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How does a cavity form?
Two main factors contribute to tooth decay -- bacteria in the mouth and a diet high in sugar and starch. There are over 500 different types of bacteria that are normally present in the mouth. These bacteria combine with food and saliva to form a sticky substance called plaque that attaches to teeth. Foods rich in starches add to the stickiness of the plaque, which begins to get hard if it remains on the teeth after a couple of days and turns into tartar or calculus. Bacteria in the plaque convert sugar into acid that dissolves the tooth structure, causing holes, or cavities. Because of these two contributing factors, dental caries have been described as a "dietobacterial" disease.
The parts of teeth that are most vulnerable to tooth decay are areas where plaque can accumulate most easily. Plaque tends to settle into the pits and fissures in the tops of teeth, into the areas in between the teeth, and next to the gum line. Where there is plaque, there are bacteria and acid, and eventually destruction of the tooth surface. The cavity starts in the outer layer of the tooth (enamel) and as it gets deeper, penetrates into the softer inner layer of the tooth (dentin). Typically, it isn't until the decay reaches the dentin that a person will start to notice signs and symptoms of the cavity.