Crowns are used to restore structure, function and esthetics of teeth. If your tooth has cracked, broken or has been damaged by decay to a large extent, you should have a crown. If your tooth is over-filled and there is not enough tooth structure left to support the filling material, a crown is also indicated.
If you tooth has had root canal treatment, the internal structure of the tooth is changed, and it is mostly hollowed out. This kind of tooth tends to split or break under stress. Your dentist usually would recommend a crown for a tooth that has had root canal treatment, especially if they are bicuspids or molars. Also, if you have a badly discolored or badly shaped tooth, your dentist may also want to put a crown on that tooth.
The average lifetime of a crown is around 10 years. Good home care and regular professional cleaning are necessary to prolong the life span of your crowns.