Here is a root canal treatment case that just came in today. The patient had pain in the tooth to bite on and tap on, and there was no reaction to hot or cold.
The tooth has been heavily filled and patched over the years. The top left and bottom right corners are actually tooth coloured filling patched onto the original metal amalgam. As there was no decay obvious we diagnosed that the tooth had cracked and bacteria had entered into the chamber within the tooth where the nerve lies. This causes the nerve tissue to swell from the infection, causing pain, and then the pressure strangles the blood supply to the tooth, leaving a dead infected space within the tooth. This was confirmed when we removed the filling.
We then opened into the root chamber and found the root canal spaces. You can see that the crack extende almost the whole way across the tooth.
I have marked the crack with a yellow arrow. Unfortunately, this crack may continue to leak bacteria back into the centre of the tooth no matter what we do, and the tooth will have to be removed. We are going to try and save the tooth with a root canal treatment and crown for now and hope for the best.