impression

IMPRESSION MATERIALS USED IN ORTHODONTICS

 

HISTORY

 

As early as the 17th century G. PURMAN of Breslau was said to have used wax for taking dental impressions.

 

During the late 1790s plaster was used to take impression for the famous dentures of Sir George Washington. (Dr. John Greenwood—a New York dentist)

 

In 1844-Westcott Dwinelle and Dunning started the use of PLASTER OF PARIS for taking impression for denture fabrication.

 

  • Elastic Impression first introduced to the dental profession in 1925 in the form of AGAR HYDROCOLLOID  called ‘Reversible Gel’. Since temperature changes its physical state.
  • ALGINATE HYDROCLLOID WERE DEVELOPED JUST BEFORE WORLD WAR 2(1930).
  • Since then, search for the ideal impression material has continued.

 

Historically, impression making was accomplished with inelastic materials for both soft and hard tissues.

 

Hydrocolloids ware initially introduced to make impression of hard tissues in place of inelastic materials.

 

After World War II, advances in polymer technology brought to the dental profession a group of synthetic rubbery materials called ‘ELASTOMERS’

 

They are capable of making impression of both hard and soft tissue.

 

In 1950s the rubber base material first in the form of Mercapton Polysulfide and later the Silicon began to be used as dental impression materials.

 

In 1994, Mitchell described a technique of occlusal registration for functional appliances using elastomeric bite registration materials.

 

The colloidal material used for making impression are either —-Agar or Algi dissolved in water –Hence the term Hydrocolloid impression materials.

 

Hydrocolloid impression materials can be classified as:-

 

  1. REVERSIBLE              eg. Agar
  2. IRREVERSIBLE         eg. Alginate

 

AGAR  – REVERSIBLE HYDROCOLLOID

 

In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type of impression material.

 

It was later described by Skinner as colloidal sols of emulsified type.

 

Sears introduced reversible hydrocolloids in 1937

 

MODE OF SUPPLY:

 

 Supplied as gel in collapsible tube or as Cylinders in a jar.

 

COMPOSITION

 

INGREDIENT WEIGHT(%) FUNCTION
Agar 13-17 To provide dispersed phase
Borate 0.2-0.5 Provides strength
Sulfate 1-2 Gypsum hardener
Wax 0.5-1 Filler
Thixotropic materials 0.3-0.5 Thickener
Water Balance Reaction medium

 

 

 

The ADA specification No 11 sets standard properties required by agar hydro colloidal material.

 

Flow:

 

    The material should be able to record the fine details

 

Gelatin temperature:-

 

After boiling for 8 min, the material should be fluid enough to be extruded from container. After tempering the solution should be homogenous and should set between 37 and 45 when cooled.

 

The ADA specification requirement for flexibility allows a range of 4% to 15% at a stress of 14.2 psi.

 

Elasticity and elastic recovery:

 

Agar hydrocolloids are highly elastic in nature and elastic recovery occurs to the extent of 98%

 

Accuracy and dimensional stability

 

Agar impressions are highly accurate at the time of removal from mouth but shrink when stored in air and expand when stored in water.  The least dimensional change occurs when stored in 100% humidity but immediate pouring of cast is recommended.

 

Working and setting time:-

 

Working time ranges between 7-15 minutes and setting time is about 5 minutes.  They can be controlled by regulating flow of the water through cooling tubes.

 

 

 

Tear and Compressive strength

 

Tear strength is 4 psi (psi = pound per square inch)

 

Compressive strength is 800 gm/cm2.

 

MANIPULATION:

 

Conditioning unit is required for manipulation of agar. The Conditioner consists of