After a root canal, teeth are weakened and prone to fracture. All posterior teeth (premolars and molars) need crowns after a root canal. Some anterior teeth may need crowns depending on the amount of missing tooth structure. Here is one example 1 year after root canal with no crown:
This tooth is now nonrestorable and must be extracted.
Here is another example. On the left is the initial root canal. On the right is 4 years later with no crown:
This tooth is now nonrestorable and must be extracted. After extraction, an implant or partial denture is necessary to prevent shifting of the opposing tooth.