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PRINCIPLES OF LASER ENERGY
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Laser (light amplification by stimulated emission of radiation) differs from common light in that it emits a more focused, or coherent beam. This specialized beam transmits energy, potentially useful to cut or vaporize tissue in surgery. Additionally, because laser beams remain compact over distances for which regular light would diffuse, lasers can be pointed precisely. Furthermore, compared to solid scalpels, laser light does not need to be sterilized, disposed, or ordered from central supply. Surgeons value lasers because they cut clean, precise, from a distance, and with depth control. However, like many powerful tools, lasers are potentially dangerous.
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Lasers contain three structural components: a power source (pump), a gain medium (tube), and an optical resonator (often two parallel mirrors or optical fibers). The power supplied to a laser may directly stimulate the population of atoms in the gain medium, thereby raising the electrons to a higher energy level, producing the traditional laser energy. The gain medium can be in any physical state; medical lasers commonly utilize gas (argon, carbon dioxide, helium) or solid-state elements (ruby, garnet). It is the gain medium that determines the wavelength emitted, thus defining the applications and risks. The wavelength emitted may be in the visible light spectrum (385–760 nm), or beyond visible light, in wavelengths from ultraviolet (200–400 nm) to infrared and far-infrared (700–10,600 nm).
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Laser energy travels as monochromatic (single wavelength), synchronous (in phase), parallel (collimated) beams deliverable to a small target. Since energy density is inversely proportional to the square of the spot size radius, halving the spot size of a laser beam, with energy constant, increases energy density by a factor of 4. Variables that relate to the degree of therapeutic effect or damage to tissue are the power density, duration of exposure, and photon wavelength.
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In addition to laser variables, tissue characteristics also modulate biologic effects of laser. These include (1) absorption, the magnitude by which excited electrons increase energy in cells, (2) scatter, the degree by which atoms disperse or remain localized, (3) thermal conductivity, how heat transmits outwardly or remains contained, and (4) local circulation, the movement of surrounding cells. As an example, the commonly used medical CO2 laser is completely absorbed by tissue water within the top few layers of skin cells. This results in vaporization of surface cell layers with minimal or no damage to the underlying layers. In comparison, the Nd:YAG (neodymium-doped yttrium aluminum garnet) laser is less absorbed by water such that the beam diffuses through several millimeters, dispersing energy to produce more coagulation or “cooking” and less vaporization. A summary of types of lasers is given in Table 9-1.
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