This operation allows the dentist to make a record of the mouth and to transfer on a lab model the work that's been done on the tooth after the filing.
As a matter of facts, a fixed prosthetics is built in a lab by a technician who is working on a patient's mouth copy made from chalk cast in the silicon impression taken by the dentist.
A) The tray
This is usually a metal container, grossly shaped like an arch and designed in various sizes to contain the impression paste and carry it into the mouth. They can have holes or not, depending on preferences in technique or on the impression materials used.
In special cases, custom trays can be built to better fit the patients' anatomy: in this case they are made of resin.
B) The impression material
In fixed prosthetics, mainly two kinds of impression materials are used: silicone (polivynilsiloxane) and poliether. Both can be used with a single or with a double impression technique.
The professional chooses the preferred material from his experience and according to the agreements taken with the lab, to optimize the various steps and to reduce the error margin.
C) The retraction thread
It's a small fiber that is introduced between the tooth and the gum surrounding it at the collar and its aim is to keep temporarily the gum displaced from the pillar, enlarging the gingival sulcus and allowing the impression material to penetrate in the thus created space, neatly recording the prosthetic filing (preparation) that has been done. A single or a double thread can be used: in the first case it must be removed a moment before introducing the impression tray, in the second case one of the two threads may stay in place during the impression and be removed only afterwards.
D) The technique
It simply consists in introducing the tray loaded with impression material into the mouth and submerge the dental arch completely. Holding the tray appropriately, the dentist waits for the material to harden and then carefully lifts it out of the mouth. A silicon or a poliether approximately have a hardening time variable between 2 and 5 minutes, depending on consistency and temperature.
During this phase, especially if taking an impression of the upper denture, the patient may experience the uneasy sensation of vomit. The dentist will suggest to calmly breathe through the nose and sometimes will help the patient by putting an alcohol soaked cotton wad under the nose.