Superficial digital flexor tendonitis (inflammation of the tendon that runs down the back of the leg from just above the knee or hock to the pastern) is a common injury in horses. Its prevalence in Thoroughbred racehorses, for example, ranges from 4-13%.

Reinjury rates are high—up to 53% in the literature—so veterinarians often turn to orthobiologic therapies such as platelet-rich plasma (PRP), autologous conditioned serum (ACS), and mesenchymal stem cells to try to improve healing. Another biologic product, autologous protein solution (APS), might hold promise, as well, but until now researchers had not studied its efficacy for treating these lesions. Previous studies have only assessed its use as a joint therapy.

Angela M. Gaesser, DVM, Dipl. ACVS, veterinary surgeon and research fellow at the University of Pennsylvania School of Veterinary Medicine, in Kennett Square, recently studied the effects of APS (Pro-Stride) on healing in tendonitis cases. She presented her team’s findings at the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee.

When tendons get injured, the resulting repair tissue is often inferior in strength and structure to healthy tissue, Gaesser explained, making it highly susceptible to reinjury. This is one of the reasons veterinarians reach for orthobiologics to improve healing characteristics.

“Several studies have shown these treatments (specifically PRP, ACS, and mesenchymal stem cells) can result in superior biomechanical, biochemical, and histological healing,” she said.

In theory, APS should do the same, because it combines the beneficial effects of PRP and ACS and can be processed stall-side within 30 minutes.

In their study, Gaesser and her team documented the prognosis of tendonitis induced with collagenase in the forelimbs of eight Thoroughbreds. Next they administered an intralesional APS injection in one leg of each horse and a placebo in another, serving as the control.

The horses remained on stall rest for eight weeks, after which they were hand-walked for five minutes a day until the conclusion of the 12-week study. A blinded clinician performed ultrasound examinations of both treatment and control tendons immediately before tendonitis induction and at two-week intervals until Week 12.

Based on the study results, Gaesser said ultrasonographic scores were not significantly different between treatment and control groups. Biomechanical testing (for tendon strength and elasticity) and histology scores on tendon samples did not differ significantly either.

Collagen type III expression, however, was significantly reduced in APS-treated tendons, which could indicate superior healing, said Gaesser. DNA content was significantly lower in treated tendons, as well, which might indicate later stages of healing than the control tendons, she said.

“In summary, intralesional injection of APS in this study in experimentally induced tendonitis resulted in some improvements in healing characteristics,” Gaesser said, adding that further studies are needed to determine which orthobiologic product works best for tendonitis cases.