A.Deciduous

1st incisor Di 1) Birth or first week

2nd incisor (Di 2) 4 to 6 weeks

3rd incisor (Di 3) 6 to 9 months

Canine (Dc )

1st premolar (Dp 2)

2nd premolar (Dp3)

3rd premolar (Dp4)

11. (11) 2¹/₂ years

12. 12 3¹/₂ years

13. 13 4¹/₂ year's (C) 4 to 5 years

(P1) 5 to 6 months

(P2) 2¹/₂ years

(P3) 3 years

(P4) 4 years

(M1) 9 to 12 months

(M2) 2 years

(M3) 3¹_/2 to 4 years

B. Permanent

1st incisor

2nd incisor

3rd incisor

Canine

1st premolar

(wolf tooth)

2nd premolar

3rd premolar

4th premolar

1st molar

2nd molar

3rd molar

The periods given for P3 and P4 refer to the upper teeth; lower ones may erupt about 6 months earlier

• (From Sisson and Grossman: Anatomy of the Domestic Animals)

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Fig 1. Skull of a 2-year-old colt sculptured to show embedded teeth. The upper first premolar (wolf tooth) is present but not visible; the lower one is indicated by an arrow. Temporary chief premolars are numbered 1. 2, 3; permanent premolars and molars are designated by Roman numerals. dc is the upper temporary canine, C is a lower permanent canine not ready to erupt, Di 2 and 3 are the 2nd and 3rd temporary incisors, and 11 is the first permanent incisor not eady to erupt. (From Sisson and Grossman: Anatomy of the Domestic Animals)

ple crown, long embedded root and very little crown cementum. Incisors and cheek teeth dif­fer in that the latter have considerable reserve crowns that erupt as the exposed crown is worn away by mastication.

Incisor teeth have a deep enamel invagina-tion partly filled with cementum. As the teeth wear, the table has a central ring of enamel surrounding the infundibulum and peripheral enamel ring. The infundibulum darkens due to food deposits and is lost as the crown is eroded away (see Determination of Age).

There is intimate contact between adjacent cheek teeth to maintain a continuous row of tooth substance. Upper cheek teeth differ from lower teeth in that the enamel is formed from an enamel organ folded vertically across the tooth as well as longitudinally. As these folds erupt and are worn away, an upper tooth is pro­duced with its distinctive enamel lakes filled with cementum and a central canal, the infun­dibulum (Fig 2).

In all horses the distance between the left and right mandible, measured at any tooth, is 30^% narrower than the space between the maxillary arcades (anisognathism). Conse­quently, the teeth in the lower arcades erode more on the buccal aspect and those of the up­per arcades on the lingual aspect, and sharp enamel edges form. As the complex enamel folds are worn away, the dental arcades take on a slight wave-like formation so that 10 transverse ridges are formed, ie, 2 for each tooth except the first and last. Corresponding valleys and ridges form on the opposing arcade.

Each tooth is composed of pulp, dentine, enamel and cementum. The pulp is a soft, ge­latinous tissue that occupies the central part of the tooth, the pulp cavity. In the upper teeth the pulp cavity has 5 main divisions within the folds of enamel and 2 main divisions in the lower teeth. Dentine forms the bulk of the tooth and progressively encroaches on the pulp cavity with age. It is hard and yellow-white. Enamel is very hard and blue-white. Cemen­tum is the peripheral tooth substance and pro­gressive cementation of peripheral enamel irregularities levels up the surface. Cementum is very similar in structure to bone. The embed­ded part of the tooth is united to the alveolus by a vascular layer of connective tissue, the al­veolar periosteum.

The position of the embedded crowns and roots of the last 4 upper cheek teeth varies at different ages and in different types of horses.

439

ALIMENTARY

Table 2. Changes in the Occiusal Surfaces of the Lower incisors

Stage	1st Incisor	2nd Incisor	3rd Incisor	Canine
deciduous	1 wk	1 mo	6 rno
permanent	2'/2 yr	31/2 yr	41/2 yr	41/2-5 yr
loss of enamel cup	6 yr	7yr	8 yr
appearance of dental star	8yr	9yr	10 yr
change from oval to triangular shape	11 yr	12 yr	14 yr
loss of enamel star	12 yr	13 yr	15 yr

Fig 2. The occlusal surfaces of maxillary (left) and man-dibular cheek teeth

All of these teeth develop in the caudal part of the maxilla and are related to the maxillary sinuses. The third and fourth teeth commonly project dorsad into the rostral maxillary sinus, and the fifth and sixth into the caudal maxil­lary sinus. This relationship is clinically im­portant in that periapical infection of these 4 teeth may lead to maxillary sinus empyema.

Determination of Age

The age of horses up to 8 years is determined by examination of the state of eruption and amount of wear of the incisors. An estimate of later age is gained by examination of incisor occlusal surface shape and changes in the an­gle of the incisor profile.¹⁴

Table 2 summarizes the changes that occur up to 15 years of age in the occlusal surfaces of the lower incisors. Variations in occlusal sur­face wear occur in cribbers, sand-eaters and horses with malocclusion.

Cementogenesis, within the base of the enamel invagination, ceases when the incisor erupts. Consequently, the depth of the enamel cup (infundibulum) within the invagination and its subsequent erosion depend on the rate of erosion and on the amount of cementum that persists up to 12, 13 and 15 years, respectively, until the enamel star is lost (Fig 3).

Paranasal Sinuses

The paranasal sinuses are air-filled, mucosa-lined spaces that communicate with the nasal cavity. Embryologically they arise as sprouts of nasal epithelium that grow into adjacent cra­nial bones and nasal turbinates. The sinuses are not fully developed at birth but continue to enlarge as the skull matures.

The sinuses certainly reduce skull weight but all their full functions are not completely understood.⁵

The horse has large rostral and caudal max­illary sinuses separated by an oblique bony septum. The rostral maxillary sinus commu­nicates with the ventral turbinate sinus and the middle nasal meatus. The frontal sinuses communicate with the dorsal turbinate sinus and the caudal maxillary sinus. The spheno-palatine sinus also communicates with the caudal maxillary sinus, which drains these si­nuses into the middle nasal meatus (Fig 4).

Because of the communication among all si­nuses but the rostral maxillary, it is usually sufficient to trephine once into this sinus and

.Fig 3. Schema of the occlusal (table) surface of I, correlated with age. (Modified after Getty: Sisson and Grossman's Anatomy of the Domes­tic Animals)

440

Fig 4. Simplified anatomy of the accessory air sacs of the head and intercommunication of paranasal sinuses. embranous bone is indicated by a thick line and turbinate bone by a thin line. RMS = rostral maxillary sinus, PO = pharyngeal orifice, OB = olfactory bulb, E = ethmoidal sinus, PR = pharyngeal recess, and SS = sphenoidal sinus. ( Courtesy of the Veterinary Record)

once into the caudal maxillary sinus when drainage of these areas is required. A third tre-phination into the main area of the frontal si­nus is used as an irrigation canal.

Dental Examination and Diagnostic Technics

Dental Examination

A complete history should be taken and in­quiry made of eating habits, quidding, bitting and riding problems, head-shaking, shyness, loss of condition or halitosis. The quality and quantity of the normal diet should be noted. By correlating the history with the animal's age and clinical signs, a presumptive diagnosis can often be made {Table 3).

It is impossible to satisfactorily examine a nervous, frightened or stubborn horse. There­fore, one must take every precaution not to in­crease the animal's natural suspicion and distrust of strangers. It is uncommon for a horse to permit a dental examination without suitable preliminaries to convince it that the examiner intends no harm. This may be impos­sible without the aid of ataractics or anesthet­ics for examination of painful disorders.

The incisors should be visually examined first by rolling back the lips to expose the inci­sors. The angle of the bite and any external ab­normalities should be noted. The deciduous incisors should be checked for looseness if evi­dence indicates they are about to be shed. The mouth is opened by reaching into the interden­tal space and withdrawing the tongue, or by applying opposing pressure on the upper and lower lips. The animal's age should be deter­mined since it can be an important clue to dis­orders encountered in further examination of the mouth and dental arcades.

The buccal edges of the first 3 cheek teeth can be assessed for sharpness by external pal­pation through the cheeks.

Examination of cheek teeth without a spec­ulum is not difficult once the technic is mas­tered. Such an examination is simpler, neater, less objectionable to the patient, and more im­pressive to the owner. Two methods may be em­ployed. In one method, both hands are required; in the other, only one hand is used. Each method has its advantages and disadvantages, and should be carefully evaluated by the indi­vidual practitioner before it is employed.

In the 2-handed method, the right side of the dental arcade is palpated by approaching the

441

horse from the left. The left labial commissure is parted with the right hand and the tongue is grasped through the left interdental space and withdrawn through the left side of the mouth. A light cotton glove on the right hand facili­tates this manipulation and keeps the tongue from slipping. At this time, the horse will open its mouth and tend to pull back, which provides a good chance to observe the table surfaces of the upper incisors and right upper cheek teeth. A flashlight held by the clinician or an assis­tant facilitates this examination. With the tongue held in the left labial commissure, the left hand is passed between the right dental ar­cade and the cheek, with the knuckles toward the cheek and palm toward the teeth. The up­per and lower cheek teeth are palpated with the fingertips. As long as the tongue is held in the labial commissure, the horse (ordinarily) will not close its mouth and there is little chance of finger injury. Do not conduct the ex­amination in a leisurely fashion because it an­noys a horse to have its tongue clutched for long periods while each tiny ridge or cavity of the cheek teeth is examined. When the right side has been examined, the process is repeated on the left using the opposite hand and labial commissure. Examiners with large hands may find this technic unsatisfactory.

For experienced operators, the one-handed technic is more suitable. It also is applicable to otherwise manageable horses who dislike spec­ula or tongue-holding. The horse is approached from the front and the right side of the mouth is palpated by inserting the right hand into the right interdental space with the palm facing laterad (Fig 5). The hand should be slightly dorsiflexed (overextended) and should lie be­tween the lingual surface of the cheek teeth and the tongue. This forces the tongue between the left rows of cheek teeth and keeps the horse from completely closing its mouth. The hand is advanced into the oral cavity, and the thumb and forefinger are used to palpate the buccal, lingual and table surfaces of the teeth (Fig 6). During this examination one should also pal­pate the buccal mucosa, gingiva and right side of the tongue. The left side of the mouth is ex-

Table 3. Equine Dentistry

AGE

EXAMINE FOR:

NECESSARY DENTISTRY:

2-3 years 1. 1st premolar vestige (wolf teeth).

2. 1st deciduous premolar (upper and lower).

3. Hard swelling on ventral surface of mandible beneath 1st premolar.

4. Cuts or abrasions on inside of cheek in region of the 2nd premolars and molars.

5. Sharp protuberances on all premolars and molars.

1. 3-4 years

   1. 2, 4 and 5 above.

2. 2nd deciduous premolar (upper and lower).

4-5 years

1.1,4 and 5 above.

2. 3rd deciduous premolar.

5 years and older

1.1.4 and 5 above.

2. Uneven growth and "wavy" arcade.

3- Unusually long molars and premolars.

1.Remove wolf teeth if present.

2Remove deciduous teeth if ready. If not, file off corners and points of premolars.

3Examine with x-ray. Extract retained temporary premolar if present.

4Lightly float or dress all molars and

premolars if necessary.

5. Rasp protuberances down to level of other teeth in the arcade.

1. 1, 2. 4 and 5 above.

-

2. Remove if present and ready.

1.1,4 and 5 above.

2. Remove if present and ready.

1.1,4 and 5 above.

2. Straighten if interfering with mastication.

3.Unusuallylong molars and premolars may have to be cut if they cannot be filed dawn

442

-

Fig 5. Hand position at entry of the mouth in the one-handed technic for dental examination.

Fig 6. Examination of the upper fourth cheek tooth by the one-handed method.

amined in the same manner using the left hand. There is danger of being bitten when us­ing this technic, particularly if one is careless. The operator's hands may be scratched by sharp edges of the upper cheek teeth. This or­dinarily results from failure to palpate the out­side of the cheeks prior to inserting the hand into the horse's mouth.

The cheek region around the labial commis­sures is readily palpated by placing the thumb in the commissure with the ball toward the buccal mucosa. Wolf teeth can be felt by insert­ing the thumb or forefinger into the interden­tal space and palpating the first upper and lower cheek teeth on the close side. Do not at­tempt to palpate the premolars on the side of the jaw opposite to the one in which your finger is inserted. Wolf teeth feel much smaller than the adjacent rostral premolar. Since wolf teeth may occur in either the maxilla or mandible, both upper and lower arcades should be exam­ined. At the same time the arcades are exam­ined for wolf teeth, the first definitive premolars should be palpated for protuberances and sharp edges. It is in this area that the bit draws the cheeks or tongue against the teeth and sharp edges of the first premolars can cause painful lacerations. Great care should be exercised if a mouth speculum (gag) is used to examine an untranquilized horse. Such specula are de signed to forcibly hold the mouth open and are usually of heavy-weight metal. The device can be a lethal weapon when attached to the swing­ing head of an excited horse.

General anesthesia enables detailed inspec­tion of molar arcades and gingival pockets, cauterization of buccal ulcers and probing of dental sinuses. For the complete investigation of tooth root infections, radiographs are in­valuable in revealing the extent of maxillary or mandibular bone diseases.

Radiography

Satisfactory radiographs can be produced with standard veterinary equipment at expo­sures equivalent to 60-70 kvp at 40 mas. Lat­eral and oblique films are taken using stationary grids, fast film and screens. The mouth is held open with the narcotized horse in lateral re­cumbency and the diseased side next to the cas­sette. In most cases, root and reserve crown detail rather than exposed crown detail is de­sirable on radiographs. This detail is achieved by using a 30-45° oblique beam to project the image of the normal arcade away from the dis­eased area.⁶

Greater detail of individual maxillary roots can be obtained from intraoral (occlusal) den­tal films wedged 45° to the hard palate, the x-ray tube head correspondingly angled, and

443

Fig 7, Erect lateral radiograph of a 7-year-old Thoroughbred wilh chronic sinus empyema and new bone formation from apical infection of the third cheek tooth

. the affected side up. A mandibular arcade oc-clusal film is placed between perspex sheets to prevent ensalivation, and wedged vertically be­tween the tongue and mandibular arcade,

Examination of the Paranasal Sinuses

The head is examined for the presence, color and odor of any nasal discharge. It is more usual for the discharge to be unilateral, muco-purulent and produced after exercise in cases of sinus empyema. Hemorrhage and malodor indicate conchal (turbinatej necrosis, dental infection or neoplasia. Palpation may reveal heat, swelling and pain, and percussion can confirm the presence of empyema by detecting areas of dullness. Endoscopic examination of the nasal chambers, using either a rigid or flex­ible fiberoptic endoscope, is a useful aid in the differential diagnosis of nasal discharges. The nasal meati, maxillary sinus drainage pas­sages, conchae (ethmoturbinates), auditory tube diverticula (guttural pouches) and pharyngeal tonsils should all be inspected. Erect, lateral radiographs of the head using a horizontal beam reveal paranasal sinus fluid lines in some cases or diffuse opacity as the si­nus air spaces are obscured by exudate, gran­ulation tissue or new bone (Fig 7).

After the application of local anesthesia, an exploratory 3-mm trephine hole may be made into the rostral and caudal maxillary sinuses using a short Steinmann pin through a stab in­cision in the skin. The presence of exudate is confirmed by aspiration using a length of 260 polyethylene tubing attached to a 6-mI sy­ringe. Suturing the tubing in place facilitates treatment by irrigation.

Dental Diseases

Maxillary and Mandibular Deformities

The most common congenital oral deformity observed is a maxilla that is relatively longer than the mandible, or so-called "parrot mouth" (Fig 8). The condition is called "sow mouth" or "monkey mouth" when the mandible is longer than the maxilla (Fig 9). These conditions are also referred to as maxillary or mandibular prognathism, respectively. These terms are

Fig 8. Parrot mouth re­sulting from mandibular brachygnathia.²

⁴⁴⁴

Fig 9. Undershot jaw (sow mouth) resulting from man-dibular prognathism.²

perhaps confusing because so-called maxillary prognathism {parrot mouth) is caused by short­ening of the mandible, ie, maxillary prognath-ism is actually mandibular brachygnathia (short mandible). Conversely, mandibular prognath-ism is caused by maxillary shortening, ie, max­illary brachygnathia.⁷ For these reasons, it is perhaps convenient to use the colloquial terms of parrot mouth, sow mouth or monkey mouth. Both conditions are thought to be inherited. Some correction of incisor malocclusion occurs up to 5 years of age.⁷ The following definitions have been adopted for use in North America and the United Kingdom.⁶ In North America, normal occlusion is complete contact of the oc-clusal (table) surface of the incisor teeth. Par­tial maxillary prognathism (overshot) or partial

mandibular prognathism (undershot) results in 10-90% contact of the table surface of the incisors. In total maxillary prognathism (par­rot mouth) or total mandibular prognathism (monkey mouth), there is gross malignment of any incisors and/or no contact of the table sur­face of the incisors.

In the UK, parrot mouth in horses should be determined at 2 years of age and is defined as that condition in which there is no occlusal con­tact between the upper and lower central inci­sors, with the lower teeth caudal to the upper teeth. Horses should be examined in a halter. The mouth should be closed and the chin ele­vated as the lips are rolled away from the in­cisors to assess the amount of occlusal contact.

Parrot Mouth: This deformity is congenital and may be heritable, and affected horses are considered unsound (Fig 8). Abnormal incisor apposition results in abnormal wear and over­growth. The central incisors develop a rabbit-toothed appearance and a characteristic parrot beak overhang of the upper incisors. There may be abnormal molar wear in grossly af­fected animals, with the formation of hooks on the first maxillary and sixth mandibular teeth, and in some cases shear mouth formation.⁹

Treatment is only palliative and involves regular rasping or sawing of the incisor arcade and chiseling of molar hooks.

Sow Mouth: This defect is not as common as parrot mouth but occurs frequently in some pony lines (Fig 9). As in parrot mouth, maloc­clusion leads to secondary infections and diges­tive disorders. The lesion is regarded as an un-soundness and treatment is palliative.

Shear Mouth: Shear mouth arises from an excessive difference in width between the max-

Fig 10. Supernumerary incisors.²

445

Fig 11. Dentigerous cyst and aural fistula in an 8-month-old Thoroughbred colt.

ilia and mandible. In normal horses the upper arcade is always wider than the lower. A slight excess of this inequality results in excessive angulation of the table surfaces of the cheek teeth and the development of long, extremely sharp edges. This condition may occur in horses of any age but is more common in old animals with irregularities in wear and age changes in­volving mandibular shape. From 1-2 cheek teeth to all premolars and molars may be in­volved. It may be unilateral or bilateral. The signs and lesions are similar to those associ­ated with sharp enamel points but are more se­vere. Treatment is usually unsatisfactory and involves cutting and rasping to achieve as nor­mal an alignment as possible. Postoperative treatment involves placing the horse on hard feed to postpone recurrence. It seldom does.

Supernumerary Teeth

A wolf tooth should not be regarded as a su­pernumerary tooth since it is part of the nor­mal permanent dentition. By definition, supernumerary teeth are those found in addi­tion to the normal number {polyodontia}.

Extra incisors occur most frequently and may arise as a result of the division of the per­manent tooth germ. In some horses there may be a complete double row but more often only 1 or 2 extra teeth (Fig 10). Treatment depends upon how the teeth develop. Those that wear more or less evenly and cause no apparent trouble should be left alone. However, if the ex­tra teeth become elongated, they should be cut off or extracted to prevent damage to the mouth. The type of operation depends upon the location of the teeth and the judgment of the veterinarian. Owners frequently want supernumerary incisors removed for cosmetic rea­sons and it may be necessary to accede to such a request when there are no other reasons for extraction.

An extra cheek tooth may be present at the caudal end of the arcade or, less frequently, may appear displaced to the lingual or buccal aspect of the normal arcade. Extra cheek teeth should be extracted to prevent malocclusion.

Dentigerous Cysts

Dentigerous cysts are abnormal tooth devel­opments of epithelial origin from Hertwig's sheath or its precursor, the enamel organ.¹⁰ Such cysts frequently contain tooth fragments and may distort the surrounding maxilla or mandible.¹¹

Dentigerous cysts arising from misplaced tooth germs of the branchial arch appear as temporal cysts and may drain into the ear.¹² The cysts are lined by stratified squamous ep­ithelium and may contain one or more teeth (Figs 11,12). Temporal teratoma or heterotopic polyodontia are terms used to describe such le­sions. Clinical signs depend upon the affected site. In rare cases cysts may form inside the cranial vault but they more commonly occur as temporal cysts with aural fistulae and as max­illary or paranasal sinus cysts.⁹ Treatment consists of careful dissection and removal of the cyst, followed by obliteration of the dead space and fistula or draining and packing the area with sterile roll-gauze.

Cystic Sinuses

This condition is also referred to as mucoid degeneration of the nasal turbinates, multiple mandibular cysts or osteodystrophia fibrosa

Fig 12. Cyst explored to show temporal tooth.

446

Fig 13. Ventrodorsal radiograph showing maxillary dis­tortion from a rnultiloculated cyst in a 1-day-old Thor­oughbred.

cystica. It is seen typically in newborn or young horses. Affected animals may have facial and/ or mandibular distortion, nasal occlusion, dys­pnea and nasal discharges from contact ulcers on the turbinates (Fig 13). The differential di­agnosis between multiple mandibular cysts and dentigerous cysts may be difficult in mild cases. The lesions in this condition are similar to those of classical hyperparathyroidism with osteopenia due to excessive resorption of bone and fibrous tissue replacement.¹³''⁴ However, there is no evidence that the condition is influ­enced by dietary factors, such as abnormal Ca:P ratios, and it is more likely that genetic defects are involved.

Although severely affected animals do not respond to treatment, they can survive satis­factorily for several years. Surgical drainage may benefit mildly affected animals.^{11 15}

Abnormal Tooth Eruption

Tooth eruption is a complex phenomenon in­volving interplay between dental morphogen­esis and the vascular forces responsible for creation of the eruption pathway. Table 1 lists the normal eruption times of the deciduous and permanent teeth. It should be noted that the third cheek tooth (fourth premolar) is the last permanent tooth to erupt and is most fre­quently impacted, rotated or misplaced.

Delayed Eruption: Wolf teeth may be dis­placed in a labial and rostral direction or in eruption. Such abnormalities result in contact ulcers at the area of bit contact. Such teeth are easily extracted in sedated horses.

Occasionally there may be marked variation in canine tooth eruption in males and one tooth may be impacted. In such cases the gingiva should be split to allow the crown to emerge.

Retention of Deciduous Teeth: Dental caps are deciduous cheek teeth that remain at­tached to the permanent teeth after the per­manent ones have erupted. Deciduous teeth are sometimes moveable on palpation; how­ever, even if they are not loose, the caps should be removed once it has been definitely deter­mined that the permanent tooth has grown be­yond the gingival line. It is best not to attempt to remove caps if it requires a great deal of ef­fort to loosen them. Age and signs of pain gen­erally dictate removal of offending caps. Sometimes the cap partially detaches and ro­tates laterally, causing buccal laceration and facial deformity.

To remove a cap in the lower arcade, me­dium-sized forceps should be applied, and the cap rocked loose and removed. Caps on the up­per arcade are best removed by inserting a screwdriver blade into the junction between the cap and the permanent tooth, and prying the cap loose. The extracted tooth should be ex­amined to make sure a portion of it has not bro­ken off and remained in the maxilla or mandible since it may hinder eating, cause the perma­nent tooth to grow unevenly, cause local gin­givitis and predispose to infection.

Dental caps should be removed before the teeth are floated because sharp edges or points are sometimes present on underlying perma­nent teeth. Removal of deciduous teeth at the proper time allows the permanent teeth to grow normally and results in less chance of training interruption due to mastication diffi­culties or interference with the bit.

Retained temporary incisors are similar to dental caps except that they may remain embedded more or less firmly in the gingiva and the permanent incisors erupt behind them rather than displacing them. Ordinarily these teeth are loose and easily removed with a pair of dental forceps but occasionally must be loos­ened with a bone gouge or dental elevator. The cavity should be searched with a probe or splin­ter forceps to make sure all tooth fragments are

447

Fig 14. Eruption pseudocyst around the apex of a pre-molar in a 3-year-old pony. Fig 15. Lateral radiograph at necropsy showing impac-tion of the third cheek tooth within the mandible.

removed; otherwise inflammation and infec­tion may ensue from irritation produced by the fragments. Occasionally the gingiva must be incised under local anesthesia and dental frag­ments removed. The surgical wound ordinarily heals without suturing, although 1 or 2 sutures of nonabsorbable material may be placed in the gum to appose the divided tissues and removed within 10 days. Most horses do not object too severely to this procedure and removal can be accomplished with minimal restraint.

Overcrowding and Displacement: If there is relative shortening of the maxilla or mandible, the third cheek tooth is unable to fit into the arcade. The upper third cheek tooth may be medially displaced into the palate or there may be diffuse alveolar periostitis and facial swell­ing. It is normal for erupting permanent teeth to cause symmetric, nonpainful swelling along the ramus of the mandible, and dorsal and ros­tral to the facial crest. Such swellings result from periapical hyperemia and alveolar bone resorption, and usually resolve spontaneously <Fig 14). In an overcrowded mouth there is ab­normal resistance to the erupting tooth.¹⁶ This may exacerbate alveolar bone changes so that swellings become painful and may progress to sinus formation and periapical infections.

The early removal of deciduous caps and fil­ing the mesial edges of adjacent teeth may fa­cilitate normal eruption of impacted teeth; however, extraction of the tooth is the only ef­fective treatment in the presence of dental si­nus formation and periapical infection.

A6sen.ce of Teeth (Oligodontia): Failure of teeth to develop is more common than poly-odontia. The canines may be absent in some stallions and are always rudimentary in mares. There normally may be fewer than normal numbers of teeth in the arcades of any horse. Pseudooligodontia may result from loss of indi­vidual teeth or from total impaction (Fig 151.

Mandibular fractures in young horses may severely damage or even destroy the perma­nent tooth germ to result in a deficiency in per­manent dentition.

Unless there is interference with normal oc­clusion, no treatment is indicated for oligodon-tia. Malocclusion should be surgically corrected, as with acquired overgrowths from loss of the opposing tooth.

Wolf Teeth: These represent the vestiges of the first premolar tooth. Only 20% of aborted Thoroughbred fetuses examined contained wolf teeth. Over 90% of plains zebras had wolf teeth in the upper arcade and 60%

in the lower ar­cade.⁷ In the latter case the teeth were large and, in some cases, occlusal contact was made. In the domestic horse the mandibular wolf tooth is rarely present. The maxillary wolf

Fig 16 Woff.tooth (left) in its normal posicion

448

Fig 17. Labial enamel points and rostral hook on the maxillary arcade of a 6-year-old Hunter.

tooth is normally in contact with the rostral margin of first cheek tooth (Fig 16).

It is commonly believed by horse owners and trainers that all wolf teeth interfere with the bit and therefore handicap the horse in train­ing. This is certainly the case if the tooth is displaced or delayed in eruption, but there is no solid evidence to support the common pro­cedure of routine wolf tooth extraction.

Abnormal Dental Erosion

Sharp Enamel Points: The normal grinding action of the dental arcades leads to the for­mation of complex transverse ridges across the occlusal surfaces of teeth. Full lateral move­ment of the mandible results in incomplete wear of the buccal surfaces of the upper arcade and the lingual surfaces of the lower arcade so that small, sharp enamel points form along the edges. At the same time there is slight caudal retraction of the mandible so the first maxil­lary and last mandibular teeth tend to develop small hooks (Fig 17), Such points and hooks can cause buccal or lingual ulcerations and may result in poor bit contact and bit-shy horses. Sharp enamel points are easily cor­rected by regular floating.

Wave Mouth: Abnormal mastication results in changes in the shape and position of the nor­mal transverse ridges across the occlusal sur­faces.¹⁷ Enhancement of some ridges and valleys occurs, and the arcades attain a wave form. In severe cases the teeth in one arcade may be worn to the gingival margin, permitting the opposing molar to lacerate the gingiva. If the history and clinical examination sug­gest minimal malocclusion and the arcades are relatively complementary, only floating is nec­essary. If mastication is grossly impaired or if there is much salivation and quidding, an at­tempt should be made to even the arcades with a dental rasp, chisels and compound molar cut­ters. In most cases this is best accomplished un­der general anesthesia, with the horse in lateral recumbency and a speculum applied. In mild cases the procedure can be accomplished with the horse standing and confined in stocks or a stall. Treatment of severe cases is seldom satisfactory despite adequate removal of the projecting portions of the wave.

Step Mouth: Step mouth is characterized by marked variation in the height of individual premolars and molars. This may be due to un­equal wear of opposing teeth in the dental ar­cade or may be a sequela to extractions. Signs of this condition are similar to those of other dental disorders, but malnutrition and emacia­tion are usually more evident since a step-mouthed animal cannot chew food properly. Treatment involves leveling projecting teeth at regular (usually semiannual) intervals.

Smooth Mouth: In extreme old age the teeth may become completely smooth or lost entirely. Smooth mouth is normally found in old horses but may be encountered in young animals. The table surfaces of the teeth become smooth through complete erosion of the crowns or through defects in the lamellar arrangement of enamel, cementum and dentine. Individual teeth or the entire arcade may be affected, al­though an animal is not ordinarily called smooth-mouthed until at least a majority of the teeth are involved. Because the smooth table surfaces of the teeth prohibit proper grinding of feed, associated signs include colic and poor condition. Smooth table surfaces are readily detected by palpation. In young animals, smooth mouth may occasionally be caused by improper floating in which the table surfaces of cheek teeth are filed smooth.

Treatment of smooth mouth in old and young animals with defective teeth is useless. Feed­ing soft mashes and chopped feed to young an­imals that have been mishandled gives the teeth an opportunity to re-establish their nor­mal rough table surfaces.

Periodontal Disease

Periodontal diseases in horses are inflam­matory and dystrophic in nature and may cause

449

Fig 18. Incidence of periodontal disease in horses

.

malnutrition, halitosis, maxillary and mandi-bular osteitis, paranasal sinusitis, quidding, na­sal discharge, colic and death in severely affected animals. It has been described as the dental scourge of horses and is the most common dental disease of horses (Fig 18).^{18 19}

The initial lesion is marginal gingivitis, with hyperemia and edema (Fig 19). The lat­eral gingival sulcus becomes eroded and a tri­angular pocket is formed, usually on the buccal aspect. This cavity harbors food material and a cycle of irritation, inflammation and erosion is established that destroys gingival tissue to the lingual aspect and deeper into the perio-dontium (Fig 20). Ultimately gross alveolar sepsis develops and the tooth is lost. Transient gingivitis may be caused by the first instars of the botfly.»

In all species the shearing forces produced by normal mastication are essential for the main­tenance of a healthy periodontium.²¹ Gingivi­tis, which may lead to periodontitis, occurs in any situation in which there is abnormal occlu­sion and alteration in shearing forces.²² The most severe form of periodontal disease, with gross pocketing, periodontal suppuration and loosening of teeth, is associated with extreme abnormalities of wear, such as irregular arcades, hooks, loss of teeth with corresponding over­growth of the occluding arcade, misplaced or split teeth, and mandibular fractures.

The disease affects all teeth during the erup­tion of permanent dentition but lesions heal once the normal grinding pattern is estab­lished. In severe cases the gross alveolar sep­sis, unthriftiness and halitosis may necessitate the animal's destruction.

The importance of this disease should be understood. Regular dental examinations and prophylaxis to maintain normal teeth and grinding action help prevent its onset.

Alveolar Periostitis (Chronic Ossifying Per­iostitis): These terms have been used to de­scribe the diffuse bone changes seen in association with periodontal disease in young horses. In particular they refer to the changes seen at eruption of the permanent cheek teeth.^{16 18 19} However, there are inflammatory periosteal changes in all horses at this stage and the differentiation between normal and pathologic changes is by no means clear. Pro­gression of inflammation to rhinitis, paranasal sinusitis and apical infections of teeth causes corresponding clinical signs.

Dental Decay

Some question has been expressed as to the use of the term caries as applied to horses.^{6 23 24} By definition, caries is a disease of the calcified tissues of teeth resulting from the action of mi­croorganisms on carbohydrates and character­ized by decalcification of the inorganic portion of the tooth. This is accompanied or followed by disintegration of the organic portion

Fig. Fig19. Marginal gingivitis between the first and second mandibular cheek teeth. Fig 20. Severe periodontitis with debris-filled pockets.

450

Fig 21. Caries of cementum of the fourth cheek tooth in an 8-year-old Hunter

Horses very commonly have an expanding, debris-filled cavity within the cementum of the enamel invaginations of the maxillary teeth. The formation of cementum within the enamel lakes is incomplete at eruption and the blood supply through the infundibulum is lost. Con­sequently, the lake cementum in all erupted maxillary teeth is dead tissue. As the surface of the tooth is worn away, these areas of hypo-plastic cementum are exposed and appear as necrotic cavities.^{7 25} Secondary to this hypopla-sia, there may be fermentation of impacted food within the infundibulum and acid disso­lution of the surrounding cementum, enamel and dentine. Such lesions are then true caries of cementum (Fig 21). ^{7. 9 .26}

The lesion is benign in many teeth and, al­though there may be local extension within an individual cementum lake, the formation of secondary dentine protects the pulp from infec­tion. Pulpitis and even splitting of the tooth re­sults when the lesion spreads (Fig 22). This may be the etiology of alveolar sepsis, sinus empyema and nasal discharge.

The primary lesion, hypoplasia of cemen­tum, is rarely detected in life and only after alveolar sepsis or sinus infection has developed do signs of disease appear. Tooth extraction, al­veolar or sinus curettage and irrigation should then be performed. Similar lesions exist in the teeth of wild equidae and, although it can be argued that some management and feeding regimens influence the development of pri­mary periodontal disease, it is not the case that caries of cementum is a disease of domestica­tion, as has been suggested.²⁷

Caries of cementum is not the only route of infection into the pulp cavity. Pulpitis may arise from tooth fracture, periodontal disease or, as seen more commonly in the mandible, as a result of maleruption and lysis of alveolar bone. Figure 23 is a schematic representation of the possible sequelae of apical infections and pulpitis in horses. Table 4 demonstrates that the second and third upper and lower teeth are most frequently affected and emphasizes the etiologic significance of dental impactions.

Apical infections cause maxillary or mandib-ular swelling and are usually of only minor dis­comfort to the horse. Clinical signs may include dental sinus formation, paranasal sinusitis and nasal discharge (Fig 24). Oblique radiographs are useful in evaluating the lesion. There may be cystic osteitis of alveolar bone and production of new bone and cementum from the irritated periodontal ligament (Fig 25).²²

Apical infections can only be treated by ex­traction of the affected tooth or teeth, followed by thorough curettage of the osteitic bone, and irrigation and cleansing of sinus tracts and paranasal sinus empyema as indicated.

Tumors

Dental tumors are classified according to their origin as epithelial or mesenchymal. Odon-toma describes any tumor of odontogenic origin and refers to a tumor in which both the epithelial and mesenchymal cells produce functional ame-loblasts and odontoblasts (and therefore enamel and dentine within the tumor). Most dental tumors occur in young animals and, if originating from a deciduous tooth germ or the first true molar tooth germ, are congen­ital.^{28. 29} They are usually benign but defy treat­ment because of their size and degree of facial

Fig 22. Fusion of rostral and caudal caries

451

Fig 23. Sequelae of pulpitis in horses

or mandibular distortion created. However, af­fected animals may survive for several months or years before euthanasia is necessary.

The most common dental tumors are ame-loblastic odontomas of the maxilla, which are mixed tumors of epithelial and mesenchymal origin. Clinical signs may include facial swell­ing, nasal obstruction, dyspnea, dysphagia, quidding and unthriftiness (Figs 26, 27).