Tooth Decay - Structure of the tooth

The variety of shapes of teeth make them specialized for the various functions in preparing food for digestion—biting, chewing, and grinding. All varieties, however, have the same basic structure. Each tooth has a crown (the part of the tooth visible above the gum line) and a root, which is embedded in a socket in the jaw.

Enamel

The outer covering of the crown is enamel , the hardest substance in the human body. Enamel is about 97 percent mineral and is as tough as some gemstones. It varies in thickness, with the greatest thickness on the surfaces that are likely to get the most wear and tear.

Enamel begins to form on the first tooth buds of an embryo at the age of about 15 weeks, depending upon substances in the food eaten by the mother for proper development. Once the tooth has formed and erupted through the gum line, there is nothing further that can be done by natural means to improve the condition of the enamel. The enamel has no blood supply, and any changes in the tooth surface will be the result of wearing, decay, or injury.

While the health and diet of the mother can affect the development of tooth enamel in the deciduous teeth, certain health factors in the early life of a child can result in defective enamel formation of teeth that have not yet erupted. Some infectious or metabolic disorders, for example, may result in enamel pitting.

Dentin

Beneath the enamel surface of a tooth is a layer of hard material—though not as hard as enamel-called dentin , which forms the bulk of a tooth. The dentin forms at the same time that enamel is laid down on the surface of a developing tooth, and the portion beneath the crown of the tooth probably is completed at the same time as the enamel. However, the dentin, which is composed of calcified material, is not as dense as the enamel; it is formed as myriad tubules that extend downward into the pulp at the center of the tooth. There is some evidence that dentin formation may continue slowly during the life of the tooth.

Cementum

The cementum is a bonelike substance that covers the root of the tooth. Though harder than regular bone, it is softer than dentin. It contains attachments for fibers of a pe-riodontal ligament that holds the tooth in its socket. The periodontal ligament serves as a kind of hammock of fibers that surround and support the tooth at the cementum surface, radiating outward to the jawbone. This arrangement allows the tooth to move a little while still attached to the jaw. For example, when the teeth of the upper and lower jaws are brought together in chewing, the periodontal ligament allows the teeth to sink into their sockets. When the teeth of the two jaws are separated, the hammock-like ligament permits the teeth to float outward again.

Pulp

The cavity within the dentin contains the pulp . There is a wide pulp chamber under the crown of the tooth and a pulp canal that extends from the chamber down through the root or roots. Some teeth, such as the molars, may contain as many as three roots, and each of the roots contains a pulp canal.

The pulp of a tooth contains the nerve fibers, lymphatic vessels, blood vessels, and connective tissue. Although the blood supply arrangement is not the same for every tooth, a typical pattern includes a dental artery entering through each passageway, or foramen , leading into the root of a tooth. The artery branches into numerous capillaries within the root canal. A similar system of veins drains the blood from the tooth through each foramen. A lymphatic network and nerve system also enter the tooth through a foramen and spread through the pulp, as branches from a central distribution link within the jawbone. The nerve fibers have free endings in the tooth, making them sensitive to pain stimuli.

Supporting Structures

The soft, pink gum tissue that surrounds the tooth is called the gingiva , and the bone of the jaw that forms the tooth socket is known as alveolar bone . The gingiva, alveolar bone, and periodontal ligaments sometimes are grouped into a structural category identified as the periodontium . Thus, when a dentist speaks of periodontal disease, he is referring to a disorder of these supporting tissues of the teeth. The ailment known as gingivitis is an inflammation of the gingiva, or gum tissue around the teeth.