Tendons are made up of thousands of collagen fibers arranged in a very precise direction and stabilized between them by bridging molecules. Tendonitis problems are common in horses. All the fibers of a tendon are surrounded by a thin fibrous membrane: the peri-tendon. The tendons of the limbs are divided into two large groups: the extensors with the dorsal extensor tendon and the lateral extensor tendon of the finger, the flexors which are divided into the superficial flexor tendon and the deep flexor tendon of the finger. As their name indicates, the flexors are used to flex the digital extremities of the limbs (fetlock, pastern, foot) as well as the carpus in the forelimbs. The extensors have an opposite action to the flexors, allowing the extension of the digital extremity but also the flexion of the tarsus
Five to six centimeters above the fetlock, the flexor tendons are surrounded by a synovial sheath. This structure is like a joint capsule in that its inner layer secretes a synovial fluid that lubricates the tendons and promotes sliding on the sesamoid bones behind the fetlock.
We should also mention three ligamentous structures that are often the cause of lameness in horses. These are the fetlock suspensory ligament (interosseous III muscle) and the accessory ligaments of the superficial flexor tendon (radial flange) and the deep flexor (carpal flange). The role of the suspensory ligament is to support the fetlock by mimicking a kind of strap behind this joint. The accessory ligaments of the flexors have the role of preventing excessive stretching of their respective tendons. Note the absence of accessory ligaments of the flexor pollicis superficialis in the hind limbs.
Tendonitis in the horse
Tendonitis and desmitis are respectively an inflammation of a tendon and an inflammation of a ligament. There is acute tendonitis, which appeared in the previous hours or days, and chronic tendonitis, which is present for a few weeks or even years. Tendonitis of the superficial flexor of the thoracic limbs is the most common condition in this group. Typically, it is responsible for mild to moderate lameness and is seen as a “banana” shaped swelling located in the palmar and middle region of the cannon, at the narrowest point of the tendon. In the acute form of tendonitis, pain and heat are detected by palpation. These signs will disappear after healing, but a swelling will often persist. In the chronic form, a sensation of hardness of the tendon will be palpated. Tendonitis of the superficial flexor is usually caused by an excessive hyper-extension of the distal extremity of the limb (the palmar region of the fetlock almost touches the ground) as observed during an important exercise (gallop, jump). This overextension leads to rupture of one or more fibers resulting in hemorrhage and fluid accumulation in the tendon. This is shown on ultrasound examination by the presence of black areas (anechoic) in the tendon. There are causes predisposing to digital hyperextension such as poor physical preparation responsible for muscular fatigue, a poor working ground (irregular, slippery), or a poor conformation of the limbs (axis of the foot less inclined than the axis of the pastern).
In the majority of cases of tendonitis, there is a rupture of fibers. Healing is then done by replacing the torn collagen fibers with new ones. This happens slowly and the scar will always be less solid than a normal tendon, exposing the affected animals to recurrences of tendonitis.
Its treatment, whether medical or surgical, must consider this long convalescence. It must include in all cases rest with daily exercise for a period of between one and twelve weeks, depending on the importance of the lesions. Resumption of full work may be delayed for up to eight to ten months after the onset of symptoms. In the first days of the acute phase, the treatment will focus on counteracting the inflammation. This is accomplished by using ice or cold water applied directly to the tendon, non-steroidal anti-inflammatory drugs and bandaging to prevent the development of swelling.
In some cases, the veterinarian will use treatments injected in contact with the tendon lesions or surgical techniques such as longitudinal tenotomy. This consists of inserting a tenotome into the areas of major hypothyroidism on a tranquilized standing horse, with local anesthesia and under ultrasound visualization. The hematoma and edema present in these hypothyroid lesions are thus eliminated, which facilitates revascularization and healing of the tendon. In some cases, desmotomy or sectioning of the radial flange will be used. This technique allows for greater involvement of the flexor pollicis superficialis muscle and the proximal tendon unit of the radial flange to the supporting apparatus of the limb to regain the elasticity lost during the healing of the tendonitis.
Tendon laceration and rupture in horses
An injury to the dorsal aspect of the limbs (often below the hock) can partially or totally lacerate the extensor tendons. With this type of laceration, the horse tends to send the fetlock forward as the injured limb touches the ground. However, this does not prevent the animal from carrying weight on the limb. Treatment consists of regular wound care and sometimes the application of orthopedic shoeing or a restraint bandage to facilitate movement of the limb. The prognosis is usually good if only the extensor tendons are affected.
Flexor laceration is most often seen in the palmar or plantar region of the pastern and is caused by sheet metal or glass. In all cases, the veterinarian should be called urgently to provide first aid, especially since the synovial sheaths surrounding the flexors may also be affected, requiring aggressive therapy appropriate to these structures. The surgeon will attempt to suture the injured tendons under general anesthesia, but this is not always possible and depends on the extent of the damage. Surgery is of no use if immobilization, adequate antibiotic, and anti-inflammatory treatment are not instituted at the same time. The prognosis for competition and even life is often reserved when lacerations involve the flexors.
Teno-synovitis in horses
Teno-synovitis is the inflammation of a synovial tendon sheath. It is observed by swelling and distension of the involved sheath which fills with synovial fluid. This condition most often affects the digital synovial sheath of the flexors located behind the fetlock. It can be acute (appearing in the previous hours or days) or chronic (present for several weeks or years), primary or secondary. In the case of a primary teno-synovitis, there is an inflammation following an abnormal stretching of the sheath itself, such as during hyperextension of the fetlock or during torsion and stretching of the distal part of the limb when it is, for example, caught in a hole. In other cases, teno-synovitis is secondary to tendinitis of one or both flexors of the finger or to a thickening of the annular ligament of the fetlock which causes, by constriction, a proximal accumulation of synovial fluid.
If the acute teno-synovitis is not treated, the synovial membrane will stretch and the cavity will fill with fluid in an irreversible way, without any obvious clinical sign except aesthetic. This is called chronic teno-synovitis. This one can also be due to a persistence or a bad healing of the primary problem. In some cases, fibrous adhesions develop in the sheath, which darkens a prognosis that is good for the sport if the problem is treated in time. The recommended treatment is like the one recommended for tendonitis, also using injections of drugs such as steroidal anti-inflammatory drugs or hyaluronic acid into the synovial sheath. In some cases, a desmotomy of the annular ligament of the fetlock will be performed, i.e., the ligament will be cut along its entire length, to release any constriction. Finally, it is important to note the case of septic teno-synovitis discussed in the paragraph on septic arthritis.
For more information: Tendonitis in horses
