Agar Impression material is an Hydrocolloid Reversible Impression material.
It is called Reversible Impression material because of its property of reversing their physical state, which in reusing the impression material for multiple number of impressions.
History of Agar Impression material:
It is was first discovered by Alphous Poller of Vienna in 1925 when he was trying to develop a material that could be sterilized and applied without pressure to the exposed surface of the dura matter for perfectly recording the convulsion and the bony margins of the skull.
Which was termed as ‘Negacol’ which was termed as “Dentacol” when introduced into dentistry.
Origin Of Agar Impression material:
- An Organic Hydrophillic Colloid (Polysaccharide) extracted from certain types of Seaweed
- It is a Sulphuric ester of a linear polymer of Galactose
- Gel in collapsible tube (Used with a Tray)
- Multiple Cylinders in a glass jar (Syringe material)
- In bulk containers (for duplication)
- Agar: 13-17%
- Borates: 0.2-0.5%
- Potassium Sulphate: 1-2%
- Wax: 05-1%
- Thixotrophic Materials: 0.3-0.5%
- Alkylbenzoates: 0.1%
- Coloring and Flavoring agents: Traces
- Water: Balance (Around 84%)
Functions of Each ingredient:
- Agar: Basic Constituent 13-17% used for tray material and 6-8%is used for Syringe material
- Borates: Improves Strength of the gel, (Also retards the setting of plaster or stone cast when poured into the finished impression – a disadvantage)
- Potassium Sulphate: It counters the retarding effect of borates, thereby ensures proper setting of the cast or die.
- Wax: It acts a Filler. Fillers affect the strength, viscosity and rigidity of the gel. Other Examples of fillers – Zinc Oxide, Diatomaceous Earth, Silica, Clay, Rubber etc..
- Thixotrpoic materials: It acts as plasticizer. Examples are: Glycerine and Thymol. Thymol acts as bactericide also.
- Alkylbenzoates: It acts as a Preservative
- Water: It acts as a Dispersion medium
- Coloring and Flavoring agents: Used for patient comfort and acceptance.
Materials Required for Manipulation of Agar :
- Agar Hydrocolloid Conditioner
- Water cooled rim lock trays
- Hydrocolloid Syringe Material
- Duplicating Mould
- Rubber Hose
How does Agar Hydrocolloid Conditioner work:
It has 3 sections:
- Boiling Section (or) Liquefaction section
- Storage Section
- Tempering Section
Boiling Section (or) Liquefaction section:
- Agar is placed in Boiling water at 100 degrees Centigrade for 10 Mins
- The Solution should be Homogenous and free of Lumps
- Everytime the material is reliquefied, three minutes should be added for the heating. This is because it is more difficult to break down the agar brush heap structures after a previous use.
- 65-68 Degrees Centigrade is ideal temperature for Storage of Agar in Sol condition.
- It can be stored in the sol condition till required for impression taking.
- 46 degrees Centigrade for about 2 minutes with the material loaded in the tray.
- This is done to reduce the temperature so that it can be tolerated be the sensitive oral tissue
- It also makes the material viscous suitable for impression taking
How to Use Water Cooled Rim Lock Impression Tray:
- Rim Lock Impression Trays have a feature of Water Circulation through the Tray.
- The Rim Lock is a Beeding on the inside edge of the tray which helps to retain the material (as Agar does not adhere to the tray)
- It has an Inlet and Outlet for connecting the water tubes
- The Tray should allow a space of 3mm occlusally and laterally and extend distally to cover all teeth
How to take an Agar Impression:
Steps to Take the Impression:
- Remove the Tray containing the Agar from the Tempering Section of the Hydrocolloid Conditioner.
- The Outer surface of the Agar sol is scraped off
- The water hoses are connected and the tray is positioned in the mouth by the dentist
- Water is circulated at 18 to 21 degrees Centigrade through the tray until gelation occurs.
- Rapid Cooling: (e.g ice cold water) is not recommended as it can induce distortion
- To guide the tray into position, 3 stops of compound are prepared on non-involved teeth
- A Post dam is constructed with compound to prevent distal flow of the impression material
- In a deep palate case, compound is placed on the palatal aspect of the tray in order to provide a uniform thickness of the Hydrocolloid
- The mandibular tray is prepared by placing compound on the distal aspect to limit the impression material
- Black Tray compound is used as it is not affected in the tempering bath
Steps to Remove the Impression:
- Wait for the Agar to turn into a Gel
- Break the Peripheral seal around the Impression
- Remove the Impression rapidly from the mouth with a Single stroke or snap (This is done to avoid distortion of the impression)
- The Impression is Rinsed thoroughly with water ans the excess water is removed by shaking the impression
How to Store Agar Impression:
- The Cast should be poured immediately
- Storage of Agar Impression material should be avoided at any cost, as there is no storage medium available
- Storage in Air results in 100% relative humidity which results in Shrinkage as a result of continued formation of the agar network agglomeration
- If Storage is unavoidable, it should be limited to one hour in 100% relative humidity
How to Pour and Separate the Cast:
- Use Gypsum products for pouring the Cast
- Mix the Gypsum product and Pour it into the impression
- When the Gypsum products have set, the agar impression must be removed promptly since the impression will Dehydrate and become Stiff and difficult to remove
- Be careful while removing the Cast as the weaker portions may fracture
- Prolonged contact will result in a rougher surface on the model, so make sure the cast is removed immediately after the setting time of the gypsum product is finished
Uses Of Agar:
- Used as a Cast Duplicating material (during fabrication of the cast metal removable partial denture)
- For Full mouth impressions without deep undercuts
- Used for Crown and and Bridge impressions before Elastomers were Discovered
- Used as Tissue Conditioners
Materials Used To Disinfect Agar Hydrocolloid Impression:
Advantages Of Agar Hydrocolloid Impression Material:
- Helps in preparing Dies Accurately
- It has Good Elastic Properties and Reproduces most undercut areas correctly
- It has Good Recovery from Distortion
- It is not Hydrophobic, hence gives good model surface
- It is Palatable ans well tolerated by the patient
- It is cheap when compared to synthetic elastic materials
- The Main Advantage is: Agar can be Reused when used as a Duplicating Material
Note: Reuse is not recommended when used as an impression material
Disadvantages Of Agar Hydrocolloid Impression Material
- Does not Flow as good as the newly developed materials
- It cannot be Electroplated
- Due to the heat it may be painful to the patient during Insertion or Gelation
- Tears relatively easily
- Greater Gingival retraction is required for providing adequate thickness of the material
- Only one model can be poured
- Expensive Equipment is required
- A Plaster Hardener should be used to get a soft surface on Gypsum Cast
- Contamination is very prevalent with this material due to Reuse and the lack of Sterilization technique