Impression materials for fixed prosthodontics

Elastic impression materials can be used for (5)

• Partial dentures

• overdentures

• Implants

• Crowns

• bridge work

All of the impression materials

Impression materials

Should be easy to mix otherwise? (4)

• Affects setting time

• May not set evenly throughout

• Affect tear strength and elongation at break

• take up both clinical and patient time

Impression materials should have what about the working time and setting time?

• Long working time: enough to mix, place in the tray, and position tray correctly in the mouth

• Quick setting time: the tray should remain in the mouth for a short time otherwise it becomes very uncomfortable for the patient

Overall 2 main properties prior to impression taking?

• Easy to mix

• Long working time and short setting time

Intra-oral, during impression making

the material should be? (5)

• Hydrophilic: so does not shy away from moist oral structures to record fine details

• Flow: Ability to flow around the preparation without slumping, otherwise certain details will not be recorded

• Capture good detail

• Good taste and odour

• Snap set: Rapid transition from the unset to the set state, in rheological terms: plastic to elastic state, should not start setting before the the working time ends, and when working time does end it should set immediately

While removing from the mouth:

the set material should be: (3)

• Easy to remove: the tray should not get stuck so that it needs to be cut away together with the impression material (depends on the rigidity of the material)

• High tear strength: So that it does not tear during removal leaving bits of material in the mouth

• Good recovery from deformation: When the material is being removed from the mouth it is elongated, after elongation it should bounce back to its original state as rapidly as possible (don’t want any permanent deformation when removing from undercuts)

After removal from the mouth, the material should have what? (2)

• Dimensional stability - in air it should not shrink

• Can be disinfected without loss of accuracy: must not swell/shrink in disinfection solutions, water or moist humid environments

In the lab, during pouring, the impression must be? (3)

• Compatible with gypsum

• Reproduction of details

• Ability for multiple pouring (to make a study cast and cast to make the actual prosthesis on)

Why are elastomers used over alginate and agar?(3)

what can they be used for?

cost?

• Strength and dimensional stability

• They produce an accurate replica of teeth and supporting tissues

• Crowns, bridges and inlays/onlays

• More expensive than alginates/agar

Elastomers vs hydrocolloids (1 property)

Alginates can be accurate if what?

Elastomers include?

• More accurate

• Alginates can be accurate if poured immediately after setting

• Condensation/addition silicones, PE, PS

Elastomeric materials are available in a range of ()

viscosities

The viscosity of the elastomeric impression materials is dependent on? (2)

• Filler content and molecular weight of the polymer

What are the different viscosities? (5)

• Putty, heavy, medium, light and wash

light/wash - least amount of filler

2 pastes can be dispensed as? (5)

• two separate tubes

• Two tubs of putty

• or tub of putty and tube

• Double barrel cartridge (dispensing gun)

• Pouches for Pentamix

Advantages of double barrel cartridges and pouches for Pentamix? (2)

• Avoid incomplete mixing

• Prevent introduction of air bubbles

Monophase technique using medium viscosity disadvantages? (3)

• Monophase technique - using on viscosity to take an impression

• Compared to the light body: Reduced ability to flow into intra coronal and gingival crevice and surface reproduction may not be as good as the light bod material

• Greater amount of polymerisation shrinkage compared to heavy body (as it has less filler in comparison)

Why can the light-bodied/wash material not be used by itself?

• Due to containing little filler, it does not have the strength and body, so it will tear easily when removing the tray/impression from the mouth

• Therefore you have to use a range of viscosities

Therefore the light bodies material should be used with what?

wash used with?

• used in combination with a high viscosity paste

• Wash is used with a putty in a stock tray

what is the role of the lighter and heavier bodies?

• Light-bodied/wash material records the fine detail

• Heavy-bodied/putty material acts as a base to support the light material

Describe the Putty and Wash twin mix/One stage technique?

• The wash material is syringed around the teeth/the prepared tooth/teeth

• The putty is loaded into the tray and inserted immediately after syringing the wash material

Disadvantage of this technique? (heavier and lightwash?

)

• Difficult to control thickness of wash material

• As higher viscosity putty displaces the wash material - critical areas captures in putty rather than the light body and so detail capture is compromised

What is the putty and wash two step/stage technique

and the two types within this?

• The putty is used to take an impression before starting the preparation

• After preparing the teeth, the wash material is syringed on the prepared teeth - can also be placed in tray over the putty

• and the tray with putty material is reinserted over the wash

• spacer or no spacer (polythene sheet over the teeth)

Disadvantages of twostep/stage technique? (2)

Difficulty to reposition the tray correctly after the first putty impression

Time - takes longer

What is the dual viscosity technique? and the role of the different viscosities?

• Heavy and light-bodied pastes are used in combination

• Heavy/medium-bodied material is extruded in the tray

• Light-bodied is syringed around the teeth of interest and can also be placed on heavy/medium material in the tray

• Tray then seated in the mouth and the material is allowed to set prior to removal

• Light body paste can flow into undercuts and record fine detail

• They heavy bodied material will force the light body material into the gingival sulcus and reparation detail

• Heavy-bodied material is more rigid so acts as a support for the light-bodied material

Which viscosity will take longer to set?

• The light bodied material will take longer to set

after extruding some of it, leave some on the side to assess set

Advantages of these techniques?

• Fine detail recorded

Dis of one stage and 2 stage technique?

• One stage : both pastes have to be mixed simultaneously

• Two stage: time taken for two impressions and difficulty in repositioning the tray after the first impression

General setting reaction (2 possible types), to form what?

What does the reaction type affect?

• When both pastes are mixed together, via a condensation or addition reaction they form a 3D network of cross-liked molecules

• The two reactions affect the dimensional stability of the set material

• Addition - no by-products

• Condensation - elimination of a small molecule resulting in shrinkage of the set material

Condensation silicone:

what type of phobic?

Dis?

• Hydrophobic

• Shrink in air (dimensional stability)

• Hydrophobic - shy away from moisture, detergents incorporated to confer a degree of wettability

• Disinfection: detergents may expand in disinfection solutions due to hydrophilic agents incorporated

• The mouth still needs to be fairly dry

• Erratic setting behaviour when a liquid catalyst is used

• Limited shelf life, mainly with liquid catalyst

Addition silicones:

phobic?

• Hydrophobic, manufacturers incorporate surfactant to confer a degree of wettability

Disadvantages of addition silicones?

2 (adv)

• Free water in plaster reacts with unreacted Si-H groups - releasing Hydrogen and giving porous models - recommended to pour the impression 30 minutes after taking (so that all the Si-H groups are consumed)

• Setting can be seriously impaired when handled with natural rubber gloves (S poisons Pt catalyst)

• Hydrophobic material - shy away from moisture (water/blood/saliva) - so detergents added to confer a degree of wettability

• the mouth still needs to be fairly dry

• Hydrophobic additions may expand during disinfection

• % elongation and tear strength are adequate

• Best elastic recovery (bounce back to original shape)

Polyether (IMPERGUM)

phobic?

setting reaction?

• Hydrophilic material

• Set via cationic addition reactions

Disadvantages of Impregum/PE?

• Original: very stiff and low elongation at break - tearing on withdrawal

• Permanent deformation

• Hydrophilic: Dimensional stability on prolonged contact with water - material swells so do not wrap in a damp napkin - disinfection also a problem

Polysulphides

setting reaction?

• CONDESNATION REACTION

Disadvantages of PS? (4)

• Dimensional stability - shrinks

• Slow setting

• Dirty to handle and unpleasant smell

• Elastic recovery not as goof as silicones and PE - permanent set (although it has good elongations at break)

Overall the desirable qualities of an ideal impression material