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7/29/2019 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