Lasers in plastic surgery

Early in the last century, Einstein theoretically explained the processes that must occur when a laser emits energy in a paper entitled "The Quantum Theory of Radiation." Maiman built the first laser in 1960. Since then, laser technology has rapidly developed, producing a variety of lasers that span the entire electromagnetic spectrum. They have since been combined with other technologies, including imaging systems, robotics, and computers, to improve the precision of laser delivery. Through collaborations in physics and bioengineering, medical lasers have become an important part of surgeons' therapeutic tools. At first, they were bulky and used only by surgeons who were specially trained in laser physics. Over the past 15 years, medical laser design has advanced to make them easier to use, and many surgeons have learned the basics of laser physics as part of their graduate training.

This article discusses: biophysics of lasers; interaction of tissues with laser radiation; devices currently used in plastic and reconstructive surgery; general safety requirements when working with lasers; issues of further use of lasers in skin interventions.

Biophysics of lasers

Lasers emit light energy that travels in waves similar to ordinary light. The wavelength is the distance between two adjacent peaks of the wave. The amplitude is the size of the peak, determining the intensity of the light. The frequency, or period, of a light wave is the time it takes for the wave to complete one cycle. To understand how a laser works, it is important to understand quantum mechanics. The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. When a photon, a unit of light energy, strikes an atom, it causes one of the atom's electrons to jump to a higher energy level. The atom becomes unstable in this excited state, releasing a photon when the electron falls back to its original, lower energy level. This process is known as spontaneous emission. If an atom is in a high-energy state and collides with another photon, when it returns to a low-energy state it will release two photons that have identical wavelength, direction, and phase. This process, called stimulated emission of radiation, is fundamental to understanding laser physics.

Regardless of type, all lasers have four basic components: an excitation mechanism or energy source, a laser medium, an optical cavity or resonator, and an ejection system. Most medical lasers used in facial plastic surgery have an electrical excitation mechanism. Some lasers (such as a flashlamp-excited dye laser) use light as the excitation mechanism. Others may use high-energy radiofrequency waves or chemical reactions to provide excitation energy. The excitation mechanism pumps energy into a resonant chamber containing the laser medium, which may be a solid, liquid, gas, or semiconductor material. The energy dumped into the resonator cavity raises the electrons of the atoms in the laser medium to a higher energy level. When half of the atoms in the resonator are highly excited, a population inversion occurs. Spontaneous emission begins as photons are emitted in all directions and some collide with already excited atoms, resulting in stimulated emission of paired photons. Stimulated emission is enhanced as photons traveling along the axis between the mirrors are reflected preferentially back and forth. This results in sequential stimulation as these photons collide with other excited atoms. One mirror is 100% reflective, while the other mirror partially transmits the emitted energy from the resonator chamber. This energy is transferred to the biological tissue by an ejection system. For most lasers, this is fiber optic. A notable exception is the CO2 laser, which has a system of mirrors on a hinged arm. Optical fibers are available for the CO2 laser, but they limit the spot size and the output energy.

Laser light is more organized and qualitatively intense than ordinary light. Since the laser medium is homogeneous, the photons emitted by stimulated emission have a single wavelength, which creates monochromaticity. Normally, light is highly scattered as it moves away from the source. Laser light is collimated: it is little scattered, providing a constant energy intensity over a large distance. Not only do the photons of laser light move in the same direction, they have the same temporal and spatial phase. This is called coherence. The properties of monochromaticity, collimation, and coherence distinguish laser light from the disordered energy of ordinary light.

Laser-tissue interaction

The spectrum of laser effects on biological tissues extends from modulation of biological functions to vaporization. Most clinically used laser-tissue interactions concern thermal capabilities to coagulate or vaporize. In the future, lasers may be used not as heat sources, but as probes to control cellular functions without cytotoxic side effects.

The effect of a conventional laser on tissue depends on three factors: tissue absorption, laser wavelength, and laser energy density. When a laser beam hits tissue, its energy can be absorbed, reflected, transmitted, or scattered. All four processes occur to varying degrees in any tissue-laser interaction, of which absorption is the most important. The degree of absorption depends on the chromophore content of the tissue. Chromophores are substances that effectively absorb waves of a certain length. For example, CO2 laser energy is absorbed by the soft tissues of the body. This is because the wavelength corresponding to CO2 is well absorbed by water molecules, which make up up to 80% of soft tissue. In contrast, CO2 laser absorption is minimal in bone, due to the low water content of bone tissue. Initially, when tissue absorbs laser energy, its molecules begin to vibrate. The absorption of additional energy causes denaturation, coagulation, and finally evaporation of the protein (vaporization).

When laser energy is reflected by tissue, the latter is not damaged, since the direction of the radiation on the surface is changed. Also, if the laser energy passes through the superficial tissues into the deep layer, the intermediate tissue is not affected. If the laser beam is scattered in the tissue, the energy is not absorbed on the surface, but is randomly distributed in the deep layers.

The third factor concerning the interaction of tissue with the laser is the energy density. In the interaction of laser and tissue, when all other factors are constant, changing the spot size or exposure time can affect the condition of the tissue. If the spot size of the laser beam decreases, the power acting on a certain volume of tissue increases. Conversely, if the spot size increases, the energy density of the laser beam decreases. To change the spot size, the ejection system on the tissue can be focused, prefocused, or defocused. In prefocused and defocused beams, the spot size is larger than the focused beam, resulting in a lower power density.

Another way to vary tissue effects is to pulse the laser energy. All pulsed modes alternate between on and off periods. Since the energy does not reach the tissue during the off periods, there is a chance for heat to dissipate. If the off periods are longer than the thermal relaxation time of the target tissue, the likelihood of damage to the surrounding tissue by conduction is reduced. The thermal relaxation time is the amount of time required for half of the heat in the target to dissipate. The ratio of the active interval to the sum of the active and passive pulsation intervals is called the duty cycle.

Duty cycle = on/on + off

There are various pulse modes. The energy can be released in bursts by setting the period in which the laser emits (e.g. 10 sec). The energy can be blocked, where the constant wave is blocked at certain intervals by a mechanical shutter. In superpulse mode, the energy is not simply blocked, but stored in the laser energy source during the off period and then released during the on period. That is, the peak energy in superpulse mode is significantly higher than that in constant or blocking mode.

In a giant pulse laser, energy is also stored during the off period, but in the laser medium. This is accomplished by a shutter mechanism in the cavity chamber between the two mirrors. When the shutter is closed, the laser does not lasing, but energy is stored on each side of the shutter. When the shutter is open, the mirrors interact to produce a high-energy laser beam. The peak energy of a giant pulse laser is very high with a short duty cycle. A mode-locked laser is similar to a giant pulse laser in that there is a shutter between the two mirrors in the cavity chamber. The mode-locked laser opens and closes its shutter in sync with the time it takes for the light to reflect between the two mirrors.

Characteristics of lasers

• Carbon dioxide laser

The carbon dioxide laser is most commonly used in otolaryngology/head and neck surgery. Its wavelength is 10.6 nm, an invisible wave in the far infrared region of the electromagnetic spectrum. Guidance along the helium-neon laser beam is necessary so that the surgeon can see the area of action. The laser medium is CO2. Its wavelength is well absorbed by water molecules in the tissue. The effects are superficial due to high absorption and minimal scattering. The radiation can only be transmitted through mirrors and special lenses placed on an articulated rod. The crank arm can be attached to a microscope for precision work under magnification. Energy can also be ejected through a focusing handle attached to the articulated rod.

• Nd:YAG laser

The wavelength of the Nd:YAG (yttrium-aluminum-garnet with neodymium) laser is 1064 nm, i.e. it is in the near-infrared region. It is invisible to the human eye and requires a guiding helium-neon laser beam. The laser medium is yttrium-aluminum-garnet with neodymium. Most tissues of the body absorb this wavelength poorly. However, pigmented tissue absorbs it better than non-pigmented tissue. The energy is transmitted through the superficial layers of most tissues and dissipates in the deep layers.

Compared with the carbon dioxide laser, the scattering of Nd:YAG is significantly greater. Therefore, the penetration depth is greater and Nd:YAG is well suited for coagulation of deep vessels. In the experiment, the maximum coagulation depth is about 3 mm (coagulation temperature +60 °C). Good results in the treatment of deep perioral capillary and cavernous formations using the Nd:YAG laser have been reported. There is also a report on successful laser photocoagulation of hemangiomas, lymphangiomas and arteriovenous congenital formations. However, the greater penetration depth and non-selective destruction predispose to increased postoperative scarring. Clinically, this is minimized by safe power settings, a point approach to the lesion and avoidance of treatment of skin areas. In practice, the use of the dark-red Nd:YAG laser has been virtually replaced by lasers with a wavelength lying in the yellow part of the spectrum. However, it is used as an adjuvant laser for dark red (port wine) coloured nodular lesions.

The Nd:YAG laser has been shown to inhibit collagen production in both fibroblast culture and normal skin in vivo. This suggests success in treating hypertrophic scars and keloids. However, clinically, recurrence rates after keloid excision are high, despite potent adjunctive topical steroid treatment.

• Contact Nd:YAG laser

The use of the Nd:YAG laser in contact mode significantly changes the physical properties and absorption of the radiation. The contact tip consists of a sapphire or quartz crystal directly attached to the end of the laser fiber. The contact tip interacts directly with the skin and acts as a thermal scalpel, cutting and coagulating simultaneously. There are reports of using the contact tip in a wide range of soft tissue interventions. These applications are closer to those of electrocoagulation than the non-contact Nd:YAG mode. In general, surgeons now use the inherent wavelengths of the laser not for cutting tissue, but for heating the tip. Therefore, the principles of laser-tissue interaction are not applicable here. The response time to the contact laser is not as directly related as with free fiber, and therefore there is a lag period for heating and cooling. However, with experience, this laser becomes convenient for isolating skin and muscle flaps.

• Argon laser

The argon laser emits visible waves with a length of 488-514 nm. Due to the design of the resonator chamber and the molecular structure of the laser medium, this type of laser produces a long-wave range. Some models may have a filter that limits the radiation to a single wavelength. The energy of the argon laser is well absorbed by hemoglobin, and its scattering is intermediate between that of a carbon dioxide and Nd:YAG laser. The radiation system for the argon laser is a fiber-optic carrier. Due to the high absorption by hemoglobin, vascular neoplasms of the skin also absorb laser energy.

• KTF laser

The KTP (potassium titanyl phosphate) laser is a Nd:YAG laser whose frequency is doubled (wavelength is reduced by half) by passing the laser energy through a KTP crystal. This produces green light (wavelength 532 nm), which corresponds to the absorption peak of hemoglobin. Its tissue penetration and scattering are similar to those of an argon laser. The laser energy is transmitted by a fiber. In non-contact mode, the laser vaporizes and coagulates. In semi-contact mode, the tip of the fiber barely touches the tissue and becomes a cutting instrument. The higher the energy used, the more the laser acts as a thermal knife, similar to a carbon dioxide laser. Lower energy units are used primarily for coagulation.

• Flash lamp excited dye laser

The flash lamp excited dye laser was the first medical laser specifically designed for the treatment of benign vascular lesions of the skin. It is a visible light laser with a wavelength of 585 nm. This wavelength coincides with the third absorption peak of oxyhemoglobin, and therefore the energy of this laser is predominantly absorbed by hemoglobin. In the range of 577-585 nm there is also less absorption by competing chromophores such as melanin and less scattering of the laser energy in the dermis and epidermis. The laser medium is rhodamine dye, which is optically excited by a flash lamp, and the emission system is a fiber optic carrier. The dye laser tip has an interchangeable lens system that allows the creation of a spot size of 3, 5, 7 or 10 mm. The laser pulses with a period of 450 ms. This pulsatility index was chosen based on the thermal relaxation time of ectatic vessels found in benign vascular lesions of the skin.

• Copper vapor laser

The copper vapor laser produces visible light of two separate wavelengths: a pulsed green wave of 512 nm and a pulsed yellow wave of 578 nm. The laser medium is copper, which is excited (vaporized) electrically. A fiber system transmits energy to the tip, which has a variable spot size of 150-1000 µm. The exposure time ranges from 0.075 s to constant. The time between pulses also varies from 0.1 s to 0.8 s. The yellow light of the copper vapor laser is used to treat benign vascular lesions on the face. The green wave can be used to treat pigmented lesions such as freckles, lentigines, nevi, and keratosis.

• Non-fading yellow dye laser

The yellow CW dye laser is a visible light laser that produces yellow light with a wavelength of 577 nm. Like the flashlamp-excited dye laser, it is tuned by changing the dye in the laser activation chamber. The dye is excited by an argon laser. The ejection system for this laser is also a fiber optic cable that can be focused to different spot sizes. The laser light can be pulsed using a mechanical shutter or a Hexascanner tip that attaches to the end of the fiber optic system. The Hexascanner randomly directs pulses of laser energy within a hexagonal pattern. Like the flashlamp-excited dye laser and the copper vapor laser, the yellow CW dye laser is ideal for the treatment of benign vascular lesions on the face.

• Erbium laser

The Erbium:UAS laser uses the 3000 nm absorption band of water. Its wavelength of 2940 nm corresponds to this peak and is strongly absorbed by tissue water (approximately 12 times more than the CO2 laser). This near-infrared laser is invisible to the eye and must be used with a visible aiming beam. The laser is pumped by a flash lamp and emits macropulses of 200-300 μs duration, which consist of a series of micropulses. These lasers are used with a handpiece attached to an articulated arm. A scanning device can also be integrated into the system for faster and more uniform tissue removal.

• Ruby laser

The ruby laser is a flashlamp pumped laser that emits light at a wavelength of 694 nm. This laser, which is in the red region of the spectrum, is visible to the eye. It may have a laser shutter to produce short pulses and achieve deeper tissue penetration (deeper than 1 mm). The long-pulse ruby laser is used to preferentially heat hair follicles in laser hair removal. This laser light is transmitted using mirrors and a articulated boom system. It is poorly absorbed by water, but is strongly absorbed by melanin. Various pigments used for tattoos also absorb 694 nm rays.

• Alexandrite laser

The Alexandrite laser, a solid-state laser that can be pumped by a flash lamp, has a wavelength of 755 nm. This wavelength, in the red part of the spectrum, is not visible to the eye and therefore requires a guide beam. It is absorbed by blue and black tattoo pigments, as well as melanin, but not hemoglobin. It is a relatively compact laser that can transmit radiation through a flexible light guide. The laser penetrates relatively deeply, making it suitable for hair and tattoo removal. Spot sizes are 7 and 12 mm.

• Diode laser

Recently, diodes on superconducting materials have been directly coupled to fiber optic devices, resulting in the emission of laser light at various wavelengths (depending on the characteristics of the materials used). Diode lasers are distinguished by their efficiency. They can convert incoming electrical energy into light with an efficiency of 50%. This efficiency, associated with lower heat generation and input power, allows compact diode lasers to be designed without large cooling systems. The light is transmitted via fiber optics.

• Filtered Flash Lamp

The filtered pulsed lamp used for hair removal is not a laser. Instead, it is an intense, non-coherent, pulsed spectrum. The system uses crystal filters to emit light with a wavelength of 590-1200 nm. The width and integral density of the pulse, also variable, meet the criteria for selective photothermolysis, which puts this device on par with lasers for hair removal.

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