Could you tell me what happens to a posterior tooth after a root canal?

Depends: A posterior tooth is much more likely to fracture following a root canal since the tooth is now more brittle and also absorbs much more force than a front tooth. Unfortunately, fracture is the most common problem seen following a root canal of a posterior tooth.

Several things...: As a rule when a molar has had root canal treatment a crown is required if there is not already one present. For a premolar, depending on the amount of missing or replaced tooth structure, a crown may also be recommended or required. If the root canal procedure was accessed through an existing crown then a filling may be able to be used to close the resultant hole if the crown is not damaged.

Crown: Usually you will want to protect it as much as possible with a crown.