AGAR(reversible)

hydrocolloid

ISHIKA GARG

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.

COMPOSITION OF COMMERCIAL REVERSIBLE HYDROCOLLOID

IMPRESSION MATERIALScomponent function composition(%)

agar brush-heap structure 13-17

borate strength 0.2-0.5

sulfate gypsum hardener 1.0-2.0

wax, hard filler 0.5-1.0

thixotropic materials thickener 0.3-0.5

water reaction medium balance

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)

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.

SUPPLIED AS

• Two forms:Syringe materialTray 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.

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.

MANIPULATION

• Manipulation includes: liquefying the gelplacing it in the impression tray tempering it to a lower temperature that the

patient can toleratemaintenance of fluid state to capture details of

oral structuresonce in the mouth, material is cooled below

mouth temperature to ensure gelation

• 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.

CONDITIONING UNIT FOR AGAR HYDROCOLLOID IMPRESSION MATERIALS. THE THREE COMPARTMENTS ARE

USED FOR LIQUEFYING THE MATERIAL, STORING AFTER BOILING, AND TEMPERING THE TRAY HYDROCOLLOID.

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.

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 3-10 minutes.• To ensure that agar temperature is <55⁰C.

• Tempering increases viscosity.• Syringe material is never tempered but always

maintained in a fluid state to enhance adaptation to tissues.

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).

AGAR HYDROCOLLOID IMPRESSION

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.

TRIPLE TRAY IMPRESSION

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.

Advantages:• 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.

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 0.8-0.9

* At 10% compression for 30 sec.** At a stress of 1000 g/cm²*** At a loading rate of 10 kg/min.

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.

• 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.

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.

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.

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.