At a patients first appointment the dentist will prepare the tooth to receive a crown, this involves removal of cairies and potential fractured portions of a tooth, the dentist will then cut the tooth using a high-speed hand piece and various diamond burs. The outer layer of a tooth is removed from all surfaces of the crown of the tooth leaving a stump which is specifically shaped to receive and retain a laboratory made crown, the prepared tooth must have a shoulder or ledge cut into it in order to create sufficient retention and to allow the final crown to fit the tooth accurately.
If there is any bleeding from the cervical margin (where tooth meets gum) of the tooth the dentist may use a special kind of string called retraction chord which is soaked in an astringent and placed around the margin until bleeding is stopped, this also keeps the affected gum away from the prepared tooth tissue allowing for more accurate impressions, the retraction chord is removed before impressions are taken.
Impressions are then taken by the dentist using elastomer impression materials (as with inlay or onlay preparations), both light and heavy bodied, which accurately records the prepared tooth, its adjacent teeth and its opposing teeth. The dentist must also provide a record of the patients’ bite, which (as with inlays and onlays) can be done using either a specific elastomer impression material or a soft wax. If the crown to be made is tooth coloured the dentist will also use a shade guide to choose the correct colour for the restoration to be made in.
The impressions and a written prescription from the dentist requesting work to be done, provide the dental technician with the appropriate information required to make a crown.
A temporary crown is then placed by the dentist in order to retain the correct space between the prepared tooth, its adjacent teeth and opposing teeth, to protect the prepared tooth, and to maintain a reasonable appearance between appointments. Most dentists use a tooth coloured temporary material to make the temporary crown, although aluminium temporary crowns are available. Usually a dentist will take an impression of a tooth before it is prepared allowing a temporary crown to be made to the original shape of the tooth.
Temporary crown material is placed into this impression (after the preparation is completed), it is then replaced into the patients mouth until the temporary material sets, the impression is then removed from the patients mouth and the temporary crown removed from the impression, it is trimmed if necessary and is then fitted with a temporary cement. This process creates an exact replica of the patients’ original tooth, providing a good fit and appearance. If the tooth is too broken down before the preparation has been done to make a temporary crown in this way, a dentist can adapt the impression once set to create a more realistic appearance and appropriate shape of the temporary crown.
Second Dental Appointment
At the second appointment, the temporary crown is removed as is any excess temporary cement. The permanent crown is then placed (without cement) and the dentist will check the appearance, the fit and the bite of the crowned tooth. If the dentist is happy with the permanent crown it is cemented into place with either a glass ionomer cement or polycarboxylate cement. Once set, any excess cement is removed and the patient is free to use the tooth again for its normal function.
