Tendinitis

Ashlee E. Watts

■ Definition and Etiology Tendinitis is one of the more common musculoskeletal injuries in athletic horses. Of the two flexor tendons, the SDFT is more commonly injured than the DDFT tendon.

The function of a tendon is to transfer activity from its associated muscle to its bony attachments on the opposite side of a joint. This allows for joint movement and locomotion. During locomotion, the elastic properties of tendons, particularly those of the SDFT, give them the ability to act like a spring, storing and releasing energy back to the limb to aid in limb advancement during movement. During gallop, substantial elongation of fibers (tendon strain) of up to 16% in the SDFT have been recorded, which is similar to the failure strains of 15% to 17% recorded in vitro, and operating this close to physiologic limits may contribute to injury of the SDFT.^7-9 The DDFT has relatively lower peak forces and strains, contributing more to flexion of the distal interphalangeal joint than to energy storage, and correspondingly has a lower injury rate.^10,11

Tendon injury occurs when fibers are stretched beyond physiologic limits, which can be due to cumulative and degenera­tive fatigue failure, as is more common for the SDFT, or to a single, sudden overloading event, as is more common for the DDFT.

■ Clinical Signs The classic appearance of metacarpal/ metatarsal superficial digital flexor tendinitis is a convex or bowed profile of the tendon when viewed from the lateral aspect. In acute injury, swelling and heat of the tendon and peritendinous tissues can be noted but may be mild in the earliest stage of injury. A painful response to direct digital palpation can be appreciated and is best performed by holding the leg and palpating the tendon between thumb and forefinger. Unless the injury is severe, most horses with superficial digital flexor tendinitis are only mildly lame initially and rapidly improve to minimal lameness.

Horses with DDFT injuries within the DFTS are moderately lame and more lame after distal limb flexion.⁴ Most horses have DFTS distention and thickening, making accurate palpa­tion of the DDFT difficult. Injuries are usually unilateral and occur more commonly in the hindlimbs.⁴ Horses with DDFT injuries within the hoof capsule have unilateral or bilateral lameness without any palpable abnormalities.² Injuries can be bilateral and are more frequently found in the forelimbs. Horses with desmitis of the accessory ligament of the DDFT have obvious swelling and are acutely moderately to severely lame.⁵ Thickening and effusion in the carpal canal may be noted in horses with desmitis of the accessory ligament of the SDFT.

■ Diagnosis Ultrasonography remains the technique of choice to confirm the diagnosis and assess the severity of SDFT injuries. The entire palmar/plantar area of the metacarpus/metatarsus should be examined in transverse and longitudinal planes. The commonly measured variables include cross-sectional area, echogenicity, and fiber alignment. The severity of the injury is determined by the length of the lesion, cross-sectional area of the tendon, cross-sectional area of the lesion, lesion echogenicity, and fiber alignment of the lesion.¹³ The severity rating score has been correlated with successful outcome. The most common type of SDFT injury is a central core lesion, which appears as an anechoic lesion.¹³ Acutely, the core lesion may not be visible, and ultrasound findings may be limited to peritendinous edema with a slight enlargement of the SDFT cross-sectional area. If clinical suspicion of a SDFT injury is high, the ultrasound should be repeated in 5 to 10 days to check for an ultrasonographically visible core lesion.

Diagnostic analgesia may assist the clinician in localizing the tendon injury if clinical signs are subtle or absent. This is particularly true for DDFT injury within the hoof capsule.

MRI is the best method of diagnosing deep digital flexor tendinitis within the hoof capsule.^2,14 Injury types include core lesions, sagittal tears, and tearing along the dorsal border; most lesions occur at or near the navicular bone.² Transcuneal (through the sole) ultrasonography has been reported but is not easy. Successful imaging requires meticulous foot prepara­tion and clinician expertise.¹⁵ Tenoscopy is the best method for detecting DDFT injury in the DFTS, although ultrasound, contrast tenograms, and MRI can also be useful.¹⁶

■ Treatment Proper treatment of tendinitis includes assessment of the injury using ultrasonography, control of the acute inflammatory episode, controlled return to function, and adjunct medical and/or surgical therapy. In acute injury, a combination of systemic NSAIDs (e.g., phenylbutazone, flunixin meglumine) and topical therapy (e.g., cold water hydrotherapy, poultice, distal limb bandage) is recommended. Box stall rest is essential during the acute phase of injury to limit propagation of the injury, especially proximally and distally.

A variety of intratendinous and peritendinous injections have been advocated. Intralesional corticosteroids dramatically decrease the swelling and pain but cannot be recommended due to delayed tendon healing and the risk of continued injury and catastrophic injury in horses that remain in training. For metacarpal/metatarsal DDFT injuries, intrathecal injections of sodium hyaluronan into the DFTS have been beneficial.¹⁷ The therapeutic role of regenerative medicine (e.g., stem cells derived from fat or bone marrow and platelet-rich plasma) in equine tendinitis is not fully understood. Long-term clinical efficacy has yet to be determined, but early clinical results are encouraging.^18,19 Other promising regenerative approaches that target specific pathophysiologic events in tendon injury are being developed and will likely become a standard treatment in the future.^20,21

The surgical treatment of tendon splitting is no longer recommended due to increased damage to surrounding fibers.²² Other surgical treatments that are recommended in specific cases are desmotomy of the superior check ligament^23,24 and desmotomy of the palmar/plantar annular ligament.²⁵ Superior check desmotomy may improve the likelihood that horses return to racing, but it remains somewhat controversial and may predispose horses to desmitis of the suspensory ligament.²⁶ Annular desmotomy has been successfully performed in horses with signs of annular ligament constriction in addition to flexor tendon injury.²⁵ Immediate decompression and improved gliding function are presumed benefits.

Other therapies for superficial digital flexor tendinitis that may speed accumulation of collagenous scar tissue include therapeutic ultrasound, low-power laser treatment, and extracorporeal shockwave therapy. Counterirritation (internal or external blisters) and pin firing in combination with controlled exercise or turnout in a large pasture are time-honored treat­ments, but their effects on tendon healing have not been helpful in studies.²² These practices can no longer be recommended.

Controlled exercise in combination with the treatments described is of paramount importance in guiding the repair process to produce a functional tendon or tendon substitute. If exercise is excessive or uncontrolled, excess fibrous tissue is produced and remodeling is delayed. If no stress is applied, the tendon will not become well adapted for performance. Therefore a controlled incremental exercise program that is carefully monitored ultrasonographically is advocated. The program should be flexible and adjusted based on the severity of injury and the quality of healing. Typically, stall rest coupled with gradually increasing amounts of hand walking and trotting are recommended. Horses are monitored by regular clinical and ultrasonographic assessment until they have returned to their previous athletic function or have failed to do so. As the tendon heals, its echogenicity increases, and short linear echoes are detected. With continued healing, longer linear echoes and improved fiber alignment appear. Decrease in cross-sectional area is a good indication that the tendon is healing and remodel­ing. Ideally, complete repair will occur, and the tendon will no longer have a discretely visible area of injury because it has filled in with tissue that is isoechoic with normal tendon and fiber alignment is parallel to normal.

Successful treatment of DDFT injuries within the foot remains a challenge. Without surgery, long-term follow-up indicates a high rate of recurrent lameness despite prolonged periods of stall rest and corrective shoeing.² Surgical debride­ment via navicular bursoscopy appears to be helpful, especially in cases without concurrent severe navicular bone degenerative changes, although long-term follow-up is not yet available in the literature.¹⁴ DDFT injuries within the DFTS also require surgical debridement for the best chance of healing and return to performance.

■ Prognosis Prognosis for horses with superficial digital flexor tendinitis depends on lesion severity and the type of athletic work performed. Although many horses treated with appropriate rest and controlled exercise programs will return to work, including racing, re-injury rates are high.^28,29 This is likely because of the permanent change in tendon stiffness due to accumulation of scar tissue in the healed area. Standardbreds, English sport horses, and Western performance horses appear to have a better prognosis. Horses with DDFT injuries remain problematic; prognosis is fair to poor, but as reports on bur- soscopically and tenoscopically treated deep digital flexor lesions come out, the prognosis may improve.^2,4,30