Laser Therapy: No One-Size-Fits-All for Horses

Scientists found that the amount of laser light reaching the tendon depends substantially on the horse’s hair and skin—in particular its thickness and color.

Photo: Courtesy Respond Systems Inc.

Low-level laser therapy (LLLT) could help treat tendon injuries in horses, but recent research has revealed that laser dosing is definitely not “one-size-fits-all.” Scientists found that the amount of laser light reaching the tendon depends substantially on the horse’s hair and skin—in particular its thickness and color.

“Different substances absorb laser light to different degrees, and one type of substance (that affect absorption) is skin pigment melanin, which causes darker skin pigmentation,” said Katja Duesterdieck-Zellmer, DrMedVet, MS, PhD, Dipl. ACVS, an associate professor of large animal surgery at the Oregon State University College of Veterinary Medicine, in Corvallis. “I have felt for a long time that there must be differences in how much of the laser light gets through skin depending on whether it has a lot of, or very little to no, pigmentation.”

As it turns out, Duesterdieck-Zellmer’s suspicions were well-founded. In her group’s study on cannon bone skin from 19 cadavers, she and her fellow researchers found that the darker the horse’s skin, the less laser light that gets through to the underlying tissues. That was also true for the horse’s hair color—the darker the hair, the less laser light that gets past the melanin “barrier.” While shaving the hair off certainly improved the penetration rate, the base color of the skin under the shaved hair could still hamper the laser light.

“It’s like sun light shining onto an asphalt road,” she said. “The black part of the surface gets much hotter than any white markings on the road. This means that sunlight is absorbed to a much greater degree by the black tar than by the white paint.”

Because the sunlight—or laser light, as the case may be—gets absorbed by the black surface, there’s less remaining light to reach the structures below it.

Perhaps less surprisingly, Duesterdieck-Zellmer said, their research also showed that skin thickness affects laser light penetration, with less light getting through thicker skin than thinner skin. But this is less of a concern in horses, she added, since leg skin thickness doesn’t vary much from one horse to another.

She recommended veterinarians consider that they'll likely have to adjust the laser energy settings depending on the skin and hair pigmentation of the target area. That being said, the goal is not to have maximum laser strength arriving into the tendons. “The therapeutic window of laser energy doses is rather narrow,” she said.

Generally speaking, despite some reports of its benefits in humans and dogs, LLLT needs further research in horses before it can be considered an effective treatment strategy, Duesterdieck-Zellmer added.

“Currently, there is no peer-reviewed publication that I know of that shows clinical efficacy of laser therapy in horses for any condition,” she said. “We have much to learn about laser therapy.”

The study, “Ex-vivo penetration of low-level laser light through equine skin and flexor tendons,” was published in the American Journal of Veterinary Research

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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