Can an uncleaned tooth begin to decay under a sealed crown?

It is always best to clean a crown before re-cementing, but sometimes dentists do it without the patient actually seeing it.  Whenever a patient comes to my office from another dental office, complaining of a crown that has fallen off, I always inspect the crown, inspect the tooth it came off of, then place the crown on the tooth to check the margins.  Once I notice that the margins are fine, I disinfect the crown, and remove any decay from the tooth surface,  After I do all of those things, I re-cement the crown.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMD

If there is no residual decay and the crown fits well, there is little concern that you will decay under your recemented crown.  However, it is always best to clean the underlying tooth before cementing any crown.  There are several ways this is done, and you might not have realized your dentist did this.  He could have by polishing it like your dental hygienist does.  Or, with liquid medications such as hydrogen peroxide or chlorhexidine applied with a special brushed applicator syringe.However, saying all that, THE VERY BEST METHOD THAT I KNOW OF TO DESTROY all the bacteria, fungus and viruses on the tooth is by using ozone gas.  Unfortunately, although this is an absolutely excellent method, less than 5% of dentists today utilize this technique yet.

In general all decay should be removed before placing or recementing a crown.  In very select situations you may leave some slight decay if removing it may result in exposing the nerve.  If your crown had a bad smell or taste as you said, then it is likely that it was decayed.  After removing all of the decay it is unlikely that the margins would be sealed adequately after recementing and thus it would just continue to decay. A properly done crown should not come off unless decay occurs at a margin leading to continued leakage until it losses enough retention and then comes off.