A crown is a laboratory construction, artificial restoration that replaces at least three-quarters of the natural tooth. There are various types, made of various materials, and they require at least two visits.
At the first visit, the tooth is prepared, using diamond discs, wheels, and tapered burs. An elastomer impression and occlusion records are then taken and, for anterior teeth, the shade is recorded. A temporary crow is then cemented and the patient is dismissed. In practice, it is the best to prepare the temporary crown before taking the impression, so that it is ready for immediate cementation at the end of the visit.
At the second visit, the temporary crown is removed and the permanent one positioned. It is then checked for the appearance and occlusion before cementing with ordinary or dual-cure GIC, zinc phosphate, or polycarboxylate. Finally, any excess cement is removed with hand instruments.
- Temporary crowns are used for the following reasons:
- To maintain appearance
- To prevent sensitivity between visits
- To maintain the correct space between adjacent teeth
- To maintain the correct occlusion between opposing teeth
A jacket crown is used for anterior teeth, which are too mutilated to be restored by ordinary fillings, and include cases with very extensive caries, fracture of the crown, and severe pitting, discoloration, or deformation of a crown.
When a crown is required on a tooth, which has been root-filled, a post crown is generally used. The filled root canal is prepared to accept the appropriate type of post, and as much of the natural crown dentine as possible is retained as the crown core. If there is insufficient core dentine left, it is built up with composite until the core is large enough to support the crown.
The post may be a gold casting or performed in stainless steel, or a tooth-coloured fibre-reinforced composite.
A veneer crown is a thin gold (or palladium alloy) shell used in the construction of bridges. On posterior teeth, it covers the entire crown and is called a full gold crown, while on anterior teeth it covers all but the labial surface and is called a three-quarter crown.
Apart from bridges, full veneer crowns are also used to restore teeth, which are unsuitable for amalgam, such as badly broken down teeth and split teeth. In the latter case, the crown acts as splint. On badly broken down teeth the full crown forms a protective shell covering the amalgam or composite core that restores most of the bulk of the natural crown.