When teeth are heavily decayed they may be too weak to survive with just a filling. By placing a crown, a tooth may be given a new lease of life.
When a tooth is heavily decayed it is more likely to fracture and lose considerable tooth substance. The greater risk is that a fracture line may go below the margin of the gum which makes restoration more difficult or near impossible.
Crowns may be made from several different materials, porcelain being the most common type. Some porcelain crowns have metal inside which gives strength and support to the porcelain. This acts as a barrier for light and gives the crown a dull colour. On posterior teeth this won't be noticed but in the front of the mouth it can occasionally be a problem as the tooth looks duller than the natural teeth. Therefore all ceramic crown have gained wide importance over the last few years. With the advent of metal free zirconia crowns, there has been a revolution in esthetics as well as functional ability of the patient.
A bridge allows the dentist to replace lost teeth without the use of a denture or dental implant. Basically the adjacent teeth on either are shaped in a miniature form to recieve joint teeth also called as a bridge. The latter is fabricated by a dental laboratory and fixed by the dentist. The disadvantage is that the teeth next to the space have to be prepared in a similiar way to a crown in order to accept the bridge. If these teeth already have crowns or big restorations then this is not a problem, the major concern however is when these teeth have small or no restorations (fillings).
One disadvantage of a bridge is that the patient should wait three months before placement as the 'gum' shrinks' after a tooth is extracted. If the bridge was fitted early a gap would appear underneath the pontic (the false tooth).