Dental Crowns And Bridges
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. Even when a single tooth is lost a denture may not be required as a bridge can be made which spans the gap with a tooth attached to those next to the space. Sometimes when a single small tooth is lost at the front of the mouth a simple bridge can be made which has ‘wings’ that are glued to the back of the adjacent teeth. This results in less tooth being damaged by the drill.
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, gold and porcelain being the most common types.
Gold is usually used on the back teeth, as it has several advantages over porcelain teeth:
- Gold is strong in thin section and less tooth needs to be drilled away before taking the impression and fitting the crown.
- The lab technician finds it easier to use gold as it shrinks less when cast and is easier to polish. Crowns of porcelain shrink when cast and the technician has to estimate this when making the crown.
Porcelain is usually always used for the front teeth but may be used for the back teeth. This material can be made to appear very natural though several factors affect this and are outlined below.
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.
A bridge allows the dentist to replace lost teeth without the use of a denture or dental implant. Basically a false tooth is held in place by being attached to a tooth next door. 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 compromise is the ‘acid etched bridge’, with this type a fine ledge is placed on the back of the adjacent teeth.
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). At the back of the mouth this may not be a problem, at the front of the mouth however this may appear as a black line along the gum.
When the top of a tooth is lost due to decay there may be very little for the crown to actually hold onto. To gain ‘retention’ as dentists call it, a post is placed inside the tooth which forms a peg on top of the tooth and acts as a seat for the crown. Post crowns have a shorter lifespan than normal crowns as the roots may be brittle and weaker. Usually a post crown is a better option than having the tooth extracted with a subsequent denture or bridge.
Normally a bridge requires the adjacent teeth to be prepared to accept the abutments of the bridge. This is destructive to these teeth especially if they have small or no fillings in them. A more conservative approach is a ‘Maryland bridge’ which uses wings that attach to the adjacent teeth. The disadvantage of these systems is that the life expectancy of the bridge is only 4-5 years when compared to the conventional type of 8 years.
How It’s Done
As a lot of tooth tissue may need to be removed your dentist will normally give you a local anaesthetic (injection) before starting treatment.
|This tooth is heavily decayed and needs a crown to prevent the top part of the tooth beaking down completely. The decay is removed and any holes are filled with amalgam or a white filling material.|
|The tooth is then prepared using the drill. A ledge is made around the tooth and the top is cut down to make space for the gold. After this the dentist will take an impression of the tooth which is then sent to the laboratory. A model of the tooth is made from this and a crown made to fit.|
|The crown is then fitted. A special cement is used to bond it to the underlying tooth. The dentist may need to adjust it before the patient leaves the surgery so that the bite is just right.|
|This diagram shows what a tooth looks like after it has been prepared. The notches cut in the side help the crown stay on when its fitted. These are called retention grooves.|