Impression materials such as alginate and hydrocolloids are used to make negative reproductions of teeth and oral structures to create study models, dies, and prosthetics. Alginate is the most commonly used impression material due to its low cost, ease of use, and accuracy for preliminary impressions and diagnostic models, though it lacks the fine detail needed for final restorations. The selection of impression material depends on the intended use, with alginate and hydrocolloids being inexpensive preliminary options and elastomers like polyvinyl siloxane and polyether used for final impressions.

1. Impression Materials
KEY TERMS
IMPRESSION TRAYS
HYDROCOLLOIDS
CRITERIA FOR CLINICALLY ACCEPTABLE IMPRESSIONS
ALGINATE IMPRESSIONS
DISINFECTION

2. Overview
 Used primarily to reproduce the form of the teeth in a negative reproduction
 This includes existing restorations, hard and soft tissues, and preparations
 Also used to fabricate prostheses and other oral devices
 Making impressions of oral structures is an almost everyday occurrence in a busy dental
practice.
 Selection of the impression material will be influenced by what the impression will be used
for.
 Typically, a material must be in a moldable or plastic state that can adapt to the teeth and
tissues.

4. Types of models made from impressions
 Diagnostic casts
 Study models
 Die: when an impression is made of a tooth that has already been
prepared for a restoration
 Used for the fabrication of the restoration in the dental laboratory
 Allows the dentist or lab tech to perform procedure by the indirect technique.

5. Types of Impressions
Dental impressions can be categorized into three
basic types based on how they will be used:
1. Preliminary impressions
2. Final impressions
3. Bite registration (occlusal) impressions
5

6. Preliminary Impressions
 Preliminary impressions are made as precursor to other treatment.
 Also called “pre-impression”
 Often used for:
 Planning purposes
 Diagnostic casts
 Temporary crowns
 Custom trays
 Working models

7. Final Impressions
 Final impressions are more accurate in their replication of
the oral structures.
 They are used to make final restorations, such as crowns,
bridges, partials, and complete dentures.
We will talk more about final impression materials in a later
lecture.

8. Bite Registration
 Bite registration is a replication of the patient’s bite and is used to
establish the proper relation between a restoration or prosthesis and
the opposing teeth.
 An impression is made that captures the relationship, so it can be
used in the office or sent to the lab where the restoration is being
fabricated.
 Used to help mount diagnostic casts in their proper relationship on
an articulator
 Used with a facebow

9. Articulator and Facebow

10. Types of Impression Materials
 Two major groups
 Elastic materials
 Hydrocolloids, polysulfides, silicone rubber materials (condensation and
addition; polyvinyl siloxane), polyethers, and hybrid of polyether and PVS
 PVS, Alginate, Polyethers are most commonly used
 Inelastic materials
 Older impression material
 Will talk about more later

11. Key Properties
 Accuracy
When the impression is made the impression material must
closely adapt to and flow over the surface of the tooth prep
and tissues to record the minute detlails in order to be
accurate.
The material will tend to flow if it has low viscosity and there
is pressure on the material as the tray is seated

12. Key Properties
 Tear Resistance
 After the impression material sets, it must have good tear resistance to
prevent tearing during removal from the mouth
 With a crown impression the material in the gingival sulcus is very thin, and
would tear if the tear resistance was poor

13. Key Properties
Dimensional Stability
After the impression is removed, the set material
must remain dimensionally stable, otherwise casts
and dies poured from it would be inaccurate

14. Impression Trays
 Trays carry the impression material to the mouth and to support
it until it sets, is removed from the mouth, and is poured into
dental plaster or stone
 They should be rigid to prevent distortion of the impression
 They can be made for arches with teeth or for edentulous ridges
 Edentulous: having no teeth

15. Stock trays
 Premanufactured trays
 Purchased in a variety of sizes
 Small, medium, large, extra large
 For adults and children
 Can be metal or plastic
 Plastic trays are inexpensive and disposable
 Metal trays are more expensive and must be cleaned and sterilized between uses
 Solid or perforated
 Perforated have holes in their sides and bottom to help retain impression material as it extrudes through the holes and
locks it into place
 Solid trays often have raised borders on the internal surface that help lock in the impression material
 Rim lock
 Require the use of adhesive to retain the impression and prevent distortion

16. Stock trays
 Because of the wide variation in size and shape of patient’s
arches, stock trays may not fit some patients well
 Ideally, the tray should conform to:
 length
size and height of the arch
Depth of the palatal vault
Position of teeth

17. Stock Trays

18. Custom Trays
 Custom made
 Usually constructed in the lab with chemical-cured, light-cured or
thermoplastic resins on casts of teeth
 Custom fit to the mouth of an individual
 They can also be made by lining a stock tray with a putty material that is
adapted to the dental arch of the individual
 Can be made for full arch or sectional impressions for part of an arch

19. Custom Trays

20. Triple Trays
 Aka: Double-bite, dual-arch, check-bite trays
 Used to make impression of teeth being treated and the
opposing teeth at the same time
 Can capture the correct centric occlusion (bite) of the
patient
 Used to take the (1) impression of the preparation, (2)
opposing arch, (3) relationship of the arches (hence TRIPLE
TRAY)

21. Bite Registration Trays
 TRIPLE TRAY
 Typically U-shaped
 Plastic Frames with thin fiber mesh stretched between the sides and is thin enough not to
interfere with the patient’s bite
 Mesh retains bite registration material
 Material is placed on both sides of the mesh
 Frame is positioned over the teeth to be recoded
 Patient closes into normal bite relationship until the material sets

22. Triple Trays/Bite Registration Trays

23. Hydrocolloids
 Colloid: glue-like material composed of two or more substances in which one
substance does not go into solution but is suspended within another substance
 Hydrocolloid: water-based colloids that function as elastic impression materials
 Two hydrocolloids used in dentistry
 Alginate
 Irreversible hydrocolloid
 Agar
 Reversible hydrocolloid

24. Agar Hydrocolloid
 Like gelatin
 When agar powder is mixed with water, it forms a glue-like suspension that
entraps the water, making a colloidal suspension called sol
 Heating will disperse the agar in the water faster
 When the agar sol is chilled, it will gel
 Becoming semisolid or jelly-like (Jell-O)
 When agar is heated it will reverse its state back into a liquid suspension (sol)
 AKA reversible

25. Alginate
 Irreversible hydrocolloid
 Alginate powder also forms a gel that sols, but chemical reaction
occurs that prevents it from reversing back to gel when heated
 By far the most widely used impression material
 Inexpensive
 Easy to manipulate
 Requires no special equipment
 Reasonably accurate for many dental procedures

26. Alginate- Irreversible Hydrocolloid
 Used for:
 Making impressions for diagnostic casts
 Partial denture frameworks
 Repairs of broken partial or complete denture
 Fabrication of provisional restorations
 Fluoride and bleaching trays
 Sports protectors
 Preliminary impressions for edentulous arches
 Removable ortho appliances
 And MANY MORE

27. Alginate- Irreversible Hydrocolloid
 Not accurate enough for final impressions for:
 Inlay
 Onlay
 Crown
 Bridge
 Does not capture the fine detail of the preparation needed for a precise fit of such restorations
 It is thick and does not flow well into the embrasures or occlusal surfaces
 Final impressions are made with one of the elastomers
 PVS or Polyether

28. More uses for Alginate Impressions
 Diagnostic casts (study models)
 Preliminary impressions for
complete dentures
 Partial denture frameworks
 Opposing casts for crown and
bridge tx
 Repairs of partial and complete
dentures
 Provisional restorations
 Custom trays for home-use
fluoride or bleaching
 Sports protectors and night
guards
 Removable orthodontic
appliances

29. Composition and Setting Reaction
 Main active ingredient in alginate is potassium or sodium
alginate
 Makes up 15%- 20% of the powder
 Produced by derivatives of seaweed
 Alginate dust contains silica particles
 Potentially a health hazard if inhaled
 What do we do to avoid this??

30. Composition and Setting Reaction
 When alginate powder is mixed with water, calcium sulfate dehydrate reacts
with sodium alginate to form calcium alginate
 Calcium alginate is insoluble and causes the sol of the mixed powder and water to gel
 Caused by chemical reaction, so it cannot be reversed
 Rapid chemical rxn so trisodium phosphate is added as a retarder to delay the
reaction
 The amount of retarder that is added will control the time of the set and will
differentiate between regular-set, fast-set, extra fast set

31. Working Times
 Regular-set alginates have a working time of 2 to 3 minutes.
 Fast-set alginates have a working time of 1.25 to 2 minutes.
Working time starts when the materials—powder and liquid—are
joined.
The longer one takes to mix the material, the less time one has to
load and seat the tray.

32. Setting Times
 Regular-set alginates take from 2 to 5 minutes to set completely.
 Fast-set alginates take from 1 to 2 minutes to set completely.
 Setting time may be adjusted by altering water temperature.
 The impression is left in the mouth beyond the appearance of set
to reduce tearing and distortion.

33. Permanent Deformation
 Alginate will be compressed when it is removed from undercuts in the mouth.
 The greater the compression, the more likely it is that the alginate will be permanently deformed
to some degree.
 The ideal thickness of the impression is 2 to 4 mm.
 Thin alginate deforms and tears easily.
 When removing the impression from the mouth, one should use a rapid snap movement to
decrease deformation.
 The impression must be kept wet until the assistant is ready to pour up the impression in dental
plaster (gypsum).
 Impressions also must be disinfected.

34. Dimensional Stability
 The impression must be kept wet.
 Once removed from the mouth, the impression should be rinsed and
disinfected.
 Wrap in damp (not dripping wet) paper towel and sealed in Ziploc
bag
 If the impression loses too much moisture, it will shrink and become
distorted
 Some moisture will still be lost, even in 100% humidity from syneresis
 Contract and some part of the liquid is squeezed out

35. Tear Strength
Alginate mixed with too much water will be weaker and
more likely to tear.
Thin sections are also prone to tear.
Slow removal from the mouth may cause the material
to tear.
Removed from mouth in rapid “snap” to prevent tear

36. Making Alginate Impressions
 The objective of impression making is to reproduce the oral structures with acceptable accuracy
while practicing good infection control and maintaining patient comfort.
 The dental assistant and the dental hygienist can make alginate impressions.
 They will need to prepare the patient for the impressions and to dispense, mix, and load the
alginate into trays.
 After removal of the impression, the assistant or the hygienist disinfects and properly handles the
impression until it is poured with the appropriate gypsum material.
 The assistant or hygienist also may be responsible for clearing residual alginate from the mouth and
face of the patient.

37. Selecting the tray
 The borders of the tray should be approximately ¼ to ½ inch away
from the soft tissues and the teeth.
 The posterior of the lower tray should extend to include the retromolar
pad
 The posterior of the upper tray should extend to include the tuberosity
and the hamular notch
 Beading wax or utility wax may be placed on the borders of the impression tray
for further extension and adaptation to the patients mouth

38. Background information
 Retromolar pad-triangular area
located behind the last molar
 Hamular notch- slender projection
of bone that lies behind/posterior
and medial to the maxillary
tuberosity
 Tuberosity- the bulging lower
extremity of the posterior surface of
the body of the maxilla, behind the
root of the last molar tooth.

39. Selecting the Tray
 The trays must be clean and sterile before placing them in the patient’s
mouth
 Tray adhesives are applied to Teflon and solid trays after they have been initially
fitted in the patient’s mouth
 When the proper trays have been selected, assemble the materials and
equipment to mix the impression material (alginate)
 Measure and mix the impression material according to manufacturers
instructions.

43. Criteria for Clinically Acceptable Alginate
Impressions
 All teeth and alveolar processes are recorded
 Peripheral roll and frenums included
 No large voids and few small bubbles present
 Good preproduction of detail
 Free of debris
 No distortion
 Alginate firmly attached to tray

44. Criteria for Clinically Acceptable Alginate
Impressions
 Maxillary Impressions:
Palatal vault recorded
Hamular notch area included
 Mandibular Impressions
Retromolar areas included
Lingual extensions recorded

45. Loading the tray (MANDIBULAR)
 Gather the impression material together
 Pick up one half of the material with spatula and place it into
the left side of the tray from the lingual
 Pick up the other half of the material with spatula and place
it into the right side of the tray being careful not to trap air
 Overfill the tray slightly and smooth material with spatula
 Remove excess material from the outside of the tray
 The tray is now ready to be inserted into the patient’s mouth

46. Loading the Tray (MAXILLARY)
 Gather the impression materials together
 Pick up material and place on the posterior palatal area of
the tray
 Press the material and place on the posterior palatal area of
the tray
 Overfill the tray slightly and smooth material with spatula
 Remove excess material from the outside of the tray
 The tray is now ready to be inserted into the patient’s mouth

47. Troubleshooting Alginate Impressions
 REVIEW TABLE 15-3 ON PAGE 251

48. A few videos…
 https://www.youtube.com/watch?feature=player_detailpage&v=QiJ5WeE96bg
 https://www.youtube.com/watch?feature=player_detailpage&v=e6sxxNuuBKk
 https://www.youtube.com/watch?feature=player_detailpage&v=ZGFCi2tcfEA