Why does my tooth all of a sudden need a root canal after having a crown done?

It is normal protocol to replace the old filling before prepping the tooth for the crown.  Unfortunately, it is very common for the replacement of this filling to cause pain which leads to the need for a root canal prior to the crown placement.  I have found that when the old filling looks completely intact and shows no signs of leakage, I give the patient the option of leaving their old filling in, then getting a crown over it.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMDSt. Louis, MOReal Self 100

Discomfort after a temporary crown is placed over the tooth is not uncommon.  The temporary cement does not seal and sooth the tooth very well and sensitivity to cold/hot as well as pressure is possible.  Remember that the crown preparation is a surgery so there might be effects, hopefully only temporary ones.  There are a lot of times this occurs and patient's worry about the need for a root canal when one is not necessary and the symptoms greatly improve or gone completely after the permanent crown is cemented on.That being said, if the tooth fractured and there was no visible sign of the nerve exposed there is really no way to tell how close the fracture did come in proximity.  If the fracture was close enough to the nerve to cause irreversible pain and infection then a root canal is indicated.

It is not uncommon after a tooth has been fractured and crowned to need a root canal. The fact that it cracked is traumatic to the tooth. Additionally working on the tooth to prepare it for the needed crown is also traumatic. Both of these can set up an inflammatory process which can cause a tooth to become irreversibly irritated requiring a root canal. On the positive side, the root canal was done before your new crown was placed permanently.