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:
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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:
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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 :
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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.