In the early stage of acne scar formation, anti-inflammatory medications are recommended to prevent further skin necrosis and obvious scar formation. Recommended active interventions in the early stage include intense pulsed light and a 560 nm or 590 nm filter or 500-600 nm mixed spectral line intense pulsed light with a low-energy mode for continuous daily treatment until the improvement or subsidence of inflammatory erythematosus. At the mature stage of acne scar formation, the recommended interventions include exfoliative lattice lasers such as CO₂ and neodymium-dopped yttrium aluminium garnet (Er:YAG) lattice lasers, fractional microneedle radiofrequency, or traditional Chinese plum needle rolling needle for the treatment of atrophic scars. Mild atrophic scars can also be treated with plasma therapy or exfoliative lattice laser therapy. Hypertrophic scars should be treated with drugs combined with photoelectric therapy, such as the injection of a long-acting glucocorticoid combined with laser stripping. For patients with keloid scar formation, the recommended treatments are drug therapy, photoelectric combined with radiotherapy, or surgery combined with radiotherapy.

Silicone improves the scar epidermal structure and prevents scar hyperplasia and contracture and is mostly used for the prevention and treatment of hypertrophic acne scars. The recommended duration of use is more than 6 months.

Heparin sodium allantoin inhibits fibroblast proliferation to make the scar surface more elastic and is used for the treatment of hypertrophic and keloid scars. The recommended duration of use is more than 6 months.

Glucocorticoids reduce inflammation, inhibit fibroblast proliferation, induce apoptosis, and reduce collagen synthesis, but may cause adverse reactions including skin atrophy, telangiectasia, and hypopigmentation. Directions for usage: 1.0 mL of compound betamethasone (7 g/L) or triamcinolone acetonide solution (10 g/L) + 1.0 mL of lidocaine solution (20 g/L) applied once a month.

Botulinum toxin A inhibits the release of acetylcholine from nerve endings, relaxes muscle fibers, reduces local tension, and reduces keloid formation. Possible adverse reactions include dysfunction caused by local muscle paralysis and subcutaneous congestion at the injection site. Directions for usage: 20-50 U/mL is usually prepared for injection into keloid dermis, and the total dose should be less than 100 U per monthly injection according to the volume of scarring.

Fluorouracil inhibits thymidylate synthase, affects fibroblast proliferation, blocks transforming growth factor b-mediated expression of the type I collagen gene, and reduces collagen production. Fluorouracil is often used in combination with triamcinolone acetonide, and patients must undergo routine blood testing during use. Fluorouracil cannot be used by pregnant or lactating women. Directions for usage: triamcinolone acetonide (10 g/L) and 2.5% fluorouracil injection are extracted to prepare a mixed solution in a ratio of 3:1-1:1. The maximum single dose is 2 mL triamcinolone acetonide and 0.5 mL fluorouracil. Patients with excessive skin lesions should be injected once every 10-14 days. If the needle wound is not healed or the drug is not completely absorbed, the interval between injections can be extended to 15-30 days.

Bleomycin is a cytotoxic drug that improves the appearance of pathological tissues and significantly relieves the itching and pain caused by keloids. Directions for usage: 1.5 IU/mL bleomycin solution is injected every 0.5 cm and repeated once or twice a month according to the patient's needs.

Oral drugs such as asiaticosides, retinoids, and tranilast can be administered to inhibit the proliferation of fibroblasts and reduce the production of collagen fibers to prevent scar formation in the early stage. However, these drugs are rarely used in clinical practice.

Laser therapy is based on the principles of selective photothermal action and focal photothermal action. The types of lasers include lattice laser, long pulsewidth Nd: YAG laser, and pulsed dye laser.

Atrophic scars: Exfoliative lattice lasers (lattice Er:YAG and lattice CO₂) form a microcolumnar gasification zone and hot solidification necrotic zone in tissue, promoting epidermal regeneration and collagen remodeling. Treatment parameters need to be individually set according to the type and location of acne scars.¹⁷ Exfoliative lattice lasers are used to treat shallow boxcar type and rolling type atrophic scars. Non-exfoliative lattice lasers include lattice Er:Glass lasers, thulium fiber lasers, and focused lattice lasers. Compared with exfoliative lattice lasers, non-exfoliative laser therapy requires a shorter recovery period, but has a weaker collagen remodeling ability, and often needs more treatment sessions to achieve similar efficacy. Long pulsewidth Nd:YAG laser therapy can also be used to treat atrophic scars. Hypertrophic scars/keloids: Pulsed dye laser therapy and long pulsewidth Nd:YAG laser therapy reduce microvessels and promote collagen degradation and remodeling, thereby improving scar erythema, thickness, and flexibility, which is useful in treating erythematous hypertrophic scars and keloids. Exfoliative laser therapy can be used to vaporize stable hypertrophic scars on areas such as the nose. Exfoliative lattice laser therapy releases scar adhesions and improve scar elasticity. For severe keloids, it is recommended to combine laser therapy with local glucocorticoid injection, superficial X-ray radiation, surgical resection, and other treatments.

Intense pulsed light acts on hemoglobin and collagen and is used to improve shallow atrophic scars and early erythematous hypertrophic scars. Intense pulsed light therapy can be combined with drug injection or laser treatment.

Radiofrequency therapy can be used in combination with laser therapy and/or for patients with atrophic acne scars with inflammatory acne lesions. Radiofrequency therapy does not aggravate the original active acne lesions and can reduce the sebum level and sebum overflow rate, which is useful in patients with inflammatory acne. The main adverse reactions include pain, transient erythema, and edema, while pigmentation is rare.

Plasma therapy mainly uses the energy released by plasma to produce heat and exfoliation to improve scarring, and is suitable for treating shallow atrophic scars. The recommended treatment methods include multiple, single round, low energy density or single, single round, high energy density; the former may have a shorter recovery time.¹⁸

Photodynamic therapy is limited to patients with moderate and severe acne caused by Propionibacterium acnes; however, there is still a lack of evidence regarding the use of photodynamic therapy for acne scars.

Skin grinding is suitable for rolling type and shallow boxcar type scars (evidence level I, recommended strength++), but has poor efficacy for icepick type and deep boxcar type scars.¹⁰ This technique enables the precise and controlled removal of scar edges without thermal damage, and the effectiveness of treatment is highly dependent on the skill and experience of the operator. A wide range of grinding requires general anesthesia, takes up to 2 weeks to heal, and is accompanied by long-term postoperative erythema, which may cause pigmentation and scar hyperplasia.

Subcutaneous separation is mainly suitable for rolling type scars (Level of Evidence III, recommended strength+++), and is partially effective for icepick type and boxcar type scars. The operation is simple and can be completed under local anesthesia, and can be combined with filling and laser technology. Multiple treatments may be required to obtain satisfactory results. Adverse reactions include temporary petechiae, swelling, bleeding, paresthesia, and inflammatory papules. Subcutaneous separation can be divided into sharp needle separation and blunt needle separation.¹⁹

A trephine with an inner diameter of 0.8-2.5 mm is often used clinically. Trephine tissue extraction is suitable for treating hypertrophic acne scars and keloids, while trephine tissue promotion is suitable for icepick type and boxcar type acne scars with clear edges and a normal basal appearance. Trephine tissue transplantation is suitable for treating hyperpigmentation and deep icepick type acne scars.⁴

Surgical resection is suitable for large scars on the mandibular angle, keloids caused by acne on the upper torso, and deep atrophic scars that cover a large area. Surgical methods include simple resection and suture, Z or W plasty, cicatrectomy, skin grafting, flap transfer, and skin dilation.²⁰ Superficial X-ray therapy is required after keloid surgery.

Filling therapy fills the skin lesions by injection or surgery and stimulates fibroblasts to produce collagen fibers and fibrous tissues to replenish the volume and smooth scars. Filling therapy is mostly used for large and deep atrophic scars, especially boxcar type and rolling type scars (Level of Evidence V, recommended strength ++). The most common fillers are autologous fat, fat glue, autologous cells, hyaluronic acid, and poly-L-lactic acid.²¹

Autologous fat grafting (AFG) supplements the missing tissue, promotes the formation of new blood vessels, and improves skin elasticity to improve the appearance of scars. AFG has the advantages of easy access and no rejection or allergies, but the fat survival rate is unstable and multiple treatments are often required to achieve satisfactory results. Stromal vascular fraction gel (SVF-GEL) is a gel-like extract obtained by mechanically cutting autologous fat and removing lipid droplets by centrifugation. The main components of SVF-GEL are extracellular matrix, adipose stem cells, and vascular endothelial cells.²² Compared with simple AFG, SVF-GEL injection results in less inflammation, less swelling, a higher survival rate, and a relatively stable effect. Comparative studies have shown that SVF-GEL is more effective than CO₂ fractional laser therapy in the treatment of acne scars. The severity of scars, percentage of scarred area, transdermal water loss, and skin water content are all significantly improved for 3 months after one SVF-GEL treatment, and the therapeutic effect of one SVF-GEL treatment is equivalent to that of three fractional laser treatments.²³

In autologous fibroblast transplantation, skin is removed from a non-exposed part of the patient. The fibroblasts from this skin are isolated and cultivated for several weeks and then injected into the scar to form new collagen to help reshape the extracellular matrix. Autologous fibroblast transplantation has the advantages of permanent filling and low immunogenicity.

Another commonly used tissue filler is low cross-linked hyaluronic acid, which has the advantages of low immunogenicity and absorbability.

Superficial chemical peeling is used to treat shallow atrophic acne scars and scars with postinflammatory hyperpigmentation (PIH). Glycol acid peeling promotes epidermal renewal and thickening, while also promoting the proliferation of acid mucopolysaccharides, collagen fibers, and elastic fibers in the papillary layer of the dermis. A mixture of 20%-70% glycolic acid, 92% lactic acid, and 25%-30% salicylic acid improves skin texture, reduces PIH, prevents scarring in the early stage, and flattens and shrinks the surface of shallow atrophic scars during the adjuvant treatment of acne lesions.²⁴ Jessner solution (14 g of resorcinol, 14 g of salicylic acid, 14 g of 85% lactic acid, and ethanol to make up 100 mL) treats acne and has a certain effect on superficial scars, and is often used in combination with other exfoliation methods to increase the depth of chemical peeling.²⁵ A compound acid composed of 10% mandelic acid and 20% salicylic acid is as effective as 35% glycolic acid, but the former is milder and has a lower risk of PIH.²⁶ Superficial chemical peeling is recommended to treat atrophic icepick type acne scars.

Although the use of trichloroacetic acid or phenol for middle or deep exfoliation is more effective in the treatment of atrophic deep boxcar type acne scars, deep exfoliation is only recommended with caution in the Chinese population because dark-skinned people are prone to PIH.

ReCell technology is also called skin active cell transplantation and is mainly used in conjunction with skin abrasion (Level of Evidence V, recommended strength +). The wound healing time is shorter after ReCell therapy than after skin abrasion alone, and the outer dressing can be removed only 5 days after ReCell surgery. ReCell therapy also has a low incidence of postoperative pigmentation and scar hyperplasia, which is most advantageous in the treatment of rolling type and shallow boxcar type scars.²⁸

Intradermal injection or topical application of platelet-rich plasma is mainly used as an auxiliary technology combined with microneedle therapy and exfoliative lattice laser therapy to improve the effectiveness of scar treatment. Platelet-rich plasma therapy is simple and safe with no reported adverse reactions, but lacks a standardized treatment plan.²⁹

There is currently a lack of high-quality research evaluating the effectiveness and safety of stem cell therapy. Injecting autologous bone marrow mesenchymal stem cells into scars may improve the appearance of atrophic scars,³⁰ and is expected to be a future research direction.

Doctors should inform the patients about realistic expectations, recovery time, treatment cost, and the importance of maintenance therapy before treatment. The types and depth of the scars and patient's skin type must be evaluated to enable the selection of the optimal therapeutic schedule and equipment. The affected area should be cleaned with a mild facial cleanser before treatment. A healthy diet is recommended before and after treatment. Pretreatment, immediately posttreatment, and follow-up photographs should be taken; the position, background, angle, direction, distance, and lighting should be optimized.

The appropriate treatment mode and parameters should be chosen based on the patient's skin type and the type and depth of the scars. Clinicians must observe the immediate treatment response, including the degree of redness, oozing, eschar formation, and penetration depth. Different photoelectric therapies result in different skin reactions. Non-exfoliative phototherapy results in only slight redness and swelling, while exfoliative laser therapy causes immediate spot-like whitening.

Clinicians must monitor the postoperative reaction, including the degree of skin swelling, peeling, and oozing, and observe the recovery to identify infection, scab formation, and erythema. Aseptic epidermal growth factor, collagen spray or gel, or a medical hyaluronic acid or collagen mask should be used as appropriate to promote repair. After exfoliative laser treatment, patients must avoid contacting water as much as possible before epidermal healing; when necessary, a sterile physiological sodium chloride solution can be used for cleaning. The posttreatment scab should be left to drop off naturally. It is particularly important to prevent infection after exfoliative photoelectric treatment. Mupiroxacin, erythromycin ointment (or eye ointment), and other antibiotics can be used to prevent infection, when necessary. It is important to moisturize the skin after non-exfoliative phototherapy and after exfoliative phototherapy once the scab has fallen off. Sun protection is essential. Patients should wear a sunscreen shirt or shelter under an umbrella and wear a mask, but should use sun cream with caution to prevent allergic reactions.

Patients are instructed to abstain from alcohol and smoking and are evaluated for heart and lung diseases, history of previous surgery, and medication allergies. The skin in the surgical area is prepared and evaluated to determine the type, size, range, color, depression or bulge, and texture of the scar. Pre-, intra-, and postoperative photographs should be taken under good conditions.

When performing skin grinding, clinicians must prevent the grinding depth from exceeding the superficial dermis to the deep dermis and monitor the amount of intraoperative bleeding and oozing. If the acne scar is removed surgically, clinicians must avoid overly deep excision, ensure that blood vessels and nerves are avoided, and completely stop the bleeding. The excision scope should not be too large but should be limited within the skin lesion boundary as far as possible to avoid excessive suture tension and later scar enlargement.

Postoperative monitoring is important to detect any adverse reactions such as oozing, exudation, redness, and swelling at the suture line. Regular dressing changes are required to prevent wound infection. After the wound has healed, patients must moisturize and apply sunscreen; anti-scar drugs can be administered to prevent scar hyperplasia.

Prior to chemical peeling, a thorough consultation and examination should be conducted to determine the skin type, degree of photoaging and sebum activity, and detect PIH, hypertrophic scarring, and local bacterial or viral infection.⁴⁰ During the operation, clinicians must use the appropriate stripping agent concentration and stripping time and observe the skin reaction. Patients should use a mild moisturizing agent for 1 week postoperatively, wait for the scab to fall off naturally without force, apply sunscreen, and consume a healthy diet.

Doctors should inquire about the patient's medical history, accurately grade the acne scar using the Vancouver Scar Scale, visual analog scale, or POSAS, and inform the patient about potential adverse reactions to reduce the psychological burden on patients when adverse reactions occur.

During treatment, the condition of the treated area should be dynamically observed and recorded, including the presence of any adverse reactions such as redness, swelling, burning, and pain. In addition, the patient's general condition and possible adverse reactions (such as allergy and embolism) should be noted.

The wound will ooze after the injection of biologics and must be kept dry and clean with medical wound dressings. Patients who have undergone ReCell therapy and biotherapy combined with laser or microneedle therapy must be vigilant about sun protection. Moreover, patients who have undergone ReCell therapy should be given anti-infection treatment to prevent local infection.