Deep caries, chemical or physical trauma may result in the loss of vitaly of pulp tissue. Nonvital pulp becomes a very convenient site for the invasion and proliferation of bacteria. Infection in this area can spread from the root tip to neighboring tissues and whole body. In such a case, it is necessary to remove the nonvital pulp tissue and to enlarge the root canals. The expanded root canal is finally filled with suitable materials. This process is called root canal treatment. If the cannal is not treated in time, it may be necessary to remove the tooth.