IMPRESSION MATERIALS

A dental impression is defined as the negative record of the tissues of the mouth . It is used to reproduce the form of the teeth and surrounding tissues.

DEFINITION:

Have pleasant taste, odor and esthetic color. Not contain any toxic or irritating ingredients. Be economical. Have adequate shelf life for storage and distribution. Be easy to use with minimum equipments. Exhibit dimensional stability. Have adequate strength so that it will not break or tear

while removing from the mouth. Posses elastic properties with freedom from permanent

deformation after strain. Be able to be electroplated Be compatible with die and cast materials

DESIRABLE PROPERTIES OF AN IMPRESSION MATERIAL:

CLASSIFICATON:

There are several classification. According to philips science of dental materials impression materials are classified on the basis of setting and elasticity.

Mode of setting rigid Elastic

Set by chemical reaction(irreversible)

Impression plaster, zinc oxide eugenol.

Alginate, polysulfide, polyether, silicone,

Set by temperature change(reversible)

Compound, waxes Agar hydrocolloid.

Impression compound:

Impression compound is rigid,reversible impression material sets by physical change.

It softens on heating and hardens on cooling.

ADA specification no 3 Supplied as sheets,sticks,cakes and

cones.

Making preliminary impression of edentulous mouth. For individual tooth impression. To make a special tray. Border moulding. To check undercuts in inlay preparations.

Composition:

Applications:

INGREDIENTS PARTS

resin 30

Copal resin 30

Carnauba wax 10

Stearic acid 5

Talc 75

Glass transitional temperature =39 degree centigrade. Fusin temperature=43.5 degree centigrade. They are poor conductors of heat. Coefficient of Linear Expansion is high. Good dimensional stability. Good flow. Fine reproduction of detail.

Properties:

Material can be reused several times. Inaccurate portions can be remade without having to

remake the entire impression. Accuracy can be improved by flaming surface material.

Disadvantages: Distortion due to poor dimensional stability. Compress soft tissue while making impression. Difficult to remove if there are undercuts. Difficult to record details because of high viscosity.

Advantages:

Applications: Cementing and insulating medium. Temporary filling. Root canal filling material. Bite registration paste. Impression material for edentulous

patients. Temporary relining material for

dentures.

ZINC OXIDE EUGENOL IMPRESSION PASTE:

Composition:

Available as:base paste(white in colour)Accelerator or reactor or catalyst paste(red in colour)

Base paste Accelerator paste

Zinc oxide-87% Eugenol-12%

Fixed vegetable oil or mineral oil-13%

Gum-50%

Filler(silica)-20%

LANOLIN-3%

Resinous balsam-10%

Accelerator solution (calcium chloride)-5%

Setting reaction is an acid base reaction to form a chelate. This reaction is also known as chelation and the product is

called zinc eugenolate. Zno + H2O Zn(OH)2 Zn(OH)2 +2HE ZnE2 +2H2O (Base) (Acid) ( Salt)

Setting time;

Setting reaction:

Initial setting time

Final setting time

Type 1 3-6 min 10 min

Type 11 3-6 min 15 min

Particle size of zinc oxide powder. Longer mixing time, shortens setting time. High atmospheric temperature and humidity accelerate

setting. Setting can be delayed by cooling mixing slab, spatula or

adding small amount of retarder or oils or waxes.

Properties: Good consistency and flow. Detail reproduction. Rigid and having good strength. Good dimensional stability.

Factors controlling setting time:

Mixing is done on an oil-impervious paper or glass slab. Two ropes of paste of same length and width, are squeezed

on mixing slab. A flexible stainless steel spatula is used for mixing. Two ropes are combined until a uniform colour is obtained. Mechanical rotary mixing are also used. Mixing time is one minute

Manipulation:

It has enough working time to complete border moulding. It can be checked in mouth repeatedly without deforming. It registers accurate surface details. It is dimensionally stable. Does not require separating media since it does not stick to

cast material. Minor defects can be corrected.

Disadvantages: Requires special tray for impression making. Sticky in nature and adheres to tissues. Burning sensation of eugenol causes tissue irritation. Highly inelastic in nature.

Advantages:

NON-EUGENOL PASTE are developed to overcome the burning sensation caused by eugenol.

Bactericides and other medicaments are also incorporated.

Used in eugenol irritant patients.

NON-EUGENOL PASTES;

Agar hydrocolloid was the first successful impression material to be used in dentistry.

It is an organic hydrocolloid extracted from certain types of seaweeds.

Agar is a reversible hydrocolloid. Since it is reversible can be reused.

ELASTIC IMPRESSION MATERIALS;AGAR;

As tissue conditioner. For full mouth impression without deep undercuts. Used extensively for crown and bridge impression before

elastomers came to the market. Widely used at present for cast duplication.

Supplied as; Gel in collapsible tube(for impression) A number of cylinders in a glass jar(syringe material). In bulk containers(for duplication)

Uses

INGREDIENTS PERCENTAGE BY WEIGHT

Agar 13-17%

Borates 0.2-0.5%

Potassium sulphate 1-2%

wax, hard 0.5-1%

Thixotropic material 0.3-0.5%

Alkylbenzoates 0.1%

water Around 84%

Coloring and flavoring agents Traces

Composition;

Agar-basic constituent. Potassium sulphate-It counters retarding effects of

borates. Wax-filler. Thixotropic material-plasticizer. Alkylbenzoates-preservative. Coloring agents-for patients comfort Water-dispersion medium.

Manipulation;Agar hydrocolloid requires special equipments ; Hydrocolloid conditioner. Water cooled rim lock tray.

Function of each ingedient;

Rim lock trays with water circulating devices are used (water cooled rim lock tray).

Rim lock has a beading on the inside edge of the tray which help to retain the material within the tray.

It also has an inlet and outlet for connecting water tubes.

The tray should allow a space of 3mm occlusally and laterally and extend distally to cover all teeth.

Impression tray;

Hydrocolloid conditioner;Boiling section or liquefaction section Ten minutes

in boiling water at 100 degree

Storage section 65-68 degree is ideal

Tempering section 46 degree for two minutes with material loaded in tray

Hydrocolloid conditioner

Syneresis and imbibition; The hydrocolloid gel may loose water by exuding of fluid.

and this process is known as synersis. Imbibition is the process of gaining of water when

immersed in water. Importance-both the phenomenon can cause dimensional

changes and produce inaccurate casts. To avoid this impressions should be poured immediately.

Flow;The material is sufficient fluid to record fine details.

Properties;

Material is sufficiently fluid to record details.

Flexibility; Material is highly flexible.Hysteresis; Temperature lag between liquefaction and gelation. Elasticity and elastic recovery; Highly elastic in nature. Elastic recovery occurs to the extent of

98.8% Fine reproduction of detail;Accuracy and dimensional change They expand when stored in water or 100% relative humidityWorking and setting time Working time- 7 to 15 min Setting time – 5 min

Flow;

Main disinfectants used include iodophor,bleach or

glutaraldehyde.

ADVANTAGES Accurate die can be prepared, if material is properly

handled Has good elastic properties. Good recovery from distortion Palatable and well tolerated by patient Cheap

IMPRESSION DISINFECTION

Not flow well compared to newly available material

It cannot be electroplated Tears relatively easily Only one model can be poured

Disadvantages

Its an irreversible elastic hydrocolloid

It is a mucous extract yielded from certain brown sea weeds

Types type 1 – fast setting type 2- normal setting Applications Used for impression making; when there are undercuts in mouth with excessive flow of

saliva For impression to make study

models and working casts For making preliminary impression

ALGINATE

INGREDIENTS FUNCTIONS

Sodium or pottasium alginate-15%

React with calcium ion

Calcium sulphate(reactor) - 16%

React with potassium alginate

Zinc oxide - 4% Acts as filler

Potasium titanium flouride -3% Gypsum hardener

Diatomacous earth -60%

Acts as filler

Sodium phosphate(retarder) -2%

React with calcium sulphate

Colouring and flavouring agents Wintergreen, orange etc.

composition

Two main reaction occur 2Na3Po4 + 3CaSo4 Ca3 (PO4)2

+3Na2So4 Here firstly the retarder is completely get

reacted to delay the reaction time. Sodium alginate + CaSo4+H2O

calcium alginate +Na2So4

SETTING REACTION

Alginate has pleasant taste and smell It is highly flexible Good elasticity and elastic recovery Fine reproduction of tissue detail Compressive strength – ranges from 0343-70

mpa Tear strength- 350-700gm/cm2 Dimensionally stable Poorly adheres to the tray Biological properties- inhalation of silica

particles cause health hazards.

PROPERTIES OF ALGINATE

steps in manipulation

fluff or aerate the powder by investing can several times.

for mixing a clean flexible plastic bowl and clean wide bladed stiff metal spatula is being used.

proper water powder ratio specified by the manufacturer should be used(1 measure water with two level scoops of powder)

mixing is started with a stirring motion. vigorous figure 8 motion can also be used.

MANIPULATION

Mixing time fast setting - 45 sec normal setting- 60sec Working time – fast setting – 1min normal setting – 2 min Gelation time fast setting - 1 to 2min normal - 2 to 4.5 min

PROPERTIES:

Easy to mix and manipulate Low cost Comfortable to patient Hygienic Good surface detail even in presence of

saliva Accuracy if properly handled Minimum requirement of equipment

ADVANTAGES

Cannot be electroplated It cannot be corrected Poor dimension stability Poor tear strength Distortion may occur if not properly handled

DISADVANTAGES

Classification A) according to chemistry 1) polysulfide 2) condensation polymerizing silicones 3) addition polymerizing silicon 4) polyether B) according to viscosity 1) light bodied(syringe consistency) 2) medium(regular) 3) heavy body(tray consistency) 4) very heavy(putty consistency)

ELASTOMERIC IMPRESSION MATERIAL

A) Base paste liquid polysulfide polymer – 80-85% inert fillers - 16-18%B) reactor paste lead dioxide -60-68% dibutyl phthalate -30-35% sulphur -3% retarders like magnesium stearate

COMPOSITION

Base and accelaration paste are mixed together which undergoes chemichal reaction to form polymer

The reaction is exothermic – 3-4 degree Celsius rise in temperature

Mercaptan+lead oxide polysulfide +water

Setting reaction

Unpleasant order and colour Material is extremely viscous and sticky and

exhibits pseudoplasticity Long setting time of 12.5min High tear strength Good flexibility Excellent reproduction of surface detail

PROPERTIES

Types; 1-condensation silicones 2-addition silicones Condensation silicones; Also known as conventional silicones. Available in three viscosities namely light,medium

bodied and heavy.

Composition;Base 1-polydimethyl siloxane. 2-colloidal silica Color pigments.AcceleratorStannous octoate

SILICON RUBBER IMPRESSION MATERIAL

Dimethyl siloxane+orthoethyl silicate silicone in presence of stannous octoate.

Addition siliconeComposition;Base paste and catalyst paste;Setting reaction;Vinyl siloxane+silane siloxane silicone rubber

Setting reaction;

Polyether impression materials;

◦ Good mechanical and compressive strength comparing other elastomers.

◦ Available in three viscosities.◦ Consist of base paste and

catalyst.