Impression Techniques For Indirects

Question 1

What do we want to record in impressions for indirects?

Answer 1

Shape of prepared tooth
Margin of preparation (tooth apical to margin)
Intra-coronal resistance features
Spatial relations
Other occlusal surfaces
Opposing arch

Question 2

Why is moisture control important for imps?

Answer 2

Crevicular fluid, blood - for visibility
Dry field for hydrophobic imp materials to be accurate

Question 3

How might we get control of the soft tissues for a crown prep?

Answer 3

 Improving the oral hygiene beforehand
 Methods aimed at reducing soft tissue trauma during tooth preparation
 Electrosurgery - Controls bleeding and dissects soft tissue
 Providing good temporary/provisional restorations to eliminate plaque-traps
 Mechanical - retraction cord
 Chemical - haemostatic solutions
 Consider CHX mouthwash for two weeks before preparation.

Question 4

What are the types/features of retraction cords?

Answer 4

 Twisted, braided or knitted (knitted on clinic, ultrapak)
 Various diameters
 Impregnated or plain
 Single or double technique
 Cord packer (don’t use a flat plastic)

Question 5

What haemostatic agents are there?

Answer 5

(mostly acidic)
 Ferric sulphate (Astringedent) - Use before refining the margins
 Aluminium Chloride (Racestyptine)
 Aluminium Sulphate Gel-cord

Question 6

Compare single and double cord techniques

Answer 6

o Single Cord
 Provides displacement and haemostasis
 Choose largest cord ± astringent
 Optimum 5 minutes
 Wash, dry, remove cord, check crevice 10s, mix impression

o Double Cord
 Gold standard technique
 1. narrow cord placed deep – haemostasis, stays in
 2. wide cord on top - opens crevice – removed before impression

o Single cord – faster, single units, minimal bleeding
o Double cord – slower, more control, multiple units, persistent bleeding

Question 7

What is the process of placing a double retraction cord?

Answer 7

Measure gingival sulcus (with perio probe) and 2 select cords of suitable diameter
Cut the cords to length
Soak both cords in astringent
Place thinner cord around the prep into the deepest sulcus area (using a serrated cord packer)
Inspect - ensure margin is visible (gingivae should not collapse over cord)
Place thicker cord - leave end out to facilitate removal

Question 8

What is the difference between the sizes/lengths of the first and second cord in the double cord technique?

Answer 8

First cord - just longer than tooth circumference, thinner, provides vertical displacement, placed into the sulcus, place before prep margins
Second cord - longer than circumference (with tail for removal), provides vertical and horizontal displacement, thicker

Question 9

What are the ideal properties of impression materials for indirect restorations?

Answer 9

 Dimensional Accuracy
 Elasticity
 Dimensional stability
 Reproduction of fine detail
 Hydrophilic
 Good tear strength
 Compatible with die stones
 Good shelf life
 Convenient setting time
 Non toxic
 Pleasant taste
 Economic

Question 10

What things should you consider when selecting the appropriate material for the clinical situation?

Answer 10

 Type of material (chemical type and setting reaction).
 Viscosity of impression material for the tray.
 Viscosity of the light-body (wash) material.
 Type of tray to be used.
 Technique used.
 Personal preference.
 Cost.
 A disinfection regime

Question 11

What material options do we have to choose from?

Answer 11

Reversible hydrocolloids
Polysulphids
Condensedation silicones
Polyethers
Addition cured silicones

Question 12

What are the most used impression materials for indirects?

Answer 12

Addition cured silicones

Question 13

Which materials do we almost never use and why?

Answer 13

Reversible hydrocolloids - very hydrophillic and need to be poured immediately
Polysulphides - unpleasant taste, messy, long set time, poor dimensional stability

Question 14

Describe condensation silicones

Answer 14

 Hydrophobic
 Condensation reaction – more shrinkage than addition
 Setting Shrinkage - 0.5%
 Low dimensional stability
 Tasteless
 Putty and light viscosity only
 Economical ?

Question 15

Describe polyethers

Answer 15

 Good wettability, but problems with dimensional stability
 Will swell in water

Question 16

Describe addition cured silicones

Answer 16

Hydrophobic - (Reason for retraction cord + moisture control!)
Addition reaction
Very low polymerisation shrinkage –(0.04%)
High dimensional stability - Can pour 7 models, very stable. Want 2, 1 for sectioning and another to test contact points etc.
Putty/Heavy/Medium/Light viscosities available
No taste (some flavored)
Rigid

Question 17

What are the different impression techniques we can use for indirect restorations (using addition silicones)?

Answer 17

Putty and wash - 1 stage
Putty and wash - 2 stage
Putty and spacer then wash
Custom tray with low viscosity and medium/heavy body
Injection molded
Impression copings
Dual arch

Question 18

Which impression technique is the gold standard for indirect impressions?

Answer 18

Custom tray with low viscosity and medium/heavy body

Question 19

Describe the process for putty and wash 1 stage

Answer 19

Tooth prepped
Remove thicker retraction cord only
Wash and dry the preparation
Putty and wash mixed at same time
Putty in tray, wash onto teeth (dry the light body on prep and reload with excess material!
Tray seated onto teeth over wash
Allow complete set before ‘snap’ withdrawal (avoid leverage that may distort the imp)

Question 20

What are the advantages and disadvantages of the 1 stage putty and wash technique?

Answer 20

Pros:
- most popular in GDP
- quick

Cons:
- putty pushes wash off prepared tooth
- reduced marginal detail
- reduced occlusal detail
- needs good coordination
- drags

Try to use similar viscosities if possible - eg soft putty and relatively higher viscosity wash

Question 21

Describe the technique for the 2 stage putty and wash

Answer 21

Place putty in stock tray - take imp, let it set, remove
Modify the putty - allow wash to escape, prevent recoil, prevent seating problems
Wash in tray, wash on prep
Replace tray

Question 22

Describe the technique for the 2 stage putty and wash space method

Answer 22

Instead of modifying the tray, we use a plastic bag or other form of space over the putty to take the impression. Then this acts as a ‘custom tray’ place light body throughout the tray and place over prep.

Question 23

Describe the advantages and disadvantages of the two stage putty and wash technique

Answer 23

Pros:
- better marginal detail
- less drags
- less cordination

Cons:
- slower
- seating problems (pressure deforms putty or tray)
- material recoils on removal
- stepped occlusal surface

Question 24

When do we consider the two stage putty and wash mainly?

Answer 24

For coronal margins eg onlays, tilted preps, and distal tooth

Question 25

How do we do custom tray gold standard technique?

Answer 25

Light body syringed onto prep and medium/heavy body in the tray Custom tray preferred (stock tray used more material)

Question 26

Advantages and disadvantages of the custom tray low visc + high body gold standard technique

Answer 26

Pros: - Most accurate imps - Used for multiple units/bridgework - Less distortion, better surface detail Cons: - Expensive

Question 27

Compare custom vs stock tray

Answer 27

Stock tray: rigid, fits arch form, 1-2 single units, putty wash, heavy/light, adhesive, no stops Custom tray: rigid, multiple units/bridgework, heavy/light, medium/light, stops, perforations, less material required

Question 28

What is a stop on a custom tray?

Answer 28

A feature on the impression tray that enables you to seat it until it hits on something. Good for locating the tray well. Placed not on the impression tooth but nearby on adjacent teeth, on non-functional cusps so not impacting on occlusion.

Question 29

What do we usually use for the opposing arch imp?

Answer 29

Alginate

Question 30

Why do we use alginate and what are we trying to achieve with the opposing arch imp?

Answer 30

Require an accurate imp of opposing arch Good reproduction of occlusal surface important Reduce air bubbles (can smear alginate over occlusal surfaces first)

Question 31

What faults might we get with impression materials and why?

Answer 31

Drags - viscosity of putty vs wash, path of insertion, tooth angulation Poor marginal definition - inadequate retraction, inadequate moisture control, poor marginal finishing Voids - air bubbles Distortion - bad technique, removal of imp Separation from tray

Question 32

How should we examine our imps once taken?

Answer 32

Cleaned, examined with magnification and good light