History and Physics

Einstein first envisioned the concept of laser radiation back in 1917. However, it was not until 1960 that the first laser was built by Theodore Maiman. It was another 25 years or so before technology advanced enough to make lasers safer, easier to use, and cost effective. Dr. Endre Mester is credited with the discovery of the biostimulative properties of red and near infrared light.

LASER is an acronym that stands for "light amplification by the stimulated emission of radiation." All lasers work in a similar manner. You must have a medium of some sort that is made up of atoms capable of reaching a metastable or "excited" state. The chemical medium will dictate the wavelength of light that is produced and the wavelength will dictate what function this particular laser is best suited. We are going to discuss medical lasers only and therapeutic lasers specifically.

Laser light in the red and near-infrared range has biostimulatory properties. Roughly, this corresponds to wavelengths between 600 nm and 1100 nm. The shorter wavelengths are absorbed more superficially and therefore do not have the ability to penetrate as readily as the longer wavelengths. From absorption spectra data we also know that the wavelengths nearer the 800 nm range (750–830) are near the peak of absorption for the cytochrome C oxidase reaction. The wavelengths near the 980 nm range have moderate increased absorption by water. This can create some thermal effects and blood will flow along these thermal gradients. It is also near the peak of the Hb dissociation curve. Therefore having multiple wavelengths may give you a wider range of treatment options and/or a synergistic effect which could result in better clinical outcomes.

We have emphasized the importance of wavelength for proper clinical applications. The second most important parameter that will dictate how effective a laser will be is the dosage. Power is the rate at which laser energy is being delivered and is measured in watts (W). There are four things that happen to the laser energy when it strikes tissue. Some energy will be reflected back. Some will penetrate all the way through without interacting with the cells. Some will be scattered. Finally, only about 20–40% actually gets absorbed by the tissue at the cellular level to have a biologic effect. Power watts or mW multiplied by time (seconds) will dictate the total amount of energy that is delivered to the tissue. This will be the dosage and is measured in Joules (J). There is a certain amount of energy or dose that is needed to elicit a clinical response. This dosage must be able to reach the target tissue. This aspect of penetration and dosage is an important concept which many texts fail to address when discussing proper laser treatment parameters. The success you will have with the Class IV therapeutic laser is a result of the wavelength of energy and the power (dosage) which it can deliver. Having adjustable power and pulsing frequencies gives you the versatility to treat a wide range of clinical conditions both superficial (dermatologic) and deep (musculoskeletal/ neurologic).

Two other parameters to keep in mind when treating patients. The first is what we call the time domain of the laser. This is related not only to hand speed during laser application but also to the pulsing frequency or "strobe" effect of how the laser is emitted. The more important thing is the pulsing rate of delivery by the laser. The pulse rate with which the laser is being delivered will have differing physiologic effects on tissue. Lower pulse rates and continuous wave for example are better for pain modulation while higher frequencies are more anti-inflammatory. Keep in mind that the protocols already set up in most therapeutic lasers simplify all these parameters in an easy to use "point-and- shoot" technique.

The final parameter is the target tissue. Hair and skin color and hair coat thickness can affect laser penetration. The shorter wavelengths in the near infrared still have some absorption in melanin. The darker animals will absorb some laser energy in the skin or coat. We need to be cautious of the superficial effects to avoid thermal injury that can occur to the coat and, rarely, to the skin. Even most dark-coated animals have light skin. In those few animals that actually have pigmentation in the skin, we need to watch for signs of discomfort if using very high powers in direct contact. As long as the proper scanning technique is used, there is little potential for any superficial irritation. The more important aspect to keep in mind is that the absorption in the skin is reducing the dosage delivered to the deeper tissues.

Photobiomodulation

Healing begins at the cellular level. Cellular chromophores within the mitochondria absorb the laser energy. Specifically, it is the cytochrome c oxidase and NADH that absorb the 500–1100 nm wavelengths. This causes an activation of the respiratory chain leading to an increased synthesis of ATP. In addition, reactive oxygen species such as NO and SOD are produced and there is a shift in the redox state. A cascade of secondary effects can then take place including DNA and RNA synthesis; activation of fibroblasts, macrophages, and lymphocytes; growth factor release; neurotransmitter release; vasodilation; collagen synthesis; improvement of cell membrane permeability and function of the Na+/K+ pump. Increased metabolic activity will increase oxygen and nutrient availability which leads to enhanced protein and enzyme production. These factors will accelerate/stimulate cell reproduction and growth which leads to faster repair of damaged tissues, moderate the inflammatory response, and provide analgesia.

These cellular reactions result in three major clinical benefits for the patient: pain reduction; inflammation reduction including swelling, edema, and bruising; and accelerated tissue healing. These events are often happening simultaneously and naturally complement each other. Inflammation is a result of both vascular and cellular consequences. The laser moderates inflammation by the following actions: there is production of NO along with other mediators which stimulate vasodilation. This facilitates removal of cellular debris along with activation of the lymphatic drainage channels; but, more importantly, it reduces ischemia and all the negative events associated with a negative oxygen balance in tissue. Angiogenesis is stimulated as well, which increases oxygen and nutrient transport to improve tissue repair and therefore reduce inflammation. Production of SOD and other ROS helps stabilize cellular membranes and balance the detrimental effects of free radical activity.

Enhanced WBC activity aids the removal of cellular debris. Lymphocyte activity is mediated to give a beneficial response between the T-helper and T-suppressor cells. There is increased production of PGI2 which has anti-inflammatory activity similar to our COX inhibitors. At the same time there is a reduction in interleukin 1, a proinflammatory cytokine.

Reducing inflammation will have a measurable effect on the level of analgesia. There is also direct pain relief by the release of endogenous endorphins and opioids both locally and centrally. Laser irradiation suppresses the depolarization of the afferent C-fibers. It helps restore the action potential of the damaged nerve back to the normal healthy level of -70 mV, thereby increasing the stimulus needed to produce a painful response. This relates to our attention to preemptive analgesia and preventing the "wind-up" effect. There is a reduction in bradykinin levels. Axonal sprouting and nerve regeneration will occur and will alleviate the pain associated with the damaged tissue. In addition, the accelerated repair process in general will reduce pain sensitivity related to structural stress and imbalance.

All the aforementioned mechanisms that help reduce inflammation, stimulate angiogenesis and vasodilation, and aid in clean-up of cellular debris, will also increase oxygen and nutrient transport to the area to accelerate tissue repair. Increased ATP production will accelerate cellular function including growth and reproduction. Fibroblast proliferation and collagen synthesis are enhanced and more organized, which leads to a reduction of scar tissue and improved tensile strength. There is more rapid epithelialization. Cellular differentiation and maturation increases the number of osteoblasts, myofibroblasts, and other muscle-regenerating precursors. Class IV Laser therapy can reduce healing times by 30–50%. It is not just faster healing; it's better healing!

Our goal for "pain management" is not to just make the patient comfortable. We want to get the patient back to their normal activity. Restore ROM and improve muscle strength and function. Become an active member of the family again. With a Class IV therapeutic laser in your armamentarium, you have the best chance of achieving these goals.