Background
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. [1]
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. [2]
Problem
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. [1]
The carbon dioxide laser quickly became the workhorse of the cosmetic laser surgeon, and its advantages and limitations are well documented. [10, 11] 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.
Pathophysiology
Pulsed laser energy causes controlled vaporization of the skin according to the principles of selective photothermolysis. [13] 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. [14]
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. [15] 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. [16] 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. [9]
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. [8] 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. [17]
