In many cases, teeth that have had a root canal treatment will require a crown. This discolored front tooth has very little internal tooth structure remaining and requires a crown to reinforce it.
This lower molar tooth has a large fracture. Teeth such as this will require a crown to prevent further fracture and breakdown, and may require a root canal and a post and core.
When teeth have large fillings within them, (generally more than 50% of the tooth), they commonly fracture. An artificial crown is recommended to protect and reinforce the remaining tooth structure, and restore its natural appearance.
Replacing an existing crown is required when the crown and/or tooth starts to deteriorate. In this example, there is bacteria around the edge of the crown, causing the tooth to decay and discolor. A new crown is required.