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Anatomy of the Mouth and Teeth of the Cat
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FELINE DENTISTRY
ANATOMY OF THE MOUTH AND TEETH OF THE CAT Paul Orsini, DVM, and Philippe Hennet, DV
Cats belong to the carnivore order and to the Felidae family. Their dentition is diphyodont, i.e., succeeding generations of deciduous teeth and permanent teeth. Cats are truly carnivorous, which is reflected in their dentition. TOOTH FORMULAS
Four types of teeth are recognized, depending on general shape and function. They are incisors (I), canines (C), premolars (P), and molars (M). The dental formula summarizes the number of each type of tooth in the upper jaw (above the line) and in the lower jaw (below the line). 26 For the cat: C 1, pp 3 or simp . l.f. II 3, C I Ie d 33 11 32 = 26 teeth 1 2 3 ' ' .. I 3, C 1, P 3, M 1 . .f'. d 3 1 2 1 h P ermanent d enhhon: I 3 C 1 p 2 M 1 or simp 1I Ie 3 1 2 1 = 30 teet ' ' ' Primitive carnivores have the general dental formula of 3143/3143 (44 teeth total). In modern carnivores, the last premolar in the upper jaw and the first molar in the lower jaw have become carnassials (dente lacerantes). 26 In a species such as the cat that has fewer total teeth than the primitive mammal, which teeth are present is determined by identifying the carnassial teeth, then counting forward and backward from the carnassial teeth. In the cat, the first premolar appearing in the upper jaw is the second premolar of the primitive dentition, and the first premolar appearing in the lower jaw is the third premolar of the typical dentition. In the following discussion, teeth are numbered according to the primitive dentition (Fig. 1). .. DeCI.duous d enhhon:
Froin the Department of Clinical Studies-Philadelphia, University of Pennsylvania School of Veterinary Medicine, Philadelphia, Pennsylvania
VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 6 • NOVEMBER 1992
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ORSINI & HENNET (203) (202) (201) (101) (102) (103) 11 12 13
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Figure 1. Anatomic nomenclature of teeth in the cat. (numbers between brackets = modified Triadan system for the cat) (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
In this article, teeth are identified by a letter for the type of tooth (I, C, P, M) and a number for position in the arcade, superscript for upper jaw and subscript for lower jaw. Teeth of the deciduous dentition can be designated by lower case letters e.g., p 3 for the deciduous upper third premolar. PERMANENT DENTITION
In comparison to dog's teeth, incisors in cats are much smaller and narrower; as in dogs, their size increases from the first (central) incisor to the third (lateral) incisor. All incisors have a single root; lower incisors are smaller than upper incisors. The crown of all incisors other than the upper lateral incisor is chisel-shaped. The cutting edge of these teeth present three small cusps; the middle cusp is the largest. The lateral upper incisor is the larger of the series; its crown is more conical, comprising the central cusp, with a mere trace of an inner cusp. The caudal (palatal) surface of upper incisors is divided by a transverse groove into a higher rostral part and a lower caudal part or cingulum. The upper canine is a strong, pointed tooth; the root is longer than the
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crown, and the neck is not well marked. The crown is slightly curved palatally and distally. The labial surface of the crown is marked by vertical grooves, one being more prominent. The crown of the lower canine is curved distally; its labial surface is ridged in its distal part. P2 is a small single-rooted tooth with a conical crown; however, one study reported that 38% have two roots, which most of the time are fused together.' 7 The P3 and the P, and P4 have a similar crown, conical in shape but strongly compressed from side to side. The crown is limited at its base by a well-marked ridge of enamel (enamel bulge). The central triangular elevation of the crown is known as the principal cusp; the distal border is interrupted by a transverse notch, which produces a second smaller cusp, the distal basal cusp; behind this cusp is a second transverse incision, beyond which the enamel bulge is elevated into a small cusp known as the talon, or heel. The mesial border is more vertical and is notched at its lower end to produce a small mesial basal cusp, which sometimes is merely an elevation of the enamel as in P2 • This mesial cusp is less prominent or may be absent in P,. These teeth have two roots, slightly greater than the height of the crown, of which the mesial is usually smaller. The distal root of P3 is much thicker than the mesial. The carnassial teeth are secondont, i.e., ~hey have sharp cutting edges that are parallel to the edge of the jaw. During jaw movement, the two cutting edges glide along one another after the fashion of scissors (Figs. 2 and 3). The upper carnassial p• is twice as large as P3 • The broader mesial part has two transversely placed roots; the greater part, however, is supported by a single large distal root. The cutting edge is composed of a principal cusp and a greatly enlarged distal basal cusp separated from the principal cusp by a narrow vertical cleft. The distal cusp has a sharp horizontal cutting edge. The broad mesial part of the crown is occupied by two small conical cusps. The distal triangular root is strongly flattened from side to side and carries the principal and distal cusps. The furcation between the mesial and distal roots is located above the tip of the principal cusp. The cutting blade of the crown of the lower carnassial is formed of two nearly equal cusps separated on the buccal surface by a vertical fissure and on the lingual surface by a deep pit. The mesial and distal edges of the blade are almost vertical. The greater part of the crown is supported by the large mesial root. The very small distal root is often angulated caudally (distally). The furcation is located at the level of a midpoint between the tip of the distal cusp and the vertical fissure. The upper molar is a very small, two-rooted tooth aligned transverse to the alveolar border, close behind and in contact with the distal end of the carnassial tooth. Very often this tooth is rudimentary. In felids, the portion of the dentition behind the carnassials has degenerated. DECIDUOUS DENTITION
The deciduous dentition in cats is composed of 26 teeth. Compared with the permanent teeth, deciduous incisors are smaller; upper canines are more slender and strongly curved; lower canines are more vertical and resemble upper deciduous canines; upper first deciduous molars are very small and resemble permanent teeth; upper second deciduous molars are smaller and sectorial; upper third molars are more complicated and resemble the permanent molar; lower first deciduous molars resemble the first permanent premolars that replace them-they are smaller and their cusp is more acute; and finally, the lower second deciduous molars resemble the permanent lower carnassial teeth and are different from the premolars that replace them. 17' 25
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Figure 2. Scissor occlusion of carnassial teeth in the cat-lingual view (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
AGE OF ERUPTION OF THE TEETH Deciduous teeth
Seven days after birth, no teeth have emerged. Between 11 and 15 days, the incisors erupt; at 17 to 19 days, the canines erupt; between 24 and 30 days, all molars except the upper first erupt. The upper first molars erupt later, between 37 and 60 days. At 60 days, the deciduous dentition is complete.17• 22• 28 Permanent teeth
See Table 1 for a breakdown of the age of eruption of the permanent teeth. TOPOGRAPHIC TERMINOLOGY OF INDIVIDUAL TEETH
Terms are described as follows:
Labial or Buccal. Surface of the tooth that faces the lip or cheek, vestibular is often used as a synonym of buccal. Lingual. Surface of the tooth that faces the tongue. Palatal. Surface of the tooth that faces the palate.
Figure 3. Diagram of the occlusal relationship of premolar and molar teeth in the cat (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
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Table 1. AGE OF ERUPTION OF PERMANENT TEETH IN THE CAT
Upper Incisors Central Middle Lateral Canine Premolar Second Third Fourth Molar
Lower
103 114 135 153
days days days days
113 119 132 149
days days days days
150 168 151 162
days days days days
173 days 174 days 130 days
Data derived from references 1, 22, and 27.
Occlusal. Surface of the tooth that faces the crown of the opposite tooth in the opposite arch. Mesial. Surface of the tooth that faces toward the midline of the dental arch. Distal. Surface of the tooth that faces away from the midline of the dental arch. DENTAL OCCLUSION IN CATS
When the jaw is closed, the lower incisors normally strike immediately caudal to the upper incisors. The lower canine occludes between the lateral upper incisor and the upper canine. The incisors and especially the canine are used for grasping and biting food, cutting being done by caudal teeth, especially by carnassials, which are placed close to the insertions of the powerful muscles of mastication. The upper jaw being wider than the lower, the lower cheek teeth (premolar and molar) are nearer the sagittal plane of the skull than are the upper teeth. The upper and lower teeth do not touch when the jaws are moved in an absolutely vertical line. When the cat chews on one side of the mouth, the lower jaw must be brought to that side, so the buccal surface of lower teeth may shear upward and forward against the palatal surface of the upper teeth. 17 Biting forces of 20 to 23.25 kg at the canine and up to 28 kg at the carnassial · teeth have been reported in cats.• TOOTH STRUCTURE
The tooth is composed of a crown and of one, two, or three roots, which normally are not visible because they are embedded in the jaw bone. The neck of the tooth is the limit between crown and roots; it is located just below the enamel bulge at the gingival margin. Two calcified tissues form the tooth itself; the dentin composes most of the volume of the crown and the root(s), whereas the enamel is the outer layer of the crown covering the dentin. The enamel is the hardest, most mineralized tissue in the body. Microhardness values of enamel in cat premolars range from 273 to 337 VHN (Vickers microhardness value). The microhardness decreases significantly from the upper part of the crown to the cervical portion and from the outer to the inner
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layer. 11 Cat enamel is less hard than that of dogs and humans. 11 Enamel in dogs and cats is composed of three layers; a surface rodless (aprismatic) layer, an outer layer of parallel rods (only at some sites), and an inner layer. 30 The cross-sectional shape of inner enamel and outer layer enamel rods differs in dogs and in cats. Owing to the rodless surface layer, enamel surface was found to be smoother in dogs and cats than in humans. Enamel structure of the cat is most similar to that described in humans. 30 Enamel is secreted during development of the tooth by ameloblasts; ameloblasts disappear when enamel has been fully formed; consequently after that period, repair will not occur if enamel substance is lost. Dentin is less calcified than enamel; it is secreted by odontoblasts, which are located on the dentinal surface in the pulpal tissue. Contrary to enamel, dentin is continuously secreted by odontoblasts, resulting in a progressive thickening of the dentinal wall and a shrinking of the pulpal tissue. Dentin can also respond to irritation by secreting reparative dentin. Dentin has a porous structure; numerous dentinal tubuli run from the pulpal tissue to the amelodentinal or cementodentinal junction. These tubuli contain cytoplasmic processes of the odontoblasts. 14 When dez:ttin is exposed, an outward flow of fluid through the tubules has been observed in cats. This flow is sufficiently rapid to limit severely the rate of inward diffusion of chemicals. 34 Vasodentin tissue, consisting of small spaces containing vascular channels, has been described in dentin of cat teeth; dentinal tubules in the area of the vasodentin tissue are disrupted and irregular. 24 Pulp is the vital, nutrient tissue of the tooth. The pulp has four main functions: (1) formation of the dentin through the activity of odontoblasts; (2) nutrition of the dentin through the dentinal tubules, which contain the odontoblastic processes/ 4 (3) innervation of the dentin through the odontoblastic processes and the sensory nerves of the pulp, which are responsible for dentinal sensitivity; and (4) protection of the pulp through secretion of reparative dentin. The pulp is a highly specialized connective tissue, with a rather homogeneous composition; it contains cells (fibroblasts, histiocytes, leukocytes, and odontoblasts), collagen fibers, ground substance, blood and lymphatic vessels, and nerves. 3, 36 Sensory nerves originating from the trigeminal ganglion innervate the teeth. The axons end in the odontoblastic layer and predentin of roots and crown; at the tip of the pulp horn of each cusp, nerve endings extend deep into dentinal tubuli. 5 Odontoblasts lie at the surface of pulpal tissue against the predentin, which they secrete and subsequently mineralize. Beneath the odontoblastic layer is a layer composed of fibers and few cells; vascular and nervous plexuses ramify (layer of Weil) in this layer. Deeper is a cell-rich layer (layer of Hohl) contiguous to the pulpal stroma. The cell-rich layer contains undifferentiated mesenchymatous cells, which are the progenitor cells of the odontoblasts. In kittens, the root apex formation is not complete, and the apex is open. 2' 12 As cats age, closure of the apex occurs through the activity of the root sheath, and pulpal volume decreases owing to secretion of dentin by the odontoblasts. 10' 29 The latter process continues throughout life. In maturing teeth, closure of the apex by dentinogenesis and cementogenesis results in the formation of an apical delta (Fig. 4). In immature premolars with an open apex, more than 20 arterioles enter the root canal through the apical foramina and generally advance through the central portion toward the coronal pulp. 19 Main venules drain along the sides of the root canal. Secondary arterioles run along the wall of the pulp canal and advance toward the coronal pulp, where they
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Figure 4. Apical delta in a lower canine of a 6-year-old cat. Numerous apical ramifications radiate from the tip of the root canal located at about 2 mm from the root surface. The scale is given by the periodontal probe graduated 1·2·3· 5·7·8·9·10 mm.
form the pulp vascular network. This is composed of a terminal capillary network (TCN) located in the odontoblastic layer and composed of true capillaries, a second capillary network (CN) composed of precapillaries and postcapillaries, and a lattice-like venular network. On the floor of the pulpal chamber, only the TCN is evident. In mature premolar teeth with a closed apex, the blood vessels passing through the apical foramina of the tooth are fewer in number: six to eight main arterioles and main venules in each root. 19 Only the TCN with an altered, coarsened flat network is apparent, and the capillaries drain directly into the main venules. 19 The size of the venules decreases as they pass through the apical foramina. 2 Lateral canals, connecting the pulpal and periradicular tissues in areas other than the apex, have been occasionally described in eat's teeth. 2• 12 PERIODONTAL TISSUE
The periodontal tissue constitutes the attachment tissue of the tooth. It is composed of gingiva, cementum, periodontal ligament, and alveolar bone. The gingiva covers the alveolar processes of the jaw bones and surrounds the tooth itself. The gingival margin has often a knife edge shape and is firm and pink or pigmented. 6 The gingival epithelium serves as a physiologic permeability barrier, which protects gingival tissue from the invasion of microorganisms and many other foreign substances. The gingival epithelium is composed of stratified squamous cells and is divided into keratinized epithelium oral epithelium of the attached and free gingiva) and nonkeratinized epithelium cervicular and junctional epithelium).27 The gingiva is demarcated from the =.h ·eolar mucosa by the mucogingival junction, recognized as a distinct furrow. The gingiva is widest (coronally-apically) over the canine teeth (up to 1 em) "nd narrowest over the premolar teeth. The epithelium of the lip and buccal :nucosa are the thinnest of the oral regions. ' 8 In cats with healthy gingiva, the :ingiva contacts the enamel surface of the tooth from its most coronal part to ::-:e cementoenamel junction (CEJ), constituting a broad junctional epithelium. Aith plaque accumulation an d subsequent inflammation, a sulcus is created; ~: the bottom of the sulcus (0 to 1 mm deep ), the cells are attached to the
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enamel surface and constitute the junctional epithelium (JE). In kittens, it is composed of about 10 layers of squamous cells oriented parallel to the enamel surface. The cells are loosely packed, with wide intercellular spaces. 27 The JE is attached to the enamel surface by hemidesmosomes. 27' 31 Cell to cell attachment in the JE is performed by desmosomesY The JE is located at the level of the CEJ; the gingiva coronal to the CEJ is called free gingiva. The sulcus is covered by a nonkeratinized epithelium, the crevicular or sulcular epithelium. It differs, however, from other nonkeratinized oral epithelium by wide intercellular spaces and the presence within it of large numbers of leukocytes, mainly neutrophils. 8 These features account for the selective permeability of the crevicular epithelium. 8 - 9 Compared with the connective tissue underlying the oral gingival epithelium, that underlying the sulcular epithelium presents fewer collagen fibrils, many degenerated neutrophils, and many vesiculated fibrocytes (which play a role in connective tissue remodeling) immediately beneath the epithelium basal lamina. 15 Even in clinically healthy gingival tissues, inflammatory changes occur in the connective tissue underlying the crevicular epithelium. 15 These changes as well as the particular features of the crevicular epithelium result from the action of bacteria or their by-products. 9 Clinically healthy gingiva, (gingival Index [GI] = 0; plaque index [PI] = 0; little gingival exudate) can be obtained after 2 weeks of toothbrushing in dogs, cats, and other species. 13 Clinically healthy gingival tissues have no or little leukocytic infiltrate and a flat epithelial-connective tissue interface at the gingival margin and beneath the crevicular epithelium. In noninflamed gingiva, the free gingiva contains a vascular network composed of vessels running parallel to the gingival margin; main afferent and efferent vessels lay at 90 degrees to the gingival margin, and efferent vessels drain into vessels in the attached gingiva. Animals that do not have their teeth thoroughly brushed daily present with plaque-induced inflammation of the gingiva. A slight inflammation of the free gingiva is a normal clinical feature in animals with healthy periodontal tissue in the absence of dental home care. With plaque-induced inflammation of the free gingiva, the network is lost and vessel loops are seen; main afferent vessels are dilated and efferent vessels constricted. The vascular changes precede epithelial and clinical changes. Leukocytic infiltrate spreads throughout the free gingiva; rete pegs form at the gingival margin and the crevicular epithelia, and gingival fluid exudation, gingival index, and plaque index increase. 13 Cat gingival blood flow is controlled by sympathic alpha-adrenergic fibers for vasoconstriction and by sensory fibers and mast cells for vasodilation. 16 Fibers of the periodontal ligament run obliquely, their attachment to the tooth being located more apical than their attachment to the bone. Compared with other carnivora, the periodontal space is very narrow. Besides collagen fibers, the periodontal space contains blood and lymphatic vessels, nerves, and cells. The innervation of the periodontal ligament comes from fibers running from the apical region and fibers entering laterally from the alveolar plates; the latter divides in fasciculi running toward the apex and others running toward the gingival margin. These fibers transmit tactile, pressure, and painful stimuli. 20- 21 The arterial blood supply to the periodontal ligament comes almost entirely from arteries of the alveolar bone, but venules leaving the tooth enter directly into the periodontal circulation. 2 Cells commonly found in the periodontal ligament are fibroblasts, osteoblasts, cementoblasts, osteoclasts, cementoclasts, rest cells of Malassez, and undifferentiated mesenchymal cells (progenitor cells). The cellular content of the periodontal ligament has not been thoroughly investigated in cats. The cementum is an avascular bone-like tissue covering the roots. It is less
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calcified than enamel and dentin and is denser than bone. Cementum deposition is continuous throughout life. At the gingival margin, it consists of acellular (fibrillar) cementum; more apically cells appear, becoming more numerous toward the apex. Only at the innermost stratum of the apical part of the root does the cementum resemble the tissue that is usually denoted as cellular cementum. Acellular cementum is particularly thick in cats. The cementum is, in cats, very thin in the furcation areas.' Cementum is an important structure involved in both resorptive and reparative processes. The alveolar bone constitutes ridges of the jaw bones. Deep depressions, the alveolar sockets, contain the roots of the teeth. The alveolar bone appears with tooth eruption and disappears with tooth loss. It consists of the three layers of regular bone, i.e., periosteum, dense compact bone, '!nd cancellous bone, plus a fourth layer, the cribriform plate covering the alveolar sockets. Radiographically this fourth layer appears as a radiopaque line called the lamina
dura.
MOUTH
In the restricted sense, the mouth or oral fissure should include only the opening between the upper and lower lips. The oral cavity, however, extends from the lips to the common pharynx. It is less variable in shape compared with the dog, being narrow rostrally, widening caudally to the last teeth, and then narrowing to form the fauces. The oral cavity is divided into the vestibule and oral cavity proper. The vestibule is the space between the teeth, with their associated gingiva, and the facial soft tissue, i.e., lips and cheeks. Owing to its relationship with the upper and lower dental arcades, it is U-shaped. The vestibule opens to the external environment by the mouth. When the jaws are closed, the vestibule communicates with the oral cavity proper through the interdental spaces, i.e., those spaces between successive teeth. The largest of these spaces is the diastema, which is the space between the canine and premolar teeth. The lips are thick folds of skin that overlie the wide orbicularis oris muscle. They are covered by hair externally and by mucous membrane internally. The philtrum is a deep external vertical groove on the midline of the upper lip. Located on either side of the philtrum are 10 to 12 small papillae arranged in two to three rows, projecting from the mucocutaneous margin. The upper and lower lips join each other caudally at the level of the fourth upper premolar, forming the commissure of the lips, thereby uniting with the cheeks. The vestibule is spacious. Inside the lips on the median line are membranous folds or frenula, which attach the lips to the jaws. Associated with the upper lip at the level of the upper third premolar is a less discrete fold that restricts mobility of the lip. There is also a frenulum in the area of the diastema of the lower lip. There is a 5-mm flap of tissue at the lower lip margin in this area. The cheeks form the caudal aspect of the lateral walls of the vestibule. They are small and thin, extending from the lips to the rami of the mandibles. They are lined internally with folded mucous membrane. This part of the vestibule is often called the buccal cavity. The majority of the muscles of mastication are adductors of the jaw, as more force is required for this action. The masseter originates from the zygomatic arch and inserts on the lateral surface of the mandibular ramus. It is covered superficially by a strong layer of fascia. This muscle has superficial and deep
parts.''
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The temporalis is the 'largest and strongest muscle of mastication. It is covered by the temporal fascia, which is a thick layer of fascia originating from the temporal line of the skull. The temporalis originates from both the temporal fascia and the temporal fossa. Both parts insert on the coronoid process of the mandible. 33 The lateral and medial pterygoid muscles lie medial to the two previously described muscles. They originate from the pterygopalatine fossa and insert on the medial aspect of the ramus of the mandible and its condyloid process. The only abductor in the group of masticatory muscles is the digastricus. This muscle originates from the jugular and mastoid processes of the skull, inserting on the ventral aspect of the mandibular body. It has a tendinous intersection that divides the muscle into a rostral and caudal body. 33 Except for the caudal belly of the digastricus, all of the aforementioned muscles are innervated by the motor component of the mandibular division of the trigeminal nerve. The caudal belly of the digastricus is innervated by the facial nerve. The parotid gland is triangular in shape and lies ventral to the ear with its apex directed ventrally. The parotid duct courses across the lateral aspect of the masseter muscle, traverses the cheek, and opens into the caudal vestibule on the small parotid papilla, which is located adjacent to the upper fourth premolar. The zygomatic gland is located medial to the zygomatic arch in the ventral orbital region. The zygomatic duct opens into the vestibule just distal to the first upper molar tooth. The roof of the oral cavity proper is formed by the hard palate rostrally and the soft palate caudally, thus separating the oral and nasal cavities. The hard palate is formed by the palatine processes of the incisive and maxillary bones and the horizontal laminae of the palatine bones. It is relatively flat, especially when compared with the extreme concavity seen in the human hard palate. The mucosa covering the hard palate consists of cornified stratified squamous epithelium, which is developed into seven to eight transverse curved ridges (palatine rugae). It has a thick tough connective tissue support, the mucoperiosteum, which is continuous with the periodontal ligaments of the upper teeth. 35 The mucosa of the hard palate ends abruptly laterally at its junction with the short palatal free gingiva. The incisive papilla is located on the midline cranial to the first transverse ridge and just caudal to the upper central incisor teeth. On either side of this papilla are the incisive ducts that extend caudadorsally through the palatine fissures into the floor of the nasal fossae. This duct communicates with the vomeronasal organ. The major palatine arteries, branches of the maxillary arteries, supply the hard palate. They exit the palatine foraminae of the hard palate at the level of and medial to the distal cusp of the fourth upper premolar. The soft palate continues the separation of the oral cavity from the nasal cavity and nasal pharynx. It is the caudal extension of the hard palate and has no supporting bone. The junction of the hard and soft palate is at the level of the first upper molar. The main blood supply to the soft palate is by means of the minor palatine arteries, branches of the maxillary arteries. The soft palate ends caudally at the common pharynx and continues caudolaterally as the palatopharyngeal arches. The soft palate, the tongue, and the mucosal folds between them (palatoglossal folds) form the borders of the isthmus of the fauces, which is defined as the passage between the oral cavity and the oral pharynx. The prominent bony hamular processes of the pterygoid bones can be palpated deep to the soft palate approximately 1 em caudal to the attachment
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of the palatoglossal arches. The palatine tonsils lie in shallow fossae located approximately 1.5 em caudal to the palatoglossal folds. The floor of the oral cavity is bordered laterally by the teeth and their associated gingiva, which overlies the alveolar bone of the mandible. The majority of the floor consists of the tongue with its laterally reflected mucosa. The tongue is attached to the hyoid apparatus caudally. On the ventral aspect of the tongue, the lingual mucosa forms a median fold, the lingual frenulum, which connects the tongue to the floor of the oral cavity. Just medial to the lower first molar on the floor of the oral cavity is a mass-like flap of oral mucosa with no known function (Fig. 5); its prominent appearance leads to an incorrect identification as an abnormal finding. The mandibular salivary gland is located caudal to the ramus of the mandible and ventral to the parotid gland. Its duct courses craniomedially to become situated in the floor of the oral cavity. It lies within a fold of oral mucosa, the sublingual fold, and opens at the apex of the sublingual caruncle or papilla, which is located just lateral to the rostral aspect of the lingual frenulum. The sublingual salivary gland lies in close opposition to the rostral aspect of the mandibular gland. Its duct courses with the mandibular duct, opening on the medial side of the sublingual papilla. The mandibular lymph nodes are located rostral to the mandibular salivary gland. They are not lobulated like the salivary gland and therefore can be distinguished from them. They, along with the parotid lymph nodes, drain the entire head. They are more superficially located than the parotid lymph nodes and therefore are more easily palpated. When the mouth is closed, the tongue fills the oral cavity. The tongue is a muscular structure consisting of a body and root. The intrinsic muscles run in three directions. They are referred to as longitudinal, transverse, and vertical muscles. They allow for the versatility of the tongue' s movement. The tongue is covered by lingual mucosa, which is heavily cornified stratified squamous epithelium. The root of the tongue is attached to the basihyoid bone and acts as the point of attachment of the majority of its extrinsic muscles. The dorsal aspect of the lingual mucosa is specialized in having five types of papillae. They are filiform, fungiform, foliate, vallate and conical. For further information on these papillae, the reader should refer to an anatomy or histology text.
Figure 5. Mucosal bulge on the lingual (medial) aspect of the lower first molar tooth of a cat.
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The oral mucosa can be pigmented, with the most common area of pigmentation being the buccal mucosa. The gingiva and mucosa of the hard palate are also frequently pigmented. Although less common, the soft palate and mucosa of the floor of the mouth can show pigmentation. Pigmentation of the tongue is uncommon. Oral pigmentation usually occurs in black, gray, and tiger cats. 6 As is true of other species, the head of the cat has an excellent blood supply, with the oral mucosa being especially well endowed. 18 The major blood supply to the upper jaw, including its dental arcade, is through the maxillary artery and its major continuation, the infraorbital artery, which becomes superficial as it emerges from the infraorbital canal. The lower jaw and its dental arcade are supplied by the mandibular alveolar artery, a branch of the maxillary artery, which courses through the mandibular canal. Terminal branches supply the mandibular incisor teeth as well as exiting through the mental foramina to supply the lower lip. These foramina usually number two on each side of the lower jaw and are located ventral to the diastema and third premolar. The lingual artery, a branch of the external carotid artery, supplies the tongue and oral mucosa of the floor of the oral cavity. Sensory innervation of the oral cavity excluding the tongue is by means of branches of the trigeminal nerve. The mandibular alveolar nerve, a branch of the mandibular division of the trigeminal nerve, courses through the mandibular canal. It supplies sensory innervation of the lower teeth. After exiting the mental foramina as the mental nerves, it innervates the lower lip. 33 The maxillary nerve, also a branch of the trigeminal nerve, sends palatine branches for sensory innervation of the soft and hard palate. The maxillary nerve and its continuation as the infraorbital nerve releases many alveolar nerves to the teeth of the upper jaw. As the nerve exits the infraorbital canal, it branches to supply sensory innervation to the soft tissue of the muzzle. References 1. Berman E: The time and pattern of eruption of the permanent teeth of the cat. Lab Anim Sci 24(6):929-931, 1974 2. Boling LR: Blood vessels of the dental pulp. Anat Rec 82:25-37, 1942 3. Brashear AD: The innervation of the teeth. J Comp Neur 64:164-183, 1936 4. Buckland-Wright: Structure and function of cat skull bones in relation to the transmission of biting forces. PhD thesis, University of London, 1975 5. Byers MR, Matthews B: Autoradiographic demonstration of ipsilateral and contralateral sensory nerve endings in cat dentin, pulp, and periodontium. Anat Rec 201(2):249-260, 1981 6. Dummett CO, Barens G: Feline oral pigmentation. J Periodont 41(12):696-701, 1970 7. Forsberg A: The periodontal tissue of mandibular premolars and molars in some mammals. Svensk Tandlarkare-Tidskrift 62(suppl1):10, 1969 8. Gavin JB: The ultrastructure of the crevicular epithelium of cat gingiva. Am J Anat 123:283-296, 1968 9. Gavin JB: Effects of systemic antibiotic therapy on the fine structure of gingival crevicular epithelium. N Z Dent J 69:85-92, 1973 10. Grant D, Bernick S: Morphodifferentiation and structure of Hertwig's root sheath in the cat. J Dent Res 50(6):1580-1588, 1971 11. Hayashi K, Hideo K: Microhardness of enamel and dentin of cat premolar teeth. Jpn J Vet Sci 51(5):1033-1035, 1989 12. Hennet PR, Harvey CE: Root canal anatomy in the cat. In Proceedings of the Veterinary Dental Forum, New Orleans, 1991 13. Hock J, Nuki K: A vital microscopy study of the morphology of normal and inflamed gingiva. J Periodont Res 6:81-88, 1971
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14. Holland GR: The dentinal tubule and odontoblast process in the cat. J Anat 120(1):169177, 1975 15. INnes PB: The ultrastructure of the connective tissue underlying the sulcular epithelium. N Z Dent J69:185-193, 1973 16. Izumi H, Kuriwada S, Karita K, et a!: The nervous control of gingival flow in cats. Microvasc Res 39(1):94-104, 1990 17. Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898 18. Johnson GK, Squier CA, Johnson WT, et a!: Blood flow and epithelial thickness in different regions of feline oral mucosa and skin. J Oral Pathol 16:317-321, 1987 19. Kishi Y, Shimozato N, Takahashi K: Vascular architecture of cat pulp using corrosive resin cast under scanning electron microscope. J Endod 15(10):478-483, 1989 20. Lewinsky W, Stewart D: The innervation of the periodontal membrane of the cat, with some observations on the function of the end-organs found in that structure. J Anat 71:232-235, 1937 21. Loescher AR, Holland GR: Distribution and morphological characteristics of axons in the periodontal ligament of cat canine teeth and the changes observed after reinnervation. Anat Rec 230(1):57-72, 1991 22. McClure RC, Pallman MS, Garrett PG: Cat Anatomy: An Atlas, Text and Dissection Guide. Philadelphia, Lea & Febiger, 1973 23. Miles AEW, Grigson C: Colyer's variations and disease of the teeth of animals. Cambridge, Cambridge University Press, 1990 24. Nalbandian J, Frank RM: Electron microscopic study of the regeneration of cementum and periodontal connective tissue attachment in the cat. J Periodont Res 15:71-89, 1980 25. Okuda A, Harvey CE: Odontoclastic resorptive lesions in cats-microscopic findings. Vet Clin North Am Small Anim Pract 22:1385, 1992 26. Peyer B: Comparative Odontology. Chicago, University of Chicago Press, 1968 27. Sasaki T, Nakagawa T, Tominaga H, et a!: Electron microscopy of the junctional epithelium of kitten gingiva. Bull Tokyo Dent Coli 22(2):139-149, 1981 28. Shapiro HH: Growth and time corrections between ossification centers in the long bones and calcification centers in the mandibular dentition of the cat. Int J Orthod 16:690-702, 1930 29. Silva DG, Kailis DG: Ultrastructural studies on the cervical loop and the development of the amelo-dentinal junction in the cat. Arch Oral Bioi 17:279-289, 1972 30. Skobe Z, Prostak KS, Trombly PL: Scanning electron microscope study of cat and dog enamel structure. J Morpho! 184:195-203, 1985 31. Thilander H, Hugoson A: The border zone tooth-enamel and epithelium after periodontal treatment. An experimental electron microscope study in the cat. Acta Odont Scand 28:147-155, 1970 32. Thomas BOA: An analysis of the inferior alveolar and mental nerves in the cat. J Comp Neuro 84(1):419-436, 1938 33. Turnbull WD: Mammalian masticatory apparatus. Fieldiana: Geology 18(2):183-193, 1970 34. Vongsavan N, Matthews B: The permeability of cat dentine in vivo and in vitro. Arch Oral Bioi 36(9):641-646, 1991 35. Wijdeveld MG, Maltha JC, Grupping EM, et a!: A histological study of tissue response to simulated cleft palate surgery at different ages in Beagle dogs. Arch Oral Bioi 36(11):837-843, 1991 36. Windle WF: Experimental proof of the types of neurons that innervate the tooth pulp. J Comp Neuro 843:347-356, 1927
Address reprint requests to Paul Orsini, DVM University of Pennsylvania School of Veterinary Medicine Department of Clinical Studies-Philadelphia 3850 Spruce Street Philadelphia, PA 19104
FELINE DENTISTRY
ANATOMY OF THE MOUTH AND TEETH OF THE CAT Paul Orsini, DVM, and Philippe Hennet, DV
Cats belong to the carnivore order and to the Felidae family. Their dentition is diphyodont, i.e., succeeding generations of deciduous teeth and permanent teeth. Cats are truly carnivorous, which is reflected in their dentition. TOOTH FORMULAS
Four types of teeth are recognized, depending on general shape and function. They are incisors (I), canines (C), premolars (P), and molars (M). The dental formula summarizes the number of each type of tooth in the upper jaw (above the line) and in the lower jaw (below the line). 26 For the cat: C 1, pp 3 or simp . l.f. II 3, C I Ie d 33 11 32 = 26 teeth 1 2 3 ' ' .. I 3, C 1, P 3, M 1 . .f'. d 3 1 2 1 h P ermanent d enhhon: I 3 C 1 p 2 M 1 or simp 1I Ie 3 1 2 1 = 30 teet ' ' ' Primitive carnivores have the general dental formula of 3143/3143 (44 teeth total). In modern carnivores, the last premolar in the upper jaw and the first molar in the lower jaw have become carnassials (dente lacerantes). 26 In a species such as the cat that has fewer total teeth than the primitive mammal, which teeth are present is determined by identifying the carnassial teeth, then counting forward and backward from the carnassial teeth. In the cat, the first premolar appearing in the upper jaw is the second premolar of the primitive dentition, and the first premolar appearing in the lower jaw is the third premolar of the typical dentition. In the following discussion, teeth are numbered according to the primitive dentition (Fig. 1). .. DeCI.duous d enhhon:
Froin the Department of Clinical Studies-Philadelphia, University of Pennsylvania School of Veterinary Medicine, Philadelphia, Pennsylvania
VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 6 • NOVEMBER 1992
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Figure 1. Anatomic nomenclature of teeth in the cat. (numbers between brackets = modified Triadan system for the cat) (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
In this article, teeth are identified by a letter for the type of tooth (I, C, P, M) and a number for position in the arcade, superscript for upper jaw and subscript for lower jaw. Teeth of the deciduous dentition can be designated by lower case letters e.g., p 3 for the deciduous upper third premolar. PERMANENT DENTITION
In comparison to dog's teeth, incisors in cats are much smaller and narrower; as in dogs, their size increases from the first (central) incisor to the third (lateral) incisor. All incisors have a single root; lower incisors are smaller than upper incisors. The crown of all incisors other than the upper lateral incisor is chisel-shaped. The cutting edge of these teeth present three small cusps; the middle cusp is the largest. The lateral upper incisor is the larger of the series; its crown is more conical, comprising the central cusp, with a mere trace of an inner cusp. The caudal (palatal) surface of upper incisors is divided by a transverse groove into a higher rostral part and a lower caudal part or cingulum. The upper canine is a strong, pointed tooth; the root is longer than the
ANATOMY OF THE MOUTH AND TEETH OF THE CAT
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crown, and the neck is not well marked. The crown is slightly curved palatally and distally. The labial surface of the crown is marked by vertical grooves, one being more prominent. The crown of the lower canine is curved distally; its labial surface is ridged in its distal part. P2 is a small single-rooted tooth with a conical crown; however, one study reported that 38% have two roots, which most of the time are fused together.' 7 The P3 and the P, and P4 have a similar crown, conical in shape but strongly compressed from side to side. The crown is limited at its base by a well-marked ridge of enamel (enamel bulge). The central triangular elevation of the crown is known as the principal cusp; the distal border is interrupted by a transverse notch, which produces a second smaller cusp, the distal basal cusp; behind this cusp is a second transverse incision, beyond which the enamel bulge is elevated into a small cusp known as the talon, or heel. The mesial border is more vertical and is notched at its lower end to produce a small mesial basal cusp, which sometimes is merely an elevation of the enamel as in P2 • This mesial cusp is less prominent or may be absent in P,. These teeth have two roots, slightly greater than the height of the crown, of which the mesial is usually smaller. The distal root of P3 is much thicker than the mesial. The carnassial teeth are secondont, i.e., ~hey have sharp cutting edges that are parallel to the edge of the jaw. During jaw movement, the two cutting edges glide along one another after the fashion of scissors (Figs. 2 and 3). The upper carnassial p• is twice as large as P3 • The broader mesial part has two transversely placed roots; the greater part, however, is supported by a single large distal root. The cutting edge is composed of a principal cusp and a greatly enlarged distal basal cusp separated from the principal cusp by a narrow vertical cleft. The distal cusp has a sharp horizontal cutting edge. The broad mesial part of the crown is occupied by two small conical cusps. The distal triangular root is strongly flattened from side to side and carries the principal and distal cusps. The furcation between the mesial and distal roots is located above the tip of the principal cusp. The cutting blade of the crown of the lower carnassial is formed of two nearly equal cusps separated on the buccal surface by a vertical fissure and on the lingual surface by a deep pit. The mesial and distal edges of the blade are almost vertical. The greater part of the crown is supported by the large mesial root. The very small distal root is often angulated caudally (distally). The furcation is located at the level of a midpoint between the tip of the distal cusp and the vertical fissure. The upper molar is a very small, two-rooted tooth aligned transverse to the alveolar border, close behind and in contact with the distal end of the carnassial tooth. Very often this tooth is rudimentary. In felids, the portion of the dentition behind the carnassials has degenerated. DECIDUOUS DENTITION
The deciduous dentition in cats is composed of 26 teeth. Compared with the permanent teeth, deciduous incisors are smaller; upper canines are more slender and strongly curved; lower canines are more vertical and resemble upper deciduous canines; upper first deciduous molars are very small and resemble permanent teeth; upper second deciduous molars are smaller and sectorial; upper third molars are more complicated and resemble the permanent molar; lower first deciduous molars resemble the first permanent premolars that replace them-they are smaller and their cusp is more acute; and finally, the lower second deciduous molars resemble the permanent lower carnassial teeth and are different from the premolars that replace them. 17' 25
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Figure 2. Scissor occlusion of carnassial teeth in the cat-lingual view (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
AGE OF ERUPTION OF THE TEETH Deciduous teeth
Seven days after birth, no teeth have emerged. Between 11 and 15 days, the incisors erupt; at 17 to 19 days, the canines erupt; between 24 and 30 days, all molars except the upper first erupt. The upper first molars erupt later, between 37 and 60 days. At 60 days, the deciduous dentition is complete.17• 22• 28 Permanent teeth
See Table 1 for a breakdown of the age of eruption of the permanent teeth. TOPOGRAPHIC TERMINOLOGY OF INDIVIDUAL TEETH
Terms are described as follows:
Labial or Buccal. Surface of the tooth that faces the lip or cheek, vestibular is often used as a synonym of buccal. Lingual. Surface of the tooth that faces the tongue. Palatal. Surface of the tooth that faces the palate.
Figure 3. Diagram of the occlusal relationship of premolar and molar teeth in the cat (Adapted from Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898; with permission.)
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Table 1. AGE OF ERUPTION OF PERMANENT TEETH IN THE CAT
Upper Incisors Central Middle Lateral Canine Premolar Second Third Fourth Molar
Lower
103 114 135 153
days days days days
113 119 132 149
days days days days
150 168 151 162
days days days days
173 days 174 days 130 days
Data derived from references 1, 22, and 27.
Occlusal. Surface of the tooth that faces the crown of the opposite tooth in the opposite arch. Mesial. Surface of the tooth that faces toward the midline of the dental arch. Distal. Surface of the tooth that faces away from the midline of the dental arch. DENTAL OCCLUSION IN CATS
When the jaw is closed, the lower incisors normally strike immediately caudal to the upper incisors. The lower canine occludes between the lateral upper incisor and the upper canine. The incisors and especially the canine are used for grasping and biting food, cutting being done by caudal teeth, especially by carnassials, which are placed close to the insertions of the powerful muscles of mastication. The upper jaw being wider than the lower, the lower cheek teeth (premolar and molar) are nearer the sagittal plane of the skull than are the upper teeth. The upper and lower teeth do not touch when the jaws are moved in an absolutely vertical line. When the cat chews on one side of the mouth, the lower jaw must be brought to that side, so the buccal surface of lower teeth may shear upward and forward against the palatal surface of the upper teeth. 17 Biting forces of 20 to 23.25 kg at the canine and up to 28 kg at the carnassial · teeth have been reported in cats.• TOOTH STRUCTURE
The tooth is composed of a crown and of one, two, or three roots, which normally are not visible because they are embedded in the jaw bone. The neck of the tooth is the limit between crown and roots; it is located just below the enamel bulge at the gingival margin. Two calcified tissues form the tooth itself; the dentin composes most of the volume of the crown and the root(s), whereas the enamel is the outer layer of the crown covering the dentin. The enamel is the hardest, most mineralized tissue in the body. Microhardness values of enamel in cat premolars range from 273 to 337 VHN (Vickers microhardness value). The microhardness decreases significantly from the upper part of the crown to the cervical portion and from the outer to the inner
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layer. 11 Cat enamel is less hard than that of dogs and humans. 11 Enamel in dogs and cats is composed of three layers; a surface rodless (aprismatic) layer, an outer layer of parallel rods (only at some sites), and an inner layer. 30 The cross-sectional shape of inner enamel and outer layer enamel rods differs in dogs and in cats. Owing to the rodless surface layer, enamel surface was found to be smoother in dogs and cats than in humans. Enamel structure of the cat is most similar to that described in humans. 30 Enamel is secreted during development of the tooth by ameloblasts; ameloblasts disappear when enamel has been fully formed; consequently after that period, repair will not occur if enamel substance is lost. Dentin is less calcified than enamel; it is secreted by odontoblasts, which are located on the dentinal surface in the pulpal tissue. Contrary to enamel, dentin is continuously secreted by odontoblasts, resulting in a progressive thickening of the dentinal wall and a shrinking of the pulpal tissue. Dentin can also respond to irritation by secreting reparative dentin. Dentin has a porous structure; numerous dentinal tubuli run from the pulpal tissue to the amelodentinal or cementodentinal junction. These tubuli contain cytoplasmic processes of the odontoblasts. 14 When dez:ttin is exposed, an outward flow of fluid through the tubules has been observed in cats. This flow is sufficiently rapid to limit severely the rate of inward diffusion of chemicals. 34 Vasodentin tissue, consisting of small spaces containing vascular channels, has been described in dentin of cat teeth; dentinal tubules in the area of the vasodentin tissue are disrupted and irregular. 24 Pulp is the vital, nutrient tissue of the tooth. The pulp has four main functions: (1) formation of the dentin through the activity of odontoblasts; (2) nutrition of the dentin through the dentinal tubules, which contain the odontoblastic processes/ 4 (3) innervation of the dentin through the odontoblastic processes and the sensory nerves of the pulp, which are responsible for dentinal sensitivity; and (4) protection of the pulp through secretion of reparative dentin. The pulp is a highly specialized connective tissue, with a rather homogeneous composition; it contains cells (fibroblasts, histiocytes, leukocytes, and odontoblasts), collagen fibers, ground substance, blood and lymphatic vessels, and nerves. 3, 36 Sensory nerves originating from the trigeminal ganglion innervate the teeth. The axons end in the odontoblastic layer and predentin of roots and crown; at the tip of the pulp horn of each cusp, nerve endings extend deep into dentinal tubuli. 5 Odontoblasts lie at the surface of pulpal tissue against the predentin, which they secrete and subsequently mineralize. Beneath the odontoblastic layer is a layer composed of fibers and few cells; vascular and nervous plexuses ramify (layer of Weil) in this layer. Deeper is a cell-rich layer (layer of Hohl) contiguous to the pulpal stroma. The cell-rich layer contains undifferentiated mesenchymatous cells, which are the progenitor cells of the odontoblasts. In kittens, the root apex formation is not complete, and the apex is open. 2' 12 As cats age, closure of the apex occurs through the activity of the root sheath, and pulpal volume decreases owing to secretion of dentin by the odontoblasts. 10' 29 The latter process continues throughout life. In maturing teeth, closure of the apex by dentinogenesis and cementogenesis results in the formation of an apical delta (Fig. 4). In immature premolars with an open apex, more than 20 arterioles enter the root canal through the apical foramina and generally advance through the central portion toward the coronal pulp. 19 Main venules drain along the sides of the root canal. Secondary arterioles run along the wall of the pulp canal and advance toward the coronal pulp, where they
ANATOMY OF THE MOUTH AND TEETH OF THE CAT
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Figure 4. Apical delta in a lower canine of a 6-year-old cat. Numerous apical ramifications radiate from the tip of the root canal located at about 2 mm from the root surface. The scale is given by the periodontal probe graduated 1·2·3· 5·7·8·9·10 mm.
form the pulp vascular network. This is composed of a terminal capillary network (TCN) located in the odontoblastic layer and composed of true capillaries, a second capillary network (CN) composed of precapillaries and postcapillaries, and a lattice-like venular network. On the floor of the pulpal chamber, only the TCN is evident. In mature premolar teeth with a closed apex, the blood vessels passing through the apical foramina of the tooth are fewer in number: six to eight main arterioles and main venules in each root. 19 Only the TCN with an altered, coarsened flat network is apparent, and the capillaries drain directly into the main venules. 19 The size of the venules decreases as they pass through the apical foramina. 2 Lateral canals, connecting the pulpal and periradicular tissues in areas other than the apex, have been occasionally described in eat's teeth. 2• 12 PERIODONTAL TISSUE
The periodontal tissue constitutes the attachment tissue of the tooth. It is composed of gingiva, cementum, periodontal ligament, and alveolar bone. The gingiva covers the alveolar processes of the jaw bones and surrounds the tooth itself. The gingival margin has often a knife edge shape and is firm and pink or pigmented. 6 The gingival epithelium serves as a physiologic permeability barrier, which protects gingival tissue from the invasion of microorganisms and many other foreign substances. The gingival epithelium is composed of stratified squamous cells and is divided into keratinized epithelium oral epithelium of the attached and free gingiva) and nonkeratinized epithelium cervicular and junctional epithelium).27 The gingiva is demarcated from the =.h ·eolar mucosa by the mucogingival junction, recognized as a distinct furrow. The gingiva is widest (coronally-apically) over the canine teeth (up to 1 em) "nd narrowest over the premolar teeth. The epithelium of the lip and buccal :nucosa are the thinnest of the oral regions. ' 8 In cats with healthy gingiva, the :ingiva contacts the enamel surface of the tooth from its most coronal part to ::-:e cementoenamel junction (CEJ), constituting a broad junctional epithelium. Aith plaque accumulation an d subsequent inflammation, a sulcus is created; ~: the bottom of the sulcus (0 to 1 mm deep ), the cells are attached to the
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enamel surface and constitute the junctional epithelium (JE). In kittens, it is composed of about 10 layers of squamous cells oriented parallel to the enamel surface. The cells are loosely packed, with wide intercellular spaces. 27 The JE is attached to the enamel surface by hemidesmosomes. 27' 31 Cell to cell attachment in the JE is performed by desmosomesY The JE is located at the level of the CEJ; the gingiva coronal to the CEJ is called free gingiva. The sulcus is covered by a nonkeratinized epithelium, the crevicular or sulcular epithelium. It differs, however, from other nonkeratinized oral epithelium by wide intercellular spaces and the presence within it of large numbers of leukocytes, mainly neutrophils. 8 These features account for the selective permeability of the crevicular epithelium. 8 - 9 Compared with the connective tissue underlying the oral gingival epithelium, that underlying the sulcular epithelium presents fewer collagen fibrils, many degenerated neutrophils, and many vesiculated fibrocytes (which play a role in connective tissue remodeling) immediately beneath the epithelium basal lamina. 15 Even in clinically healthy gingival tissues, inflammatory changes occur in the connective tissue underlying the crevicular epithelium. 15 These changes as well as the particular features of the crevicular epithelium result from the action of bacteria or their by-products. 9 Clinically healthy gingiva, (gingival Index [GI] = 0; plaque index [PI] = 0; little gingival exudate) can be obtained after 2 weeks of toothbrushing in dogs, cats, and other species. 13 Clinically healthy gingival tissues have no or little leukocytic infiltrate and a flat epithelial-connective tissue interface at the gingival margin and beneath the crevicular epithelium. In noninflamed gingiva, the free gingiva contains a vascular network composed of vessels running parallel to the gingival margin; main afferent and efferent vessels lay at 90 degrees to the gingival margin, and efferent vessels drain into vessels in the attached gingiva. Animals that do not have their teeth thoroughly brushed daily present with plaque-induced inflammation of the gingiva. A slight inflammation of the free gingiva is a normal clinical feature in animals with healthy periodontal tissue in the absence of dental home care. With plaque-induced inflammation of the free gingiva, the network is lost and vessel loops are seen; main afferent vessels are dilated and efferent vessels constricted. The vascular changes precede epithelial and clinical changes. Leukocytic infiltrate spreads throughout the free gingiva; rete pegs form at the gingival margin and the crevicular epithelia, and gingival fluid exudation, gingival index, and plaque index increase. 13 Cat gingival blood flow is controlled by sympathic alpha-adrenergic fibers for vasoconstriction and by sensory fibers and mast cells for vasodilation. 16 Fibers of the periodontal ligament run obliquely, their attachment to the tooth being located more apical than their attachment to the bone. Compared with other carnivora, the periodontal space is very narrow. Besides collagen fibers, the periodontal space contains blood and lymphatic vessels, nerves, and cells. The innervation of the periodontal ligament comes from fibers running from the apical region and fibers entering laterally from the alveolar plates; the latter divides in fasciculi running toward the apex and others running toward the gingival margin. These fibers transmit tactile, pressure, and painful stimuli. 20- 21 The arterial blood supply to the periodontal ligament comes almost entirely from arteries of the alveolar bone, but venules leaving the tooth enter directly into the periodontal circulation. 2 Cells commonly found in the periodontal ligament are fibroblasts, osteoblasts, cementoblasts, osteoclasts, cementoclasts, rest cells of Malassez, and undifferentiated mesenchymal cells (progenitor cells). The cellular content of the periodontal ligament has not been thoroughly investigated in cats. The cementum is an avascular bone-like tissue covering the roots. It is less
ANATOMY OF THE MOUTH AND TEETH OF THE CAT
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calcified than enamel and dentin and is denser than bone. Cementum deposition is continuous throughout life. At the gingival margin, it consists of acellular (fibrillar) cementum; more apically cells appear, becoming more numerous toward the apex. Only at the innermost stratum of the apical part of the root does the cementum resemble the tissue that is usually denoted as cellular cementum. Acellular cementum is particularly thick in cats. The cementum is, in cats, very thin in the furcation areas.' Cementum is an important structure involved in both resorptive and reparative processes. The alveolar bone constitutes ridges of the jaw bones. Deep depressions, the alveolar sockets, contain the roots of the teeth. The alveolar bone appears with tooth eruption and disappears with tooth loss. It consists of the three layers of regular bone, i.e., periosteum, dense compact bone, '!nd cancellous bone, plus a fourth layer, the cribriform plate covering the alveolar sockets. Radiographically this fourth layer appears as a radiopaque line called the lamina
dura.
MOUTH
In the restricted sense, the mouth or oral fissure should include only the opening between the upper and lower lips. The oral cavity, however, extends from the lips to the common pharynx. It is less variable in shape compared with the dog, being narrow rostrally, widening caudally to the last teeth, and then narrowing to form the fauces. The oral cavity is divided into the vestibule and oral cavity proper. The vestibule is the space between the teeth, with their associated gingiva, and the facial soft tissue, i.e., lips and cheeks. Owing to its relationship with the upper and lower dental arcades, it is U-shaped. The vestibule opens to the external environment by the mouth. When the jaws are closed, the vestibule communicates with the oral cavity proper through the interdental spaces, i.e., those spaces between successive teeth. The largest of these spaces is the diastema, which is the space between the canine and premolar teeth. The lips are thick folds of skin that overlie the wide orbicularis oris muscle. They are covered by hair externally and by mucous membrane internally. The philtrum is a deep external vertical groove on the midline of the upper lip. Located on either side of the philtrum are 10 to 12 small papillae arranged in two to three rows, projecting from the mucocutaneous margin. The upper and lower lips join each other caudally at the level of the fourth upper premolar, forming the commissure of the lips, thereby uniting with the cheeks. The vestibule is spacious. Inside the lips on the median line are membranous folds or frenula, which attach the lips to the jaws. Associated with the upper lip at the level of the upper third premolar is a less discrete fold that restricts mobility of the lip. There is also a frenulum in the area of the diastema of the lower lip. There is a 5-mm flap of tissue at the lower lip margin in this area. The cheeks form the caudal aspect of the lateral walls of the vestibule. They are small and thin, extending from the lips to the rami of the mandibles. They are lined internally with folded mucous membrane. This part of the vestibule is often called the buccal cavity. The majority of the muscles of mastication are adductors of the jaw, as more force is required for this action. The masseter originates from the zygomatic arch and inserts on the lateral surface of the mandibular ramus. It is covered superficially by a strong layer of fascia. This muscle has superficial and deep
parts.''
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The temporalis is the 'largest and strongest muscle of mastication. It is covered by the temporal fascia, which is a thick layer of fascia originating from the temporal line of the skull. The temporalis originates from both the temporal fascia and the temporal fossa. Both parts insert on the coronoid process of the mandible. 33 The lateral and medial pterygoid muscles lie medial to the two previously described muscles. They originate from the pterygopalatine fossa and insert on the medial aspect of the ramus of the mandible and its condyloid process. The only abductor in the group of masticatory muscles is the digastricus. This muscle originates from the jugular and mastoid processes of the skull, inserting on the ventral aspect of the mandibular body. It has a tendinous intersection that divides the muscle into a rostral and caudal body. 33 Except for the caudal belly of the digastricus, all of the aforementioned muscles are innervated by the motor component of the mandibular division of the trigeminal nerve. The caudal belly of the digastricus is innervated by the facial nerve. The parotid gland is triangular in shape and lies ventral to the ear with its apex directed ventrally. The parotid duct courses across the lateral aspect of the masseter muscle, traverses the cheek, and opens into the caudal vestibule on the small parotid papilla, which is located adjacent to the upper fourth premolar. The zygomatic gland is located medial to the zygomatic arch in the ventral orbital region. The zygomatic duct opens into the vestibule just distal to the first upper molar tooth. The roof of the oral cavity proper is formed by the hard palate rostrally and the soft palate caudally, thus separating the oral and nasal cavities. The hard palate is formed by the palatine processes of the incisive and maxillary bones and the horizontal laminae of the palatine bones. It is relatively flat, especially when compared with the extreme concavity seen in the human hard palate. The mucosa covering the hard palate consists of cornified stratified squamous epithelium, which is developed into seven to eight transverse curved ridges (palatine rugae). It has a thick tough connective tissue support, the mucoperiosteum, which is continuous with the periodontal ligaments of the upper teeth. 35 The mucosa of the hard palate ends abruptly laterally at its junction with the short palatal free gingiva. The incisive papilla is located on the midline cranial to the first transverse ridge and just caudal to the upper central incisor teeth. On either side of this papilla are the incisive ducts that extend caudadorsally through the palatine fissures into the floor of the nasal fossae. This duct communicates with the vomeronasal organ. The major palatine arteries, branches of the maxillary arteries, supply the hard palate. They exit the palatine foraminae of the hard palate at the level of and medial to the distal cusp of the fourth upper premolar. The soft palate continues the separation of the oral cavity from the nasal cavity and nasal pharynx. It is the caudal extension of the hard palate and has no supporting bone. The junction of the hard and soft palate is at the level of the first upper molar. The main blood supply to the soft palate is by means of the minor palatine arteries, branches of the maxillary arteries. The soft palate ends caudally at the common pharynx and continues caudolaterally as the palatopharyngeal arches. The soft palate, the tongue, and the mucosal folds between them (palatoglossal folds) form the borders of the isthmus of the fauces, which is defined as the passage between the oral cavity and the oral pharynx. The prominent bony hamular processes of the pterygoid bones can be palpated deep to the soft palate approximately 1 em caudal to the attachment
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of the palatoglossal arches. The palatine tonsils lie in shallow fossae located approximately 1.5 em caudal to the palatoglossal folds. The floor of the oral cavity is bordered laterally by the teeth and their associated gingiva, which overlies the alveolar bone of the mandible. The majority of the floor consists of the tongue with its laterally reflected mucosa. The tongue is attached to the hyoid apparatus caudally. On the ventral aspect of the tongue, the lingual mucosa forms a median fold, the lingual frenulum, which connects the tongue to the floor of the oral cavity. Just medial to the lower first molar on the floor of the oral cavity is a mass-like flap of oral mucosa with no known function (Fig. 5); its prominent appearance leads to an incorrect identification as an abnormal finding. The mandibular salivary gland is located caudal to the ramus of the mandible and ventral to the parotid gland. Its duct courses craniomedially to become situated in the floor of the oral cavity. It lies within a fold of oral mucosa, the sublingual fold, and opens at the apex of the sublingual caruncle or papilla, which is located just lateral to the rostral aspect of the lingual frenulum. The sublingual salivary gland lies in close opposition to the rostral aspect of the mandibular gland. Its duct courses with the mandibular duct, opening on the medial side of the sublingual papilla. The mandibular lymph nodes are located rostral to the mandibular salivary gland. They are not lobulated like the salivary gland and therefore can be distinguished from them. They, along with the parotid lymph nodes, drain the entire head. They are more superficially located than the parotid lymph nodes and therefore are more easily palpated. When the mouth is closed, the tongue fills the oral cavity. The tongue is a muscular structure consisting of a body and root. The intrinsic muscles run in three directions. They are referred to as longitudinal, transverse, and vertical muscles. They allow for the versatility of the tongue' s movement. The tongue is covered by lingual mucosa, which is heavily cornified stratified squamous epithelium. The root of the tongue is attached to the basihyoid bone and acts as the point of attachment of the majority of its extrinsic muscles. The dorsal aspect of the lingual mucosa is specialized in having five types of papillae. They are filiform, fungiform, foliate, vallate and conical. For further information on these papillae, the reader should refer to an anatomy or histology text.
Figure 5. Mucosal bulge on the lingual (medial) aspect of the lower first molar tooth of a cat.
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The oral mucosa can be pigmented, with the most common area of pigmentation being the buccal mucosa. The gingiva and mucosa of the hard palate are also frequently pigmented. Although less common, the soft palate and mucosa of the floor of the mouth can show pigmentation. Pigmentation of the tongue is uncommon. Oral pigmentation usually occurs in black, gray, and tiger cats. 6 As is true of other species, the head of the cat has an excellent blood supply, with the oral mucosa being especially well endowed. 18 The major blood supply to the upper jaw, including its dental arcade, is through the maxillary artery and its major continuation, the infraorbital artery, which becomes superficial as it emerges from the infraorbital canal. The lower jaw and its dental arcade are supplied by the mandibular alveolar artery, a branch of the maxillary artery, which courses through the mandibular canal. Terminal branches supply the mandibular incisor teeth as well as exiting through the mental foramina to supply the lower lip. These foramina usually number two on each side of the lower jaw and are located ventral to the diastema and third premolar. The lingual artery, a branch of the external carotid artery, supplies the tongue and oral mucosa of the floor of the oral cavity. Sensory innervation of the oral cavity excluding the tongue is by means of branches of the trigeminal nerve. The mandibular alveolar nerve, a branch of the mandibular division of the trigeminal nerve, courses through the mandibular canal. It supplies sensory innervation of the lower teeth. After exiting the mental foramina as the mental nerves, it innervates the lower lip. 33 The maxillary nerve, also a branch of the trigeminal nerve, sends palatine branches for sensory innervation of the soft and hard palate. The maxillary nerve and its continuation as the infraorbital nerve releases many alveolar nerves to the teeth of the upper jaw. As the nerve exits the infraorbital canal, it branches to supply sensory innervation to the soft tissue of the muzzle. References 1. Berman E: The time and pattern of eruption of the permanent teeth of the cat. Lab Anim Sci 24(6):929-931, 1974 2. Boling LR: Blood vessels of the dental pulp. Anat Rec 82:25-37, 1942 3. Brashear AD: The innervation of the teeth. J Comp Neur 64:164-183, 1936 4. Buckland-Wright: Structure and function of cat skull bones in relation to the transmission of biting forces. PhD thesis, University of London, 1975 5. Byers MR, Matthews B: Autoradiographic demonstration of ipsilateral and contralateral sensory nerve endings in cat dentin, pulp, and periodontium. Anat Rec 201(2):249-260, 1981 6. Dummett CO, Barens G: Feline oral pigmentation. J Periodont 41(12):696-701, 1970 7. Forsberg A: The periodontal tissue of mandibular premolars and molars in some mammals. Svensk Tandlarkare-Tidskrift 62(suppl1):10, 1969 8. Gavin JB: The ultrastructure of the crevicular epithelium of cat gingiva. Am J Anat 123:283-296, 1968 9. Gavin JB: Effects of systemic antibiotic therapy on the fine structure of gingival crevicular epithelium. N Z Dent J 69:85-92, 1973 10. Grant D, Bernick S: Morphodifferentiation and structure of Hertwig's root sheath in the cat. J Dent Res 50(6):1580-1588, 1971 11. Hayashi K, Hideo K: Microhardness of enamel and dentin of cat premolar teeth. Jpn J Vet Sci 51(5):1033-1035, 1989 12. Hennet PR, Harvey CE: Root canal anatomy in the cat. In Proceedings of the Veterinary Dental Forum, New Orleans, 1991 13. Hock J, Nuki K: A vital microscopy study of the morphology of normal and inflamed gingiva. J Periodont Res 6:81-88, 1971
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14. Holland GR: The dentinal tubule and odontoblast process in the cat. J Anat 120(1):169177, 1975 15. INnes PB: The ultrastructure of the connective tissue underlying the sulcular epithelium. N Z Dent J69:185-193, 1973 16. Izumi H, Kuriwada S, Karita K, et a!: The nervous control of gingival flow in cats. Microvasc Res 39(1):94-104, 1990 17. Jayne H: Mammalian Anatomy. Philadelphia, JB Lippincott, 1898 18. Johnson GK, Squier CA, Johnson WT, et a!: Blood flow and epithelial thickness in different regions of feline oral mucosa and skin. J Oral Pathol 16:317-321, 1987 19. Kishi Y, Shimozato N, Takahashi K: Vascular architecture of cat pulp using corrosive resin cast under scanning electron microscope. J Endod 15(10):478-483, 1989 20. Lewinsky W, Stewart D: The innervation of the periodontal membrane of the cat, with some observations on the function of the end-organs found in that structure. J Anat 71:232-235, 1937 21. Loescher AR, Holland GR: Distribution and morphological characteristics of axons in the periodontal ligament of cat canine teeth and the changes observed after reinnervation. Anat Rec 230(1):57-72, 1991 22. McClure RC, Pallman MS, Garrett PG: Cat Anatomy: An Atlas, Text and Dissection Guide. Philadelphia, Lea & Febiger, 1973 23. Miles AEW, Grigson C: Colyer's variations and disease of the teeth of animals. Cambridge, Cambridge University Press, 1990 24. Nalbandian J, Frank RM: Electron microscopic study of the regeneration of cementum and periodontal connective tissue attachment in the cat. J Periodont Res 15:71-89, 1980 25. Okuda A, Harvey CE: Odontoclastic resorptive lesions in cats-microscopic findings. Vet Clin North Am Small Anim Pract 22:1385, 1992 26. Peyer B: Comparative Odontology. Chicago, University of Chicago Press, 1968 27. Sasaki T, Nakagawa T, Tominaga H, et a!: Electron microscopy of the junctional epithelium of kitten gingiva. Bull Tokyo Dent Coli 22(2):139-149, 1981 28. Shapiro HH: Growth and time corrections between ossification centers in the long bones and calcification centers in the mandibular dentition of the cat. Int J Orthod 16:690-702, 1930 29. Silva DG, Kailis DG: Ultrastructural studies on the cervical loop and the development of the amelo-dentinal junction in the cat. Arch Oral Bioi 17:279-289, 1972 30. Skobe Z, Prostak KS, Trombly PL: Scanning electron microscope study of cat and dog enamel structure. J Morpho! 184:195-203, 1985 31. Thilander H, Hugoson A: The border zone tooth-enamel and epithelium after periodontal treatment. An experimental electron microscope study in the cat. Acta Odont Scand 28:147-155, 1970 32. Thomas BOA: An analysis of the inferior alveolar and mental nerves in the cat. J Comp Neuro 84(1):419-436, 1938 33. Turnbull WD: Mammalian masticatory apparatus. Fieldiana: Geology 18(2):183-193, 1970 34. Vongsavan N, Matthews B: The permeability of cat dentine in vivo and in vitro. Arch Oral Bioi 36(9):641-646, 1991 35. Wijdeveld MG, Maltha JC, Grupping EM, et a!: A histological study of tissue response to simulated cleft palate surgery at different ages in Beagle dogs. Arch Oral Bioi 36(11):837-843, 1991 36. Windle WF: Experimental proof of the types of neurons that innervate the tooth pulp. J Comp Neuro 843:347-356, 1927
Address reprint requests to Paul Orsini, DVM University of Pennsylvania School of Veterinary Medicine Department of Clinical Studies-Philadelphia 3850 Spruce Street Philadelphia, PA 19104