Hard tissues are affected

Depending on the hard tissues affected, caries may be described as involving enamel, dentin, or cementum. Early in its development, tooth decay may affect only the enamel. Once the extent of decay reaches the deeper layer of dentin, the term “caries” is used. Because cementum is the hard tissue covering the roots of teeth, it is not usually affected by decay unless the roots of the teeth are exposed to the mouth. Although the term “cement caries” can be used to describe tooth root decay, dental caries rarely affects cement alone. The roots have a very thin layer of cementum over a large layer of dentin, and so most types of caries affecting cementum also affect dentin.