What would be the cause of area under a bridged tooth feeling like it is deteriorating?

I had a root canal some 40 years ago. Finally that tooth had to be removed and I had a bridge placed there about 10 to 15 years ago. Recently it feels like the area where the tooth was is deteriorating, like the bone there is wasting away. There is a "dent" in that area where the root canal tooth had... show more I had a root canal some 40 years ago. Finally that tooth had to be removed and I had a bridge placed there about 10 to 15 years ago. Recently it feels like the area where the tooth was is deteriorating, like the bone there is wasting away. There is a "dent" in that area where the root canal tooth had been before. What could be causing this?
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