Chapter 54 Lasers in dermatology
2. What does “stimulated emission of radiation” mean?
Stimulated emission is a complicated phenomenon of physics, first described by Albert Einstein. Atoms must be in an excited state (an electron is in an elevated orbit). Normally, adding a photon with energy equal to the energy between orbits would raise this electron to a higher orbit. Instead, in special circumstances found in laser systems, two photons are released from the atom, and the electron returns to its lower, resting state. These two photons are then able to enter two other atoms with excited electrons, allowing rapid multiplication of photons in a process similar to a chain reaction. This process accounts for the stimulated emission of radiation used in the acronym laser.
3. How is the light amplified in the laser system?
The laser system uses an optical resonator to amplify and orient the light. This is a cylindrical chamber filled with the laser medium. There are mirrors on each end and an absorptive lining. The photons of light are reflected between the mirrors. The lining will absorb any light that is not perfectly parallel. These parallel photons continue to enter additional atoms, producing more photons by stimulated emission. By this process, the laser light is amplified. One end of the optical resonator has a mechanism to release the light periodically from the chamber.
4. What types of medium are used in laser systems?
The ability of an atom to be used in a laser system is a complicated function of quantum mechanics and the physical characteristics of the lattice in which it is constructed. The basic requirement of any medium is to be able to support a population inversion so that there may be stimulated emission. Some types of lasers include the following:
5. What are the special features of laser light?
• Coherence: Coherence is the property that represents a uniform wave front, that is, the peaks and valleys of the waves are aligned as the light exits the laser, which allows the light to be in phase and focused to very small areas. This also allows the energies to be additive.
6. Why is monochromatic light useful?
Many lasers target specific chromophores, which are biologic structures with a specific absorption spectrum. Two common chromophores are hemoglobin within the red blood cell and melanin within melanosomes. The absorption spectrum is the amount of light absorbed at various wavelengths. The idea is to match a peak absorption wavelength with the wavelength of the laser.
7. What is selective photothermolysis?
The theory of selective photothermolysis assumes the laser light will pass through tissue until it targets a specific chromophore with an absorption spectrum corresponding to the wavelength of the laser. The target then absorbs the light, generating heat in the target tissue. The spread of heat is determined by the thermal relaxation time (TRT) of the tissue. This is the amount of time necessary for 50% of the peak heat to diffuse out of the target. It is important for the laser pulse duration not to exceed the thermal relaxation time of the target or the heat will diffuse into surrounding tissue, causing damage and possibly scarring. Thus, the TRT depends on the actual size of the target. The smaller the target, the shorter the TRT, and thus the need for shorter laser pulse durations.
8. What is an ablative laser?
An ablative laser vaporizes tissue. Different lasers cause different types of ablative reactions. Er:YAG lasers, due to their very high water absorption, cause almost pure ablation with very little collateral thermal heating. CO2 lasers have less water absorption and therefore deliver less pure ablation and have more collateral heating. This heat causes thermal damage in a positive and negative manner. An example of a positive effect is collagen contraction and new collagen stimulation. Other positive effects of CO2 lasers are sealing of blood vessels leading to less bleeding and sealing of nerve endings leading to less pain. An example of a negative effect is hypopigmentation due to inadvertant melanocyte damage. Scarring from laser procedures is usually due to excessive collateral heating.
9. What is a nonablative laser?
Most lasers are capable of delivering laser energy in a nonablative or nonvaporizing manner. Pulsed dye lasers, long-pulsed alexandrite, and Nd:YAG and many diode lasers deliver energy so as not to ablate tissue. The only lasers that truly ablate are those listed above and are used to remove tissue in a physical manner. The entire concept of selective photothermolysis is the ability to deliver energy in a nonablative manner.
11. What is a fractional laser?
Fractional lasers deliver short pulses of laser that are separated by space on the surface of the skin. These lasers may either by nonablative, leaving a column of heat-damaged skin, or ablative, leaving a column of true ablation. This will be discussed in detail in a later question. The concept of fractional lasers was first proposed by Dr. Rox Anderson.
13. What lasers have historic interest but are seldom used?
The argon laser (488 nm, 514 nm), was one of the first dermatologic lasers. This is seldom used because of scarring issues. The krypton laser generates dual wavelengths: 520-nm (green) and 568-nm (yellow) light. The continuous-wave thermal nature of the laser caused too much surface heating. Copper vapor lasers generate dual wavelengths of 511 nm (green light) and 578 nm (yellow light). These lasers are not currently used. There was a pulsed dye pigment lesion laser with a wavelength of 504 nm. This laser effectively treated surface pigmentation but was mechanically unsound and is no longer available (Table 54-2).
14. What are the basic features of the carbon dioxide (CO2) laser?
The CO2 laser emits radiation at 10,600 nm, in the far-infrared region. All water in tissue absorbs this wavelength of light, and this absorption is not dependent on selective absorption by any biologic tissue. As the water absorbs energy, the temperature rapidly rises, vaporizing the tissue. The amount of tissue damage is related to the energy setting and the amount of time the laser impacts on the target. There is some true ablation of tissue and this is surrounded by a zone of thermal damage. This area of thermal damage is used in resurfacing by causing immediate collagen contraction and later collagen remodeling.
| LASER MEDIUM | WAVELENGTH | PROBLEMS |
|---|---|---|
| Argon | 488 nm, 512 nm | Scarring |
| Krypton | 520 nm, 568 nm | Scarring |
| Copper vapor | 511 nm, 578 nm | Ineffective |
| Pulsed pigment | 504 nm | Mechanical nightmare |
The new superpulsed and ultrapulsed CO2 lasers have powers up to 60 W and pulse duration in the range of 250 μsec to 1 msec. There are now many fractional CO2 lasers designed to reduce the side effects of CO2 laser ablation (Table 54-3).
15. What are some uses for the standard carbon dioxide laser?
Treatment of warts has been the hallmark of CO2 laser therapy. Large plantar and periungual warts are effectively treated. The main advantage of CO2 laser treatment in these larger warts is the ability to decrease the bleeding during the laser excision and a slight decrease in scarring. Any benign lesion may be removed using CO2 laser but this is not a common method used in dermatology. CO2 lasers in a high-power focused mode are very effective at surgical cutting, especially in patients on anticoagulants or those with other bleeding disorders.
Olbricht SM: Use of the CO2 laser in dermatologic surgery, J Dermatol Surg Oncol 19:364–369, 1993.
16. How is the CO2 laser used for resurfacing?
Historically, the ultrapulsed CO2 laser effectively removed the top layers of the epidermis and caused collagen contraction in the superficial dermis. This combination of effects allows the treatment of moderate facial wrinkles and sun damage. This procedure is seldom used at this time because of very prolonged healing and many side effects.
17. What precautions must be used with the CO2 laser?
The main precaution with the CO2 laser is smoke evacuation. During the procedure, the laser generates a large amount of smoke that may harbor viral particles. Studies have suggested the presence of live viral particles or genetic material of human papillomavirus (HPV), hepatitis viruses, and human immunodeficiency virus (HIV). Adequate vacuum devices and surgical masks are a must for use with this laser. The CO2 laser will also burn any cloth or paper that it contacts; therefore, appropriate fire precautions must be observed. Clear plastic or glass eyewear is adequate to protect the eyes from this wavelength of laser light. Patients having skin resurfacing with CO2 laser must be treated both before and immediately after the laser treatment with antiherpes simplex medication.
18. What are the basic features of the erbium:YAG laser?
YAG crystals are composed of yttrium and aluminum in a garnet crystal matrix. The Erbium:YAG crystal has some of the atoms of yttrium replaced with erbium atoms. The laser output is at 2490 nm. This wavelength is absorbed by water 10 times better than the 10,600-nm light of CO2 lasers. This more efficient effect leads to little collateral damage to surrounding collagen and more efficient ablation of tissue. The clinical result is less effect on wrinkles but a smoother, faster-healing resurfacing procedure. By itself, the erbium:YAG laser has been used to treat mild facial sun damage, some sun damage on necks and hands, and acne scarring. In treating scars, the laser has the ability to plane down the edges of acne scars. Er:YAG lasers are also often used as a fractional device to decrease the healing time and side effects (Table 54-4).
19. What are pulsed dye lasers?
The most common pulsed dye lasers use a flashlamp to energize the laser. The active medium is a fluorescent dye, often rhodamine. This will generate a wavelength between 200 and 700 nm. The older vascular lesion pulsed dye lasers used either 577 nm or 585 nm as the preferred wavelength, corresponding to a small peak in the oxyhemoglobin absorption spectrum in an area that does not have much competition from melanin. Newer pulsed dye lasers offer wavelengths that range from 585 to 600 nm. The flashlamp generates pulses with a duration of 300 to 500 msec in older lasers and up to 40 msec in newer lasers.
20. What is the flashlamp pulsed dye vascular lesion laser used to treat?
The pulsed dye lasers may be the most effective lasers in treating the thin, lightly colored port wine stains, especially those in children. These lesions have been treated without scarring in children as young as 1 week of age. Increasing the wavelength to 595 nm theoretically allows the treatment of many port wine stains that were resistant to treatment with the shorter wavelengths. Pulsed dye laser treatment is effective for facial telangiectasias, cherry angiomas, childhood hemangiomas, poikiloderma of Civatte (a mottled vascular condition on the necks of adults), warts, scars, and possibly stretch marks. Leg veins less than 1 mm in diameter are also effectively treated with pulsed dye lasers using 595- to 600-nm light and a pulse duration of up to 20 msec. One of the newer pulsed dye lasers even has a handpiece to treat pigmented lesions. This is a compression handpiece designed to physically compress out the blood, thereby removing one of the competing chromophores of this wavelength. This leaves the laser energy able to treat the remaining melanin targets.
21. What is nonablative resurfacing and how does a pulsed dye laser accomplish this?
Theoretically, gentle heating of the superficial dermal layer will stimulate the production of new collagen. This helps to fill in fine wrinkles on the face. There is some controversy about this procedure, but most patients think it is helpful. Pulsed dye lasers at low to moderate fluences (nonpurpuric settings) are used for this nonablative resurfacing. The low fluences help to stimulate collagen production. Many other lasers and intense pulsed light (IPL) machines are also purported to be effective.
Goldberg DJ: Nonablative resurfacing, Clin Plastic Surg 27:287–292, 2000.
22. What are the disadvantages of the pulsed dye laser?
The pulsed dye lasers are not very useful in treating thicker vascular lesions, because the short pulse duration is not usually sufficient to damage the target vessels without increasing the power to a level that could lead to generalized damage and scarring. There may be significant posttreatment purpura that takes 7 to 10 days to resolve. This purpura results from the explosive opticoacoustic pulse generated by the pulsed dye lasers. This is a cosmetic problem that limits the use of the laser in some patients. The newer pulsed dye lasers are able to treat lesions without purpura by using longer pulse durations in the range of 10 msec (Table 54-5).
23. What is an Nd:YAG laser?
The Nd:YAG laser uses an yttrium-aluminum-garnet crystal in which neodymium has been dispersed into the crystal. The standard wavelength of light emitted is 1064 nm, which is in the near-infrared spectrum. The light from these lasers may be passed through a potassium-titanyl-phosphate (KTP) crystal to double the frequency, halving the wavelength to 532 nm, in the green spectral region. These lasers are usually referred to as KTP lasers. Nd:YAG lasers may be long-pulsed in the 3- to 100-msec pulse duration range or Q-switched with pulses in the 5- to 50-nsec pulse range.
24. How are the long-pulsed Nd:YAG (1064-nm) lasers used?
The long-pulsed Nd:YAG laser (at 1064 nm) has been used for hair removal in dark-skinned patients (Fitzpatrick types IV to VI). This laser has also been used for hair removal in tanned patients, but I would recommend avoiding laser treatments with recently tanned skin. These long-pulsed lasers are also very useful for treating vascular lesions including leg veins and larger vessels on the nose. Long-pulsed Nd:YAG lasers have also been reported to help with skin tightening.
25. How are the long-pulsed KTP lasers used?
26. How are the Q-switched Nd:YAG lasers used?
The Q-switched Nd:YAG lasers are primarily used to remove tattoos. The 1064-nm laser effectively removes black, blue-black, and blue tattoo pigment. This laser has also been useful in treating dermal pigmented lesions such as nevus of Ota and nevus of Ito. The 532-nm laser effectively removes red tattoo pigment. The 532 Q-switched Nd:YAG laser is also used for removal of brown lesions such as lentigines, nevus of Ota, and café-au-lait spots (Table 54-6).
27. What is the alexandrite laser?
Alexandrite crystal, a rare gemstone, is named after the Russian tsar Alexander II. The most sensational feature about this stone, however, is its surprising ability to change its color. Green or bluish-green in daylight, alexandrite turns a soft shade of red, purplish-red, or raspberry red in incandescent light. An alexandrite laser is a solid state laser in which chromium ions (Cr+3), at the amount of 0.01% to 0.4%, are embedded in a BeAl2O4 crystal. Ions of chromium are contaminants in the crystal. Both Q-switched and flashlamp-pumped alexandrite lasers are available. These lasers deliver a wavelength of 755-nm light.
28. How are the alexandrite lasers used?
The Q-switched alexandrite laser is effective in removing green and black tattoo pigment. This laser has also been used to remove brown macules. The long-pulsed alexandrite laser is the gold standard for laser hair removal. Dark hair in Fitzpatrick types I to IV skin is very effectively removed. The long-pulsed laser has been reported to effectively remove leg veins that are 1 to 2 mm in diameter and blue in color. This laser has recently also been reported to effectively treat brown lentigines on the face and hands (Table 54-7).
| LASER | MODE | USES |
|---|---|---|
| Nd:YAG, 1064 nm | Long-pulsed | Hair removal Blood vessels, nose Leg veins Skin tightening |
| Nd:YAG, 1064 nm | Q-switched | Black, blue tattoos Nevus of Ota |
| KTP, 532 nm | Long-pulsed | Facial telangiectasias |
| KTP, 532 nm | Q-switched | Red tattoos Epidermal pigmentation |
Table 54-7. Alexandrite Lasers
| LASER | MODE | USES |
|---|---|---|
| 755 nm | Q-switched | Tattoos (green, black) Brown spots |
| 755 nm | Long-pulsed | Hair removal Leg veins (1–2 mm blue) Brown spots |
29. What is the ruby laser?
The ruby laser has an active medium of aluminum oxide (Al2O3) that has been chromium-doped. Some of the aluminum (Al+3) atoms have been replaced with chromium (Cr+3) atoms. This laser emits a wavelength of 694 nm that is in the red visible light spectrum. These lasers may be Q-switched or long-pulsed.
30. How are the ruby lasers used?
The Q-switched ruby laser light is well absorbed by black, blue, and occasionally green tattoo pigment. The Q-switched ruby lasers have also been used in treating some dermal pigmentation abnormalities such as nevus of Ota. The long-pulsed ruby laser has been used primarily for laser hair removal (Table 54-8).
| LASER | MODE | USES |
|---|---|---|
| 694 nm | Q-switched | Tattoos (black) Nevus of Ota |
| 694 nm | Long-pulsed | Hair removal |
31. What is a diode laser?
Diode lasers use microscopic chips of gallium-arsenide or other semiconductors to generate coherent light. The energy of the light is generated by the differences between energy levels within the semiconductor. The wavelength may be varied to a wide spectral range. The high energy of medical lasers is generated by stacking arrays of the diodes. The advantage of the diode lasers is lower cost, a wide range of wavelengths, and smaller lasers.
32. How are the diode lasers used?
The 810-nm diode lasers are primarily used for hair removal and, occasionally, for treatment of vascular lesions. The 1320-nm diode lasers have been used for nonablative laser resurfacing and treatment of acne scarring. The 1450-nm diode lasers have been used for nonablative laser resurfacing, treatment of acne scarring, and the treatment of active acne (Table 54-9).
33. What are nonablative fractional lasers, and for what are they used?
The Fraxel (Reliant Technologies, Palo Alto, CA) was the first fractional laser. This was a nonablative 1550-nm erbium-doped fiber laser that generated what was termed microscopic treatment zones (MTZ). Being nonablative, the epidermis is thought to remain intact. Within the area of the thermal damage, there is heat-coagulated tissue termed microepidermal necrotic debris (MEND) that is exfoliated through the intact epidermis. The theory is that there will be melanin and elastic tissue removed, and the thermal injury will lead to collagen stimulation.
| WAVELENGTH | PULSE DURATION | USES |
|---|---|---|
| 532 nm | 10–150 msec | Vascular lesions |
| 810 nm | 10–1000 msec | Hair removal Vascular lesions |
| 1320 nm | 20 msec | Nonablative resurfacing Acne scarring |
| 1450 nm | 3 msec | Nonablative resurfacing Acne scarring Acne treatment |
Since this first laser, there have been many nonablative lasers with a variety of wavelengths and theoretical mechanisms. These include 1410-nm, 1540-nm, and a combination of 1440- and 1320-nm light. These lasers are purported to treat acne scars, surgical scars, and facial photodamage, including fine rhytids and dyschromia, melasma, and striae distensae. Table 54-10 lists the nonablative fractional lasers.
Table 54-10. Nonablative Fractional Lasers
| NAME | TYPE | WAVELENGTH |
|---|---|---|
| Fraxel re:fine | Single-mode fiber | 1410 nm |
| Affirm | Nd:YAG | 1320 nm/1440 nm |
| Lux1440 Fractional | Nd:YAG | 1440 nm |
| Lux1540 Fractional | Er:glass | 1540 nm |
| Fraxel re:store | Erbium fiber | 1550 nm |
Narurkar VA: Nonablative fractional laser resurfacing, Dermatol Clin 27:473–478, 2009.
34. What are ablative fractional lasers, and how are they used?
Ablative fractional lasers fire a vaporizing pulse into the skin. There is a column of tissue ablation formed. The fractional lasers spread the pulses out using a variety of scanning devices. The theory of these fractional ablative lasers is to markedly reduce the healing time by spreading the pulses out, leaving normal epidermis between the pulses. As with nonfractional laser pulses, Er:YAG is more purely ablative than CO2. The latter uses the surrounding tissue heating to help collagen contraction and remodeling. The first ablative fractional laser used was a 2940-nm (Er:YAG) laser called PIXEL. This has been joined by other Er:YAG fractional devices and many CO2 10,600-nm fractional lasers. There is even a diode 532-nm ablative fractional laser.
Fractional lasers are primarily used for cosmetic purposes. They help with mild to moderate facial rhytids, hyperpigmentation, and lentigines, and are very effective in helping acne scarring. Table 54-11 lists the ablative fractional lasers.
35. What is an intense pulse light machine?
Intense pulse light (IPL) is a system that uses a flashlamp to generate a high-energy pulse of noncoherent light. The spectrum usually runs from about 550 nm up to 1200 nm. This light, having a wide spectrum, is then subjected to “cutoff” blocking filters to give specific starting wavelengths to each handpiece.
Raulin C, Greve B, Grema H: IPL technology: a review, Lasers Surg Med 32:78–87, 2003.
Key Points: Lasers in Dermatology
1. The wavelengths of common lasers are as follows: KTP, 532 nm; pulsed dye, 585 nm; ruby, 684 nm; alexandrite, 755 nm; diode, 810 to 1450 nm; Nd:YAG, 1064 nm; erbium:YAG, 2490 nm; and carbon dioxide, 10,600 nm.
3. The pulsed dye laser is the gold standard for treatment of vascular lesions, but the KTP, Nd:YAG, and alexandrite lasers are also useful.
4. The long-pulsed alexandrite laser is the gold standard for laser hair removal, but the long-pulsed Nd:YAG and the long-pulsed ruby lasers are also used.
36. What are IPL machines used to treat?
The wide range of available wavelengths allows the IPLs to be very versatile. They have been used for hair removal, treatment of vascular lesions, treatment of pigmented lesions, and nonablative resurfacing. IPL machines have a base platform that delivers the energy to the handpiece. These handpieces now have not only pure IPL but have a variety of laser handpieces. These deliver many different wavelengths based on the different handpieces.
38. What is radiofrequency resurfacing?
These machines deliver radiofrequency to the skin at a frequency of about 6 megahertz. This energy heats the dermal layer and stimulates collagen formation. The end result is similar to many of the other lasers and IPLs that accomplish nonablative resurfacing. The difference with radiofrequency is that some collagen denatures and thereby contracts immediately. This allows rapid apparent changes in the superficial wrinkles.
40. What new technologies will soon be available?
There are now radiofrequency machines that do what is called sublative resurfacing. The machines focus the radiofrequency energy just below the epidermis and cause some coagulative ablation. The newest technology is that of intense focused ultrasound for sublative resurfacing. Here, the ultrasound energy is focused just below the epidermis for ablation similar to that above.
