impression

IMPRESSION MATERIALS USED IN ORTHODONTICS

Reduced working time

More uniform mix

A recent development in the automatic mixing and dispensing device is a dynamic mechanical mixer. Materials are supplied in plastic bags housed in a cartridge. Device uses a motor to drive parallel plunger that force the materials into a mixing tip and the spiral inside the mixing tip rotates as the materials are extruded through the tip.  With this uniform mix of even higher viscosity material can be achieved

       Making of impression

Initially tray is coated with adhesive that forms bond between tray and impression material. Slightly roughened surface of tray increases adhesion.

      METHODS OF MAKING IMPRESSION

Three techniques

Single mix technique

Multiple mix technique

Reline technique

Single Mix technique:

Viscosity used is regular body

Method:

The paste is mixed and part of it is loaded into tray and part into syringe.  The syringe material is then injected on to the prepared area.  Then tray is seated over it.

Multiple mix technique

Viscosity used is heavy body and light body

Method

            The two viscosities are mixed simultaneously on separate pads. Heavy body is loaded onto the tray and light body in to syringe. The syringe material is injected into area of preparation. Tray with heavy body is seated over it.

Reline technique:      

Viscosity used is putty and light body

Two approaches are present

Two stage procedure

Single stage procedure

—  For two stage procedure, thick putty material is placed in stock tray and a preliminary impression is made.  This forms an intra oral custom tray.  Space for light body or was is provided whether by cutting away some of the putty or by using thin polyethelene sheet as a spacer between putty and prepared teeth.

—  A mix of thin wash material is placed into putty and putty with combination tray is seated finally to make impression.

For single stage procedure,  material is syringed into place and then unset putty is seated over light body but the disadvantage is that putty may displace light body.

Removal of the impression:

The material is checked for set by probing with the blunt instrument.  Then impression is dislodged from the mouth with a steady pressure

APPLICATIONS:

  1. Impressions for fixed partial dentures
  2. Impressions for removal partial dentures
  3. Impressions for complete dentures
  4. polyether is used for Border moulding
  5. To make impressions for implant procedure
  6. In maxillofacial prosthesis
  7. To make impression in orthodontic cases for study model

REVIEW OF LITERATURE

# Andree, Alfred and Christoph evaluated the dimensional accuracy of monophase elastic impression making with addition silicone and polyether and found no significant change.

 int J Prosthodont . 2002;15:168–174

# Steven O Hondrum assessed the shelf life of different elastomers like polyether, polysulfide, additive silicone and found all the materials were fairly efficacious. But addition silicone showed better value during 72 month period and polysulfide with a little change.

Journal of prosthodontics 2001 Jan;85(1):73-81.

#  The influence of mixing methods and disinfectant on the physical properties of alginate impression materials.

The European Journal of Orthodontics Advance Access published May 10,2012