Why does my tooth need a crown and not a filling?

In order to fix a tooth with a filling (whether it was due to a cavity or a fractured tooth), there are certain parameters that must be met. First, internal walls of a tooth are necessary in order to hold a filling within the tooth. If a fracture or a cavity has caused more than two thirds of a tooth structure to be lost, a filling will not suffice. If, however, at least two thirds of the tooth is intact, using a composite or amalgam material to replace the void will work. 

It is possible to get a new cavity around an existing filling. When recurrent cavities require frequent removal and replacement of tooth structure, the remaining tooth becomes extremely brittle. Large fillings and prolonged clenching or grinding habits put teeth at a high fracture risk. When fractures occur, they can be difficult to deal with. Small cracks in our teeth can be similar to a small crack on the windshield of our car. The crack usually starts small and then gets bigger as time progresses. If the cracks are identified early on, we can usually prevent it from propagating into the nerve or root of the tooth. If, however, the fracture continues into the center of the tooth, a root canal or extraction may be necessary. To prevent the advancement of an existing fracture, a crown is placed over the tooth to support the remaining tooth structure and prolong the life of the tooth.

For specific questions or concerns, consult with your dentist. He or she will make recommendations based on your specific needs and risk factors.