Unusual Tendon Injuries in Sport Horses

Injuries to the weight-bearing front digital flexor tendons are some of the most common tendon issues in athletic horses. As such, we talk about these frequently while other, less-common tendon injuries get less press. That doesn’t mean they’re any less important, however.

Roger Smith, MA, VetMB, PhD, DEO, FHEA, LAAssocECVDI, Dipl. ECVS, FRCVS, from the Royal Veterinary College, in London, U.K., shed some light on out-of-the-ordinary injuries during the 2018 British Equine Veterinary Association Congress, held Sept. 12-15, in Birmingham. First, he explained what he classifies as “unusual.”

The digital flexor tendons (DFT) that run down the back of the horse’s forelimbs and attach at the sesamoid bones are particularly vulnerable to injury because of their high load-bearing and their lack of protection by other tissues, he explained.

“Therefore, unusual injuries are those that affect either these tendons at unusual sites, other tendons, or where the nature of the pathology is unusual,” said Smith. He discussed each of these categories separately.

Strange Sites for DFT Injuries

Intrasynovial injuries to the superficial digital flexor tendon (SDFT): The SDFT inserts on the back of the horse’s pastern, between the fetlock and the hoof. Unlike the common extrasynovial (outside the joint sheath) injury involving this tendon, intrasynovial injuries are those occurring lower within the digital flexor tendon synovial sheath or higher within the carpal sheath.

Smith said these are slow-healing due to their lack of vasculature and their communication with the synovial cavity and are difficult to assess via veterinarians’ typical soft tissue imaging modality, ultrasound. Conservative treatment is particularly lengthy. He said treatment options to try to speed healing include intralesional platelet-rich plasma or mesenchymal stem cell injections, but only if the lesions don’t communicate with the synovial cavity.

Hind SDFT injuries: Hind-limb SDFT injuries are far less common than those in the forelimb, said Smith, but when they do occur, it’s typically in the metatarsal (cannon bone) region. If injury occurs at the fetlock level, the compressive load in this area makes it difficult to heal, he said. However, once healed, their prognosis is positive.

Extrasynovial injuries of the deep digital flexor tendon (DDFT): The DDFT lies beneath the SDFT and runs all the way down around the navicular bone to insert at the back of the coffin bone. Smith said extrasynovial injuries to this structure are extremely rare and are usually found in conjunction with injury to the DDFT’s accessory ligament. Similar to the usual treatment for the common SDFT injury, he recommended intralesional injections or a lengthy period of conservative treatment.

DDFT injuries in the carpal sheath: This very rare injury occurs when the radial head (coming from another muscle belly of the same deep digital flexor muscle that is connected to the radius, the bone between the knee and the elbow) tears. In the few cases that have been reported on, said Smith, debriding the torn tissues has resulted in a good prognosis.

DDFT injuries in the tarsal sheath: This traumatic (e.g., following a kick to the hock) or overstrain injury can result in swelling of the DDFT’s sheath as it passes over the hock. Treatment and prognosis are similar to that of injuries to the carpal sheath, said Smith.

Uncommonly Affected Tendons

Extensor tendon: These tendons that run down the front of the cannon and pastern bones aren’t heavily loaded and, thus, don’t experience overstrain injuries as frequently as other structures, said Smith. When they do get injured, it’s more often due to external trauma. They’re usually inconsequential, he said, and prognosis for return to performance is good.

Bicipital tendon: When this tendon in the shoulder gets injured, it can significantly affect the horse’s ability to advance his limb, said Smith. While swelling might not be obvious, damage on ultrasound is. Veterinarians typically treat this injury conservatively and/or with intrabursal steroids.

Gastrocnemius (“Achilles”) tendon: While this injury is common in humans, it is rare in horses. It can be treated either conservatively or with injections, although the prognosis is uncertain, said Smith.

Popliteal tendon: No record exists of injuries to this tendon in the horse’s stifle, but Smith said he’s seen two convincing cases—one due to overstrain and another due to infection.

Ulnaris lateralis: This elbow structure lies adjacent to the joint’s collateral ligament and, therefore, avulsion fragments from the humerus (the bone between the elbow and shoulder) in this region can be confused with that ligament, said Smith. Rare cases have done very well with conservative treatment alone, he said.

Uncommon Injury Types

Degenerative rupture of the SDFTThis dramatic injury is seen mostly but infrequently in older horses. Many of these horses go on to do well with conservative treatment alone, as their athletic requirements are usually less, said Smith.

Tendon mineralizationThis occurs as a result of atypical ‘failed’ healing in tendons, although it can also be induced by corticosteroid injections. Smith said it has limited treatment options.

Paratendinitis: This is inflammation of the outer layer of the tendon rather than the tendon itself. While common in humans, it’s rare in horses, as are foreign bodies and tumors such as hemangiosarcoma.