Orthopedics

Managing tendon injuries, from diagnosis to rehabilitation

By Adam Marcus

Equine tendon injuries are a significant challenge in veterinary medicine. Without proper diagnosis and treatment, injury to these tissues can lead to chronic and even life-threatening lameness.

In a recent lecture at Cornell University, Shannon Connard, DVM, PhD, DACVS, an assistant professor in the section of large animal surgery in the Department of Clinical Sciences at Cornell College of Veterinary Medicine, in Ithaca, N.Y., discussed the pathophysiology, clinical management and rehabilitation strategy for common equine flexor tendon injuries.

Mechanism and healing constraints
Injury typically occurs through 3 pathways:

1. Strain: Minor stress allows the tendon to snap back like a rubber band, but excessive force leads to microscopic fiber damage, partial tears or complete rupture.
2. Degeneration: Cumulative microdamage and chronic degeneration, such as degenerative suspensory ligament desmitis, weaken the structure over time, often acting as a precursor to acute overstrain injury. “We most commonly see this in Peruvian Paso horses, but it affects many horses,” Dr. Connard said.
3. Trauma and infection: Due to the lack of protective muscle on the lower limb, tendons are highly vulnerable to lacerations. If an infection involves a synovial structure, such as tendon sheaths or bursae—which provide vital lubrication for locomotion—they should be treated as a medical emergency, as they can lead to potentially life-threatening damage requiring humane euthanasia if untreated. “The sooner we see these [cases], the better the chance we have of getting them back to a full recovery,” she said.

Locating the lesion
“It’s important to localize the source of the pain when you notice that your horse is lame,” Dr. Connard said. “I’ve had many horses present that are very lame in 1 leg. They have a very obvious bump or abnormality somewhere on the leg where it seems like that has to be the explanation for what’s going on,” but the real problem turns out to be a foot abscess.

Proper diagnosis starts with localization of pain using physical examination, palpation, lameness assessment, appropriate diagnostic analgesia (ie, nerve blocks) and imaging. Monitoring starts with ultrasonography.

“This is really our first line of defense,” Dr. Connard said. “It’s really helpful for us to see the change in the size of the tendon and the size of the lesion over time. Following injury, a lesion will look like a black hole where the fibers have torn. This region will fill in with short, irregular fibers as the tendon heals, and by the end it starts to resemble normal tendon more closely.

However, don’t be fooled by an innocent-looking scan. Following the injury, they take time to mature and remodel. “When they happen [initially], they typically don’t look that bad on ultrasound. When you see a lesion in the very beginning of an injury, it’s usually worse than it actually appears,” she said.

Magnetic resonance imaging (MRI) is particularly helpful for visualizing tendon lesions that occur in the hoof, which is not amenable to ultrasound,  Dr. Connard noted. “The deep digital flexor tendon runs all the way down through the hoof to then insert on the coffin bone. There’s a long area of tendon that we cannot ultrasound at all that frequently likes to get injured,” she said.

Positron emission tomography (PET) and computed tomography (CT) scans may also provide useful information, Dr. Connard said. “We can combine these imaging modalities and overlay them to fit the puzzle pieces together.”

Treatment options
Although tendon injuries can heal with medical management, surgery may be indicated in more serious cases. Healed tendons often never fully regain their original strength. The primary goals of treatment are to control infection, manage inflammation and minimize the formation of disorganized scar tissue, according to Dr. Connard.

“It’s good to have a bacterial culture and sensitivity where we can get a sample of the infection, grow the bacteria and know exactly what bacteria we’re treating and what antibiotics they’re susceptible to,” Dr. Connard said. Treatment may include systemic antibiotics, local antimicrobial therapy, and surgical lavage or debridement, especially when a tendon sheath or bursa is involved.

Another key step is to realign and stabilize the tendons, if necessary. Surgery and/or external stabilization may be required in the event of a complete laceration or large tears.

In the absence of infection, initial management often relies on nonsteroidal anti-inflammatory drugs (NSAIDs), including phenylbutazone or flunixin meglumine for acute pain. For longer term management, firocoxib is sometimes preferred because it is a more targeted, COX-2 selective NSAID and may be easier on the gastrointestinal tract and kidneys than some traditional NSAIDs—although it still must be used carefully and under veterinary guidance.

Healthy tendon is composed predominantly of type 1 collagen. Following injury, the body frequently replaces this tissue with scar tissue containing increased type 3 collagen, which is stiffer and weaker and creates a mechanical mismatch at the junction of healthy and scarred tissue—making the area vulnerable to reinjury.

Biologic therapies, such as platelet-rich plasma and mesenchymal stromal cells, can help improve tissue quality and reduce reinjury rate. These may be injected directly into the lesion under ultrasound guidance.

Technological advancements have introduced minimally invasive options like Tenex, which uses high-frequency ultrasound to debride injured tissue while preserving healthy fibers. Extracorporeal shockwave therapy can reduce pain and promote healing at the cellular level, although it is largely applied for ligament injuries. High-power laser therapy has been shown to promote healing, too, although questions remain about the ability of low-power lasers to penetrate deep enough to reach tendons. Therapeutic shoeing is also important for mechanically reducing strain during the healing process.

For injuries to the deep digital flexor tendon, heel elevation is helpful and can be accomplished with a graduated shoe or wedge pad. But for the superficial digital flexor tendon, at least 1 study found elevating the heel may increase the tension on the tendon. “So, not all tendon injuries are created equal, and you really want to work together as a group [with your veterinarian and farrier] to figure out the best way to shoe your horse,” Dr. Connard said.

Advanced age and the presence of metabolic conditions, such as pituitary pars intermedia dysfunction, can impair healing, Dr. Connard added.

Rehabilitation: A 6-phase process
Recovery is a distance race, often spanning 8 to 14 months. Success depends on a controlled, graduated return to exercise that prevents reinjury.

• Phase 1 (0–1 month): Focus is on stall rest, pain relief, and cryotherapy to reduce inflammation. Brief (5 to 10 minute) hand-walks once or twice a day can start in this period.
• Phase 2 (1–3 months): Back off the systemic analgesics, if possible, and gradually increase hand-walking by about 5 minutes a day until the animal can handle 30 minutes twice daily.
• Phase 3 (3–6 months): Reduce pain relief and try heat therapy to improve flexibility in the developing scar tissue. Add gradual weight loading or ground poles to the hand-walking.
• Phase 4 (6–8 months): Progression to walking under saddle. “On the off days where they’re not under saddle, continue hand-walking and then eventually add in ground poles,” Dr. Connard said.
• Phase 5 (8–10 months): Introduction of controlled trotting. “I would begin with something very low like 2 minutes, and then gradually increase by 2 to 3 minutes per week until they’re trotting for 5 or 10 to 15 minutes,” Dr. Connard said. “If at any point they become more lame or this is not working for them, you need to stop, go back to the lighter version of what you were doing, and [have a veterinarian] re-evaluate them.”
• Phase 6 (10–14 months): The final period of healing and remodeling occurs here, and a return to full exercise. “You can gradually start increasing cantering just like you did trotting and then really return them to full exercise if they stay comfortable,” she said.

Prognosis
The prognosis for equine tendon injuries is generally better for lesions on the front of the leg than for those on the back, which must bear significantly higher loads. “If there’s an injury in the muscle area vs the tendinous portion that tends to heal better,” Dr. Connard said. Smaller lesions generally fare better than larger or complete transections.

Infection and the formation of adhesions also are associated with worse outcomes, the latter preventing the tendon from moving fluidly as it heals. MeV