The dental laboratory is the place where indirect dental restorations are practically manufactured. The laboratory stages, which in most cases are not visible to patients, require great skill and precision. The dental technician works in close collaboration with the dentist and sometime he may participate at certain clinical stages.
The dental technician receives from the dental office the impressions together with other important information: shade, type of restoration, case study impressions, sometimes pictures or videos etc. The first thing he must do is to create a dental cast for each impression.
The dental cast (dental mould or dental model) is the positive reproduction of a patient's teeth and surrounding tissues obtained from a dental impression (which is a negative imprint of teeth and soft tissues).
dental impression :
a prepared tooth is marked with an arrow
the dental cast obtained from the impression :
the prepared tooth is marked with an arrow
How is the dental cast constructed ?
Most often, the laboratory receives from the dental office a complete impression. A complete impression captures all teeth and is made up of 3 distinct impressions :
The impression of the upper arch - from this, the upper dental cast is obtained
The impression of the lower arch - from this, the lower dental cast is obtained.
Bite registration - this is used to position the 2 casts in a situation similar to the patient static occlusion ( or correct bite )
There are situations when a partial impression was made. Normally, a partial impression is used when small restorations are designed (crowns or small bridges).
a complete impression captures
all teeth from the dental arch
a partial impression captures only a part
(several teeth) of the dental arch
All impressions are thoroughly dried with air.
A specific plaster (gypsum) is prepared by mixing the powder and water together.
When gypsum reaches optimum consistency, it is poured inside the impression so it reaches all its details.
After the plaster sets, the impression is carefully removed from the cast.
Positioning the two dental casts
When both dental casts (upper and lower) are ready, they have to be positioned in the correct bite. This was recorded with the bite registration. For this operation, a specific device is utilized : the dental articulator.
An articulator is a mechanical device to which casts of the maxillary (upper) and mandibular (lower) teeth are fixed, reproducing recorded positions of the mandible in relation to the maxilla. Moreover, the articulator can mimic various movements of the mandible.
dental articulator before the casts are fixed
dental articulator after
the upper and lower casts are fixed
in the position recorded by the bite registration
Dental cast preparation
The dental casts are used by the technician in manufacturing the designed restorations. Basically, the dental crowns, bridges or removable dentures will be constructed on the dental cast and then fitted in the mouth by the dentist.
Before the modelling can begin, certain preparations are performed. The purpose of these operations is getting the best possible access to the prepared teeth during the various stages of the manufacturing procedure.
All plaster around abutment teeth is removed so that the technician has complete access to all portions of the teeth that were prepared (including the area bellow the gum).
Before the plaster is poured inside the impressions, small metal rods are inserted at the level of the abutment teeth.
After the plaster sets, the rods allow the abutment teeth to be removed and reposition on the dental cast. This gives the technician a very good view of all sides of the prepared teeth during the next stages.
The small imperfections on the surface of the plaster are closed using special waxes or other specific substances.
a prepared tooth on the dental cast
after all preparations are over
Other types of models
Besides the "working cast" on which the dental restoration is manufactured, other types of casts can be constructed.
Preliminary cast : obtained from the preliminary impression. This is used to manufacture the custom tray.
Case study model : obtained from the case study impression. It is used for observing various existing details prior to tooth preparation. Several study models may be constructed in various stages of the treatment.
Didactic or documentation cast : use for presenting various clinical cases at faculties, congresses, in scientific studies, forensic documents etc.
Last review and update: November 2017