Tooth cracked under crown?

Years ago, #19 had a large amalgum restoration.  About 5 years ago, it cracked and a permanent crown was placed.  About 6 months ago I started experiencing aching in that tooth when chewing.  2 months ago the pain progressed to a sharp pain that is worse when the bite is released than when I bite down.   I have seen my dentist twice in the last 6 months.  Each time he adjusted my night guard and tooth #19, but it is not getting better.  My last visit was one week ago.  How long should I wait before insisting on further evaluation?  X-rays were negative.

[love1221]

You should go back and tell him that the tooth still isn't getting any better.  X-rays aren't always the best for detecting problems under crowns, as you can't see through the metal in the crown.  There is a possibility that the whole crack wasn't removed from the tooth and it has started to crack further.