I had a filling replaced a few months ago and even tho I've been back to the dentist twice I still can't bite on the tooth w/o unbearable pain. When he was drilling I almost came out of the chair when he hit the nerve. Now he tells me I will have to have a root canal. If he had to drill that deep why didn't he just do a root canal to begin with? The x-rays would have surely shown this. I'm beginning to think they do these things on purpose just for more money.

Infection of the root is always a possible consequence of replacing a filling... and the deeper the hole the greater the chance of this happening. Some dentists will tell you this up front before they do the work. Of course, if the filling they're replacing is no good, you've got no choice but to do it. Obviously we can't comment on your particular dentist, but what you describe isn't unheard of.

An ethical dentist will not drill into the nerve just for a filling. I don't think they will do that on purpose except for those very few crooked dentist. Was your old filling pretty large and close to the nerve? If that is the case your dentist should've told you ahead of time that it's a potential root canal. You should've been numbed during the procedure unless you opted not to. If you are numb you would not feel anything he's doing even if he drilled to the nerve. When a filling is being replaced, your new filling will be slightly larger than the existing filling. Ok to answer your questions: Usually a dentist will try to do a filling first to be on the conservative side. If the filling does not work then they will go with the next step which is a root canal. X-rays are helpful but its very hard to tell sometimes how close to the nerve the old fillings are. If it's a small filling and no where near the nerve then of course you can tell it definitely do not need a root canal just by looking at the x-ray. But if it's a large filling and pretty close to the nerve it's hard to tell how close to the nerve it is until he is actually drilling. I hope that helps. Also if you need a root canal is the same dentist doing the RC or he is sending you to an endodontist? If you don't trust him with that filling then you should probably have an endodontist do your RC.