If the natural crown of a tooth is too small to receive and retain a laboratory made crown, either due to fracture or because of large cairies being removed, it is necessary for a core to be built up for the crown to be placed over.
A core is basically a restoration which mimics a natural tooth that has been prepared for a crown.
Dentists can build up a core using either glass ionomer cement or composite, as both of these materials can effectively bond to dentine. Because the core will be covered with a permanent crown it is unlikely that the core will fail due to forces it endures in its normal function. There must be a sufficient amount of natural tooth tissue present in order for this procedure to be a viable option.
If a tooth is fractured at gum level, leaving none of the natural crown available to prepare for a crown, it is possible to have a root filling placed and a post retained crown. A post retained crown is also an option for teeth which have already had a root filling placed, however, as all root filled teeth become brittle over time most dentists will place a crown routinely on these teeth once they are satisfied that the root filling has been successful. Post crowns are most often used on anterior teeth, but are occasionally used on posterior teeth however, the long term prognosis of these can be poor.
A post can either be a pre- made, fairly flexible type, which is made of fibre reinforced composite or can be made of metal, such as gold or stainless steel, which is specifically and individually made by a dental technician.
The post is placed into a prepared canal within the remaining tooth tissue; this usually involves the area of the tooth that would normally house the nerves and blood vessels – the pulp or root canal, which is why it is important that the tooth has a root filling before a post is considered, without one the tooth would become non-vital (die off) and be painful, if untreated would also become infected.
Once the dentist has made appropriate measurements, e.g. length of root canal, the canal made to receive the post is created using a low speed handpiece and specifically shaped and sized burs.
If a fibre reinforced composite post is to be placed this can be done in surgery, and once the correct size has been chosen it can be cemented into place using a specific cement which is usually light cured and is provided with the post kit. When this type of post is used a dentist can build up a core around the post and then prepare it as they would with a natural tooth. One advantage of these posts is that they are usually transparent or dentine coloured, therefore if a ceramic crown for example is placed the colour of the post will not darken the appearance of the final crown.
If a metal post is to be placed the dentist prepares the post canal in the same way as they would for a fibre reinforced composite post but then places a plastic coloured temporary post into the canal and an impression is taken over this, this post is held in the set impression allowing the dental technician to see the size, length and angle of the canal for the post to be made for. A burnout post can also be provided for the technician to use but this is not necessary as technicians keep these in stock in the laboratory.
Posts made by the dental technician also include a metal core which the crown is then fitted on to. They are cemented into place using an appropriate luting cement such as polycarboxylate cement or a reinforced glass ionomer such as Ketac cement. Metal posts and cores are usually covered with bonded crowns therefore the colour of the metal does not affect the colour of the permanent crown.
Once the core or post canals have been prepared the dentist finishes the procedure as they would if doing a straightforward crown preparation (see in surgery procedure for crowns).