Tendinitis most commonly occurs in the superficial digital flexor tendon in the midcannon bone region. Deep digital flexor tendinitis and injury of the inferior check ligament occur less frequently.

Tendons are comprised of collagen fibers that are arranged in such a manner to allow the tendons to be elastic in nature. This elasticity stores energy that is used during the horse's stride.

During exercise, tendons normally are able to withstand strains of 3 to 5 percent (similar to being able to stretch 3 to 5 percent of the tendon's original length) during weight bearing. When strains of 8 to 12 percent occur, the collagen fibers that comprise the tendon break and the ability of the tendon to withstand exercise is compromised. This breakage results in inflammation, swelling, pain and lameness.

Primarily, injury is initiated by overstretching of the tendon 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.

Predisposing factors to injury include muscle weakness and fatigue during competitions which lead to hyperextension of the fetlock joint, and overstretching of the tendons. Inadequate training and conditioning may also predispose to tendon injury. Tendons and ligaments are known to develop strength with appropriate conditioning exercises.

The superficial digital flexor tendon has the smallest cross-sectional area in the midcannon bone region and as such this location is most at risk to incur a tendon injury. Due to the weight distribution of the horse being the greatest on the forelimbs versus hind limbs, overloading of the superficial digital flexor tendon is most common in the forelimbs.

Injury of the deep digital flexor tendon and of the inferior check ligament is more likely to occur in middle aged or older individuals. Deep digital flexor Tendinitis can occur in both the front limbs and in the hind limbs with equal incidence. Lesions develop near the navicular bone and within the digital flexor tendon sheath. Visual enlargement of the tendon is rare due to the location of most lesions.

Inferior check ligament desmitis is reported more frequently in Standardbred racehorses, jumpers and draft horses. The severity of symptoms exhibited by horse with Tendinitis or inferior check ligament desmitis is variable. Small lesions may cause a lameness but no visible or palpable changes in the leg while other lesions may result in a non-weight bearing lameness.

Moderate to severe injuries will result in a horse that is visibly lame at the walk while less severe injuries will only cause lameness at the trot or faster. Lameness can persist at the trot for several weeks to months.

What to Watch For

Swelling of the tendon or surrounding tissues

Increase in warmth to the area

Increase in sensitivity to palpation of the tendinous structures

Changes in tendon size

Soft tissue swelling

Sensitivity to palpation

Lameness

Bowed swelling when observed from the side

Healing of tendinous injuries can require six months to more than a year depending on the extent of damage. For this reason it is very important to diagnose and treat injuries quickly before damage is extensive. 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 which is less elastic than originally.

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.

Diagnosis

Clinical signs, such as swelling, pain on palpation and lameness during exercise, often lead the way to making an accurate diagnosis.

Lameness evaluation 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 is evaluated at the gait in which he appears unsound. Sequential anesthetic nerve and joint injections are performed to rule in or rule out the tendon as the source of the horse's lameness.

Ultrasound evaluation of the tendinous structure is considered the gold standard in making the diagnosis of a tendon injury, determining the severity of the lesion, monitoring the healing of an injury, and developing a prognosis for future soundness.

Thermography is an adjunct diagnostic imaging technique that can be used to identify sites of inflammation within a tendon.

Nuclear scintigraphy is another adjunct diagnostic imaging technique that can be useful in identifying tendinitis.

Treatment

Initial therapy is directed at reducing the inflammation. 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. Other treatments include:

Stall rest. 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. This procedure is recommended in acute superficial digital flexor tendon lesions where the lesion is located in the center of the tendon.

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. It 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). This procedure is recommended for those horses that have Tendinitis in the lower third 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. The lesion is injected with B-aminoproprionitrile, polysulfated glycosaminoglycan, hyaluronan, or corticosteroid.

Home Care and Prevention

A veterinary consult 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. 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 very important that owners adhere to the veterinarian's treatment plan. Failure to do so often leads to protracted healing and unsatisfactory long term results.

Prevention is not always possible in competitive horses. The chances of injury can be reduced by making sure horse are physically trained prior to participating.