Alginate impression is done to receive study modals and diagnostic cast for crown or bridges.

1. How To Make Good Alginate
Impressions?
Dr. Ibadat Jamil
Assistant professor
Dep. Of Prothodontics

3. A recurring requirement in the dental office is of
study models or diagnostic casts , opposing arch
models for crown and bridge or even bleaching
trays. Alginate is one of the most frequently used
dental materials for the same and the reason for
its popularity being its cost effectiveness , good
reproducibility if handed well, and easy
availability. Taking an alginate is such a routine
simple job that some times one wrongly
overlooks mediocre impression; and the
resultant study models and dental prosthesis are
below desired standards.

4. Alginate impressions form a major bulk of
our clinical practice; therefore, it becomes
mandatory to follow certain fundamental
guidelines for flawless, predictable
impressions. The purpose of this article is to
provide clinical do’s and don’ts while using
alginate material for impressions.

5. Six keys to obtain accurate alginate
impressions and study models :-
1. Tray selection.
2. Choosing the right alginate and accessory
impression material.
3. Manipulation of the material.
4. Tray insertion and intraoral positioning.
5. Handling of impression prior to pouring of
study model.
6. Selection of gypsum product and
fabrication of study model.

6. 1) Tray Selection :-
 Material –
 Trays are available primarily in metal(image 1) and
plastic(image 2)

7.  Stainless steel rim lock perforates trays are the gold
standard as the extensive number of perforation and rim
lock design offers great anchorage and retention for the
impression material.
 Plastic trays that have slots instead of holes and work
well with heavy bodied alginate as the slots allow for
good extrusion of the viscous material. For patients with
communicable diseases, it is better to use plastic
disposable trays to avoid cross contamination.

8.  Metal and plastic trays without retentive features
(Image 3) must be painted with an adhesive to prevent
the impression material from separating.

9.  If the teeth are malpositioned or dental arch form is
irregular, it is advisable to use thermoplastic trays as
they are moldable in hot water around 70 degrees
Celsius and stay rigid after cool (Image 4)

10.  Size :-
 A proper tray size (Image 5 and 6) should be selected
covering all the teeth completely with a 5mm
clearance. The hamular notches, retromolar pads and
lingual sulcus should also be covered.
Image 5
Trail fit of impression tray
Image 6
A well fitting tray should
Have no tray-tooth contact

11.  The stock trays may require some customization in the
form of tray modification to ensure well extended
impression. Modification can be done with modeling
wax, tracing stick impression compound, acrylic resin
or heavy-bodied silicone, depending on the operators
convenience. Greenstick compound is preferable as
wax is nonrigid. Impression trays and there
modifications should be rigid (Image 7).
(Image 7)
Diagrammatic representation
Of tray extension using low
fusing compound.

12.  Modified impression tray must be placed in the mouth
and muscle trimmed prior to impression making.
Orthodontics pliers or metal trimming burs can be
used for flange modifications.

13. 2) Choosing the right Alginate and Accessory
impression materials:-
 A variety of alginate impression materials are available
in the market. Considerations in selecting the right
one are accuracy, dimensional stability, setting time,
taste, compatibility with the gypsum or stone.
 For edentulous patients , a high viscosity alginate
should be used.

14.  Tri- or Bi - colour changing alginates are available
indicating the correct time for spatulating, loading the
tray, and its placement, simplifying the impression
prcess for the dentist.
 Different flavours can be produced, depending on the
most liked one by the patients on an average

15. Certain accessory impression materials may be
required.
 If there are any existing fixed partial dentures, the pontix
required to be blocked out with modeling wax so that
the impression does not tear in this region.
 If the patient has a high peletal vault, low fusing
compound or putty can be used in the center of the
maxillary tray to reduce the bulk of alginate impression
material.

16. 3) Manipulation of the material :-
 Commonly used alginate materials are supplied in
containers. The material should be rolled or tumbled
within the container to fluff the powder and mix the
ingredients unless it is dustless alginate.

17. Mixing :–
 Alginate may be mixed by hand spatulation,
mechnical spatulation, under vacuum. A scoop is used
for measuring the powder; and a plastic measuring
cylinder for measuring the water propotion.(Image 8)
Showing scoop and cylinder to measure correct quantities of
alginate and water respectively.
(Image 8)

18.  Mixing is initiated by adding measured quantity of water
to a clean, flexible rubber bowl. This is followed by the
addition of correctly proportioned powder. Mixing
should begin slowly till all the powder is wetted, after
which the speed of spatulation should be rapid with a
wide- bladed, stiff spatula.
 The spatula should be used to press the alginate against
the sides of the bowl or in figure of eight motion. Hand
spatulation time is 45-60 seconds, while mechanical
spatulation time is about 30 seconds.

19.  The resultant mix should be creamy in consistency but
must not be driffed of the spatula when lifted from the
bowl (Image 9).
 Colder water can be used if longer working time is
desired. Setting time should be controlled by varying
water temperature, and not the consistency of mix.
Image 9
Smooth consistency
of mix.

20. Loading –
 The tray must be filled with the impression material
upto the tray borders and any excess unsupported
material (over;filled tray) at the periphery must be
removed with the mixing spatula. The surface of the
alginate should be smoothed with a wet gloved finger
and the tray should be inserted intraorally.
(Image 10)
Correctly loaded
mandibular tray

21. 4) Tray insertion and intraoral positioning –
 During tray insertion , the operator should stand in
front of the patients for lower impressions and behind
the patient for upper impressions. The lower
impressions should be made first as it is more
comfortable for the patient . prior to tray insertion,
occlusal surfaces of teeth and prepared teeth , if any ,
should be blown off gently with an air syringe to
remove debris and saliva . Having the patient rinse
with water and mouth-wash mixture will eliminate
mucin and lower the surface tension , thereby
eliminating air bubbles.

22.  The mixed alginate should be first rubbed onto the
occlusal surface of teeth with a gloved finger , allowing
accurate reproduction of the tooth anatomy(image 11).
(Image 11)
Rubbing some alginate onto occlusal surfaces
of teeth using gloved finger.

23.  Some alginate must be placed in the palatal vault ,
especially in deep palates . impression tray is positioned
in the mouth by retracting the patients lip on one side
with a mouth mirror / gloved finger ; and on the other
side , by rotating the tray into the mouth. The tray has to
be centered in position in the mouth ; and with light ,
even pressure , the impression tray is held in place till
the material set . the setting time is 2 -3 minute . Any
movement of the tray at this stage will result in an
inaccurate impression. The soft tissues, especially
labially , should be relieved and manipulated for the
alginate to flow into the sulci and record the details.

24. Removal of impression :–
 The seal between tissues and the impression should be
broken before removal of impressions, by gently
pushing with the gloved finger(image 12) or by using
air syringe into the buccal sulcus.
(Image 12)
Index finger used to break the seal.

25.  The tray should be supported with the other fingers or
hand during removal too . once set , the impression
has to be removed with a firm , quick snap . the
impression should not be rocked or twisted before or
during removal of the impression . this is to minimize
the time for which the set material is distorted as it
moves over the teeth

26. 5)Handling of impression prior to pouring of
study model :-
 Cleaning :-
 Upon removal of the impression from the mouth ,
impression is inspected for defects under good lighting
before it is rinsed to remove any saliva or blood . Most
patients have thin , serous saliva . this type of saliva can
be removed by holding the impression under gently
running , cool tap water . But thick , ropy saliva is
difficult to remove . A thin layer of dental stone powder
can be sprinkled onto the surface of the impression . The
stone adheres to saliva and acts as a disclosing agent.
When impression is placed under running tap water , the
saliva will be seen and can be removed by light brushing
with wet camels hair brush .

27.  The impression should be covered in a damp gauze/
napkin to prevent syneresis ( generally not
recommended to place in water , which would cause
imbibition – expansion ).

28.  Excess unsupported alginate should be removed with a
sharp knife(image 13) . if the tray is left on a firm table
surface with unsupported material , the impression
would distort as the weight of the impression acts
directly on the unsupported material . hence the tray
should be suspended by its handle in a tray
holder(image 14) , or a slightly open drawer with the
heels of the impression unsupported .
(Image 13)
Unsupported alginate cut
using sharp instrument.
(Image 14)
Tray holder

29. Disinfection :–
 Following the cleaning process each impression
should be sprayed with an appropriate disinfectant like
1% sodium hypochlorite or 2% glutaraldehyde ,
depending on the manufacturers instructions and kept
for 2-10 minutes depending on the type of disinfectant
used after which it can be covered with damp gauze
and left in a zip – lock plastic bag until the cast is
poured .
(Image 15)
Impression placed in a
sealed plastic bag to maintain
Dimensional accuracy.

30. 6) Selection of gypsum product and fabrication
-of study model :-
 There are a variety of gypsum products available for
fabricating models. If appliances like RPDs , clear
aligners , orthodontic retainers or final crown and
bridge restorations are being fabricated , die stone is
preferable because it provides excellent detail and
hardness . otherwise , dental stone is used .

31.  Pouring of the cast should be initiated within 12
minutes of impression removal , unless it is extended
pour alginate. It is done by the two stage pour
technique . in this technique , an initial mix of stone is
used to fill the impression , in increments , upto 6-8
mm height , against a vibrator machine(image 16,17) .
(Image 16)
Pouring of impression using stone.
Only small quantities are initially
applied in to the deepest areas
without bubble.
(Image 17)
Pouring stone using vibrator.
Larger quantities are then added
until the impression is filled
to the margin.

32.  After the initial set of 12 -15 minutes , a second mix of
dental stone is poured to form the base (image 18,19).
(Image 18) (Image 19)
Fabrication of the base. Two – stage : after the stone in the dental has set,
turn over the impression and press into the newly mixed base stone using slight
Vibration and remove any excess with a plaster knife.

33.  The impression should be separated between 45 – 60
minutes after the first pour .
 Alginate impressions often develop small voids on the
occlusal surfaces during impression taking . these air
bubbles in the alginate impression produces plaster
bubbles on the occlusal surface of the stone
model(image 20) .
(Image 20)
Plaster blobs

34.  These prevent accurate articulaton of the models .
these are to be removed prior to mounting and
prosthesis fabrication(image 22).
(Image 21)
The plaster bubbles prevent correct
Articulation of the upper and lower models.

35. Conclusion :-
 An impression is the first step of the treatment plans
of most of our cases. It is essential to begin correctly to
end well . following proper protocol in making
impressions ensures accuracy , predictability and
consistency in clinical situations and enhancement of
this skill is a necessity – unfortunately overlooked –
which would help us enjoy constant successful results .

36. THANKYOU