The term "tendinitis" refers to acute inflammation/injury of tendons (soft tissues that connect muscles to bones). Inflammation of a tendon can be acute or chronic, with varying degrees of tendon fibril disruption. Acute injuries are also almost always preceded by microdamage to the constituent collagen fibers.
Chronic and recalcitrant tendon injuries are more appropriately referred to as "tendinopathy."
Tendinitis is the most common in horses used for fast work, particularly racehorses.
Superficial digital flexor tendon (SDFT) injuries commonly occur in the forelimb of Thoroughbred racehorses. These are overstrain injuries that typically occur in the mid-metacarpal region. The primary pathological finding is a central core lesion.
Deep digital flexor tendon (DDFT) injuries are more common within compressed regions of the limb, such as overlying the palmar fetlock joint and proximal palmar border of the navicular bone (see DDFT linear tear image).
Courtesy of Dr. Sushmitha Durgam.
Etiology of Tendinitis in Horses
SDFT tendinitis in horses usually appears after fast exercise and is associated with overextension and poor conditioning, fatigue, poor racetrack conditions, and persistent training when inflammatory problems in the tendon already exist. Other predisposing factors are improper shoeing, poor conformation, and poor training.
DDFT injuries are linked to wear and tear from repetitive stress and biomechanical forces.
Clinical Findings of Tendinitis in Horses
Acute tendon injuries are characterized by heat, swelling, pain, and varying degrees of lameness depending on the severity and location of the injury. Similar clinical signs, as well as fibrosis and thickening, can be evident in chronic injuries. Some horses with chronic tendinitis become less lame while walking or trotting; however, lameness can recur under hard work.
Specific clinical signs of tendinitis in horses vary by the tendon that is injured. Posture and function of the limb can be altered, depending on the structure damaged. With severe SDFT injuries, fetlock hyperextension might be evident. With severe DDFT injuries, a "toe flip" might be evident at weight bearing.
Diagnosis of Tendinitis in Horses
History
Clinical signs
Ultrasonography
MRI
Ultrasonography (high-frequency [7.5–15 MHz] linear probes) is the most commonly used and available diagnostic tool for documenting tendon injury in horses. Tendon enlargement, focal or generalized hypoechogenicity, and varying degrees of tendon fiber disruption or remodeling can be visualized.
Advanced imaging modalities such as MRI are beneficial for diagnosis of DDFT lesions within the hoof capsule and at the proximal metacarpal/metatarsal regions.
Treatment of Tendinitis in Horses
Cold hydrotherapy
Compression bandaging
Corrective shoeing
Controlled exercise
Tendinitis in horses is best treated in the early, acute stage. The horse should be stall-rested, and the swelling and inflammation should be treated aggressively with cold packs and systemic anti-inflammatory agents. Some degree of support or immobilization might be indicated, depending on the amount of damage to the tendon. Intratendinous corticosteroid injections are contraindicated because they inhibit repair mechanisms.
Pearls & Pitfalls
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In cases of a distinct hypoechoic or anechoic core lesion present on ultrasonographic examination, tendon splitting was sometimes performed in the past (the rationale was to decrease intratendinous pressure due to serum or hemorrhage); however, it is no longer a common practice. Shock wave therapy and intralesional injection (eg, of stem cells or platelet-rich plasma products) are now common forms of treatment.
The amount of evidence for various treatment modalities varies. The horse should be rehabilitated via a regimen of increasing exercise. Superior check ligament desmotomy has been used as an adjunctive treatment for SDFT tendinitis, to minimize recurrence of the problem before the horse is returned to training.
Other treatments for chronic tendinitis in horses have included superficial point firing (which is of questionable benefit) and percutaneous tendon splitting. When tendinitis occurs within the digital tendon sheath, annular ligament desmotomy might be beneficial.
The prognosis for a flat-racing Thoroughbred racehorse to return to racing after an injured SDFT is guarded, regardless of treatment. The prognosis is better for sport horses, and generally better for those that do not jump or compete at high speed.
Key Points
Tendon injuries are common musculoskeletal injuries that occur in all disciplines of equine athletes; superficial and deep digital flexor tendons are structures commonly affected.
Heat, swelling or thickening, and pain on palpation are commonly present; lameness depends on the severity and duration of the injury.
Diagnosis is based primarily on history, clinical signs, and ultrasonography or MRI.
Treatment is largely conservative and includes cold treatment, compression bandaging, shoeing, and controlled exercise.
For More Information
Kümmerle JM, Theiss F, Smith RKW. Diagnosis and management of tendon and ligament disorders. In: Auer JA, Stick J, Kümmerle JM, Prange T, eds. Equine Surgery. 5th ed. Elsevier; 2019:1411-1445.
Bubeck KA, Aarsvold S. Diagnosis of soft tissue injury in the sport horse. Vet Clin North Am Equine Pract. 2018;34(2):215-234.
Ortved KF. Regenerative medicine and rehabilitation for tendinous and ligamentous injuries in sport horses. Vet Clin North Am Equine Pract. 2018;34(2):359-373.
Also see pet owner content regarding joint disorders in horses.
