Impression Materials – Agar

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Agar is an aqueous impression material used for recording maximum details; for example, as in the production of dies for fixed restorations.

Agar is also known as a reversible hydrocolloidal impression material. It gives good detail reproduction than any other material. However, it has been replaced by rubber-based impression materials because of the costly armamentarium required and prolonged chair time.

 

DO YOU KNOW THE MEANING OF ‘AGAR’?

The word “agar” comes from the Malay word agar-agar – meaning” jelly”.

Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work.

* Wikipedia, the free encyclopaedia.

Properties:

• Agar is a hydrophilic colloid extracted from certain types of seaweed.
• It is a complex sulfated polymer of galactose units.
• It is a mucilaginous substance that melts at high temperature (about 100 °C) and solidifies into a gel at low temperature (about 36 °C).

 

Composition:

1. Agar (8-15 %) – The basic constituent
2. Borates – Improves strength but acts as a gypsum retarder
3. Potassium sulfate – Acts as a gypsum hardner
4. Fillers – Hard waxes which improves strength
5. Plasticizer – e.g. glycerine and thymol
6. Alkyl-benzoates (0.1 %) – Preservative that increases shelf-life.
7. Colouring and flavoring agents
8. Water (> 80%) – Dispersion medium

 

Usage:

1. Primary use of agar impression material: It is used to make secondary/final impression in dentolous patients requiring removable and fixed partial dentures.

For impression material usage, it comes in the form of:

• Gel in tubes (tray material)
• Number of cylinders in a glass jar or cartridges (Syringe material)

2. Widely used at present for cast duplication (during fabrication of cast RPD).

For laboratory duplication material usage:

• Bulk containers

 

Advantages:
1. Detail reproduction is very good.
2. Can record undercut areas correctly.
3. Distortion on removal is prevented due to elastic recovery.
4. Well tolerated by the patient.
5. Can be re-used.

 

Disadvantages:
1. Cannot be electroplated.
2. Thin sections of impression tears easily.
3. Multiple models cannot be poured like elastomeric impression materials.
4. Special armamentarium required.
5. Gypsum hardener required.
6. Sterilization of impression is difficult.

 

WHAT IS GELATION?

Gelation is a sol-gel transformation.

Liquefaction Temperature:
Agar gels have fibrils held together by weak forces. They break at increasing temperature to the from sol.

This increased temperature is called liquefaction temperature and is 71 to 100 °C.

 

Gelation Temperature:
the sol can be converted to gel by decreasing the temperature below liquefaction.

It is called gelation temperature and it varies from 36 to 43°C.

 

WHAT DO YOU NEED TO KNOW ABOUT HYSTERESIS?

Hysteresis is the temperature lag (slow) between Liquefaction Temperature & Gelation Temperature. This helps in using agar as an impression material. This time is the manipulation time of the material.

The gelation temperature is critical when:

• If too high: Injury to oral tissues due to increased temperature.
• If too low: Temperature for gelation would be made difficult or impossible to attain.
• Exactly at mouth temperature, surface stress may develop causing syneresis later.

 

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Manipulation

Armamentarium:

i. Conditioning unit has 3 chambers

• Liquefying chamber: Boiling (100 °C) for 10 minutes. For reused material, additional 3 minutes is required.
• Storage chamber: Stored at about 65 °C.
• Tempering chamber: Tempering at 45 °C for 3 minutes after it has being placed in the tray.

ii. Water-cooled rim-lock tray

 

Impression Making:

1. The syringe material is taken from the second chamber and injected into the prepared tooth.
2. The tray material is taken from the third chamber and the tray is seated in position.
3. Cool the tray with water at 13 °C for 3 minutes.
4. The tray should be held with little pressure and should not be distributed until gelation is complete (convert sol to gel).
5. The impression is removed with a single stroke along the long axis of the tooth. Twisting or torquing should be avoided.

 

Dimensional Change:

Depends on water content of the hydrocolloid,
Decrease water content = Shrinkage = Syneresis – an exudation of fluid onto surface of set gel

Increase water content = Expansion = Imbibition – absorption of water

 

Disinfection:

it is done to prevent contamination of gypsum models by viruses like AIDS, Hepatitis B, etc.

It can be done through the following steps:
i. Thoroughly rinse the impression under tap water to remove any blood or saliva.

ii. Disinfect the impression by:

• Submerging it for 10 minutes in a fresh 0.5 % solution of sodium hypochlorite or glutaraldehyde.
• Spraying with antimicrobial agents.
• Wrapping the impression in the disinfectant soaked paper towel and placing it in a sealed plastic bag for 10 minutes.

 

Storage of the impression:

After disinfection, pour the model with stone immediately. Only in unavoidable conditions, storage for short period is done.

Methods of storage include:

• Impression may be wrapped in a water soaked towel.
• Placed in a plastic bag which is convenient for storing impressions under humid conditions.
• It may be placed in a humidor (100% humidity) or 2% potassium sulfate solution.

 

Precautions:

• Even under proper storage conditions the cast should be poured within an hour.
• The impression should not be wrapped too tightly which incorporates stress.
• Excess water in the towel may lead to imbibition.

 

Removal of the cast:

• The contact of stone with the impression should be for 60 minutes before the cast is removed.
• A chalky stone surface may be produced, if the cast is allowed to remain in contact with the impression overnight as set stone absorbs water from the impression.

 

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Laboratory Duplication Procedure

In the construction of partial dentures and orthodontic appliances it is often necessary to produce more than one cast. It is not always possible or advisable to pour two or more casts from one impression and in such cases the first cast is duplicated using a reversible hydrocolloid duplicating material.

The duplicating hydrocolloid, which is normally thinner in consistency than the impression hydrocolloids.

The agar materials are widely used as laboratory duplicating materials.

For this application their main advantage is that the material can be reused.
A significant factor in this application where the products are used in relatively large bulk.

The technique for duplication involves standing the cast surrounded by a metal
duplicating flask which is designed to allow an even thickness of material all round.

Duplicating Flask

Manipulation:

1. The duplicating material is liquefied at 70 °C to 100 °C, tempered to 50 °C and then poured through an opening and filled.
2. When gelation is complete, the master cast is removed with a rapid movement rather than by easing it away, in order to optimize elastic recovery within
   the gel.
3. The duplicate cast should be poured immediately after removal of the master cast in order to avoid dimensional changes in the hydrocolloid.

   

   Dentures are suspended in the metal duplicating flask. Then, molten agar is being poured.

The mold space after removal of the denture.

Auto-polymerized acrylic resin is then poured in the mold space to produce template dentures for modifications.

 

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