This document discusses secondary impression materials used in fixed prosthodontics. It defines an impression as a negative reproduction of prepared teeth that provides information to fabricate a crown or fixed prosthesis. Impressions can be physical materials or digital scans. Physical impressions include reversible hydrocolloid, condensation silicone, polysulfide, polyether, and addition silicone. Custom trays are often used and are fabricated from autopolymerizing or light-cured resin. Ideal impressions accurately record all prepared surfaces and maintain dimensional stability until the laboratory casts are made.

1. Secondary impression materials in
fixed prosthodontics
Charles J. Goodacre, DDS, MSD
Professor of Restorative Dentistry
Loma Linda University School of Dentistry
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2. Definition of an Impression
• It is a negative reproduction of the prepared teeth or
implant(s), adjacent teeth, and surrounding soft
tissues that provides the information required for
fabrication of a crown or fixed prosthesis.
• It is used to produce a positive replica of the oral
structures recorded in the impression.
• It is used as a permanent record of the mouth and
for the fabrication of a restoration or prosthesis

3. How Impressions are Made
• It can be made using a physical material or
• The information can be recorded digitally
Cerec Bluecam by Sirona

4. Physical Impression

5. Direct Digital Impression
• Digital scan using a camera
• CAD design (Computer Aided Design)

7. Indirect Digital Scanning
Wax Pattern Made on Cast

8. Scan of Cast & Scan of Wax Pattern

9. CAD/CAM Milled Zirconia Crown
It is milled in an
oversized form
due to the
shrinkage that will
occur upon firing.

10. Fabrication Sequence
Scan of cast Scan of wax pattern
Milled green-state crown
Fired zirconia crownSurface colors applied

13. 3Shape scanner was used to scan the cast
Katana CAD/CAM system was used to
design and mill the zirconia copings

16. Ideal Requirements of
a Good Impression
– Record all the prepared tooth surfaces and some
of the unprepared tooth cervical to the finish line.
– With implants, record their position in the bone
and relationship to teeth.

17. Ideal Requirements
of a Good Impression
• Record unprepared teeth so their shape can be
used to establish the morphology of the crown /
prosthesis and their lingual & occlusal surfaces
can be used to establish the proper occlusion

18. Ideal Requirements of
a Good Impression
• Be made using a material that accurately
records detail and maintains its
dimensional stability for sufficient time to
permit arrival at the dental laboratory so
accurate fabrication procedures can be
completed.

19. Impression Materials
• Aqueous elastomers
Irreversible hydrocolloid (alginate)
Reversible hydrocolloid
• Nonaqueous elastomers
Polysulfide
Condensation reaction silicone
Additional reaction silicone
Polyether
Alginate substitutes

20. Alginate (Irreversible) Hydrocolloid
• Mixed with water to form a Sol and it hardens
chemically to form a Gel.
• Undergoes distortion by syneresis (gives off
water) and imbibition (imbibes or takes on water)
• Detail reproduction is not excellent
• Not well suited for fixed prosthodontics

21. Reversible Hydrocolloid
• Agar = 8-15%
• Water = >80%
• Borax to strengthen the gel
• Potassium sulfate (an accelerator of the
setting of gypsum) is added to counteract
borax which is a retarder of gypsum setting
• Sol-Gel transformation (Sol = softened form
and Gel = hardened form)
• Hardening produces “brush-heap” intertwining
of fibrils

22. Reversible Hydrocolloid
• Hysteresis = Ability to be heated the gelled form
to soften it and then cool it to create the gel form
again.
• Gel is boiled to soften, then held in ready to use
state at 150 degrees F, then put in 110 degree
bath to temper for placement in syringe, tray,
and use in the mouth.
• Uses special metal tray through which water
circulates to cool & create gel
• Undergoes syneresis and imbibition like
alginate.

23. Characteristics of Reversible
Hydrocolloid
• Good detail reproduction and dimensional
accuracy but must be poured before the
syneresis or imbibition occurs with water
changes that create distortion.
• Today, it is not widely used in fixed
prosthodontics.

26. Condensation Silicone
• Catalyst is Tin Octoate
• Reaction by-product is Ethyl Alcohol
• Dimensional stability - Rapid
evaporation of alcohol means you must
pour impression immediately to
maintain optimal accuracy

27. Condensation Silicone
• Mainly used today for duplication
procedures and templates for fabrication
of provisional crowns

28. Polysulfide (Rubber base,
Thiokol, Mercaptan)
• Polysulfide base contains mercaptan groups
(-SH) that produce sulfur smell
• Catalyst is Lead Dioxide
• Adding drop of water accelerates reaction
• Reaction by-product is water
• Dimensional stability - Evaporation of water
produces need to pour right away but not as
critical as condensation silicone

29. Polysulfide
• It used to be the most commonly used
material (1970’s & 1980’s) but is not
commonly used today in fixed prosthodontics.

30. Polysulfide
• It It is still used in complete denture
prosthodontics today by some clinicians

31. Polyether
• No by-products of reaction but is hydrophilic and
therefore can give up or take on moisture
• Dimensional stability – Excellent unless
substantial humidity change occurs
• Very stiff originally

32. Addition Silicone
Poly (vinyl siloxane); Vinyl polysiloxane
• Catalyst is chloroplatinic acid
• Hydrogen gas byproduct was initial problem
that caused voids on surface of gypsum.
This problem has been virtually eliminated by
use of palladium but still should wait a few
minutes before pouring
• Dimensionally accurate for a long time

33. Sample impressions

34. Custom Trays & Their
Fabrication

35. Requirements of a Tray
• Rigid
• Dimensionally stable
• Provide space for impression material
(2-3 mm minimum)
• Promote retention of impression
material
• Possess a handle that allows a
positive grasp of tray

36. Types of Impression Trays
Stock trays
1. Plastic
2. Metal
Custom trays
1. Autopolymerizing resin
2. Light-polymerized resin

37. Advantages of Custom Trays
• Made to fit the arch – needed for some
patients
• Carries and confines the material to
certain difficult areas – distal surfaces of
last molar
• Uses less impression material

38. Tray Extension, Form, & Thickness
• Should extend to cover required areas
without impinging on other areas such as
bony undercuts
• Extend 5 mm beyond gingival margin
• Do not have to cover palate
• Thickness of 3 mm

39. Tray Fabrication
• Fabricate autopolymerizing resin trays well in
advance of appointment so all polymerization
shrinkage has occurred – 24 hours.
• Some clinicians use stops to control the tray seating
while minimizing tooth contact with the tray because
contact affects the accuracy of the impression in the
area where the tray contacts the teeth.

40. Custom Tray Fabrication Technique
Autopolymerizing Resin
• Diagnostic cast with extension marked

41. • Diagnostic cast with extension marked
• Covered with two layers of denture
baseplate wax or beeswax
Custom Tray Fabrication Technique
Autopolymerizing Resin

42. • Diagnostic cast with extension marked
• Covered with two layers of denture baseplate
wax or beeswax
• Small notches can be cut into the wax for those
who wants stops
Custom Tray Fabrication Technique
Autopolymerizing Resin

43. • Diagnostic cast with extension marked
• Covered with two layers of denture baseplate
wax or beeswax
• Small notches can be cut into the wax for those
who wants stops
• Cover the wax with aluminum foil
Custom Tray Fabrication Technique
Autopolymerizing Resin

44. • Diagnostic cast with extension marked
• Covered with two layers of denture baseplate
wax or beeswax
• Small notches can be cut into the wax for those
who wants stops
• Cover the wax with aluminum foil
• Mix the autopolymerizing resin and mold when
it is no longer “stringy” and has a “doughy”
consistency
Custom Tray Fabrication Technique
Autopolymerizing Resin

47. Place a drop of monomer to
enhance bonding of the tray
handle

49. Custom Tray Fabrication Technique
Light-Polymerized Resin
• Same marking of tray extension on cast
• Sheet of resin material applied over cast
• Excess trimmed with a knife & handle
applied
• Air-barrier coating material is applied to
enhance the polymerization process
• 5 minutes of polymerization in light unit

50. Custom Tray Fabrication Technique
Light-Polymerized Resin
• Same marking of tray extension on cast
• Sheet of resin material applied over cast
• Excess trimmed with a knife & handle applied
• Air-barrier coating material is applied to enhance the polymerization process
• 5 minutes of polymerization in light unit

51. Custom Tray Fabrication Technique
Light-Polymerized Resin
• Same marking of tray extension on cast
• Sheet of resin material applied over cast
• Excess trimmed with a knife & handle applied
• Air-barrier coating material is applied to enhance the polymerization process
• 5 minutes of polymerization in light unit

52. Blocking out spaces between teeth
Ultradent LC Block-Out Resin
Wax can be used but
there is a better solution

55. The Continuum of Treatment:
Examples Of Good Tooth
Preparation Followed By A Good
Impression

56. Two Metal Ceramic Crowns

59. Ten All-Ceramic Crowns

60. Diagnostic Wax Patterns

61. Vacuum-Formed Template
Flowable
composite
resin

62. Resin application using template
– moist or lubricated tooth
surfaces with no etching

63. Esthetic evaluation of
resin veneers

70. Thank You For Your
Kind Attention
Charles J. Goodacre, DDS, MSD
Professor of Restorative Dentistry
Loma Linda University School of Dentistry

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