Engineering phases - Restorative dentistry
We have already seen that the dental laboratory pours the prints into hard plaster. The stub (s) prepared for crowns are provided with a wax pattern by the dental technician. A wax pattern is actually a copy of the crown to be manufactured.
The metal restorations for crown and bridge work are then cast. To this end, the manufactured wax cartridge is placed in a special cylinder. This cylinder is filled with a special embedding compound (comparable to plaster). The wax pattern is therefore immovably anchored in the mass after curing of the embedding mass. The cylinder is placed in the oven and the wax burns through heating, but the embedding mass remains unchanged.
The space originally occupied by the wax pattern in the embedding mass is now empty. In this space, a metal is poured through a pre-arranged access channel in the embedding compound and after cooling the casting is ready. The casting is removed from the investment material and, in the case of a full metal crown, cleaned and polished by the technician. If it is a metal-porcelain crown, porcelain is baked on the desired metal part on the desired surfaces. This is done in thin layers and in the desired color. The crown is then polished.
In the case of a whole porcelain crown, porcelain is layered by the technician on the working model on the stump.
All-metal crowns, metal parts of a metal-porcelain crown or bridge, as well as superstructures, produced by the direct or indirect method, are cast (see figure below).
Engineering phases. a On the plaster model a restoration is modeled in wax. This wax model is placed on a rubber foot with a casting pen. b Cast steel cylinder. c The cylinder is placed over the wax model; a rubber cap is placed on top of the casting cylinder. d The casting cylinder is filled with embedding plaster, the casting pin has been removed (this is now the casting channel), the rubber foot and cap have been removed. The cylinder is placed in an oven from which the wax is burned. e The hollow space in the embedding plaster is filled with metal. After this, the embedding mass is removed, the casting pin is sawn off and the restoration is polished.
Recent developments in crown manufacturing include scanning the preparation, digitally designing the crown and milling the crown.
- Nowadays, instead of making a spray print, it is also possible to scan the preparation in the mouth. The digital scanner transmits information to a computer, which creates a three-dimensional image with the help of a special program. A crown can be designed on this, again digitally.
- The digital crown design is passed on to a milling machine, which mills a crown from a block of ceramic material.
With this system, the mouth scan can be sent by e-mail to the dental technician. There are also practices that have a milling machine themselves. After the mouth scan, the patient can wait in the waiting room until his crown is designed and milled. Printing with print paste and the production of an emergency crown are no longer necessary.
A plaster model can also be scanned by the dental technician, so that he can design and mill the crown digitally (see image below).
a Device for milling ceramic crowns and caps. b Several crowns and caps are milled from a ceramic block.