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Caring for the Horse's Teeth and Mouth
By Chris Hannes
Trafalgar Square Books
Copyright © 2009
All right reserved.
Chapter One The History of Equine Dentistry
The first type of horse in existence-the Hyracotherium-lived 55 million years ago in South and Central America. The ancestor of today's horse was then about the size of a dog and mainly fed on soft leaves from the abundance of shrubs present. At that time, this precursor of the horse still had teeth with a short crown-just as humans do. The horse of that period could successfully survive because its teeth were barely worn down by the soft food.
Due to great changes in climate, the vegetation in America changed into vast grasslands of hard pampas grass with low energy yields. The horse also became bigger, so gradually, its energy requirements increased. In order to still take in enough food for energy, the horse needed to graze and chew its food for 18 hours a day. In the following evolutionary phase, due to wear on the teeth, the horse developed a set of teeth with a bigger grinding surface and long "reserve" crowns deep into the socket (see p. 6); these teeth were worn down by the chewing of hard vegetation but at the same rate were continually replaced by new tooth from the socket so that the visible crown stayed the same size in the mouth. After 25 to 30 years, the teeth were completely used up.
It appears from cave drawings in Spain and France that the first contact between humans and horses was around 15,000 BC. At that time, the horse was a source of food for humankind. The cave drawings show images of horses stuck with spears. The first proof that man made use of the horse dates from 4000 BC. In the Black Sea area, the skull of a horse was found from this period that clearly had abrasions of the first molars caused from the use of a bit.
Documentation of horse teeth dates back as early as the ancient Greeks (Xenophon, Aristotle), and the most important aspect of which referred to determining the age of the horse by the "replacement" of the incisors. Even at that time, the age of a horse was relevant for its market value. The wearing down of the incisors was described in the Roman period and permitted the age of a horse to be determined up to the age of 10 years. This information was handed down and taken for granted for more than 10 centuries.
In 1770, in Lyon in France, at the school of veterinary sciences, there was mention of the cutting of the milk teeth and their replacement by permanent teeth. For the first time in many ages, people gained new insights into the teeth of horses by studying the heads of cadavers. Later in 1885, in Alfort, a substantial book was published about the build of the horse, which also fully described the set of teeth of a horse and the determining of age.
At the end of the 19th century, a book was published in England about dental care for horses. This was a project in collaboration between a veterinarian and a dental specialist (note, some people are still calling this a new profession today!). Several firms began making instruments for treatment of horse's teeth. Hand floats designed at that time are still excellent instruments for use in treatment today.
In 1904, the first course in equine dentistry was offered at the Detroit Veterinary College. As a means of transport and as a source of power for industry, the horse was extremely important. This is the likely explanation for the growing interest in equine dentistry in the 19th and at the beginning of the 20th century.
After the Second World War, this interest nearly completely disappeared: companies that made the dental instruments were converted to other uses, and interest in horses radically declined due to new forms of transport and the growth of mechanization.
In the 1980s, we witnessed-once again-a return of dental care, despite the fact that it was considered pointless by many horse owners. Nevertheless, from that time, there has been an enormous increase in scientific research on horse's teeth in universities worldwide. Thus, the advantages of caring for the horse's teeth has now been scientifically proved and new treatment protocol developed based on this research.
The array of instruments has expanded enormously in the last twenty years and this has lead to more possibilities for treatment. As well as the existing hand floats, all kinds of electrical floats have been developed. Better dental speculums, portable light sources and such have now made it possible to carry out a thorough examination of the mouth in an animal- friendly manner. The use of anaesthesia for horses has now made the treatment quicker and more efficient.
Within the coming 10 years, every horse owner will consider annual dental treatment as something that goes without saying-just as the bimonthly visit from the farrier already is.
Chapter Two The Anatomy of a Set of Teeth
The set of teeth of a horse consists of 12 incisors and 24 molars; male animals also have four stallion or gelding teeth (the canines). Sometimes, horses may also have two wolf teeth just in front of the molars in the upper jaw; and, there may also be two wolf teeth found in the lower jaw.
There is a very big difference between horse teeth and human teeth. Horse teeth are of the hypsodont type, which have a very long crown that, in a young horse, goes very deeply under the gums into the socket to a relatively short root. The crown is divided into the visible crown (the part you see in the mouth) and the reserve crown (the part of the crown that is still in the socket). Horses need to grind their rough, fibrous rations sufficiently finely with their molars in order to produce a good enzymatic and bacterial digestion in the stomach and the intestines. (The average length of fiber found in a horse's manure is 4 mm). The teeth are worn down every year by about 2 or 3mm because of the rough fibrous structure of forage.
This wearing down is compensated for by the tooth continually emerging further from the socket-this is called the "eruption" of the teeth. In this way, the part of the tooth above the gums-the visible crown-always remains the same. Human teeth are of the brachydont type, which mean that once the permanent teeth of humans have appeared in the mouth, the tooth has finished growing. The crown is much shorter and ends at the gum where it becomes the root. The crowns present in our mouths must be able to serve us for the rest of our lives. We do not need "horse teeth" because we have a diet of soft food.
In order to avoid misunderstanding, a form of the Triadan system is used to denote specific teeth. Triadan numbering is used worldwide in human dentistry. As horses have more teeth than humans, the Triadan system has been adapted but it remains, in principle, the same. This system consists of three numbers and will be used in this book to indicate the tooth denoted.
The first figure denotes which half of the jaw is indicated. The right upper jaw is designated as number 1; the left upper jaw as number 2; the left lower jaw as number 3, and the right lower jaw as number 4. Please note that when stating "left," we mean left from the horse's viewpoint: thus not left as you are standing in front of and facing your horse.
The two numbers that follow thus indicate the specific tooth and in which part of the jaw it is situated. The incisors go from the center to the outside from .01 to .03. If the stallion or gelding tooth is present, it is called .04. Any possible wolf teeth are designated by the number .05. The first to the last molars then receive the numbers .06 to .11. So if we are talking about tooth 310, we are talking about the last but one molar in the left lower jaw.
The two central milk incisors (.01) emerge soon after birth, the next pair-the lateral incisors (.02)-after about 4 to 6 weeks, and the two farthest back-the corner incisors, (.03)-after about 6 to 9 months. The milk incisors are whiter and smaller and have a more clearly defined neck (division between crown and root) and narrower roots than the permanent incisors. The permanent incisors push out the milk incisors from behind and underneath, the roots of which waste away causing the milk teeth to fall out and the permanent incisors to appear. With the central incisors (.01) this happens at the age of 3 years, the lateral ones (.02) at 4 years, and the corner incisors (.03) at 5 years old. These are immutable facts thus they can be soundly used to determine the age of a young horse.
On its grinding surface or "table," the incisor has a depression or enamel cup called the "mark." The mark is, in the beginning, about 10mm deep. As the incisor wears down, the mark gradually becomes shallower and finally disappears. On the lip side of the tooth, the dental star appears on the table as the horse gets older. This is actually the top of the root canal, which gradually gets filled up with dentine. Initially the dental star looks like a line parallel with the lip and then, as the horse gets older, the dental star shifts toward the middle of the table and it becomes rounder. The dental star often has a yellow-to-brownish tint due to the effect of staining from grass pigment. As the horse ages, a white fleck appears in the center of the dental star. The mark in the crown and the dental star are also used to determine age. The rapidity of the wearing down is indeed determined by, among other things, the makeup of the diet (little or much roughage) and the differences in tooth composition and sometimes by external factors such as crib biting. Formerly, this knowledge was also regularly used to falsify the age of a horse by filing away the surface of the incisor.
Galvayne's groove is a vertical groove on the lip side of the upper corner incisors. This groove originates at the edge of the gum and becomes longer with age until the groove reaches the nipping surface. Formerly Galvayne's groove was also used to determine age. However, this groove is absent in 50 percent of horses.
Horses often develop a hook on the back edge of the nipping surface of the upper corner incisor called the échancrure or notch. It used to be thought that these horses were 7 to 13 years old: this parameter is, however, totally untrustworthy as a method of determining the age of a horse.
The gap between the incisors and the first molar is called the inter-dental space or bar. The bit rests in this place. The thin soft tissue of the mouth covers the hard underlying bone. We often see injuries on the bar of the lower jaw in horses that are ridden with a curb bit.
The canines, also known as the tushes, stallion, or gelding teeth (.04), are found in male animals a few centimeters behind the last incisor. Sometimes mares have extremely small canines that can be seen or felt under the soft tissue. Originally, this tooth was used by males as a defence weapon but, with the advance of evolution, it has become defunct in the modern horse. These teeth erupt at about 4 1/2 to 5 years old in both the upper and lower jaw. It is usually pointed and quite sharp. Seeing that the lower canine is further forwards than the upper canine, there is no wearing out of the visible crown by grinding. After their eruption in the mouth, these teeth do not continue to emerge from their sockets as do the incisors and molars. During the whole life of the horse, the long reserve crowns of the canines remain buried in their sockets.
The Wolf Teeth
Wolf teeth (.05) are usually small teeth of about 1 to 2 cm long, which are situated just in front of the upper molars. The visible crown is mostly only a few millimeters long. Wolf teeth are present in the upper jaw of 20 to 30 percent of horses. Very occasionally, wolf teeth can be seen just in front of the first molar in the lower jaw; they are usually much smaller than the wolf teeth in the upper jaw.
Wolf teeth appear in the mouth between 8 months and 1 1/2 years. Long ago in the evolution of the horse, these teeth formed an extra set and were then much bigger. In the modern horse they are completely redundant.
"Blind" wolf teeth are situated 1 to 2 cm in front of the first molar and are mostly implanted slanting toward the front and thus, do not erupt from the soft tissue; they can only be felt as a knob under the skin at the level of the bar.
Wolf teeth can be a reason why some sport horses "lean on the bit" and the ensuing problems. As these teeth no longer have a function in the mouth, they are often removed at the beginning of a sport horse's career.
The first three milk molars (.06 to .08) are already present in the first week of the foal's life. These teeth are replaced by the permanent teeth at 2 1/2 years (.06), 3 years (.07), and 4 years old (.08). The force of the emerging permanent teeth destroys the roots of the much shorter milk teeth and pushes the remaining parts of the milk teeth into the mouth. These remains of milk teeth-the "caps"-can sometimes be found in the horse's feeding container. These caps are usually thin, rectangular slivers with a few sharp remains of the root on the edge.
The last three molars (.09 to .11) appear only once, that is, they arrive as permanent teeth. These appear in the mouth as a 1-year-old (.09), 2 years old (.10) and 31/2 years old (.11). Thus, in an adult horse, the fourth molar (.09) is the oldest tooth in its mouth.
The upper molars have two depressions centrally placed on the grinding surface or table of the crown. As opposed to the incisors, in the beginning these "marks" or enamel cups are not concave but completely filled with cementum and they also reach to just above the root of the tooth. The lower molars do not have enamel cups.
The molars fit beautifully together and form a nearly level surface that gradually curves up toward the back of the mouth. This upward curve is known as the "Spee curvature." The root of the first molar is implanted a little forwards and the roots of the last two molars are implanted slightly backward. With this pressure from both ends of the arcade of teeth, the molars remain nicely compact and the six individual teeth act as a single unit.
The table of most of the molars is rectangular. The tables of the upper teeth are much broader than those of the lower teeth; the upper teeth are also spread 30 percent wider than the lower teeth. This means that, when at rest, only half of the grinding surface of the upper teeth is placed above the outside third of the grinding surface of the lower teeth. Additionally, the tables aren't horizontal but slanted outwards at an angle of about 10 to 25 percent. All these factors in the construction of the upper teeth explain why we so often see injuries in the soft tissue of the cheek that are caused by sharp glazed points on the outside of the chewing surface of the upper molars.
On each complete arcade of teeth there are 11 to 13 transverse ridges on the grinding surface. These ridges ensure a multiplication of the chewing surface of the molars and are a normal and useful phenomenon in a horse.
In the young horse, the roots of the last four upper molars reach to just under the bottom of the sinuses. The sinuses are hollow spaces in the forehead of a horse. This proximity can mean that a bacterial infection of these roots could also cause an infection of the sinuses.
A mature molar is up to 10 cm long of which the greater part is to be found in the socket. As mentioned, during the lifetime of the horse, the reserve crown gradually and continually erupts from the socket at a rate of 2 to 3 mm a year. At about 30 to 35 years old, the roots appear at the surface and the rest of the tooth falls out by degrees.
The skull is made up of two halves: the lower half consists of only two bones that join together in the front, that is, the branches of the lower jaw or rami mandibulae together form the lower jaw. The upper half consists of a collection of several bone parts including the upper jaw part. The lower jaw can, via the jaw joint, move in three directions in relation to the upper jaw. The horse can move its mouth quite far-up and down and sideways. The forward and backward movement of both jaws, in relation to each other, is limited (6 to 8 mm). With each of these movements it is always the lower jaw which moves in relation to the upper jaw. The outer side of the jaw joint can be felt on the outside of the head just in front of the base of the ear.
The Salivary Glands
On each side of its head the horse has three salivary glands of which the biggest is the parotid gland. This is found behind the jaw and under the ear, and when the horse is chewing, it produces 50 ml of saliva a minute: thus, a production of 3 liters of saliva per hour. The gland discharges into the mouth cavity at the level of the third upper molar via the cheek. The drainage channel of this salivary gland commences along the inside of the lower jaw. At the front of the masseter (the muscle for chewing), the channel goes under the lower jaw through to the outside of the lower jaw where it lies just below the skin. An injury at this spot can cause an opening in the channel; you will then see saliva flow from the wound when the horse is eating.
In the spring, when horses are first put out onto grass, this gland can sometimes swell up a lot. The reason for this is still not known. Fortunately, after a night of rest in its stall with its head in the normal raised position, the swelling usually subsides.
The smaller, submandibular salivary gland is situated behind the lower jaw curve and it discharges into the mouth cavity in the inside at the level of the canines. The sublingual salivary gland under the tongue lies on both sides just under the soft tissue and has several openings into the mouth cavity.
Excerpted from Caring for the Horse's Teeth and Mouth by Chris Hannes Copyright © 2009 by Chris Hannes. Excerpted by permission.
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