Dentistry

Elastic Dental Impressions PDF

Elastic dental impressions, Introduction to Elastomers, Silicones

Silicones

The silicone impression materials are classified according to the type of chemical reaction by which they set:

  1. Addition
  2. Condensation

Addition silicones

Can be used as a one or two stage technique. May be used in special or stock trays. The very heavy bodied materials are measured in scoops and are mixed by hand until homogeneous in colour.

Elastic Dental Impressions

Example of an addition silicone

Elastic Dental Impressions

An impression taken in Xantopren Green                 Xantopren impression with beading

Elastic Dental Impressions

Addition silicone impression material being used to take an impression of implants

Properties of Addition Silicones

CHEMISTRY
These materials are often termed vinyl polysiloxanes. Supplied in 2 pastes or in a gun and cartridge form as light, medium, heavy and very heavy bodied. One paste contains a polydimethylsiloxane polymer in which some methyl groups are replaced by hydrogen. The other paste contains a pre-polymer in which some methyl groups are replaced by vinyl groups, this paste also contains a Chloroplatinic acid catalyst. On mixing, in equal proportions, crosslinking occurs to form a silicone rubber. Setting occurs in about 6-8 minutes.

PROPERTIES
Good shelf life
Dimensionally stable
Moderate tear strength
Excellent surface detail
No gas evolution
Non toxic and non irritant

ADVANTAGES
Accurate
Ease of use
Fast setting
Wide range of viscosity’s

DISADVANTAGES
Hard to mix
Sometimes difficult to remove the impression from the mouth
Too accurate in some circumstances (cast produced is not sufficiently oversized)

Condensation Silicones

CLINICALLY
Used for crown and bridge work mainly, but also for partial dentures, implants and overdentures. Used in stock trays or special trays. One or two stage impression stage. Although dimensionally stable the impression should be cast within 24 hours.

CHEMISTRY
Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).

BASE PASTE
Silicone polymer with terminal hydroxy groups
Filler

CATALYST PASTE
Crosslinking agent (organohydrogen siloxane)
Activator (dibutyl-tin dilaurate)
On mixing the two pastes react, cross linking occurs and setting takes about 7 minutes.
The setting reaction is a condensation reaction.
Hydrogen gas is evolved on setting which leads to surface pitting, and a roughened surface to the resulting model.

PROPERTIES
Hydrophobic
Hydrogen gas evolution on setting
Moderate shelf life
Moderate tear strength
Good surface detail
Shrinking of impression over time
Non toxic and non irritant
Very elastic (near ideal)

ADVANTAGES
Accurate
Ease of use
Can be used on severe undercuts

DISADVANTAGES
Hydrogen evolution
Liquid component of paste/liquid system may cause irritation