DENTAL MATERIALS LECTURE #3

Done by: 1*Latifa Sabbah

Corrected by: Hussam Hamad

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In this lecture we will talk about impression materials and their

properties.

Definition

(A dental impression may be described as an accurate

representation of part or all of a person's dentition and other

areas of the mouth. The dental impression forms an imprint

(i.e. a 'negative' mould) of those teeth and gums, which can

then be used to make a cast or 'positive' model of the

dentition.)

So, we use impression materials to take measurements of

teeth, soft tissues and any details which I want to take its

measurements. We can get these impressions by using

different materials each one has its own properties.

After taking measurements, pouring the material in plaster or

stone then we have a cast (model) for studying purposes or

working on it. They may be used for the fabrication of

dentures, crowns or other prostheses.

1 2

Impression Materials and Technique

(Well-fitting indirect restorations can only be made if there are

accurate models of the oral tissues available, made from high

quality impressions) Indirect restoration )التعويضات( in cases of

bad or destruction tooth, missing tooth, crown, bridge,,,

These restorations must be well-filling, we can achieve this by

having an accurate cast which done by high quality of impression

which done by perfect using of materials and trays.

(Waiting for an impression to set may be more stressful for the

dentist than the patient. Should the impression need to be

repeated there is the embarrassment of having to explain this to

the patient, the cost implications of material and time wasted

and the aggravation of running late for the next appointment)

At the beginning all materials are (soft) but after a setting time

they will become rigid. This process is stressful for the dentist

because he should wait until a setting time and repeat the

In the 1st picture you can see

impression trays, but here

they aren’t custom made

trays means not suitable for

the patient, they are already

made trays. using them with

impression material to have a

cast (2nd pic). We have

negative…4:33…… then

positive…………… .

impression if it wasn’t accurate, this leads to a high cost.

(Sometimes impression defects only come to light after the cast

has been poured. Thus laboratory inspection is an important part

of quality control, but many technicians find it difficult to feed

back to their dentists for fear of the messenger being shot)

(Feedback between laboratory and dentist is critical to the

establishment of an open and honest relationship. Hopefully, an

improved understanding of impression materials coupled with

techniques to overcome problems will encourage higher

standards and the confidence to accept appropriate advice)

Requirements of dental impression materials

1- Accuracy (which is the most important requirement and affected by

7 factors)

Viscosity: indirect relation with accuracy, low viscosity gives

more accurate details.

Interaction with saliva: saliva consists of 99% of water, so we

should use a hydrophilic materials because hydrophobic ones

will go away from saliva leading to wrong readings, so we have

to dry the teeth and required areas very well (doing moisture

control) which is difficult.

Wetting ability (contact angle): contact angle between

impression material and teeth (tissues), to understand remember

the water and honey, if I pour water on the table, it will wet the

surface easier and better than honey could, so water has higher

wetting ability.

This factor related to (viscosity, polarity and surface tension).

Dimensional stability (Dimensional change on setting):

the material should maintain its shape even after become rigid

and after removing it from the patient mouth.

Thermal changes: the temp. of patient mouth is 36-37 degrees

and the room temp. differs seasonally. Some of these materials

may be affected by changing in temp.

Adhesion to the tray: we don’t want the impression material to

get stuck to the tooth and not come out on the tray.

Two types of retentions:

-chemical: chemical reactions.

-physical: as adhesion and cohesion "mainly mechanical

adhesion"

Adequate elastic properties, rigidity and tear resistance:

Tearing resistance ( ت المادة بس أطلعها)عامل مهم عشان ما تتفت

elasticity

2-Dimensional stability

3-Ease of handling, setting characteristics, etc. the material

should be easy to handle (easy to deal with), not messy and

don’t need lots of time.

4-Low cost

5-Good taste

Impression

materials are

mainly divided

into two

categories:

Hydrocolloid impression materials

2 types: 1- reversible (Agar) 2- irreversible (Alginate).

(Agar is a reversible hydrocolloid because it can pass

repeatedly between highly viscous gel and low viscosity sol

simply through heating and cooling. However, alginate once

converted to the gel form cannot be converted back into the

sol, and is therefore said to be irreversible hydrocolloid

material)

*These materials have two stages:

Gel with increasing Tem. ⇾ Sol stage.

Sol with decreasing Tem. ⇾ Gel stage.

Characteristics:

Viscoelastic: viscosity and elasticity increase with pressure.

More accurate in bulk: (3-5 mm) in alginate the proper

thickness is 3-5 mm to get accurate details. If less or more ⇾ poor details. Less than 3mm the impression will tear easily.

Which makes a proper space between alginate impression and

the tooth.

Advantages:

Reasonable surface details: not excellent nor bad

Hydrophilic: no need to remove the saliva

Cheap

Disadvantages:

Low tear resistance: Alginate has low tear resistance; so if you

applied a force (tear force) during the removing of Alginate

impression it will easily fragmented, so remove it in single

strong pull (slam).

Alginate should not be thin (this important in thin sections).

Very poor dimensional stability:

There are 2 phenomena happen for gel materials as alginate:

* Syneresis: we should not put alginate (gel) impression in dry

environment, because it will lose water.

* Imbibition: we should not put this impression in water because

it will absorb water so increasing in size (not accurate details)

This means, when we use disinfectant solution for the

impression it should be between 30 sec or reach 1 min according

to its guide line to avoid the imbibition. Latest studies said that

we have to pour plaster or stone on the impression in not more

than 15 min only, we wrap the impression with a wet gauze and

put it in a sealed plastic pocket.

Reversible hydrocolloids (agar)

Chemistry:

* Agar (colloid) 12.5%, a complex polysaccharide which is

extracted from seaweed

* Borax (strengthen gel) 0.2% function: body, filler

* Potassium Sulphate 1.7%

* Water (dispersion medium) 85%

*Normally supplied as a gel in a flexible,

toothpaste-like tube or syringe.

*Reusable!

Now we don’t use it in dental clinics but used in labs for

duplication of the cast because it is already disinfected so no

need for disinfecting again.

◆ The main cause that we don’t use it is disinfection, when use

it for a patient it will change from gel to sol stage and another

reason is needing special equipment:

*A special water-bath for conditioning the impression material.

*A water cooled tray.

Irreversible hydrocolloids (alginates)

*Supplied as powder (in packets) that is mixed with water.

*Use measured powder-liquid quantities… Rapid spatulation in

a plastic mixing bowl to get a creamy consistency.

*Each factory has special measurements .stick to a ratio between

water and powder (as one spoon of water to a spoon of powder)

as if we put more water, the viscosity will decrease so the tear

resistance and accuracy will be affected.

*Initial low viscosity

*Temperature of the water controls working and

setting times (increasing in temp. will decrease

setting time means fast setting)

The synthetic elastomers

*First introduced in the late 1950s, synthetic elastomeric

impression materials quickly became popular as dental materials

because they significantly reduced the two main problems

associated with the hydrocolloids, namely poor dimensional

stability and inadequate tear resistance.

* Available in different consistencies or viscosities.

* Mixing systems: hand mixing, static auto mixing, and dynamic

mechanical mixing.

Polysulfide

* The polysulfide impression materials have the longest history

of use in dentistry of all the elastomers.

* Interestingly, they were first developed as an industrial sealant

for gaps between sectional concrete structures.

ويستخدم في كاوشوك السيارات

* They are available in a range of viscosities namely, light

bodied (low viscosity), medium or regular bodied and heavy

bodied (high viscosity). We use each of them according to the

need as we use light ones for more accuracy.

* These are now relatively unpopular materials because of its

bad smell and other disadvantages.

The blue line is atray with pores inside it the

impression polysulfide material which give a

mechnical adhesion.

◆2 types of polymerization reactions:

1/Addition reaction: without a by product

2/condensation reaction: with a by-product mainly H2O, OH-

from a monomer and H+ from another monomer. Which does a

small effect on the dimensional stability of these rubbers

(polymers).

*Supplied as two pastes: catalyst and base.

*Setting reaction: condensation polymerization (water by-

product)

*Lead dioxide is the catalyst enhancing the oxidation reaction.

Special considerations:

* A special tray, providing a 4 mm uniform space, is needed to reduce

distortion from the shrinkage of a large bulk of material.

*The recommended maximum storage time of the set impression is

about48 hours.

Advantages:

Long working time.

Excellent tear resistance, undergoing considerable tensile strain

before tearing.

Disadvantages:

The setting reaction of poly-sulfides tends to be long with setting

times often in excess of10 minutes (acceleration is possible by adding a

small drop of water to the mix).

They are also messy to handle and have an objectionable odor.

Unfortunately, their elastic properties are not ideal and some of this

strain may not be recovered (high value for stress relaxation 2 minutes

after setting time. To optimize the recovery of these viscoelastic

materials, the impression should be removed with a single, swift pull as

the strain imparted on the material is a function of the time for which

the load is applied. This method of removal of impressions should be

adopted when using any impression material, irrespective of its elastic

properties.

Shrinkage: Continued setting reaction after the apparent setting time

.Evaporation of water produced as a by-product of the setting reaction.

Polyether

A popular polyether impression material, Impregum (Espe

GmbH, Germany), was the first elastomer to be developed

specifically for use in dentistry and introduced in the late 1970s.

Initially available only in a single ‘regular’ viscosity. A heavy

and light bodied systems have been recently introduced.

They undergo an addition cured polymerization reaction on

setting which has no reaction by-product resulting in a material

with very good dimensional stability.

Advantages:

Hydrophilic: Polyether impression materials are more

forgiving of inadequate moisture control than the hydrophobic

polysulfides and silicone rubbers.

Polyether impression materials tend to have a fast setting time

of less than 5 minutes and, for this reason, have been popular for

the recording of single preparations in general practice.

Polyether impression materials have adequate tear resistance

and very good elastic properties.

Disadvantages:

Rigidity: they do have a high elastic modulus and

consequently are relatively rigid when set, hence considerable

force may be required to remove the impression from both the

mouth and the stone cast. This may preclude their use in cases

where severe undercuts are present.

Special considerations: The set material may swell and

distort because of the absorption of water on storage in

conditions of high humidity; therefore, impressions should be

stored dry. They should also not be stored in direct sunlight.

Ideally, impressions should be poured within 48 hours of them

being recorded.

Back to the tables and memorize them 😊

Best wishes