Impression Materials: Linking of Laboratory to Clinics

Question 1

Steps to Fabricate an Indirect Restoration:

Answer 1

Take an accurate impression.

Cast a model.

Invest the pattern using the lost wax technique.

Construct the final appliance.

Question 2

When Is an Indirect Restoration Indicated?

Answer 2

When significant tooth structure is lost and a direct filling isn’t suitable.

Question 3

4 Benefits of Indirect Restorations:

Answer 3

Less shrinkage due to lab-based curing.

More accurate shape and fit.

Higher polymerisation from controlled lab curing.

Stronger mechanical performance (generally).

Question 4

One Drawback for the Patient of indirect restoration

Answer 4

Higher cost, though benefits often outweigh this.

Question 5

Why Is the First Step (Impression) Crucial?

Answer 5

It’s essential for accuracy — any errors here affect the entire restoration.

Question 6

Preferred Material Properties for Indirect Restorations:

Answer 6

Rigid

High modulus of elasticity can resist high stress without permanent distortion.

Many behave as a brittle material

Question 7

Material-Specific Properties:

Answer 7

Ceramics: Rigid and brittle.

Pure gold: Ductile (can deform without breaking).

Prefabricated composites: Better cured than direct composites due to lab conditions.

Question 8

Role of an Impression:

Answer 8

Capture the shape and relationship of teeth and oral tissues.

Provide a base to create external appliances (e.g., dentures, crowns).

Question 9

Appliances Made from Impressions:

Answer 9

Dentures, bridges, crowns, inlays, onlays

All require accurate models of both hard and soft tissues.

Question 10

Types of Impression Trays:

Answer 10

Stock trays (metal, polystyrene, nylon – reusable or disposable)

Custom trays (individually made – disposable)

Question 11

What are the 2 main classes of impression materials?

Answer 11

1. Elastic materials - easier to remove if there are undercuts than non elastic.

Synthetic elastomers e.g. silicones and polyethers.
Hydrocolloids (natural) e.g. agar and alginates.

1. Non elastic materials - impression plaster that lacks any rubbery nature.

Impression compound, zinc oxide pastes and impression waxes.

Question 12

7 Ideal Requirements of Impression Materials:

Answer 12

Accuracy

Proper flow (rheological properties)

Dimensional stability (short + long term)

Suitable thermal properties

Good adhesion to trays

Allow for disinfection

Compatible with model/die materials

Question 13

11 Key Physical/Mechanical Properties:

Answer 13

Flow properties

Wettability

Working and setting time

Tear resistance

Compressive strength

Biocompatibility

Shrinkage

Dimensional accuracy

Question 14

what is rheology?

Answer 14

The study of flow and deformation of materials.

Focuses on how materials respond under applied forces.

Question 15

what is viscosity?

Answer 15

Viscosity = a fluid’s resistance to flow.

It varies with shear rate (how fast layers move over each other) and time.

Question 16

Flow Behaviours:

Answer 16

Newtonian Flow:
Viscosity stays constant as shear rate increases.

Shear stress ∝ shear rate (linear relationship).

Flow index = 1

🟨 Pseudoplastic Flow (Shear Thinning):
Viscosity decreases with increasing shear rate, then levels off.

Initial rise in shear stress is exponential, then becomes proportional.

Common in many biological fluids and impression materials.

🟥 Dilatant Flow (Shear Thickening):
Viscosity increases with shear rate.

Starts slow, then both viscosity and shear stress increase exponentially.

Seen in materials like some suspensions or heavily filled composites.

Question 17

Why is a long working time and a short setting time preferred for impression materials?

Answer 17

Long working time → Allows for proper mixing and tray loading.

Short setting time → Minimizes patient discomfort.

Question 18

Why do impressions set more rapidly in the patients mouth?

Answer 18

Impressions set faster in the mouth due to higher temperature, which accelerates setting reactions.

Question 19

Give the 3 types of viscosity impression pastes can have.

Answer 19

high viscosity - may not flow over the undercuts provides a body for the impression
medium - flow improves not adequate
low - good flow but may drip

Question 20

What is meant by an impression materials wettability?

Answer 20

Refers to how well the material interacts with saliva and moist tissues.

Question 21

If an impression material is highly hydrophobic what does this mean?

Answer 21

Repelled by moisture; can cause voids or blow holes.

Question 22

If an impression material is highly hydrophilic what does this mean and why is a balance needed?

Answer 22

Better flow in moist conditions, but may swell on storage.

A balance is needed to combine good flow with stability.

Question 23

Define accuracy in terms of impressions.

Answer 23

Ability to replicate the intraoral surface details

Question 24

Why are elastic and viscoelastic materials preferred to plastic materials?

Answer 24

Plastic materials deform permanently on removal.

Elastic/viscoelastic materials can rebound to original shape, maintaining accuracy.

Question 25

An impression sets in one of 2 ways which are?

Answer 25

Chemical reaction (e.g. addition silicone) Physical change of state (e.g. cooling of reversible hydrocolloid)

Question 26

Managing Impression Shrinkage:

Answer 26

Apply light-bodied material over the tooth (flows into detail). Use heavy-bodied material in the tray for support. Shrinkage tends toward the tray — the technique helps capture detail accurately.

Question 27

Why is it advised to use a fast snap technique to remove elastomers from the patients mouth?

Answer 27

Elastomers are strain-rate sensitive. Quick removal stores energy elastically, allowing the material to rebound properly.

Question 28

Why should there be a uniform thickness of impression material?

Answer 28

Ensures even dimensional changes during setting, reducing distortion.

Question 29

How thick should the impression be?

Answer 29

2–4 mm is ideal for accuracy and handling.

Question 30

What are the implications if the impression material is too thick?

Answer 30

Leads to increased polymerisation shrinkage, which can distort the impression.

Question 31

What are the implications if the impression material is too thin?

Answer 31

Results in higher strain during removal and more viscoelastic flow, increasing the risk of deformation.

Question 32

5 Factors Affecting Dimensional Stability (Post-Setting):

Answer 32

Continued setting reaction after removal. Slow elastic recovery (may change shape over time). Internal stress release, causing distortion. Loss of volatile components (like water or solvents). Moisture uptake, leading to swelling or warping.

Question 33

- Should the coefficient of thermal expansion of impression materials and stock trays be low or high?

Answer 33

Low — and they should be similar to each other.

Question 34

Why do impressions shrink upon removal from the mouth?

Answer 34

Thermal contraction. The temperature drop from mouth (32–37°C) to room temp (21–25°C) causes the material to contract.

Question 35

- Give 2 reasons why tear strength is important for impression materials:

Answer 35

Sulcus reproduction – to capture fine details without tearing Sulcus response – to avoid leaving residual material in the sulcus. A severe periodontal reaction

Question 36

What are the 4 currently available delivery systems for impression materials?

Answer 36

Pad-mix – Manual mixing of 2 putties Automix – Uses 2 syringes with 1 mixing tip; produces fewer bubbles Syringes – Narrow diameter maximizes pseudoplasticity (viscosity decreases under shear stress) Bulk mixers – For 500 ml cartridges; tip is replaced with each use

Question 37

What delivery system is best for dilatant and newtonian fluids and why?

Answer 37

1. Mixing pads 2. Due to applied stress affecting viscosity with the use of syringes

Question 38

What delivery system is best for pseudoplastic fluids and why?

Answer 38

1. Auto-mixing 2. Viscosity decreases on external stress.

Question 39

What does impression plaster consist of?

Answer 39

calcined b calcium sulphate hemihydrate mix with water rigid calcium sulphate dihydrate

Question 40

How can expansion of impression plaster occur and how can this be reduced?

Answer 40

plaster expands on setting reduced by addition of potassium sulphate, but accelerates creation between impression plaster so borax is used to retard the rate of reaction.

Question 41

Give 3 positives and 1 drawback of impression plaster.

Answer 41

pros: record fine details intially has low viscoirty and small dimensional changes cons: brittle

Question 42

What is the reaction that occurs with zinc oxide-eugenol and what is yielded?

Answer 42

two molecules of eugenol reacts with zinc oxide - zinc eugenolate can be used in impression taking only use 1mm to minimise dimensional changes

Question 43

Give a drawback and why it’s a drawback of zinc oxide-eugenol as an impression material.

Answer 43

non elastic impression material lacks elasticity can fracture when removed from undercuts