Could you tell me what happens when you do a root canal and don't put the crown on?

Possible fracture: Teeth that have had root canal therapy are more brittle than vital teeth. A crown helps protect the tooth and restores lost structure that a filling can't.

Possible fracture: In molar and premolar teeth there is about a 30% incidence of tooth fracture over a 5 year period if a crown is not placed (and is simply filled instead). Crowns help prevent tooth fracture by directing the chewing forces down the long axis of the tooth.

Possible fracture: Most posterior (back teeth) should have crowns after root canal treatment unless you are under 18 years of age. Back teeth are used for grinding food and are more prone to fracture after root canal treatment. If you are young it's typical to wait for your to finish growing before a crown is placed on your tooth.