1. AGAR (reversible) hydrocolloid

2. DEFINITION
An aqueous impression material used for recording maximum detail; e.g. as required in the production of dies for fixed restorations.
An organic hydrophilic colloid (polysaccharide) extracted from certain types of seeweed.

3. COMPOSITION OF COMMERCIAL REVERSIBLE HYDROCOLLOID IMPRESSION MATERIALS
component
function
composition(%)
agar
brush-heap structure
13-17
borate
strength
sulfate
gypsum hardener
wax, hard
filler
thixotropic materials
thickener
water
reaction medium
balance

4. CLASSIFICATION (ISO 1564: 2001)
TYPE I – high consistency (for use as tray material)
TYPE II – medium consistency (for use as tray or syringe material)
TYPE III – low consistency (for syringe use only)

5. USES
For cast duplication (e.g. during fabrication of cast metal removable partial dentures, etc.)
For full mouth impressions without deep undercuts.
Previously used for FPD impressions prior to elastomers.
As a tissue conditioner.

6. SUPPLIED AS Two forms: Syringe material Tray material
Tubes are used to fill the water-cooled trays and cartridges for use with the syringes.
Differences between two forms are the color and a greater fluidity of the syringe material.

7. CARTRIDGE OF AGAR HYDROCOLLOID AND SYRINGES USED FOR INJECTING ONTO THE PREPARED TOOTH. ALSO SHOWN IS THE HOLDER FOR CARRYING THE AGAR INTO THE CONDITIONING UNIT.

8. MANIPULATION Manipulation includes: liquefying the gel
placing it in the impression tray
tempering it to a lower temperature that the patient can tolerate
maintenance of fluid state to capture details of oral structures
once in the mouth, material is cooled below mouth temperature to ensure gelation

9. The process requires proper equipment.
Three compartments in conditioning unit make it possible to simultaneously liquefy, store, and temper the hydrocolloid.
Temperatures in each of the step are critical.
Equipment should be calibrated weekly.

10. CONDITIONING UNIT FOR AGAR HYDROCOLLOID IMPRESSION MATERIALS
CONDITIONING UNIT FOR AGAR HYDROCOLLOID IMPRESSION MATERIALS. THE THREE COMPARTMENTS ARE USED FOR LIQUEFYING THE MATERIAL, STORING AFTER BOILING, AND TEMPERING THE TRAY HYDROCOLLOID.

11. Step 1: Preparation and conditioning of the agar material
First step is to liquefy the hydrocolloid gel in boiling water for at least 10 minutes.
After liquefaction, the material may be stored in the sol condition at 65⁰C until needed for injection into prepared tooth or for filling a tray.
The material can be stored for several days, therefore, a number of tubes and syringes can be prepared for use throughout a week or so.

12. Step 2: Tempering of the material
65⁰C is too hot for the oral tissues, therefore, the material used to fill the tray must be tempered.
For immediate preparation step : hydrocolloid sol removed from storage bath, tray filled with sol, gauze pad placed over it, placed in water filled tempering container at 45⁰C.
Tempering time minutes.
To ensure that agar temperature is <55⁰C.

13. Tempering increases viscosity.
Syringe material is never tempered but always maintained in a fluid state to enhance adaptation to tissues.

14. Step 3: Making the agar impression
Just before completion of tempering for tray material, low viscosity material syringed directly from storage compartment is applied to prepared tooth (first applied to the base, then rest of the preparation covered).
Gauze pack and outer layer of agar is removed from tempering bath, transferred into stock tray and seated with light pressure.
Gelation is accelerated by circulating cool water (18-21⁰C) through the tray for 3-5 min.
Lower the temperature, more rapid is the gelation and stronger the material.
Impression removed suddenly with snap (viscoelastic behavior).

15. AGAR HYDROCOLLOID IMPRESSION

16. TRIPLE TRAY TECHNIQUE
Even sufficiently viscous material sometimes does not offer much resistance to seating making easy for the patient to bite through it.
This is why, triple tray is commonly used with agar.
With this technique, one impression records oral structures of both the arches as well as occlusal relationship.
Technique-sensitive procedure.
Dentist must guide the patient into centric occlusion as he/she bites into the material.
Optimal consistency of agar allows its successful use.

17. TRIPLE TRAY IMPRESSION

18. LAMINATE TECHNIQUE Recent modification to traditional agar procedure.
Hydrocolloid in the tray is replaced with a mix of chilled alginate that bonds with the agar expressed from a syringe.
Alginate gels by a chemical reaction, whereas, agar gels by means of contact with the cool alginate.
Since agar is in contact with the prepared teeth, maximum detail is reproduced.

19. Advantages: Disadvantages: Equipment cost is lower.
Less preparation time is required.
Produces an impression with adequate detail.
Disadvantages:
Bond between agar and alginate is not always sound.
Higher viscosity of alginate displaces agar during seating.
Dimensional inaccuracy of alginate limits its use to single units.

20. PROPERTIES OF AGAR HYDROCOLLOID IMPRESSION MATERIALS
Gelation
(⁰C)
Elastic recovery*
(%)
Flexibility**
Compressive strength***
(MPa)
Tear
strength
(kN/m)
agar
37-45
99.0
4-15
0.78
* At 10% compression for 30 sec.
** At a stress of 1000 g/cm²
*** At a loading rate of 10 kg/min.

21. Other properties Accuracy:
Highly accurate at the time of removal from the mouth, but shrinks in air and expands in water.
Viscosity of the sol:
Sufficiently viscous to record every detail of the teeth and soft tissues if correctly manipulated.

22. Compatibility with gypsum:
More compatible with gypsum model materials than alginates.
Washed of saliva or blood.
Excess liquid blown with an air syringe.
Dimensional stability:
If stored in air, lose water and contract.
Replacement in water leads to absorption and swelling.
Best stored in 100% relative humidity.

23. DUPLICATING IMPRESSION MATERIALS
Duplicating is done for two reasons:
The cast on which wax pattern of metal framework is formed must be made from a refractory investment, as it has to withstand casting temperatures.
Original cast is needed for checking accuracy of metal framework and for processing denture base portion of partial denture.
Most commonly used material – agar hydrocolloid.

24. Use of agar as duplicating material
Reversible material.
Can be continuously stored at 54 to 66 ⁰C and used when needed without converting from gel to sol state.
Have adequate strength and elastic properties to duplicate undercut areas.
However, dimensional change can occur.
Hydrolysis at storage temperature is accompanied with loss of elasticity and strength.

25. ADVANTAGES DISADVANTAGES
Accurate dies can be prepared.
Good elastic properties help reproduce most undercut areas.
Has good recovery from distortion.
Hydrophilic.
Palatable and well tolerated.
Economical.
Reusable.
Cost effective.
Does not flow well.
Cannot be electroplated.
Thermal discomfort on insertion or gelation.
Dimensional instability.
Tears easily.
Only one model can be poured.
Needs special equipment and technique.
Risk of contamination and deterioration of properties on reuse.