The depth of the decay and the sensitivity or pain the patient is in, are the factors in determining whether a tooth needs a root canal
When the cavity is too big and very close to the nerve, the right treatment is to have a root canal done. If it's not that close to the nerve, we can try some medication as a base, a permanent filling, and observe the tooth. If the pain comes back, then that tooth really needs a root canal.
I hope I made it clear.
Dr. David Wiseman