Think the number of innovative new therapies for tissue healing in horses can't get any higher? Think again. Kim A. Sprayberry, DVM, Dipl. ACVIM, an internal medicine specialist at Hagyard Equine Medical Institute in Lexington, Ky., introduced attendees of the Oct. 18-19 National Thoroughbred Racing Association Safety and Integrity Alliance held at Keeneland Race Course in Lexington, Ky., to a new tissue healing system currently undergoing testing for efficacy in equine tendons.
How Horses Move
Sprayberry discussed briefly how the horse's body moves and which body parts are stressed most as movement is initiated and the horse proceeds to work at speed.
"Think of the hind end as the motor, the source of propulsion," Sprayberry said. "The shoulders and forelimbs support approximately 60% of the horse's weight during standing, and their soft-tissue attachments to the trunk mean they are designed to accept and absorb the body weight as it is thrown forward by the hind end."
Specifically, she noted, the horse's fetlocks act as shock absorbers as the front legs absorb the weight, store it as potential energy in the elastic ligaments and tendons, and return it to the moving forelimbs as kinetic energy (the energy in motion).
In healthy horses moving in a balanced manner, the body will handle this stress on the forelimbs with little trouble. But if a horse is moving in an unbalanced manner or if his body is fatigued, the fetlocks will stretch to the extent that the pasterns lie parallel to the ground, rather than holding the more upright angle they generally do - see any finish line racing photograph and look at the weightbearing limb when one limb is bearing weight and the other three are airborne. This stress subjects the horse's sesamoid bones to extraneous ground forces and causes the superficial digital flexor tendon (SDFT) to stretch further than its innate elasticity has the capacity to withstand.
"The SDFT is located on the back of the forelimb," Sprayberry explained. "It is a straplike structure that is thinnest and weakest in the middle third of its length, which is also where the blood supply is most sparse."
This feature of the tendon makes injury healing difficult, as healing necessitates good blood flow to the damaged area. The top and bottom thirds of the structure are more substantial and have a better blood flow, she noted.
Tissue Healing Methods
Sprayberry explained that the traditional methods of managing SDFT injuries, including ice or cold therapy, bandaging for light support, administering nonsteroidal anti-inflammatory drugs (such as phenylbutazone or flunixin), restricting exercise, and rest, still hold. But in addition to these inflammation-limiting measures, it is increasingly common to look at what can be done to promote the actual healing phase, and this is where regenerative medicine finds application.
She discussed some of the more recently developed therapeutic options, including stem cell therapy, platelet-rich plasma, growth factors, and scaffold-based healing; all the aforementioned therapies have been investigated and some are actively in use for healing equine tendon injuries. Anecdotal evidence is abundant for some of these treatment modalities, and they are the focus of a great deal of research as treatment options for horses with tendon injuries.
Finally, Sprayberry introduced attendees to a new therapy currently undergoing research to potentially help heal equine tendon injuries. Nonsurgical exogenous crosslinking therapy (NEXT) has been shown to stabilize degenerative vertebral disc disease in other species. It works by providing a form of internal bracing to collagen tissues (the main substance of which tendon is composed)--in the case of a degenerating or collapsing disc, injecting the material "plumps up" the disc and helps it retain normal configuration while also increasing the flow of nutrients into the tissue and waste products out of it. These effects are important in relatively avascular tissues such as tendon collagen. Sprayberry explained the complex process behind NEXT, beginning with the agent the treatment uses: a plant-derived substance called genipin. She relayed that the protein crosslinking activity of genipin confers a high tensile strength when injected into collagen.
The protein crosslinks--which Sprayberry noted begin forming immediately after injection--help bind the edges of the lesion together, add strength to the bonds between individual collagen bundles inside the tendon, and support the body's own healing mechanism.
In other species and in in vitro (in the lab) administration in equine tendons, NEXT has been shown to increase tissue strength while not interfering with tissue flexibility, Sprayberry said.
While NEXT hasn't yet been used in horses, Sprayberry, among other veterinarians, is optimistic that further testing will confirm the treatment as a viable therapeutic option for tendon lesions on horses.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.