The anchor tooth on my lower bridge has developed an infection. Can a root canal be done through existing crown?

Thank you for your inquiry. To answer your question in general, yes your root canal can be done through your current crown/bridge. This is common practice but please keep in mind that if/when this is done, there is a risk that your crown/bridge may fracture in doing so. If you're having jaw discomfort, you may have another issue occurring aside from your root canal, as root canals are typically associated with discomfort with temperature. If your crown/bridge was recently placed, then make sure your occlusion (bite) is ideal. You can also look into wearing a night guard that may help with possible grinding you may be doing to cause your jaw discomfort. Either way, give it a bit of time to all subside but continue to monitor and treat as needed. We hope this helps and wish you all the best! 

Ideally root canal therapy is completed before final restorations are permanently inserted, but sometimes the issues are not present until AFTER the initial treatment.  It is not unusual to do root canal therapy through a crown or bridge, and sometimes the access hole is simply filled (so the original restoration is not replaced).

Root canals is an ideal treatment for saving your tooth. Once the root canal is done, the infected tooth should start feeling better within a few days. Most likely your dentist will make a small hole through your anchor tooth to perform the root canal and once root canal is complete, the small hole will be sealed with a permanent filling material.  Just remember that sometimes it is possible for the porcelain of the anchor tooth to crack or break off, in which case a new bridge may be needed.