Tendons and Horses



Tendons and Horses

During the evolution of the horse from an animal about the size of a dog to the large animal we know today, many changes took place in the limbs to allow the horse to maintain its speed and outsprint predators.

The horse has had to develop large, powerful and efficient limb muscles, which for mechanical reasons, had to be positioned close to the body. It also had to develop long limbs so that a great distance could be covered with each stride. For these changes to occur, it required long, fine, lightweight structures to connect the strong upper limb muscles to the bones of the distant lower limbs. These structures are tendons.

The superficial digital flexor tendon.

This lies just beneath the skin at the back of the cannon bone, its major function is to flex the fetlock joint. The muscle to which is it attached, the superficial flexor muscle, begins at the level of the elbow and joins the superficial flexor tendon at the back of the knee. The tendon continues under the skin at the back of the cannon bone, and attaches to the bones of the pastern. Not only does this tendon flex the fetlock joint when its muscle contracts, but it also supports the fetlock joint at its normal angle when the horse places full weight on the leg.

In the forelimb, just below the knee ( carpus), a ligament attaches the top of the tendon to the radius. This is the proximal check ligament and its function is to prevent excessive stretching of the muscle-tendon junction.

At the top of the pastern, the tendon divides into two branches, each of which attaches to the bones at the back of the pastern.

The deep digital flexor tendon.

This lies deeper in the limb than the superficial digital flexor tendon throughout most of its course. It also has an attached muscle that begins at the elbow and joins the tendon at the back of the knee. Between the knee and the fetlock, the tendon lies between the superficial digital flexor tendon and the suspensory ligament. Behind the fetlock joint, the tendon passes over the back of the sesamoid bones, and then down the back of the pastern, between the branches of the superficial digital flexor tendon. This tendon connects to the underside of the pedal bone in the foot. When the muscle contracts, the coffin, pastern and fetlock joints all flex.

Like the superficial digital flexor tendon, the deep one also has a check ligament, and it connects the tendon to the back of the cannon bone. The ligament is called the distal check ligament, and it connects the tendon to the back of the cannon bone. The ligament attaches to the deep digital flexor tendon between a third and a half of the way down the cannon bone and helps prevent excessive stretching of the tendon.

The suspensory ligament.

This is a part of the suspensory apparatus which also includes two sesamoid bones and the distal sesamoidean ligaments. The suspensory apparatus supports the fetlock at its normal angle.

What is tendon made of ?

Tendons are complex structures that have evolved to withstand the great forces exerted on them by heavy, fast running animals. Their main component is a protein, collagen, which provides the strength. The collagen is present as long, thin fibrils lengthwise along the tendon, arranged in bundles. The fibrils in a tendon vary in size, the larger ones are thought to be stronger, the smaller ones provide elasticity. In adult equines, both small and large fibrils exist in tendons, giving a combination of strength and elasticity.

When injured, tendons heal with a weaker type of collagen. The fibrils are smaller and they are aligned randomly, rather than along the length of the tendon. For these reasons, healing tendons are much weaker than normal tendons, and they take many months to regain near-normal strength.

TENDON INJURY

How are tendons injured ?

Tendons can be cut by sharp objects, or bruised by being struck by another limb, or even damaged by overly tight bandages. Hobbling a horse above the fetlock is a very common reason for tendon injury. Excessive strain on the tendons caused by sudden or repeated overstretching ( in overloading for example) will also cause damage. Sudden overstretching can be caused by a tired animal putting the limb awkwardly on uneven ground. Young animals suddenly put into work, can easily repeatedly strain tendons, causing minor injuries within the tendon structure that gradually weaken it.

Poor limb or hoof conformation may predispose to tendon injury by causing uneven strain on the tendons. For example horses with a long toe, low heel conformation and horses that have uneven wall lengths on the hoof wall, seem particularly prone to tendon injury. Horses with lameness for whatever reason may overload the tendons of the “sound” leg and damage them.

How is tendon injury recognised ?

Damage to body tissues results in signs of inflammation, which include swelling, heat, pain and, if the injury is severe enough, loss of function of the body part. Tendons react to injury by showing these typical signs of inflammation.

1) Swelling

Swelling is one of the most important indications that damage has occurred to a tendon. Even minor swelling of a tendon can indicate serious injury. Swelling of the superficial digital flexor tendon can be seen at the back of the cannon area. When there is severe swelling, the tendon has a bowed appearance, instead of appearing straight up-down when viewed from the side. Minor swelling of the tendon is not so easily seen, but can be detected by feeling the tendon, and comparing it with a normal tendon on the other front leg or on another horse.

Suspensory ligament swelling can be seen in two places. It may occur just behind the cannon bone in front of the tendons, or it may be limited to swelling of the suspensory branches near the side of the fetlock joint. In the normal horse, a vertical depression or hollow is present just behind the cannon bone. With suspensory ligament injury, there may be generalised filling or swelling in this area.

When the check ligament of the deep flexor tendon is damaged, swelling is commonly seen or felt behind the top of the cannon bone just below the knee. The swelling may be similar to that seen with suspensory ligament injury.

Occasionally tendons are damaged behind the pastern. Swelling in this area may indicate that the branches of the superficial digital flexor tendon have been injured where they attach to the pastern, or that the deep digital flexor tendon or tendon sheath have been damaged.

2) Pain

Horses exhibit pain by showing lameness, or by flinching when a painful area is palpated. Horse with tendon and suspensory ligament injuries may show very little pain. Lameness is not usual or may go unnoticed with damage to the superficial digital flexor tendon, unless the damage is recent or severe. Lameness commonly occurs with injury to the suspensory ligament or check ligament, but the lameness may be mild.

A horse with a recent injury of the superficial digital flexor tendon will show pain when the tendon is palpated with the leg held off the ground. When the horse is standing with weight on the tendon, palpation may not cause pain, even if the tendon is damaged. Within about a month of injury, the pain associated with the injury may no longer be present. The fact that the horse shows no pain does not necessarily mean that the tendon injury has healed.

A horse with significant injury to a tendon may not show any lameness. A horse, which has recent swelling of a tendon, should not be worked, even if it is not lame.

3) Heat

An inflamed tendon will feel warm to the touch, when compared with a normal tendon on the same horse. The heat, like the acute pain, tends to disappear within a few weeks of the injury, but this does not necessarily indicate that the tendon damage has healed.

4) Loss of function.

If a superficial digital flexor tendon or suspensory ligament is severely injured, there may be a loss of support of the fetlock joint. When the horse puts weight on the injured limb, the back of the fetlock joint may sink further to the ground than usual. Sinking of the fetlock joint, in association with damage to a tendon or suspensory ligament, indicates very severe injury. Occasionally complete rupture of the tendon or suspensory ligament can occur. This can happen if an animal with tendon injury continues to do hard work.

DIAGNOSING THE PROBLEM

Prior to examining the horse, it is important to ask a few questions of the owner:

• What type of work has the horse been doing.

• When was the injury first noticed.

• Whether the horse has had any previous tendon problems.

• Whether the horse has had any other leg problems or signs of lameness recently.

A general examination of the animal should then take place to detect any possible conditions that might have predisposed the horse to tendon injury, eg: poorly shod, poor conformation, other lameness etc. Occasionally horses with tendon problems will have back pain or hind limb lameness which have contributed to overload of one leg. The extent of lameness examination will vary with each individual case.

Close examination of the affected tendon or ligament mainly consists of palpating the affected area, although in some countries other diagnostic equipment such as ultrasound or thermography is also used.

The tendons are palpated with the horse standing on the injured leg and, then again, with the foot held off the ground. Gentle pressure is applied with the fingers up and down each of the main tendons, including the superficial and deep digital flexor tendons and the distal check ligament and suspensory ligament. The superficial digital flexor and deep digital flexor tendons should be easily separated with the fingers, and the superficial digital flexor can be rolled gently away from the deep one. Enlargement of part of a tendon, pain on gentle palpation, or adhesion of the superficial digital flexor and deep digital flexor tendons to each other suggest that there has been tendon injury.

The tendons on both front legs should be examined because it is common for tendon injuries to occur in both forelegs at the same time, although one injury may be worse than the other.

MANAGING THE INJURED TENDON

The importance of immediate treatment.

In many cases of tendon injury, particularly those in which severe damage has occurred, the healing process can be greatly influenced by the immediate treatment given. Treatment should be started as soon as the first sign of tendon injury ( swelling) is noticed.

Reducing the amount of swelling can help stop the progressive damage, which occurs in a tendon after the initial injury.

The immediate first aid treatment aims to:

• Restrict the amount of swelling. ( by reducing bleeding and accumulation of inflammatory fluids in the tendon)

• Prevent further damage to the tendon.

• Relieve pain.

This is carried out by :

• Cooling the tendon.

• Applying pressure.

• Restricting a horse’s activity.

Cooling the tendon with cold water or ice packs will slow the bleeding and reduce the amount of inflammatory fluid pouring into the injury site. Cold will encourage vasoconstriction, slowing bleeding and delivery of inflammatory products. Cold should be applied for 10 - 20 minutes at a time ( any longer and vasodilation will start again due to oxygen depletion in the tissues). Cold application can be repeated 2-3 times a day, several hours apart, for 3-5 days.

After the leg has been hosed or iced, a firm padded bandage should be applied. This will also restrict the amount of swelling that occurs because of the injury. It is very important to use sufficient padding, as constriction of blood vessels may lead to pressure sores or further tendon injury. The leg should be evenly wrapped using a roll of cotton wool for example. Bandaging can be continued for up to 2-3 weeks. It is very important to change the bandage at least every two days, even if it is in good condition, as pressure points can develop under even the most expertly applied bandage.

The horse’s activity should be restricted by confining it to a box, this is particularly important if there are indications that the injury may be severe: eg, the horse shows lameness or the fetlock appears to be dropped in comparison with the normal leg.

Non steroidal anti-inflammatory drugs, such as phenylbutazone, are recommended to help control pain and inflammation. These will be most effective if given very soon after the injury. They make the horse feel better, reinforcing the need for restricted activity.

Usually box rest needs to continue for at least 3 weeks depending on the severity of the injury. Reintroduction to work should, ideally, be very gradual. As mentioned earlier, healing tendons are not as strong and the likelihood of re-injury is significant if animals are put back to work too soon.

Adjusting foot balance.

Horses recovering from tendon injuries should have their feet carefully trimmed or shod, to ensure that the tendons are loaded evenly. Any abnormalities of hoof conformation, which may have contributed to the tendon injury, should be corrected during this time. In some cases of severe tendon injury, eg if the fetlock is dropped, it may be necessary to have special shoes made.

Surgery.

Surgery is sometimes carried out to try to speed the healing process, and to try to prevent recurrence of the injury when the horse returns to work. Techniques include tendon splitting and proximal check ligament desmotomy. In many opinions the surgical procedures offer little advantage over rest and controlled return to full work.

Older treatments.

Firing is the application of heat to burn the skin and, in some cases, the tissue under the skin, including the tendon. Blistering is the application of chemicals to achieve the same effect. The result of both “treatments” is inflammation with pain, swelling and blistering of skin. Both are likely to cause delay in healing and scarring can result between the skin and the tendon beneath, reducing normal movement and predisposing to further tendon injury. Research has not shown any benefits from firing or blistering and they should not be used.

PREVENTING TENDON STRAIN.

Maintain good foot balance.

Poor foot balance is thought to be one of the most important factors contributing to tendon injury in horses. Low heels, long toes and uneven hoof wall lengths can predispose to tendon injury by loading the tendons unevenly. Good farriery can therefore help to prevent damaged tendons.

Avoid overloading the tendons.

Therefore do not overload the horse ! Young horses, particularly, should be introduced to work gradually.

Don’t overwork the horse.

Fatigue is an important cause of tendon injury, especially if it is working on rough terrain.

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