I had a root canal and crown put on my left lower molar about 4 years ago. Went for my cleaning/checkup today to be told that my crown is showing bacteria that is "eating" away at my tooth nub and eventually will break off and so more damage to my jaw bone if left untreated! My options at this point are to pull tooth, bridge or implant. One, how does this happen? Two, they say implant is my best option which happens to be most expensive. Is this true?