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Bowed Tendons - Why tendons are tough to repair

Ken Marcella, D.V.M.

The classic bowed tendon is easy to recognize. The entire cannon bone area, when viewed from the side, resembles an archer's bow rather than the straight structure it resembled before the injury.

The animal is in pain, with one leg holding less weight than the other and only the toe of this foot contacting the ground. The back of the leg is noticeably swollen, hot and tender to the touch.

Owners fear these problems even more than some fractures, with good reason. Bone heals much faster than tendon, and horses can return from some fractures to perform at the same level of activity, even higher, without threat of future injury.

Horses with severe tendon injuries, however, take a long time to heal and generally do not return to the same level of competition--at least not without the worry that another potential tear is just a misstep away.

Most tendon injuries occur when the horse places so much force on its leg that the normal load strength of the tendon is exceeded. This can occur because of improper positioning of the leg in relation to the weight of the horse's body, such as when a horse clears a jump and lands wrong. Any unbalanced loading of weight on a leg, such as uneven footing, poor conformation or improper shoeing, can harm a tendon.

When loaded or stretched beyond its capacity, some of the fibers that compose the tendon tear. Since a tendon is one large structure composed of many smaller fibers, a tear consists of individual fibers in one area pulling apart from others in the same area.

This initial damage results in fluid buildup within the tendon itself, the result of leakage from damaged cells. Blood is also possibly present as it leaks from damaged vessels. These accumulations of fluid or blood appear as holes in the tendon when viewed with ultrasound and are called core lesions.

Soon after the initial trauma the body begins the difficult task of healing. Fibroblasts, which are specific cells from the connective tissue in tendons, begin to produce new cells to repair the damage. These fibroblasts go through a series of changes to become tenocytes or tendon cells.

Normally these tenocytes are separate bundles of cells throughout the tendon and the connective tissue between these bundles keeps the entire structure flexible and elastic.

According to Dr. Nat White, professor of surgery and assistant director of the Marion Dupont Scott Equine Center in Leesburg, Va., the biggest » difference in tendon repair vs. repair of most other tissues is that newly produced tenocytes do not have the same properties as the original cells they replace.

These new cells have much less elasticity, and the scarring that results from the healing process combines with altered cell properties to produce a stiffer, less elastic tendon. This is why repaired tendons are potential sources of trouble to equine athletes.

Also, the new tenocytes produce type III collagen rather than type I collagen found in normal, healthy tendon. Collagen is the matrix or support substance of the tendon and type III is smaller and weaker than type I. Type I collagen fibers are also ordered in a precise array within the tendon. The scarring that occurs when tendons heal is the result of the misaligned, disorganized, mesh-like array of the new collagen. This often produces a shortened, thickened tendon.

Overall, though the tendon attempts to repair itself, the resulting repair is weaker and inelastic. This leads to a horse that often cannot function at its previous level of performance.

Ultrasound has been the greatest advancement in the diagnosis and treatment of tendon injuries in the last 10 years. Use of this machine allows the veterinarian to see the structure and fiber pattern of the individual tendons. Swelling and inflammation of the tendon, a precursor to actual damage, can be diagnosed via ultrasound, and the horse can then be rested rather than exercised. Injured tendons can be scanned, and the extent of the core lesion can be measured and recorded.

Ultrasound can also be used to assess healing. After the initial injury, the heat, swelling and tenderness of the tendon may completely disappear. The horse may even begin to walk without any signs of lameness.

Before ultrasound, the owner relied on feel, experience and a bit of luck to decide when to put the horse back into work. Guess wrong and the horse may suffer a re-injury that is likely to be much worse than the first one. Ultrasound allows the veterinarian to accurately see the healing process. This allows recommending rest until the entire tendon has healed, regardless of how the horse has been acting or how wonderful the tendon looks and feels. The ultrasound evaluation of the tendon keeps over-anxious owners, trainers and veterinarians from pushing a horse into work too soon.

A new drug, called BAPN-f (beta-amino propionitrile fumarate, pronounced Bapten), which has been used to treat scarring in humans, has found use in the treatment of tendon injuries in horses. Derived from seeds of the wild sweetpea plant, it prevents the cross-linking or binding together of collagen fibers during the tendon repair process.

BAPN-f is injected following tendon injury and ultrasound is used to monitor the repair process. It is crucial that this drug be combined with a detailed rehabilitation process combining leg exercise, hand walking and progressively more intense exercise over a 10-month period.

The schedule is set up on an individual basis and is largely determined by ultrasound evaluation of healing. Studies have yielded encouraging results, but it is an expensive, time-consuming program.

Prevention is the best medicine. Insist on good trimming and shoeing. Proper nutrition produces a sound, healthy, balanced hoof.

Adequate and appropriate conditioning prevents tendon injuries.

Horses should first be conditioned with long slow-distance work. This conditions the cardiovascular system and also helps tone the tendons and ligaments. However, because tendon injuries occur when those structures are overloaded with work or speed, conditioning must also include elements of heavy work such as jumping, pulling, sharp turns and speed.

Include gradually faster and more difficult periods of exercise in a conditioning program to allow the tendons to adapt to that level of stress. Horses that have been out of work are especially at risk for tendon injuries if exercised without sufficient tendon conditioning.

Also, care should be taken when your horse begins to tire. At this stage, the horse cannot compensate as well. Many tendon injuries occur at the end of a ride or workout, so pay attention to your horse and learn its limitations.

Wrapping it up

Bandages applied too tightly to the legs can result in "bandage bow." These are generally not as severe, but are still cause for concern. Ultrasound should be used to evaluate the degree of damage and to decide on a course of treatment.

Bandages must always be applied carefully and the tension adjusted so that there is never excessive pressure on any area of the tendon. Learning to correctly apply leg bandages is one of the most important skills that a horse owner can have, so learn from a professional and practice until perfect.

©Southern States Cooperative, Inc., Reprinted from Mane Points, with permission of Southern States Cooperative, Inc.