Zirconia is one of the hardest materials and certainly the hardest/strongest ceramics utilized intraorally. In order to have the root canal accomplished, a small opening will have to be accessed on the chewing surface of the crown. The Dr will most likely use 2-3 new diamond drill bits with a heavy flow of water to cool and keep the ceramic from cracking. Once the access is open the root canal treatment can progress, followed by a tooth colored filling to seal and restore the opening that was created.
Removing an existing zirconia crown will require the same care and patience as drilling through it. Once a "slit" is created along the side of the crown a special instrument can separate the pieces. Unfortunately the crown is no longer usefull. Occasionally a "gummy" substance is placed on the crown, the patient bites into it, and opens rapidly. Sometimes the crown will pop of and is reusable.