Oral Anatomy and Physiology

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 shared connection between the oral cavity and the digestive tract.

Around day 21 of development, branchial arches I and II are present. By day 23 the paired maxillary and mandibular processes of branchial arch I have become distinct. The mandibular processes grow rostrally, forming the mandible and merging at the mandibular symphysis, which in the dog and cat normally remains a fibrous union throughout life. The paired maxillary processes form most of the maxillae, incisive, and palatine bones.

Initial development of the dental structures occurs during embryonic formation. Rudimentary signs of tooth development occur approximately at the 25th day of development when the embryonic oral (stratified squamous) epithelium begins to thicken. This thickening, known as the dental lamina, forms two U‐shaped structures, which eventually become the upper and lower dental arches. The enamel organ, which evenutally is responsible for enamel formation and has a role in induction of tooth formation, arises from a series of invaginations of the dental lamina into the adjacent mesoderm. The oral epithelium, dental lamina, and enamel organ originate from the outer embryonic germ layer known as ectoderm. The dental papilla and sac appear in coordination with the enamel, but originate from mesoderm (ectomesenchyme of the neural crest).

The enamel organ develops through a series of stages known as the bud, cap, and bell (Figure 1.1). The bud stage is the initial budding off from the dental lamina at the areas corresponding to the deciduous dentition. The bud eventually develops a concavity at the deepest portion, noting the start of the cap stage. As the enamel organ enters this stage it is comprised of three parts: the outer enamel epithelium (OEE) on the outer portion of the cap, the inner enamel epithelium (IEE) lining the concavity, and the stellate reticulum within the cap. The onset of the bell stage occurs as a fourth layer to the enamel organ, the stratum intermedium, emerges between the IEE and the stellate reticulum.

Micrographs of 5 important stages of tooth development: placode stage, bud stage, cap stage, late bell stage, and root development. Ep and mes are indicated the first stage, sr and dm in the second stage, etc.

Figure 1.1 Histology of important stages of tooth development. Note that all early development is directed at creating the crown and only then root formation is initiated. Ameloblasts differentiate from the epithelium and odontoblasts from the mesenchyme and they deposit the matrices of enamel and dentin, respectively. Ameloblasts and enamel are missing on the root, which is covered by the softer dentin and cementum. Ep, epithelium; mes, mesenchyme; sr, stellate reticulum; dm, dental mesenchyme; dp, dental papilla; df, dental follicle; ek, enamel knot; erm, epithelial cell rests of malassez; hers, Hertwig’s epithelial root sheath.

Source: from Thesleff, I. and Tummers, M. Tooth organogenesis and regeneration: stembook.org/node/551; accessed November 2017.

Each layer of the enamel organ has specific functions to perform. The OEE acts as a protective layer for the entire organ. Stellate reticulum works as a cushion for protection of the IEE and allows vascular fluids to percolate between cells and reach the stratum intermedium. The stratum intermedium apparently converts the vascular fluids to usable nourishment for the IEE. The IEE goes through numerous changes, ultimately being responsible for actual enamel formation.

The dental lamina buds that form the primary dentition develop lingual extensions referred to as successional lamina. The successional laminae progress through bud, cap, and bell stages to eventually form the successional permanent dentition. The non‐successional teeth, those permanents not succeeding deciduous counterparts, develop directly from the dental lamina.

During the late bud stage, from an area adjacent to the IEE, mesenchymal cells begin development of the dental papilla and dental sac. The mesodermal cells of the dental papilla form the dentinal and pulpal tissues of the forming tooth. The dental sac is comprised of several rows of flattened mesodermal cells covering the dental papilla and attaching part of the way up the OEE of the bud. It gives rise to cementum, periodontal ligament (PDL), and some alveolar bone.

The frontal prominence, the forehead area of the embryo, occurs in coordination with the stomodeum and mandibular processes. Nasal pits, the beginning of the nasal cavities, are first revealed by two small depressions found low on the frontal prominence. On either side of the nasal pits are the medial and lateral nasal processes. The two medial nasal and two maxillary processes form the upper lip. The groove between the two fills with connective tissue in a process known as migration. If migration fails to occur the tissues will be stretched thin and will tear. This results in a separation between the medial nasal and maxillary process, which causes a cleft lip.

The left and right maxillary processes and the single medial nasal processes also form the palate. The incisal portion (maxilla) of the hard palate is the part from the maxillary incisor teeth back to the incisive foramen. The area of the incisive bone (the premaxilla in some species, and formerly in the dog) is also known as the primary palate and is formed solely by the medial nasal process. The medial nasal process forms the philtrum and helps form the nasal septum. The left and right maxillary processes form two palatal shelves that grow inward toward the midline, beginning rostrally, and then attaching to the primary palate and growing together. This is known as the secondary palate.

Cleft lips and palates are not uncommon. Clefts are generally designated as unilateral or bilateral. A unilateral cleft lip occurs when migration fails to occur between one of the maxillary processes and the medial nasal process. A bilateral cleft lip occurs when both maxillary processes fail to migrate. A unilateral cleft palate occurs when one of the palatal plates of the maxillary processes fails to fuse with the nasal septum. A bilateral cleft palate occurs when both palatal plates of the maxillary processes fail to fuse with the opposite plates at the nasal septum. Clefts of hard or soft palates develop in a wide range of varying degrees of severity.

1.2.1 Enamel, Dentin, and Pulp

These three structures have an intimate relationship during early development, although they do not all develop from the same foundation cells. Enamel is produced by the enamel organ, which is derived from ectoderm. In contrast, the dentin and pulp develop from the dental papilla, which is derived from mesoderm.

During the bell stage, the IEE cells evolve into a taller form and become preameloblasts. The peripheral cells of the dental papilla bordering the preameloblasts transform into low columnar or cuboidal shapes and form odontoblasts. As the newly formed odontoblasts move toward the center of the dental papilla and away from the preameloblasts they leave behind a secreted matrix of mucopolysaccharide ground substance and collagen fibers. This substance appears to stimulate a polarity shift in the preameloblasts of the nucleus from the center of the cell toward the stratum intermedium. It is thought that this shift in polarity is caused by an alteration in the nutritional supply route to the cells. With this shift in polarity, the cells now become ameloblasts and begin secretion of enamel matrix. As this enamel matrix (mucopolysaccharide ground substance and organic fiber) is laid down next to the dentinal matrix, the dentinoenamel junction (DEJ) is formed. As the ameloblasts lay down matrix they move away from the dentin and toward the OEE. Both the dentin and enamel begin to lay down crystal and mineralize at this point into hard tissue.

The enamel matrix is laid down at the end of the bell stage. All of the crystal placed within the rods are laid down at this time. This is known as the mineralization stage of calcification of the enamel rod. The next is the maturation stage of calcification. It is during this stage that the crystals grow in size, becoming tightly packed together within the enamel rod. Should the crystals fail to grow to full size, the rods will be poorly calcified and have less than 96% inorganic composition; this results in a condition known as hypomineralization. As enamel is produced by the ameloblasts, a change occurs in the enamel organ. The ameloblasts gradually begin to compress the two middle layers of the organ, the stratum intermedium and the stellate reticulum. The middle layers are eventually lost and the ameloblasts make contact with the OEE. This activates the final two functions of the ameloblasts to commence. First, a protective layer is laid down on top of the enamel known as the primary enamel cuticle or Nasmyth’s membrane. This cuticle remains on the teeth for weeks to months, until it is worn away by abrasion. The cuticle is laid down upon the crown from the tip toward the cementoenamel junction (CEJ). Once the cuticle is formed the ameloblasts merge with the OEE to form the reduced enamel epithelium. The reduced enamel epithelium is produced on adhesive‐like secretion known as the secondary enamel cuticle or epithelial attachment. The epithelial attachment functions to hold the gingiva and tooth together at the bottom of the gingival sulcus. During enamel development, several abnormalities may develop. These are sometimes found on clinical, radiological, or histological examination. Amelogenesis imperfecta is the general term that includes any genetic and/or developmental enamel formation and maturation abnormalities. Enamel hypoplasia refers to inadequate deposition of enamel matrix, i.e., when the density or mineralization is generally normal, but the enamel is thinner than normal. Enamel hypomineralization refers to inadequate mineralization of enamel matrix, resulting in white, yellow, or brown spots in the enamel. This often affects several or all teeth. The crowns of affected teeth may be soft and wear faster than normal teeth.

Mesodermal tissue from the dental papilla forms the pulp. Once developed, it consists of blood vessels, lymphatic vessels, nerves, fibroblasts, collagen fibers, undifferentiated reserve mesenchymal cells, other cells of connective tissue, and odontoblasts. Odontoblasts are an integral part of the dentin, but are also the peripheral cells of the pulp. The pulpal nerves are primarily sensory and transmit only the sensation of pain. There are some motor nerves that innervate the smooth muscles within the blood vessels. These result in constriction of the vessels in response to irritation. Young pulps have a large volume, which is considered primarily cellular, with a small concentration of fibers. The large number of cells allows for repair from trauma. As the pulp ages, it loses volume and reserve cell capacity. This loss of reserve cells is thought to be the reason that older patients are more susceptible to permanent pulpal damage.

1.2.2 Root Formation

Formation of the root begins after the general form of the crown has developed, but prior to its complete calcification. At the point where the OEE becomes the IEE, the stellate reticulum and stratum intermedium are missing from the enamel organ at this deepest point, and is referred to as the cervical loop. These two layers of cells become the epithelial root sheath or Hertwig’s epithelial root sheath (Figure 1.2). This sheath begins to grow into the underlying connective tissue by rapid mitotic division, initiating root formation. This growth advanced deep into underlying connective tissue, but at some point, angles back toward the center of the forming tooth. The portion of the sheath that turns back in is known as the epithelial diaphragm. The growth pattern of the epithelial diaphragm determines the number of roots a tooth develops. The point at which the epithelial diaphragm meets will be the apex of a single rooted tooth but the furcation in multirooted teeth. As the root sheath makes contact with the dental papilla, it stimulates the peripheral contact cells to differentiate into odontoblasts. Once the odontoblasts begin to produce dentin, the root sheath trapped between the dental sac and the dentin begins to break up. As Hertwig’s epithelial root sheath dissolves, the dental sac comes into direct contact with the newly formed dentin. Some of the dental sac cells differentiate into cementoblasts and initiate cementum formation. The cementum that contacts the dentin becomes the dentinocemental junction (DCJ).

Diagram of epithelial root sheath with arrows depicting outer enamel epithelium, inner enamel epithelium, stellate reticulum, dental mesenchyme, odontoblast, dentin, enamel, and ameloblast.

Figure 1.2 Fate of the stem cell progeny in the epithelial stem cell niche of the continuously growing tooth, the cervical loop. Stem cells divide in the stellate reticulum compartment giving rise to cells that will become inserted to the basal layer of epithelium looping around the stellate reticulum. Here the cells proliferate, migrate toward the oral cavity and differentiate into ameloblasts, depositing enamel matrix. ; accessed November 2017.

Source: From Thesleff, I. and Tummers, M. Tooth organogenesis and regeneration: stembook.org/node/551 – animation

The epithelial root sheath cells that move away from the dentin, but fail to dissolve, become entrapped in the PDL and are referred to as epithelial rests or epithelial rests of Malassez. These cell rests are a normal finding, but under the influence of various stimuli, they could proliferate later in life to form epithelial lining of various odontogenic cysts, such as the radicular cyst. When epithelial root sheath cells fail to dissolve and remain in contact with the dentin, they typically convert to ameloblasts. These may secrete enamel on the roots, forming what is known as enamel pearls. If the root sheath’s epithelial diaphragm malfunctions, accessory roots may be formed.

1.2.3 Tooth Eruption

The emergence and movement of the crown of the tooth into the oral cavity is typically termed tooth eruption. The eruptive sequence is generally divided into three stages. The pre‐eruptive stage commences with crown development and the formation of the dental lamina. With the onset of root development, the eruptive stage begins. This is also sometimes referred to as the pre‐functional eruptive stage. When the teeth move into actual occlusion it is termed post‐eruptive stage or functional eruptive stage. This stage is considered to continue until tooth loss occurs, or death. In the hypsodont species, this stage may function to serve occlusion in several ways. As the jaws grow, the mandible and maxilla spatial relationship becomes further apart and the teeth continue to erupt to maintain occlusion. With time, attrition results in loss of dental occlusal contacts and it is this further eruption that maintains the occlusal balance. In some cases, this can cause an imbalance in occlusion when teeth are lost and supraeruption of the opposing teeth occurs. Supraeruption is when teeth erupt beyond the normal occlusal line.

Four major theories for eruption have been expounded upon in the literature. Most likely none are totally correct in themselves, but the most accurate picture is probably a combination of them. The theory of root growth is the belief that root growth pushes the crown into the oral cavity. Experiments of removing Hertwig’s epithelial root sheath on developing teeth has stopped root formation. However, these rootless teeth still erupt, thus disproving this as a major factor in eruption. The theory of growth of pulpal tissue proposes that continued growth of the pulp tissue while the hard sides of the tooth are forming provides apical propulsion. Yet developing teeth in which the pulp dies or is removed will still erupt, also disproving this as a major factor in eruption. The theory of bone deposition in the alveolar crypt is the precept that bone deposition within the alveolar crypt forces the tooth to erupt. This deposition is not constant and even when the crypt undergoes resorption due to various factors teeth generally still erupt, making this theory a dubious major factor. The theory of PDL force is the hypothesis that it is the PDL’s driving force that maintains occlusal contact also thrusts the tooth into the oral cavity. This is the most plausible postulate, although the exact mechanism is unknown. Eruption times are variable not only with size and breed but also within the breeds themselves. Average eruption times of deciduous and permanent teeth can be found in Table 4.1 in Chapter 4 – Developmental Pathology and Pedodontology.

Exfoliation of deciduous dentition is a complex function and not fully understood. It is believed that as the permanent tooth root begins development, the crown makes contact with the deciduous tooth root structure. The pressure of the permanent tooth crown on the deciduous tooth root, and possibly the contact of the permanent tooth’s dental sac or the OEE with the deciduous root, stimulates the resorptive process of the deciduous tooth root. Deciduous root resorption occurs in cycles or stages, and is not constant. Once sufficient root support is lost, the crown is shed or exfoliated. Although it is common for deciduous teeth to persist when a permanent successor does not develop, this is not always the case, indicating that other factors may play a part in root resorption.

Persistent deciduous teeth are commonly attributed to four causes. The first is the lack of a permanent successor. The second is ankylosis of the tooth to the alveolus. This may occur during root resorption when holes in Hertwig’s root sheath develop and the tooth’s cementum makes contact with the alveolar bone and fuses to it. In these cases, it is common to find teeth with almost the entire root structure dissolved, but with the crown still firmly in place. Once the ankylosis is relieved, typically the crown rapidly exfoliates. The third cause for persistent deciduous dentition is failure of the permanent crown to make contact with the deciduous root during eruption. This occurs if either tooth is in an improper position, in comparison to each other. Finally, the fourth reason is hormonal influences, which can affect growth or metabolism.

1.3 Basic Anatomy of the Dental‐Periodontal Unit

1.3.1 Directional, Surface, and Ridge Nomenclature

Prior to discussing dental anatomy, a general understanding of directional, surface, and ridge nomenclature is required.

Rostral and caudal are anatomical terms of location applicable to the head in a sagittal plane in non‐human vertebrates. Rostral refers to a structure closer to, or a direction toward, the most forward structure of the head. Caudal refers to a structure closer to, or a direction toward, the tail. Anterior and posterior are the synonymous terms used in human dentistry. The term caudal teeth refer to premolars and molars, as opposed to incisors and canines, which are rostral teeth. Incisors, canines, and premolars have four exposed surfaces and a ridge or cusp, making a total of five surfaces. Molars have five exposed surfaces. Sometimes a ridge may be referred to as a surface.

As a general rule, the surfaces of the teeth facing the vestibule or lips are the vestibular surfaces [1] (Figure 1.3). For the incisor and canine teeth, the surface directed toward the lips is commonly called the labial surface. With premolars and molars, the surface facing the cheek is known as the buccal surface. The term “facial” has been used traditionally in human dentistry to refer to the surfaces of the rostral teeth visible from the front. All surfaces facing the tongue are described as lingual, although for the maxillary teeth this surface is often described as the palatal surface. For premolars and molars, the surface making contact with the teeth in the opposite jaw during closure is known as the occlusal surface. The ridge of the premolars that does not make contact with opposing teeth is typically referred to as the occlusal ridge. For the incisors, the ridge along the coronal‐most aspect is referred to as the incisal ridge. The cusp is the point or tip of the crown of a tooth. For the canine tooth, the cusp is generally called the cusp surface. Premolars and molars may have multiple cusps. Surfaces facing toward adjoining teeth within the same jaw quadrant or dental arch are collectively called the contact or proximal surfaces. Proximal surfaces may be either distal or mesial. The term distal indicates a proximal surface facing away from the median line of the face. In contrast, the term mesial designates the proximal surface facing toward the median line. The space between two facing proximal surfaces is known as the interproximal space. Apical is a term used to denote a direction toward the root tip. Coronal is a term used to indicate a direction toward the crown tip or occlusal surface. The terms incisal for incisors and occlusal for premolars and molars is also used to indicate the coronal direction. The term cervical either means the juncture of the tooth crown and root or a direction toward that point.

Illustration of directional nomenclature with labels median line, interproximal or contact surfaces, distal, mesial, labial, facial, buccal, palatal, and maxilla.

Figure 1.3 Directional nomenclature of the maxillary teeth of the cat.

Source: Courtesy of Josephine Banyard.

To further break down tooth locations, combinations of the above terms are sometimes used, with one additional term, middle (Figure 1.4). The term middle means at or toward the middle of a designated portion of the tooth and can indicate either a horizontal or vertical middle area.

Illustrations of 2 sets of crown with lines indicating mesial, middle, and distal and lingual, middle, and facial (top) and 2 crowns and roots with lines for incisal, middle, cervical, cervical, middle, and apical (bottom).

Figure 1.4 Division into thirds.

Source: Courtesy of Josephine Banyard.

1.3.2 Crown Line and Point Angles

For the purpose of identifying and classifying distinct areas on teeth in operative dental procedures, the coronal surfaces can be divided and classified by eight line angles and four point angles (see Chapter 17 – Restorative Dentistry). These lines and points are also sometimes used for identification of cavity prep areas.

There are five crown surfaces: vestibular, lingual/palatal, mesial, distal, and occlusal/coronal/incisal. The line angles are simply the dividing lines formed between the surface areas. They are named from two of the five surfaces that divide them. Where the surface terms are joined, the “ar” or “al” ending is dropped and “o” is added. The eight line angles are (i) mesiovestibular (mesiolabial, mesiobuccal), (ii) mesiolingual (mesiopalatal), (iii) mesioincisal (mesiocoronal, mesio‐occlusal), (iv) distovestibular (distolabial, distobuccal), (v) distolingual (distopalatal), (vi) distoincisal (distocoronal, disto‐occlusal), (vii) linguoincisal (linguocoronal, linguo‐occlusal), and (viii) vestibuloincisal (vestibulocoronal, vestibulo‐occlusal).

The point angles are the junctures of three of the line angles. There are four coronal point angles, each named for the three surfaces that actually make the juncture or point. The four point angles are (i) mesiovestibuloincisal (mesiovestibulocoronal, mesiovestibulo‐occlusal, mesiolabioincisal, mesiolabiocoronal, mesiolabio‐occlusal, mesiobuccocoronal, mesiobucco‐occusal), (ii) mesiolinguoincisal (mesiolinguocoronal, mesiolinguo‐occusal, mesiopalatoincisal, mesiopalatocoronal, mesiopalato‐occlusal), (iii) distovestibuloincisal (distovestibulocoronal, distovestibulo‐occlusal, distolabioincisal, distolabiocoronal, distolabio‐occlusal, distobuccocoronal, distobucco‐occusal), and (iv) distolinguoincisal (distolinguocoronal, distolinguo‐occusal, distopalatoincisal, distopalatocoronal, distopalato‐occlusal).

1.3.3 Contact Points and Areas

Contact points and areas are the sites where adjacent or opposing teeth make contact. The term contact area is considered a more correct term than contact point, since an area is typically making contact rather than a specific point. Adjacent teeth have proximal contact areas, where opposing teeth have occlusal contact areas.

1.3.4 Embrasures

Projecting away from the proximal contact areas are V‐shaped areas termed embrasures. They are named for the surface from which they are derived and the direction they radiate toward. There are theoretically four embrasures between each tooth with proximal contacts. The embrasures are the (i) vestibulogingival (labiogingival, buccogingival), (ii) vestibuloincisal (vestibulocoronal, vestibulo‐occlusal, labioincisal, labiocoronal, labio‐occlusal, buccocoronal, bucco‐occlusal), (iii) linguogingival (palatogingival), and (iv) linguoincisal (palatoincisal, palatocoronal, palato‐occlusal, linguocoronal, linguo‐occlusal).

1.3.5 Tooth Function and Terms

Teeth are multifunctional organs that play an important part in overall animal health and activity. Their shape aids physiologically in protection of the oral mucosa, as well as reduction of stress forces on the teeth and the alveolar process. Teeth are used to catch, hold, carry, cut, shear, crush, and grind sustenance. Besides their masticatory functions, they are used in protection, aggression, and sexual attraction. Sexual dimorphism, such as length of tooth, may play a part in sexual attraction and social behavior for defense.

Each tooth has a crown and a root, except for aradicular hypsodonts (see Chapter 21 – Small Mammal Oral and Dental Diseases). Generally, the brachydont crown is covered with enamel and the root with cementum. Where the enamel of the crown and cementum of root meet is known as the CEJ. The line formed by the CEJ is commonly called the neck, cervix, or cervical line. In many cases, especially during eruption and in hypsodont dentition, not all of the crown may be fully exposed. The entire crown, whether exposed or not, is the anatomical crown. The supragingival portion of the crown is the clinical crown and the subgingival portion is the reserved crown. The reserved crown is occasionally referred to as the clinical root as compared to the anatomical or true root. The incisor teeth are designed to cut, scrape, scoop, pick at or up, and groom. The term incisor means “that which cuts.” The actual biting edge of the incisor is the incisal edge or ridge. The incisal edge picks up and cuts food, scrapes meat off bone, grooms the hair, and is used to catch parasites. The concave lingual surface acts as a scoop and, along with the tongue, aids in carrying food into the oral cavity. The canine teeth are designed to pierce and hold a victim. They can also be used to slash and tear when used as weapons in fighting. In the carnivores, canines have the longest crowns and roots. These large roots make them very stable and good anchorage points. Premolars resemble a cross between canine teeth and molars. They are not as long as canine teeth and generally have multiple functional cusps. Being a cross between a canine tooth and a molar, they are designed to function similarly to both. They help to hold and carry, while also helping to break food down into smaller pieces. Molars have an occlusal surface that can be used to grind food or break it down into smaller pieces. The incisors and canine teeth are referred to as rostral teeth, while the premolars and molars are caudal teeth. The carnassial teeth are considered to be the largest shearing teeth in the upper and lower jaws. In the dog and cat these are the maxillary fourth premolars and the mandibular first molars. The term carnassial (commonly used, not an accepted anatomic term) means flesh cutting.

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Aug 15, 2020 | Posted by in GENERAL | Comments Off on Oral Anatomy and Physiology

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