1 Impression Materials

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Impression Material

Dr.Mohammed.M.Heskul, DDS,MSc.

Impresstion MaterialThe function of impression material is to record accurately the dimensions of oral tissues and their spatial relationships. In making an impression, a material in the plastic state is placed against the oral tissues to set . After setting, the impression is removed from the mouth and is used to make a replica of the oral tissues

Impresstion MaterialThe impression gives a negative reproduction of these tissues. A positive reproduction is obtained by pouring dental stone or other suitable material into the impression and allowing it to harden. The positive reproduction is called a model or cast when large areas of the oral tissues are involved or a die when single or multiple tooth preparations are recorded the relationships between a tooth.

Impresstion MaterialImpression materials are classified as:

Flexible Rigid

Undercuts

Impresstion MaterialRigid: is restricted to applications in areas where no undercuts exist.

- If there is undercut: on setting, it would be locked in place and could not be removed over the bulge of the tooth without fracturing. - A rigid material can be removed from a tooth prepared for a full crown or from an edentulous.

Impresstion MaterialFlexible : can be used in there are or (are not) undercuts,so itused most frequently. (Dentulous-edentulousprepared non prepared teeth)

Impresstion MaterialDesirable Properties of Impression Materials1- Ease of manipulation and reasonable cost 2- Adequate flow and wetting properties 3- Appropriate setting time and characteristics 4- Sufficient mechanical strengthelastic recovery and resistant to tearing during removal. 5- Good dimensional accuracystable over time 6- Taste and odor acceptable to patient 7- Safenot toxic or irritating. 8- No significant degradation of properties as a result of disinfection. 9- Compatibility with all die and cast materials. 10- good shelf life.

PlasterNon-elastic

Compound Waxes

Impression Materials

ZnO - Eugenol

Agar (reversible)Aqueous Hydrocolloids Alginate (irreversible) Polysulfide Condensation

Elastic Non-aqueous ElastomersSilicones

AdditionPolyetherOBrien, Dental Materials & their Selection 1997

ALGINATE Hydrocolloid Impression MaterialsAlginate is one of the most widely used aqueous dental impression materials.

ALGINATE

The wide use of alginates results from : 1) The ease of mixing and manipulate. 2) The minimum equipment necessary.

3) The flexibility of the set impression. 4) Their accuracy if properly handled. (5) Their low cost.

ALGINATEMIXINC AND SETTING TIMES

when properly mixed by hand, should develop a smooth, creamy , no voids consistency free of graininessNormal set : 2 4m.15s

Fast set 1-2 mBecause the setting occurs as a result of a chemical reaction, an increase in the temperature of the water used to prepare the mix shortens the working and setting times.

ALGINATE

The proportions of powder and water also affect the setting times.

Thinner mixes increase the time required for the material to set.

ALGINATE

A problem with alginate impressions is loss of accuracy with increased time of storage.

The set alginate is a hydrocolloid gel that contains large quantities of water. This water evaporates if the impression is stored in air, and the impression shrinks. If the impression is placed in water, it absorbs water and expands. Therefore, storage in either air or water results in serious changes in dimensions and a loss of accuracy. Storage in humid air approaching 100 relative humidity results in the least dimensional change

ALGINATE

Alginate gels, however, shrink even under conditions of 100 relative humidity as a result of a process caned syneresis, in which water forms on the surface of the impression.

Fortunately, syneresis occurs rather slowly, and alginate impressions prepared from modern products usually can be stored under conditions of 100 relative humidity for up to 5 days without serious dimensional changes.

INGREDIENTS

Na/K alginate Calcium sulfate to react with Na alginate. Sodium phosphate retarder Diatamaceous earth control flexibilty Pigments colour Phenylanaline sweetner

ELASTIC RECOVERY

Alginates have elastic recovery of 96-98 prcnt. 2 - 4 percent permentant deformation THUS . Alginate is flexible but not perfectly elastic. ANSI-ADA specifications require at least 95 percent elastic recovery when Alginate is compressed 20 prcnt for 5 sec.

FLEXIBILITY ANSI-ADA set limit of 5 20 percent at the time a model or die is prepeared in the impression. Relative amount of water powder ratio affects flexibility. Thicker mixes result in low flexibility

REPRODUCTION OF DETAILS. ADA Specifications recommend that alginate must have the minimum capabilities of transferring a line only 0.075 mm wide to a gypsum model or die material.

Objectives for Taking Alginate Impressions

All teeth in the upper and lower arch The entire alveolar process The retromolar area of the lower arch The area of the hamular notch in the upper arch . A detailed, undistorted, and bubble-free reproduction of the oral tissues

Mixing the Alginate

With a stiff, wide-bladed spatula. the alginate powder is stirred with water. The mix is spatulated vigorously against the side of the rubber bowl until smooth, creamy mix results, which is free of voids.

Steps after Making the Impression

1. Rinse the impression with cool water. 2. Cut away any unsupported alginate. 3. Disinfect alginate impression. One method is to soak it in hypochlorite bleach Diluted 10:1 with water for 10 minutes. 4. Rinse disinfected impression with cold water. 5. Remove excess water.

Steps after Making the Impression

6. Proceed to pour gypsum model , or if storage is necessary, wrap loosely in a wet paper towel or seal in a plastic bag for up to 5 days. 7. If stored, the impression should be placed so that the tray supports the alginate. Thus, the tray side of the impression is against the laboratory bench.

Troubleshooting Alginate ImpressionsA- If the alginate set before a satisfactory impression was obtained, several factors could be contributing to this problem:

incorrect dispensing of powder and liquid (not aerating the powder before dispensing) .An unusually high room temperature .

Too much time used in loading the tray and inserting the impression.

Troubleshooting Alginate ImpressionsA- If the alginate set before a satisfactory impression was obtained, several factors could be contributing to this problem: solution for delaying the setting: is to reduce the temperature of the mix water rather than use a thinner mix, which results in lower tear strength.

Troubleshooting Alginate ImpressionsB- If the alginate impression contained several large voids in critical areas, what can be done to avoid this problem? 1- During the dispensing and mixing of the alginate, the powder is add to the water and often an initial wetting of the powder by water is accomplished by squeezing the alginate between the blade of the spatula and the side of the rubber mixing bowl, which minimizes incorporation of voids.

Troubleshooting Alginate ImpressionsB- If the alginate impression contained several large voids in critical areas, what can be done to avoid this problem? 2-When filling the tray, add alginate at one location, and force the alginate to flow into other portions of the tray.

Troubleshooting Alginate ImpressionsB- If the alginate impression contained several large voids in critical areas, what can be done to avoid this problem? 3- As a special precaution, a portion of alginate can be spread over the occlusal surface of the teeth before the loaded tray is inserted.

Troubleshooting Alginate ImpressionsC- If the alginate impression had a large amount of material in the posterior palatal area and did not record adequately the area of the vestibule, several factors could be involved: 1- The posterior of the tray may not have been seated first followed by the anterior, which allowed a large portion of the alginate to flow into the palatal area (which also could make the patient gag).

Troubleshooting Alginate ImpressionsC- If the alginate impression had a large amount of material in the posterior palatal area and did not record adequately the area of the vestibule, several factors could be involved: 2- Inadequate alginate may have been placed in the tray.

Troubleshooting Alginate ImpressionsC- If the alginate impression had a large amount of material in the posterior palatal area and did not record adequately the area of the vestibule, several factors could be involved: 3- The lip may not have been pulled out gently to allow alginate to flow into the vestibule.

Troubleshooting Alginate ImpressionsC- If the alginate impression had a large amount of material in the posterior palatal area and did not record adequately the area of the vestibule, several factors could be involved: 4- The impression must be seated gradually to give the alginate time to flow.

Alginate Substitute Impression Materials

Alginate substitute impression materials are addition silicone impression materials that are formula ted to provide characteristics similar to alginate impression materials.

Alginate Substitute Impression Materials

Agar Hydrocolloid Impression Matyerials

Agar hydrocolloid was the first successful flexible aqueous impression material used in dentistry.

Agar Hydrocolloid Impression Matyerials

The flexibility of the material at the time it is removed from the mouth allows impression of undercut are