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Equine March 2009 (Vol 4, No 2)

From the Horse's Mouth — The First Premolar Teeth

by Cleet Griffin, DVM, DABVP

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    Abstract

    The first premolar teeth (wolf teeth) of horses are considered a component of the permanent dentition and may be identified as Triadan ’05. Through study of the fossil record, it has been determined that wolf teeth are dental vestiges from ancestors of the modern horse. Although wolf teeth may not be problematic for the horse, many believe that harm arising from the presence of wolf teeth in some horses may account for oral discomfort, abnormal behaviors during performance, and bitting problems. Rationale exists for veterinarians to remove wolf teeth when necessary to alleviate discomfort and bitting problems.

    Morphologic features of the modern horse's teeth represent adaptations to ecologic changes.1 In the Eocene period, the first ancestor of the horse—a small, leaf-eating mammal called Hyracotherium—had four toes on each front foot and three toes on each hind foot2 and bore little resemblance to modern horses. Fossils show that Hyracotherium had three incisors; one canine tooth; and seven primitive, short-crowned cheek teeth (four premolars and three molars) in each side of the upper and lower jaws.3 Over time, the second, third, and fourth premolars as well as the three molars progressively became (through "molarization") long-crowned, complex, well-developed grinders as the horse adapted to prolonged periods of grazing.1,3 The first premolar progressively became smaller and is now small, cone shaped, and usually situated just rostrally to the first well-developed cheek tooth3-5 (FIGURE 1). Thus, the first premolar (wolf tooth) is a functionless remnant from ancestors of the modern horse.1-5

    There are several possible origins of the term wolf tooth, some of which appear to be based on superstition. In one publication from the 1700s, the term wolves' teeth was used as a general description of teeth with sharp edges that could prick and wound the oral mucosa or tongue while eating.6 At about the same time, Daubenton7 described the horse's first premolars as "supplementary premolars,"8 and some early 20th-century texts suggested that the term wolf teeth may have originated from the resemblance of these teeth to the incisors of carnivores.8,9 Merrillat10 used the terms remnant teeth and supernumerary teeth as synonyms for wolf teeth. He suggested that these teeth were retrogressive representatives of prehistoric horses' premolars that were degenerating under the influence of selection. He further theorized that the use of the bit over many generations had rid the interdental space of premolars.10 Easley11 retold a remarkable story about a 16th-century military general's horse that had turned and reared, refusing to move forward into a battle. According to the story, removal of a small tooth in the area of bit contact alleviated the embarrassing behavior of the general's horse, which became a favorite mount for battle. Unable to leave well enough alone, the general's horseman concluded that the horse had sensed the presence of wolves when the bit was worked against the tooth, which instinctively caused the horse to shy in order to protect the general.11

    In some older publications, the first premolar teeth were blamed for eye problems, including excessive watering, inflammation, and blindness.12,13 It was even speculated that wolf teeth adversely influenced ophthalmic branches of the fifth cranial nerve.12 Therefore, wolf teeth were commonly removed from horses to alleviate eye ailments, resulting in use of the synonym eye tooth.5,10,12,13

    Apart from these myths and misconceptions, the presence of wolf teeth in horses remains an area of importance and interest to veterinarians. In general, wolf teeth are not considered to serve a useful purpose. Although wolf teeth may not be problematic for the horse, many horsemen and horsewomen believe that harm caused by the presence of wolf teeth may account for oral discomfort, problems with the bit, and abnormal behavior of some horses at work.11,14-20

    Anatomy and Eruption

    Anatomically, the wolf tooth is a component of the permanent dental formula of the horse and is designated as Triadan '05. Because practitioners are using the modified Triadan tooth numbering system more frequently to identify specific teeth, a review of the numbering system is provided21,22 (BOX 1 and FIGURE 2).

    The first premolars have a simple short-crown (brachyodont) structure22 and are composed of enamel, dentine, cementum, and a pulp chamber.23 These teeth usually erupt between 6 and 12 months of age and may be present in up to 90% of yearlings.11,22 When horses are approximately 3 years of age, supporting structures of the wolf tooth become sclerosed and calcified and the pulp chamber starts to obliterate with secondary dentine.23 Wolf teeth are usually no more than 1 to 2 cm in length and have a single root of variable length and substance, ranging from short, loose attachments to the gingiva to 30 mm in length.4,14,22 During eruption of the adjacent second premolar (which occurs at approximately 30 months of age), some wolf teeth may undergo root resorption or be shed.4,24,25 This normal shedding process may account, in part, for the 13% to 32% reported incidence of wolf teeth in mature horses22,25,26 (FIGURE 3).

    In my experience, the incidence of mandibular wolf teeth (Triadan 305 and Triadan 405) is very low; however, it has been reported that these teeth may be more common in some lines of Standardbred horses.27 Lower wolf teeth are usually very small but can be large with a sharp point25 (FIGURE 4). Careful examination is required to detect short, small, splinter-like lower wolf teeth because they can be positioned close to the second premolar and may be partially concealed by the mucosa of the cheek and gingiva (FIGURE 5).

    The upper wolf teeth usually erupt in a vertical plane close to the first cheek tooth. Occasionally, the crown of the upper wolf tooth may be positioned further rostral to or palatal or buccal to the second premolar. The crown may also be angled slightly in some instances.22 Wolf teeth have been reported to be quite large and even "molarized" in Clydesdale horses or other draft breeds.16,25 In some horses, a wolf tooth can be large, rostrally displaced, and mistaken for a supernumerary canine tooth.28 Supernumerary wolf teeth are reported to be rare; however, small, superficial remnant portions of the deciduous second premolar may occasionally be found just rostral to the first cheek tooth and may be erroneously identified as a supernumerary wolf tooth.14,19,28

    Unerupted wolf teeth are occasionally detected in the upper arcades. Synonyms for unerupted wolf teeth include blind or impacted wolf teeth.27 An unerupted wolf tooth is positioned along the maxilla several centimeters rostral to the first cheek tooth in a relatively horizontal orientation within the submucosa of the interdental space. Because of this orientation, the crown fails to erupt vertically through the mucosa. With careful palpation, a blind wolf tooth can usually be detected as a firm, nodular, submucosal enlargement. These teeth are generally small, so radiography can be useful for confirming their presence25 (FIGURE 6 and FIGURE 7). Unerupted wolf teeth of the mandible are reported to be rare and are probably very difficult to detect without the use of radiography.22

    Rationale for Removing Wolf Teeth

    Horses may experience oral pain as a result of pressure from the bit forcing the cheeks against sharp dental points.29 In some horses, a wolf tooth may cause discomfort as a result of the bit working against the tooth and forcing cheek mucosa into the sharp point of the tooth.14,16,17 Behaviors associated with oral discomfort caused by wolf teeth include bitting problems, head tossing, and head shaking.14,30 The presence of a wolf tooth also makes it difficult to adequately float and bevel the rostral portion of the second premolar (i.e., creating "bit seats"). Wolf teeth that are enlarged, displaced, loose, fractured, or diseased are considered to be a source of pain.14,18,25,30 Blind wolf teeth may cause discomfort when the mucosa is hit and compressed against the tooth by the bit.27

    Erupted and unerupted lower wolf teeth have also been associated with bitting problems in horses.14,22 Therefore, I prefer to remove wolf teeth from young riding horses and from horses of any age with a history of performance problems that may be related to oral discomfort. On occasion, erupted upper wolf teeth may be present in mature horses that are being ridden successfully with a bit and have no history of oral discomfort.14,31 If wolf teeth are detected during examination of this type of horse, the owner should be made aware of potential behavioral problems related to the teeth. Extraction is not recommended in these horses unless the wolf teeth begin to cause a problem.

    Acknowledgment

    The author acknowledges the following individuals at Texas A&M University's College of Veterinary Medicine for their assistance in preparing this article: Kathrin R. Burke, DrMedVet, Department of Veterinary Pathobiology; Kyle Westfall, veterinary technician, Veterinary Medical Teaching Hospital; Betsy McCauley, veterinary radiologic technologist, Veterinary Medical Teaching Hospital; and Larry Wadsworth, medical photographer, College of Veterinary Medicine Media Resources.

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    1. Soana S, Gnudi G, Bertoni G. The teeth of the horse: evolution and anatomo-morphological and radiographic study of their development in the foetus. Anat Histol Embryol 1999;28:273-280.

    2. Matthew WD. Evolution of the Horse. New York: Guide Leaflet Series; 1932:36.

    3. MacFadden BJ. Equine dental evolution: perspective from the fossil. In: Baker GJ, Easley J, eds. Equine Dentistry. 2nd ed. Philadelphia: Elsevier; 2005:1-8.

    4. Sisson S, Grossman JD. Anatomy of Domestic Animals. 4th ed. Philadelphia and London: WB Saunders; 1953.

    5. Jones WE. The Teeth of the Horse. Fort Collins, CO: Caballus Publishers; 1972:31.

    6. Bartlet J. The Gentleman's Farriery: or, a Practical Treatise on the Diseases of Horses. 8th ed. London; 1773:283-284. Based on information from English Short Title Catalogue. Eighteenth Century Collections Online. Accessed May 2008 at http://galenet.galegroup.com/servlet/ECCO.

    7. Daubenton LJM. Histoire Naturelle, Générale et Particulière, avec la Description du Cabinet du Roi. Paris: Imprimerie Royale; 1763:344.

    8. Huidekoper RS. Age of the Domestic Animals. Chicago: Alexander Eger; 1903:33.

    9. Goubaux A, Gustave B. The Exterior of the Horse. 2nd ed. London: JB Lippincott; 1904:621-622.

    10. Merrillat LA. Animal Dentistry and Diseases of the Mouth. Chicago: Alexander Eger; 1905:202-204.

    11. Easley KJ. Equine canine and first premolar (wolf) teeth. Proc AAEP 2004;50:13-18.

    12. Korinek CJ. The Veterinarian. 2nd ed. Cedar Rapids, IA: The Veterinarian Publishing Company; 1916:85.

    13. Baker AH. Livestock and Complete Stock Doctor, A Cyclopedia. St. Louis: ND Thompson Publishing Company; 1916:116.

    14. Dixon PM, Dacre I. A review of equine dental disorders. Vet J 2005;169:165-187.

    15. Lane JG. A review of dental disorders of the horse, their treatment and possible fresh approaches to management. Equine Vet Educ 1994;6:13-21.

    16. Linkous MB. Performance dentistry and equilibration. Clin Techniques Equine Pract 2005;4(2):124-134.

    17. Easley J. Guidelines to extracting wolf teeth. Vet Pract News 2002:39-40.

    18. Gaughn EM. Dental surgery in horses. Vet Clin North Am 1998;14(2):381-397.

    19. Dixon PM. Equine dental disease, part 1: a long term study of 400 cases: disorders of incisor, canine, and first premolar teeth. Equine Vet J 1999;31:369-377.

    20. Scrutchfield L, Schumacher J. Examination of the oral cavity and routine dental care. Vet Clin North Am 1993;9(1):123-131.

    21. Easley KJ. Dental and oral examination. In: Baker GJ, Easley J, eds. Equine Dentistry. Philadelphia: WB Saunders; 1999;(1):151-169.

    22. Dixon PM. Dental anatomy. In: Baker GJ, Easley J, eds. Equine Dentistry. 2nd ed. Philadelphia: Elsevier; 2005:25-48.

    23. Stelzer P. Die Extraktion des Wolfszahnes beim Pferd (Extraction of the wolf tooth in horses). Der Praktische Tiearzt 2004;85(3):188-189.

    24. Nickel R, Schummer A, Seiferle E. The Viscera of Domestic Mammals. New York: Springer-Verlag; 1979;2:95.

    25. Easley KJ. Corrective dental procedures. In: Baker GJ, Easley J, eds. Equine Dentistry. 2nd ed. Philadelphia: Elsevier; 2005;2:221-248.

    26. Dixon PM. The gross, histological, and ultrastructural anatomy of equine teeth and their relationship to disease. Proc AAEP 2002;48:421-437.

    27. Scrutchfield WL. Dental prophylaxis. In: Baker GJ, Easley J, eds. Equine Dentistry. Philadelphia: WB Saunders; 1999;1:185-205.

    28. Dixon PM, Easley J, Ekmann A. Supernumerary teeth in the horse. Clin Techniques Equine Pract. Philadelphia: Elsevier; 2005;4:155-161.

    29. Manfredi J, Clayton HJ, Rosenstein D. Radiographic study of bit position within the horse's oral cavity. Equine Comp Exerc Physiol 2005;3:195-201.

    30. Lowder M. Dental conditions affecting the young horse birth to 2 years. Focus Dent Proc AAEP 2006:203-205.

    31. Scrutchfield WL. Wolf teeth: how to safely and effectively extract and is it necessary. Focus Dent Proc AAEP 2006:56-60.

    References »

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    didyouknow

    Did you know... The difference between attrition and abrasion pertains to the object that causes wear on the tooth. Attrition is caused by tooth-on-tooth contact over time. Abrasion is due to wear by anything other than a tooth.Read More

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