Internal resorption is a rare condition where the living cells within a tooth attack and eat away the inside of the tooth. Initially, the process is slow and painless and may only be detected with dental x-rays. If left untreated, the tooth can weaken to the point of breakage. Although it is still unknown why cells attack their own teeth, the most common cause of internal resorption is due to trauma. Non-surgical root canal therapy can remove the harmful cells before they do more damage.