Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Laser resurfacing
Brief introduction
A logical approach to rejuvenation follows an understanding of skin anatomy and physiology, as they relate to skin aging. Any assessment of the face should include the surface, where sun and aging result in pigment inhomogeneities, wrinkles, and telangiectasia. Epidermal changes include basilar hyperpigmentation, hyperkeratosis, and thinning of the “living” portion of the epidermis. Another component of skin aging derives from changes in the dermis, where decreased glycosaminoglycans (GAGs), decreased elastin fibers, and changes in the character of collagen result in fine lines, sallowness, and eventually in cobblestoning of the skin. Microscopically, the changes present as solar elastosis. Also, weakened blood vessels dilate and present as telangiectasia. In some patients, hyperpigmentation results from both dermal and epidermal pigment dyschromias. The third component of aging skin results from bone regression, weakening of connections from the hypodermis to the surface, and volume loss. ,
Most energy-based interventions address one or more of these components of skin aging.
Epidermal pigmentation can be addressed by pigment-specific lasers or laser peels. In the first case, visible light is used to selectively heat the epidermis. Proper parameter selection allows for preferential targeting of the hyperpigmented lesion, whereas the normal background “innocent” bystander skin is unharmed. Traditional “nonfractional” laser peels, on the other hand, target water and therefore heat a confluent “slab” of the uppermost skin. The depths of ablation of heating are determined by wavelength, power density, fluence, and pulse-width.
Blood vessels are heated by three broad categories of devices: (a) visible light technologies (520–600 nm); (b) near-infrared I (NIR I) technologies (755, 800 nm); and (c) NIR II (940, 980, 1064 nm). The former is associated with very strong hemoglobin (HgB) and melanin heating, the second by moderate HgB and melanin heating, and the third by moderate HgB heating but relatively weak melanin heating. Deep-heating devices have included laser, halogen lamps, xenon flash lamps with long wavelength cut-off filters, radiofrequency (RF), ultrasound (US), and combination technologies.
The word “laser” is an acronym for the term light amplification by stimulated emission of radiation. The device itself consists of an energy source, a laser medium, and a resonating tube. The medium can be a gas, liquid, or solid, and this will often be used to name the type of laser (e.g., ruby laser and CO 2 laser). The light emitted is composed of photons that travel in the same direction, making laser light highly directional. Laser light is monochromatic, which means that all photons have the same wavelength. By contrast, intense pulsed light (IPL) consists of many different wavelengths. The specific wavelength of each laser will determine how the laser beam interacts with tissue. The light can be reflected by tissue, scattered by the tissue, or transmitted through tissue. The intention is that the laser light be absorbed by a specific target tissue (the “chromophore”). The mechanism by which lasers are used to target specific tissue is called selective photothermolysis (SPT; photo = light; thermolysis = decomposition by heat).
Lasers can be broadly broken into two categories, ablative and nonablative. Until recently, ablative (meaning “to remove”) lasers have been the “gold standard” of care for wrinkle reduction. The carbon dioxide laser with a wavelength of 10,600 nm and the Er : YAG (erbium-doped yttrium aluminum garnet) with a wavelength of 2940 nm are mainstays of ablative laser treatment. A relative newcomer is Er : YSGG (erbium: yttrium–scandium–gallium–garnet) (at 2790 nm). With each of these lasers, an intense burst of energy is delivered onto the skin. The energy heats water in the skin and causes both the water and tissues to vaporize. With each pass of the laser, a controlled depth of skin is vaporized and/or coagulated. In response to the injury and subsequent healing, new layers of collagen are produced. While ablative nonfractional lasers can be very effective and have a firm place in laser skin rejuvenation, each can be associated with risks of infection, scarring, hypopigmentation, and unnatural alterations in the texture and sheen of the skin. Moreover, complex aftercare is required until the skin is fully healed. Resolution of erythema may take months.
Nonablative nonfractional treatments are safer than their ablative counterparts but require epidermal cooling, which may reduce efficacy of the treatment. In general, small therapeutic windows are associated with nonablative treatments and, outside of dyschromia reduction with visible light approaches, only modest cosmetic enhancement is achieved. The Nd : YAG (neodymium-doped yttrium aluminum garnet) 1320 nm pulsed laser is an example of a nonablative laser in wide clinical use. IPL, monopolar RF skin tightening, light-heat energy (LHE), and light-emitting diode (LED) are all examples of nonablative treatments.
Skin rejuvenation with fractional photothermolysis represents a newer class of therapy ( Fig. 5.1 ). Thousands of microscopic wounds surrounded by viable tissue permit rapid healing and are made with a variety of laser wavelengths and delivery systems. Immediate and delayed therapeutic results are seen through a combination of epidermal coagulation for surface enhancement and dermal heating for deeper remodeling. Unlike SPT, in which targets are damaged based on color contrast, fractional photothermolysis only damages specific zones based on the pattern of the microbeams, leaving other zones completely intact. Fractional laser techniques began with a 1550 nm wavelength. The concept of a fractional laser can be applied, however, to almost any wavelength of light and can be used with both ablative laser resurfacing and nonablative laser rejuvenation. With increasingly aggressive densities and depths of injury, the fractional approach may achieve comparable results to nonfractional approaches, without the associated side effects.
Background information/anatomic pearls
Biology of wound healing
The three main stages of wound healing include reepithelialization, scar formation, and wound contraction.
In superficial insults to the skin, the basal layer of epidermis is intact and is capable of proliferating and repopulating the epidermis. This form involves minimal involvement of the underlying dermis, and thus no scar formation ensues. Following deeper injuries to skin, wound healing relies on keratinocytes from the wound edge and skin adnexa to migrate and proliferate. Collagen within the dermis is affected and undergoes some change. The migration of keratinocytes from the wound edge begins within hours of injury and involves four phases: (1) Mobilization: epithelial cells immediately adjacent to the wound enlarge, flatten, and detach from neighboring cells and the basement membrane; (2) Migration: as marginal cells migrate, the cells immediately behind them also tend to flatten, break connections, and drift along. The epithelial stream continues until advancing cells contact cells from the other side, whereupon motion stops abruptly—a process called contact inhibition ; (3) Proliferation/mitosis: fixed basal cells away from the wound edge begin mitosis to replace the migrating cells. The cells that have migrated in turn start to divide and multiply; (4) Differentiation: once the wound gap is bridged by advancing cells from the perimeter, normal differentiation of basal cells occurs. The stimuli for keratinocyte migration and proliferation include loss of cell-to-cell contact, growth factors (epidermal growth factor, transforming growth factor alpha [TGF-α], keratinocyte growth factor, TGF-β), loss of contact with normal components of a basement membrane (type IV collagen and laminin), and contact with proteins of the provisional matrix (fibrin, fibronectin, type I collagen). ,
Reepithelialization is facilitated by a moist environment (the proper dressing), debridement of scabs (fibrin, dead neutrophils, and other debris), growth factors, and high concentration of skin adnexa (the face and scalp). ,
Once contact of keratinocytes occurs and contact inhibition is achieved, hemidesmosomes re-form, cells become more basaloid, and cellular proliferation generates a multilaminated neoepidermis that is slightly thinner. ,
Wound healing that occurs following deeper insults to the skin (deeper chemical peels) results in the scar formation pathway of wound healing. The phases of this process include inflammation, proliferation, and remodeling.
Laser-tissue interactions and properties of lasers
An understanding of light-tissue and electrical-tissue interactions allows physicians to expand their laser repertoire and optimize outcomes. Lasers as light sources are useful because they allow for exquisite control of where and how much one heats. However, tissue reactions are not intrinsically specific to the heating source. In principle, a large number of nonlaser devices (i.e., IPL) can be used for heating skin. In many cases, laser is simply a way to convert lamplight to a more powerful monochromatic form. With respect to lasing media, there are diode lasers, solid-state lasers, and gas lasers. An example of a solid-state laser is the erbium : glass laser. These lasers have a solid rod that is pumped by a flash lamp. Miniaturized diode lasers have become popular. Some diode lasers are housed separately from the handpiece and delivered by fiberoptics. Others are configured with the laser diodes in the handpiece. IPL devices are increasingly comparable to lasers that emit millisecond (ms) domain pulses. Absorption spectra of skin chromophores are broad, and therefore a broadband light source is a logical approach for certain cosmetic applications.
Basic parameters for any procedure using light are power, time, and spot size, for continuous wave lasers; and for pulsed lasers, the energy per pulse, pulse duration, spot size, fluence, and repetition rate. All of these parameters should be considered in characterizing a laser procedure. Energy is measured in joules (J). The amount of energy delivered per unit area is the fluence, sometimes called the dose or radiant exposure , given usually in J/cm 2 . The rate of energy delivery is called power , measured in watts (W). One watt is 1 joule per second (W = J/s). The power delivered per unit area is called the irradiance or power density , usually given in W/cm 2 . Laser exposure duration (called pulse-width for pulsed lasers) is the time over which energy is delivered.
Fluence is equal to the irradiance times the exposure duration. Other important factors are the laser exposure spot size (which for wavelengths from 400–1200 nm greatly affects intensity inside the skin), whether the incident light is convergent, divergent, or diffuse, and the uniformity of irradiance over the exposure area (spatial beam profile). The pulse profile, that is, the character of the pulse shape in time (instantaneous power vs. time), is another feature that can impact the tissue response.
Molecular basis of light-tissue interaction
In any light-tissue interaction, the thermal or photochemical effects depend on the local absorbed energy density at the target. Spatial localization of temperature elevation is possible when: (1) the absorption coefficient of the target exceeds that of surrounding tissue (SPT); (2) when the “innocent bystander” tissues are cooled so that their peak temperatures do not exceed some damage threshold; or (3) by applying very small (usually <500 μm in diameter) beamlets or microbeams (i.e., fractional methods— vide supra ). Localized heating, for example, in telangiectases and lentigines, follows from the relative excess of HgB and melanin, respectively, in the lesions versus surrounding skin. In contrast, “nonfractional” mid-infrared (MIR) lasers spatially confine temperature elevation by using heating and cooling schemes that allow for selective dermal heating.
Targeting discrete chromophores offers advantages over targeting tissue water, especially where the ratio of light absorption between the chromophore and surrounding tissue is large (i.e., >10). For example, at least in lighter skinned patients, targeting dermal HgB can be achieved with minimal surface cooling. Whereas cooling is desirable, even in these cases, when the primary indication is analgesia rather than epidermal protection. Also, the risk of a severe injury to the skin is lessened, as there is no bulk heating. Lastly, because temperature elevations are localized, there is often less pain than with devices targeting ubiquitous tissue water. Thermal injury is determined by time/temperature combinations. Protein denaturation is dependent linearly on exposure time and exponentially on temperature. That is, cell death is more sensitive to temperature than time. Most devices for rejuvenation are based on photothermal or “electrothermal” mechanisms, that is, the conversion of light or electrical energy to heat. More recently, US devices have been applied to tighten the skin. Two processes govern all interactions of light with matter: absorption and scattering. The absorption spectra of major skin chromophores dominate laser-tissue interactions. If tissues were clear, then only absorption would be required to characterize light propagation in skin. However, the dermis is white because of light scatter (milk is a reasonable model for the dermis with regard to scattering). Scattering is responsible for much of the light’s behavior in the skin (beam dispersion, spot size effects, etc.). The main scattering wavelengths are between 400 and 1200 nm (those where tissue water absorption is poor).
There are three chromophores of interest in skin (water, blood, and melanin). Water makes up about 65% of the dermis and lower epidermis. There is some water absorption in the ultraviolet (UV). Between 400 and 800 nm, water absorption is quite small (which is consistent with our real-world experience that visible light propagates quite readily through a glass of water). Beyond 800 nm, there is a small peak at 980 nm, followed by larger peaks at 1480 and 10,600 nm. The water maximum is 2940 nm (Er : YAG).
Selective photothermolysis
With the exception of cases where water is heated, rejuvenation of the skin is based on discrete heating by chromophores of relatively low concentration (i.e., melanin, HgB). Anderson described the concept of SPT more than 25 years ago. He noted that extreme localized heating achieved with SPT relies on: (1) a wavelength that reaches and is preferentially absorbed by the desired target structures; (2) an exposure duration less than or equal to the time necessary for cooling of the target structures; and (3) sufficient energy to damage the target. The heterogeneity of the skin with respect to HgB and melanin allows for very selective injury in thousands of microscopic targets.
The thermal relaxation time is the time it takes for a target to cool to a certain percentage of its peak temperature (after laser exposure). Larger targets take longer to cool, and therefore spatial selectivity is preserved with a wider range of pulse durations. Even so, as a general rule, assuming adequate fluences are applied, longer pulse durations will result in greater collateral damage. In laser scenarios, we assume instantaneous heating of the target, so that τ is usually thought of as the time for cooling after the pulse.
Reaction types
Photothermal
Most laser applications rely on heating. Photothermal approaches depend on the type and degree of heating, from coagulation to vaporization. There is a range of effects on tissue based on temperature. Below 43 °C, the skin remains unharmed, even for exposures as long as 20 min. Thus, one can exceed body temperature by about 5 °C without a measurable change in the skin. The first change is a conformational change in the molecular structure. These typically occur at temperatures from 42 to 50 °C. At higher temperatures, very short times (seconds or in extreme cases [>100 °C] less than 1 ms) can induce cell death. ,
Photochemical
Photochemical reactions are governed by specific reaction pathways, and the most common photosensitizer (PS) in skin is protoporphyrin IX. This PS is formed in skin cells by the prodrug, aminolevulinic acid (ALA). ,
Biostimulation
Biostimulation (also called low-level laser therapy ) belongs to the group of photochemical interactions. Most biostimulation studies involve low-power lasers, and this field continues to be a subject of controversy. Home-use devices that use LEDs are now available in a wide range of wavelength ranges. Typical fluences are in the range of 1 to 10 J/cm 2 , and normally there is no acute temperature elevation, nor any clinical endpoint. ,
Cooling
Before the addition of surface cooling, fluence thresholds for efficacy and epidermal damage were often close. Visible light technologies (especially green-yellow light sources such as IPL, potassium titanium oxide phosphate [KTP] laser, and pulsed dye laser [PDL]) are the wavelength ranges where epidermal damage is most likely. The epidermis is an innocent bystander in cutaneous laser applications where the intended targets, such as hair follicles or blood vessels, are located in the dermis. ,
Beyond visible light (green, yellow, and red) sources, surface cooling also has been used in NIR and MIR lasers. With NIR lasers, surface cooling is important, but not only because of dermal/epidermal junction-derived epidermal heating. In addition, deep beam penetration may cause catastrophic bulk heating. With MIR lasers (1.32, 1.45, and 1.54 μm), the chromophore is water. It follows that with even very low fluences, surface cooling is imperative. Without cooling or a fractional design, water’s ubiquitous nature in the skin causes a laser-induced top-to-bottom injury. All techniques are susceptible to operator error and device failure. It follows that as physicians rely more heavily on cooling devices, any lack of their proper deployment unveils the dark side of cooling. Fractured sapphire windows, disabled cryogen spray apparatus, and crimped forced air chiller tubes have all contributed to unintended epidermal injury. ,
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
