There are several issues with performing root canal retreatment through an existing crown.
1) Reduced visibility.
If the crown has a metal base/is completely made of metal, this can reduce the amount of light entering the pulp chamber and make visibility limited during retreatment.
Use of a dental operating microscope will help.
2) Higher risk of perforation/procedural errors.
The crown may not replicate the size and orientation of the original tooth exactly, so the position of important surface anatomical features may be distorted. This increases the risk of procedural errors during root canal access such as perforations of the tooth/root.
3) Inability to assess remaining tooth structure.
The remaining tooth structure is obscured by the crown, and may hide an unrestorable tooth.
4) The crown will be damaged.
During the access, a hole must be drilled into the crown. This may cause microcracks if the crown is made of porcelain. At the end of the root canal treatment, the hole will be filled with a separate material.
5) Does not allow treatment of decay underneath the crown.
Old crowns may harbour decay underneath that may not be visible on xray. The crown should be removed so that this can be seen and treated. Leaving decay underneath the crown will cause it to fail eventually and also compromise the success of the root canal retreatment.
