First of all it is very difficult for the dentist to access that tooth as long as the crown is present. So it is extremely important that the doctor takes a diagnostic x-ray, check what is happening underneath and then delicately cut through the crown in order to reach the decayed tooth. There are times when the tooth structure under the corn is perfectly okay but there is some residual infection or repeat infection at the tip of the roots . In such cases the doctor can make a small opening, an orifice through the metal crown, access the root and do the treatment that is required, debride the tooth disinfect it and refill the tooth without disturbing the crown at all. So next time when you please ask the dentist if that s an option for the tooth.
