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Please click here to read more about metal alloys used for PFM crowns and bridges


PFM Crowns
PFM
stands for porcelain fused to metal. The base of the crown is made from a metal alloy and covered with a porcelain.
After the doctor takes an impression of the prepared tooth in the office, he sends this impression to the dental lab.

  • First the lab technician will pour the models from the impression from dental stone or plaster. More than one model could be poured for further use in different steps of production. Dental stone is agypsum product that, when combined with water in proper proportions, hardens in a plaster-like form. The model is poured the certain way, making certain parts removable, thus allowing dental technician with easier access to the tooth. This model with the removable parts is called dental die. Using of high quality products is very important, as lower grade plasters shrink too much during the pour and if it's not hard enough it will scrape too easily during the siting step and lead to discrepancies between the model and mouth cavity.
  • The next step is to open or trim the die. Using the microscope ( sorry to say, but not all technicians do that) the technician trims the die, up to the margins of the preparation. That's why the good impression done by doctor  is a key is successful restoration. If the margins of the tooth preparation are not clearly seen on the impression, the margins on the crown will be according to the dental technician guess, and this guess could be far from the truth. It takes a lot of experience and knowledge of a technician to trim the die correctly.
  • Next step is topaint the trimmed die with the special spacer ( something what resembles nail polish ). The spacer is painted on the die to allow the space for the cement the doctor will use at the office.
  • The special dental wax is used to model the base of the crown. this process is called wax up. Technician has to imagine the finished product, as the minimum thickness of the metal should be 0.5 mm and the ideal thickness of porcelain should be 1 mm. In more cosmetic areas and areas where not much stress put on the tooth slightly thicker porcelain layer is acceptable (1.5 -1.8 mm). Not following this guidelines can lead to chipped porcelain once the crown is cemented in. Basically what the dental technician should do is to build the whole tooth with the wax and cut back to allow the ideal thickness of porcelain. ( how many problems would be resolved if they all do that!!!)
  • After wax up is done, the wax pattern carefully removed from a die, invested and casted from a metal of choice. To read more about dental alloys click here.
  • The casting then removed and fitted back on the die, fit and the thickness of casted copping verified. One of the mistakes which I have noticed  is that the technician would try to make thickness of metal 0.5 ( or even less) all around the copping, forgetting that the porcelain thickness shouldn't exceed 1.8 mm. After the metal was sited it is washed with acids, sandblasted and transfered to the porcelain department.
  • The porcelain is applied on metal copping in layers. Ceramics used in PFM is usually supplied in powder which is mixed with water and then fired at a high temperature. The ceramic powder comes in different shades to allow fabricate the custom porcelain restoration which matches the rest of the teeth. The first layer placed on the metal copping of the PFM crown is the opaque layer. It's special porcelain layer which fuses to metal, allowing for the bond between the metal copping and porcelain. Another reason of the opaque layer is to mask dark metal under the translucent layers of porcelain to prevent from dark metal to shine from porcelain.
  • After the the opaque layer was baked on porcelain, the tecnician using the mixture of different shades of porcelain builds up the crown. This process will involve more than one bake as porcelain shrinks during the firring process. After all the porcelain has been build up, the technician finishes and shapes the crown with the bur to make it look like a natural tooth. The use of second solid model is highly advisible at the final stage to proper adjust the contacts and the oclusion (height) of the PFM crown.
  • The final step in the process is glazing and staining of the crown. The glaze as some stain (paint) is apliied to the crown, and the final firring is done. PFM crown is ready to go back to the dentist.