Download pdf - 1 Impression Materials

Impression Material

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

The 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 Material

The 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.


Impression materials are classified as:


• Flexible

• Rigid

Undercuts


Rigid: 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.


Flexible : can be used in there are or (are not) undercuts,so it

used most frequently.

(Dentulous-edentulous…prepared – non prepared teeth)

Desirable Properties of Impression Materials


1- Ease of manipulation and reasonable cost

2- Adequate flow and wetting properties

3- Appropriate setting time and characteristics

4- Sufficient mechanical strength—elastic recovery and resistant to

tearing during removal.

5- Good dimensional accuracy—stable over time

6- Taste and odor acceptable to patient

7- Safe—not 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.

Imp

ress

ion

Ma

teria

ls

Non-elastic

Elastic

Aqueous

Hydrocolloids

Non-aqueous

Elastomers

Polysulfide

Silicones

Polyether

Condensation

Addition

Agar (reversible)

Alginate (irreversible)

Plaster

Compound

ZnO - Eugenol

Waxes

O’Brien, Dental Materials & their Selection 1997

ALGINATE Hydrocolloid Impression

Materials…

Alginate is one of the most widely used aqueous dental impression

materials.

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.

ALGINATE…

when properly mixed by hand, should develop a smooth, creamy ,

no voids consistency free of graininess

Normal set : 2 – 4m.15s

Fast set 1-2 m

Because 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…

MIXINC AND SETTING TIMES

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.

ALGINATE…

Na/K alginate

Calcium sulfate to react with Na alginate.

Sodium phosphate retarder

Diatamaceous earth control flexibilty

Pigments colour

Phenylanaline sweetner

INGREDIENTS

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.

ELASTIC RECOVERY

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

FLEXIBILITY

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.

REPRODUCTION OF DETAILS.

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

Objectives for Taking Alginate Impressions

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.

Mixing the Alginate

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.

Steps after Making the Impression

A- 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 Impressions

A- 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.


B- 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.




2-When filling the tray, add alginate at one location, and force the

alginate to flow into other portions of the tray.




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.


C- 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).





2- Inadequate alginate may have been placed in the tray.





3- The lip may not have been pulled out gently to allow alginate to flow

into the vestibule.





4- The impression must be seated gradually to give the alginate time to

flow.


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

Alginate Substitute Impression Materials

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 areas .

It yields accurate impressions.


Agar impression material is supplied as a gel in

1- a collapsible tube: is used with a water-cooled impression tray .

2- a number of cylinders in a glass jar: is used with a syringe.


The agar gel is converted to a sol by heating in water, usually boiling,

100°C, and becomes a gel again by cooling to 43.3°C.

Once the gel has been converted to a sol, it remains fluid for extended

periods (all day) by being stored at 65.7°C.


Agar impressions are highly accurate at the time of removal from the

mouth but shrink when stored in air or 100% relative humidity and

expand when stored in water as with alginate. The least dimensional

change occurs when the impressions are stored in 100% humidity;

however, prompt pouring of plaster or stone models is recommended.


polysulfides,

condensation silicones

polyethers,

addition silicones

addition silicone-polyether hybrid

* elastomeric impression materials dominate the market mainly as a

result of their greater accuracy, dimensional stability with time, and

ability to record detail as compared with the hydrocolloid materials.

Elastomeric Impression Materials

-vinyl polysiloxanes .

-Devloped as alternative to polysulfide and condensation silicones.

- supplied as low-, medium-, high-, or very high-viscosity (putty) material.

- The material is supplied as a two-paste or a two-putty system.

Addition Silicone Impression Materials

1- A low-molecular-weight silicone with terminal vinyl groups

2- reinforcing filler

3- chloroplatinic acid catalyst

1- low-molecular-weight silicone with silane hydrogens

2- reinforcing filler

First Second

Because no volatile byproduct (such as water or ethanol) is formed in this reaction, minimal dimensional change occurs during polymerization.

Increases in temperature lengthen the rate of reaction and shorten the setting time.


If hydroxyl groups are present in the addition silicone, a side reaction occurs that results in the formation of hydrogen.

The hydrogen is released gradually from the set impression material and produces bubbles in gypsum dies prepared less than 1 hour, or epoxy dies less than 24 hours, after the impression is taken.


Hydrophilic silicones wet the oral tissues better than earlier hydrophobic

materials and permit gypsum models and dies to be prepared with fewer

air bubbles.


Hand Mixing

Automixing or dual- cartridge systems


Automixing or dual- cartridge systems:

During extrusion through the static mixing tip, the two pastes are

folded over each other and exit the tip in a mixed condition.

The mixing tip is left on until the next mix, at which time it is

replaced by a new tip


Properties:

Less dimensional change

higher elastic recovery


Properties:

The dimensional change in 24 hours of about -0.1% is very low.

The elastic recovery at the time of removal from the mouth of about

99.8% (permanent deformation of 0.2%) is the highest of all the

impression materials.

The percent flow values of the addition silicones are likewise low.

These properties indicate the superior accuracy of the addition

silicones.


- The working time is short for the addition silicones.

- Setting time: 2-10 (according to the manufacturer).

- Flexibility is low to moderate.

- Removal of addition silicone impressions from undercut areas may

present difficulties because of this stiffness, and extra space should be

provided for the impression material when a custom tray is used.


Condensation silicone impression materials are not used commonly in

the dental office but rather as an accurate duplicating material in the

dental laboratory. This impression material is supplied as a base and an

accelerator

Condensation Silicone Impression Materials

The dimensional change during 24 hours after setting is relatively high.

About 50% of the dimensional change occurs during the first hour after

setting, and the remaining 50% occurs between 1 and 24 hours.

Condensation Silicone Impression Materials

Three types are available and are classified as low, medium, and high

viscosity

The permanent deformation values of 2% to 3% (elastic recovery of 97%

to 98%) indicate that compression during removal of the impression

material should be kept to a minimum.

shrinkage 0.3-0.4 during the first 24 hours

Polysulfide Impression Materials

The Impression immersed in appropriately diluted solution for 10 M.

Except for chlorine dioxide (3M)

Disinfection of Elastomeric impression

Principle application for a check impression to determine whether the

cavity preparation contains UNDERCUTS that would cause problem in

cast gold alloy restoration.

Rigid Impression Materials

Impression compound

- Heated over aflame or hot water until softened.

- Pressed.

- Held firmly.

- Removed.

- Fracture or deformation indicates

undesirable undercut areas in the cavity.

Rigid Impression Materials

Impression compound

Elastomeric impression materials eliminated the use of Zeo for

edentulous impressions.

Two pastes

Set time:3-5

Material is not strong

Zinc Oxide-Eugenol

Bite Registration : In the past: by wax.

Factors limited using wax:

1- Distortion on removal

2- release of internal stress on storage.

3- High flow properties.

4- large dimensional change.

Elastomeric Material for Bite Registeration

- Elastomeric impression materials ideally should :

Be fast setting.

Be mousselike in the tray.

No taste or odor.

be Rigid on set.

Be easy to trim

Elastomeric Material for Bite Registration

THanks