2.4 impression material

What are the two types of impression material?

• non elastic rigid

• elastic non rigid

what are the types of non elastic rigid dental impressions?

• hard

• Thermoplastic

What are types of elastic rigid dental impressions?

• elastomers

• hydrocolloids

define impression materials

• creates negative replica of soft/hard tissue which can be filled in by dental stone or other model materials for a positive copy to be achieved

What are the two types of self curing/photo resins?

Type 1 and Type 2

What is Type 1 self curing/photo resins used for?

• full crown impression

• impression of partially and completely edentulous jaws

• impressions trays

Can self curing/photo resins record undercuts?

no because it’s non elastic which means fractures can be caused during removal

what are ingredients of self curing/photo resins?

• natural resin

• waxes

• stearic acid

• fillers

• inorganic pigments

what can cause dimensional differences for self curing/photo resins?

It has high coefficient of thermal expansion which can cause dimensional differences between the impression and mouth

what are the actual non elastic rigid dental impressions?

• impression plaster

• Zinc oxide eugenol

• self curing/ photo resin

what is main use of zinc oxide eugenol?

• dentures on edentulous ridges with minor/no undercuts

• wash impression over compound

• bite registration

composition of zinc oxide eugenol?

base paste: zinc oxide, oil and hydrogenated rosin

Accelerator paste: 12-15% eugenol, oils, rosin and filler (talc or kaolin)

how does the temperature and humidity affect the setting time of zinc oxide eugenol?

Zinc oxide, in presence of moisture, reacts with eugenol to form zinc eugenolate

increase in temperature and humidity causes shorter setting time

what is the percentage shrinkage experienced by zinc oxide euugenol?

0.1% shrinkage

how can you determine the two pastes of zinc oxide eugenol have been thoroughly mixed?

the two pastes have contrasting colours

What is a non eugenol paste?

it is a paste used for sensitive patients as eugenol can cause a burning sensation in some

eugenol is replaced by carboxylic acid

what is eugenol replaced by in non eugenol paste?

carboxylic acid

what should model or cast of non eugenol paste ONLY be made of?

What should be done once stone is set?

gypsum-type plaster or stone. After stone has set, impression is immersed in warm water, 600 C, to ease its removal from the cast

Advantages of zinc oxide eugenol

• high accuracy of soft tissue impression, due to low viscosity

• stable after setting

• good surface detail reproduction

• inexpensive

• adheres well to dental impression compound

Disadvantages of zinc oxide eugenol

• Messiness

• Variable setting due to temperature or humidity

• Eugenol is irritating to soft tissues

• Nonelastic and may fracture if undercuts are present

What are the elastic non rigid impression materials?

reversible agar hydrocolloids, irreversible alginate hydrocolloid Polysulfide, polyether, silicone,

What is reversible agar hydrocolloid used for?

• full-mouth impression

• quadrant impressions without deep undercuts

• Single impression

• Fixed partial denture impression

what forms is agar hydrocolloid available?

• tray

• syringe consistencies

what does the gel of agar hydrocolloid consist of and what do they help with?

• 12-15% Agar - gelling agent

• 1-2% Potassium sulfate - for good surfaces on gypsum models and dies

• 0.2% Borax - improve strength

• 0.1% Alkylbenzoates - preservatives

• Traces of colouring and flavouring agents - ease of reading impression and aesthetic

• ~85% water - balances formulation

How is the syringe consistency of agar hydrocolloid prepared?

by increasing water content and decreasing the agar content

How can the solid gel of agar hydrocolloid be turned into a liquid?

What are the steps to the use of a water bath for agar hydrocolloid?

1. Heat in 100 C water for 8-12 minutes

2. Store in 65 C water

3. Place in tray (with cooling coils) at 65 C

4. Temper in 46 C water for 2 minutes before taking impression

5. After seating the tray, cool it with water at no less than 13 C until gelation occurs

What is a potential issue faced when using agar hydrocolloid?

Thermal shock

what does thermal shock entail?

• When warm agar comes into contact with cool water during gelation process it can cause sudden temperature change.

• Can be uncomfortable or painful for patients with metal restorations (crowns, fillings etc)

• Metal conducts heat and col more effectively, lead to sensitivity or discomfort

How to minimise the risk of thermal shock?

Ensure gradual transition from warm to cool temperatures during gelation process, this can be done by controlling water temperature and allowing enough time for the agar to adjust.

Advantages of agar hydrocolloid

• Inespencive

• No unpleasant odour

• Nontoxic, nonstaining

• No custom tray required

• No mixing

• Hydrophilic, can be used in moist conditions

• Displaces water and body fluids

• Easily poured in stone

• Easy removal of stone cast from impression

Disadvantages of agar hydrocolloid

• Requires expensive equipment

• must be prepared in advance

• tendency to tear

• immediate pour required

• dimensionally unstable

• loss of detail possible due to tray movement

• impression stability crucial for accurate results

What is non reversible alginate hydrocolloid used for?

• removable partial denture

• preliminary impressions for complete dentures

• orthodontic models

• study models

can alginate hydrocolloid be used for fixed partial dentures?

no because they are not accurate enough

what form does alginate hydrocolloid come in and what does it consist of?

comes in powder form

Contains: 12-15% sodium/potassium alginate, 8-12% calcium sulfate dihydrate

what are the two main reactions that occur when the alginate hydrocolloid powder is mixed with water?

1. Sodium phosphate reacts with calcium sulfate which provides adequate working time for the impression (2Na3PO4 + 3CaSO4 → Ca3(PO4)2 + 3Na2SO4)

2. Remaining calcium sulfate react with sodium alginate. Forms calcium alginate which combines with water to create a cell-like consistency

How accurate is alginate hydrocolloid and what is its elastic recovery?

• Less accurate than agar hydrocolloid, silicone and polyether

• 97.3% elastic recovery value

What is the combination impression technique?

• Combining alginate tray material with agar syringe material to create dental impression, this is done to utilize the detailed reproduction and compatibility of agar hydrocolloid with gypsum qualities

Are additional eqipments’s needed in combination impression technique?

No, a simple heater can be used to prepare the syringe material

water cooler tray is no longer needed

what is the actual procedure of combination impression technique?

• Placing alginate in a tray

• syringing agar around the preparation

• seating alginate on top of agar

What is important to be wary of when choosing agar and alginate for combination impression technique?

To choose an agar-alginate pair with suitable bond strengths, best to follow the manufacturers recommendations for combinations

Advantages of alginate hydrocolloids

• Inexpensive

• easy to manipulate

• pleasant tasting

• displaces blood and body fluid

• hydrophilic

• easily poured in stone

• compatible withs stock trays

Disadvantages of alginate hydrocolloids

• Tendency to tear

• Immediate pouring required after removal

• Limited detail reproduction

• dimensionally unstable

• suitable only for single casts

• Variable gypsum compatibility

• Incompatibility with some epoxy resin die materials

Why is polysulfide rubber called a thiokol rubber?

as it is derived from thiols

What is polysulfide rubbers also known as?

Mercaptan

what is polysulfide rubbers commonly used for?

• fixed partial denture applications

Why is polysulfide rubbers preferred?

high accuracy and low cost

What viscosities is polysulfide available in?

low, medium and high

when is polysulfide rubbers particularly useful?

for multiple impressions that require extra time

What form is polysulfide rubber available?

Two-paste form

what does the base paste and catalyst paste of polysulfide rubber consist of?

Base paste - polysulfide polymer, fillers and plasticiser

Catalyst paste - lead dioxide 30%

What is the process that polysulfide rubbers go through?

Condensation polymerisation

What is the condensation polymerisation of polysulfide rubber catalysed by?

lead dioxide

What occurs during condensation reaction of polysulfide rubbers?

The -SH (sulfhydryl) groups at the ends and branches of the rubber molecules react with other -SH groups on different molecules. This leads to lengthening of the polymer chains and the formation of cross-links

Causes material to change form paste-like consistency to a rubbery one

What is the by-product of teh condensation polymerisation os polysulfide rubbers?

water

How is the condensation polymerisation of polysulfide rubbers accelerated?

higher temperatures and the presence of moisture accelerates the reaction

What is the elastic recovery of polysulfide rubbers?

about 96%, which si slightly lower that other rubber impression materials

What is the tear strength of polysulfide rubbers?

It has the highest tear strength of the rubber materials, which allows their use in deep subgingival areas where removal is difficult

What is polyether used for?

for accurate impression of a few teeth without sever undercuts

in what viscosities is polyether available in?

low, medium and high

what form is polyether available in?

2 paste system

what does the base plate and catalyst paste of polyether consist of?

Base plate - low molecular weight polyether with ethylene imine terminal groups

Catalyst paste - contains aromatic sulfonic acid ester

what is the shrinkage value of polyether?

0.3% in 24 hours

What is the elastic recovery of polyether?

average 98.5%

What is the flexibility of polyether like?

low

advantages of polyether

• pleasant handling

• ease of mixing

• higher accuracy compared to polysulfide or condensation silicone materials

• good surface detail reproduction

• easy to pour in stone

• dimensionally stable up to 1 week if kept dry

disadvantage of polyether

• high cost

• short working and setting time

• high stiffness after setting

• limitations on use due to stiffness

• bitter taste to some patients

• cannot be left in disinfectant solutions for long periods

• tearing may occur if rubber thickness is inadequate (at least 4mm)

what is condensation silicone rubber used for?

• mainly for fixed partial denture impressions

• ideal for single-unit inlays

what viscosities is condensation silicone rubber available in?

low, medium, high and very high (putty)

what forms is condensation silicone rubber available in?

• two-paste

• paste-liquid

what is the base paste and catalyst of condensation silicone rubbers?

base paste - contains poly(dimethylsiloxane) with terminal hydroxy groups -OH

Catalyst paste or liquid - metal organic esters, such as tin octoate

what are the two reactions that occur for condensation silicone rubbers?

chain extension

cross-linking

what does the two reactions of condensation silicone rubbers entail?

Chain extension occurs through condensation of the terminal -OH group in a siloxane

Cross-linking happens between chains with the help of orthoalkylsilicate molecules

alcohol by-product

what is the elastic recovery of condensation silicone rubber?

99%, excellent

what is the shrinkage of condensation silicone rubber?

0.2-1% in 24 hours

how is accuracy of condensation silicone rubbers improved and shrinkage reduced?

• first taking impression with a highly filled silicone putty, after setting, taking a second impression with a light-bodied silicone. thus the final shrinkage is lower

advantage of condensation silicone rubbers

• clean and pleasant for patient

• highly elastic

• setting time controllable with accelerator

• putty-wash system improves accuracy

• eliminates need for custom tray

disadvantage of condensation silicone rubbers

• very hydrophobic

• requires a dry field

• difficult to pour in stone

does addition silicones have higher accuracy than condensation silicones?

yes

how is addition silicones more accurate than condensation silicones?

Instead of polymerisation to addition and eliminating alcohol by-product which evaporates and causes shrinkage

what forms is addition silicone rubbers avaialble in?

two paste system

what viscosities is addition silicone rubber available in?

low, medium, heavy and very heavy (putty)

What are addition vinyl silicones based on?

silicone prepolymers

what does prepolymers (which addition vinyl silicones is based on) consist of?

vinyl and hydrogen side groups, allowing them to undergo addition polymerisation

how is addition polymerisation initiated in addition vinyl silicones?

the platinum catalyst, chloroplatinic acid, initiates the reaction

how is hydrogen produced by addition vinyl silicone rubbers?

other than addition polymerisation, other reactions may occur during polymerisation process

what is the issue with hydrogen being released by other reaction during polymerisation process of addition vinyl silicones? How can its effects be reduced?

it can rarely cause porosity in materials

Some manufacturers include hydrogen absorbers in their formulation to eliminate the problem.

Some products have finely divided palladium added to absorb the hydrogen to prevent bubbles forming on stone die surfaces

what is the difference between addition silicones compared to condensation silicones?

addition silicones don’t produce a volatile by-product, so it has greater dimensional stability compared to condensation silicone

does condensation or addition silicone have greater dimensional stability?

addition silicones have greater dimensional stability

what are some properties of addition silicones?

• really stretchy and don’t shrink much when stored

• easy to work with (like condensation silicone)

what you avoid wearing when handling addition silicones?

avoid mixing addition silicones with laterx gloves