Journal of the American Academy of Dermatology
Volume 49 • Number 1 • July 2003
Copyright © 2003 American Academy of Dermatology, Inc.




Lasers in dermatology: Four decades of progress




Elizabeth L. Tanzi MDa
Jason R. Lupton MDb
Tina S. Alster MDa


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From the Washington Institute of Dermatologic Laser Surgerya and Dermatology Associates of San Diego County.b

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Funding sources: None.

Conflict of interest: None identified.


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Reprint requests: Tina S. Alster, MD, Washington Institute of Dermatologic Laser Surgery, 2311 M St NW, Suite 200, Washington, DC 20037. E-mail: talster@skinlaser.com.

Copyright © 2003 by the American Academy of Dermatology, Inc.



0190-9622/2003/$30.00 + 0




Washington, DC, and San Diego, California


Advances in laser technology have progressed so rapidly during the past decade that successful treatment of many cutaneous concerns and congenital defects, including vascular and pigmented lesions, tattoos, scars, and unwanted hair-can be achieved. The demand for laser surgery has increased substantially by patients and dermatologists alike as a result of the relative ease with which many of these lesions can be removed, combined with a low incidence of adverse postoperative sequelae. Refinements in laser technology and technique have provided patients and practitioners with more therapeutic choices and improved clinical results. In this review, the currently available laser systems with cutaneous applications are outlined, with primary focus placed on recent advancements and modifications in laser technology that have greatly expanded the cutaneous laser surgeon's armamentarium and improved overall treatment efficacy and safety. (J Am Acad Dermatol 2003;49:1-31.)
Learning Objective: At the completion of this learning activity participants should be able to identify the various types of dermatologic lasers currently available, to list their clinical indications, and to understand the possible side effects of laser treatment.




Abbreviations used
APTD
argon-pumped tunable dye
CO2
carbon dioxide
CW
continuous wave
Er:YAG
erbium:YAG
FDA
Food and Drug Administration
IPL
intense pulsed light
KTP
potassium titanyl phosphate
LP
long-pulsed
Nd
neodymium
PDL
pulsed dye laser
PDT
photodynamic therapy
QS
quality-switched
YAG
yttrium-aluminum-garnet


Laser history
The term laser is an acronym for light amplification by the stimulated emission of radiation. Although the first laser was developed by Maiman[1] in 1959 using a ruby crystal to produce red light with a 694-nm wavelength, the concept of stimulated light emission was initially introduced by Einstein[2] in 1917. Einstein[2] proposed that a photon of electromagnetic energy could stimulate the emission of another identical photon from atoms or molecules that are in an excited state. In 1963, Dr Leon Goldman pioneered the use of lasers for cutaneous applications by promoting ruby laser treatment for a variety of cutaneous pathologies.[3] [5] The development of the argon and carbon dioxide (CO2 ) lasers soon followed and served as the focus of cutaneous laser research during the next 2 decades.[6] The argon laser produced blue-green 488-/514-nm light that was primarily used to treat benign vascular birthmarks. Although most port-wine stains and hemangiomas could be effectively lightened, there was an unacceptably high rate of hypertrophic scar formation.[7] [8] Emitting infrared light at 10,600 nm, the CO2 laser was used for tissue vaporization and destruction of various epidermal and dermal lesions.[9] Unfortunately, the continuous-wave (CW) CO2 laser also yielded high rates of hypertrophic scarring and pigmentary alteration as a result of prolonged tissue exposure to laser energy resulting in excessive thermal injury to the skin.[10] [11]

Cutaneous laser surgery was revolutionized in the 1980s with the introduction of the theory of selective photothermolysis by Anderson and Parrish.[12] Application of their theory effects specific destruction of a target in the skin with minimal unwanted thermal injury. During the past decade, greater understanding of the complex laser-tissue interaction coupled with extensive advances in laser technology have refined cutaneous laser surgery to the point that it is now considered a first-line treatment for many congenital and acquired cutaneous conditions.

Laser principles
The therapeutic action of laser energy is based on the unique properties of laser light itself and complex laser-tissue interactions.[13] [16] Laser light is monochromatic-the emitted light is of a single, discrete wavelength determined by the lasing medium (eg, solid, liquid, gas) in the optical cavity of the laser through which the light passes. At certain wavelengths of light, specific absorption of laser energy can be achieved by distinct cutaneous targets or chromophores such as melanin, hemoglobin, or tattoo ink. The second property, coherence, refers to laser light traveling in phase with respect to both time and space. Lastly, collimation of laser light indicates emission of a narrow, intense beam of light in parallel fashion to achieve its propagation across long distances without light divergence. Thus, collimated light can be focused into small spot sizes allowing for precise tissue destruction.

When a laser is used on the skin, the light may be absorbed, reflected, transmitted, or scattered. The first law of photobiology, the Grotthus-Draper law, states that light must be absorbed by tissue for a clinical effect to take place, whereas transmitted or reflected light has no effect. The energy absorbed is measured in joules per square centimeter and is known as the energy density or fluence. The amount of absorption is determined by the chromophore present in the skin and whether a wavelength that corresponds to the absorptive characteristics of that chromophore is used. The principle endogenous chromophores of the skin are water, melanin, and hemoglobin, whereas tattoo ink is an example of an exogenous chromophore. Once laser energy is absorbed in the skin, 3 basic effects are possible: photothermal; photochemical; or photomechanical. Photothermal effects occur when a chromophore absorbs the corresponding wavelength of energy and destruction of the target results from the conversion of absorbed energy into heat. Photochemical effects derive from native or photosensitizer-related reactions that serve as the basis of photodynamic therapy (PDT). Extremely rapid thermal expansion can lead to acoustic waves and subsequent photomechanical destruction of the absorbing tissue. Although all 3 types of laser effects can occur, photothermal and photomechanical reactions are most commonly observed in current cutaneous laser surgery practice.

The depth of penetration of laser energy into the skin is dependent upon absorption and scattering. Although scattering is minimal in the epidermis, it is greater in the dermis as a result of the presence of collagen fibers, which are responsible for the majority of tissue scatter in the skin. The amount of scattering of laser energy is inversely proportional to the wavelength of incident light. In general, the depth of penetration of laser energy increases with wavelength until the midinfrared region of the electromagnetic spectrum. Penetration of 300- to 400-nm wavelengths are limited by strong scattering of the beam whereas scattering is minimal at longer wavelengths (1000-1200 nm), allowing greater penetration into the skin. However, wavelengths in the mid- to upper-infrared range of the electromagnetic spectrum penetrate superficially as a result of high absorption by tissue water, the principle chromophore at this range. As such, selective vaporization of water-containing tissue serves as the basis of ablative laser skin resurfacing.

The world's understanding of laser-tissue interactions was greatly enhanced with Anderson and Parrish's[12] theory of selective photothermolysis. The theory describes how controlled destruction of a targeted lesion is possible without significant thermal damage to surrounding normal tissue. To achieve selective photothermolysis, a proper wavelength that is absorbed preferentially by the intended tissue target or chromophore is selected. To limit the amount of thermal energy deposited within the skin, the exposure duration of the tissue to light (pulse duration) must be shorter than the chromophore's thermal relaxation time (defined as the time required for the targeted site to cool to one half of its peak temperature immediately after laser irradiation). Finally, the energy density (fluence, measured in joules per square centimeter) delivered by the laser must be sufficient to achieve destruction of the target within the allotted time. Therefore, on the basis of these principles, laser parameters (wavelength, pulse duration, and fluence) can be tailored for specific cutaneous applications to effect maximal target destruction with minimal collateral thermal damage.

There are several types of lasers used in cutaneous laser surgery. CW lasers, such as the CW CO2 and older argon technology, emit a constant beam of light with long exposure durations that can result in nonselective tissue injury. Quasi-CW mode lasers, including the potassium-titanyl-phosphate (KTP), copper vapor, copper bromide, krypton, and argon-pumped tunable dye (APTD) lasers, shutter the CW beam into short segments, producing interrupted emissions of constant laser energy. The pulsed laser systems emit high-energy laser light in ultrashort pulse durations with relatively long intervening time periods (0.1-1 second) between each pulse. The laser systems may be either long-pulsed (LP), such as the pulsed dye laser (PDL) with pulse durations ranging from 450 μs to 40 milliseconds, or very short-pulsed (5-100 ns), such as the quality-switched (QS) ruby, alexandrite, or neodymium (Nd):yttrium-aluminum-garnet (YAG) lasers. The QS lasers have electro-optical shutters that permit release of stored energy within the laser cavity in short high-energy bursts, delivering power outputs as high as 109 W. “Superpulsed” is a term specific to CO2 lasers that are modified to produce very short pulses in a repetitive pattern to reduce the amount of thermal damage that occurs adjacent to the vaporized tissue. Pulsed and quasi-CW systems (as opposed to CW systems) are better adapted for cutaneous laser surgery on the basis of the principles of selective photothermolysis because the thermal relaxation times of most cutaneous chromophores are very short. Because cutaneous lasers have different clinical applications related to their specific wavelengths and pulse durations, the choice of laser should be on the basis of the individual absorption characteristics of the target chromophore (Table I).[17] [18]
Table I. Types of lasers and their cutaneous application Laser type Wavelength Cutaneous application
Argon (CW) 418/514 nm Vascular lesions
Argon-pumped tunable dye (quasi-CW) 577/585 nm Vascular lesions
Copper vapor/bromide (quasi-CW) 510/578 nm Pigmented lesions, vascular lesions
Potassium-titanyl-phosphate 532 nm Pigmented lesions, vascular lesions
Nd: YAG, frequency-doubled 532 nm Pigmented lesions, red/orange/yellow tattoos
Pulsed dye 510 nm Pigmented lesions

585-595 nm Vascular lesions, hypertrophic/keloid scars, striae, verrucae, nonablative dermal remodeling
Ruby 694 nm

QS
Pigmented lesions, blue/black/green tattoos
Normal mode
Hair removal
Alexandrite 755 nm

QS
Pigmented lesions, blue/black/green tattoos
Normal mode
Hair removal, leg veins
Diode 800-810 nm Hair removal, leg veins
Nd:YAG 1064 nm

QS
Pigmented lesions, blue/black tattoos
Normal mode
Hair removal, leg veins, nonablative dermal remodeling
Nd:YAG, long-pulsed 1320 nm Nonablative dermal remodeling
Diode, long-pulsed 1450 nm Nonablative dermal remodeling, acne
Erbium:glass 1540 nm Nonablative dermal remodeling
Erbium:YAG (pulsed) 2490 nm Ablative skin resurfacing, epidermal lesions
Carbon dioxide (CW) 10,600 nm Actinic cheilitis, verrucae, rhinophyma
Carbon dioxide (pulsed) 10,600 nm Ablative skin resurfacing, epidermal/dermal lesions
Intense pulsed light source 515-1200 nm Superficial pigmented lesions, vascular lesions, hair removal, nonablative dermal remodeling
C
CW, Continuous-wave; Nd, neodymium; QS, quality-switched; YAG, yttrium-aluminum-garnet.



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Vascular-specific lasers
Vascular-specific laser systems target intravascular oxyhemoglobin to effect destruction of various congenital and acquired vascular lesions. The 3 primary absorption peaks for oxyhemoglobin are within the visible range of the electromagnetic spectrum: 418, 542, and 577 nm. Lasers that have been used to treat vascular lesions include the argon (488-514 nm), APTD (577 and 585 nm), KTP (532 nm), krypton (568 nm), copper vapor/bromide (578 nm), PDL (585-595 nm), and Nd:YAG (532 and 1064 nm).

The argon laser emits a continuous blue-green beam with wavelength peaks at 488 and 514 nm. Although it has been used in the past for a variety of vascular lesions, several histologic studies have shown that the tissue effect of the argon laser is a result of nonspecific thermal injury resulting from exposure intervals exceeding the thermal relaxation time of the vessels.[7] [19] [22] Consequently, the risk of scarring and dyspigmentation is increased.[23] For this reason, the argon laser is no longer commonly used to treat vascular lesions.

Quasi-CW systems such as the APTD,[24] [26] krypton,[27] copper vapor/bromide,[28] [34] and KTP[35] [37] lasers are CW systems that can be mechanically shuttered to deliver pulses as short as 20 ns to treat facial telangiectasias. However, because of the rapid delivery and short interval between laser pulses, targeted vessels are not allowed to cool adequately, rendering the tissue reaction similar to that of a CW system. Although the APTD and copper vapor/bromide lasers have been used to treat other cutaneous vascular conditions such as rosacea, poikiloderma of Civatte, and port-wine stains, their quasi-CW nature is often associated with higher incidences of hypertrophic scarring and textural changes than is seen with pulsed laser systems.[38] [40]

The KTP laser uses a Nd:YAG crystal (1064 nm) to produce light that is passed through a KTP crystal that frequency-doubles the wavelength to 532 nm. Several investigators have reported good results after 532-nm KTP or Nd:YAG treatment of facial telangiectasias. [36] [38] The most common side effects include mild erythema, edema, and transient crusting. Because purpura is avoided and erythema is minimized with KTP laser treatment, its use may be preferable to the PDL in patients with facial telangiectasia even though additional treatments may be needed to achieve vessel clearance.[36] Compared with longer wavelength vascular-specific lasers, potential limitations of the 532-nm wavelength include decreased tissue penetration of its shorter wavelength resulting in diminished absorption by deeper vessels. In addition, the 532-nm wavelength is more avidly absorbed by melanin than is the 585- to 595-nm wavelength of a PDL, thereby limiting its use for patients with darker skin types.

The flashlamp-pumped PDL was the first laser specifically developed for treatment of vascular lesions based on the principles of selective photothermolysis.[12] Although original investigators used a 577-nm system,[41] [50] the wavelength was later modified to 585 nm to effect deeper tissue penetration while maintaining vascular specificity.[51] [54] Although PDL use was initially recommended for treatment of lesions in pale skin, [55] recent reports have shown that darker skin tones can be safely and effectively treated.[56] [57] In addition, dynamic cooling devices were incorporated in most pulsed dye systems to reduce intraoperative discomfort and postoperative occurrence of epidermal damage or pigmentary change.[58] [60] With a pulse duration (450 μs) shorter than the thermal relaxation time of small- to medium-sized blood vessels (1 millisecond), the PDL conforms to the principles of selective photothermolysis leading to selective vascular injury without unwanted thermal damage to the surrounding tissue. The PDL has revolutionized the treatment of many vascular lesions and is considered the laser of choice for most benign congenital and acquired vascular lesions because of its superior clinical efficacy and low risk profile.[17] [18] [39] [61] This laser has been used to successfully treat a variety of vascular lesions such as port-wine stains, [40] [42] [43] [45] [47] [60] facial telangiectases,[62] [66] hemangiomas,[67] [71] pyogenic granulomas,[72] Kaposi's sarcoma,[73] and poikiloderma of Civatte.[74] [75] Other conditions successfully treated with PDL irradiation include hypertrophic and keloid scars,[76] [79] striae distensae,[79] [80] warts,[81] [83] angiofibromas,[84] lymphangiomas, [85] Goltz's syndrome,[86] inflammatory linear verrucous epidermal nevus,[87] atrophoderma vermiculata,[88] multiple eccrine hidrocystoma,[89] lupus pernio,[90] lupus erythematosis, [91] morphea,[92] granuloma faciale,[93] necrobiosis lipoidica diabeticorum,[94] [95] elastosis perforans serpiginosa,[96] sebaceous gland hyperplasia,[97] [98] and molluscum contagiosum[99] [100] (Figs 1 and 2).

 

Adnan Alabdulkairm, MD