Dental Materials

Impression Materials

Impression Materials


A. General Comments

Impression materials are used to make replicas (models or casts) of teeth and other oral

In dentistry, we take impressions of teeth and their supporting structures. These supporting structures include gingiva, alveolar bone or residual ridge, hard and soft palate, and
frenums, which are muscle attachments.

The replicas are used to construct restorations and other appliances.

The impression is a negative reproduction, whereas the replica (model or cast) is a positive reproduction.

The impression must be an accurate duplication of the hard and soft tissues of interest and be stable enough to allow disinfection and production of a model.

Not all impression materials are compatible with all model materials.

– Because impression materials are used for many purposes, a wide variety of products are available to make impressions of oral tissues.

Some uses are simply to produce a physical model of the oral tissues for study, called a study model.

Study models are used in diagnosis and treatment planning.

Other uses require very exact (within 0.1%) replication of the size and shape of a preparation for the construction of a restoration or appliance.

These replicas are called casts.

A replica of a single tooth is called a die.

B. Impression Material Systems

Impression materials are supplied in a variety of forms.

Some are powders that are mixed with water; others are paste–paste systems.

– Several materials are softened or melted by heating.

Regardless of their form, impression materials are mixed (or heated) to make a thick paste or liquid.

– They are then loaded into an impression tray, placed in the mouth, and seated onto the
tissues of interest.

– The tray functions as a carrier and can stabilize the set impression material.

It is interesting to note that dental impression materials are also used to make impressions for medical prostheses (such as artificial eyes) and forensic investigations (bite marks).

Paste–paste impression materials (and many other dental materials) come in tubes, much like toothpaste tubes.

– The orifice of each tube is sized to provide the proper ratio of the two pastes when equal lengths are dispensed.

Thus, if the size of the two orifices is the same, one should dispense equal lengths.

– If the orifice of one tube is larger than that of the other tube, then again,
one should dispense equal lengths.

C. Cost

The cost of impression materials varies greatly, from pennies per impression to as much as several dollars.

An accurate cost analysis needs to include the percentage of first impressions that are
acceptable and the number of restorations that need to be remade.

Unacceptable restorations made on casts from cheap impression materials can make
dentistry a non profit enterprise.

D. Impression Trays

1. Use of Impression Trays

Impression trays are used to carry the impression material into the mouth, and the handle of the tray is used to remove the impression.

The tray can also support a weak material impression and improve accuracy.

Trays are supplied in a variety of shapes and sizes and are made from several materials.

Plastic disposable trays are very popular and work well with current infection control

Plastic trays are inexpensive and 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 and, thus, are less likely to distort when removed from the mouth.

Cleaning and sterilizing metal trays adds to the cost of their use.

2. Stock Trays

Stock trays are “off-the-shelf” items that come in a variety of materials, shapes, and sizes.

-The different-shaped trays are designed to take impressions of different oral conditions:
edentulous mouths, partially edentulous mouths, and mouths with a full complement of teeth.

– Stock trays also come in a range of sizes, from very small trays for pediatric patients to very large trays for large adult mouths.

– Stock trays can be metal or plastic.

3. Custom Trays

The most accurate impressions are made with custom trays.

Custom trays are made on a model of the patient’s arch with acrylic or other resin.

Two impressions are necessary when a custom tray is used.

A preliminary impression is taken with a stock tray and an inexpensive material.

– A gypsum product is poured into the impression and the resulting model is used to construct the custom tray.

The custom tray is then used to take a final impression.

Custom trays are used to take final impressions for full dentures, inlays, crowns, bridges, and some removable partial dentures.

– Custom trays use less impression material than stock trays because they fit around the teeth better than a stock tray does.

– Some say that custom trays are cost-effective because they use less material.

Others say that custom trays are cost-effective because they make the impressions easier to take and accurate impressions result more frequently than with stock trays.

It is important to note that proper mixing and handling of the impression material are the factors most critical for success.

4. Special-Use Trays

A variety of special-use trays have been developed.

– Bite registration trays record the occlusal surfaces of both arches.

Bite registrations are used to relate the upper and lower casts in the dental laboratory in
precisely the same manner as they come together in the patient’s mouth.

Another popular impression tray is the triple tray, which is used with the dual-arch or closed-mouth impression technique.

The triple tray is a quadrant tray with a “J” shape that curls around the posterior of the most distal molars.

With the dual-arch technique, the loaded tray (on both sides) is seated on the arch with the prepared tooth.

The patient then closes the mouth to his or her normal bite and the tray records
an impression of the prepared tooth, an impression of the opposing teeth, and a bite
registration, all at the same time

E. Classification of Impression Materials


1. Chemical Reaction or Physical Change

Impression materials set either by a chemical reaction or by a physical change.

– Impression materials set by chemical reactions to form elastic rubber materials are called thermoset.

– The chemical reaction involves chain lengthening, cross-linking, or both.

– Other impression materials set by a physical change when they cool, either by solidification or by gelation.

– Solidification occurs when molten wax cools and goes from a liquid state to a solid state.

Gelation is the process by which gelatin, such as Jell-O, changes from a liquid state to
a semisolid state when it cools.

Impression materials that undergo a physical change when they cool
are called thermoplastic.

In general, thermoplastic materials are not as stable as thermoset materials.


2. Use

(A) Elastic/inelastic

Because impression materials are used for a variety of purposes, the requirements of
a particular procedure determine which impression materials can be used.

– Impressions of dentulous mouths use trays and materials different from those for
edentulous patients.

– Teeth typically have undercuts (nooks and crannies) that require an elastic material to be used.

– Edentulous impressions may use elastic or inelastic impression materials.

Inelastic impression materials set hard and rigid compared with elastic impression materials and would “lock” around teeth if used on a dentulous patient.

(B) Accuracy

The accuracy required by the restoration or prosthesis determines which impression
materials can be used.

Not all impression materials have sufficient accuracy for crown and bridge impressions.

-Study models are not considered to be highly accurate reproductions of oral tissues.

Therefore, alginate is an acceptable impression material for study models, but it is
an unacceptable impression material for the fabrication of bridges, crowns, and inlays.

Nearly all impression materials are acceptable for full-denture impressions.

(C) Flow and detail reproduction

Many kinds of impression materials come in a variety of viscosities.

Viscosity is a measure of a liquid’s ability to flow. There are (from thinnest to thickest
consistency) light-body, medium-body, monophase, heavy-body, and putty materials.

Light-body materials are typically used with an impression syringe and are injected around preparations.

They are the most “runny” of the impression materials and best record the details of tooth preparations (small grooves, edges, and margins).

Many times, a retraction cord is placed in the gingival sulcus to facilitate crown and bridge impressions.

– In most instances, the patient will have had local anesthesia to ensure his or her comfort.

– The retraction cord pushes the gingiva away from the tooth.

– Frequently, the cord will contain a hemostatic or astringent medicament to control bleeding.

– After the cord has been in place for several minutes, it is usually removed, just before the
impression is taken.

There are exceptions, however, as the cord may be kept in place during the impression.

The use of a retraction cord allows the low-viscosity material to flow to, and thus reproduce, areas of a preparation that are difficult to reach.

– The cord moves the gingival tissue slightly away from the tooth and also controls the
moisture in the area.

Putty materials are the thickest impression materials, but they can still record the details of a fingerprint.

– Heavy-body and putty materials are placed in an impression tray, and their high viscosity
reduces running and dripping of the impression material out of the tray and onto the
operator or patient.

They are typically used with a light-body material.

The light-body material records the preparation and its margins, while the thicker material becomes the bulk of the impression.

The impression tray, which is filled with the high-viscosity material, is immediately seated over the light-body material.

– The materials then set together into one mass and are removed as one impression.

Medium-body impression materials may be injected or used in the tray depending on the dentist’s preferences.

– Monophase materials are designed to be used for both injection and in the tray.

– Therefore, only one mix of monophase material is needed.


3. Types of Impression Materials

Each impression material has its own advantages and disadvantages for use in dentistry.

 (A) Inelastic materials

1. Plaster

2. Wax and impression compound

3. Zinc oxide–eugenol (ZOE)

(B) Aqueous elastomeric impression materials

1. Alginate (irreversible hydrocolloid)

2. Agar (reversible hydrocolloid)

(C) Nonaqueous elastomeric impression materials

1. Polysulfides

2. Polyethers

3. Addition silicones

4. Condensation silicones

Classification and Use of Impression Materials


Type of





                       Used for the Construction of

Full Denture


Inlay, Crown
Or Bridge









Wax or




















hydrocolloid (agar)






Not used














































Addition silicone















I. Inelastic impression material


1. Plaster

Plaster would seem to be an unlikely impression material.

However, impression plaster is still sold.

– It has the same setting reaction and properties as the gypsum products that are used for models and casts.

– The major difference is that impression plaster has flavors added and that it sets more quickly to minimize the time that the material is in the mouth.

– When impression plaster sets, it is hard and brittle.

– Its major use is for an impression of an edentulous ridge in the fabrication of a denture.

Impression plaster is inexpensive compared with other materials, but its unpleasant taste makes it a seldom-used option.


2. Wax and Impression Compound


A. Wax

Wax was probably the first impression material used in dentistry.

It is cheap, clean, and easy to use.

A multitude of waxes are used in dentistry.

– Some are hard, much like paraffin wax that is used in canning jellies and jams.

– Others are soft and moldable at room temperature, similar to Play-Doh.

Wax used for taking impressions is solid at mouth temperature but is moldable at
a temperature that is tolerated by oral tissues.

Wax comes in many forms (sticks, strips, tubes, etc.).

– Wax can be thought of as a low-molecular-weight polymer.

It is a thermoplastic material.

It is also very weak, and the procedure technique must compensate for wax’s poor
mechanical properties.

– Wax is used by some dentists to take impressions for full dentures.

– Wax is often used to extend tray borders or adapt a stock tray when taking impressions.

More commonly, a variety of waxes are used as adjunctive materials in the dental laboratory for the fabrication of crowns, bridges, and other restorations.

Wax is softened or melted and then formed into the desired shape.

Then, the wax piece is surrounded by a mold material.

– Later, the wax is melted away and the mold is filled with a restorative material, such as gold.

These processes are described in more detail in subsequent chapters.

B. Impression Compound

Impression compound is wax with added filler to improve handling and stability.

It is stronger and more brittle and, when softened, flows much less compared with wax.

Impression compound is supplied as sticks or cakes of material.

It is softened in a warm water bath.

Because the thermal conductivity of impression compound is low, time and patience are
required to soften the material properly.

The heated, softened, and moldable material is placed in an impression tray, resoftened, and then seated in the mouth.

– When the material has cooled to mouth temperature, it returns to a rigid state and is

Impression compound is a stiff, thermoplastic material and is used by many dentists to make preliminary impressions for complete dentures.

–  Other impression materials that record fi ne details are better suited for final impressions.

3. Zinc Oxide–Eugenol (ZOE)

Zinc oxide–eugenol (ZOE) has been formulated for a wide variety of uses in dentistry,
including as an impression material.

A. Form of the Material

The ZOE materials come as two pastes.

– One paste contains eugenol and inert fillers; the other is formed by using zinc oxide powder mixed with vegetable oil.

Eugenol is a major component in oil of cloves.

Therefore, ZOE materials smell and taste like cloves.

For some patients, this is unpleasant.

The two pastes come in tubes, much like toothpaste.

Equal lengths of material from each tube are dispensed.

– Typically, the two pastes of ZOE (and other materials that come in tubes) are of different

– The pastes are swirled, stropped, and scraped together during mixing until one
homogeneous color is obtained.

B. Use

The ZOE impression materials set to a hard and brittle mass, which limits their use to
impressions of edentulous ridges for removable dentures.

– They are inexpensive and easy to use and were once very popular.

They are commonly used in a custom tray for the final impression for a complete denture.

Today, however, ZOE materials have been replaced by newer materials, such as addition

Although the newer materials have little advantage in terms of performance and are much more expensive, the simple advantage of stocking one less impression material in the
dental office is likely the reason for the declining use of ZOE impression materials.


 II . General Aspects of Hydrocolloid Impression Materials

Two impression materials are considered to be hydrocolloid materials because their major component is water.

Both materials change from a viscous liquid state, called the sol, to a semisolid, rubbery state, called the gel.

The sol state is a solution of one material dissolved in another.

In the gel state, two phases exist, much like a foam of soap bubbles.

– The first phase is a solid carbohydrate polymer network, like the soap used to create a foam.

The second phase is water trapped in very small pockets of the material, like the air trapped in the foam formed by soap.

Hydrocolloid materials that set via a chemical reaction are called irreversible hydrocolloids, or more commonly, alginate.

Heating alginate that was set results in warm alginate; it does reverse back to the sol (fluid) state.

Hydrocolloid materials that gel by a physical change (cooling) are called
reversible hydrocolloids.

These impression materials actually reverse back to the sol state when heated, then change again to the gel state when cooled; hence, the name “reversible hydrocolloid.”

Reversible hydrocolloid is also called agar or agar-agar (or sometimes simply hydrocolloid).


1. Alginate (Irreversible Hydrocolloid)

Alginate (Irreversible Hydrocolloid) Impression Materials


2. Agar (Reversible Hydrocolloid)

Agar (Reversible Hydrocolloid) Impression Materials

III. General Aspects of Nonaqueous Elastomeric
(Rubber) Impression Materials

A number of rubber-like impression materials have been developed for dentistry.

– A variety of names are used to describe this group of materials: non aqueous elastomeric
impression materials, rubber base materials, elastomers, and others.

– These materials set via polymerization reactions and are more stable than hydrocolloid
materials, but they are also more expensive.

– They are named based on their polymerization chemistry: polysulfide, condensation silicone, polyether, and addition silicone.


1. Polysulfides

Polysulfides as impression material


2. Polyethers

Polyethers as impression material


3. Addition Silicones

Addition Silicones as impression material


4. Condensation Silicones

Condensation silicone impression materials were the next elastomeric impression material
to be developed for dentistry.

They are based on silicone rubber, which is commonly used in other industries.

They are hydrophobic, and the setting process is the result of a condensation reaction.

An alcohol by-product rather than water is formed.

They are cleaner materials to use, but it is difficult to pour a model without voids
and bubbles.

As with polysulfide materials, loss of the reaction by-product through evaporation can result in distortion.

Condensation silicone impressions must be poured without delay.

These materials are not popular at present because newer materials provide better results.


Miscellaneous Comments


A. Other Uses of Impression Materials

1. Bite Registration Materials

Most elastomeric impression materials can be used as a bite registration material.

– The mixed material is placed on the occlusal surfaces of the mandibular arch.

– The patient bites into the material.

– The impression material sets, recording the relationship of the maxillary arch to the
mandibular arch.

– The bite registration is then removed and used in the dental laboratory to relate an upper cast to a lower cast in the same manner as the patient’s natural bite.

2. Mold for Temporary Crowns

Putty can be used to make a “mold” for the fabrication of temporary crowns.

– Before the tooth is prepared for a crown, the mixed putty is placed over the tooth

– The putty sets and is removed, and the tooth is then prepared.

– The area of the prepared tooth of the putty impression is filled with a plastic temporary

– The most common temporary material is chemically activated acrylic resin.

– The putty impression is filled with the acrylic resin and is reseated in the patient’s mouth.

The prepared tooth forms the inside of the temporary crown, and the putty impression forms the outside of the mold of the temporary crown.

– After the acrylic resin sets, it is removed from the mouth and is trimmed, polished, and then temporarily cemented.

B. Compatibility of Impression Materials with Die Materials

Not all impression materials are compatible with all model materials.

– All impression materials can be poured with gypsum products, but not all impression
materials can be electroplated to form dies.

Hydrocolloid materials are not compatible with epoxy die materials.

C. Optical Impressions

Dentistry has embraced digital technology when it is economically advantageous.

– One example is digital radiography, which has been adopted by many practices.

Several CAD/CAM systems have been developed for dentistry.

CAD/CAM stands for computer-aided design/computer-aided manufacture.

CAD/CAM requires a computer model of the tooth preparation.

Several techniques are used.

One involves the usual dental impression with an impression material.

– The resulting cast is digitized at the dental laboratory.

– The restoration is then designed using interactive software and machined by milling
(grinding) the restoration out of a piece of solid material.

– The dental laboratory assumes the financial burden of the cost of the equipment.

The cost is spread among many dental practices.

The second technique uses an “optical” impression integrated with a CAD/CAM milling

The optical impression is taken with an intraoral infrared camera.

The “optical” impression is stored digitally in a computer.

– The restoration is designed and milled in the dental office.

– The cost of the equipment is born by the individual dental office.

– In-office CAD/CAM systems are quite expensive but are increasing in popularity.

– In the future, optical impressions may be more common when just the camera is purchased and the resulting data file is sent to a dental laboratory for construction of the restoration.

– The common crown and bridge impression may become a memory, as has the eight-track
audio tape.

D. Performance of Impression Materials

Impression materials are the best performing of all dental materials.

The use of impression materials has fewer restrictions than are found with most other dental materials.

– Impression materials only need to function for a short time (several hours to a few days).

In addition, simulating clinical use of impression materials is quite simple compared with
restorative materials, such as amalgam and composite.

Research, product development, and quality control are greatly facilitated when clinical use can be easily simulated in the laboratory.

E. Biocompatibility of Impression Materials

Some assistants have had a skin rash on their hands that has been attributed to the
manipulation of impression material.

– With the current practice of wearing gloves when treating patients, skin problems are first blamed on latex gloves.

– However, one must not forget that other dental materials can cause skin irritation, even though this is rare.

– Polymeric dental materials are much more likely to be irritating before they are mixed; the cured or set polymer is typically much less of a problem.

– Because impression materials are used in the mouth only for several minutes,
biocompatibility is usually not a significant concern.

– However, if residual pieces of impression material remain subgingivally or between teeth,
significant irritation will occur.

The clinician should inspect the oral cavity for residual material after impression removal.


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