Tendon injuries, ranging from mild sprains to complete tendon ruptures, occur commonly in sport horses. With so many therapeutic options, the goal of rehabilitation is to achieve optimal functional healing using treatments and controlled exercise.
“I am seeing more and more superficial digital flexor tendon (SDFT) injuries in sport horses, especially jumping horses with anything from microdamage to complete tissue breakdown,” said Duncan Peters, DVM, Dipl. ACVSMR, Cert ISELP, of East-West Equine Sports Medicine LLC, in Lexington, Kentucky, during his presentation at the Northeast Association of Equine Practitioners Convention, held Sept. 21-24, 2022, in Saratoga Springs, New York.
Despite tendon injuries being so common, Peters acknowledged that we, as an industry, expect sport horses to have long careers, raising the question, “Can the tendons keep up?”
“Tendon maturation occurs around 18 to 24 months of age, and these are the strongest tendons a horse will probably have in their lifetime,” said Peters. “Tendon strength and elasticity slowly decrease with age, beginning at about 5 years of age. At this point, horses begin experiencing tendinous microdamage. The SDFT supports the fetlock. It stores elastic energy that allows for more efficient movement, working with the common digital extensor tendon so that minimal energy is needed to move the leg forward.
“Tendons like a ‘pull’ on them, but they do not like sudden impact,” he continued. “So, the hoof hitting the ground is potentially the biggest factor causing damage to the SDFT. The SDFT is like a rope that is supposed to hold 75 pounds. But if you put a 100-pound weight on it and drop it … the rope is going to fail.”
So why aren’t the tendons repairing the microdamage? Are we asking our older horses to do more? “Probably not,” Peter said. “And we are seeing minimal signs of that microdamage in terms of exercise performance, so there are no indications that we need to back off until there is a problem.” The mechanisms involved in tendinous micro-breakdown are unclear, he added.
Risk Factors for Injury
As noted, increased age is a risk factor for SDFT injuries. Peters said conformation, fitness, and underlying metabolic issues such as pituitary pars intermedia dysfunction (PPID, aka equine Cushing’s disease) or equine metabolic syndrome are also risk factors contributing to tendon injuries.
“If a horse is not fit for their task or is fatigued, then they are more likely to be injured,” he said. “Fatigue can be due to overtraining or even just after trailering and being at a competition away from home for a few days.”
The footing a horse works on can also increase SDFT injury risk. “Uneven loading of the fetlock due to uneven ground, synthetic, or ‘grabby’ footing can be hard on horses,” Peters explained. “If jumpers aren’t able to slide as they land after a jump, there is an excess spike of force. And without the slide after the jump the horses tend to go faster. They put more torque on the tendons.”
Diagnosis Is Typically Definitive
Peters politely pointed out that diagnosing tendon injuries isn’t typically challenging but does require a thorough physical examination and digital palpation.
“Use a systematic and attentive approach to identify subtle changes in enlargement, surface temperature, and retraction response,” he said. “You’d be surprised that there is sometimes very little swelling. Even with complete rupture of the SDFT, they are remarkably not painful. I see horses that are much more painful, a Grade 3 out of 5 lame, with a small core lesion than complete rupture.”
No Recipe for Successful Healing
Peters said in his experience, many treatment modalities make a lot of promises, but there is no cookbook to refer to for appropriate and guaranteed successful tendon healing. He recommends considering the three phases of healing and what to do during each:
1. Acute, inflammatory phase of healing. This usually lasts seven to 14 days following the injury.
“You want to decrease inflammation, minimize mechanical stress with rest, and initiate the reparative process,” advised Peters.
Cold therapy, bandages/support, and anti-inflammatory medications are all appropriate during this time frame. Peters is not, however, a proponent of injecting the area too early in the healing process.
“Get the heat and pain out instead,” he said. “Don’t add good soup to bad soup.”
2. Subacute, fibroblastic phase of healing. This extends from about three to 10 weeks following the injury, during which time we want to optimize “functional” healing.
“Use all your modalities here,” said Peters. “Laser, shock wave, hydrotherapy, platelet-rich plasma (PRP) for scaffolding, interleukin-1 receptor agonist protein (IRAP), stem cells for directing repair process, amnionic products, alpha 2 macroglobulins … this is where the ‘art’ of healing comes in.”
This is also typically the time to start a graduated exercise loading program.
3. Chronic, tissue maturation phase of healing. This is the bulk of the reparative process, taking up to 12 or more months.
Peters said during this time we need to embrace the four Rs of rehab: reducing pain, restoring range of motion, restoring strength, and reducing re-injury risk.
In this extended phase of healing, the veterinarian should reevaluate the horse’s preparedness to return to competition frequently. Adjustments to the treatment plan can be made during those exams based on the client’s desires, time, and economics.
Monitoring Injury Long-Term
After the horse is cleared to return to work, how do we monitor his risk of re-injury?
“There is currently no way,” said Peters. “We need this!”
Solidifying this point, Peters ended his presentation relaying the story of a grand prix jumper with a marginal lesion of the proximal SDFT. The horse was treated appropriately and extensively, coming back 10 months later to jump clean and sound around a grand prix. Two weeks later, he completed another clean grand prix—finishing with another SDFT lesion proximal to the original lesion.
In short, Peters said, success is never guaranteed.