at the cusps, thinning to a knife edge at the cervical line. It is formed only once and cannot regenerate or repair itself. Thus, when enamel is destroyed by decay, operative dentistry is required to reconstruct the tooth. Enamel has no nerve fibers and cannot register sensations.
Dentin is the light yellow substance that makes up the bulk of the tooth. It is softer than enamel but harder than bone and is located inside the crown under the enamel. The point at which the dentin and the enamel meet is called the dentinoenamel junction. Dentin is also found inside the root of the tooth under the cementum. The inner surfaces of the dentin forms a hard-walled cavity that contains and protects the pulp.
Unlike enamel, dentin continues to form throughout the life of the tooth. When the dental pulp is mildly stimulated as a result of caries, cavity preparation, abrasion, attrition, or erosion, a protective layer of secondary dentin is formed on the pulp wall.
Even though dentin is not sensitive to stimuli, sensation may result when mechanical, thermal, or chemical stimuli are applied to it. The sensation comes not from the dentin itself but from cells that extend into it. These cells are actually part of the pulp, not the dentin, and they are sensitive to stimuli.
Cementum is a bonelike substance, although it is not as hard as bone. It forms a protective layer over the root portion of the dentin. The cementum joins the enamel at the cervix of the tooth.
The main function of cementum is to anchor the tooth to the socket by attaching to the principle fibers of the periodontal ligament.
Cementum is formed continuously throughout the life of the tooth. Thus, it compensates for the loss of tooth substance due to wear by attaching new fibers of the periodontal ligament to the root.
Pulp is soft tissue that fills the pulp cavity. This tissue contains numerous blood vessels and nerves that enter the tooth through the apical foramen. It is enclosed within the hard, unyielding dentin walls of the pulp cavity. The cavity has two parts: the pulp chamber and the root, or pulp canal. The chamber is located inside the crown. The canal is located inside the root.
An important function of the pulp is to form dentin. It provides the cells from which dentin is formed and supplies the dentin with blood.
Pulp responds to external stimuli, providing sensation to the tooth. It responds to irritation either by forming secondary dentin or by becoming inflamed. Since the walls of the pulp chamber and root canal permit no expansion of the pulp tissue, any inflammatory swelling of the tissue will compress the blood vessels against the walls. This results in a condition known as hyperemic pulp, which can lead to necrosis of the pulp tissue.
The tissues that surround and support the teeth are the cementum, the alveolar process, the periodontal ligament, and the gingivae. Collectively, these tissues are known as the periodontium. Throughout the following discussion, refer to figure 2-5.
The alveolar process is the portion of the maxillae and mandible that forms and supports the sockets (alveoli) of the teeth. The alveolar process can be divided into two parts: the alveolar bone proper and [he supporting alveolar bone.
The alveolar bone proper is a thin layer of bone that lines the tooth socket and attaches the principal fibers of the periodontal ligament. The supporting alveolar bone is the portion of the alveolar process that surrounds the alveolar bone proper and gives support to the tooth socket.
The periodontal ligament consists of hundreds of tissue fibers that, except at the apical foramen, completely surround the tooth root. The ligament acts as a shock absorber, reducing the impact of the teeth as they occlude.
The gingivae are the soft tissues that cover the alveolar process and surround the necks of the teeth. They consist of an outer layer of epitheliums and an inner layer of connective tissue.
Healthy gingivae are pink, firm, and resilient. They have a stippled appearance. Stippling refers to the orange peel texture of the healthy tissue. Inflammation causes a loss of stippling. When inflamed, the gingivae may become sore and swollen, and they may bleed.