Impression Techniques

Question 1

What are the aims of a good impression?

Answer 1

Create an exact duplicate of the prepared tooth/teeth including all of the preparation.

Other teeth and tissue adjacent to the prepared tooth must be accurately reproduced to permit suitable articulation. Must be free of bubbles, air blows, voids, and drags.

Question 2

What factors to consider when taking an impression?

Answer 2

Impression material for working and opposing casts, tray selection, adhesive selection, handling properties, tissues to be recorded, ST management, patient tolerance, interocclusal record.

Question 3

Which impression materials are most commonly used in fixed prosthodontics?

Answer 3

Synthetic elastomers - mostly addition silicones.

Question 4

What are the two classifications of hydrocolloids?

Answer 4

Reversible - agar; Irreversible - alginate.

Question 5

When is alginate used for fixed prosthodontics?

Answer 5

Study models prior to the preparation; Opposing arch impression.

Question 6

What are the advantages of hydrocolloids for opposing impressions?

Answer 6

Hydrophilic, good record of surface detail, cheap.

Question 7

What are the cons of hydrocolloids, making them unsuitable for crown impressions?

Answer 7

Poor dimensional stability; lower tear resistance; unsuitable for recording gingival sulcus.

Question 8

Why are polysulphides no longer used in dentistry?

Answer 8

Slow set, messy, unpleasant odour, impression shrinkage, requires special tray with 4mm spacing, 48h max storage time prior to casting.

Question 9

What are the advantages of polysulphides?

Answer 9

Long working time, excellent tear resistance.

Question 10

What is polyether (e.g., Impragum)?

Answer 10

Developed specifically for use in dentistry; fast set time; good dimensional stability when stored dry; rigid when set - can be difficult to remove in cases of undercuts; adequate tear resistance; more hydrophilic than silicones.

Question 11

What is condensation silicone?

Answer 11

Produces byproduct of alcohol on setting; less dimensionally stable than addition silicones; more dimensionally stable than alginate.

Question 12

What is a risk of condensation silicone?

Answer 12

Sensitisation reactions therefore not used in the mouth.

Question 13

Which materials are largely used for crown and bridge work impressions?

Answer 13

Addition silicones.

Question 14

What different forms can addition silicones come in?

Answer 14

Light, medium, heavy body; putty.

Question 15

Which elastomer has the best elasticity?

Answer 15

Addition silicone.

Question 16

What are the different techniques for addition silicone impressions?

Answer 16

One stage, two stage unspaced, two stage spaced.

Question 17

What is the one stage technique of addition silicones?

Answer 17

Either putty and wash or heavy body and light body material.

Question 18

What is the two stage unspaced technique for addition silicones?

Answer 18

More dense material recorded first followed by light bodied material.

Question 19

What is the two stage spaced technique for addition silicones?

Answer 19

Can be spaced by polyethylene separator over teeth, recording putty impression before preparation or by gauging away part of putty to enable space for light bodied material.

Question 20

Which technique of addition silicone do we use mostly?

Answer 20

One stage.

Question 21

What properties should the impression tray have?

Answer 21

Good fit; sturdy enough not to be displaced during the impression process.

Question 22

Which trays should we not use for working impressions?

Answer 22

Sectional trays; trays which take impression of upper and lower arch together (triple trays).

Question 23

When would we use custom impression trays for crown and bridge work?

Answer 23

Only in exceptional circumstances, e.g., if there is something meaning a regular tray can’t be used.

Question 24

Why do we need to use gingival retraction?

Answer 24

To pick up the margins accurately.

Question 25

What are different methods of gingival retraction?

Answer 25

Retraction cords, retraction paste, electrosurgery, gingettage, copper ring (these ones are rarely used).

Question 26

What are retraction cords?

Answer 26

Knitted, interlocking chains of cotton; available with and without epinephrine; can be used with astringent; they come in different thicknesses.

Question 27

What can be used as astringent for retraction cords?

Answer 27

Ferric sulphate, aluminium chloride.

Question 28

Which size retraction cord do we mostly use?

Answer 28

00.

Question 29

What are the two techniques for using retraction cords?

Answer 29

Single cord technique; two cord technique.

Question 30

What is the two cord technique?

Answer 30

Pack a narrow cord into the sulcus; pack another thicker cord on top; take out thicker cord but leave in the narrow cord for the impression.

Question 31

What are the disadvantages of the two cord retraction cord technique?

Answer 31

More traumatic to gingiva.

Question 32

When can the two cord technique be helpful?

Answer 32

Subgingival margins.

Question 33

What are the advantages of retraction paste over cords?

Answer 33

Less pressure, less invasive; good for digital impressions; quicker to place.

Question 34

What are the disadvantages of retraction paste over cords?

Answer 34

Less physical displacement of tissues.

Question 35

What are some more invasive methods of tissue retraction?

Answer 35

Electrosurgery; gingettage (removing the gingiva); copper ring.

Question 36

When can retraction pastes be helpful?

Answer 36

Digital impressions.

Question 37

What is the purpose of interocclusal records?

Answer 37

Accurate recreation of interocclusal contacts to accurately articulate casts.

Question 38

How far should interocclusal records extend?

Answer 38

Not beyond contact point.

Question 39

What can be used for interocclusal records?

Answer 39

Beauty wax; occlusal registration paste.

Question 40

What to be careful of when taking interocclusal records?

Answer 40

Not to open the contacts - can be more of a risk if covering all the occlusal contacts.

Question 41

What are some common errors in impression techniques?

Answer 41

Tray too small; contact of tray with soft tissue may cause distortion; impression tray positioned too anterior; inadequate capture of adjacent tooth; distortion of dual arch technique; step due to incompatible impression materials; set of surface material inhibited -> loss of surface detail; poor marginal detail; internal bubbles due to saliva, air, blood; marginal tears.

Question 42

What can cause internal bubbles in the impression?

Answer 42

Saliva, air, or blood.

Question 43

What can cause a loss of surface detail of the impression?

Answer 43

Inhibition of set of surface material - this can occur due to use of latex gloves.

Question 44

What can cause a step in the impression?

Answer 44

Differential set of different materials which do not blend; sectional impression which does not enable full arch articulation.

Question 45

What can cause tray distortion?

Answer 45

Contact of tray with soft tissues.

Question 46

How to assess the quality of the impression?

Answer 46

Magnification; good lighting.

Question 47

When to use a facebow?

Answer 47

Lone standing molar.

Question 48

Which impression can you take sectionally rather than full arch?

Answer 48

Interocclusal record.