Impression materials

What are impression materials?
 A dental impression is a negative record of the tissues
of the mouth. It is used to reproduce the form of the teeth &
surrounding tissues.
The negative reproduction of the tissues given by the
impression material is filled up with dental stone or other model
materials to get a positive cast.

 The positive reproduction of a single tooth is described as a
‘die’, & when several teeth or a whole arch is reproduced, it is
called a ‘cast’ or ‘model’.
 Impression materials are used to make replicas of oral
structures.
 All impression materials must be in a plastic or fluid state
while the replica is being made.
…….

Dental Impression (negative replica)

There are several ways of classifying impression materials
1. According to mode of setting & elasticity
2. According to their uses in dentistry

According to their uses in dentistry
1. Impression Materials used for Complete Denture Prosthesis
 Impression plaster, impression compound & impression paste set to a hard rigid mass
& hence cannot be removed from undercuts without the impression being
fractured or distorted. Therefore these materials are best suited for edentulous mouth.
2. Impression Materials used for Dentulous Mouths
 On the other hand alginates & rubber base impressions are sufficiently elastic to
be withdrawn from undercut areas. Such elastic impression materials are suitable for
impressions for fabrication of removable & fixed partial denture prostheses,
where the impressions of the ridge & teeth are required.

Use of Impression Trays
− Impression trays are used to:
1. Carry the impression material into the mouth, & the handle of the tray is
used to remove the impression.
2. Support a weak material impression & improve accuracy.

Trays are supplied in a variety of shapes & sizes and are
made from several materials.
− Plastic disposable trays are very popular & work well with current infection
control practices. Plastic trays are inexpensive & convenient, but they do not
support the impression as well as metal trays do.
− Metal trays are more expensive but reusable. In addition, metal trays are
stiffer &, thus, are less likely to distort when removed from the mouth. Cleaning
& sterilizing metal trays adds to the cost of their use.

Different types of impression trays

1. metal impression trays
• For use with edentulous patients
• Autoclavable
• Available in different sizes
• Not easily adapted
• Available in perforated & non-perforated trays
• May be used in conjunction with an adhesive

2. plastic impression trays
• For use with edentulous patients
• Single use
• Available in different sizes
• Easily adapted using a heat source &/or impression compound
• Available in perforated and non-perforated trays
• May have a separate metal handle that is autoclavable

3. metal impression trays
• For use with dentate patients
• Autoclavable
• Available in different sizes (the size is usually engraved on the handle of the tray)
• Not easily adapted

4. plastic impression trays
• Single use
• Available in different sizes (usually denoted by colours)
• Easily adapted using a heat source and/or impression compound

5. Universal sectional impression tray
• Used for impressions of a specific areas of the mouth
• Easily adapted as it is fabricated from flexible metal

6. Custom / special tray
• Fabricated from a plaster model of the patient’s dentate or edentulous arch
• Fabricated from an acrylic material
• Single use

7. Triple tray
• Used to take an impression of both arches simultaneously
• Fabricated from a combination of plastic &flexible webbing material
• Different sizes available
• Single use

The rigid impression materials
The rigid impression materials are:
1. Impression plaster
2. Impression compound
3. Zinc oxide eugenol impression paste
4 Impression waxes

Impression
compound
One of the oldest of the dental impression materials.

 Impression compound can be described as a rigid, reversible impression
material which sets by physical change.
 On applying heat, it softens & on cooling it hardens.
 It is mainly used for making impressions of edentulous ridges.
 A more viscous variety of compound (tray compound) is used to form a
tray in which a second more fluid material is placed to make a more
detailed impression compound.

Type I: Impression compound  for making an impression in an edentulous
mouth
Type II: Tray compound  is used to prepare a tray for making an
impression. A second material is then carried in it in order to make an
impression of oral tissues. Since reproduction of the fine details is not
essential, it is generally stiffer & has less flow than regular impression
compound.

Supplied as
1. Sheets,
2. Sticks,
3. Cakes
4. Cones in a variety of
colors

Applications
1. For making a preliminary impression in an edentulous mouth (mouth without teeth)
2. For individual tooth impression
3. Peripheral tracing or border molding
4. To check undercuts in inlay preparation
5. To make a special tray

Advantages
1. The material can be reused a number of times (for the same patient only) in
case of errors.
2. Inaccurate portions can be remade without having to remake the entire
impression.
3. Accuracy can be improved by flaming the surface.
4. The material has sufficient body to support itself especially in the peripheral
portions. It does not collapse completely if unsupported by the tray.

Disadvantages
1. Records less detail because of its high viscosity.
2. Compresses soft tissues during impression.
3. Distortion due to its poor dimensional stability.
4. Difficult to remove if there are severe undercuts.
5. There is always the possibility of overextension especially in the peripheries.

Zinc oxide
eugenol
impression paste

Zinc oxide & eugenol based products are widely used in dentistry:
1. Cementing and insulating medium.
2. Temporary filling material.
3. Root canal filling material.
4. Surgical pack in periodontal surgical procedures.
5. Bite registration paste.
6. Temporary relining material for dentures.
7. Impressions for edentulous patients.

zinc oxide eugenol is popular as an impression material for making
impressions of edentulous arches for the construction of complete dentures.
It is classified as a rigid, irreversible impression material.

Classification
ADA specification No. 16.
Type I or Hard
Type II or Soft

Available as
In paste form in two tubes
Base paste (white in color).
Accelerator or reactor or catalyst paste (red in color).

Advantages
1. It has sufficient body so as to make-up for any minor under extensions in the tray
itself during impression making. (Co-effective)
2. It has enough working time to complete border molding.
3. It can be checked in the mouth repeatedly without deforming.
4. It registers accurate surface details.
5. It is dimensionally stable.
6. Does not require separating media since it does not stick to the cast material.
7. Minor defects can be corrected locally without discarding a good impression.

Disadvantages
1. It requires a special tray for impression making.
2. It is sticky in nature and adheres to tissues.
3. Eugenol can cause burning sensation and tissue irritation.
4. It cannot be used for making impression of teeth &undercut areas as it is inelastic in nature.

Types of elastic impression materials
Two systems are used:
1. Hydrocolloids
2. Elastomeric materials

Irreversible hydrocolloid
Alginate

 The word alginate comes from ‘alginic acid’.
 Alginate is the most widely used impression material in the world.
 Currently alginate is more popular than agar for dental impressions, because it
is simpler to use.
 TYPES
Type I — Fast setting.
Type II — Normal setting.

Supplied as
 A powder that is packed:
1. Commonly in bulk packing (tins, bins or sachets).
2. In preweighed packets for individual impression.
3. A plastic scoop is supplied for dispensing the bulk powder and a plastic
cylinder is supplied for measuring the water

1. It is used for impression making
— When there are undercuts.
— In mouths with excessive flow of saliva.
— For partial dentures with clasps.
2. For making preliminary impressions for complete dentures.
3. For impressions to make study models & working casts.
4. For duplicating models.
Applications

 Mixing equipment includes:
— A clean flexible plastic bowl &
— A clean wide bladed, reasonably stiff metal spatula.

 For fast set alginate - 45 seconds.
 For normal set alginate - 60 seconds.
 Over Mixing Results In
— Reduction in final strength as the gel fibrils are destroyed.
— Reduction in working time.
 Under Mixing Results In
— Inadequate wetting, lack of homogeneity and reduced strength.
— The mix being grainy and poor recording of detail.
Mixing time

Working time
Fast set alginate — 1¼ minutes.
Normal set alginate: — 2 minutes.
Gelation time (setting time)
Type I (fast set) — 1.5-2.0 mins.
Type II (normal) — 3-4.5 mins.

Rinsing removes traces of saliva, bacteria and other contaminants.

Advantages and disadvantages of alginate

Advantages
1. It is easy to mix and manipulate.
2. Minimum requirement of equipment.
3. Flexibility of the set impression.
4. Accuracy if properly handled.
5. Low cost.
6. Comfortable to the patient.
7. It is hygienic, as fresh material must be used for each impression.
8. It gives a good surface detail even in presence of saliva.

Disadvantages
1. Cannot be electroplated so metal dies are not possible.
2. It cannot be corrected.
3. Distortion may occur without it being obvious if the material is not held
steady while it is setting.
4. Poor dimensional stability—it cannot be stored for long time.
5. Poor tear strength.
6. Because of these drawbacks and the availability of better materials, it is not
recommended where a higer degree of accuracy is required, e.g., cast RPD,
crowns and FPDs, etc

ELASTOMERIC IMPRESSION MATERIALS

Uses of elastomeric impression materials
1. In fixed partial dentures for impressions of prepared teeth.
2. Impressions of dentulous mouths for removable partial dentures.
3. Impressions of edentulous mouths for complete dentures.
4. Polyether is used for border molding of edentulous custom trays.
5. For bite registration.
6. Silicone duplicating material is used for making refractory casts during
cast partial denture construction.

Polysulphides
Impression Materials
(Polysulfides) (Rubber Base)

Material constituents/composition
• Supplied in a two-paste system
• Available in low, medium and high viscosity

Properties
• The working and setting times are altered with heat and humidity
◦ Higher the temperature and humidity, the shorter the working & setting time
◦ Lower the temperature and humidity, the longer the working & setting time
• Hydrophilic
• Useful when a long working time is needed

Advantages
• High accuracy
• Long working time
• Good tear strength
• Low cost
Disadvantages
• Material will stain clothes
• Very unpleasant odor
• Must be used in conjunction with a custom/special tray

Indications
• Fixed partial denture impressions
• Crown and bridge impressions
• Implant impressions
Contraindications
• Patients with a strong gag reflex
• Mixing time → 45–60 seconds
• Working time → 5–7 minutes
• Setting time → 8–12 minutes

Silicone Rubber
Impression Materials

● There are two types of Silicone Impression Materials available:
1. Condensation &
2. Addition or vinyl silicones.
●They are categorized in relation to the type of reaction responsible for
their setting.

• Available in two-paste or liquid/paste systems
• Available in light, medium and heavy viscosities as well as a putty
• May be difficult to achieve a uniform mix as base and catalyst pastes are not
of the same consistency
Properties
• Moderate shelf-life
• Moderate tear strength

Advantages
• Accurate
• Easily manipulated
• Fast setting time
• Non-toxic and non-irritant
• Very elastic
Disadvantages
• Setting may be affected by changes in temperature and humidity
• Hydrophobic
• Prone to shrinkage on storage– condensation impressions must be poured up within an hour

Indications
• Inlay and onlay impressions
• Implants
Contraindications
• May cause irritation with some patients

•Supplied in:
1. Two-paste System &
2. Auto mix cartridge system (dispensing gun)

• Available in:
Light, medium and heavy viscosities as well as a putty
• Base & catalyst pastes are the same consistency, which makes manipulation easy
• May be referred to as vinyl polysiloxane impression materials

Properties
• Accurate
• Good shelf-life
• Dimensionally stable
• Non-toxic and non-irritant
• Moderate tear strength

Advantages
• Accurate
• Easily manipulated
• Range of viscosities
• Fast setting time
• Dimensionally stable – they are not prone to shrinkage on storage –
addition silicones can be poured up multiple times. They are more
expensive than condensation silicones

Disadvantages
• Setting may be affected by changes in temperature and humidity
• Hydrophobic
• Powdered latex gloves may retard the setting properties in the putty
form of addition silicone impression materials

Indications
• Inlay and onlay impressions
• Implants
Contraindications
• Using powdered latex gloves during the manipulation of the putty form of
material will affect the material properties adversely

Waxes have a variety of uses within dentistry & are manufactured from
various materials, including plants, minerals, animals and synthetic
waxes.
They can be used both intra- and extra-orally.

Waxes can be divided into three categories:
1. Pattern waxes: are used to pattern moulds of various restorations and
dental prosthesis. They all burn out without leaving a residue, create
smooth surfaces and carve easily.
2. Processing waxes: have an auxiliary function in the fabrication of
impressions and casts.
3. Impression waxes: are used intra-orally for taking impressions

• They are thermoplastic materials that present as solids at room
temperature; they can be softened with heat and hardened with cooling.
• Waxes are often used in conjunction with dies and models to facilitate
construction of various restorations, appliances and prostheses.

Instruments and materials used in set-up
• Choice of wax
• Choice of heat source
• Wax knife
• Le Cron carver
• Flat plastic instrument
• Bowl of cool water

Impression materials

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