The web shows endless examples of tooth crown preparation onto teeth with 100% of tooth still far above gum-line. My tooth in contrast is half lost, down to the gum-line. The other half is intact. How to attach a crown on the side where nothing can be prepped? Do they simply cement it directly onto the broken half with minimal preparation? They can still prep and shape the unbroken half of the tooth. Is such irregular shaped crowns normal? Is there greater risk of decay under the crown?