Dental caries, also known colloquially as tooth decay, is a disease of the teeth resulting in damage to tooth structure. The cavities formed because of dental caries are called dental cavities.

Dental caries is one of the most common disorders affecting humans [1] [2]. It usually occurs in children and young adults, but can affect any person. It is the most significant cause of tooth loss in younger people.

The mouth contains a wide variety of bacteria, but only a few specific species of bacteria cause dental caries: Streptococcus mutans and Lactobacilli. These bacteria convert foods—especially sugar and starch—into acids such as lactic acid created through fermentation processes. These acids seep into the tooth and can wear away tooth structure. If conditions in the mouth are favorable, S. mutans and Lactobacilli will continue to thrive and continue to secrete these acids. Bacteria, acid, food debris and saliva combine in the mouth to form a sticky substance called plaque that adheres to the teeth.

It is most prominent on the grooved chewing surfaces of molars, just above the gum line on all teeth, and at the margins of fillings. Plaque that is not removed from the teeth may mineralize into calculus (formerly known as tartar). Plaque and calculus irritate the gums, resulting in gingivitis. Periodontitis occurs when there is loss of gingival attachment.
The acids secreted by S. mutans and Lactobacilli in the plaque dissolve the enamel surface of the tooth. As the bacteria become more prolific, the bacteria will follow the advancing front of acid damage and infect the dentin within the tooth. Left untreated, carious lesions will increase in severity from small discolored stains to actual holes in the tooth (cavities). Cavities are usually painless until they grow very large inside the internal structures of the tooth (the dentin and the pulp at the core) and can kill the nerve and disrupt the blood vessels in the tooth. If left untreated, complications may occur such as acute irreversible pulpitis (infection of the pulp) or acute apical pulpitis (abcesses within the mandible.)

Plaque and bacteria begin to accumulate within 20 minutes after eating, the time when most bacterial activity occurs. If plaque and bacteria are left on the teeth, cavities can develop, and untreated tooth decay can result in death of the internal structures of the tooth and ultimately the loss of the tooth.

Dietary sugars and starches (carbohydrates) increase the risk of tooth decay. The type of carbohydrate and the timing and frequency of ingestion are more important than the amount. Sticky foods are more harmful than nonsticky foods because they remain on the surface of the teeth. Frequent snacking increases the time that acids are in contact with the surface of the tooth.

• toothache (technically called odontalgia or odontalgy)-- particularly after sweet or hot or cold foods or drinks (sensitivity)
• visible pits or holes (cavities) in the teeth
• halitosis (bad breath)

The earliest sign of a carious lesion is the appearance of a chalky white spot on the surface of the tooth (aka white spot lesion), indicating an area of demineralization caused by acid. As the lesion continues to demineralize, it begins to turn brown. If the lesion's progress continues unchecked, the demineralization can turn into a gross cavitation (a 'cavity'). Brown spot lesions that look shiny indicate that the demineralization process has stopped; this is now just a stain. Brown spot lesions that look dull indicate that active caries are present.

Most dental caries are discovered in the early stages during routine checkups. The surface of the tooth may be soft when probed with a sharp instrument, such as a dental explorer. Pain may not be present until the advanced stages of tooth decay, when the bacterial infection reaches the deeper layers of the tooth and begins to involve the nerve fibers at or near the pulp. Dental radiographs, produced when X-rays are shot through the jaw and picked up on film, may show some cavities before they are visible to the eye.

An image showing various stages of dental caries is shown here

Carious lesions may form on different surfaces on a tooth. Occlusal caries are on the chewing surface, facial caries are on the surface of the tooth opposite the tongue, and lingual caries are on the surface facing the tongue. Interproximal cavities occur in between two adjacent teeth. Facial caries can be further subdivided by nomenclature: facial surfaces of posterior teeth are termed buccal while those of anterior teeth are termed labial.

Cavities that occur on the root usually occur when gross decay on the facial or lingual surfaces extends apically (towards the root, or the apex) past the CEJ (cementoenamel junction) or when the root surfaces have been exposed due to gingival recession. Because the cementum enveloping the root surface is not nearly as durable as the enamel encasing the crown, root caries tends to progress much more aggressively than decay on other surfaces.