Week 1 - Primary Impressions

Question 1

What is an impression

Answer 1

a negative replica of a subject

Question 2

What are guidelines to achieve a successful full impression

Answer 2

1. Oral tissue should be healthy
2. Maximum extension within functional and anatomical limitations
3. Physiological type of border molding should be performed by the patient or dentist
4. Suitable try should be selected and correctly positioned in the mouth ( the most important factor)
5. Enough space for the selected impression material should be provided in the tray
6. Impression should be removed without damage to the soft tissue
7. Tray and impression material should be stable
8. The external shape of the tray should be similar to the denture’s external surface

*The mouth should be free of inflammation:

Question 3

How do you take impressions of someone who wear dentures

Answer 3

If the patient is wearing a denture the tissue should rest for 24hr before taking an impression or use tissue conditioner

Question 4

What are the types of impressions

Answer 4

1. Preliminary (primary) impressions
2. Final (or definite) impression using a special tray

Question 5

What is the difference between a primary and final impression

Answer 5

Primary
First impression taken to obtain a rough replica of oral structures. More detailed impression for precise replication of tissues.

Final Impression
Purpose Used for diagnostic models, custom tray fabrication, and preliminary assessment. Used for definitive restorations (e.g., dentures, crowns, bridges)

Question 6

What is the impact of using a tray which is too large

Answer 6

large tray pulls away the surrounding tissue stretching the underlying tissue - will stretch the soft tissue

Question 7

What is the impact of using a tray which is too small

Answer 7

small trays can not prevent surrounding tissues from collapsing inward onto the residual ridge - will also hit the hard tissue

Question 8

What is a hydrocolloid

Answer 8

substance that forms a gel when mixed with water
e.g. agar, alginate

Question 9

Explain alginate as a hydrocolloid

Answer 9

Irreversible hydrocolloid (can’t change back)
Uses a chemical reaction
Sol –> Gel

Question 10

What is the composition of alginate

Answer 10

• Sodium alginate (hydrogel former)
• Calcium sulphate dihydrate (provides calcium ions)
• Sodium phosphate (controls working time) - PO4 ions bind to the Ca increasing working time

Question 11

What is the purpose of calcium sulphate dihydrate in alginate

Answer 11

provides calcium ions
undergoes crosslinking reaction as it displaces the sodium ions in the sodium alginate polymer

Question 12

What is the purpose of sodium phosphate in alginate

Answer 12

produces PO4 which controls working time
- increasing working time as it binds to Ca2+

Question 13

What is an impression compound

Answer 13

• thermoplastic material
• comprised of resin and waxes
• used to create mostly primary impressions, boarder molding in complete dentures and sometime in special tray fabrication

Question 14

How do you do an upper impression

Answer 14

1. Place the tray in the patient mouth and the border is checked in terms of under and overextensions
2. check the vestibules are filled with borderline materials

Patient to say “E” and “O” and “ahs”

Question 15

How do you do a lower impression

Answer 15

1. Ask patient to raise the tongue then place the tray in patient mouth
2. Check the vestibules are filled with border line materials
3. Patient is instructed to stick out tongue
4. Press tongue on the rugea area, touch the upper lip and move left and right
5. Say “E” “O”

Question 16

What is permanent deformation

Answer 16

lack of recovery of the elastic impression material when removing the impression due to strain of the undercuts

Question 17

What are gypsum products

Answer 17

Products derived from calcium sulfate dihydrate and when mixed with water, undergo a setting reaction to form a hard structure.

Gypsum product with chemical name of calcium sulphate hemihydrate (CaSO4). Water produced from calcium sulphate dihydrate (CaSO4. 2H2O)

Gypsum products are to make cast or models, dies.

Question 18

What is the impact of heating on gypsum products

Answer 18

reduces working time and setting time by accelerating sodium phosphate consumption

Question 19

what tray space does gypsum products require

Answer 19

3-4 mm

Question 20

what is the impact of any excess gypsum impurity in the mixing bowl

Answer 20

reduces working and setting time

Question 21

Where does the setting of a gypsum product start

Answer 21

tissue side towards tray side (distortion due to immature removal of impression)

Question 22

does refrigeration of impression impact stability of impression

Answer 22

very limited effect

Question 24

what is a type 1 gypsum product

Answer 24

Dental plaster (impression)

Question 25

What is a type 2 gypsum product

Answer 25

Dental plaster (model)

Question 26

what is a type 3 gypsum product

Answer 26

Dental stone (die, model)

Question 27

what is a type 4 gypsum product

Answer 27

Dental stone (Die, high strength, low expansion)

Question 28

what is a type 5 gypsum product

Answer 28

Dental Stone (die, high strength, high expansion)

Question 29

What counts as a failure with impressions

Answer 29

1. Poor reproduction of surface detail - rough or uneven surface - Air bubbles - distorted - cast too big - cast too small

Question 30

What increases the working time of alginate

Answer 30

using cold water

Question 31

What decreases the working time with alginate

Answer 31

hot water

Question 32

What would happen if you leave the impression stored in a moist condition for too long

Answer 32

smaller cast Because the impression will soak in the water and expand in both sides - so there's less space for the plaster to flow making the cast smaller

Question 33

What would happen if you leave the impression stored in a dry condition for too long

Answer 33

large cast