I had this same problem for years. I finally started a dental clean-up a couple months ago with a dentist who follows the proper protacol in removing mercury Amalgams and root canals. I had four teeth with root canals extracted with the ligament removed, as well as having my cavitations cleaned out. It was the first pain free dentist visit I have had in a long time! I think the infection in my jaw bones from having my wisdom teeth out 30 years ago (cavitations) would not allow the novicaine to do its job. The dentist was really fighting to get my teeth out because teeth that have root canals are very brittle and they shattered into fragments. He had to "dig" to get every piece. I told him when he was done how painless the whole experience was because I had never been to the dentist before without feeling the pain. But then maybe he just knew what he was doing too!?