Photo Therapy: AKA Therapeutic Laser

Over the past decade, physical therapy tools have gained a place of respect in the management of equine injuries. As we move toward the 21st Century, a medical philosophy is blossoming based on advice from Hippocrates, a physician who lived in 400 BC. Although he is known for his admonition to doctors to "First, do no harm," Hippocrates also is quoted as having said, "Honor the healing power of nature."

Physical therapy tools stimulate the natural healing power in the cells of the body. One tool uses the energy of light, called photon energy, to stimulate the activity of certain cell components. By using photo energy, the veterinarian or the horse owner has a simple, effective, non-pharmacologic medical alternative. Conservative management of acute and chronic injuries and post-operative wounds has come to include the use of photo energy because it is quickly effective, cost effective, and easy to administer.

What Is Photo Energy?

Light is energy in the form of radiation. It moves in wave formation and is characterized by its wavelength. Light forms a small part of the continuous spectrum we call the electromagnetic spectrum. Other wavelengths are found in this spectrum of radiation-radio and TV waves, X rays, gamma rays, and cosmic rays, to name a few. Therapeutic photo energy utilizes wavelengths that are found just outside the visible red portion of the light spectrum, in the infrared region.

Infrared wavelengths are longer than visible red waves and shorter than microwaves. They are considered to be therapeutic because they are absorbed by components in tissue and blood cells and stimulate normal cell activity that has been disrupted by injury. It is the photon, the energy packet of the light wave, that is the catalyst for increased cell activity; therefore, the therapy should be called photo therapy.

The tools that are popularly used for photo therapy have been victims of mistaken identity. Because helium-neon (HeNe) lasers were used in early research and are the best-documented of all the therapeutic lasers, we see names such as low level laser, infrared laser, cold laser, low level light, and low power laser attached to all photo therapy devices. Not only is there no consensus on the proper name for these tools, but the term laser might not be correctly applied.

HeNe lasers produce a brilliant, visible red light.They require a gaseous medium and a sensitive glass tube for the production of the laser light. They are large, and they are expensive. One such laser recently was priced for the veterinary market at $22,000. They are not used commonly in equine therapy today. The more popular tools available today do not require this type of light emitting mechanism, but use semiconductor diodes. These light-emitting diodes are less expensive and tiny, by comparison.

Some manufacturers of photon energy devices call their light emitting diode a laser diode; some call them superluminous diodes. Laser light is different from all other light in the following ways: It is always monochromatic, or light of one wavelength. It is always coherent and tightly collimated.

Unless the emitted light fits this description, it is not truly laser light.

Although laser is the familiar term, it is not entirely accurate for the popular light therapy used today. And after all, it is not the device itself, but the type of energy produced that creates the biological effect. Perhaps photon therapy would be a more accurate term, as it is the interaction of light photons and molecules within the cell that activates cellular activity.

The importance of the characteristics of coherency and collimation is perhaps the most often debated aspect of photo therapy. Coherency means that the photons, or energy packets, in the beam move in an organized wave formation. The photons do not collide with each other in the beam, which would cancel out some of the energy. A true laser beam contains a high level of energy because of coherency.

But can the photons maintain their organized formation once they encounter solid tissue?

This is a controversial question and has received considerable research attention over the past ten years. Living tissue has its own coherency factor, which is higher than that of the light that is used for therapy. Therefore, it cancels or overcomes the coherency factor of the incoming light, and photons are scattered. If coherency is mostly lost as the light wave passes through the first millimeter of tissue, the target tissue, the nerve, muscle, blood, or connective tissue, absorbs light that is no longer coherent. The interaction between the photon and the cell molecule has nothing to do with coherency. There appears to be no difference between the effects of coherent light and non-coherent light on cell structures.(Karu, 1987)

Collimation means that the light does not spread into a wider and wider beam as it moves farther away from the source. Imagine the light of a flashlight aimed at a wall. The spot of light on the wall is much larger than the end of the flashlight and gets bigger as you move the light source away from the wall. The spot of light also grows dimmer as you move away because the light energy is not as concentrated. A collimated light beam produces a small spot size with no loss of photon energy as the beam travels through space.

But what happens to the beam as it encounters living tissue?

Again, the issue of photon scatter makes one question the need for a true laser. Important features of photo therapy are strength of the light source, or its power, and the wavelength of light emitted.

How Much Photon Power?

Power output has a great influence on the effectiveness of photon therapy. Thinking back to our flashlight example, a bigger battery could produce a brighter spot--a spot with more photon energy. A therapeutic device must be powerful enough to deliver adequate energy to the target tissue to create a therapeutic effect. Too little power will create no effect, but too much power can cause a disruption in cell function, even cell death.

Semiconductor light emitting diodes have gone through a growth spurt in terms of power output and have increased in brilliance over the past 15 years. The term superluminous, really a marketing term, has been applied to this new generation of more brilliant diodes. A therapeutic device offering 8mW peak power per diode 15 years ago now offers 28mW peak power per diode.

Unfortunately, peak power does not tell us much about the actual amount of power delivered to the tissue. The actual power amount is a function of the pulse width (the number of nanoseconds the pulse is on) and the frequency (the number of pulses per second). A device advertising 30W peak power might deliver only 3mW to the tissues due to low pulse frequency. (Ten- to 30-watt lasers are used for surgery. The sale of surgical lasers is regulated.)

When considering which tool to buy, consider the actual average power output. This will greatly influence the treatment time necessary to deliver an effective photon dose. Power density is important to know to determine if units have the power to get photon energy to the cellular level. You have to know the tissue effect of the light.

Which Wavelength?

In photon therapy, wavelength of the light is as important as power output from the unit. Wavelength is another word for frequency, the frequency of the photon's vibrations. Photons, those packages of vibrating energy, are absorbed by molecules in the cells. Molecules, the smallest particles of the cell, are made up of atoms (very small!). The photon wavelength is described in nano meters (one billionth of a meter--that's very, very small!). Molecules are said to be wavelength specific because they will absorb only the energy that matches exactly the energy needed by the electrons they contain.

When the cell molecule absorbs photon energy, its own energy level is increased. This can have the effect of changing chemical bonds or exciting chemical activity in the cell structure. Skin cells will absorb light of shorter wavelengths, in the 632 nm red range. Infrared waves are longer, around 810nm, and will penetrate to deeper tissues. Acupuncture point stimulation has been done with longer wavelengths. As we learn more about the nature of these points, we might learn the exact wavelength for optimal stimulation.

Several photo therapy devices offer a single semiconductor diode probe designed for acupuncture point stimulation. We are standing at the threshold of a new frontier, a frontier that is so tiny it cannot be seen with an ordinary microscope. Many controlled studies now underway are looking at the effectiveness of semiconductor diodes. As we learn how to make the best use of these tools, we should be glad for the safe, painless, sterile, and effective opportunity to diminish the effects of injury that these tools provide.

Photon Therapy Tools

Some photon therapy devices offer multi-diode arrays, or 'cluster heads,’ where many diodes are grouped together, increasing the photon density. One device offers four pads with 60 superluminous diodes per pad. Each diode has an average power of 10mW, giving this device considerable output power. These pads can be wrapped in place, freeing the veterinarian or therapist to apply massage or some other therapeutic modality to associated trigger points.

Multi-diode arrays cover a greater surface area, making the treatment of a large wound more efficient. Because of the lack of collimation, the power density from semiconductor diodes decreases with increasing distance between the diode and the target tissue (Baxter, 1994). By using a wrap to hold the cluster pads in place, the veterinarian or therapist knows that the photon energy is in contact with the target area, the most efficient method of delivery. Photon density also is diminished if the light source is not held perpendicularly to the skin surface. When using a cluster head, always maintain contact with the skin surface and apply the light perpendicularly to the skin.

What Does Photon Energy Do?

The most common sports medicine uses for photon therapy are in treating myofascial pain, joint pain, edema, and open wounds. New research might help explain the mechanism of action when photons and cell components interact. A gas molecule (identified in the 1980s) called nitric oxide might provide an important piece of the cellular physiology puzzle. This gas molecule plays a role in intercellular signaling, the cell to cell communication that sets in motion such physiological processes such as vasodilation, immune response, and neurotransmission.

Wound repair relies on local capillary vasodilation to bring oxygen and nutrients to the cells and to carry away waste products of injury. Nitric oxide was first identified as a natural relaxing factor for vascular smooth muscle. A study, using humans as subjects, sought to determine if nitric oxide levels in the blood could be raised by applying photon energy (Horwitz and Burke). A 60 diode cluster pad, delivering 810nm, monochromatic infrared light, was applied to a human subject's arm for 30 minutes. In blood drawn from the experimental area, nitric oxide was seen to have elevated 16-25% on the treated limb, compared to 8-18% on the control limb.

To study the systemic effects (effects on the body in general) of monochromatic infrared light, pads were placed over the jugular veins. Blood was again drawn from the arm. After 30 minutes of treatment, blood and plasma levels of nitric oxide were seen to increase in both forearms, sampling sites that were remote from the site of light application.

Proteins in the blood absorb photon energy very effectively, as can be observed in the photo on the facing page. Hemoglobin, the protein molecule that transports oxygen to the tissues, also carries nitric oxide (Horwitz and Burke). The researchers proposed that photon energy releases nitric oxygen from hemoglobin, resulting in better tissue nutrition and less ischemia. More research is needed toknow how nitric oxide stimulates tissue nutrition.

Photon therapy is most impressive when it is used to treat wounds that either have failed to heal or worsened during conventional medical treatment. In our clinical observation, infection in a wound resolves quickly and closure proceeds without an accumulation of proud flesh. Photon therapy might have a bactericidal effect, but also appears to enhance the patient's 'host response’ by controlling infection through the immune system.

Proud flesh is the over-production of collagen from cells that have differentiated due to injury. These cells normalize and become healthy skin cells, no longer forming a barrier to wound closure. Therapists, including myself, have treated many superficial wounds on horses that have all responded well to photo therapy. Many of the wounds had persisted for months and were infected. Even such severe cases responded to photo therapy, healing with strength and little scar tissue. To stimulate wound closure, photo therapy has the advantage over electrical stimulation in another effective way. Photo therapy cannot be felt by the horse. This makes treating wounds on foals or fractious horses safer for horse and handler.

Skin cell malformation, such as lick granulomas on dogs, can be treated successfully with photon therapy. Although lick granulomas are not problems faced by horses, the cell differentiation is similar to proud flesh and prevents wound closure. Lick granulomas are difficult to manage and often do not heal despite bandaging and aggressive medical treatment. With photon therapy, the dog's behavior of relentless licking has shown to cease in one or two treatments. A recent reported case involved a deep wound with a four by seven centimeter surface area. Within 10 treatments, given every other day, the wound closed to an area of one square centimeter and was not as thick. Only monochromatic infrared light was used in treatment of this wound.

A further mechanism of action in the story of photon-stimulated wound repair includes the production of collagen. Collagen, the cells from which tissue is formed, is produced by fibroblasts. Fibroblasts increase in number under the influence of photon energy (Mester, 1989). Collagen provides the healing tissue with increased tensile strength and resilience, enabling it to withstand the stresses of movement. Wounds treated with photon energy do not break down over time and have less scar tissue to weaken them.

Many injury problems respond well to photon therapy, but of particular interest to me is laminitis. According to experts on this disease, when the acute stage of laimintis has subsided, an important element in limiting damage to the foot involves enhancing circulation to the hoof tissues. Once again, the nitric oxide molecule is thought to come into play. If we stimulate the release of nitric oxide, the horse's own vasodilating mechanism could be activated. Nitric oxide might be the molecule responsible for the effects of nitroglycerin in treating the post traumatic laminitis. Chris Pollitt, BVSc, PhD, points to this relationship by explaining that nitric oxide is now known to be the universal and most potent of the body's natural vasodilators. It is manufactured from argenine, a simple amino acid, by cells lining all blood vessels. Nitroglycerin creams or patches act as an external source of nitric oxide with the effect of rapid vasodilation, he said. After the metabolic crisis of laminitis is over, nitroglycerin is used to aid healing.

But administering the patches or creams poses human risk.

Rick Redden, a veterinarian and world renowned laminitis expert, reports that despite wearing two pairs of rubber gloves, he felt an immediate and severe headache and increased heart rate while working on a horse which had been treated with nitroglycerin cream. If infrared light can be used to release nitric oxide from hemoglobin, this risk could be avoided.

The therapeutic approach I use is to place a cluster pad over the coronary band in front and on the lateral and the medial sides of the lower leg, over the digital artery and vein. The coronary band is dense with capillaries that nourish hoof tissue. The digital artery is the main vessel that perfuses the foot. This setup allows these important structures to receive photon energy directly. Treatment is given for 30 minutes every day, or twice per day if the condition is severe. After the discomfort has subsided, which could be in as few as three or four treatments, the treatment frequency can be reduced to every other day. Photon therapy also can be used on acupuncture points in the musculature of the hips and back, which often are sore in laminitic horses because of the animal's shifting weight off the front feet.

In our experience treating several horses with chronic laminitis, we observed a normalization of the digital pulse and a reduction in the horse's discomfort when standing. (This was not a scientific test, merely observations of actual horses.) The horses spent less time lying down and were able to tolerate turnout. Every horse we have treated was able to have Butazoladin eliminated from its care regimen. One stallion could not tolerate walking on the asphalt barn aisle before treatment. He willingly did this after two weeks of photon therapy and was often seen trotting along his fenceline, something he had not done for years.

Often our patient is a mare with a history of increased discomfort from laminitis as pregnancy advances. With photon therapy to the coronary band for 30 minutes three times per week, we have seen an increase in comfort level. This therapy has been of great benefit to these mares.

Because photon therapy devices are easy to use and are sold to the lay horse owner, no doubt many treatments are given at incorrect dosages and to undiagnosed conditions. For effective use of these valuable tools, one must seek a complete veterinary evaluation and diagnosis before therapy is begun.

Perhaps the biggest danger in using these devices is that one will mistake symptom relief as cure for the problem. Unfortunately it is all too common to see a trainer or owner resume the horse's training before the tissues have had time to heal sufficiently. Photon energy quickly reduces pain and tissue swelling, but time still is needed for cell repair to be complete and for tissue maturation to take place. The strength that comes from maturation of the repair allows the injury to withstand the stress of athletics.

Dosage of light therapy is determined by the power available from the unit and duration of treatment. The effects of photon therapy are cumulative, but each treatment must be sufficient to cause biological changes. Repeated doses of the appropriate strength give beneficial results over a wide range of conditions. Many of the devices available offer treatment manuals that give doses for various problems. A reputable company also will have someone with whom you can discuss treatment protocols and work out appropriate treatment dosage. Remember, light therapy should only be used on a horse that has been diagnosed by a licensed veterinarian and used by a trained therapist or horse professional.


Karu, T. Photobiological Fundamentals of Low-Power Laser Therapy. J of Quantum Electronics. 23-1703-1713. 1987.

Mester, AF, Mester A. Wound Healing. Laser Therapy. 1:7-15. 1989.

Horwitz, LR, Burke, TJ. Accelerated Wound Healing with Monochromatic Infrared Light: Possible Mechanism of Action. unpublished.


  • Light is energy in the form of radiation.
  • Therapeutic photo energy utilizes wavelengths that are found in the infrared region.
  • Infrared wavelengths are longer than visible red waves and shorter than microwaves. They are considered therapeutic because they are absorbed by components in tissue and blood cells and stimulate normal cell activity.
  • The photon is the energy packet of the light wave and the catalyst for increased cell activity.
  • Laser light is monochromatic, or light of one wavelength.
  • Laser light is coherent, meaning that the photons, or energy packets, in the light beam move in an organized wave formation.
  • Laser light is collimated, meaning that the light does not sperad into a wider and wider beam as it moves farther away from the source.
  • Power output has a great influence on the effectiveness of photon therapy. Too little power will create no effect and too much power can cause a disruption in cell function -­ even cell death.
  • Power is a function of the pulse width (number of nanoseconds the pulse is on) and frequency (number of pulses per second).
  • Molecules are said to be wavelength specific because they absorb only the energy that matches the energy needed by the electrons they contain.

Because photon therapy devices are easy to use and are sold to the lay horse owner, no doubt many treatments are given at incorrect dosages and to undiagnosed conditions. For effective use of these valuable tools, one must seek a complete veterinary evaluation and diagnosis before therapy is begun.

About the Author

Mimi Porter

Mimi Porter lives in Lexington, Ky., where she has practiced equine therapy since 1982. Prior to that, she spent 10 years as an athletic trainer at the University of Kentucky. Porter authored The New Equine Sports Therapy, published by Eclipse Press and available at or by calling 800/582-5604.

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