When my crown came off, my dentist took x-rays with it in place and saw a tiny area on one edge where there was a gap. The crown is at least 15 years old. There was no decay on the tooth. The dentist said it was an area where decay might eventually occur, that the gap was larger than previously seen, and that I should replace the crown. How would the gap occur, why would it get larger if there is no decay, and why shouldn't I wait to see if there should be some decay before getting it replaced?