W4. Impression taking for fix appliences, sectional models

Question 1

1. What does it mean when we take Impression?

Answer 1

soft, semifluid material placed in the mouth and allowing that material to set

Question 2

2. What are the 3 requirements of impression?

Answer 2

1. It should be an exact duplication of the prepared tooth, including all of the preparation, and enough uncut tooth surface beyond the preparation to allow the dentist and technician to be certain of the location and configuration of the finish line.
2. Other teeth and tissue adjacent to the prepared tooth must be accurately reproduct to permit accurate articulation of the cast and to allow proper contouring of the restoration.
3. The impression of the preparation must be bubble free, especially in the area of the finish line.

Question 3

3. What is Die-cast impression for FPD?

Answer 3

1. Impresson for die= impression of the prepared surfaces and finishing line
2. Impression for cast = approximal contours and correct occlusion

Question 4

4. Why is it important for impression materials to adhere firmly to the impression trays?

Answer 4

1. It is essential that impression materials adhere firmly to the impression trays.
2. perforations or by the use of adhesives

Question 5

5. What is the two cord technique?

Answer 5

1. A piece of fine retraction cord is placed in the gingival sulcus
2. A thicker cord is placed over the first leaving a tag for removal
3. The thicker cord is removed after washing (note clearly defined sulcus) and impression
4. The buccal tissues are relatively thin and great care is needed to avoid recession.
5. Sufficient haemostasis can usually be achieved with ferric sulphate solution to allow the impression to be recorded

Question 6

6. What are the roles of pastes and gels of Gingival Retraction?

Answer 6

1. paste used for gingival retraction opens the sulcus, physically displacing the tissue and leaving the field dry,
2. place it directly into the sulcus, leave it for one to two minutes, then rinse. When you use it before taking the final impression,
3. little or no pressure required to apply, that minimizes the risk of rupturing the epithelial attachment.
4. bleeding and crevicular seepage are controlled through the presence of aluminum chloride.

Question 7

7A. For FPD, What are the characteristics of elastic impression materials?

Answer 7

These materials can be stretched and bent to a fairly large degree without suffering any deformation.
- Used for recording the patient’s mouth where undercuts are present.

Question 8

7B. For FPD, What are the examples of elastic impression materials?

Answer 8

1.Hydrocolloid:
- Alginate
2. Synthetic elastomers:
- Polysulphide
- Polyether
- Silicone

Question 9

7C. For FPD, What are the characteristics and examples of elastic impression materials?

Answer 9

Non undercut areas
- Impression
- Plaster
- Compound
- Zinc-oxide eugenol paste
- Wax

Question 10

8. Describe Opposing arch impression-Alginate

Answer 10

• hydrocolloid impression materials:
• poor dimension stability and
• low tear resistance

Question 11

9A. What is the single mix - impression technique?

Answer 11

• Regular viscosity - intermediate flow
• Monophase pastes/ mono phase technique
• Custom tray

Question 12

9B. What is the double mix - impression technique?

Answer 12

1. Low viscosity
   - correction material or wash
2. High viscosity
   - heavy-bodied or putty or tray material or preimpression material
   +) One stage
   +) Two stage

Question 13

10. What is the procedure of escape channels?

Answer 13

1. Cut back impression material to the borders of the tray
2. Remove undercuts
3. Remove material between unprepared teeth
4. Cut in grooves up to the occlusal level at unprepared teeth alternating orally and buccally/facially 2mm wide und max. 1mm deep

Question 14

11A. Explain Double mix - ONE STAGE IMPRESSION technique

Answer 14

• Light bodied impression material is placed in a syringe, and placed over the areas where high detail is required (e.g. over a crown preparation).
• Some is then squirted over the heavy-bodied impression material which has been loaded into an impression tray.
• The impression is then taken as normal.
  => This technique saves time, but it can be very labour intensive because the two need to mixed at the same time.

Question 15

11B. What are visible flaws of Double mix - ONE STAGE IMPRESSION technique?

Answer 15

Visible flaws related to impression technique which occur commonly include:
- Finish line not visible
- Air bubbles in critical places
- Voids or drags
- Unset impression material on surface of impression and cast

Question 16

11C. What are invisible flaws of Double mix - ONE STAGE IMPRESSION technique?

Answer 16

Invisible impression flaws, resulting in an apparently good fit of the restoration on the die but a poor fit on the tooth, may also occur because of:
- Tray and impression recoil Detachment of impression from tray Permanent deformation

Question 17

12. What is cast?

Answer 17

the positive likeness of a patient’s upper or lower
jaw

Question 18

13A. What is die?

Answer 18

small model of prepared teeth jaw
- The dies of the prepared teeth or roots can be removed separately from the cast to make a very accurate or precise wax pattern for making a fixed dental appliance.

Question 19

13B. What are the 3 materials for preparing dies?

Answer 19

1. die stone: most widely used
2. Acrylic
3. Polyurethan

Question 20

13C. What is the function of preparing dies?

Answer 20

Essential function of the die is to allow a restoration to be constructed in the laboratory which has an accurate internal or fitting surface.

Question 21

14A. How do we make die casts?

Answer 21

• the base of the die provides a means of locating the die within a larger localizing model which includes the other teeth on the arch adjacent to the prepared tooth.
• the die can be removed and replaced in its original position

Question 22

14B. What are the 3 method used for making sectional casts?

Answer 22

1. Pindex method
2. Kiefer method
3. Crystal method