DM MIDTERM - DENTAL IMPRESSION MATERIALS

Dental impression is defined as -

A negative likeness of a dental structure

• record accurately the dimensions of oral tissues

• material in the soft state is placed against the oral tissues to set (harden)

+ vs - replica

• (model or cast of the oral tissues) = POSTIVE REPLICA

• (dental impression) = NEGATIVE REPLICA

Impression material is defined as -

negative likeness of a dental structure

• accurately record dimensions of oral tissues

• material in the soft state is placed against the oral tissues to set (harden)

What are the ideal properties of impression materials?

• 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

Which impression materials set by chemical reaction (irreversible) and are rigid (edentulous)?

Impression plaster, Zinc oxide eugenol

Which impression materials set by chemical reaction (irreversible) and are elastic (non-rigid, dentulous)? - most commonly used

Alginate, Polysulfide, Polyether, Silicone

Which impression materials set by temperature change (reversible) and are rigid (edentulous)?

Compound, Waxes

Which impression materials set by temperature change (reversible) and are elastic (non-rigid, dentulous)?

Agar hydrocolloid

What is impression plaster?

Basically a GYPSUM product (Type-1)

What is impression plaster used for?

Used as mucostatic impression material for making final impressions for edentulous patients

When is impression plaster applicable?

Applicable to patients with displaceable soft tissues that should be recorded in a passive state

What are the advantages of impression plaster?

• Cheap and long shelf life, easy to prepare

• Very good surface detail reproduction

• Excellent dimensional stability

What are the disadvantages of impression plaster?

• Very rigid -

• Fractures if undercuts are present

• May dry soft tissues, unpleasant to patients

• Separation from cast may become difficult

What is zinc-oxide eugenol impression paste used for?

Secondary (final) impression material for edentulous patients

Temporary relining material for dentures

Bite registration paste

Surgical pack paste for periodontal wound healing

How is zinc-oxide eugenol impression paste supplied?

Supplied in two separate pastes dispensed in tubes.

What is the composition of Tube no 1 (Base) in zinc-oxide eugenol impression paste?

• Zinc oxide

• Fixed vegetable or mineral oil

What is the composition of Tube no 2 (Accelerator) in zinc-oxide eugenol impression paste?

• Oil of cloves or eugenol

• Gum or polymerised rosin

• Filler (silica type)

• Lanolin

• Resinous balsam

• Accelerator solution (CaCl2)

What is the setting reaction of zinc-oxide eugenol impression paste?

Ionic in nature

What does the setting reaction of zinc-oxide eugenol impression paste require?

Requires ionic medium in which it can proceed at any desired rate

What is the first reaction in zinc-oxide eugenol setting?

Hydrolysis of zinc oxide to its hydroxide form

What happens when the two zinc-oxide eugenol pastes are mixed?

The phenol -OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide

What salt forms during zinc-oxide eugenol setting?

Zinc eugenolate.

Setting reaction of zinc eugenol chemical equation:
Base -
Acid -
Salt -

Base - zinc hydroxide
Acid - eugenol
Salt - zinc eugenolate.

Why can the zinc-oxide eugenol setting reaction also be described as autocatalytic?

As water is used for initiation and obtained as a by-product, the reaction can also be described as an autocatalytic reaction.

What is the initial setting time of Type I zinc-oxide eugenol impression paste?

3-6 min

What is the final setting time of Type I zinc-oxide eugenol impression paste?

10 min

What is the initial setting time of Type II zinc-oxide eugenol impression paste?

3-6 min

What is the final setting time of Type II zinc-oxide eugenol impression paste?

15 min

Consistency and Flow

According to ADA specification No. 16, what is the spread of Type I zinc-oxide eugenol?

30 to 50 mm

Consistency and Flow

According to ADA specification No. 16, what is the spread of Type II zinc-oxide eugenol?

20 to 45 mm

RIGIDITY AND STRENGTH:
What is the compressive strength of hardened ZOE two hours after mixing?

Approximately 7 MPa

DETAIL REPRODUCTION:
Why does zinc-oxide eugenol reproduce surface details accurately?

It registers surface details quite accurately due to the good flow

DIMENSIONAL STABILITY:
What dimensional change may occur during zinc-oxide eugenol hardening?

A negligible shrinkage (less than 0.1%) may occur during hardening.

What biological consideration applies to zinc-oxide eugenol?

Some patients experience a burning sensation in the mouth due to eugenol. - allergic

What are the advantages of zinc-oxide eugenol impression paste?

• Low viscosity - no compression of soft tissues (flows slowly)

• Dimensional stability (shrinkage less than 0.1%)

• Good surface detail reproduction

• Low price

What are the disadvantages of zinc-oxide eugenol impression paste?

• Cannot be used in deep undercuts

• Eugenol allergy in some patients

How is zinc-oxide eugenol impression paste mixed?

Mixed on oil impervious paper or glass mixing slab

How is the proper proportion of zinc-oxide eugenol pastes obtained?

By squeezing 2 strips of paste of the same length, one from each tube, onto the mixing slab

What spatula is used to mix zinc-oxide eugenol pastes?

Flexible stainless steel spatula

How long is zinc-oxide eugenol paste mixed?

Approx 1 min, until a uniform color achieved.

What are the two main groups of elastic impression materials?

Aqueous Hydrocolloids; Non-aqueous elastomeric impression materials

Which materials are aqueous hydrocolloids?

Agar-agar; Alginate

Which materials are non-aqueous elastomeric impression materials?

Polysulfides; Silicones; Polyethers

What does the agar conditioning unit allow?

Liquefaction, storage and tempering

How many compartments does the agar tray material conditioning unit have?

3 compartment conditioning unit

In which compartments is syringe material used when making the agar impression?

Syringe material used only in liquefaction and storage compartments

What is alginate?

Irreversible elastic hydrocolloid

What is alginate derived from?

Mucous extract yielded from certain brown seaweeds.

How is alginate described in the lecture?

Most widely used impression material

Why was alginate developed?

Developed as a substitute for agar

What are the types of alginate?

Type 1- Fast Setting; Type 2 - Normal Setting

When is alginate used for impression making?

When there are undercuts; in mouth with excessive flow of saliva

What are the applications of alginate?

• For impression to make study models and working casts

• For making preliminary impression

What is the function of sodium or potassium alginate?

React with calcium ion

What is the function of calcium sulphate (reactor) in alginate? **

React with potassium alginate

What is the function of zinc oxide in alginate? **

Acts as filler

What is the function of potassium titanium flouride in alginate? **

Gypsum hardener

What is the function of diatomaceous earth in alginate? **

Acts as filler

What is the function of sodium phosphate (retarder) in alginate? **

React with calcium sulphate

What are the colouring and flavouring agents in alginate? **

Wintergreen, orange etc.

What are the two main reactions in alginate setting?

2Na₃PO₄ + 3CaSO₄ → Ca₃(PO₄)₂ + 3Na₂SO₄

Sodium alginate + CaSO₄ + H₂O

Calcium Alginate + Na₂SO₄

What happens first during the alginate setting reaction?

Firstly the retarder is completely reacted to delay the reaction time.

How is alginate manipulated?

• The measured powder is sifted into pre-measured water

• A vigorous figure - 8 motion, with the mix being stropped against the sides of rubber mixing bowl

What is the mixing time for alginate?

Between 45 secs - 1 min

What causes dimensional change in alginate? **

Dimensional change due to syneresis and imbibition.

What is syneresis?

gel contracts and expels liquid (water) from its structure, leading to shrinkage and dimensional reduction.

What is imbibition?

gel absorbs water from the surroundings, resulting in swelling and dimensional increase.

Why does an alginate impression shrink slightly?

due to thermal differential between mouth temperature (37℃) and room temperature (23℃)

What are the elastic properties of alginate?

Good elasticity and elastic recovery

How is alginate flow described?

Highly flexible

How is alginate accuracy described?

Fine reproduction of tissue detail

What is the compressive strength of alginate?

500-8000 gm/cm²

What is the tear strength of alginate?

350-700 gm/cm²

How does sodium sulfate affect gypsum setting after alginate gelation?

Gelation produces sodium sulfate which acts as an gypsum-setting accelerator at low concentrations and a retarder at higher concentrations.

What is used as a gypsum surface hardener with alginate?

Potassium titanium fluoride

What are the advantages of alginate?

• Ease of manipulation

• No need of expensive equipments

• Relatively low cost

• Comfort to patients

What are the disadvantages of alginate?

• High viscosity

• Ability to displace tissues

• Tears easily

• Poor dimensional stability

What is the working time of normal set alginate?

2 min

What is the working time of fast set alginate?

1¼ min

What is the setting time of normal set alginate?

4½ min

What is the setting time of fast set alginate?

1-2 min

In what increasing order of filler content are elastomer consistencies formulated? **

Extra light body → Light body → Medium or regular body → Heavy body → Putty (extra heavy)

What was the first synthetic elastomeric impression material?

Polysulphide, NOT used anymore.

What were polysulphides used for?

record impressions for crowns, bridges, inlays, onlays, and partial dentures.

What are addition silicones commonly referred to as?

Polyvinyl siloxanes (PVS) or Vinyl poly siloxane

Composition of base paste of polysulphides:

• Fillers

• Strengthener

• Plasticizer (confers viscosity to base, gives elastic property)

Composition of accelelator paste of polysulphides:

• Reactor

• Promoter, accelerates the reaction

• Retarder, controls setting reaction

Why are polysulphides NOT used anymore?

• water = byproduct, low dimensional stability, easily distorts

• longest setting time out of all elastomers (7-10 mins)

• hydrophoic

• unacceptable odor

• untidy stains clothing

What is condensation silicone also known as?

Conventional silicones or room temperature vulcanizing (RTV) silicones.

How is condensation silicone supplied?

• two-paste system

• base-paste with low-viscosity liquid catalyst

• two-putty system

What is the putty-wash technique for condensation silicone impressions?

Using putty as tray material with a low-viscosity silicone.

Give an example of a condensation silicone brand:

Speedex

What are the uses of condensation silicone?

Impressions for:

crowns, bridges, inlays, onlays, and complete dentures.

What is in the base paste in condensation silicone?

• Hydroxyl-terminated polydimethyl siloxane

• Colloidal silica or titanium dioxide

What is the function of hydroxyl-terminated polydimethyl siloxane in condensation silcone?

Low-molecular-weight silicone polymer

What are the fillers in condensation silicone?

Colloidal silica or titanium dioxide

What are the liquid accleators in condensation silcone?

• Alkyl silicate

• Stannous octoate

What is the function of alkyl silicate in condensation silicone?

Cross-linking

What is the function of stannous octoate in condenstaion silcone?

Activator

What is the by-product of condensation silicone?

Ethyl alcohol