Bowed tendon

Tendinitis/tendonitis is inflammation of a tendon. Many times, the tendon tissue is torn. A bowed tendon is a horseman's term for a tendon after a horse has sustained an injury that caused the tendon fibers to be torn, and then healed with "bowed" appearance.

Tendinitis usually involves disruption of the tendon fibers. It is most commonly seen in the superficial digital flexor tendon (SDFT) in a front leg—the tendon that runs down the back of the leg, closest to the surface. Tendinitis is uncommon in the deep digital flexor tendon (DDFT) of a front leg or either the SDFT or DDFT in a hindleg.

When the SDFT is damaged, there is a thickening of the tendon, giving it a bowed appearance when the leg is viewed from the side. Bows usually occur in the middle of the tendon region, although they may also be seen in the upper third, right below the knee or hock (high bows), and lower third, just above the fetlock (low bows).

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 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.

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