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