Hi JamesSmith,

I am going to re post some of your post and reply in bold type. Makes it easier to follow.

<< I got my first RCT done 10 years ago on the lower jaw left premolar and although it got infected shortly afterwards, it settled down eventually and I got it crowned.>>

There are several reasons why root canal therapy cannot cure an infected tooth but the easiest reason to understand is because it is not possible to remove all of the nerve tissue and sterilize the tooth. Your symptoms may have calmed down with this upper tooth, but the tooth itself is not healthy. Crowning the rc tooth does not alter the bacterial status of the tooth. It is more like putting a hard blanket over it to help prevent the fragile tooth from cracking when chewing on it.

<<My dentist initiated my second RCT on the upper left molar number 26 this November (2015) and since then the nightmare has started! My dentist cleaned out the infection and killed the root in the first sitting, prescribed painkillers and told me to come back after a week.>>

Again, the nerve tissue cannot be removed entirely. Some teeth will act up right away, while others won't. Also, the dentist did not kill the root of the tooth. The root system is still intact in the jaw bone. She scraped out visible nerve tissue from inside the root system. The tooth may still have partial vitality because not all of the nerves can be removed. Sometimes the remaining nerves flare up and cause pain, other times they become necrotic and may or may not show symptoms of infection until sometime later.

<< A few days later, I felt extreme pain in the tooth and could not sleep for 3 days! I went back to my dentist and she said it was because of a reaction to the disinfectant used to clean the canals. >>

A reaction to the disinfectant?? Did she perforate the tooth with the disinfectant? If she did, then bacteria and inflammation can build up around that perforation. You should find out if she did that because that problem can lead to complications involving the nerve bundles that reside in the bone surrounding that tooth.

<<She prescribed augmentin antibiotic along with pain killers and told me to come back after a week. The pain reduced a bit, but it was still bothering me so I went back to her and she realized that one of the canals had not been cleaned out properly. She then recleaned the canals and the pain went away. However, the sensitivity remained.>>

It is possible that she missed a visible canal and/or perforated the tooth. The number of visible canals (one to four depending on the tooth) are significantly less than the many hundreds of non accessible microscopic canals.

<< A week later, she filled the canals with gutta percha and sealed the tooth with a temporary filling. She wanted me to come back after a week for the post and core filling, but I told her that I wanted the tooth to settle down and for all the sensitivity to go away and then continue. It's been almost 2 weeks since then. The tooth sensitivity has increased and even the molar next to it is quite sensitive.>>

This indicates vitality in the tooth as well as inflammation in the accessory canals. Also, the tooth is intricately connected to a live periodontal ligament which becomes inflamed from the infected tooth. The inflamed ligament can literally push the tooth down from the original position causing the tooth to hit the lower tooth too hard. An inflamed ligament can cause pain, sensitivity, soreness, etc.

<< Also, the tooth on which this RCT tooth is biting on in my lower jaw has become sensitive also! They are reacting to touch both with my tongue and finger! What could the problem be?>>

This may be due to the inflamed ligament as I stated above. Both the rc tooth and the one it is biting against will become sore as both of their ligaments become inflamed.

An xray should help determine if there was a perforation of the tooth during the rc procedure... it can also help determine an inflamed perio ligament... and also infection if it is large enough to be seen on the 2 dimensional xray.

Bryanna