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Conformation Clinic Part 5 – The hoof

Before we can begin our discussion of the hoof, it is important that you understand its anatomy.  From the outside, it looks like a one hard, dense growth, like a horn or tusk.  In reality, the hoof is a very complex structure of bones, cartilage, tendon, tissue, blood and nerves. 

What you see from the outside is the hoof wall.  This part is hard and dense with no nerve endings, much like your fingernail.  However, it is only about one-half inch thick.  If you lift the foot and look at it from the bottom, you can see a “V” shaped structure.  The widest part of the “V” starts at the back of the hoof and the tip of the “V” ends in the middle of the hoof.  This structure is called the frog.  Where the frog meets the leg is called the heel.  On either side of the frog the hoof raises and forms a ridge called the bars of the hoof.  Sometimes you can see the hoof wall running along the outer edge of the hoof.  If the horse is wearing shoes, they will be nailed to the wall and covering it.  The rest of the underside of the hoof is called the sole.

Inside the hoof, the frog and sole are comprised of very elastic tissue and huge amounts of blood.  The elastic nature of the tissue allows it to expand and absorb the weight and shock of each step or stride.  Some have said each hoof is like another heart in the horse, pumping the blood with every step.  There are also two important bones in the foot.  The coffin bone is the larger bone, angled down toward the toe of the foot. A smaller bone tucked in behind the coffin bone is the navicular bone.  These bones work in conjunction with the short and long pastern bones connecting the leg to the hoof.  The deep flexor tendon runs from the coffin bone up the back of the leg.  When you hear reference to the laminae of the horse’s hoof it is most often pertaining to a sensitive, highly vascular layer of tissue that attaches to the coffin bone and lower edges of the lateral cartilages.  The outer layer becomes the insensitive laminae and forms the hoof wall.  There are other, more specific structures within the hoof, but this information will suffice to give you an idea of some of the problems that occur within the hoof. 

So what should you look for in a correct, healthy hoof?  A correct hoof is round and large enough to support the weight of the horse. The angle of the foot follows the angle from the pastern.  If you measure the angle along the front of the hoof from the end of the hair (coronet) to the ground it should be about fifty five degrees.  On the underside, the sole of the foot should be slightly concaved and the widest part of the frog should be about half the width of the hoof.  Standing in front of the horse, the toes should point straight forward.  Standing behind the horse, both heels should touch the ground.  What makes it difficult to determine conformational correctness in a hoof is that a farrier can make many changes to the hoof’s appearance, both good and bad.

For example, when I purchased my young horse Greta, she initially did not pass the pre-purchase inspection by the veterinarian because he felt that she was “club footed.”  We knew that her mother had a club foot and that the condition could be inherited.  A club foot is a conformational flaw where an imaginary line drawn from the pastern through the toe is not a continuous angle, but breaks downward just above the foot at the coronet.  This puts more weight and pressure on the front of the hoof than on the back.  The result is that the heel of the foot grows faster than the toe and eventually the elevated heel and consistent pressure on the toe causes the deep flexor tendon to shorten, rotating the coffin bone out of position.  Knowing that my intention was to do dressage and eventing, the veterinarian felt that she would not stay sound.  However, Greta had never had a proper trim job done on her feet (she was only 1 year old, would not stand to be trimmed because she had not been properly handled, and was being trimmed by a less than competent farrier).  To the surprise of my veterinarian, X-rays revealed that the bone structure of the foot was in perfect condition.  After a year of proper trimming to gradually reshape the hoof she showed no sign of excess heel growth, her foot had a perfect angle and she remained sound.  When I sold her 3 years later, almost everyone (including other veterinarians) commented at what sound hooves she had. 

Horses that toe in or toe out slightly can have the hoof shaped to make them look more balanced and help them travel straighter.  An extra piece of metal on the inside of a hind horseshoe can help horses that travel too close behind.  Corrective shoeing can sometimes make a horse that has a conformational flaw or diseased hoof more comfortable and usable. An improper trimming or shoeing may result in sheared heals where both heels do not meet the ground equally.

A horizontal ridge around the hoof is an indication that a horse has foundered.  Founder (technically called Laminitis) is one of the most serious conditions of the horse’s foot.  It results from the pooling of blood and swelling in the tissue (or laminae) of the hoof which is trapped against the solid hoof wall.  It can be caused by sudden change in diet (too much grass or grain), ingesting too much water when over heated, riding on hard surfaces (such as pavement or cement) for extended periods of time, as a complication of pregnancy after birth, or anytime the horse suffers from a high fever.  The horizontal ridges are formed from the wall of the hoof trying to separate at the cornet band (where hoof meets leg).  All of the internal structures of the hoof are compromised under the intense pressure that occurs during foundering.  Foundered hooves may appear to curl up at the toes due to the damage of the tissue in the sole of the foot.   In most cases the coffin bone suffers some rotation.  In the most severe cases the coffin bone can fall through the sole of the foot.  Occasionally the entire foot is lost and the horse must be destroyed.

  Horses that suffer a mild case of founder can often be serviceably sound.  However, if a horse has foundered once he will be predisposed to do it again.  Special care must be taken to keep his diet free from excess amounts of grain, lush green pastures or fresh hay.  Special care must also be taken in maintaining the feet. A farrier and veterinarian will need to work together to develop corrective shoeing or trimming methods to make the horse comfortable and support the coffin bone.

The second most common and serious condition of the hoof is called Navicular disease.  In navicular disease the tissue is stressed beyond its limits and the navicular bone begins to degenerate.  Erosion of the cartilage and adhesions between the bone and flexor tendon also occur.  These changes may also be accompanied by swelling in the bursa (fluid around the joint) and tendons. It is not known exactly what causes navicular disease, but conformational defects such as small hooves, straight pasterns and straight shoulders are predisposing conditions.  Horses with navicular disease often have contracted heels due to poor circulation and a reluctance of the horse to bear weight over his heels. “Contracted heels” is a conformational flaw of the hoof where the heels, or back of the hoof become narrow.  As you look at the conformation of a horse, be sure that he is standing squarely on all four feet and that he carries the majority of his weight over the center of the hoof. Pointing of the toes is observed in horses suffering from both navicular disease and founder.  Both of these diseases are more prevalent in the front hooves.

   Navicular disease is difficult to diagnose as the navicular bone is obscured by the coffin bone.  A veterinarian will use x-rays, the horse’s reaction to pressure applied when using hoof testers, predisposing conformational flaws, predisposing types of work (hard stops, twists at speed, forceful landings) and a temporary block of the nerve endings to target the area. Treatment is limited to corrective shoeing and the use of pain relieving drugs such as butazolidin (bute).  The drug Isoxsuprine hydrochloride has also proven helpful, although no one is quite sure why or how it works.  In advanced stages sometimes surgery is performed to permanently deaden the affected nerve endings.  The procedure is called a palmar digital neurectomy.    

Bacteria, yeast and fungus are responsible for many undesirable hoof conditions.  Most of them can be successfully treated therefore placing them in the category of blemishes rather than unsoundnesses.  Left untreated however, all of these conditions can cause permanent damage.  The most common of these is thrush.  Thrush is a disease of the frog.  It is a painful bacterial infection where the frog literally rots away.  The characteristic stench is unmistakable.  The frog will appear black, and with further cleaning will reveal white cheesy diseased material.  The infection may spread into the laminae and other structures in the foot.

Any type of open sore or drainage from around the cornet band is a sign of infection within the hoof.  These may include white line disease, quitters, and gravels.  Of these gravels are the most common and least destructive.  A gravel is the result of a puncture wound to the white line.  The “White Line” refers to an area between the hoof wall and the laminae.  All foot wounds are predisposed to infection, and since the infection can not travel down through the sole of the foot, it travels upwards through the cornet band.  Most infections and abscesses will “blow out” at the cornet band.

White line disease is more serious because the condition is often not discovered until the infection is quite progressed.  It is also known as “Seedy Toe.”  The infection is caused by a bacteria, yeast or fungus at the ground level and works its way up the white line to the coronary band.  The disintegration of this white line creates a hollow space between the hoof wall and the sole.  The hollow space becomes mealy or “seedy” and fills with cheesy material and debris.  The loss of the supporting white line against the pull of the flexor tendon can cause rotation of the coffin bone, at which point the condition would be considered an unsoundness (permanent impairment of function).  The only way to treat the condition is to remove all of the affected area.  This often requires removal of so much of the hoof that prosthetic hoof repair material and special shoes must be used to preserve the foot.  It is believed that horses that have suffered white line will be predisposed to contract it again.  

A quitter is a deep seated infection of the cartilages around the coffin bone.  The inflammation will eventually blow out the coronet band.  Injuries to the sole of the foot often lead to this infection.  The only treatment is to surgically remove the affected cartilage.  Many horses have a complete recovery, but if the surrounding structures have been damaged there may be permanent lameness.

The last unsoundness you should look for in the hoof area is ringbone.  Ringbone is more accurately a condition of the pastern, but because its effects can often be seen at the very top of the hoof on the coronet band I have included it here.  Ringbone is a type of arthritis of the pastern joints.  Low ringbone affects the joint where the coffin bone meets the short pastern bone.  High ringbone affects the joint where the short and long pastern bones meet.  Ringbone will look like a hard lump on the coronet band.