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Acne scar is "a result of damage to the skin during the healing of active acne." The three basic types of acne scar are atrophic scar, hypertrophic scar, and keloid. Atrophic scar formations are associated with losses of collagen. Hypertrophic scar and keloid formations are associated with gains of collagen. Atrophic scar can be further subclassified into icepick scar, boxcar scar and rolling scar.

Three common techniques for diagnosis of acne scars are scar lesion counting, Goodman & Baron qualitative scarring grading system, and ultrasound imaging. They determine the total number and the severity of acne scars. Different treatments can be employed depending on the type and the severity of acne scar. Treatments for atrophic scars include laser treatment, chemical peeling, dermabrasion and microdermabrasion, dermal grafting, punch techniques, needling, and tissue augmenting agents. Treatments for hypertrophic scars and keloids include cryotherapy, intralesional steroid therapy, pulsed dye laser, silicone gel, and surgery.

Classification
The three basic morphological types of acne scars are atrophic scars, hypertrophic scars, and keloids. The majority of acne scars are atrophic scars, with a prevalence of 80 - 90%.

Atrophic Scar
Atrophic scars are formed due to a net loss of collagen in the skin layers. Atrophic acne scars can be further sub-classified into three types -- icepick scar, boxcar scar, and rolling scar. Often times, a combination of the three types of atrophic scars can manifest simultaneously in one patient.

Icepick scar is the most common type of atrophic scar, representing 60 - 70% of total atrophic scar cases. Icepick scars are deep and narrow (<2 mm). They extend vertically to subcutaneous tissue or the deep dermis in the form of sharply marginated epithelial tract.

Boxcar scar represents 20 - 30% of total atrophic scars. Boxcar scars are round to oval depressions that are sharply demarcated at the edges. They can be shallow (0.1 - 0.5 mm) or deep (≥0.5 mm), and their diameters are often 1.5 - 4.0 mm. Compared to icepick scars, they are wider at the surface. Rolling scar is the least common type of atrophic scar, only representing 15 - 25% of total atrophic scars. Rolling scars are usually wider than 4 - 5 mm. Visually, rolling scars have "a rolling or undulating appearance to the overlying skin" with superficial shadowing. The major cause of rolling scar is dermal tethering of normal skin.

Hypertrophic Scar and Keloids
Hypertrophic scars and keloids are formed due to a net gain of collagen in the skin layers. Hypertrophic scars are "typically pink, raised, and firm, with thick hyalinized collagen bundles that remain within the borders of the original site of injury." In contrast, keloids are "reddish-purple papules and nodules that proliferate beyond the borders of the original wound." Hypertrophic scars and keloids occur predominantly on the trunk. Individuals with high melanin that have darker tone skin are more susceptible to hypertrophic scars and keloids.

Pathogenesis
In general, acne formation is triggered by inflammation within the pilosebaceous units of the body. The most commonly affected pilosebaceous units are the face, chest, upper arms, and back. With excess sebum production, Cutibacterium acnes bacterial infection proliferates easily and causes inflammation of the skin. This inflammation often results in a bump or a protrusion of the skin, a common visual characteristic of all acne cases. As time passes, continual inflammation or physical lesion will lead to either follicular rupture or perifollicular abscess, eventually stimulating the skin's wound healing process. The altered uneven wound healing response to the ruptured inflamed pilosebaceous fundamentally leads to all types of acne scar formation. Atrophic scars, hypertrophic scars and keloids differ in terms of healing responses to ruptured acnes.

Atrophic scar proliferation is mainly attributed to the net destruction of collagen within the dermis layer during the healing process. A commonly identifiable characteristic of atrophic scars is the indentations left on the skin. Due to the net loss of collagen, skin tissue around the boundaries of the follicle rupture is removed creating an indentation.

Hypertrophic scars and keloids are formed because of the net gain of collagen within the dermis layer during the healing process, forming a protrusion of the skin. Heightened production of collagen creates a mass that protrudes away from the skin. Often times, hypertrophic scars and keloids have a slightly translucent red colouration.

Diagnosis
Scar lesion counting, Goodman & Baron qualitative scarring grading system and ultrasound imaging are three commonly used approaches to diagnose and grade the severity of acne scars. All these approaches cater specifically towards diagnosis of acne scars residing on the face.

Scar Lesion Counting
Scar lesion counting is a physical examination counting the total number of acne scars on the face. The number of scars determines the overall severity of the acne scarring condition. Lesion counting diagnostic tests do not take into account the severity of each individual scar, and merely takes the total number of scars.

Goodman & Baron Qualitative Scarring Grading System
Goodman & Baron qualitative scarring grading system is the most commonly used global acne scoring system. The grading scale of this system ranges from grade 1 to grade 4. The severity score of certain scarring conditions depend on whether the scarring condition matches certain categorical descriptions at each grade.

- Grade 1 conditions: Macular cases of acne scars that are localised on the epidermis with minor erythematous, hyperpigmentation, or hypopigmentation.

- Grade 2 conditions: Mild cases of atrophic or hypertrophic scars that require careful visual examination to identify.

- Grade 3 conditions: Moderate cases of atrophic and hypertrophic scars with significant border indentations that are easily identifiable to the naked eye.

- Grade 4 conditions: Severe cases of prominent acne scars such as deep atrophic scars, highly protruded hypertrophic scars or keloids.

The usage of the Goodman & Baron quantitative grading system can also be combined with scar lesion counting methods. This quantitative system combines both the total number of scars and the severity of each scar, yielding a more holistic method of acne scar diagnosis as the approach covers both overall severity and distinct individual severity of acne scars.

Ultrasound Imaging
Ultrasound imaging is a computational imaging technique that is used to evaluate scar thicknesses and determine whether scars are thinning or not. The technology highlights scar patterns on the facial surface corresponding uneven surfaces to a specific colour. With the penetrating nature of ultrasound, the 3D topographic images generated can visualize density, thicknesses, and severity of acne scars that are hidden to the naked eye.

Treatments
Treatments for acne scars vary among different types of acne scars as each condition either requires treatments to reduce collagen deposits or treatments to stimulate collagen production.

Treatments for for Facial Atrophic Acne Scar
All treatments for facial atrophic acne scar fundamentally attempt to reduce the duration and the intensity of inflammation and stimulate the regeneration of collagen. Commonly utilised treatments are laser treatment, chemical peeling, dermabrasion and microdermabrasion.

Laser Treatment
Laser treatment utilize ablative laser or non-ablative laser aiming to tighten collagen fibres at atrophic scarring sites. This type of treatment is most effective on boxcar or rolling scar patients. In ablative laser treatment, ablative lasers are used to remove damaged skin tissue by melting, evaporating or vaporizing. Two most commonly used ablative lasers are Erbium YAG lasers and carbon dioxide lasers. In non-ablative laser treatment, non-abrasive lasers stimulate new collagen formation without removing the tissue. Two most commonly used non-ablative lasers are diode lasers and Nd:YAG lasers.

Chemical Peeling
Chemical peeling involves the use of chemicals on the skin to destroy outer damaged layers and stimulate a subsequent repair process. Different types of atrophic acne scars with different severity require different chemical peelings to treat. Commonly used chemical peelings include pyruvic acid, glycolic acid, salicylic acid, trichloroacetic acid, and Jessner’s solution.

The level of improvement expected from chemical peeling vary among different types of acne scars and different patients. For example, icepick and rolling scars will not vanish completely and require follow-up peelings and homecare treatments with alpha hydroxy acids and topical retinoids.

Dermabrasion and Microdermabrasion
Dermabrasion and microdermabrasion are ablative facial resurfacing techniques that promote reepithelialization by skin ablation. Although both dermabrasion and microdermabrasion physically abrade skins, their instruments and technical executions differ.

Dermabrasion induces remodelling of the skin’s structural proteins by completely removing the epidermis. In the treatment, under local or general anesthesia, a motorized handpiece rotates a wire brush or a diamond fraise to abrade skins and produce wounds. After the treatment, appropriate wound care is needed for small pinpoint bleeding of the wounds. Darker-skinned patients may suffer from permanent skin discoloration after dermabrasion because of improper repair process.

Microdermabrasion only exfoliates the skin surfaces. In the treatment, a pump will generate a stream of aluminium oxide or salt crystals on the skin surface. Then, a hand piece will be used to remove the crystals under vacuum and exfoliate the skin surfaces. Compared to dermabrasion, microdermabrasion can be repeated in shorter intervals with fewer rare and severe complications. Also, microdermabrasion does not require anesthesia because it is painless. However, microdermabrasion is ineffective in treating deep scars.

Treatments for Facial Hypertrophic Acne Scar and Keloid
Treatments for facial hypertrophic acne scar and keloid are distinctly different from treatments for scars on the back or chest. Treatments for facial hypertrophic acne scar and keloid require less intrusive methods as the procedures are done on the face. All treatments fundamentally attempt to taper out the protrusion of the scar by reducing the excess collagen deposits within the scar. Commonly utilised treatments are cryotherapy, intralesional steroid treatment, and pulsed dye laser.

Cryotherapy
Cryotherapy treatment involves the exposure of the skin to extremely low temperatures. Low temperatures reduce blood flow of the skin, causing a state of anoxia to the scar tissue eventually triggering scar tissue necrosis. Necrosis of scar tissue causes the cells and collagen within the hypertrophic scar to slowly die out, which in turn reduces the size of the protruding scar.

Cryotherapy is very effective on younger and smaller hypertrophic scars as these scars are more susceptible to the effects of extreme temperatures. Cryotherapy treatment takes 2-3 cycles with each cycle of cold temperature exposure lasting 25 seconds. Potential adverse reactions to cryotherapy on facial skin are temporary hypopigmentation or hyperpigmentation.

Intralesional Steroid Treatment
Intralesional steroid treatment involves the injection of anti-inflammatory steroids into protruded hypertrophic acne scars and keloids. The use of anti-inflammatory steroids aids in scar treatments through promotion of vasoconstriction and antimitotic effects. Firstly, vasoconstriction effects of steroids reduce the blood flow, and hence the supply of oxygen and nutrients to the scar region. The lowered levels of nutrients and oxygen then depletes the number of fibroblasts and keratinocytes, agents that maintain collagen deposits within the hypertrophic scar. Additionally, antimitotic effects of steroids block collagenase inhibitors whereby the blockage of collagenase inhibitors induces more collagenase activity therefore facilitating the digestion of collagen deposits within the scar.

The most frequently used steroid for intralesional steroid treatment is triamcinolone acetonide. The steroid is injected into the scar tissue to take full effect. Common temporary adverse reactions to intralesional steroid treatment are hypopigmentation, skin atrophy, telangiectasia, and infection.

Pulse Dye Laser
Pulse dye laser treatments use depth penetrating lasers to treat regions within hypertrophic scars and keloids. By carefully adjusting the strength of pulse dye lasers, they can progressively destroy fibroblasts and collagen fibers within the scar. Furthermore, pulse dye lasers can stimulate MMP-13 activity within the scar tissue and increase collagen digestion.

Common side effects of pulse dye laser treatments include temporary purpura of the skin lasting roughly 7-10 day, blistering of the skin, hypopigmentation, or hyperpigmentation of the skin for patients with high melanin content.