The abundance of facial skin care regimens and their enthusiastic promotion in the popular media is the modern expression of an ancient desire to attain beauty and recapture a youthful appearance. Facial rejuvenation has been practiced for thousands of years, as can be seen in documents from previous millennia that describe the topical application of substances such as soured milk, vegetable extracts, and mud packs. 
Modern agents include preparations of retinoic acid, ascorbic acid alpha-hydroxy acids, and antioxidants used in conjunction with sunscreens. These treatments are useful for restoring and maintaining the health of the epidermis, but they generally are not effective for deeper, long-lasting rejuvenation. A more penetrating removal of the outer layers of aged and sun-damaged skin, such as with chemical peels, is required to induce reepithelialization and new collagen formation. Laser resurfacing was developed as an alternative to chemical peels to operate in these upper layers of the dermis, and its popularity increased quickly and enormously (see the image below). Most recently, however, the popularity of laser resurfacing has declined, probably because of a desire to avoid its complications. 
As the face ages, skin quality deteriorates. Intrinsic aging from the genetically determined, natural, chronological degradation of metabolic processes leads to epidermal thinning, dermal hypocellularity, decreased numbers of dermal blood vessels, and decreased amounts of collagen and elastic tissue. These changes manifest as skin atrophy, pallor, and loss of elasticity. [3, 4, 5, 6]
Extrinsic aging from years of sun exposure and other external factors leads to the deposition of abnormal elastic fibers, the degeneration of collagen, and the twisting and dilation of microvasculature. These are compounded with the intrinsic changes and result in a rough surface texture with wrinkling, scaling, dyspigmentation, telangiectasias, and skin laxity. [7, 8, 1, 9] The accumulation of free radical damage probably plays a major role in both intrinsic and extrinsic processes.
Underlying anatomic structures sag as deep layers loosen and subcutaneous fat accumulates or atrophies. Furrows develop in the skin that overlies facial muscles. Surgically lifting the skin and subcutaneous tissue, rearranging the distribution of facial fat deposits, and paralyzing facial muscles with botulinum toxin are effective methods of addressing the underlying structural changes associated with aging, but they do not directly address the degradation in the quality of skin. One method of improving the condition of the skin is by "resurfacing" it, by removing the outer layers to the level of the papillary dermis. Resurfacing can be used as an adjunct to facial surgery, or it can replace facial surgery when surgery is inappropriate or not desired by the patient.
Removing the outer layers of skin to the level of the papillary dermis induces reepithelialization and new collagen formation, which can create a smoother, pinker, and more youthful appearance. Chemical removal of skin layers with peels (eg, trichloroacetic acid, phenol) and mechanical removal (ie, dermabrasion) are effective modalities for facial rejuvenation.
In the late 1980s, laser technology applied to skin resurfacing was discovered to yield more predictable depths of injury when compared with chemical peels or dermabrasion. The first laser used for skin resurfacing was a pulsed carbon dioxide laser that Fitzpatrick et al modified from a device that had been developed for otolaryngological and gynecological use. Its cosmetic uses were initially limited to the periorbital and perioral regions, but dramatic clinical results quickly led to its use for full-face resurfacing. 
The carbon dioxide laser quickly became the workhorse of the cosmetic laser surgeon, and its advantages and limitations are well documented. Although long-term skin tightening and improvement of facial rhytides is unparalleled, marked erythema persists for several weeks or months and permanent hypopigmentation occurs at a rate that is unacceptable to many patients. Even without complications, the early period of recovery until full reepithelialization can leave the patient housebound for up to 2 weeks.
Other lasers were developed for resurfacing; with these, more precise light energy can be applied to the skin, resulting in less intense adverse effects from collateral damage. The erbium:yttrium-aluminum-garnet (Er:YAG) laser was introduced as a bone-cutting tool in the United States in 1996. Its unusual name derives from the Swedish town of Ytterby, which is the site of a quarry where the silvery rare-earth elements erbium and yttrium were discovered. The cutaneous absorption of the Er:YAG laser energy by water is 10-fold more efficient than that of the carbon dioxide laser, allowing for more superficial tissue ablation and finer control. Other qualities of the Er:YAG laser are best appreciated in comparison to the carbon dioxide laser, as discussed below.
In 2007, 647,707 laser-resurfacing procedures were performed in the United States. 
Pulsed laser energy causes controlled vaporization of the skin according to the principles of selective photothermolysis.  The target tissue contains a chromophore with an absorption peak that selectively absorbs the particular wavelength of the laser pulse, whereas the tissue surrounding the chromophore absorbs the energy to a much lesser degree.
The interaction of the target tissue with the energy of the carbon dioxide laser is transformed mostly into a thermomechanical reaction that destroys dermal vessels and denatures dermal proteins. In the case of the Er:YAG laser, the interaction involves a photomechanical reaction. Absorption of the energy causes immediate ejection of the desiccated tissue from its location at a supersonic speed, creating a characteristic and almost startling "popping" sound. This translation of Er:YAG laser energy into mechanical work is an important factor that protects the surrounding tissue; minimal thermal energy remains to dissipate and cause collateral damage.
Immediately after the target tissue reaches its peak temperature, it begins to cool. The thermal relaxation time is the amount of time required for it to cool to half its peak temperature. When the duration of the carbon dioxide laser pulse is greater than the thermal relaxation time, a stacking of the laser energy and rapid heat accumulation occur. This stacking effect is much less important with the Er:YAG laser, despite a thermal relaxation time of 1.9 microseconds and a pulse duration of 250-350 microseconds, because the laser energy dissipates so rapidly and penetrates so shallowly. 
Three important variables in laser technology are wavelength, pulse duration, and fluence. (Fluence, or energy density, is the amount of energy delivered.) They are optimized to achieve maximal ablation of the target tissue with minimal collateral damage. The newer pulsed carbon dioxide lasers ablate tissue to a depth of 20-30 µm with each pass and cause collateral damage to a surrounding area of 20-70 µm.
Collagen contracts by approximately 15-25% during carbon dioxide lasing, producing a shrunken form that serves as a template for tighter, more organized new collagen formation.  Char forms in the wound during the procedure. This char must be wiped away before subsequent passes, but it marks the depth of ablation. The characteristics that produce the immediate contraction of collagen also create an injury that often causes prolonged erythema, lasting up to 6 months, and can lead to permanent scarring and dyspigmentation.
The Er:YAG laser operates at a more superficial level and with greater precision. Similar to the carbon dioxide laser, its chromophore is water; however, the energy is absorbed by a different absorption peak at a different wavelength. The Er:YAG emits a wavelength of 2940 nm, which is absorbed by water because of its 3000-nm absorption peak. The passes of short-pulse lasers (250 µm) penetrate to a depth of only 10-15 µm, and several passes only cause collateral thermal necrosis to a distance as thin as 20-50 µm.
Collagen contraction is 1-2% during lasing, and it may only reach 14% in the long term.  No char forms, and only a transient white discoloration of the wound bed occurs. Dermal vessels treated with the laser dilate and cause transudation of fluid; this increases the water (chromophore) content in the treated area and allows for consistent ablation with each subsequent pass. 
The clinical manifestations of laser treatment depend on the ability of the skin to resurface itself. After lasing, the vaporized, atypical, disorganized epidermal cells are replaced with normal, well-organized keratinocytes from the follicular adnexa. The irregular, disorganized collagen and elastin of the upper papillary dermis are replaced with normal, compact collagen and elastin organized in parallel configurations.  This manifests as a more youthful appearance and improved skin texture. Patients in the most favorable preoperative categories generally show a 50% improvement in rhytides and skin lesions. Whereas collagen remodeling and further clinical improvement often continue for up to 18 months after carbon dioxide laser resurfacing, the reduced photothermal effect of the Er:YAG laser allows the resurfacing process to end before 12 months. 
A disadvantage of the superficial and fleeting energy absorption of the Er:YAG laser is its poor ability to cause hemostasis. Although thermal necrosis does not significantly interfere with subsequent passes of the laser, blood in the wound bed makes controlling the wound depth difficult. Only several passes may be possible, which may not ablate the tissue to the desired depth. The carbon dioxide laser can generally produce the same effect in a third of the number of passes, with better hemostasis. The carbon dioxide laser is a more reliable modality for deeper tissue ablation than the short-pulsed Er:YAG laser. 
Newer Er:YAG lasers with longer (500 µm) and variable pulses have been developed. They have better tissue penetration, which makes deeper tissue ablation less difficult. They create larger zones of thermal necrosis, leading to more collagen contraction and better remodeling. Although the postoperative erythema is greater and lasts longer than with the short-pulsed Er:YAG lasers, it is still less severe than after the carbon dioxide laser. [16, 17, 18] Another improvement is in the shape of the energy distribution within the laser beam; some lasers distribute the energy in a uniform, or "top hat," pattern rather than in a gaussian pattern. The uniform pattern is thought to provide better hemostasis.
The selection of patients who are best suited for laser resurfacing and the education of those who are at higher risk for complications are the most important parts of the initial consultation. The best results are achieved in patients with a fair complexion and few wrinkles. Fitzpatrick and Glogau developed classification schemes that help predict the effectiveness of skin resurfacing.
Fitzpatrick classes of skin phototypes are as follows:
- Always burn, never tan
- Light-eyed, fair-skinned northern European persons
- Always burn, sometimes tan
- Fair-skinned European persons
- Sometimes burn, always tan
- Mediterranean origin (eg, Spanish, Italian, or Greek persons)
- Never burn, always tan
- Hispanic and Asian persons
- Darkly pigmented skin
- Hispanic and Asian persons
- Black skin
- Darkly pigmented African and southern Indian persons
Glogau classes of photodamage are as follows:
- Few wrinkles, no keratosis, require little or no makeup
- Usually aged 28-35 years
- Early wrinkling, sallow complexion with early actinic keratosis, require little makeup
- Usually aged 35-50 years
- Persistent wrinkling, discoloration of the skin with telangiectasias and actinic keratosis, always wear makeup
- Usually aged 50-60 years
- Severe wrinkling, photoaging, gravitational and dynamic forces affecting skin, actinic keratosis, wear makeup with poor coverage
- Usually aged 65-70 years
Patients categorized in Fitzpatrick and Glogau types I and II obtain better results than those in higher types. After carbon dioxide laser resurfacing, patients with higher Fitzpatrick skin types are likely to experience scarring, hypopigmentation or hyperpigmentation, and a prolonged recovery period. Complications are generally less frequent, milder, and of shorter duration after Er:YAG laser resurfacing, and the Er:YAG laser has been advocated for use in patients with darker skin. 
Indications for use of the Er:YAG laser are expanding.  Early indications included mildly photodamaged skin lesions (eg, solar keratoses); mildly atrophic facial scars (eg, from acne or varicella); dyschromias (eg, melasma, lentigines); and mild-to-moderate facial wrinkles in the perioral, periocular, and cheek areas. Deeper facial wrinkles and abnormalities, as are seen in persons in the higher Glogau classes, have conventionally been treated with the carbon dioxide laser because of its greater and wider tissue ablation.
Recent combined use of carbon dioxide and Er:YAG lasers has taken advantage of the properties of each system to extend their indications for use. Full-face resurfacing is safe with the combined system, which also decreases the lines of demarcation and textural differences between treated and untreated areas and allows uneven areas of photodamage to be treated at the same time. Combined laser use on the eyelids causes less coagulative dermal damage and results in reepithelialization in almost half the time than after the carbon dioxide laser alone. [8, 20, 21, 22] Many other skin lesions have been successfully treated with the Er:YAG laser, including compound nevi, sebaceous hyperplasia, trichoepitheliomas, miliary osteomas, syringoma, telangiectasia, rhinophyma, adenoma sebaceum, hidradenoma, xanthelasma, and the cutaneous manifestations of Hailey-Hailey disease and Darier disease. [23, 8, 24, 9]
The Er:YAG laser can also be cautiously used on the neck, arms, and hands, where fewer pilosebaceous units are found compared with the face. These pilosebaceous units are the source of new keratinocytes in the reepithelialization process, and resurfacing modalities more damaging than the Er:YAG laser can lead to fibrosis, hypertrophic scar formation, and prolonged healing. [25, 26] Even with the Er:YAG laser, surface changes are unpredictable.
The newer Er:YAG lasers with longer and variable pulses even allow patients with deeper wrinkles and scars to be successfully treated. Rhytides in the glabellar region and nasolabial folds are difficult to treat with any laser; they are associated with movement and are probably best approached with botulinum toxin therapy or surgical skin and muscle adjustment.
Perioral rhytides reflect generalized atrophy. Restoration of a "level playing field" with subdermal grafts ("fresh," nonrefrigerated autologous fat,) and intradermal filling (hyaluronic acid, human collagen injections), as well as mild derma planning, reduce the resurfacing required, thus reducing the number and severity of complications.
Absolute contraindications to Er:YAG laser skin resurfacing include active bacterial or viral infections, an inflammatory condition in the area to be treated, the presence of ectropion (in the case of infraorbital resurfacing), unrealistic patient expectations, and patient unwillingness or inability to care for the wound. Isotretinoin use within the preceding 12-24 months, which diminishes the sebaceous unit source of keratinocytes for reepithelialization, is also a contraindication.
Relative contraindications include extreme or habitual ultraviolet light exposure, collagen-vascular disease, immune disorders, prior lower blepharoplasty (in the case of infraorbital resurfacing), prior radiation therapy or burns with loss of cutaneous adnexal structures, extensive fibrosis from previous cosmetic procedures, and a tendency to form hypertrophic scars or keloids. Koebnerizing diseases such as psoriasis, lichen planus, or pyoderma gangrenosum are also relative contraindications.
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