Shaping the future of animal health
Dentistry & Oral health

The tooth and its supporting structures

General Dental Anatomy

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Fig. 8

Macroscopically, the tooth is composed of a crown and one to three roots. Incisor, canine and first premolar teeth in dogs have one root. The second maxillary premolar tooth and the first maxillary molar tooth in cats have, in 2/3 of cases, two roots or two partially fused roots. Other premolar and molar teeth have two roots except the fourth upper premolar, the first and second molar teeth in dogs which have three roots. The crown of the teeth is roughly conical in shape. The crown of small premolar teeth as well as carnassial teeth in cats forms a cutting edge (secodont teeth). Carnassial teeth in dogs are more crushing teeth. A flat occlusal surface can be seen on the distal part of the crown of the lower first molar and on other lower molar and upper molar teeth. This is an adaptation to a more omnivorous diet compared to felines, which have a stricter carnivorous diet. In dogs, during grinding of the food, the occlusal surfaces of the lower molar teeth are in contact with the palatine occlusal surface of the upper molar teeth.

The demarcation between the crown and the root(s) is called the neck of the tooth. The crown is covered by enamel under which the dentin constitutes most of the volume of the tooth. The layer of enamel is thinner in dogs and cats (0.1 to 1 mm) than in humans (up to 2.5 mm). The crown contains the pulp chamber with pulpal horns extending into the cusp(s) of the crown. The root is covered by cementum but is made of dentin and contains a pulp canal where pulpal tissue runs. The pulp canal(s) is connected to the pulp chamber. The root canal does not directly opens to the outside, it divises into multiple small ramifications forming an apical delta; each ramification of the delta opening on the outside surface of the root. The pulp tissue is a specialised connective tissue containing blood and lymphatic vessels, nerves and cells, including odontoblasts that secrete dentin (Figure 8).

Periodontium

The periodontium or periodontal tissue constitutes the attachment tissue of the tooth. It is composed of the gingiva, the cementum, the alveolar bone, and the periodontal ligament (Figure 9).

The gingiva covers the alveolar processes of the maxilla and mandible and surrounds the tooth itself. The gingival margin often has a knife-edge shape and is firm and pink or pigmented. In animals exposed to dental plaque accumulation at the time of tooth eruption, a physiologic gingival sulcus is found around the tooth (depth 0-3 mm in dogs and 0-1 mm in cats). At the bottom of the sulcus, the cells are attached to the enamel surface and constitute the junctional epithelium (JE). The JE ends at the level of the cemento-enamel junction (CEJ). The gingiva coronal to the CEJ is called the marginal or free gingiva. Below it, the attached gingiva is tightly bound to the underlying periosteum of the alveolar bone. The attached gingiva is demarcated from the alveolar mucosa by the mucogingival junction. The oral sulcular epithelium (OSE) forms the lateral wall of the sulcus. The inner surface of the sulcus is covered by a thin non-keratinized stratified squamous epithelium. The interface between the JE and subepithelial connective tissue follows a fairly smooth line with usually no interdigitations (rete pegs) compared to the epithelial-connective tissue interface of the oral gingival epithelium. About 70% of the gingival connective tissue is composed of collagen fibres

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Fig. 9

The cementum is bony-like avascular tissue covering the roots. Less calcified than enamel and dentin, it does not contain lacunae and channels and is therefore denser than bone.

Apposition and resorption phenomena are slower than in bone. Cementum deposition is continuous throughout life.

More deposition occurs in the apical area where the cementum is the thickest. Cellular cementum deposed continuously has an irregular rough surface. Cementum is a very important structure involved in both resorptive and reparative processes.

The periodontal ligament is composed of collagen fibres which anchor the tooth to the alveolar bone. Sharpey fibres are compact bundles of smaller collagen fibres which perforate the cementum and the bone where they are tightly embedded. These principal fibres are covered by a dense meshwork of undifferentiated fibres. The periodontal ligament is about 100 - 250 microns wide. Besides collagen fibres, it contains blood and lymphatic vessels, nerves, elastic fibres and cells. Fibroblasts, osteoblasts, cementoblasts, osteoclasts, cementoclasts, and MalassezÕs rests are found in the periodontal ligament. Fibroblasts can differentiate into cementoblasts or osteoblasts.

The alveolar bone constitutes the ridges of the jawbones. Deep depressions, the alveolar sockets, contain the roots of the teeth. The alveolar bone appears with tooth eruption and disappears with tooth loss. Bone generally consists of 3 layers : periosteum, dense compact bone, and cancellous bone (spongiosa); alveolar bone is composed of a fourth layer, the cribriform plate covering the alveolar sockets. Radiographically, it appears as a radioopaque line called the lamina dura. The alveolar crest is located about 1 mm apical to the CEJ. The surface of the alveolar bone proper adjacent to the periodontal ligament is made up of multiple layers of bone parallel to the surface of the alveolar wall, which are penetrated by bundles of Sharpey fibres embedded almost perpendicular to the surface. Vessels and nerves run through the alveolar bone and perforate the cribriform plate. Some are connected to the pulpal tissue through lateral canals. Alveolar bone responds readily to externaland systemic influences by a process of resorption which may be accompanied by a process of apposition.

fig10.gif 1- Supra-periosteal vessels
2- Periodontal ligament vessels
3- Alveolar bone vessels