History and presenting complaint

Horses presented for superficial digital flexor (SDF) tendinitis are usually in full athletic use. Affected horses often have a workload that consists primarily of galloping and/or jumping (Fig. 20.1). Most tendon injuries are due to cumulative damage rather than a single event, the exception being a fall or a strike from another limb. Horses often develop clinically apparent pain, swelling and lameness 2–3 days following SDF injury.

Physical examination

Lameness ranges from grade 1 to 3 of 5 and is often transient or intermittent. Lameness often resolves rapidly with the use of ice, rest and anti-iflammatory agents. There will acutely be pain on palpation, heat and swelling at the lesion site. Pain on flexed palapetion is often the sign with longest duration as the lameness resolves. Affected horses frequently exhibit a decrease in performance ability after the initial lameness resolves.1,2

Luxation of the SDF tendon from its attachment to the calcaneus often presents as an acute injury during exercise, particularly in polo ponies and eventers. An audible pop may be heard, followed by acute lameness. As luxation usually occurs laterally, the tendon can be seen coursing down the lateral aspect of the tarsus. Within a few days, massive swelling of the limb obscures anatomic details if the injury is not treated aggressively for inflammation.

Differential diagnosis for lesions in the metacarpal/metatarsal region is limited as the SDF tendon is readily palpated and separated from deeper structures. Carpal sheath synovitis may be associated with proximal SDF tendinitis. Digital sheath synovitis may be associated with SDF tendinitis in the fetlock and pastern regions. The manica flexoria portion of the SDF tendon may be damaged deep to the deep digital flexor tendon in conjunction with digital sheath synovitis. Differential diagnosis in the pastern region includes damage to the sesamoidean ligaments or the collateral ligaments of the proximal interphalangeal or distal interphalangeal joints.

Special examination

For SDF branch lesions, a low palmar digital nerve block generally provides analgesia. For SDF lesions in the metacarpal/metatarsal region, a high palmar nerve block resolves the lameness.³

Increased size of the affected tendon region combined with loss of echogenicity and normal parallel linear fiber pattern observed with diagnostic ultrasonography provides the definitive diagnosis (Fig. 20.2, Fig. 20.3, Fig. 20.4, Fig. 20.5, Fig. 20.6).^4–10

Tenoscopy of carpal and digital sheaths provides diagnostic information as well as further treatment options. For example, under arthroscopic guidance debridement of frayed tendon fibers and resection of proliferative synovium and adhesions may be performed.11,12

Therapeutic aims

Therapy

Initial therapy is directed at controlling excessive inflammation and should include anti-inflammatory therapy such as 1% diclofenac cream applied twice daily to the affected area.

Cold therapy, such as ice for 20 minutes 2–3 times/day for three weeks, is an effective adjunct to reduce inflammation. Treatment with intramuscular polysulfated glycosaminoglycan, 500 mg every four days for seven treatments, has been shown to improve tendon healing.¹³

Foot conformation should be evaluated early in the course of injury. Good caudal heel support should be provided using an egg bar shoe that extends to the level of the heel bulbs unless the horse has sufficient support from his foot shape. Any break in pastern/hoof axis should be corrected at the same time to minimize tension on healing soft tissues.

Although stem cell and platelet rich plasma (PRP) injections are in common use, no contolled study has yet shown that they produce collagen within injured tendon or ligament. A recent paper has shown that mesenchymal stromal cells survivied for approximately 10 days when injected into damaged tendon, and that they did not migrate, indicating little viable activity.¹⁴ Embryonic stem cells were able to survive for 120 days and to migrate indicating that futher research is warranted to determine if embryonic cells might produce viable tendon collagen. Another recent paper has shown that insufficient amounts of growth factor are released from platelets using current techniques to induce collagen growth.¹⁵

Native stem cells present in the patient’s injured tendon are capable of producing collagen under appropriate circumstances including controlled exercise, which is the likely reason that horses following a controlled exercise program and whole horse care have as good a prognosis to return to work as those which have received stem cell or PRP injections in addition to controlled exercise. Horses that receive stem cell or PRP injection without adequate time to heal or appropriate rehabilitation have a poor prognosis to return to work without re-injury.

Stall (12′ by 24′) confinement with hand-walking should be initiated following examination. Unless the tendon is so severely damaged that rupture appears likely (rare), hand-walking is an important part of treatment and should not be delayed. If a free or box stall type of mechanical exerciser is available, horses can exercise up through the trot phase on this equipment. Walking times can be moderately increased above hand walking times. All exercise should be under supervision. Aquatic treadmills are also useful for horses when they reach the level of healing equivalent to the trot phase; earlier use may be excessive work for healing soft tissues.¹⁶

Bandaging is useful to decrease edema in the surrounding tissues. Bandages will not provide significant tendon support. Any concurrent lameness, even slight or chronic, should be treated at the same time.

Following an initial period of stall rest and hand-walking, the SDF tendon should be re-evaluated using physical examination and ultrasonography to assess healing.¹⁷ Increasing exercise should be based upon examination findings rather than time elapsed. The horse should be confined to a space small enough so that it can only walk when not undergoing controlled exercise until healing is sufficient to allow canter work under saddle for one week. Please refer to Tables 20.1–5 for detailed exercise protocols.

Surgical options

If an anechoic or mostly anechoic core lesion is seen on ultrasonographic examination, ultrasound-guided tendon splitting to decompress the core lesion should be performed as soon as possible, ideally 2–14 days following injury, to prevent further compression injury of the surrounding normal fibers.

If a subcutaneous hematoma is detected, injection of hyaluronic acid between the tendon and the skin should be performed as soon as possible, ideally 2–14 days following injury to attempt to prevent development of adhesions between the subcutaneous tissue and the SDF tendon.

Superior check ligament desmotomy has been shown to improve SDF appearance in the short term; long-term results have been less encouraging. Superior check desmotomy has been associated with significantly increased strains on the SDF tendon and SL.¹⁸ Horses treated surgically were 1.2 times more likely to develop recurrent or new injuries after returning to training than horses managed non-surgically. Racehorses that have undergone a superior check ligament desmotomy are 5.5 times as likely to suffer a subsequent suspensory ligament injury as are horses without desmotomy.¹⁹

Palmar annular ligament (PAL) desmotomy is very helpful in treatment of SDF injuries that extend to the distal metacarpal/metatarsal region. Resection of the PAL relieves compression of the enlarged SDF and allows for improved tendon gliding. This procedure is most effective when performed soon (within four weeks) after SDF tendon injury. If the procedure is not performed, the proximal portion of the SDF on subsequent ultrasound exams will appear to be healing at a normal rate, while the portion of the SDF just proximal to and at the level of the PAL will heal poorly. Resection is still useful as a treatment for chronic distal SDF tendon injury, although improved healing of the tendon is less dramatic. Although several surgical repair methods have been described for SDF tendon luxation, aggressive medical therapy provides a good long-term outcome without the complications that can occur following surgical attachment/ stabilization at the original site on the calcaneus.

Prognosis

The prognosis is excellent for life and good for return to full athletic use, if aggressive treatment followed by controlled rehabilitation is performed. If the tendon appears 95% healed based on size, echogenicity and fiber pattern on the final ultrasound exam, the horse is at no greater risk of re-injury than of any athletic use injury upon return to full work. A superficial digital flexor tendon that has luxated from its calcaneal attachment also has a good prognosis with aggressive medical treatment for inflammation and a full rehabilitation program.

History and presenting complaint

Forelimb deep digital ßexor (DDF) damage occurs most frequently in the pastern region of horses used for athletic pursuits which involve twisting and turning, such as polo, endurance riding in rough terrain or roping. In the foot region of fore- and hindlimbs, DDF tendinitis often occurs in middle-aged horses. It is often concurrent with navicular disease, particularly navicular bursitis. Hindlimb metatarsal region DDF tendinitis is most frequently seen in hunters/jumpers, primarily middle-aged geldings that have had years of work and is often accompanied by tarsal sheath synovitis.

Physical examination

Grade 1–3 out of 5 lameness is observed and is often intermittent or transient. Heat, swelling and pain will be present on palpation of the DDF.

Differential diagnoses depend on location; in the metacarpal/ metatarsal region, inferior check ligament(ALDDF or accessory ligament of the deep digital flexor tendon) desmitis is the major differential. Tarsal sheath synovitis may be associated with hindlimb DDF tendinitis. In the pastern the SDF tendon and the sesamoidean ligaments must be included in the differential diagnoses. Digital sheath synovitis is often associated with pastern region DDF tendinitis. In the foot, navicular bursitis, navicular bone changes and other causes of heel pain should be considered as differential diagnoses.

Special examination

Diagnostic nerve blocks will localize the region of the DDF involved and begin the process of elucidating the source(s) of heel pain, if present. Radiographs of the foot will provide information regarding bone and joint involvement. MRI will define both hard and soft tissue injuries and their inter-relationship MRI is providing a large amount of new information that is important in a complete diagnosis but that also needs to be interpreted in light of clinical significance.

Diagnostic ultrasonography will define the presence and extent of DDF damage (Fig. 20.7).^23,24 For a diagnostic examination of the DDF within the hoof, as well as the navicular bursa and the impar ligament, the shoe should be removed and the frog should be pared to provide a level surface for good transducer contact. The foot should be soaked in water overnight to further soften the tissues. The insertion of the DDF on the solar aspect of the distal phalanx, the navicular bursa and the impar ligament can then be evaluated for disease (Fig. 20.8). Ultrasonography will also allow for sequential examinations, which are critical for the accurate assessment of healing stage of injured tendon so that an appropriate treatment and rehabilitation plan can be formulated for each stage of healing.¹⁷

Therapeutic aims

As for SDF tendinitis above.

Therapy

For general tendinitis medical treatment, please refer to the section on SDF tendinitis.

DDF tendinitis in the region of the navicular bone is frequently associated with navicular bursitis. If this is the case, therapy should include injection of the navicular bursa with a symptom modulating and preferably a disease modulating agent. Steroids remain the gold standard for anti-inflammatory activity. Hyaluronic acid, IRAP, or polysulfated glycosaminoglycan are also useful. Injection of the coffin joint rarely provides sufficient clinical and ultrasonographic signs of resolution of bursitis, even if the horse responds to coffin joint anesthesia. Reasons include:

Tarsal sheath synovitis may be treated medically in a similar fashion to the digital sheath (see following section. Tenoscopy of the sheath is an effective treatment for chronic/complicated cases with extensive synovial membrane proliferation and/or adhesions.²⁶

Prognosis

Prognosis is good for return to athletic use if the tendon alone is involved. Concomitant tarsal sheath synovitis, digital sheath synovitis, and navicular bursitis need to be aggressively managed or they may prove to be the limiting factor(s) in future athletic performance. (Fig. 20.9).

History and presenting complaint

Desmitis of the accessory ligamentof the deep digital flexor tendon (AL-DDFT) is usually observed in horses that are starting into regular work if the condition is developmental, or in middle-aged horses (10–14 years) which have been in regular use, particularly as jumpers.

AL-DDFT desmitis can also occur secondary to moderately severe or severe SDF tendinitis, if the SDF tendon becomes sufficiently enlarged to contact the medial and lateral borders of the AL-DDFT. Adhesions may subsequently develop which cause AL-DDFT inflammation and damage.²⁷

Physical examination

Grade 2–3 out of 5 lameness is present and is usually persistent. Heat, swelling and pain are felt on palpation, most often at the accessory ligament-DDFT junction in the palmar/lateral midmetacarpal region. AL-DDFT desmitis occurs less frequently at the origin of the ligament.

Differential diagnoses include DDF tendinitis and suspensory ligament desmitis. The AL-DDFT is often damaged at the midmetacarpal level, where its fibers join those of the DDF tendon.

Special examination

A high two-point nerve block (lateral and medial palmar nerves) should ablate the lameness and will confirm that the lameness is localized to the metacarpal region.

Diagnostic ultrasonography will reveal AL-DDFT enlargement, loss of echogenicity and deterioration of fiber pattern (Fig. 20.10).²⁷ AL-DDFT desmitis and associated enlargement of the ligament may cause the DDF tendon to be completely encircled and compressed by the SDF and the AL and may be an additional source of the chronic pain often associated with AL desmitis (Fig. 20.11).