Thirty-six out of 38 Thoroughbreds and Standardbreds with subchondral bone cysts, mainly in the stifle, became completely sound within months of having a newly designed prosthesis implanted into their cysts. The horses then resumed their racing careers “as though they’d never had a cyst to begin with,” said Paolo Ravanetti, DVM, of the Equitecnica Equine Hospital in Parma, Italy.

“There was no difference in performance in the racing careers of the treated horses versus horses without bone cysts,” Ravanetti told The Horse. While his study, published in Equine Veterinary Journal, only reports on a small group of horses, in reality the treatment—which he developed in 2017—has led to full recovery in hundreds of horses in Europe, Australia, and the U.S., he said.

Subchondral bone cysts, which create a cavity filled with fluid and/or necrotic material in the bone, can occur in any horse, but they’re most frequent in yearling racehorses, said Ravanetti. Although scientists still don’t know why they develop, many suspect it’s because of leaking synovial (joint) fluid, local inflammation, an injury to the bone, and/or even osteochondrosis. Therapies have included stall rest, joint injections (steroids, stem cells, or plasma products), and surgical repair, including the insertion of a removable metal screw—with the idea that it relieves strain on the cyst area, allowing it to heal. However, no studies have reliably compared the different methods, and the scientific community hasn’t agreed on how to manage these cases, he said.

Ravanetti decided to try a composite screw-shaped implant that would—like the metal screw—provide bone support, as well as encourage bone growth, he said. But because of its composite materials, it could help fill the cavity while providing a matrix structure that would allow the bone to rebuild. And unlike the metal screw, it would get reabsorbed by the horse’s body over time, making removal surgery unnecessary. In theory, new, healthy bone would eventually replace it entirely, he said.

Ravanetti and his fellow researchers tested the procedure in 38 Thoroughbred and Standardbred racehorses aged 10 to 24 months. Most of the horses had subchondral bone cysts in their stifles, but some had them in their pasterns or fetlocks. Surgeons drilled a hole into the bone to reach the cyst and then, after cleaning out (debriding) the cyst area, they implanted a composite screw, sized according to the cyst’s dimensions. Because the surgery requires great precision, the horses underwent general anesthesia during the half-hour operation, and veterinarians took several X rays during the surgery to ensure proper drill and screw positions, Ravanetti said.

After four weeks of strict stall rest followed by a progressive return to exercise over the next two to three months, all but two study horses had returned to grade 0/5 lameness, he said. The 36 healthy horses picked up their racing training where they’d left off, and follow-up reports showed that even up to four years later, the horses continued to run as if they’d never had a cyst. No horses had infections, implant rejection, or cosmetic problems. And overall, the researchers saw that within three months, the cyst area had refilled an average of 77% with new bone. Nearly three-fourths of the horses went on to race, including about half that raced already as 2-year-olds—which are statistics equivalent to horses of the same age without cysts, said Ravanetti.

In addition to the study horses included from 2017 to 2018, more than 200 horses have since benefited from the technique in various hospitals across the globe, he said.

“This is a minimally invasive technique, with no contact with the joint itself, which appears to actually heal the lesion itself and thereby put the horse back on his regular sporting track,” Ravanetti said.