Can a badly decayed tooth be saved?

The first line of defense is a filling, but if the tooth decay is serious you may need a root canal. But you can only do this if the root is still healthy. If not, there is no choice but to extract the rotten tooth. With a root canal, the dentist will drill down the tooth to clean out the decay.

At what point can a cavity not be fixed?

If the decay reaches your tooth's main structure, called dentin, then a filling can replace the lost tooth structure after your dentist has cleaned the cavity of bacteria and infection. However, if it reaches the tooth's center chamber, called the pulp, a filling may no longer suffice to address it.