Written by Kristin Kirkby, DVM, MS, CCRT, Diplomate ACVS
Lasers were first used surgically in high doses to cut or seal tissue. It is only recently that therapeutic lasers have gained wide recognition in veterinary medicine.
Lasers, along with the sun, ordinary light bulbs, x-ray machines, and microwave ovens, emit electromagnetic radiation. Energy from these sources travels at the speed of light in packets known as photons. Photons travel in waves, and the type of radiation is distinguished by its wavelength. Electromagnetic radiation is typically described as a spectrum from very short (gamma rays, 1000-1 fm) to long (radio waves, 1000-1m) wavelengths. Within the center of this spectrum lies visible light (400-800 nm) and infrared (1000-0.8 um) and laser radiation falls within these wavelengths.
Laser is an acronym for “light amplification by stimulated emission of radiation”. In this process, electromagnetic energy is harnessed into an intense, coherent, monochromatic beam of light. The properties of monochromaticity (all waves are same length/ color) and coherence (all waves in phase) are characteristics of all lasers.
A laser is made using a material (gas, liquid, solid) that when stimulated by an external energy source such, such as electricity, will release photons of a single color or wavelength. Wavelength is inversely proportional to energy.
Therapeutic lasers are commonly referred to as “low level lasers” or “cold lasers”, in contrast to high-powered lasers that are used to cut tissue. Therapeutic or low level lasers have a power output less than 500 miliwatts (mW) and cannot cut tissue. Power output is significant, because a laser with a higher wattage will reach the desired dose more quickly. Amongst low level lasers, power output (watts, W) varies greatly, anywhere from 3.5mW to 500 mW. Recently, lasers with power output greater than 500 mW (typically around 10 W) have been marketed as therapeutic lasers. These lasers have the capacity cause considerable tissue heating and risk to skin tissue.
Lasers are divided into four classes with additional subclasses (I, II, IIIA, IIIB and IV). A common misconception is that these classes distinguish the efficacy or quality of the laser. Rather, laser class is determined by the ability to cause eye injury and is based on power output. Class I-IIIA lasers, including supermarket scanners, laser pointers and remote controls, are considered safe. Class IIIB lasers pose a risk of eye injury, and eye protection is recommended. Any laser with greater than 500 mW of power falls into class IV, and while these lasers provide therapeutic properties, they have the potential to cause tissue burning and significant eye injury and should only be used by trained medical professionals.
Dosage (also known as energy density or fluence) appears to be the most important laser parameter in clinical application. Dosage is measured in J/cm2, and can be calculated using the following formula:
Dose = P x t / A
P= laser’s output power (W)
t= treatment time (seconds)
A= area treated (cm2)
1 J= 1 W/s
Lasers that are available commercially and marketed for medical or veterinary use will likely come with recommended doses for various conditions pre-programmed into the unit or listed in the instruction manual. While the optimal treatment dose has not been established for any condition, generally recommended doses fall between 2-10 J/cm2.
One last consideration is the depth of laser penetration. This depends on the wavelength of the laser, with longer wavelength resulting in deeper penetration. A GaAs laser (904 nm) can reach tissue depths of 3-5 cm, while a HeNe (633 nm) will have more superficial penetration, near 1cm.
The effects of therapeutic lasers in wound healing include vasodilation, angiogenesis, increased collagen synthesis by fibroblast, differentiation of fibroblast into myofibroblasts, stimulation of leukocytes, and enhanced antioxidant activity. The end results of these processes are improved tissue healing and regeneration, increased wound contraction, increased strength of repaired tissue, improved immune function, and defense against ischemia/ reperfusion injury. Furthermore, laser therapy can reduce pain through several mechanisms, including increased secretion of seretonin, increased release of endogenous opiates, and blockage of afferent C fiber depolarization.
Based on the aforementioned properties of laser, the indications for use in veterinary medicine are numerous, but primarily include enhancement of cutaneous wound and tissue healing and amelioration of acute and chronic pain. Because laser is recognized to enhance neovascularization, irradiation of tumors or wounds that may contain cancer cells is contraindicated. Furthermore, lasers pose a known risk to the eye, so irradiation of or near the eye should not be performed. Additional contraindications include irradiation over a pregnant uterus and open growth plates.
Finally, it is important to note that a therapeutic window exists for photobiostimulation. At sub-therapeutic doses, cells will not be stimulated and no reactions will occur; at extremely high doses, detrimental effects can be seen. Interestingly, it is believed that low-level lasers, at any dose, have minimal effect on normal, uninjured tissue. The modulatory effects may also be wavelength specific and vary with other laser parameters such as polarization or pulse frequency.
In conclusion, therapeutic lasers are gaining acceptance in veterinary medicine as an adjunctive tool in wound healing and pain management. Therefore, it is important for practitioners to increase their knowledge in this area in order to make informed recommendations for such therapy. While a plethora of anecdotal reports, case reports and experimental evidence exist in support of therapeutic lasers, large clinical studies are currently lacking. It is anticipated that clinical veterinary studies will emerge in the near future that enhance our understanding of the usefulness of this modality.
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Published on February 7, 2011.