Tendon injuries, ranging from minor sprains to complete tendon ruptures, are common in sport horses. With so many therapeutic options, the goal of rehabilitation is to achieve optimal functional healing through treatments and controlled movement.
“I’m seeing more and more superficial digital flexor tendon (SDFT) injuries in sport horses, especially show jumpers, which can range from micro damage to complete tissue breakdown,” said East-West Equine’s Duncan Peters, DVM, Dipl. ACVSMR, Cert ISELP Sports Medicine LLC, of Lexington, Kentucky, during its presentation at the Northeast Association of Equine Practitioners Convention held September 21-24, 2022 in Saratoga Springs, New York.
Although tendon injuries are so common, Peters acknowledged that as an industry we expect sport horses to have a long career and asked 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 is likely to have in its lifetime,” said Peters. “Tendon strength and elasticity slowly decreases with age, beginning around age 5. At this point, horses begin to suffer tendon microdamage. The SDFT supports the shackles. It stores elastic energy that allows for more efficient movement by working with the common digital extensor tendon so that minimal energy is required to move the leg forward.
“Strings like a ‘pull’ on them, but they don’t like a sudden impact,” he continued. “So the hoof touching the ground is possibly the biggest factor causing damage to the SDFT. The SDFT is like a rope meant to hold 75 pounds. But if you put a 100-pound weight on it and drop it… the rope will fail.”
Why don’t the tendons repair the microdamage? Are we asking more of our older horses? “Probably not,” said Peter. “And we’re seeing minimal evidence of these microdamages in terms of exercise performance, so there’s no indication that we need to back down until there’s a problem.” The mechanisms involved in tendon microdegradation are unclear, he added .
risk factors for injuries
As previously mentioned, older age is a risk factor for SDFT injury. Peters said that conformation, fitness and underlying metabolic issues such as pituitary pars intermedia dysfunction (PPID, also known as Cushing’s disease in horses) or equine metabolic syndrome are also risk factors contributing to tendon injuries.
“If a horse is unfit or tired for its job, it’s more likely to get injured,” he said. “Fatigue can be due to overtraining or even just after towing a trailer and a few days at a competition away from home.”
The surface a horse works on can also increase the risk of injury from SDFT. “Uneven pastern loading due to uneven ground, synthetic or ‘grippy’ foot can be hard on horses,” explained Peters. “If jumpers are unable to slide while landing after a jump, an excessive force spike is created. And without the slide after the jump, the horses tend to run faster. They apply more torque to the tendons.”
The diagnosis is usually definitive
Peters politely pointed out that diagnosing tendon injuries isn’t usually difficult, but does require a thorough physical exam and digital palpation.
“Use a systematic and vigilant approach to identify subtle changes in magnification, surface temperature, and retraction response,” he said. “You would be surprised that sometimes there is very little swelling. Even with complete rupture of the SDFT, they are remarkably painless. I see horses that are much more painful, a grade 3 out of 5 lameness, with a small core lesion rather than a full fracture.”
No recipe for successful healing
Peters said in his experience that many treatment modalities show promise, but there is no cookbook to refer to for adequate and guaranteed successful tendon healing. He recommends considering the three stages of healing and what to do at each stage:
1. Acute, inflammatory healing phase. This usually lasts seven to 14 days after the injury.
“You want to reduce inflammation, minimize mechanical stress from rest, and initiate the repair process,” advised Peters.
Cold therapy, bandages/support, and anti-inflammatory medications are appropriate during this period. However, Peters is not a proponent of injecting the area too early in the healing process.
“Instead, let out the heat and the pain,” he said. “Don’t add good soup to bad soup.”
2. Subacute fibroblastic healing phase. This extends from about three to 10 weeks after the injury, in which we want to optimize the “functional” healing.
“Use all your opportunities here,” said Peters. “Lasers, shock waves, hydrotherapy, platelet-rich plasma (PRP) for scaffolding, interleukin-1 receptor agonist protein (IRAP), stem cells to direct the repair process, amniotic products, alpha-2 macroglobulins… this is where the ‘art’ lies.” of healing occurs.”
This is also usually the time to begin a graded training load program.
3. Chronic tissue maturation phase of healing. This is the bulk of the repair process, which can take up to 12 months or more.
Peters said during this time we must embrace the four Rs of rehab: reduce pain, restore range of motion, restore strength and reduce the risk of re-injury.
During this extended healing period, the veterinarian should frequently reassess the horse’s readiness to return to the show. Adjustments to the treatment plan can be made during these examinations based on the client’s desire, time and economy.
Long-term monitoring of injuries
How can we monitor the risk of re-injury after the horse has been cleared to return to work?
“That’s not possible at the moment,” said Peters. “We need this!”
Peters reinforced this point and ended his presentation by telling the story of a Grand Prix jumper with a marginal lesion of the proximal SDFT. The horse was treated appropriately and comprehensively and returned 10 months later to compete clean and healthy for a Grand Prix. Two weeks later he completed another clean Grand Prix – and ended up with another SDFT lesion proximal to the original lesion.
In short, Peters said, success is never guaranteed.