Internal bleaching is unlike traditional tooth whitening where whitening agents are applied to the outside of the tooth. Internal bleaching is completed from the inside out. Sometimes a traumatic injury or an old root canal procedure can cause a small amount of blood to enter the porous areas inside a tooth, causing it to become discolored. A yellowing gray or dark tooth may occur if this happens. When this happens, common external teeth whitening methods can't restore the tooth because the discoloration is happening from the inside. Instead, a minor restorative procedure can whiten the tooth from the inside out. After a dental X-ray is taken to determine the extent of discoloration and its cause, a dental dam is used to isolate the tooth from further contamination during the bleaching procedure. If diseased or damaged pulp is to blame, the material is removed and the whitening paste is applied inside the tooth. If the tooth already has a root canal, a small hole is made in the back of the tooth to gain access to the inside. Then a bleaching agent is added inside the tooth and the hole is carefully filled. After two or three days, or when the desired tooth color shade is reached, your doctor will remove the bleaching agent and seal the tooth. If the desired color is not reached or the discoloration is substantial, additional bleaching treatment may need to be performed. It's important to seek treatment for a dark tooth so your doctor can evaluate it, rule out additional injury, and restore the discolored tooth to its optimal appearance and function.