Equine Digital Flexor Tendon Sheath Injuries
The digital flexor tendon sheath (DFTS) is far more complex than what one equine surgeon gathered in vet school. The additional details he’s discovered in practice have been crucial to his success in diagnosing and treating pathology—disease or damage—to this structure.

The DFTS is more than just the synovial casing that surrounds the flexor tendons as they course around the bottom third of the cannon bone and the fetlock joint and disperse throughout the pastern region, said Florent David, DVM, MS, Dipl. ACVS & ECVS, Dipl. ACVSMR, ECVDI Assoc., specialist in Surgery, Sports Medicine & Rehabilitation, and Diagnostic Imaging at the Equine Veterinary Medical Center, in Doha, Qatar. He reviewed the anatomy and described how he diagnoses and treats closed DFTS injuries at the 2019 Northeast Association of Equine Practitioners Symposium, held Sept. 25-27 in Saratoga Springs, New York.

DFTS Anatomy

The three continuous, communicating (shared fluid space) regions throughout the length of the sheath meet several bony prominences along the way, where the superficial digital flexor tendon (SDFT) and the deep digital flexor tendon (DDFT) encounter considerable changes in direction and friction.

Other parts—mesotenons, thin stabilizers connecting the tendons to the sheath and providing nutrients and oxygen to the tendon via blood vessels, and tendinous rings called manica flexoria or annular ligaments, for instance—help with the intricate mechanics. Some of these parts are difficult to see on imaging except in horses with DFTS inflammation.

In cases of DFTS tenosynovitis (inflammation of the membrane surrounding the tendon sheath) David sees a combination of two or more of the following:

  • Effusion (fluid swelling of the sheath itself);
  • Deformation (change in the sheath’s shape);
  • Heat (focal, paired with increased digital pulses in acute cases);
  • Pain on palpation;
  • Pain on limb flexion;
  • Lameness; and
  • Fever in horses with DFTS sepsis (infection of the structure).

Diagnosis

David said divides his thorough DFTS tenosynovitis ultrasound exam into several parts:

  • He examines the tendons’ core and then margins with 2D ultrasound (proximal to distal—top to bottom), focusing on one structure at a time in the weight-bearing leg. He also assesses synovial fluid volume and echogenicity (solid matter—let’s say bits of injured DFTS–are visible as white; fluid appears black). “If it is not well-distended, you can add fluid (to the tendon sheath) … it will help you find out if the surface of the tendon is abnormal or not.” He checks the mesotenons, the manica flexoria, the sheath wall, and the annular ligaments, as well.
  • He performs dynamic ultrasound scanning of the same structures with flexion/extension of the limb with the leg nonweight-bearing.
  • He uses Doppler ultrasound of the same structures in the nonweight-bearing leg, focusing especially on the tendon margins.
  • He sometimes uses ultrasound elastography to look at tissue hardness or softness.
  • An optional step: He performs another 2D scan after adding fluid/ultrasound contrast within the DFTS.

David gave veterinarians some tips to help their diagnostic efforts:

  • Pay close attention to the outline of the tendons. “Protruding tendon fibers may not jump to your eyes, but if you pay attention closely, you should be able to see it.”
  • Shoot from a different angle and appreciate protruding material (think of the tendon as a frayed rope).
  • Move the sheath wall, synovial fluid, and tendons around to get a better understanding of what you’re seeing.

“Everything becomes really thick on these horses,” he said, showing ultrasound images of digital flexor tenosynovitis.

In addition to the ultrasound techniques, David performs synoviocentesis when he suspects infection, during which he collects and analyzes synovial fluid from the sheath. Normal DFTS fluid is pale yellow, clear, and doesn’t clot, he explained. Veterinarians might measure white blood cell count, percentage of neutrophils, and total proteins and look for the presence of intracellular bacteria. These values can help them screen for sepsis. In addition, he measures serum amyloid A, a protein the liver and synovial lining produce in response to inflammation; levels can help vets discern whether a horse is septic or merely inflamed. He also said measuring cartilage oligomeric matrix protein (COMP), which is a biomarker for tendon and cartilage damage, can be useful, though there’s no commercial test for .

Then there’s diagnostic analgesia—synovial blocks—where the veterinarian injects local anesthetic into the DFTS to numb it and confirm it as the cause of lameness. Other options include contrast tenosynoviography (injecting contrast material into the DFTS, walking the horse for 20 seconds, then shooting several X rays of the loaded limb to see highlighted areas of damage) and MRI and contrast-enhanced CT.

Some of these methods have more science behind them than others. Veterinarians can review David’s proceedings paper for more information.

Treatment

David said horses with primary acute and subacute (not a fresh injury, but not chronic, either) DFTS tenosynovitis—that is sheath inflammation but no tendon or manica damage—are best treated with a conservative approach: rest, local (at the injury site) and systemic anti-inflammatory treatment, cryo- or hydrotherapy, and bandaging.

In subacute cases he also recommends injecting short-mid duration corticosteroids and hyaluronic acid into the sheath, because conservative approaches alone won’t be sufficient to resolve or take hold of the inflammation; he uses four approaches, which veterinarians can review in his proceedings paper. Researchers have shown that injecting stem cells, platelet-rich plasma (PRP), or a combination into superficial digital flexor tendon core lesions seems to enhance healing and reduce re-injury risk, but David said there’s no evidence that these help marginal lesions (ones on the tendon margins in the sheath where fibers are exposed) such as longitudinal tears of the DDFT. (Research in a sheep model showed bone-marrow-derived stem cells could engraft into the synovial membrane with this type of injury, but they didn’t alter or improve the healing process.)

When there’s no response or just a partial response within four to six weeks—the horse has persistent effusion and/or lameness—his cases go to surgery.

David acknowledged that there aren’t many new treatment approaches in veterinary medicine for equine tendon injuries within the DFTS: “These cases need the skills of an experienced surgeon, but even in great hands the outcome can still be disappointing—for longitudinal tears, for instance. We need some better instrumentation and better techniques to increase our success rate post-surgery for certain lesions in the DFTS.”

Rehab Approaches

In DFTS synovitis cases, David recommends orthopedic farriery, which includes trimming and shoeing to increase (DDFT lesion) or decrease (SDFT lesion) the angle between the solar surface of the coffin bone and the ground.

Next, there’s rest and progressive exercise. Swimming and underwater treadmills can be useful for increasing the horse’s activity without loading the tendon too quickly. “Horses under exercise in water protocols do not get overweight and usually go through their rehabilitation faster,” said David.

In horses that don’t handle stall rest well, he has found success using an equine tendon and suspensory ligament injury recovery system (EqueStride), along with orthopedic farriery. The device is designed to control a horse’s amount of fetlock drop, and the veterinarian can set it in different positions, depending on the patient’s needs. “This device is brilliant, as it allows rehabilitation of tendons and ligaments in a dynamic and controlled way,” David said.

“I see so many cases that do poorly after being cast for three months following a complete tendon rupture or laceration, (but with this new device horses can) remobilize much faster in a controlled way,” he said. “The healing time and outcome are much better.”

In cases where the tendon sheath is still effused at three weeks post-surgery, David injects hyaluronan and a small dose of short-acting corticosteroids, then repeats the treatment at six weeks.

Finally, he pointed out that pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease) has been associated with tendon degeneration. He recommended testing horses with tendon injuries in case the horse needs pergolide treatment for that .

David concluded saying, “DFTS injuries should be treated with a clock in hand. Reaching a proper diagnosis can be tricky but is very important, as important as a staged treatment approach. Surgical exploration is strongly recommended if a sheath does not settle within four to six weeks from injury. Secondary lesions such as adhesions and constriction syndrome can develop rapidly in an angry DFTS, worsening the chance of your horse returning to soundness or full potential. DFTS injury should not be taken lightly.”