Medical Laser Applications

Despite their relative novelty to medicine and an undeniable "high tech" appeal, medical lasers are simply tools used to solve a problem or accomplish a task. A thorough understanding of the interaction of directed energy on tissue is essential to choose the best modality to complete the task with maximum safety and elegance.

Skin Rejuvenation-True Ablative Systems: True ablative laser systems ablate epidermis and dermis and thermally injure skin in a controlled manner.

• The arrival of pulsed and scanned CO2 lasers in the early 1990s marked the beginning of the laser skin rejuvenation revolution, and CO2 laser resurfacing remains the standard against which other skin rejuvenation techniques are measured. The mid-infrared emission at 10600 nm is absorbed by water, and delivery through a computer-controlled scanner allows precise vaporization and ablation of soft tissue. For resurfacing, the epidermis and upper dermis is ablated, inducing a powerful collagen remodeling and regeneration of the epidermis from residual cells in the hair follicles and sweat glands.

Because of its simplicity and consistency, CO2 Laser resurfacing has almost completely replaced "traditional" resurfacing methods such as dermabrasion and deep chemical peels. However, with all of these deep resurfacing techniques, the entire epidermal layer is removed and the barrier function of the skin is compromised, with prolonged healing and a low but significant chance of infection and scarring.

• Er:YAG or "erbium" laser energy at 2940 nm was introduced as an alternative resurfacing technique because of its rapid healing and low incidence of scarring and infection. 2940 nm is an absorption peak of water, and the strong absorption and short pulse of the original erbium lasers induced a minimal thermal effect on skin, stripping the epidermis but with little thermal effect on the dermis, and correspondingly limited results. Newer erbium lasers with their somewhat longer pulses are somewhat more effective because of their increased thermal effect. However, compared to CO2 laser resurfacing, Erbium laser resurfacing is faster healing with less risk of complications, but significantly less effective.

Skin Rejuvenation-Minimally Ablative Systems: Minimally ablative systems dessicate epidermis, which remains in place as new epidermis forms, and induce a mild thermal injury in the upper dermis.

• Plasma skin resurfacing uses nitrogen plasma to deliver non-optical thermal energy to the skin. A flow of nitrogen gas is excited by pulses of radio frequency energy, producing a stable nitrogen plasma. Applied in pulses through a disposable handpiece, the intense heat from the plasma dessicates the epidermis and superficial dermis, which remain in place as healing and repithelialization occurs under the dessicated area, which acts as a biologic dressing. Healing is rapid with a minimal risk of infection and scarring. The thermal effect on the dermis induces collagen remodeling with mild skin tightening.
• YSSG Laser resurfacing exploits the absorption of 2790 nm light in water. With less water absorption than the Er:YAG, the YSSG emission is delivered through a scanning device, heating and dessicating the epidermis and upper dermis while leaving the dessicated layer in place as reepithelialization takes place below. Heating of the upper dermis induces mild collagen remodeling over the next few weeks.

Skin Rejuvenation-Fractionally Ablative Systems: Fractionally ablative systems deliver a pattern of microscopic zones of thermal injury, each zone surrounded by normal uninjured tissue to facilitate rapid healing and minimize complications.

• Er:Fiber laser energy at 1550 nm is strongly absorbed by water, and delivered through a "smart" scanner deposits a regular array of microscopic columns of coagulated tissue. Because the stratum corneum (external surface) of the epidermis is dry, the epidermal barrier is not compromise, minimizing downtime and facilitating rapid healing.

Depending on the density and diameter of the "microthermal zones", up to 25% of the targeted skin surface can be treated in a single session in this "pixelated" fashion. Multiple treatments are necessary for full treatment of a given area.

• Er:YAG fractional systems using a non-contact scanners are becoming available and work in a similar fashion; Fractional systems using masks or microlens arrays, although not true "fractional" systems because intervening tissue is heated as well, are also being marketed.
• Fractional CO2 Laser systems offer the advantage of tissue ablation with the safety and rapid healing of fractional delivery, generating a microscopic columnar lesion with normal, uninjured interalesional tissue. Fractional CO2 resurfacing offers rapid healing with low risk of complications, modest downtime and results approaching that of traditional ablative CO2 laser resurfacing. Systems with both contact and non-contact scanners are becoming available.

Skin Rejuvenation-Non-Ablative Systems: Non-Ablative systems are intended to rejuvenate skin without coagulating or ablating tissue and with no or miminal downtime. Multple treatments are the rule, and results are necessarily limited.

• Visible light systems typically employ green (532 nm) or yellow (585-595 nm) wavelengths to induce a mild vascular injury with release of cytokines and growth factors that stimulate collagen remodeling. These wavelengths are also well absorbed by oxyhemoglobin and melanin, making them useful for treating dyschromia, "brown spots", redness, and telangiectasia, but often at the expense of modest downtime with redness or superfical crusting. Use of these systems is limited in darker skinned patients because of competition from epidermal melanin.
• Near infrared (NIR) systems at 1320 nm and 1450 nm use water as the chromophore without competition from hemoglobin or melanin, heating collagen in the upper dermis with subsquent healing and remodeling. Typically these systems use crygen cooling to prevent epidermal damage and mitigate patient discomfort.
• Intense Pulsed Light devices emit broadband light in the 550-1200 nm range, targeting multiple chromophores including hemoglobin, melanin, and to some extent water, but with less selectivity than monochromatic laser light. Typically, IPL systems are used to treat redness and dyschromias, with collagen remodeling an incidental effect.
• LED Narrowband Light exploits photobiomodulation to increase celluar activity and stimulate collagen and epidermal regeneration. As with other non-ablative modalities, multiple treatments are necessary and results are modest.

Skin Tightening Systems-Radiofreqency Devices: Radiofrequency current flow is used to heat dermal and subdermal collagen which tightens as it remodels.

• Monopolar Radiofrequency uses the body as a volume conductor, with current flowing from a grounding plate to a chilled electrode applied to the skin. Controlled heating of collagen fibers in the dermis and subdermal fibrous septae induces thermal injury and subsequent remodeling with tightening over weeks to months. Treatment is painful and results are variable and modest at best.
• Electro-optical Synergy (ELOS) devices use bipolar radiofrequency passed through a volume of skin preheated by laser or IPL energy. A series of treatments is recommended for best results. Treatment is painful and results are variable and modest at best.

Skin Tightening Systems-Infrared Intense Pulsed Light use an infrared lamp delivering NIR energy to the dermis through a chilled lens to prevent epidermal injury. Multiple treatments are necessary for best results. Treatment is painful, results are modest at best.

Treatment of Vascular Lesions: Laser and light-based treatment of vascular lesions is dependent on the absorption of light energy by hemoglobin. Considerations when treating vascular lesions include the size, depth and location of the vascular target and competion from epidermal melanin.

Vascular targets include:

• Facal telangiectasia
• Rosacea
• Vascular malformations and hemangioma
• Leg veins

As a general rule, smaller, superficial, redder vascular targets are best treated with devices that deliver shorter wavelengths and pulse widths, and larger, deeper, more burgundy/blue targets are best treated with longer wavelengths.

• KTP Lasers: The Potassium Titanyl Phosphate (or Frequency doubled Nd:YAG laser) emits a lime green light at 532 nm, coinciding with an absorption peak of hemoglobin, in millisecond pulses, making it well suited for treated facial telangiectasia and rosacea without the purpura or bruising commonly seen with Pulsed Dye Lasers. Strong melanin absorption at this wavelength makes it useful for the treatment of pigmented lesion, but may limit its use in darker skinned patients.
• Copper Vapor Laser: The greenish yellow light from the copper vapor laser emitted in millisecond domain pulses makes it useful for the treatment of telangiectasia and small pigmented lesions; however, the long warm-up time and high maintenance requirements make this laser a less popular alternative for the treatment of vascular lesions.
• Pulsed Dye Laser (PDL): Arguably the most popular vascular laser, the PDL uses an organic dye in a liquid solvent as the gain medium to produce a sub-millisecond pulses of 585-595 nm yellow light. The peak absorption of hemoglobin at this wavelength and short pulses ruptures small vessels and causes purpura, a harmless but cosmetically objectionable pupura, or bruising. This problem can be minimized by using pulse "trains" to simulate a longer pulse.

As with the KPT laser, the vascular specificity of these wavelengths allows these lasers to be used for skin rejuvenation at low fluences, and to treat superficial pigmented lesions at high fluences.Although PDLs are very effective for superficial vessels, telangiectasia and redness, they are maintenance-intensive, and purpura remains problematic.

• Alexandrite Laser: Widely used for laser hair reduction, the Alexandrite laser's 755 nm deep red emission coincides with a shallow hemoglobin absorption peak. Relatively low melanin absorption (compared to the KTP and PDL) allows treatment of deeper vessels and leg veins.
• Diode Laser: The 800-810 nm pulsed output of the Diode Laser is fairly well absorbed by both oxygenated (red) hemoglobin at high fluences and deoxygenated (blue) hemoglobin at lower fluences. Low melanin absorption allows deep penetration in all but the darkest skin types. 940 nm diode lasers also have weak hemoglobin absorption and are marketed for dental use.
• Nd:YAG Laser: 1064 nm light generated by the Nd:YAG laser coincides with minimal absorption of all tissue chromophores, but treatment of both blue and red vessels is possible at high fluences. Deep penetration and preferential absorption by deoxyhemoglobin and methemoglobin make the Nd:YAG especially useful for the treatment of leg veins in all skin types.
• Dual Wavelength Vascular Lasers use both 585 nm and 1064 nm light to target vascular lesions of variable depth and vessel diameter. The 585 nm wavelength is preferentially absorbed by oxyhemoglobin and converts oxyhemoglobin in deeper, larger vessels to methemoglobin, which is targeted by the 1064 nm wavelength a few milliseconds later. The combination allows lower fluences of both lasers, and is especially effective for mature port wine stains and vascular malformations.
• IPL: Broadband light from Intense Pulsed Light devices targets multiple chromophores, although for treatment of vascular lesions, IPL is most useful for treating small facial vessels and facial erythema (redness or flushing). Many IPL devices designated for vascular treatment incorporate a Nd:YAG handpiece for the treatment of larger vessels or leg veins.

Hair Removal: Laser and light based hair removal systems all depend on the selective targeting of melanin in the hair shaft and follicle, while sparing epidermal melanin.

As wavelength increases, absorption by melanin in both hair and epidermis decreases. Epidermal melanin content drives the selection of the most appropriate wavelength in a given clinical situation. Patients with fair skin and dark hair may be safely treated with any available wavelength; patients with dark skin and light hair are poor candidates for laser hair reduction. Additional epidermal protection can be achieved by cooling the epidermis.

• Ruby Laser light at 694 nm has the highest absorption by melanin, making its use problematic for darker skinned patients, but useful for the treatment of fine, light, or blond hair. Although the ruby laser was the original laser used for light-based hair removal, the devices tend to be bulky, relatively slow, and their use is limited to fair skinned patients without an unacceptable risk of epidermal blistering.
• Alexandrite Laser: The most popular laser for laser hair removal, the Alexandrite laser emits 755 nm light, just at the limit of visibility. Although slightly less well absorbed by melanin than ruby laser light, the 755 nm wavelength offers the best balance between efficacy for all hair types and epidermal safety for darker skinned patients. Alexandrite lasers are capable of high repetition rates, reducing operator and patient fatigue, and large spot sizes, allowing lower effective fluences and increased epidermal safety.
• Diode Laser: Consisting of little more than an 800-810 nm laser diode with regulated power supply, the diode laser is mechanically the least complex hair removal laser. The slightly longer wavelength offers slightly more epidermal safety than the alexandrite laser, to some extent offset by the higher fluences necessary to achieve an effective fluence at the hair follicle. Treatment is somewhat more painful than with shorter wavelength lasers.
• Nd:YAG Laser: The 1064 nm wavelength is poorly absorbed by melanin, but the amount of melanin in hair versus epidermis allows this wavelength to be safely used even in the darkest skin types, although at the expense of efficacy for fine, light, and red hair. Nd:YAG lasers are capable of large spot sizes and high repetition rates, although discomfort during treatment largely negates these advantages.
• IPL devices: When used for hair removal, these broadband light sources use a cutoff filter to limit the spectrum to wavelengths longer than 640 nm, minimizing absorption by chromophores other than melanin. IPL devices typically have large spot sizes but slow repetition rates, and are more likely to cause epidermal injury than laser devices. Recent peer-reviewed articles comparing IPL to Alexandrite laser treatment for laser hair reduction demonstrated greater efficacy and patient satisfaction with Alexandrite laser treatment.
• ELOS (electro-optical synergy) devices: Because these devices rely on bipolar RF as well as optical energy, theoretically there should be less dependence on melanin absorption. One manufacturer claims effciacy for treatment of white/gray (unpigmented) hair.

Q-Switched Laser Applications: All of the Q-Switched medical laser systems available today are solid-state flashlamp pumped bulk lasers using an active, and in a few cases, a passive Q-Switch. These laser systems emit nanosecond-domain pulses to photomechanically target submicron structures, primarily melanosomes in pigmented skin lesions and pigment in decorative tattoos.

• Q-Switched Ruby Laser
• Q-Switched Alexandrite Laser
• Q-Switched Nd:YAG Laser

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Medical Lasers and Light-based Devices: An Editorial

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