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Oral Anatomy and Physiology

Matthew Lemmons1 and Donald Beebe2

1 MedVet Medical and Cancer Centers for Pets, Indianapolis, IN, USA

2 Apex Dog and Cat Dentistry, Englewood, CO, USA

Within this chapter, the dog will be discussed primarily, although some comparative information will be covered. Related anatomy and variations for other species will be discussed within chapters covering those. It is intended that this chapter serve to provide the foundation knowledge for the chapters that follow.

The practice of veterinary dentistry is concerned with the conservation, reestablishment and/or treatment of dental, paradental, and oral structures. In dealing with their associated problems a fundamental awareness of anatomy and physiology is essential for an understanding of the presence or absence of the abnormal or pathologic structure. Anatomy and physiology are acutely interactive, with anatomy considered the study of structure and physiology that of its function. These deal with bones, muscles, vasculature, nerves, teeth, periodontium, general oral functions, and their development.

1.1 General Terms

• Dentes decidui - deciduous teeth.
• Dentes permanentes - permanent teeth.
• Dentes incisivi - incisor teeth.
• Dentes canini - canine teeth.
• Dentes premolares - premolar teeth.
• Dentes molares - molar teeth.

1.1.1 Three Basic Types of Tooth Development

• Monophyodont. Only one set of teeth that erupt and remain in function throughout life (no deciduous teeth), such as in most rodents (heterodont) and dolphins (homodont), as currently accepted.
• Polyphyodont. Many sets of teeth that are continually replaced. Most of these are homodonts. In sharks, the replacement is generally of a horizontal nature with new teeth developing caudally and moving rostrally. In reptiles, the replacement is generally of a vertical nature with new teeth developing immediately apical to the teeth in current occlusion and replacing them when lost.
• Diphyodont. Two sets of teeth, one designated deciduous and one permanent. Common to most domesticated animals and man.

1.1.2 Common Terms Used with Diphyodont Tooth Development

• Deciduous teeth (Dentes decidui). Considered to be the first set of teeth that are shed at some point and replaced by permanent teeth.
• Primary teeth (Dentes primarui). Considered to be the first set of teeth that are shed at some point and replaced by permanent teeth. Some distractors feel this term is not totally correct because in some species primary teeth are also their permanent teeth, and even in diphyodonts some permanent teeth (i.e., the dog: first premolar and molars) may theoretically also classify as primary, since all teeth may eventually be exfoliated. The term primary is acceptable when speaking to the layperson, but not acceptable in the professional setting.
• Permanent teeth (Dentes permanentes). The final or lasting set of teeth, that are typically of a very durable nature (opposite of deciduous).
• Nonsuccessional teeth (Nonsuccedaneous). Permanent teeth that do not succeed a deciduous counterpart. Classically molars of dogs and cats.
• Successional teeth (Succedaneous). Permanent teeth that replace or succeed a deciduous counterpart. Typically certain diphyodont incisors, canines, or premolars.
• Mixed Dentition. The transient complement of teeth present in the mouth after eruption of some of the permanent teeth but before all the deciduous teeth are absent. Commonly seen in diphyodonts during the early stages of permanent tooth eruption, until all deciduous teeth have been exfoliated.

1.1.3 Two Basic Categories of Tooth Types or Shapes

• Homodont. All teeth are of the same general shape or type, although size may vary, such as in fish, reptiles, sharks, and some marine mammals.
• Heterodont. Functionally different types of teeth are represented in the dentition. The domestic dog and cat have heterodont dentition, characterized by incisors, canines, premolars, and molars.

1.1.4 Three Common Types of Vertebrate Tooth Anchorage

• Thecodont. Teeth firmly set in sockets typically using gomphosis, such as dogs, cats, and humans. Gomphosis. A type of fibrous joint in which a conical object is inserted into a socket and held.
• Acrodont. Teeth are ankylosed directly to the alveolar bone without sockets or true root structure. This type of attachment is not very strong; teeth are lost easily and are replaced by new ones. This formation is common in the order Squamata (lizards and snakes) with the only other teeth formation in this order being pleurodont. Acrodontal tooth attachment is also seen in fish.
• Pleurodont. Teeth grow from a pocket on the inner side of the jawbone that brings a larger surface area of tooth in contact with the jawbone and hence attachment is stronger, as in amphibians and some lizards. However, this attachment is also not as strong as the codont anchorage.

1.1.5 Two Basic Tooth Crown Types

• Brachydont. Dentition with a shorter crown to root ratio, as in primates and carnivores. A brachydont tooth has a supragingival crown and a neck just below the gingival margin, and at least one root. An enamel layer covers the crown and extends down to the neck. Cementum is only found below the gingival margin.
• Hypsodont. Dentition with a longer crown to root ratio, as in cow, horses, and rodents. These teeth have enamel that extends well beyond the gingival margin, which provides extra material to resist wear and tear from feeding on tough and fibrous diets. Cementum and enamel invaginate into a thick layer of dentin.
• Radicular hypsodont (subdivision of hypsodont). Dentition with true roots, sometimes called closed rooted, that erupts additional crown through most of life. These teeth eventually close their root apicies and cease growth. As teeth are worn down new crown emerges from the submerged reserve crown of the teeth, such as in the molars and premolars of the equine and bovine. Known as continually erupting closed rooted teeth.
• Aradicular hypsodont (subdivison of hypsodont). Dentition without true roots, sometimes called open rooted, that produces additional crown throughout life. As teeth are worn down new crown emerges from the continually growing teeth, such as in lagomorphs and incisors of rodents. Known as continually growing teeth or open rooted teeth.

1.1.6 General Crown Cusp Terms of Cheek Teeth

• Secodont dentition. Having cheek teeth with cutting tubercles or cusps arranged to provide a cutting or shearing interaction, such as premolars in most carnivores, especially the carnassial teeth.
• Bunodont dentition. Having cheek teeth with low rounded cusps on the occlusal surface of the crown. Cusps are commonly arranged side by side on the occlusal surface for crushing and grinding, such as molars in primates (including man), bears, and swine.
• Lophodont dentition. Having cheek teeth with cusps interconnected by ridges or lophs of enamel, such as in the rhinoceros and elephant.
• Selenodont dentition. Having cheek teeth with cusps that form a crescent-shaped ridge pattern, such as in the even-toed ungulates, except swine.

1.1.7 Two Types of Jaw Occlusal Overlay

• Isognathous. Equal jaw widths, in which the premolars and molars of opposing jaws aligned with the occlusal surfaces facing each other, forming an occlusal plane. Man is an imperfect isognathic, or near equal jaws.
• Anisognathous. Unequal jaw widths, in which the mandibular molar occlusal zone is narrower than the maxillary counterpart, such as in the feline, canine, bovine, equine, etc.

1.1.8 The Dog and Cat Dentition

Dogs and cats have diphyodont development, heterodont teeth types, brachyodont crown types, secondont teeth (all premolars, feline mandibular molar and a portion of the canine mandibular first molar), bunodont (feline maxillary molar, canine molars, including a portion of the mandibular first molar), thecodont tooth anchorage and anisognathic jaws.

1.2 Development

Note that the following section will give a brief overview of the embryologic development of the mouth and associated structures. The same tissues in the adult animal will be discussed later in the chapter.

Development of the gastrointestinal tract begins early in embryonic formation. The roof of the entodermal yolk sac enfolds into a tubular tract forming the gut tube, which will become the digestive tract. It is initially a blind tract being closed at both the upper and bottom ends. The bottom ultimately becomes the anal opening and the upper portion connects with the primitive oral cavity known as the stomodeum, or ectodermal mouth. The stomodeum and foregut are at this time separated by a common wall known as the buccopharyngeal membrane. It is located at a level that will become the oropharynx, located between the tonsils and base of the tongue. This pharyngeal membrane eventually disappears, establishing a...