Lecture 6----------------------------------------------------------------crown and bridges
Impression techniques: د.احمد غانم
1. Single stage technique.
2. Two stages technique.
3. Putty wash technique.
1. Single stage technique:
Most of time we use this technique when we have impression material with single type of viscosity (mostly: medium viscosity material) e.g. polyether. After we mix the impression material , a part of impression material loaded to the impression syringe from the mixing pad ,& we place the reaming material in the special tray , by the impression syringe we inject the material over preparation , we start from the most critical part(pin holes, finishing lines) followed by the remaining part of the dental arch , after this application is completed we place the special tray over the dental arch , waiting for complete set, then removed &should be inspected then poured .
2. Two stages technique:
This technique is followed with materials those have two viscosities (light & heavy consistency) we mix separately the two types at the same time. The light body type is loaded in the impression syringe; on the other hand the heavy body is loaded to the special tray, then we inject the light body type on the dental arch starting with prepared teeth, after finish we seat the special tray with its content into the patient 's mouth .the pressure created by the heavy body will create intimate contact between the light body & the prepared tooth & will make direct flow of the light body into the preparation .
3. Putty wash technique:
In this technique we use a high viscosity material, we start to take impression with heavy body before tooth preparation , we remove the impression after it set inside the patient's mouth after completion of the preparation we start to mix the light body & load it in the impression syringe & then we inject it over the prepared area & the surrounding dental arch followed by reseating of the tray with previous impression inside the patient's mouth & we wait until setting occurs, on the other hand if we do the impression after the preparation is done, we use it with spacer made of polyethylene material, placed over the heavy body & inserted it inside the patients mouth & wait until setting , then removed the polyethylene spacer .
***All the techniques mentioned can be used in any type of preparation but it depend on the type of the material,
Impression for the post crown:
we need duplication for the root canal space so it’s difficult to insert the impression material inside the tiny canal & even when it fills the canal it may tear during removal or there may be distortion during pouring die material ( working cast ) for that we use a type of reinforcement either by plastic pin or using stainless steel wire pushed inside the canal & each of them are inserted after injecting the impression material inside the canal , we remove the canal impression after setting , this technique is used to prevent the impression not to be torn or distorted , we make a scratches on the surface of the wire by the bur in order not to be pulled away from the impression.
Inspection of the completed impression:
After the impression is completed we must inspect it for the following points:
1. The finishing line replica must be continuous from one surface to another.
2. No air bobbles present on the surface of the impression, especially at the area of the finishing line.
3. The attachment of the impression to the tray must be firm.
To have a good impression we must have:
1. Special tray: to reduce the volume i.e. thickness so; reducing the polymerization shrinkage & the thermal contraction.
2. Good knowledge about the physical properties of the impression material.
3. Dry field during taking the impression
4. It is necessary to do gingival retraction.
It's important that the prepared tooth should be protected and the patient kept comfort while the restoration being fabricated.
The temporary crown must completely cover the prepared teeth without applying any pressure on the gingival tissue (without any over extension).
It must restore the shape of the natural teeth with good proximal and occlusal relationship to prevent the tilting of the adjacent teeth and over eruption of the opposing teeth.
It should be able to withstand the force to which it subjected and have some strength and retention.
The margins should be well adapted to the finishing line to provide good marginal seal.
It must be done from the material that prevent the conduction of heat and cold and other chemicals of the mouth (not harmful to the pulp).
Protect the prepared teeth from fracture.
Function and esthetic.
Protection of gingival tissue from irritation and prevent over growth of the gingiva.
Facilitate plaque removal from the area.
Types of temporary crowns:
Preformed temporary crowns.
Chair side temporary crowns.
Laboratory made temporary crowns.
1. Preformed temporary crowns:
A. Metal crowns: Mainly used, it comes in different shapes and sizes made of stainless steel or aluminum. The Aluminum T.C. is either anatomical or non anatomical types.
Technique for application of metal T.C.:
1. Select the proper size which fit the prepared tooth.
2. Trim (cut) the gingival margin with scissor to confirm it to the gingival finishing line of the preparation & to accommodate the vertical height of the occlusion.
3. Seat the T.C. on the prepared tooth inside the patient's mouth & check the margin, occlusion & the proximal relation with opposing & adjacent teeth, if it fit properly.
4. Remove the T.C. from the patient mouth; smooth the margin with stone bur.
5. Cement T.C. with ZOE cement on the prepared tooth.
B. Plastic crowns:
Celluloid direct or indirect.
1. Select the proper size of T.C.
2. Try the crown on the prepared tooth if it is not fit on the prepared tooth , you should select the most suitable size T.C. & follow the steps.
3. Cut the margin of the crown according to the finishing line.
4. Put a thin layer of separating medium on the preparation.
5. Mix cold cure acrylic.
6. Fill the crown with acrylic & seat it on the prepared tooth.
7. Remove the crown before it completely set; this is due to polymerization reaction of acrylic which is exothermic reaction & this irritant the pulp
(On the polymerization reaction there is shrinkage in the acrylic).
8. Seat the crown on the prepared tooth.
9. Check the high spots.
10. Relief should be done from the inside of the T.C.
Is done on the study cast & preferred over the direct one because of the accuracy & done away from the pulp and oral tissues.
2. Chair side temporary crowns:
The technique called direct costumed temporary crown or over impression:
An alginate impression is made resulting in diagnostic cast while waiting for anesthesia before starting the preparation.
When the tooth preparation completed, another alginate impression is made and poured in quick set plaster.
Coat the prepared tooth with separating medium, a colored acrylic is mixed and loaded in the impression.
Reseat the alginate impression with acrylic inside the patient mouth on the prepared tooth.
Remove the impression before complete polymerization otherwise it can't be removed from the patient mouth due to shrinkage.
Check the occlusion and the margins, cement it.
3. Laboratory made temporary crowns:
A preoperative impression is made, and we prepare the tooth on the cast, and temporary crown or bridge is done.
Post crown temporary restoration:
It is often difficult to fabricate T.C for a tooth which have been prepared to receive post crown, because there is so little of tooth structure left standing supragingivally, in such case a piece of paper clip or stainless steel wire can be placed & adapted to the prepared canal then select ordinary readymade T.C (acrylic, polycarbonate or celluloid) trim it & adapt it to the prepared tooth. T.C was then filled with a cold cure acrylic (tooth colored type). The acrylic filled the temporary crown were then seated on the prepared tooth & over the wire. Remove the T.C (with wire attach to it) after complete setting of the acrylic resin .remove any excess acrylic & cement the T.C.
Cementing of the T.C. or Bridge: Zinc oxide eugenol is the most commonly used cement as cementing medium for T.C and T. Bridge, this cement provide less irritation to the pulp & allow easy removal of the temporary restoration.