Impressions (for indirect restorations)

Question 1

How to get a good impression?

Answer 1

Impression tray choice
Good soft tissue management
Appropriate technique
Appropriate choice and use of material

Question 2

Which tray should I use?

Answer 2

Size of tray - trim the tray in the mouth, adequate seating? Room for rotation?
Rigid tray - supports material, maintains pressure on teeth and soft tissues, prevents distortion, retention with perforated holes

Question 3

Define accuracy

Answer 3

Ability to replicate hard and soft tissues

Question 4

Define resolution/precision

Answer 4

Ability to reproduce fine surface detail

Question 5

Define dimensional stability

Answer 5

Maintenance of the material in terms of size and shape over a prolonged period of time

Question 6

Define hydrophobic materials

Answer 6

Don’t like water, may lead to bubbles, voids, inaccuracies if inadequate moisture control

Question 7

Types of hydrocoloids?

Answer 7

Reversible (agar)

Irreversible (alginates)

Question 8

Types of synthetic elastomers?

Answer 8

Polysulphides
Polyethers
Silicones - addition silicones, condensation silicones

Question 9

Ideal properties of impression materials?

Answer 9

High accuracy
Elastic
Good dimensional stability
Good tear strength
Easy to use
Acceptable taste and smell
Cheap
Tolerant to disinfection
Good working and setting times

Question 10

Why is alginate not used for indirect restorations?

Answer 10

Insufficient reproduction of surface details
Poor dimensional stability
Not suitable for definitive indirect restorations
Used for the opposing arch

Question 11

Uses of addition silicones/polyvinyl siloxanes (PVS)? How does it work?

Answer 11

Impression material for fabrication of indirect restorations
Addition reaction between silane and vinyl groups when catalyst and base pastes are mixed
No byproducts produced - very good dimensional stability
Hydrophobic - increased risk of air entrapment

Question 12

Advantages of addition silicones?

Answer 12

Excellent dimensional stability
Excellent surface detail reproduction
Wide range of material choice and techniques
Biocompatable
Simple mixing and reasonable working time

Question 13

Disadvantages of addition silicones?

Answer 13

Expensive
Hydrophobic - but getting better
Setting time affected by temp
Can be difficult to remove from casts
Some latex gloves could inhibit setting

Question 14

Formulations of addition silicones?

Answer 14

Putty
Heavy bodied silicone
Medium bodied silicone
Light bodied silicone
Super light bodied silicone

Question 15

Positives of heavy bodied/putty addition silicone?

Answer 15

High filler
High viscosity
High dimensional stability

Question 16

Positives of light/medium bodied addition silicone?

Answer 16

High detail reproduction
High thermal contraction
High setting contraction

Question 17

Polyether features?

Answer 17

Good for fixed prosthodontics
Good accuracy and hydrophilicity allows it to compete with addition silicones
Rigidity - difficult to remove from the mouth and may remove periodontically involved teeth
Rigidity is favoured - popular material, commonly used for impressions of implant copings

Question 18

Advantages of polyethers?

Answer 18

Excellent dimensional stability
V good surface detail reproduction
Only one viscosity but undergoes shear thinning allowing it to flow
Hydrophilic

Question 19

Disadvantages of polyethers?

Answer 19

Very stiff/rigid difficult to remove from mouth and cast
Swells with water if kept for long time in damp environment
Bitter taste

Question 20

How to disinfect impressions?

Answer 20

Trim off excess
Contaminated in water and blood
Risk of infecting surgery and lab staff is cross infection procedure not followed
Rinse under tap to remove debris
Put in disinfectant bath - soak for 10 mins - longer can affect the accuracy of the impression

Question 21

Soft tissue management?

Answer 21

Retraction cord
Retraction pastes
Haemostatic agents
Electrocautery
Soft tissue laser
Rotatory curettage

Question 22

How to retract the gingivae?

Answer 22

Downward and outward movement of the free gingival margin
Allows us to syringe impression material beyond preparation margin
Does not control inflam or allow us to take an impression when the gingivae are inflammed - manage this beforehand

Question 23

What is a retraction cord?

Answer 23

Cord that’s packed into the gingival sulcus to retract the gingivae to improve visibility, access and moisture control
Single cord technique most common (removed before impression)
Cord often soaked in haemostatic agent to prevent bleeding

Question 24

What are retraction pastes?

Answer 24

Viscous agent syringed into gingival sulcus to provide retraction and haemostasis
Useful in simple cases with equi-gingival finish lines
More expensive

Question 25

Electrosurgery and soft tissue lasers features?

Answer 25

Widen or re-contour gingival profile
Widen gingival sulcus 
Remove overgrown tissue
Used in combination with cord or paste usually
Haemostasis

Question 26

Techniques for addition silicones?

Answer 26

Twin mix or one stage (most common) = at least 2 viscosities used, seated in mouth together

Two stage

• First impression taken in putty, removed just prior to full setting
• Then reline with lower viscosity (light body alone or in combo with medium body)

Question 27

Protocol for the one stage addition silicone impression?

Answer 27

Adequate tray, adhesive applied
Select material: combo of putty or heavy body with light body
Soft tissue management - retraction cord, retraction paste, electrocautery
Place putty/heavy body into tray
Remove retraction cord (damp) if used
Dry teeth
Syringe light body around preparation - starting within gingival sulcus and ending at coronal part of prep - use controlled pressure to force material against tooth
Place some light body into putty/heavy body on tray
Seat impression
Cafeful removal in 1 direction - do not rotate
Inspect, rinse and disinfect
Send to lab

Question 28

Two stage addition silicone impression protocol?

Answer 28

Adequate tray, adhesive applied
Select material - combo of putty/heavy body with light body
Spaced placed (thin sheet of plastic)
Take impression
Essentially creates a special tray
Follow previous steps: remove retraction cord, dry teeth, syringe light body around prep, reseat impression

Question 29

Negatives of the two stage addition silicone impression technique?

Answer 29

If you dont sit the original impression in the same way - get a step or inaccurate

Question 30

Should you use the single stage or two stage addition silicone impression technique?

Answer 30

Case dependent
Single stage easier, quicker
Putty - high viscosity so a lot of pressure is needed to seat the tray which can distort or push away the lower viscosity material
If you don’t have a rigid tray and you can see the lower viscosity material is being pushed from the prep margin - you could chose the 2 stage technique - however ensure you fully seat the original impression

Question 31

What to look for when assessing your impression?

Answer 31

Well defined margins
Good surface detail
No drags or voids
No tray exposure
All prep in appropriate material (light body)
Impression adhering to tray

Question 32

Why may you not be able to see your finish line?

Answer 32

Problem with prep, may need refining or smoothing to improve the surface feature
Or it may be problem with impression - excessive bleeding or gingivocrevicular fluid affecting the set of the material? Are the gingivae healthy enough? Is my gingival retraction technique suitable?

Question 33

Why may your impression have bubbles/voids/drags?

Answer 33

Alginate mixing
Drying the tooth is important to allow adequate wetting and to avoid bubbles/voids
Keep tip of syringe within the material and starting from the finish line and ending occlusally should minimise the problem
Whole impression tray needs to be filled with material so the impression is supported in all directions
Chose the correct tray size