Structure for the benefit of crown and bridge work - Restorative dentistry
Structure for the benefit of crown and bridge work
If an element is affected and weakened by caries and / or trauma in such a way that there is insufficient support for a crown, the stump (the part that remains after caries removal) must be built up. It is important here whether the element is vital or avital.
Construction of a vital element
A vital element can be built with glass ionomer cement or composite. If there is insufficient support for the restoration material on the residual stump, umbrella pins can be used. These are pins that are fixed in the dentine. With a special drill a channel is made in the dentine, into which the pin is screwed. A matrix band is then placed around the stump and further built up with amalgam or composite (see image below).
Construction of an avital element
With an avital element, if there is insufficient support for the material with which one wants to build up the stump, one can use the present root canals. Carrot sticks can be made suitable for this. Before we start constructing, we assess the amount of residual stump. If this is still reasonable, you can choose to cement a root pin in the channel and then build the element up with GIC or composite. We then speak of a root pin with a plastic structure.
When there is almost no crown remnant material left, the decision is made to have a cast post built up. All this is made in the laboratory. The pin and the superstructure form a whole.
Structure with root pin and plastic filling material
There are various root pin systems made of metal and fiberglass on the market. Markers are supplied by the manufacturers with corresponding drills of the same diameter. The root pin can be provided with ribs or grooves for retention of the plastic filling material. With the root canal drill in the green corner piece, the channel is drilled to the required length, so that the matching pin fits firmly into the channel. The depth of the root pin in the channel is determined by the thickness of the root and the channel closure. It is recommended to leave 5 to 7 mm of the channel filling.
If the pin is made to fit into the root canal, the pin is fixed with cement. A matrix tape is then applied around the stump and the stump is constructed with a plastic filling material (see image below).
Structure with root pin and plastic material.
Superstructure with root pin and cast superstructure
The molded structure is made in the laboratory on a model of the residual stump. There are two ways to offer a model for the superstructure to the laboratory: the model for the superstructure is made in the mouth (direct method) or an impression is taken of the residual stump with the pin in the root canal (indirect method).
- With the direct method, the root canal is first prepared to make a pin fit. The pin can be a metal pin from a root pin system, a plastic pin or a self-made pin. The pin is placed in the channel. With the help of synthetic resin, a synthetic resin structure is made on this pin in the mouth. The element is then filled with a matrix band filled with red self-polymerizing synthetic resin. After the resin has hardened, the resin is ground until an ideal preparation form for the crown is reached. The superstructure, consisting of marker and synthetic resin, is removed from the element and sent to the laboratory. In the laboratory, the marker and the red synthetic resin are replaced by a metal that is cast on the marker (see images below).
Making a molded structure according to the direct method. a The element has undergone an endodontic treatment and offers too little retention and resistance to a crown. A structure must be made. b The filling has been removed and the root canal is prepared. c A matching root pin has been selected. Self-polymerizing synthetic resin is attached to this and the desired crown preparation is roughly polished (dotted). Small spaces that are not completely filled are filled with wax (dark). d The shape obtained in this way is removed from the element and sent to the engineering laboratory. e The structure is cast in one piece in metal and can now be cemented into the element.
Synthetic resin construction as sent to the dental technician.
- In the indirect method, a sprayed print is taken, identical to the way described in the manufacture of a crown. The difference is that the impression is taken with the stylus in the root canal. In order to give the post sufficient hold in the impression material, so that the post remains in the impression after hardening and a reproduction of the prepared root canal is obtained, a ball of adhesive wax is melted at the end. The print is sent and after pouring out, you have an exact representation of the situation in your mouth. The dental technician models a structure of wax. This wax pattern is poured in metal (see images below).
Making a molded structure according to the indirect method. The root canal is prepared and a suitable stylus has been selected. The marker is placed in the channel and a sprayed print is made of this situation. To ensure that the marker remains in the impression, a ball of adhesive wax is melted on the marker. The print is sent to the laboratory, where a molded structure is made.
Spray print for a cast structure according to the indirect method.
As long as the cast structure has not returned from the laboratory, the residual stump must be protected. This is done by making an emergency crown on the stump, with an aluminum pin as a hold in the root canal.
The cast superstructure comes back and in the next session the emergency crown is removed and the superstructure on the stump cemented with glass ionomer cement. Since the old emergency crown no longer fits, a new one must be manufactured. After the superstructure has been placed, a new print must be made for the crown, as described in the crown procedure.