Determining that a tendon has healed completely is a challenge, as is knowing when to initiate therapy (beyond rest). But as imaging technology improves, so do scientists’ opportunities to meet these challenges. According to Italian researchers, the “power Doppler” (which visualizes amplitude, or power, of Doppler signals, rather than frequency) provides highly sensitive “views” of the equine tendon healing process, with better results than those provided by less-advanced Doppler ultrasound technology.
“Power Doppler is a useful tool for clinicians to assess the severity of the injured tendon, establish if the lesion is acute or chronic, and help to choose the right time for any therapy (particularly for biological therapy like stem cell therapy and PRP),” or platelet-rich plasma, said Luca Lacitignola, DVM, PhD, an associate professor of veterinary radiology at the Universita degli Studi di Bari “Aldo Moro,” in Valenzano, Italy.
“It also permits them to monitor the healing process after therapy and consequently modulate the post-treatment training program as needed,” he said.
Blood Flow: The Biology of Tendon Healing
Healthy tendons have very little blood in them, said Lacitignola. Because tendons maintain their structure and elasticity in a stable environment that requires only minimal oxygen and nutrients from flowing blood, blood vessels are few and far between.
However, when a tendon sustains damage, those requirements change. The ruptured tissue can’t reconnect and rebuild properly without help from the circulatory system, which transports the aforementioned oxygen and nutrients necessary for healing. Therefore, the body responds to tendon damage by creating special blood vessels that channel blood flow into these tendon areas that didn’t have blood vessels before the injury. That process, Lacitignola said, is known as “neovascularization” (new blood vessel creation).
When the horse suffers a tendon injury, he experiences neovascularization as well as pain in the injured area due to the tissue damage, Lacitignola said. While the neovascularization doesn’t cause pain, it’s associated with pain—simply because they happen at the same time.
“Neovascularization and pain both gradually reduce over the healing process,” said Lacitignola. “But if a chronic lesion persists, neovascularization is still detectable,” even if pain is not.
The Search for Ongoing Injury: Looking for Flowing Blood
By “detectable,” Lacitignola said he means Doppler technology can pick up the blood flow coming through those new—and theoretically temporary—blood vessels. In other words, if scientists detect blood flow in tendons through Doppler, they know the tendon is still in the healing process, he said.
“In a healthy tendon, because of scant blood supply, the power Doppler signal is low or undetectable,” said Lacitignola. “But if there’s injury, the signal reveals the quantity of blood flow.”
Riding on the Power of Color Doppler
In their study, Lacitignola and his fellow researchers ran power Doppler and grayscale ultrasound analyses on 10 healthy horses and 25 horses with confirmed tendinopathies. As anticipated, the power Doppler gave readable signals in injured tendons but no signals in healthy tendons, indicating (a healthy) absence of blood flow.
While the ultrasound alone provided feedback about tissue integrity, it was the Doppler that gave data on pulsating blood, Lacitignola said. The power Doppler was particularly useful in evaluating tendon neovascularization because it reads the power coming off the signal of color Doppler technology.
“Power Doppler is based on color Doppler physics, but they’re not the same,” he explained. “Power Doppler is based on the energy of the color Doppler signal, but it’s independent of the direction of the signal and is more sophisticated and sensitive than color Doppler alone.”
Color Doppler provides information about blood flow direction, which isn’t important when evaluating tendon healing, he said. What matters is the intensity of the flow.
“We paid attention to the presence or not of neovascularization,” he said. “We also evaluated the quantity of the neovascularization, assigning a score. The score is correlated to the severity of the lesion.”
Respecting Time off to Avoid Re-Injury
Knowing when to restart exercise after a tendon injury is a critical component of ensuring tendon health, said Lacitignola. The risks of getting back to exercise too soon—even when the horse seems to be sound again—are significant.
“Tendon injury is one of the high-rate reoccurring diseases in equine orthopedics,” he said. “When you suspect a tendon injury, call your vet soon so that your horse can get the best diagnostic and therapeutic options. And meanwhile, you need to commit to giving your horse the time he needs to recover.”