Cavities that expose the dental pulp cannot be directly filled in one session. Treatment of the tooth must be carried out before filling. In the early stages of a cavity, only the hard tissue of the tooth is damaged. If the decayed part is removed promptly and the dental nerve is not exposed, the tooth can be directly filled in one session. However, if the cavity progresses to the point where the dental pulp, or nerve, is exposed, this requires a nerve removal procedure. After the nerve is killed, the root canal is sealed with medication, and following inflammation control, the root canal is filled. Only after root canal treatment can the tooth be filled. Otherwise, the inflammation may further develop into apical inflammation, or even lead to space infections causing facial swelling.