Tendon Injuries in Horses

If your horse frequently suffers from lameness, he may have a case of tendinitis, an inflammation of the tendons and the tendon-muscle attachments. Tendons are composed of collagen fibers arranged to make the tendons elastic. This elasticity stores the energy that is used during the horse's stride.

When a muscle contracts, it pulls on the tendons. During exercise, tendons can normally withstand strains of 3 percent to 5 percent (similar to being able to stretch 3 percent to 5 percent of the tendon's original length). But if a horse overexerts himself and puts a strain of 8 percent to 12 percent, the collagen fibers can break. The breakage results in inflammation, swelling, pain and lameness.

The primary cause of tendinitis is overstretching, either as a single excessive loading strain (a single bad step on uneven ground) or as a repetitive strain associated with frequent overloading of the limb (such as with a galloping race horse). Tendinitis is more likely to occur if the horse's muscles are weak and fatigued during competitions, which can lead to hyperextension of the fetlock joint, and overstretching of the tendons.

Inadequate training and conditioning may also predispose a horse to tendon injury. Tendons and ligaments are known to develop strength with appropriate conditioning exercises. The injury most often occurs in the back part of the forelimbs, where the leg bends, no matter the breed or gender. However, older horses can develop tendinitis in both the front and hind legs.

What to Watch For

• Lameness. A horse suffering from moderate to severe tendinitis will be obviously lame. Less severe injuries may only cause lameness at a trot or faster. Lameness can persist at the trot for several weeks to months.
• Changes in the leg. These include swelling of the tendon or surrounding tissues, increase in warmth in the area, and increase in sensitivity to palpation.
• "Bowed tendon." Lesions within the superficial digital flexor tendon cause the characteristic swelling that appears "bowed" when observed from the side.

Diagnosis

Recovery from tendinitis can take 6 months to more than a year, depending on the extent of damage. For this reason it is important to diagnose and treat injuries quickly before damage is extensive.

• Clinical signs. Swelling, pain on palpation and lameness during exercise often lead the way to making an accurate diagnosis.
• Lameness evaluation. This may be required in some horses that do not have obvious visual and palpable evidence of tendon injury or in horses that appear to have more than one problem. During the examination the horse will be evaluated at the gait in which it appears unsound. Sequential anesthetic nerve and joint injections will be performed to rule in or rule out the tendon as the source of the horse's lameness.
• Ultrasound. An ultrasound evaluation of the tendinous structure is considered the "gold standard" in making the diagnosis of a tendon injury, in determining the severity of the lesion, monitoring the healing of an injury, and developing a prognosis for future soundness.
• Thermography. This is an adjunct diagnostic imaging technique that can be used to identify sites of inflammation within a tendon.
• Nuclear scintigraphy. This adjunct diagnostic imaging technique that can be useful in identifying tendinitis.

Treatment

Initial therapy is directed at reducing the inflammation present in an acute injury. Rest, cold therapy (water, ice pack), bandaging, and the use of systemic anti-inflammatory medications are recommended. Cold therapy should be applied frequently throughout the day but the duration of application limited to less than 20 minutes at a time.

Stall rest is important until the horse is able to walk soundly. Once walking soundly, controlled hand-walking exercise can be started. Increase in intensity and duration of exercise will be directed by the progression of healing seen in sequential ultrasound evaluations. Tendon splitting is recommended in acute superficial digital flexor tendon lesions where the lesion is located in the center of the tendon.

There are a number of procedures available to treat tendinitis:

• Superior check ligament desmotomy. This surgical procedure results in transection of a fibrous band that connects the superficial digital flexor tendon muscle unit to the horse's forearm. This procedure may be useful in increasing the "elasticity" of the superficial digital flexor tendon following injury.
• Inferior check ligament desmotomy. This is a surgical procedure that results in transection of the check ligament between its attachment to the back of the cannon bone and where it joins the deep digital flexor tendon. The surgery is recommended in some horses that have inferior check ligament desmitis.
• Desmotomy of the palmar annular ligament (annular desmotomy) is recommended for those horses that have tendinits in the lower 1/3 of the deep digital flexor tendon or the superficial digital flexor tendon. Transection of the ligament prevents its constriction around the swollen or enlarged injured tendons.
• Intralesional therapy with B-aminoproprionitrile, polysulfated glycosaminoglycan, hyaluronan, or corticosteroid may also be performed under certain circumstances.
• A veterinary consultation is recommended for all horses that develop swelling in the region of the tendons or within the digital flexor tendon sheath, regardless of the duration of the swelling or the degree of the animal's soundness.

The injured tendon initially heals with scar tissue that is later remodeled to tendon collagen. The collagen fibers are not in their original alignment, which results in a tendon that is less elastic. Prognosis for recovery and return to use depends on the severity of the injury and the horse's occupation. Horses that are returned to less strenuous endeavors generally have a more favorable outcome.

Horses should be seen on an emergency basis if they develop acute swelling of the tendon and lameness. Early intervention can minimize progression of the lesion. It is important that owners adhere to the veterinarian's treatment plan. Failure to do so often leads to protracted healing and unsatisfactory long-term results.