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Today's Veterinary Practice May/June 2014 62 | RECOVERY & REHAB tvpjournal.com the skin, treating only a small area of the body, while minimizing/avoiding unwanted effects to other tis- sues, such as heating and/or damaging the skin. 1 Tissue Interaction with Lasers Light Reaction Tissues interact with lasers in varying ways, allowing light to be reflected, scattered, transmitted, or absorbed. • Reflection of photons takes place at the epidermis; reflected photons not only lack clinical effect, but can also be responsible for tissue damage (eg, to the eyes). • Scattering occurs once the photons penetrate the tissue. Each time the scattered photons strike an ob- ject outside the target tissue, the amount of photon energy is reduced. • Transmitted photons also lack clinical effect because they pass through the tissue without being absorbed. • Absorption of photons by the target tissue realizes the therapeutic benefit of lasers. See The Benefits of Absorbed Photons. Wavelength Wavelengths are typically measured in nanometers (nm). Wavelength is important when determining the biological effect of lasers on tissues. Tissues, such as melanin and pro- teins, absorb ultraviolet light (100–400 nm). Light on the other end of the spectrum (1400–10,000 nm) is absorbed by water. Therefore, optimum wavelength ranges of 600 to 1200 nm—which minimize scatter and maximize absorp- tion—are recommended for tissue penetration (Figure 1). Power The power density or intensity indicates the amount of power in a given surface area, while the spot size of the laser indicates the surface area size that can be treated when the laser is held stationary. Lasers with larger spot areas have a more homogeneous passage of the photons with less scatter. The energy of the laser characterizes the power emitted over time, measured in joules. Frequently, energy density is used to report dosage of the laser in joules per cm 2 (J/cm 2 ). Continuous or Pulsed Emission of Photons Photons can be emitted either continuously or by pulse. • Continuous emission implies that radiation is emitted at a constant power for the entire duration of use. • Pulsed therapy implies that radiation is delivered in cy- cles over the entire duration of use, with time spent one of 2 ways: radiation emitted or no radiation emitted. Currently, there is debate for superiority of continuous versus pulsed therapy. Some have suggested that there is no difference, 1,6 while others have shown that pulsed therapy may be more effective. 7 WHEN SHOULD LASER THERAPY BE USED? While veterinar y studies are sparse, currently, most studies evaluating laser therapy focus on wound healing and pain management. From a biologic perspective, photons absorbed through cellular pathways allow production of adenosine triphos- phate (ATP). This process is similar to photosynthesis in plants: light is absorbed and converted into chemical energy (ATP) by reduction of CO 2 to useful organic com- pounds, such as glucose. ATP not only alters cellular metabolism, but also acts as a cell-signaling molecule 8 and/or neurotransmitter. • ATP's role as a neurotransmitter helps explain some of the pain modulation effects of lasers. 1 • Due to enhanced cellular metabolism, lasers potential- ly accelerate tissue repair and cell growth . • Additional effects of laser therapy are stimulation of stem cells 8 and anti-inflammatory effects that decrease prosta- glandin E2 (PGE2) and cyclooxygenase-2 (COX-2). 9 Figure 1. Electromagnetic spectrum: Most lasers for therapeutic uses fall in the infrared or near red portion. THE BENEfITS Of ABSORBED PHOTONS A chromophore is responsible for a molecule's color and, in biologic molecules, undergoes a conforma- tional change when hit by a light, such as a laser. This change in the chromophore excites cells and can pos- sibly alter, or speed up, cellular reactions. Commonly noted chromophores include hemoglo- bin, water, melanin, proteins, and amino acids. 1 The thought process is that these compounds—when exposed to laser light—cause alteration of cellular functions, allowing increased healing and/or recruit- ment of secondary mediators to facilitate healing. KEY RECOMMENDATIONS » Hold the laser 90 degrees to the skin surface to minimize reflection of the laser. » To help negate the scatter effect, use wave- lengths in the range of 600 to 1200 nm, which pass deeper into tissue and minimize this effect, and apply the laser directly to the skin. » Use lasers with larger spot areas, which allow more homogeneous passage of photons, less scatter, and greater treatment area. TVP_2014_0506_RecovRehab_LaserTherapy.indd 62 5/23/2014 6:02:08 PM