Impressions

Question 1

Terminology: accuracy

Answer 1

Ability to replicate the hard and soft tissues

Question 2

Terminology: resolution/ precision

Answer 2

ability to reproduce surface

detail

Question 3

Terminology: dimensional stability

Answer 3

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

Question 4

Terminology: hydrophobic materials

Answer 4

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

Question 5

Classification of impression materials

Answer 5

Hydrocolloids
-reversible (agar)
-irreversible (alginates)
Synthetic elastomers
-polysulphides
-polyethers
-silicones (addition and condensation silicones)

Question 6

Ideal properties of impression materials

Answer 6

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

Question 7

Alginate?

Answer 7

Insufficient resproduction of surface details
Poor dimensional stability
Not suitable for definitive indirect restoration impressions
We do use it for opposing arch impressions

Question 8

Addition silicones

Answer 8

Also known as PVS (polyvinyl siloxanes)
Most commonly used imp material for fabrication of indirect restorations
Addition reaction between silane and vinyl group when catalyst and base pastes are mixed
No by-products: v good dimensional stability
Historically silicones have been very hydrophobic. This
leads to poorer wetting of the surfaces and increased risk of air entrapment - new materials better

Question 9

Advantages and disadvantages of addition silicones

Answer 9

Advantages
-biocompatible
-simple mixing and reasonable working time
-excellent surface detail reproduction
-wide range of material choice (viscosities) and techniques
-excellent dimensional stability
Disadvantages:
-not very flexible (can be difficult to remove from casts)
-expensive
-setting time affected by T
-hydrophobic - but getting better
-some latex gloves could inhibit setting

Question 10

Addition silicones - formulations

Answer 10

Putty
Heavy-bodied silicone
Medium-bodied silicone (monophase)
Light-bodied silicone
Super light-bodied silicone
*low viscosity vs high viscosity*

Question 11

Why do we have so many formulations of addition silicones?

Answer 11

Light/ medium bodied: 
-setting contraction
-thermic contraction
-detail reproduction
Putty/ heavy bodied:
-filler
-viscosity
-dimensional stability

Question 12

Polyethers

Answer 12

Good options for fixed prostho
Good acuracy and hydrophilicity allows it to compete with addition silicones
Rigidity makes it difficult to remove from the mouth and
may remove periodontally involved teeth.
This rigidity is favoured and therefore this material is
popular and commonly used for impressions of implant
copings
IMPREGUM?

Question 13

Advantages and disadvantages of polyethers

Answer 13

Advantages:
-excellent dimensional stability
-very good surface detail reproduction
-only one viscosity but undergoes shear-thinning allowing it to flow
-hydrophilic
Disadvantages
-very stiff/ rigid difficult to remove from mouth and cast
-swells with water if kept for a long time in damp environment
-bitter taste

Question 14

Disinfection

Answer 14

Imps contaminated with saliva and often blood
Risk of infecting surgery and lab staff if strict-croos-infection procedure not followed
Rinse under tap to remove visible debris.
In CCDH we disinfect by immersing the impression in a disinfectant bath
Soak for 10 mins, no longer as this can affect the
accuracy of the impression (particularly alginates)

Question 15

Soft tissue management

Answer 15

Mechanical:
-Retraction cord
-Retraction pastes
Chemical:
-Haemostatic agents
-Electrocautery
Surgical:
-Soft tissue laser
-Rotary currettage

Question 16

Gingival retraction

Answer 16

Downward and outward movement of free gingival margin
Allows us to syringe imp material beyond our prep margin
These techniques do not control inflammation or allow to take imp when gingivae are inflamed - this should be managed beforehand
need to equi- and subgingival preparation finish lines

Question 17

Retraction cord

& most common technique

Answer 17

Cord packed into gingival sulcus to retract gingivae to improve visibility, access and moisture control
Single-cord technique most common (removed before imp)

Question 18

Retraction pastes

Answer 18

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

Question 19

Electrosurgery and soft tissue lasers

Answer 19

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

Question 20

Imp technique for addition silicones

Answer 20

Twin mix or one stage (most common)
-at least two viscosities used, seated in mouth together
Two stage:
-first imp taken in putty, removed just prior to full setting
-then reline with lower viscosity (light body alone or in combination with medium body)

Question 21

Twin mix/ one stage imp (addition silicones)

Answer 21

Adequate tray selected, adhesive applied
Select material: usual combination putty or heavy body with light body
-however medium and light body can be used
Soft tissue management
Place putty/ heavy body into tray
Remove retraction cord (dampen) - if used
Dry teeth (still hydrophobic material)
Syringe light body around prep: starting within gingival sulcus and ending at coronal par of prep
-use controlled p to force material against tooth
Place some light body onto putty/ heavy-body on tray
Seat imp
Careful removal in 1D - don’t jiggle or rotate
Inspect, if adequate, rinse and disinfect
Send to lab

Question 22

Two stage impression (addition silicone)

Answer 22

Adequate tray selected, adhesive applied.
Select material: usual combination is putty or heavy body
with light body

Question 23

Should i use one stage or two stage impression?

Answer 23

Case dependent
Single stage easier, quicker
Problem esp with putty (high viscosity), 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 prep margin - you could chose the two stage technique - however ensure you fully seat the original impression!

Question 24

Assessing imp

Answer 24

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

Question 25

Digital imps

Answer 25

**

Question 26

Problem solving: I can’t see my finish line

Answer 26

This is either a problem with your finish line, this
may need refining or smoothing to improve the
surface feature
Or it may be a problem with your impression - is
there excessive bleeding or gingivocrevicular fluid affecting the set of the material? Are the gingivae healthy enough? Is my gingival retraction technique suitable?

Question 27

Problem solving: my impression has air bubbles/ voids or drags

Answer 27

It is rarely related to the mixing technique as
syringe-mixed silicones rarely have air-bubbles
but it can be a problem with alginates
Drying the tooth is important to allow adequate wetting and to avoid bubbles and voids
Keeping the tip of the syringe within the material
and starting from the finish line and ending occlusally should minimise this problem
The whole impression tray needs to be filled
with material so the impression is supported in
all directions.
Chose the correct tray size - in my opinion, this
is the single biggest factor in why students end
up repeating impressions

Question 28

How do I get a good impression?

Answer 28

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

Question 29

Which tray should I use?

Answer 29

Size of tray - spend time trying the tray in the mouth! Is there anything impeding adequate seating? Is there room for a bit of rotation?
Rigid tray - supports material, maintains pressure on teeth and soft tissues, prevents distortion
Retention?

Question 30

Polysulphides

Answer 30

Not used nowadays as we have better materials.

Still used for major impressions for complete dentures in some institutions around the world

Question 31

Advantages of polysulphides

Answer 31

Long working time (multiple preps)
Flexible - easy to remove
Good tear resistance
Long shelf-life

Question 32

Disadvantages of polysulphides

Answer 32

Long working time (single preps)
Poor dimensional stability
Sticky to handle
Unpleasant smell
Need special tray

Question 33

Condensation silicones

Answer 33

Also known as Polyvinyl Siloxanes (PVS)
Condensation polymerisation - there is a by-product (alcohol)
Poor dimensional stability than addition silicones means the use of this material is limited
May still be used in some practices as cheaper than
addition silicones
Also used by dental labs to make matrices

Question 34

Advantages of condensation silicones

Answer 34

Easy to use
Good surface detail reproduction
Wide range of material choice (viscosities) and techniques
Good tear resistance

Question 35

Disadvantages of condensation silicones

Answer 35

Dimensionally unstable

Hydrophobic

Question 36

Gingival retraction

Answer 36

The downward and outward movement of the free
gingival margin.
This allows us to syringe impression material beyond our preparation margin.
These techniques do not control inflammation or allow us to take an impression when the gingivae are inflamed -
this should be managed beforehand!
Need for equi- and subgingival preparation finish lines