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Impression Materials

Q: List the desirable properties of an impression material.

Easy to handle/mix Accurately record detail Good tear resistance Easy to remove particularly where undercuts exist Dimensionally stable Has ability to disinfect Compatible with oral fluids No adverse effects to patient Pleasant smell/taste Short setting time Cost-effective

Q: List the factors that can affect the accuracy of an impression materials.

Viscosity Hydrophobic/hydrophilic materials Wet-ability of impression surface by dental stone/plaster Setting Reaction (dimensional change) Impression retained in the tray

Q: Which of the three (original oral environment, impression, and model) are positive and negative spaces?

Original oral environment and model are positive spaces , filling up a negative space—impression.

Q: What does a higher modulus of elasticity mean?

Higher modulus of elasticity = greater resistance to elastic deformation under stress

Q: What are the essential characteristics of a stock tray for a good impression?

Rigid and non flexible Extend sufficiently to support the impression material Fit loosely ( Correct size and shape of the dental arch to allow for support of impression material) Have a robust handle If not for single use be able to disinfect Retain the impression material

Q: What are the two methods of retaining the impression material in the tray?

Perforations - holes in the tray Adhesives

Q: What are cautions to take for better adhesive application to the stock tray?

Apply sparingly and allow to dry Avoid pooling – adhesive wont dry and will interfere with bond Ideally 2 coats with drying between applications Don't let solvent evaporate = thickening of the adhesive Adhesive specific to each generic group of impression materials

Q: List the non rigid/elastic impression materials

1. Hydrocolloids - reversible (agar), irreversible (alginate) 2. Non-aqueous elastomeric materials (elastomers) - polysulphides, polyethers, silicones

94 important questions on Impression Materials

List the desirable properties of an impression material.

Easy to handle/mix
Accurately record detail
Good tear resistance
Easy to remove particularly where undercuts exist
Dimensionally stable
Has ability to disinfect
Compatible with oral fluids
No adverse effects to patient
Pleasant smell/taste
Short setting time
Cost-effective

List the factors that can affect the accuracy of an impression materials.

Viscosity
Hydrophobic/hydrophilic materials
Wet-ability of impression surface by dental stone/plaster
Setting Reaction (dimensional change)
Impression retained in the tray

Which of the three (original oral environment, impression, and model) are positive and negative spaces?

Original oral environment and model are positive spaces, filling up a negative space—impression.

What kind of model does a contracted impression give rise to?

Oversized model

What kind of model does an expanded impression give rise to?

Undersized model

What is modulus of elasticity?

Modulus of elasticity = stress/strain.

What does a higher modulus of elasticity mean?

Higher modulus of elasticity = greater resistance to elastic deformation under stress

What are the typical values for modulus of elasticity of brittle and flexible materials?

Brittle - high modulus of elasticity
Flexible - low modulus of elasticity

What are the essential characteristics of a stock tray for a good impression?

Rigid and non flexible
Extend sufficiently to support the impression material
Fit loosely (Correct size and shape of the dental arch to allow for support of impression material)
Have a robust handle
If not for single use be able to disinfect
Retain the impression material

What are the three common variations of stock tray materials?

Plastic
Metal
Special tray

What are the two methods of retaining the impression material in the tray?

Perforations - holes in the tray
Adhesives

What are cautions to take for better adhesive application to the stock tray?

Apply sparingly and allow to dry
Avoid pooling – adhesive wont dry and will interfere with bond
Ideally 2 coats with drying between applications
Don't let solvent evaporate = thickening of the adhesive
Adhesive specific to each generic group of impression materials

List the non rigid/elastic impression materials

1. Hydrocolloids - reversible (agar), irreversible (alginate)
2. Non-aqueous elastomeric materials (elastomers) - polysulphides, polyethers, silicones

List the rigid/non-elastic impression materials

1. Impression Plaster
2. Zinc Oxide and Eugenol
3. Impression Compound

What are the two phases that a colloid can exist in?

A colloid can exist as a viscous liquid (sol) or a solid (gel)

What are the two variations of hydrocolloids and what are their differences?

1. Alginates (irreversible) - can't go back to sol once it becomes gel
2. Agar (reversible) - can go back to sol from gel

What is a reversible hydrocolloid (agar)?

A galactose sulphate which forms a colloid with water.

How does reversible hydrocolloid go from gel to sol and sol to gel?

From gel to sol - gel is heated at 70-90 degrees C
From sol to gel - sol is cooled at 30-50 degrees C

Explain how each compartment of the water bath is used for reversible hydrocolloid

1st compartment of water bath near boiling point = liquify agar for 10 minutes
2nd compartment of water bath between 63-66 degrees C = storage of agar
3rd compartment of water bath at 46 degrees C = tempering (increase toughness, reduce brittleness) of agar after it is placed in special tray

What are the steps that follow after the reversible hydrocolloid on the special tray is removed from the 3rd water bath compartment?

1. Cool sufficiently to take the impression
2. After 5 minutes, remove the solidified agar from patient's mouth

Which part of the reversible hydrocolloid sets the quickest?

Reversible hydrocolloid closest to the tray sets first

List the properties of reversible hydrocolloid

1. Hydrophilic
2. Low tear resistance
3. Visco-elastic
4. Technique sensitive
5. Non-toxic and non-irritant
6. Dimensionally stable right after removal but needs to be kept at 100% humidity with damp paper towel to prevent dimensional change from water loss

How should reversible hydrocolloid be removed from the patient's mouth due to its low tear resistance and visco-elasticity?

Low tear resistance - needs care when removal from undercuts
Viscoelasticity - remove tray with snap action to cause near elastic response

Within how many hours should the model be poured in reversible hydrocolloid?

Models should be poured within 1 hour due to risk of syneresis and imbibition.

Why is reversible hydrocolloid rarely used now?

Expense with start up and maintenance of baths
Risk of contamination from water bath
Technique sensitive procedure

What are the indications of reversible hydrocolloid?

Fixed indirect restorations or duplicating models in lab due to high accuracy of the material.

What is the most commonly used dental impression material?

Irreversible hydrocolloid (alginate)

Explain the setting reaction of irreversible hydrocolloid

An irreversible chemical reaction cross links the polymer chain, producing calcium alginate gel + potassium/sodium sulphate from potassium alginate + sodium alginate + calcium sulphate dihydrate + water.

What element of irreversible hydrocolloid extends the working time?

Sodium phosphate slows the dissolution calcium sulphate by making calcium ions preferentially react with phosphate ions, reducing the calcium ions available for cross linking and extending working time.

How much of irreversible hydrocolloid does potassium/sodium alginate make up?

18%

How much of irreversible hydrocolloid does calcium sulphate dihydrate make up?

14%

What does potassium/sodium alginate do in irreversible hydrocolloid?

Forms hydrogel

What does calcium sulphate dihydrate do in irreversible hydrocolloid?

1. Provides Ca ions for cross linking

2. Reacts with soluble alginate to form insoluble calcium alginate

What does sodium phosphate do in irreversible hydrocolloid?

Reacts with calcium ions (from calcium sulphate dihydrate) to give working time retarder

What does potassium sulphate do in irreversible hydrocolloid?

Reduces inhibition of setting of model

What do fillers do in irreversible hydrocolloid?

Control consistency and flexibilty of irreversible hydrocolloid

What does sodium silicoflouridedo in irreversible hydrocolloid?

Controls pH of irreversible hydrocolloid

What do organic glycols do in irreversible hydrocolloid?

Reduces dustiness of powder

What do oil and pigment do in irreversible hydrocolloid?

Provides pleasant taste and color

List the properties of irreversible hydrocolloid

1. High recovery of 98.5% from permanent deformation
2. Poor dimensional stability due to loss or uptake of water
3. Hydrophilic
4. Low tear resistance

How can permanent deformation of irreversible hydrocolloid be reduced?

1. Reduce compression by increasing the bulk of impression material
2. Reducing the time the impression is compressed i.e. snap removal
3. Allowing longer time for recovery, not pouring the model for about 1 hour

Within which time frame should the model be poured in irreversible hydrocolloid?

Model should be poured within 2 hours of impression taking

What is the advantage of a hydrophilic impression material?

Can produce detailed impression in a wet field

How can tear resistance be altered for irreversible hydrocolloid?

For greater tear resistance
1. Increase powder/liquid ratio
2. Sufficient spatulation for consistency and all particles being
dissolved
For weaker tear resistance
3. Over spatulation can reduce strength by disruption of the
forming of calcium alginate gel

What are the advantages of irreversible hydrocolloid?

1. Easy flow
2. Reproduction of adequate detail
3. Fast set
4. Minimal tissue displacement
5. Cheap
6. Good patient tolerance

What are the disadvantages of irreversible hydrocolloid?

1. Poor dimensional stability
2. Poor tear strength
3. Distorts if unsupported
4. 3mm thickness required to prevent distortion
5. Easy to include air from mixing of paste

What are the indications of irreversible hydrocolloid?

1. Study casts
2. Opposing arch
3. Removable orthodontic appliances
4. Partial dentures
5. Occlusal splints/Bleaching trays

What are the contraindications of irreversible hydrocolloid?

1. Fixed indirect cast restorations
2. Full dentures

What percentage of strain should a correctly mixed irreversible hydrocolloid have?

Maximum 10% strain without tearing

What is the mixing time of irreversible hydrocolloid?

60 secs = 15 secs of incorporation + 45 secs of spatulation

What is the working time of irreversible hydrocolloid?

45 secs for fast set
1 min 15 secs (75 secs) for regular set

What regulates the working time for alginate (irreversible)?

Amount of sodium phosphate
ex) smaller amounts lead to shorter working time

What is the setting time of alginate (irreversible hydrocolloid)?

1-4.5 mins

What factors can regulate the alginate's setting time?

1. Temperature of water (most important factor)
cold water = slower setting
warm water = faster setting
2. Amount of sodium phosphate

What can immersion of hydrocolloid impression in disinfectant do?

Can affect the dimensional stability of hydrocolloid impressions

How much sodium hypochlorite is incorporated in the disinfectant for hydrocolloid impressions?

1% of sodium hypochlorite

How long is each hydrocolloid impression suspended in the 1% sodium hypochlorite disinfectant for?

10 minutes

What is the purpose of elastomeric impression materials?

Address the shorcomings of alginates and agars
Required for fine detail and accuracy

What are the three major types of elastomeric impression materials?

Polysulphides
Polyethers
Silicones (Condensation silicones, addition silicones, vinyl polyether silicone)

What are polysulphide impressions?

A type of elastomeric impression based on a paste/paste system with unpleasant smell and taste.

What is the setting reaction of polysulphides?

Long condensation polymerisation reaction that is 3-4 degrees C exothermic and produces water as a byproduct.

What is the most hydrophilic elastomer?

Polyethers

What problem does a lower viscosity polyether address?

Addresses stiffness problem

What does the base paste of polyether contain?

Polyether polymer
Plasticizer such as glycoether or phthalate
Colloidal silica as an inert filler

What does the accelerator paste of polyether contain?

Aromatic sulphonate ester
Plasticizer
Inert filler

What is the setting reaction of polyether?

Cross linking reaction between aziridine at the end of each polyether molecule
Chain lengthening occurs by cationic polymerization of the imine groups on the polymer chain

What is the base to acclerator ratio in polyether?

4 (base) : 1 (accelerator)

Why does polyether have good dimensional stability?

Because polyether doesn't have byproducts from its setting reaction

What environment should polyether be placed in?

Dry and room temperature environment

How does cold temperature affect the viscosity of polyether?

Cold temperature = increased viscosity

When can the high stiffness of polyether cause problems?

When removing from patient's mouth, especially around crowns and bridges

What are cautions to take when using 'soft' polyther variants that have been produced to address the problems associated with high stiffness of original polyether?

Select larger trays to allow for greater bulk of material
Care on removal from model as thin dies can fracture

What are the indications of polyether?

Indirect cast restorations
Functional impression
When moisture control is an issue

What is the contraindication of polyether?

Small thin preparations

What are the two types of silicones?

1. Addition cured silicone
2. Condensation cured silicone

Are silicones hydrophobic or hydrophilic?

They are hydrophobic because silicone oxygen bonds are hydrophobic in nature.

What system is a condensation cured silicone?

Base paste (putty) + low viscosity accelerator paste kneaded together

How can different amounts of accelerator affect condensation cured silicones?

Too little accelerator
1. Incomplete cure
2. Poor mechanical properties
3. Decreased tear resistance
Excess accelerator
1. Incomplete cure
2. Unreacted ethyl end gps

What are the ingredients of the base paste of condensation cured silicone and what do each ingredients do?

1. Dimethyl siloxane - Reactive terminal hydroxyl end groups
2. Colloidal silica - Filler

What is the ingredient of the accelerator paste of condensation cured silicone and what does that ingredient do?

Tetra ethyl silicate - Cross linking agent

How does the condensation cured silicone set?

Condensation reaction that releases ethyl alcohol, which evaporates as the material shrinks significantly

List the properties of condensation cured silicone

1. Correct base to activator ratio and incorporation is critical (affects mechanical properties and working & setting times)
2. Tetra-ethyl silicate (TES), the activator, is susceptible to hydrolysis

What are the indications for condensation cured silicones?

1. Most commonly used in removable prosthodontics (ex: impressions for metal based dentures or relines)
2. Indirect cast restorations

What is the most popular material used for impressions in advanced restorative dentistry and why?

Addition cured silicone because it is dimensionally stable and accurate due to the absence of byproducts produced in the setting reaction.

What are the available forms of addition cured silicone?

Putty and heavy bodied
Universal (medium) bodied
Light bodied
Extra light bodied

What are the ingredients of the base/putty for addition cured silicone and what do each ingredient do?

Poly methyl hydrogen siloxane - Polymer
Siloxane prepolymers - Cross linking polymer
Quartz - Filler

What are the ingredients of the catalyst paste for addition cured silicone and what do each ingredients do?

Poly vinyl siloxane - Polymer
Chloroplatinic salt - Catalyst
Quartz - Filler
Surfactants - Regulates setting
Retarders - Aids wetting of hydrophobic silicone

Explain the setting reaction of addition cured silicone

Hydrogen from hydrogen siloxane backbone + vinyl gp on the other siloxane = cross linked polymer

Why should the addition cured silicone be left for a period of time to pour the model?

Because hydrogen gas is released from some addition silicones during setting, causing porosities in the model. To reduce porosities in model, impression must be left for a period of time before model is poured

Why shouldn't latex gloves be used when 2 putties for addition cured silicones are being mixed by hand?

Because sulphur containing materials will inhibit the platinum catalyst.

List the properties of addition cured silicone

1. Hydrophobic = needs moisture control
2. Excellent dimensional stability
3. Increased stiffness up to 3 hours after clinical set
4. Poor tear resistance
5. Needs separator when patient has recently placed composite (because material may react with oxygen inhibited layer)