ALGINATE IMPRESSION

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Alginate Impressions

&

Diagnostic Casts


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Background Information

A recurring requirement in the dental office

is for study models or diagnostic casts of

patients teeth and adjacent tissues. Variousrequirements necessitate these models, the

most common being for diagnostic

purposes. Models are also required infabricating crowns and dies for

prosthodontic appliances.


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A diagnostic cast is an accurate replication

of the anatomic form of the maxillary or

mandibular dental arch showing therelationships of the remaining teeth and the

surrounding tissues.

Diagnostic casts are a supplement to theoral examination, but in some ways they

can reveal more.


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Diagnostic casts permit inspection from

perspectives that are impossible to obtain

when looking in the patients mouth.

Diagnostic casts permit extended

observations and comparasions far beyond

the patients endurance for holding his/hermouth open and are available for study

during the patients absence.


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Diagnostic casts are used in educating the

patient about his/her dental needs and

corresponding treatment plan.


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Discussion

An alginate impression is a ____________

mold of the teeth.

A. Positive

B. Negative


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Three steps of obtaining

diagnostic casts An alginate impression is made directly in

the patients mouth producing a negative

mold. The impression is poured in dental stone,

producing a positive cast.

The cast is inspected for defects, repaired ifpossible, and trimmed to the proper size andshape on a model trimmer.


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Physical Properties of Alginate

Alginate is an impression material that is

supplied in powder form and mixed with

water.

When set, the material is a flexible gel

resembling rubber.

Its most important characteristic is itsability to rebound from stresses.


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Composition of Alginate

Alginate is classified as irreversible

hydrocolloid.

Hydrocolloid because it consists of particles

of a gelatinous (colloidal) state in water

(hydro) and irreversible because once it has

jelled it cannot be returned to a liquidsolution.


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Packaging of Alginate

Bulk form, the most popular

Preweighted package, the more desirablechoice when temperature change andmoisture are of concern.

All types of alginate packaging must bestored in a cool, dry place.


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Alginate is sensitive to higher temperature

and moisture contamination. Those

variables cause the alginate to lose strengthduring mixing and be more apt to distort

upon removal from the mouth.


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Mixing Alginate

Fluff or shake the powder to distribute thepowder particles evenly

Use the specific measuring devices (water& powder) provided by the manufacturerfor mixing

Follow the manufacturers directionregarding the ratio of water to powder.

Use exact measurements


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After the water and powder have been

measured, place the water in a clean dry

bowl. Sift the powder into the water.Adding the powder to the water ensures the

powder particles are wet evenly. If mixed in

reverse (the water is added to the powder)the chemical reaction will start early with

some particles setting faster than others.


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Mix the alginate for the specific amount of

time and using a stiff spatula swipe the

alginate mass against the sides of the bowlto avoid entrapment of air in the mix.


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Discussion

Care should be taken in shaking a dustless

can of alginate.

A. Correct

B. Incorrect


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Discussion

So increasing the temperature of the

water used to mix alginate material will:

A. Decrease the setting time

B. Increase the setting time


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When ideal conditions exists a fast-setting

type of alginate will gel in 1 to 2 minutes

and a normal type gels in 2 to 4.5 minutes.The fast-setting type is the most widely

used.


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Dimensional Stability

Alginate has a tendency, after it sets to lose

(syneresis)or absorb (imbibition) water,

depending on the atomosheric conditionssurrounding it.If conditions are dry it loses

water & shrinks; if immersed in water, it

imbibes moisture or swells.


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Advantages of Alginate

It makes an accurate impression

It allows for undercuts

The process is not time consuming

It is easy to work with, has good viscosity & is

low in adhesive qualities

It causes no great pain to the patient Inexpensive to the dentist

Requires little armamentarium


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Alginate Impression Procedure

Assemble the armamentarium

Patient drape

Impression trays Flexible rubber bowl

Stiff spatula

Saliva ejector

Alginate of choice

Utility wax

Disinfecting agent


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Seat & drape the patient

Operator should wear appropriate PPE

Instruct patient of procedure

Position the patient upright & inspect mouth

Lubricate the patients lips

Select the appropriate trays


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Install the wax

Mold the wax to the vestibule

Have patient rinse mouth

Measure water & test temperature

Fluff alginate in can & measure into cup

Prepare the alginate Load tray (always taking the mandibular arch

first)


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Semi-dry the teeth & mucosa

Position & seat the tray

Remove excess alginate

Hold the tray in place for the prescribed

time

Remove the tray by breaking the seal at the

periphery then giving a quick pull


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Label impression & store in humidor

Repeat procedure for opposite arch

Pour impressions immediately after


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Criteria for Selecting Appropriate

Trays Tray clears all tissues by at least 4 mm.

Tray is long enough to cover part of retromolar

pads or tuberosity Tray sides fall at least 4 mm short of periphal turn

Tray sides do not grossly depress any frenum

Patient should not feel pain or excessive pressure


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Evaluation Criteria

All detail is reproduced, including thecomplete peripheral turn and a portion of

the retromolar pads or tuberosities Detail is sharp, not blurred or indistinct

Free of voids in critical areas

Free of large folds of alginate No areas where alginate has pulled away

from the tray


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Free of rips and tears except in interproxinal

areas

Alginate thoroughly covers the tray, no trayvisible through alginate

Free of bulges or depressions that indicate a

sub-surface bubble

Alginate is smooth, not sponge-like

ALGINATE IMPRESSION

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