AGAR IMPRESSION MATERIAL is a reversible hydrocolloid composed of agar weed polymer chains. These polymer chains align to form fibrils, which trap water in the interstices between the chains. This conversion from sol to gel is a thermal process, making agar a thermo-reversible material. Notably, agar was the first elastic impression material used in dentistry.
The addition of potassium sulfate (K2SO4) is aimed at improving the surface quality of the gypsum model. Borax, which is commonly used in gypsum materials, has the effect of retarding the setting reaction. Potassium sulfate is added to counteract this retardation, ensuring that the gypsum sets properly and improving the surface quality of the final model.
Characteristics of AGAR IMPRESSION MATERIAL
Composition
The primary constituent of agar, as with alginate, is water. The composition of agar impression material typically includes:
- Water (83-85%): Provides the colloid medium necessary for the material’s properties.
- Agar (12-15%): A complex polysaccharide derived from seaweed, serving as the gelling agent.
- Borax (0-2%): Added to strengthen the gel.
- Potassium sulfate (1-7%): Included to improve the surface of gypsum models by counteracting the retarding effect of borax.
- Alkylbenzoate (0-1%): Acts as a mold inhibitor.
- Colors and flavors: These are sometimes added for patient acceptance, improving the overall experience of using the material.
Manipulation Technique
The manipulation of agar involves several steps to ensure its proper use for recording dental impressions:
- Preparation of Agar: Agar is typically supplied in its gel state, packaged in tubs or tubes. A 3-chamber conditioning unit is commonly used for its manipulation.
- Liquefaction: The first chamber of the conditioning unit is used to liquefy the gel by heating it to 100°C for 10 minutes. This converts the gel to a sol, making it easier to manipulate.
- Storage: The sol is then transferred to the second chamber, where it is stored at 65°C until ready for use.
- Tray Preparation: When the operator is ready to take the impression, the sol is placed in an impression tray and kept in the third chamber at 46°C for 3 minutes. This ensures that the material is at the appropriate temperature for recording the impression.
- Impression Recording: The tray containing the sol is seated in the patient’s mouth and allowed to cool. Cooling channels in the special stock tray facilitate this process. Gelation of the agar begins at 43.3°C, converting the sol back into a gel. Once the material has set, the impression can be removed from the mouth.
It’s important to note that the manipulation of agar is a lengthy and technique-sensitive procedure, which has led to its decreased preference in clinical applications.
Salient Properties of Agar
Agar possesses several key properties that make it suitable for dental impressions:
- Reversible Setting Reaction: Agar undergoes a reversible setting reaction, allowing it to transition from a sol (liquid) to a gel (solid) state and back again. This property enables agar to be reused multiple times.
- Thermo-Reversible: The setting reaction of agar is triggered by temperature changes. Heating the sol liquefies it, while cooling it causes gelation, making agar thermo-reversible.
- Dimensional Stability: Agar exhibits excellent dimensional stability during setting, ensuring accurate reproduction of dental structures.
- Tissue Compatibility: Agar is gentle on oral tissues and does not cause irritation or adverse reactions.
- High Detail Reproduction: It has the ability to capture fine details of dental anatomy, providing precise impressions.
These properties make agar a suitable choice for certain dental impression procedures, particularly when high accuracy and detail reproduction are required.
Properties of Agar Impression materials
Agar exhibits several properties that make it suitable for certain dental applications:
- Biocompatibility: Agar is non-toxic, non-irritant, non-allergenic, and non-carcinogenic, making it safe for use in the oral cavity. However, its slow setting reactions can be cumbersome.
- Accuracy: When manipulated correctly, agar is capable of accurately reproducing fine details of dental structures.
- Elasticity: Agar is elastic, allowing it to be withdrawn from undercut areas. However, it has poor tear strength, particularly in large interdental spaces.
- Polymer Structure: The polymer chains in agar form fibrils, contributing to its unique properties.
- Dimensional Changes: Agar undergoes dimensional changes due to imbibition (absorption of water from the environment) and syneresis (loss of water), emphasizing the need for immediate casting after impression taking.
- Adhesion: Agar does not adhere well to stock trays. Perforations in the stock tray provide mechanical retention for the material.
- Effect on Gypsum: The borax component in agar can soften the surface of gypsum, affecting model quality.
- Reusability: Agar can be reusable, but this is not always practical due to its handling characteristics.
- Sterilization: It can be sterilized using 2% hypochlorite for 10 minutes.
Applications
Despite the advent of elastomeric materials, agar still finds use in certain laboratory applications in dentistry:
- Model Duplication: Agar is commonly used in the duplication of models in dental laboratories.
- Crown and Bridge Impressions: Although elastomeric impression materials have largely replaced agar for clinical impressions, agar is still used in certain crown and bridge procedures, particularly in laboratory settings.
Agar-Alginate Laminate Technique
To address the challenges posed by the lengthy preparation and setting reaction of agar, an innovative method known as the agar-alginate laminate technique was developed. This technique offers a more efficient and practical way of handling agar while still harnessing its benefits.
Procedure:
- Preparation of Agar Sol:
- Agar sol is prepared by heating agar gel in a three-chamber conditioning unit until it liquefies. The sol is then stored at a controlled temperature of around 65°C.
- Preparation of Alginate Sol:
- Alginate, an irreversible hydrocolloid, is mixed with cold water to form a sol. The cold temperature of the water initiates gelation in the alginate.
- Application of Agar Sol:
- The agar sol is syringed onto the oral structures that need to be recorded, ensuring adequate coverage and accurate placement.
- Application of Alginate Sol:
- Next, a tray loaded with alginate sol is placed over the agar-coated oral structures. The alginate sol, being cold, provides a gelation environment for the agar.
- Setting:
- Both the agar and alginate undergo their respective setting reactions simultaneously.
- Removal:
- Once set, the impression tray containing the agar-alginate laminate is carefully removed from the mouth.
- Separation:
- The agar-alginate laminate is separated, revealing the impression of the oral structures captured in the agar layer.
- Casting:
- The impression can then be cast with gypsum or other suitable materials to create a replica of the oral structures.
Advantages:
- Improved Handling: The agar-alginate laminate technique simplifies the handling of agar, making it more practical for clinical use.
- Faster Set: The combination of agar and alginate allows for a faster setting time compared to agar alone.
- Accuracy: Despite the accelerated setting, the technique still offers accurate reproduction of fine details.
- Cost-Effective: Alginate is a cost-effective material, and using it in combination with agar reduces the overall material cost.
Applications:
- The agar-alginate laminate technique is commonly used for various dental impressions, including:
- Preliminary impressions for denture fabrication
- Diagnostic impressions for treatment planning
- Impressions for provisional restorations
- It is particularly useful in situations where a quick, accurate impression is required without the need for complex equipment or procedures.