EQUUS - - Con­for­ma­tion In­sights -


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.


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.


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.


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.



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.


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.


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.

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