BIOLOGIC EFFECTS OF LASER LIGHT

Living tissue is a complex aqueous solution containing a variety of molecules that absorb light. When an atom interacts with a photon whose energy does not exactly match a possible electron transition, the photon's energy may be translated into atomic vibrations that we recognize as heat. The degree to which a particular wavelength of light is absorbed and converted to heat in the target tissue determines its effect on that tissue.

Long infrared wavelengths are absorbed with great efficiency by water, the main constituent of tissue. Long-wavelength laser light, such as that from CO₂ (operating at 10,600 nm), is therefore completely absorbed by water in the first few layers of cells. With powerful, focused CO₂ beams, this results in explosive vaporization of the surface tissue of the target with surprisingly little damage to underlying cells. Excimer lasers have an extremely precise effect because they produce ultraviolet light that is absorbed more intensely by water and other molecules. These lasers are of great value in office-based refractive surgery. Near-infrared light from an Nd:YAG laser at 1064 nm is less absorbed by water, and the beam diffuses through several millimeters, scattering through a volume of tissue perhaps 100 to 1000 times the volume through which a CO₂ beam diffuses. Consequently, the energy of an Nd:YAG beam is more widely disseminated ( Fig. 67-5 ), producing less vaporization and more thermal coagulation (i.e., a cooking effect). Some of the effects of this bulk coagulation may not be apparent for hours or days after exposure. The red light produced by a ruby laser (694 nm) is poorly absorbed, except by cells containing dark pigment. The green and blue light produced by argon (514,488 nm) or krypton (476,521,568 nm) gas lasers is transmitted by water but is intensely absorbed by hemoglobin, providing the ability to penetrate skin or ocular structures and selectively coagulate vascular or pigmented regions. Although lasers that produce infrared or visible light have exclusively thermal effects on tissue, the photons produced by ultraviolet excimer lasers are energetic enough to disrupt chemical bonds directly and cause ionization, which can result in mutation and carcinogenesis.

Figure 67-5 Different wavelengths of laser light cause different patterns of tissue destruction. The destructive effect of laser light on tissue depends on laser parameters and tissue factors.