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.
Description of tendinitis in horses.
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).
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.
Each of these factors encourage the overextension of the fetlock and knee during work. Several of these factors at once can add up.
Bandage bows.
Bandage bows are caused by applying a bandage too tightly, creating an acute pressure injury to the tendons. The compression may cause the area to swell once the bandage is removed, giving a "bowed" appearance. However, the damage is usually just to the skin and not to the tendon itself.
Horses with bandage bows usually respond to sweats or poultices. These treatments must be applied under a bandage that is not tightly fitted and the bandage should only be left on for a few hours. Cold hosing, NSAIDs and DMSO may also help.
Signs of tendinitis in horses.
Signs of acute tendinitis include swelling, heat, and pain when the affected area is palpated. If mild, swelling may not be readily apparent, although there will still be heat and pain in the area, as well as mild lameness. If more severe, the injury is usually accompanied by moderate lameness (2-3 on a scale of 5) with obvious swelling.
It is important not only to palpate the SDFT but the branches of the SDFT, the DDFT, check ligament, and suspensory ligament as well. These structures could have been damaged at the same time as the SDFT. Both legs should be checked, although tendinitis usually only occurs in one leg.
When the tendon is healed, it will still have a thickened, bowed appearance that feels firm and woody. However, all heat, lameness, and pain should disappear.
Treatment of tendinitis in horses.
Initial treatment of a bowed tendon should concentrate on anti-inflammatory therapies, including cold water or ice therapy, and anti-inflammatory medications on the direction of a veterinarian. The horse should be confined to a small area until the severity of the injury can be assessed with ultrasound. Standing bandages are helpful, but care should be taken to avoid applying them too tightly.
Controlled exercise.
Generally speaking, the most important aspect of long-term therapy is controlling the level of exercise. A balance must be struck between two competing ideas: 
For the first several months, large area turnout is discouraged, since even a small amount of running or playing could easily re-injure the weakened tendon. As the tendon heals, it is loaded in increasing amounts. The use of diagnostic ultrasonography is extremely valuable in guiding changes in workload.
Adjunctive therapies.
Many adjunctive therapies have been attempted to improve the quality of healing or to speed the recovery process. There is no consensus in the veterinary community as to which treatments are the most effective.
Prognosis of tendinitis in horses.
The prognosis for return to full work depends on:
The best way to ensure that an injured horse returns to full work is to rehabilitate the animal correctly. This includes slowly bringing the horse back into training, and giving the horse light exercise each day as the tendon is healing. An impatient trainer who rushes to bring the horse back to intense training is likely to cause re-injury of the tendon.
Sources.
King, Christine, BVSc, MACVSc, and Mansmann, Richard, VDM, PhD. "Equine Lameness." Equine Research, Inc. 1997. Pages 400-415, 532-533.