Impression materials (2)

Non-aqueous elastomeric impression materials:They are also called rubber base materials.

They all undergo cross-linking polymerization reactions.

They have similar but not identical mixing and handling properties. One very important difference between types of elastomers is the adhesive used to bond the impression material to non-perforated metal stock tray or custom tray.

1-polysulfides:Composition:They are supplied as two pastes in tubes:

White paste Brown paste - polysulfide polymer. - Lead oxide.

- inorganic filler. - oily organic chemicals. - sulfur.The working and setting times are accelerated by heat and humidity. Advantages:-The longest working time of any other elastomers.

-inexpensive.

-Easy to pour with gypsum materials.Disadvantages:-Disagreeable smell and taste.

-Stain clothing.

-long working and setting time(15 min).

-Evaporation of water (by-product of reaction) results in distortion.

Uses:-impression of crowns and bridges (light body is injected around teeth and heavy body in the tray).

-complete denture impressions (light and medium body material).

2-Condensation silicones:It supplied in two tubes:

Base catalyst

-silicone and fillers -stannous octoate , alkyl silicate.

It comes in light, medium, heavy and putty viscosities.

The condensation reaction produces ethyl alcohol as by-product that rapidly lost by evaporation dimensional instability.Advantages:It is better than polysulfide in :

1- Ease of mixing.

2- Pleasant taste and odorless.

3-Shorter working and setting times.

Disadvantages:-Contract with time and must be poured within a few minutes.

-It has poor wetting characteristics make it difficult to pour without bubbles unless a surfactant is used. Uses:It has been used in crown and bridge procedures but it has been replaced by addition silicone. 3-Addition silicone (polyvinyl siloxane or vinyl polysiloxane): The most popular type of impression material especially for crowns and bridges.It comes in five viscosities: light, medium, heavy body, monophase (more viscous than regular) and putty.Each viscosity comes in two different colored pastes. The pastes (except putty) are mixed in the same manner as polysulfides.

Also it supplied in double-barreled cartridges for use in automix gun that forces the pastes through a tip which contain spiral-shaped baffle.Jenis material cetak yg digunakan. Brand: Exaflex (GC), aquasil (Dentsply), dsb

The baffle causes the material to swirl around and mixes two pastes together. Putty material:It is mixed by kneading the two colors together with the fingers.

Vinyl gloves should be worn over hands washed with soap and water. Latex gloves should not

be worn when mixing because sulfur in latex gloves inhibit the polymerization reaction.

It sets faster than light, medium and heavy body materials.

Uses of putty: It is used in wash technique(putty is mixed and placed in stock tray, the light body is syringed around the prepared tooth).

It is also used to form a provisional restoration. Advantages of addition silicones:-They are clean.

-Have no unpleasant odor or taste. -Accurate and stable impression material for a least a week without distortion.

-can be poured up several times.

Uses:-They are the most popular impression material for crown and bridge impression.

-They used also for bite registration either by injecting the mixed material directly on the occlusal surfaces of mandibular teeth then the patient bite on it or by using bite tray.

Additives to the material:1-surfactants:

A chemical that lowers the surface tension of a substance so it increases wetting and decreases the likelihood of bubbles. 2-Hydrogen absorbers:If the components are not formulated in the proper ratio or with the proper purity, hydrogen gas is produced as secondary reaction.

If a material is poured quickly, hydrogen gas is formed between impression material and the model so hydrogen absorber is added to the formulation to prevent this problem. 4-Polyethers:Polyethers are supplied as light, medium and heavy body viscosities.

Equal lengths of material are dispensed from two unequal sized tubes of base and catalyst onto a mixing pad.

It also comes in pouches of base and catalyst that are placed in mechanical mixer then delivered into the impression tray. Advantages:-They set quickly.

-clean materials.

-short working and setting times.

-very accurate and easy to pour.

-can be poured up to a week and remain stable for several days.

Disadvantages:-unpleasant taste and very stiff.

-expensive.

-very sensitive to temperature and moisture.Features of elastic impression materials:Impression material cost Surface detail Dimensional stability Ease of use Pour within Ability to repour

Agar low high low low Immediately no

alginate low lowest low high 1 hr no

Polysulfide Medi-um highmedium medium 30 min-several hrs Yes, second not as accurate

Condensation silicone Medi-um highLow to medium medium 1 hr yes

Addition silicone high highhighesthigh 1 wk yes

Polyether high highhigh high 1 wkyes

Disinfecting the impression:Steps:1-After removal from the mouth, impression should be rinsed with water to remove saliva, blood and other debris.

2-Excess water should be shaken off before using the disinfectant as not to dilute it.

3-Disinfectant can be applied by :

Immersion of impression or spraying

4-They should be placed in a closed container or sealed plastic bag before transported to the lab.

5-Inspect the oral cavity for residual material upon removal( use explorer and floss to remove alginate from embrasures).

6-clean the patients face with damp towel and rinse patients mouth.

Disadvantages of : Immersion sprayingCan cause distortion of 1 -It creates airborne particles of materials that prone to chemicals that could be inhaled imbibe water and swell by staff or patients.as polyethers and alginate.

2-It may not reach all surfaces if sever tissue undercuts are present. Different impression materials require different materials and handling for disinfection:

Impression material compatible disinfectants Immersion time

Alginate & Agar NaHCl, iodophors 10-30 min or spray

Polysulfides NaHCl, iodophors, glutaraldehydes. 10-30 min

Silicones (condensation and addition) NaHCl, iodophors, glutaraldehydes, complex phenolics. 10-30 min

Polyethers NaHCl, iodophors, complex phenolics. > 10min or spray

compound NaHCl, iodophors. 10-30 min

Zinc oxide eugenol iodophors, glutaraldehydes. 10-30 min

Spraying of the impression should be done inside a plastic bag or headrest cover to contain the spray and protect the handler from inhaling droplets.

Wax may distort when it is immersed. So the recommended procedure is to rinse-spray-rinse-spray-rinse and then placed in the container for transport.

Good Luck9