Each one of your teeth is made up of a crown and a root. The crown is the white portion you see in your mouth, the root is the portion hidden in your gum. If the natural crown of your tooth becomes badly damaged through for example tooth decay, whereby it is heavily filled, or through an accident, it may need to be replaced with a laboratory made crown. The crown is made to fit preciscely over the specifically prepared remaining portion of the natural crown of the tooth and the natural root is retained. It is made of metal, ceramic or a combination of both depending on the requirements for that tooth. Ceramic or porcelain is used towards the front of the mouth where aesthetics are really important. Often a combination of both metal and ceramic, whereby the metal is covered with a layer of ceramic, is used towards the back of the mouth where additional strength is required. Crowns can also be screwed into or cemented onto implants.
Root treated teeth are often crowned because these teeth have usually been extensively damaged by tooth decay and they are also more brittle than non-root treated teeth and so a crown may be required to protect what remains of the tooth.
Teeth do not necessarily require root treatments prior to crowning.