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
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.