Bowed Tendon - Causes and Factors of Tendinitis in Horses

Causes and Factors of Tendinitis in Horses

Excessive Strain

Excessive strain on a tendon can damage its collagen fibers. This is most commonly seen in performance horses that gallop or jump, who usually strain a tendon as a result of fetlock overextension when their weight is loaded on one leg. The overextension of the fetlock causes overstretching of the flexor tendons, resulting in the rupture of tendon fibers. Horses in intense training, especially those that were not conditioned properly, may damage many collagen fibers. This may occur gradually or suddenly.

After the fibers are torn, the tendon hemorrhages and collects fluid (edema), creating swelling and lameness in the area as well as increasing the pressure. The increase in pressure may damage the tendon further by destroying the cross-linking of undamaged collagen fibers and preventing the flow of blood to the area.

The SDFT

The middle third of the SDFT is most likely to suffer from tendinitis for several reasons.

The SDFT is narrower in its middle third than its top or bottom sections, making it weaker. The top and bottom of the SDFT has a better supply of blood as well, with the top third supplied by the vessels from the knee, and the bottom third supplied by the vessels in the fetlock. The middle third has a poor supply of blood, relying on the tiny vessels of the peritendon (the membrane that surrounds the tendons). If this supply is for some reason compromised, the collagen fibers in the area may die, weakening the tendon in that area and making it more likely to tear.

The SDFT branches at the fetlock, creating a sling under the back of the joint. Thus, overextension of the fetlock is more likely to overstretch the SDFT than the DDFT, which simply travels straight down behind the fetlock and pastern, to attach to the coffin bone.

Other Factors

• Poor conformation: long, sloping pasterns and a long-toe, low-heel shape to the hoof (seen commonly in Thoroughbreds) predisposes a horse to tendinitis.

• Poor trimming and shoeing: such as a farrier that causes a hoof shape that predisposed the horse to tendon injuries (such as a long-toe and low-heel), or one that shoes a horse with toe grabs, which artificially create a long-toe and low heel by lifting the toe up.

• Improper conditioning: such as working a horse at an intensity that it has not yet been conditioned for, working an unfit horse, and continuing to work an extremely fatigued horse.

• Poor footing: working a horse on uneven or slippery footing can cause tendon strain, as well as deep, “thick” footing.

Each of these factors encourage the overextension of the fetlock and knee during work. Several of these factors at once can add up.

• Direct trauma to a tendon: such as when a horse hits its front leg with a hind hoof.