Constructing a model or cast is an important step in numerous dental procedures. Various types of cast & models can be made from gypsum products using an impression mold or negative likeness of a dental structure. An impression is a negative replica of the tissue of the oral cavity. It is used to register or reproduce the form and relation of the teeth & surrounding tissue.
l 1558 - Celline described a wax model to make impression in his book MEMORIES.
l 1700 - Matheus G. Purenam suggested that wax models can be used in prosthetic work.
l 1728 – Pierre Fauchard described various impression materials in his book LE CHIRURGEEN DENTISTE.
l 1756 – Persian Phillip Pfaff first used plaster models prepared from sectional wax impression of the mouth.
l 1810 – R C Skinner describe about many materials and techniques in his first American book.
l 1928 – ADA developed specification No. 19 for impression material
l 1930 – Zinc Oxide Eugenol a rigid impression was introduced
l 1930 – Polysulfide was first used as a commercial synthetic rubber as a copolymer of Ethylene Chloride & sodium.
l 1937 – Agar was introduced by Sears.
l 1949 – Alginate was developed during World war II.
l 1950 – Polysulfide was used in dentistry.
l 1955 – Condensation Silicone was introduced in Germany.
l 1975 – Addition Silicone was introduced.
l 1977 – Costell introduced Dual tray technique.
l 1980 – A Visible light cure Polyether Urethane Dimethacrylate rubber impression material was introduced.
l 1996 – Blare & Wassed considered a no. of solution used to disinfecting impression material.
IDEAL REQUIREMENTS OF IMPRESSION MATERIAL:
1. Pleasant taste & odor
2. Not contain any toxic & irritating ingredients
3. Adequate shelf life
4. Satisfactory consistency and texture
5. Easy to disinfect with out loss of accuracy
6. Compatible with die and cast material
7. Dimensional stability
8. Good elastic properties
9. Easy manipulation
10. Adequate setting characteristics
11. High degree of reproduction details
12. Adequate strength
13. Should not release any gas
15. Should not be technique sensitive.
1. Based on rigidity/elasticity: -
a. Rigid (non-elastic).
2. Based on viscosity: -
3. Based on setting of material: -
1. a. chemical reaction.
b. Physical change of state.
2. a. reversible.
4.Based on interaction with saliva/water: -
5.Based on chemistry:-
1. Impression Plaster.
2. Impression compound.
3. Metal oxide (zinc oxide eugenol).
4. Reversible hydrocolloid.
5. Irreversible hydrocolloid.
6. Poly sulfides.
7. Condensation silicones.
8. Addition silicones.
10. Visible light curing polyether urethane dimethacrylate.
6.Based on use: -
1. Primary impression materials.
2. Secondary impression materials.
3. Duplicating materials.
· Colloids are often classified as fourth state of matter, the Colloidal state. In a solution of sugar in water, the sugar molecules are uniformly dispersed in the water & there is no visible physical separation between the solute & the solvent molecules. If sugar molecules replaced with large & visible particles such as sand, the system is Suspension or if molecules are liquid such vegetable oil, then system is Emulsion.
· True solution exists as a single phase. However, both the colloid & the suspension have to phase- The dispersed & Dispersion phase. In the colloid, the particles in the dispersed phase consist of molecules held together either by primary or secondary force. The size of the particles range is 1 – 200nm.
· Colloids with a liquid as the dispersion medium can exist in two different forms known as Sol & Gel.
· A sol has the appearance & many characteristic of a various liquid.
· A gel, on the contrary, is a semi solid & produced from a soldering process of gelation by the formation of fibrils or chains called Micelles.
· Gelation is the conversion of a sol to gel, & the temperature at which this occurs is called Gelation Temperature.
Gelation may be brought about in one of these ways:
l Lowering the temperature:- It is done by reducing the thermal energy of effectively. These forces are secondary molecular forces. The bond between the fibrils is weak & they break at slightly elevated temperature. Gelation temp. is 37-50degree centigrade.
l Liquefaction temperature:- It is considerable higher than gelation temp. & this property is known as Hysteresis. This temp. is between 70 -100 degree centigrade.
l Chemical reaction:- Gelation may also induced by chemical reaction, where in the dispersed phase of soil is allowed to react with a substance to give a different type of dispersed phase. The process is not reversed by an increased temperature.
Types of Hydrocolloids:
REVERSIBLE HYDROCOLLOIDS: - Reversible hydrocolloids are those sol can be changed to gel, but gel can’t be reversed back to the solution.
Eg: Alginate impression material.
IRREVERSIBLE HYDROCOLLOIDS: - Irreversible hydrocolloids are those materials, where the change from the sol to gel can be brought by lowering the temp. of the sol & the gel can be converted back to sol condition by heating.
Eg: Agar – Agar.
ALGINATE IRREVERSIBLE HYDROCOLLOID
• The word alginate comes from Algin & named by a chemist of Scotland. It was identified as a liner polymer with numerous carboxyl acid groups. It is called as irreversible hydrocolloids because gelation is induced by chemical reaction & transformation is not possible. It is the most widely used dental materials.
• The principal factors responsible for the success of this type of impression materials are:-
• Easy to manipulate
• Comfortable for the patients.
• Relatively inexpensive.
Ø Potassium or Sodium Alginate:- 15% It is the chief active ingredient
Ø Calcium Sulfate:- 16% It is used as a reactor.
Ø Zinc Oxide & Diatomaceous Earth:- Zinc oxide – 4% & Diatomaceous
Ø Potassium Titanium Fluoride :- 3%
Ø Sodium Phosphate :- 2%
l The typical sol-gel reaction is a soluble alginate with calcium sulfate & the formation of an insoluble calcium alginate gel. Calcium sulfate reacts rapidly to produce the in soluble Ca alginate from the potassium or sodium alginate in a aqueous solution. The production of calcium alginate is rapid that it does not allow sufficient working time. Thus third water soluble salt, such as a trisodium phosphate, is added to the solution to prolong the working time. Thus the reaction between the calcium sulfate & the soluble alginate is prevented as long as there is uncreated trisodium phosphate.
l 2Na3Po4 +n CaSo4 →nKaSo4 +Ca. alginate
l When the supply of the trisodium phosphate is exhausted, the ca ions begin to react with the potassium alginate to produce calcium alginate. Kzn Alg +n CaSo4→ n KaSo4 + Cal. alginate
l The added salt is retarder.
Controlling of gelation time:-
Ø An increase in the temp. of water used for mixing, shorten the working & setting time.
Ø The proportion of powder & water also effect the setting time.
l Alginate is of two types
Type I: - Fast setting
Type II: - Normal setting.
According to ADA Specification No. 18 properties are:-
1. Mixing time:- Creamy consistency come in 45 – 60 sec.
2. Working time:-
- Fast setting material: - 1.2 – 2 min
- Normal set material: - 2 – 4.5 min
3. Setting or gelation time:-
- Optimum gelation time: - 3-4 min at room temp.
- For fast setting material: - 1 -2 min
- For normal setting material: - 2 – 4.5min
4. Permanent deformation: - ADA specification requires 97% recovery & where 3% is permanent deformation. Alginate has 98.8% recovery & 1.5 % permanent deformation.
5. Flexibility:- ADA specification permits10 – 20% at the stress of 1000gm/ cm2. Hard material has value of 5-8%.
The compressive strength:- 5000-8000gm/cm2
Tear strength :- 350-700gm/cm2
7.Viscoelasticity: Usually an alginate impression material does not adhere to the oral tissue as strongly as some of the non aqueous elastomers, so it is easy to remove the alginate impression rapidly.
8.Accuracy: Most alginate is not capable of reproducing the finer details that are absorbed in impression with Agar & other elastomeric impression material.
9. Dimensional effects: - The gel may loss water by evaporation from its surface & it shrinks.
- If the gel placed in the water it absorbed water & gel swells.
- Thermal change also contributes to dimensional change. The alginate shrinks slightly due to difference in temp. between mouth temp.(30·c) & room temp(23·c).
10. Biocompatibility :-
No chemical or allergic reaction associated with alginate.
11. Shelf life :-
Alginate impressions have shorter shelf life. Strong temp. & moisture contamination are two factor which effect the self life of alginate.
MANIPULATION OF ALGINATE:
PREPARING THE MIX
A measured powder is shifted into pre measured water, that has been placed in a clean rubber bowl. Care should be taken to avoid whipping air into the mix. A vigorous figure 8 motion is best, with the mix being swiped or stropped against the side of the mixing rubber bowl with intermittent rotation ie.180 degree of the spatula to press out air bubbles. The mixing time is 45sec -1 min.
A variety of mechanical devices are also available for mixing impression material. There benefits are convenience, speed & elimination of human error.
MAKING THE IMPRESSION
Before setting the impression, the material should have developed sufficient body so that it doesn’t flow out of the try. A perforated tray is generally used. If plastic try or metal rim lock try is selected a thin layer of adhesive should be applied & allowed to dry before mixing and loading the try. The thickness of the alginate between the tray & tissue should be at least 3mm.
ADVANTAGES & DISADVANTAGES:
- Easy to mix and manipulate.
- Minimum requirement of equipment.
- Accuracy (if properly handled)
- Low cost
- Comfortable to the patient
- Hygienic (as fresh material is used for each impression)
1. Cannot be electroplated.
2. Distortion occurs easily
3. Poor dimensional stability (poured within 15 min.)
4. Poor tear strength
AGAR – REVERSIBLE HYDROCOLLOIDS
· When agar hydrocolloids heated, they liquefy or go into the sol state & on cooling they return to the gel state. Because this process can be repeated, a gel of this type is described as Reversible hydrocolloid. The preparation of the agar hydrocolloids for clinical use requires care full control & yields accurate impression. It has been largely replaced by alginate hydrocolloids & rubber impression materials.
§ Borates :- 0.2 – 0.5% Its works as retarder.
§ water :- 85.5% It is reaction medium.
§ Agar:- 13-17% It is main active constituent of reversible hydrocolloid impression material.
§ Sulfates :- 1 – 2% Accelerators
§ Fillers :- 0.5 -1% Diatomaceous earth, silica, wax rubber etc used as filler.
§ Bactericide :- Thymol & glycerin are used
§ Color & flavor
l The setting of the reversible hydrocolloid is called gelation. The reaction can be expressed a sol- gel reaction. The physical changes from to gel does not return to the sol at the same temp. at which it solidified. The gel must be heated at the liquefaction temp. ie 70 - 100·c to form sol. Sol transform into a gel at 37-50·c. the exact gelation temp depended on several factor, including the molecular wt. the purity of the material & the ratio of agar to other ingredients.
l The gelation is critical. If the temp. is high, the heat from sol may injury to the soft tissue, or if the surface of the sol transforms to a gel as soon as the sol may injure the oral tissues, a high surface stress may develop. If the gelation temp. is too far below the oral temp. it will be difficulty or even impossible to chill the material sufficiently to obtain a firm gel to adjacent to the oral tissue. The temp. lag between the gelation & liquefaction temp. of the gel makes it possible to use as a dental impression material.
l Gelation temperature :-
After tempering, the sol should be homogenous and should set to a gel between 37 – 500c when cooled.
l Viscoelastic properties :-
It demonstrates the necessity of deforming the impression rapidly when it removed from the mouth which reduces the amount of the permanent deformation. The elastic recovery of the hydrocolloid is never complete & it does not return entirely to its original dimension after deformation. The amount of permanent deformation in clinical is negligible, provided that :-
a) The material has adequately gelled.
b) The impression has been removed rapidly.
c) The under cuts present in the cavity preparation are minimal.
l Permanent deformation :-
The ADA specification requires that the permanent deformation should be less than 1.5% after the material compressed 10% for 30sec. This impression material readily meets this requirement with the value of about 1%.
l Distortion during gelation :-
If the material is held rigidly to the tray, then the impression material shrinks toward the center of its mass. Rapid cooling may cause a concentration of stress near the tray where gelation first takes place.
· Flexibility :-
The ADA specification requirement for flexibility allows a range of 4-14% & most agar materials meet this requirement.
l Strength :-
The compressive strength of Agar impression material is 8000gm/cm2
The tear strength of Agar is 7000gm/cm2
l Flow :-
Agar is sufficiently fluid to allow detailed reproduction of hard & soft tissue.
l Compatibility with gypsum :-
Not all the agar impression materials are equally compatible with all gypsum products. The impression should be washed of saliva & any trace of blood, which retarded the setting of gypsum.
Manipulation of agar impression:
The use of agar hydrocolloid involves special equipment called conditioning unit for agar. The hydrocolloid is usually supplied In two forms: syringe and tray materials. The only difference between the materials is the color and the greater fluidity of the syringe material.
It is rim locked trays with water circulating device. This types of trays should allow a space of 3mm occlusally and laterally and extend distally to cover all the teeth. After the tray has been properly positioned, water is circulated at 13 degrees through the tray until gelation occurs.
Preparation of material:
Proper equipment of liquefying and storing the agar impression material is essential.
At first reverse the hydrocolloid gel to the sol stage. Boiling water is a convenient way of liquefying the material. The material must be held at this temperature for a minimum of 10mins. Propylene glycol can be added to the water to obtain 100 degrees. After it has been liquefied, the material must be stored in the tray. The material can be stored for several days. Usually, there are three compartments in the conditioning unit, making it possible to liquefy, store and temper the material.
Conditioning and tempering:
Because 55 degree is the maximum tolerable temperature, the storage temperature of 65 degrees would be too hot for the oral tissues, especially given the bulk of the tray material. Therefore, the material that is used to fill the tray must be cooled and tempered. Eliminating the effect of imbibition is the purpose of placing the gauze pad over the tempering tray materials. When the tray material is placed into the tempering bath, the gauze is removed and the contaminating surface layer of material clings to the gauze and is removed as well.
Making the impression:
The syringe material is taken directly from the storage compartment and applied to the prepared cavities. It is first applied to the base of the preparation and then the reminder of the tooth is covered. By the time the cavity preparation and adjoining teeth have been covered, the tray material has been properly tempered and is now ready to be placed immediately in the mouth to form the bulk of impression. Gelation is accelerated by circulating cool water, approximately, 18 – 21 degrees through the tray for 3 – 5 minutes.
Disinfection of hydrocolloid impression:
As the hydrocolloid impression material must be poured within short time after removal from the mouth. The disinfection procedure should be relatively rapid to prevent the dimensional change. Most manufacturers recommended a specific disinfectant. The agent may be iodophor, bleach or gluteraldehyde. The distortion is minimal if the recommended immersion time is followed and if the impression is poured properly. The irreversible hydrocolloid may be disinfected by 10 minute immersion in/or spraying with the antimicrobial agent such as NaOCl and glutaraldehyde without sufficient dimensional change. The current protocol for disinfecting hydrocolloid impression is to use household bleach, iodophor, and synthetic phenols as disinfectants. After the immersion, it is thoroughly rinsed. The disinfectant is sprayed liberally o the exposed surface. The immersion should not be submerged or soaked in the disinfectant solution.
ADVANTAGES & DISADVANTAGES:
- Only one model can be used
- Extensive and expensive equipment required
- It can not be electroplated
- Impossible to sterilize for reuse
- Low dimensional stability & tear resistance