What is a crown?
A crown is a laboratory made tooth shaped cap.
They may be recommended when a tooth is so heavily broken down that a conventional filling will not be strong enough or is not cosmetically suitable.
Crowns are the strongest restoration which a dentist can put on your tooth, cosmetically they give a good result as they cover the whole tooth and they are better than what can be made by the dentist during a filling session.
Can crowns be made from different materials?
Yes, crowns can be made from several different materials. Depending on where in the mouth the tooth is will depend on what material is used. There are three main types of crown.
These are placed where appearance is important and loads on the teeth are low, usually front teeth. Recent advances in crown technology now mean that they can also be placed on back teeth.
Porcelain crowns are brittle and can fracture, when this occurs they usually need to be replaced.
The porcelain also causes wear on the opposing teeth.
Porcelain and Metal Crowns
These are placed where appearance is important but where the teeth also are required to withstand chewing forces.
More of the tooth has to be removed as a layer of metal is first laid down on the tooth before it is covered with porcelain.
These crowns are stronger, however sometimes the layer of porcelain breaks off the layer of metal and although the tooth is functionally fine, visually it looks untidy. Also the porcelain will wear the opposing teeth
These are placed on back teeth where high forces are generated. Gold has excellent strength, does not wear the opposing teeth, and only requires a minimum of tooth removal.
What are the disadvantages of crowns?
Often to prepare the crown a lot of healthy tooth may need to be removed. If the underlying filling is large and deep sometimes the nerve of the tooth will die (the nerve would probably have died if a large filling had been placed), the tooth will need a root canal therapy which is made more difficult with the crown being in place.
As the laboratory is involved the costs are higher and at least two visits are required (one to prepare and one to fit the crown). Getting the correct colour can be difficult especially if the crown is on a single front tooth.|
Crowns fail for many different reasons; such as the remaining natural tooth and filling breaks away from the root of the tooth, decay is allowed to form around the outside of the crown, the nerve of the tooth under the crown dies (see Root Canal Treatment), the porcelain of a crown fractures off (this may or may not make it necessary to replace the crown)
A crown is not a life long fix for a tooth. Although it is the strongest fix for a tooth, how long a crown will last depends on a number of factors such as how much natural tooth remains, the force applied to the crown while eating, and how clean the crown is kept. Many of these are not in the control of the dentist.
What is the process for making a crown?
Visit one- the preparation.
Usual time 60 to 90 minutes
Anaesthetic will usually be applied.
The initial condition of the tooth will determine what happens first.
If the tooth is heavily broken down the dentist will remove all the old fillings and place a new one. If this filling is extremely large or deep the dentist may choose to leave the tooth for a period of time until it is certain that the nerve of the tooth will not die.
Assuming the dentist is happy to continue, the tooth will then be reduced in size on all its sides. The end shape will be a smaller sized cone shape.
Impressions are then taken. An impression tray is loaded up with material and then placed in the mouth around the tooth until the material sets. This could take up to 5 minutes. It is very important that an accurate impression is taken and it can take several visits to achieve this.
When the dentist is happy with the impression a temporary crown is then made. The temporary crown is cemented on the tooth with temporary cement.
This is one of the hardest parts of making a crown. Often the dentist will ask you to go directly to the laboratory so they can decide what colour or colours should be used- you will be amazed at how many different colours of white there are and also how many different colours there are in a tooth!
Visit two –the fitting.
Usual time 30 minutes
Anaesthetic is usually applied.
The temporary crown is removed and the new crown is seated on the tooth.
You and the dentist will then decide if it acceptable to cement on.
• Is the appearance acceptable?
• Does it sit on the tooth correctly?
• Is the bite acceptable?
There are some minor adjustments which can be made at the chair side, however if you or the dentist are not happy then the crown is not cemented. Instead the appropriate steps are taken which may require another impression being taken
Providing everyone is happy the crown is then cemented on to the tooth.