TALL-CROWNED TEETH WITH COMPLEX ENAMEL PATTERNS
The teeth of all mammals are formed of three hard materials: enamel, dentine and cementum. The arrangement of embryonic tissues dictates the final form of the tooth, which is vastly different in humans vs. equines. Human teeth are low-crowned (brachydont) and can be compared to cream-filled cupcakes: The substance of the tooth is the “cake”--dentine---while a layer of “icing” coats the top, the denser, harder enamel. “Cream filling”---the pulp which contains tooth-repair cells and a rich circulatory supply---enters through the apical foramina at the tips of the roots and is massed in the center of the tooth.
Horse teeth are high-crowned (hypsodont). Crown height in teeth is measured from the cingulum---a thin zone of dentine that overlies the tooth roots below and supports the cusps above. In horses, the crowns of the cheek teeth, instead of measuring less than half an inch from cingulum to chewing surface (as in a person or a pig), measure six inches or more.
Horse and human teeth differ not only in height but in the arrangement of enamel, dentine and cementum. Human teeth are “bunodont,” which means “teeth with little hills on them”---the “hills” being the cusps that are individually distinguishable. As the person ages, the hard enamel “icing” coating the crown of the tooth wears thin and may eventually be worn entirely off the tops of the cusps, exposing the dentine. The pattern of exposure consists of little dots or arcs scattered over the chewing surface of the tooth which enlarge as more enamel is worn off as the cusps wear down.
Horse teeth are lophodont rather than bunodont. While the “crown pattern” of a worn bunodont tooth consists of dots centered over individual cusps, that of a lophodont tooth is formed by parallel lines and low basins. The enamel bands on the chewing surfaces of the teeth represent the tops of enamel tubes that are standing on end.
Think of a horse tooth as being essentially like a bundle of pipes: Looking at them end-on, you would see the pattern formed by the top edges of the pipes. In horse teeth, the pipes are not round but stretched out wide, and their walls are corrugated so that the top edges appear wrinkled. Each tube is filled nearly to the top by either dentine or cementum. Because enamel is the hardest material, as the horse chews it wears away more slowly. This creates a rough surface like a washboard that not only efficiently grinds fodder but continually self-sharpens.
TALL-CROWNED TEETH CONTINUOUSLY ERUPT
The lifespan of a mammal in nature is ultimately dictated by its teeth; if disease, predation or mishap doesn’t end the horse’s life, he will die when his teeth wear out. Horse teeth are designed to withstand a diet made gritty not only by dust particles that adhere to the surfaces of grass blades but by the silica (glass-hard) spicules that many grasses secrete within their blades as structural support. A human attempting to eat such a diet would soon wear all the enamel off his teeth, but the horse’s teeth will last for up to about 20 years because they are not only tall---with six inches or more of length available to wear down---but also lophodont, which ensures that bands of hard enamel will continually be present on the surface.
Of course, all six inches of crown are never present in the horse’s mouth. Only about half an inch of crown is exposed at any one time in the oral cavity, while the rest is stored in the vertically deep skull and jaws. This can be seen in the difference between the roots of equine teeth and the stored portion, which is called “reserve crown”: The roots are no longer in hypsodont than in brachydont teeth. It is only the crown---the portion above the cingulum---that has lengthened. The reserve crown and roots of most of the upper cheek teeth are stored in the maxillary sinus cavity, while those of the lower cheek teeth are stored in the horizontal ramus of the mandible.
In the mouth of a normal adult male horse, one may observe at least 40 teeth---20 above and 20 below. Females may have as many as four fewer teeth, because they sometimes lack canines. Both sexes may have as many as four additional teeth in the form of the socalled “wolf teeth,” which are vestigial anterior premolars, not canines. Horse teeth fall into four categories based on shape and position: incisors, canines, premolars and molars. Each tooth develops within, and erupts from, a silolike tube in the skull---the tooth socket (alveolus). Ligaments extending inward from its rim “grab” the tooth, supporting it and acting to regulate its height in the mouth.
In people, each tooth goes through a short period of eruption that stops once the tooth breaks through the gum and comes into occlusion with its mate in the opposite jaw. In the incisor and cheek teeth of horses, there is an initial “push” similar to that in humans, but the process in the horse never completely stops. As the horse’s gritty diet wears down the surfaces of its teeth, the alveolar ligaments continually pull
the teeth into the mouth. You can think of tooth wear in horses as being similar to using the sort of eraser that comes in a plastic tube: As the eraser wears down to the rim, you click out a little more length as needed.
DECIDUOUS AND PERMANENT TEETH
Like people, horses have “baby teeth” (technically, called “deciduous” because they are eventually shed, like leaves from a tree). In the embryo, the alveoli influence tooth development, so that it is the position of the alveolus in the mouth that determines the identity of the tooth that develops within it. Thus, a premolar is a “grinder” that develops in one of the first three alveoli along the cheek tooth row---the prefix “pre-” in this case indicating the forward position of the tooth. Deciduous teeth, however, are anterior in time, i.e., they develop and erupt first. In horses,
deciduous teeth develop only in the alveoli for the incisors and those for the first three cheek teeth, the premolars. During embryonic life and foalhood, each of these alveoli contains two masses of germ cells---tooth buds--that have the potential to develop into working teeth. The first of the buds to mature is that for the deciduous tooth. The second bud, which lodges deeper in the alveolus, matures later. When the tooth that forms from it finally begins to erupt, it will push the (by then) expired deciduous tooth out. The second tooth in a two-bud alveolus is the “permanent” tooth.
In mammals, the alveoli for the molars---that is, the rear three alveoli of the cheek tooth rows---never contain more than one tooth bud, that for the permanent tooth. In most mammals, both deciduous and permanent teeth develop in the alveoli for the canine teeth, but in horses it appears that the
deciduous tooth bud has been lost, so that only one tooth develops and erupts from the canine alveolus. In all mammals, teeth erupt only one or a few at a time, which preserves the youngster’s ability to eat when it is teething. There is a regular schedule of eruption, which differs from species to species.
PRESSURE CHANGES AND RESULTING MALOCCLUSIONS
Horses are highly unusual in having incisor teeth that are meant to meet end to end. When a person with normal dentition closes her mouth, the upper incisors close in front of the corresponding lower incisors. Cattle lack upper incisors altogether. When the jaws are closed with either of these configurations, contact between the upper and lower jaws happens in only two areas: the
cheek-tooth row and the jaw joint.
In horses, however, there are three zones of contact: the jaw joint, the whole length of the cheek-tooth row, and the incisors. As a normal horse closes its mouth to crunch down on the bolus of feed, the jaws are pulled sideways and upward until they are fully closed, at which point the central incisor pair of the lower jaw will be directly beneath, and in contact with, the central incisor pair of the upper jaws. In the normal equine mouth, the incisors do not touch at all during the chewing stroke, and even when centered at full closure they touch only lightly. They play no part in the grinding of food.
It is useful to visualize the chewing stroke as a sheet like the top surface of a wide, flat pipe that parallels the long axis of the animal’s head. When the horse moves its lower jaw to chew, the lower cheek teeth are drawn up and across the corresponding surface of the upper cheek teeth. Malocclusions tend not to develop so long as even pressure is maintained along this “surface of grind.”
Unfortunately, this balance can be upset in several ways. A common one is the late shedding of the “caps”---wornout deciduous teeth---pertaining to the premolar alveoli. Having a cap clinging on is about as comfortable for the horse as having a piece of hard candy stuck atop one of your bicuspids: It forces the jaws to tilt to one side as you attempt to chew and creates discomfort in your TMJ (temporomandibular or jaw joints) as well. If the retained cap isn’t removed by an equine dentist, or doesn’t fall off of its own accord within a few days of the time its twin from the opposite side of the mouth was shed, the teeth will already have begun to wear unevenly. The lopsided grind will get worse and worse, because once the process starts there is no way for the horse to correct it on his own.
A second way to deform the “surface of grind” is to make it difficult for the horse to fully close his mouth. This commonly happens when the horse has limited access to pasture. When a horse grazes, he grasps blades of grass between his incisors---since the incisor teeth are blunt, the horse does not cut the blades but rather tears them off by a backward or sideways action of the head. This style of grazing drags grass blades across the occlusal surfaces of the incisors. This is the only way that the incisors of a horse can experience significant wear, because they do not participate at all in the grinding of the bolus---that’s the job of the cheek batteries---and only come into contact at the end of each chewing stroke.
The horse kept in a stall or dry lot is maintained upon hay and possibly also grain or other bagged feeds. Video studies have shown two facts: First, when a horse eats either hay or something out of a bucket, he picks up the feed with his lips and tongue, and he thus incurs almost no wear upon the incisor teeth. And second, the horse uses a different chewing stroke for hay and grain than he does for grass, one that is different in length, frequency and force.
Getting little or no wear on the incisor teeth has serious consequences over the long term. Remember those alveolar ligaments, the ones that assist the tooth in erupting and which maintain the height of the working crown in the mouth? The fibroblast cells that secrete and maintain those ligaments are “under orders” from the horse’s very DNA to maintain the continuous eruption of the teeth through the gums until there is no more reserve crown left. So long as the fibroblasts “feel” the pressure of the teeth in the opposing jaw when the mouth is closed, they will continue to pull tooth out of the alveolus at a preordained rate. The amount of this daily eruption has been estimated to be from a quarter to a half centimeter per year, a minuscule amount on a daily basis, no greater or more noticeable from day to day than the growth of your fingernails, your hair, or your horse’s hooves. Nonetheless a horse whose hooves neither get worn off naturally nor get trimmed off by a farrier will inevitably accumulate excess length in them until he develops highly abnormal “ski feet.”
A similar process in the dentition creates two kinds of abnormalities. First, excess accumulated length in the incisors acts to prop the horse’s mouth open in front, thus creating uneven pressure along the cheek battery. This in turn throws the “surface of grind” into transverse ripples whose amplitude gradually increases over time until the horse has what is called a “wave mouth” malocclusion of the teeth. In some horses, what develops
When a horse grazes, he grasps blades of grass between his incisors–since the incisor teeth are blunt, the horse does not cut the blades but rather tears them off by a backward or sideways action of the head.
is one big wave, creating a “Viking funeral ship” malocclusion, in which unworn anterior and caudal lower molars stand up so high they gouge into the gums. Other individuals develop a sinusoidal occlusal surface with a high, sharp “hook” at the front end, or the reverse curve, with the hook at the
caudal end. These variations are all ultimately due to pressure perturbations; the variety in shape is driven primarily by different skull shapes, as in Arabians vs. Andalusians, Lipizzans vs. Saddlebreds, Thoroughbreds vs. draft breeds.
The second effect that excess accumulated incisor length has is to distort the way the incisors themselves meet. Here again two effects are commonly seen. First, the superior incisors may come to “dominate” or overhang those of the lower jaw; this condition looks like parrot mouth but isn’t congenital and wasn’t present when the horse was young. The inclination of the incisor table in such a horse will be abnormally steep, and if the upper incisors overhang enough, every time the horse closes his mouth, his upper teeth will act to force the lower jaw painfully backward against the “stop” (glenoid process) that forms the back wall of the TMJ.
The second variant produces exactly the opposite result---instead of having a very steep orientation to the incisors, they become almost horizontal. Think of the process as being the reverse of orthodontic treatment for buckteeth in humans: The human patient presents with incisor teeth that are oriented too much horizontally, and the dentist uses appliances that put steady pressure on the teeth to cause them to rotate into a more vertical orientation. Some geriatric horses do just the opposite: They begin with incisors that meet more or less vertically, but as excess incisor length accumulates it becomes more and more difficult for them to bring the cheek teeth into occlusion. The horses respond by using greater bite force, which over time rotates the incisors into a horizontal orientation.
NORMAL VS. ABNORMAL PARTICLE
FLOW IN THE MOUTH
The chewing stroke of a horse with normal teeth reduces long grass blades to little pieces in an orderly manner. Once a horse takes grass blades into its the mouth, the tongue acts to orient them along the bars--indeed, this is why grazing mammals all have toothless bars: They act as a smooth “receiving platform” where the tongue and cheeks orient blades lengthwise before pushing them up onto the cheek batteries.
The cheek batteries themselves work like an old-fashioned single-belt combine: The enamel bands that structure the occlusal surfaces of the teeth alternate in thickness so that they not only self-sharpen with use but wear into a pattern of transverse ridges---like the crossbars on the combine belt---which help to prevent food particles from falling out the front. With each chewing stroke, incoming food material pushes the part of the bolus already on the “belt” farther back.
If a horse is euthanatized while having its last earthly meal (as many of the specimens donated to my laboratory are), the animal dies in mid-stroke, so to speak. We can thus observe the particle distribution just as it was at the moment of death. There is an orderly diminution in the size of food particles from the front to the back of the mouth: long blades in front; toward the middle of the arcade, blades that have received one or two chews so that they have been reduced to around a half inch in length; material over the rear cheek teeth reduced to particles the size of cornmeal. The horse is meant to swallow, and its digestive tube is designed to handle, cornmeal-sized particles; ideally nothing longer than a quarter-inch.
Contrast this with the effect of a “wave mouth” malocclusion on mastication. In an extreme case, high, pointed teeth (superior 2, inferior 3, superior 5) are so protuberant that they beat the opposing tooth to a stump and gouge the surrounding gum. The distribution of particle sizes is chaotic: long sections of stem and blade stuck to superior 5, three-quarter-inch long pieces at the base of inferior 4 and 5, powderfine backwash at the front of the mouth packed into the interproximal space between inferior 1 and 2 and jammed into the receding gum around inferior 2.
The horse is meant to swallow— and its digestive tube is designed to handle—cornmeal-sized particles; ideally nothing longer than a quarter-inch.
DO HORSES GET “CAVITIES”?
The simple answer is yes, they do---but the location of decay is typically different in horses vs. humans. Tooth decay---“cavities” or caries---involves the localized destruction of enamel and dentine by acids produced by the bacterial breakdown of sugar contained in food particles that are stuck to, or lodged between, the teeth. Bacteria may also invade the space between the gums and teeth, giving rise to infection that causes reddened, sore, and eventually receding gums (gingivitis and periodontitis).
Because horse teeth are larger than those of people and have more protection in the form of a complete external coating of cementum and multilayered enamel, equine tooth decay does not usually manifest as it does in people, as pinprick holes “drilled in” from the outside. Indeed, in 40 years of examining horse teeth from fossil and archaeological digs I can count on the fingers of one hand the number of gum-line or external wall caries that have turned up in ancient animals.
Because of the narrowness of the spaces formed by the infundibula (“enamel lakes” or “fossettes” in the cheek teeth; “cups” in the incisor teeth), the embryological tissue layer responsible for depositing cementum inside the infundibula during the period of development may not be able to entirely fill it. This leaves a pit or deficit in the center that may extend all the way to the bottom of the infundibulum. Inevitably, food will become packed into this deep pit, where it will ferment and cause tooth decay which weakens the tooth from the inside.
Caries also occur in the dentine exposed on the occlusal surfaces. The decayed area will be dark brown in color and rough-edged. As bacterial acids continually deepen the cavity, food will be packed into it. Eventually it may become so deep that it extends all the way through the tooth, so that a wire threaded in at the chewing surface will emerge through the corresponding root. This is dangerous, providing a direct route for food material, bacteria and fungi to travel from the external environment into the maxillary sinus or the substance of the mandible.
As with people, horses also pack food into the “cracks” between adjacent teeth (the interproximal spaces). In healthy equine dentition, the teeth abut so tightly that no food can pack in, but as the amplitude of wave mouth increases, high teeth act to wedge opposing teeth apart. This provides yet another route for infection, because the interproximal spaces also give bacteria easy access to the interior of the skull or mandible.
EARLY METHODS OF DENTAL TREATMENT: MANIPULATION OF TOOTH SHAPES
In ancient times, dating back to at least the Middle Ages in Europe, India and China, dental treatment involved only the mechanical reshaping of the teeth, cleaning and extractions. Equine dentistry was performed by the farriers who made and owned tools meant for rasping, grasping, chiseling, drilling, pounding and pulling. By the end of the 18th century, grooms working in the stables of the wealthy knew how to “float” (rasp) cheek teeth, how to use a pair of pliers to remove calculus (crusty tartar) from the canines of male horses, and how to use a bent screwdriver to pry “wolf teeth” out of their shallow sockets. Stallions and geldings in well-run stables commonly had their canines buffed to remove sharp edges.
Advanced thinking in equine oral biomechanics, and the basis for modern equilibrative dentistry, arose during the 19th century in Germany. German and Dutch farriers were the first to make and use dental speculums ---ratcheted braces--designed to hold the horse’s mouth open for thorough manual and visual inspection. The Germans were the first to understand how the occlusal surfaces or “tables” of the cheek teeth---not just their edges---could be reshaped by rasping to balance or “equilibrate” bite pressure and thus diminish or prevent the development of some malocclusions. R. S. Huidekoper and T. D. Hinebauch brought the knowledge of how to reduce excess accumulated incisor length to America in the 1890s, but as we will see in our analysis of Black Hawk who died in 1856, knowledge of dental care apparently existed in some communities here much earlier.
Expertise at “floating” led to the realization that horses’ teeth could be reshaped for the purpose of enhancing performance. Traditionally, colts and fillies destined for the track are not given a long period to become acclimated to the bit. When a horse does not
accept the presence of a foreign object in its mouth or thinks that it may hurt him, he will try to get rid of it by spitting it out: Over and over again, he will push the bit forward and lift it with his tongue. When this doesn’t work, some horses retract the tongue in an attempt not to touch it at all. The tongue is actually quite a thick muscle, which roots behind and below the mouth in the pharynx. When the horse retracts the tongue, he pulls much of its mass back
into the pharynx. Inspired air must pass through the pharynx on its way to the lungs, so that when a horse retracts the tongue he also cuts off his own air---as well as any chance of winning
a race. Two solutions---other than actually taking the time to educate the horse to the bit---have commonly been tried. The first and simplest, the tongue-tie, dates to ancient times. The trainer grasps the horse's tongue
pulling it down and forward and holding it firmly in place while an assistant puts a leather thong through the horse’s mouth to tie the tongue in a forward position. Modern race trainers sometimes use surgical tubing or a heavy rubber band instead of a leather strap. It must be emphasized that tongue-tying, while inelegant, does not hurt the horse, and the tie is left on only for the short duration of the race. The tongue is stretched forward and tied down to enhance the horse’s ability to breathe while galloping.
A second and more sophisticated
Expertise in “floating” led to the realization that horses’ teeth could be reshaped for the purpose of enhancing performance.
solution, bit seating, was invented by equine dentists in Germany at the end of the 19th century. To create bit seats, the dentist rasps across the anterior cheek teeth in order to ramp the anterior third of each tooth. The lower teeth are ramped upward while the opposing upper teeth are ramped downward, so that when the horse’s mouth is closed, the anterior cheek teeth present a forward-facing, “V”-shaped notch.
Bit seating works along with the tradition of teaching the racehorse to accelerate when the reins are pulled back against the commissures of the lips. In a horse without bit seats, firm backward pressure from a snaffle bit can pinch the frenula of the cheeks--fleshy webs that tie the lips to the gums at the level of the commissures--and it can also apply fairly high pressure to the tongue. To relieve this, the horse may adopt dangerous behaviors such as throwing his head, gapping the mouth, or “taking the bit between his (cheek) teeth.”
When the bit is pulled back in a horse with bit seats, it slides up the tongue and bars and lodges in the “V”-shaped notch that has been carved into the teeth. The horse’s jaws remain comfortably closed; bit seating reduces the likelihood that the horse will either open its mouth or “grab” the bit. Further, when the bit is lodged in the “V,” it is held above the level of the tongue and thus prevented from pinching either the tongue or frenula. The horse then relaxes his tongue and runs with an open airway.
Coming next: Please keep this issue handy for reference! In our next installment, we’ll use the knowledge you gained by reading this article to analyze and understand extreme dental pathologies in the mouths of four famous stallions.
This 13-year-old Thoroughbred mare shows steepened incisor table angles which resemble, but are not, parrot mouth. The dominant upper incisors act to force the lower jaw backward as the mouth is closed, causing pain where the articular condyle of the mandible impinges the bony “stop” that forms the caudal aspect of the jaw joint (blue arrow). This mare’s dentition presents several other points of interest: Red arrows show tiny “blind” canines that in life never erupted through the gums. Mares with this condition typically object to bitting because the blind teeth feel like splinters under the flesh. Note her irregular cheek teeth, forming a moderate “wave mouth” with dominant, protuberant premolars and a particularly weak first molar tooth. Without dental intervention this pattern will amplify over time. The orange arrow shows the anterior premolarsreshaped to form a “bit seat.”
Red arrows point to the unworn crown of the just-erupting fourth premolar in an image of the upper left cheek tooth row, photographed with the skull turned upside-down so that the chewing surfaces face upward (A). Atop the tooth like a “cap” (blue arrows) sits the nearly-expired deciduous fourth premolar. Image B shows the same teeth from the side.
Even though modern jockeys use the crouch or “forward” seat, many still often “water ski” with the whole of their weight bearing against the horse’s mouth. The animal defends himself by retracting his tongue; the trainer responds with atongue-tie (green).
This image shows the normal dentition of a 9-year-old mare with orderly particle flow. Note the chaotic particle flow, caused by the “wave mouth” malocclusions of a 30-year-old mare. Numbers mark the positions of the cheek teeth: #3 is the last premolar, #4 is the first molar.
Because excess accumulated incisor length was not removed either by natural wear or by skilled dentistry, this 20-year-old Welsh Pony mare could not bring her cheek teeth into occlusion except by exerting great force. Over time, this pattern of use causes the incisors to rotate into a near-horizontal orientation.