FUNCTION OF TEETH

Teeth are important in many functions of the body. They are essential for protecting the oral cavity, in acquiring and chewing food, and in aiding the digestive system in breaking down food. The teeth form a hard physical barrier that protects the oral cavity. This shield not only affords protection to the oral structures, but the teeth themselves are formidable weapons. One group of mammals classified to the order Carnivora demonstrates this particularly well. Lions and tigers are members of this order and have well-developed canines that they use as weapons to defend themselves and to attack and kill their prey. The teeth also function in communication. They are necessary for proper speech, phonetics and even whistling. In many cultures their appearance can be a very positive sexual attraction. In dental anatomy the teeth are studied individually and collectively, including their functions, anchorages, and relations to one another. Our study therefore begins with a discussion of the individual tooth.

CROWN AND ROOT

Each tooth has a crown and root portion. The crown is covered with enamel, and the root portion is covered with cementum. The crown and root are joined at the cementoenamel junction (CEJ). The line that demarcates it is called the cervical line, a line that is formed by the junction of the cementum of the root and the enamel of the crown (Fig. 2-1).

The crown portion of the tooth erupts through the bone and gum tissue. After eruption it will never again be covered with gum tissue. Only the cervical third of the crown in healthy young adults is partly covered by gingiva (gum tissue). The tooth continues to erupt from the bone and gingival tissue until all of the crown is exposed (Fig. 2-2).

A clinical difference is evident between the amount of crown that could be erupted and the actual amount that is visible in the mouth. The anatomic crown is the whole crown of the tooth that is covered by enamel, regardless of whether it is erupted. The clinical crown is only that part seen above the gingiva. Any nonerupted area is not a part of the clinical crown of the tooth. Therefore, if all of the anatomic crown does not erupt, the part that is visible is considered the clinical crown, and the unerupted portion is part of the clinical root (Fig. 2-3). Eruption of a tooth is thus the moving of that tooth through its surrounding tissues so that the clinical crown gradually appears longer. The tooth may have a single root (see Fig. 2-1) or a multiple root with bifurcation or trifurcation—that is, division of the root portion into two or three segments (Figs. 2-4 and 2-5). Each root has one apex or terminal end. The root is held in its position relative to the other teeth in the dental arch by being firmly anchored in the bony process of the jaw. The portion of the jaw that supports the teeth is called the alveolar process. The bony socket in which the tooth fits is called the alveolus (Fig. 2-6). Teeth in the upper part of the jaw are called maxillary teeth because they are anchored in the maxillary bone. In the lower jaw they are called mandibular teeth because they are anchored in the bone called the mandible.

TOOTH TISSUES

The four tooth tissues are enamel, dentin, cementum, and dental pulp (Fig. 2-7). The first three are hard tissues; the pulp is soft tissue.

TYPES OF TEETH

The functions of teeth vary, depending on their individual shape and size and their location in the jaws. The three basic food processing functions of the teeth are cutting, holding or grasping, and grinding.

Incisors

The incisors are designed to cut (incisor means “that which makes an incision, or cut”), and the biting edge is called an incisal edge (Fig. 2-8). The tongue side, or lingual surface, is shaped like a shovel to aid in guiding the food into the mouth (Fig. 2-9).