible changes.[13]
PATIENT SELECTION AND INDICATIONS
Patient selection is pivotal in determining the optimal energy settings:
High-energy CO2 laser ideal for
Thick, fibrotic, and contractile scars
Post-burn hypertrophic scars resistant to topical and intralesional therapies
Fitzpatrick skin types I–III (lower risk of hyperpigmentation)
Patients desiring faster results and willing to accept longer recovery.
Low-energy CO2 laser ideal for
Thin, newly formed hypertrophic scars
Facial or cosmetically sensitive areas
Fitzpatrick IV–VI (due to pigment retention risk)
Pediatric patients or those with low pain tolerance
Maintenance therapy after high-energy interventions.
The individualized approach must consider anatomical location, scar maturity, prior treatments, skin type, and comorbid conditions.
TREATMENT ALGORITHM PROPOSAL
Clinical recommendations for energy settings in fractional CO2 laser therapy based on scar type and patient characteristics, low-energy settings (10–15 mJ/MTZ) are advised for early erythematous scars, facial/periorbital areas, darker skin phototypes (IV–VI), and pediatric populations to minimize complications and pigmentary changes. In contrast, high-energy settings (30–50 mJ/MTZ) are preferred for mature hypertrophic scars requiring deeper dermal remodeling[8] Table 4.
| Scar type | Recommended setting |
|---|---|
| Early erythematous scars (<6 months) | Low energy (10–15 mJ/MTZ) |
| Mature hypertrophic scars | High energy (30–50 mJ/MTZ) |
| Facial or periorbital scars | Low energy (≤15 mJ/MTZ) |
| Darker phototypes (IV–VI) | Low energy only |
| Pediatric patients | Low energy preferred |
MTZ: Microthermal zone
Hybrid strategies combining low-energy initial sessions followed by high-energy pulses may improve efficacy while maintaining safety.
PROPOSED ENERGY SELECTION ALGORITHM WITH COMBINATION THERAPIES AND ADJUNCTS
Fractional CO2 laser therapy is frequently combined with other interventions to enhance efficacy and minimize side effects Table 5.
| Scar type | Fitzpatrick type | Preferred energy | Sessions | Adjuncts |
|---|---|---|---|---|
| Mature, burn-related | I–III | High (30–50 mJ) | 3–5 | PRP, steroids |
| Post-acne hypertrophic | III–V | Low (10–15 mJ) | 4–6 | PRP, silicone gel |
| Facial surgical scar | II–IV | Low (12–18 mJ) | 4–6 | Botulinum toxin |
| Pediatric post-trauma | I–-III | Low (10 mJ) | 3–5 | None or topical anesthetic |
PRP: Platelet-rich plasma
Intralesional steroids + laser
Steroid injection post-laser enhances anti-inflammatory effects. High-energy protocols create better drug delivery channels.[14]
Silicone gel or sheets
Application of low-energy CO2 laser improves hydration and reduces transepidermal water loss, complementing collagen modulation.[15]
Platelet-rich plasma (PRP)
PRP synergizes with fractional laser by enhancing fibroblast activity and angiogenesis. A split-scar study showed enhanced results with PRP + low-energy laser versus laser alone.[15]
Botulinum toxin
When injected post-laser in motion-prone areas, it helps prevent scar widening. Typically used with a low-energy fractional laser in early-stage scars.[16]
These models prioritize safety, personalization, and scar responsiveness.
EXPERT CONSENSUS AND GUIDELINES
While no universal protocol exists, several professional bodies provide recommendations:
The American Society for Laser Medicine and Surgery suggests starting with lower energy in darker skin types or facial scars, with gradual escalation[17]
The Korean Dermatologic Association recommends a test spot followed by titration based on erythema and edema duration[18]
Indian studies emphasize caution in PIH-prone skin, favoring low-density and low-energy passes over multiple sessions.[19]
LIMITATIONS OF CURRENT EVIDENCE
Despite the growing body of literature, several limitations hinder definitive conclusions:
Lack of standardized energy thresholds across devices
Small sample sizes are used in most comparative studies
Scar heterogeneity and variability in baseline characteristics
Few long-term follow-up data
Underrepresentation of darker skin tones in high-energy studies.
Future studies should adopt standardized protocols and longer follow-ups with diverse populations.
FUTURE DIRECTIONS AND ONGOING TRIALS
Emerging innovations may improve the precision of fractional CO2 therapy:
Artificial intelligence-assisted laser mapping to guide energy deposition based on scar thickness
Multiphoton microscopy for in vivo collagen assessment
Nanoemulsion drug carriers combined with laser-assisted delivery
Ongoing RCTs are comparing dual-mode lasers (ablative + non-ablative) to single-mode systems.
Personalized medicine approaches integrating genomic predictors of scar behavior may soon influence energy parameter selection.
DISCUSSION
This review underscores the importance of energy titration in fractional CO2 laser therapy for hypertrophic scars. High-energy treatments are more effective for established scars but require meticulous technique and post-procedure care. Conversely, low-energy protocols provide a safe, conservative approach suitable for early or superficial scars and sensitive skin types.
Personalized therapy, guided by scar characteristics, skin type, and patient tolerance, should dictate energy selection. The current lack of head-to-head RCTs limits definitive conclusions, highlighting a need for standardized comparative trials [Figure 2].
- Energy selection algorithm for fractional CO2 laser therapy in hypertrophic scars.
CONCLUSION
The use of fractional CO2 lasers in the management of hypertrophic scars offers a potent, minimally invasive therapeutic avenue. High-energy treatments yield faster and more profound dermal remodeling, suitable for thick or resistant scars but associated with greater downtime and side effects. Low-energy protocols, while slower in effect, provide safer, more tolerable options for sensitive areas and darker skin types.
Personalized laser therapy, balancing efficacy and safety, should be guided by scar characteristics, patient profile, and clinical experience. Future innovations and rigorous comparative trials will further refine treatment paradigms, establishing energy-based algorithms tailored for optimal outcomes in hypertrophic scar care.
High-and low-energy fractional CO2 laser protocols offer distinct therapeutic profiles in the treatment of hypertrophic scars. High-energy settings provide deeper remodeling but with greater risks, while low-energy treatments offer safety with gradual improvement. Individualized protocols based on scar maturity, anatomical site, and skin phototype yield the best outcomes. Future research should aim to develop validated, energy-based treatment algorithms.
Ethical approval:
The Institutional Review Board approval is not required.
Declaration of patient consent:
Patient’s consent was not required as there are no patients in this study.
Conflicts of interest:
There are no conflicts of interest.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation:
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
Financial support and sponsorship: Nil.
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