secondard impression materials

Secondary impression materials in fixed prosthodontics

Charles J. Goodacre, DDS, MSD Professor of Restorative Dentistry

Loma Linda University School of Dentistry

This program of instruction is protected by copyright ©. No portion of this program of instruction may be reproduced, recorded or transferred by any means electronic, digital, photographic, mechanical etc., or by any information storage or retrieval system, without prior permission.

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

How Impressions are Made •  It can be made using a physical material or

•  The information can be recorded digitally

Cerec Bluecam by Sirona

Physical Impression

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

Indirect Digital Scanning Wax Pattern Made on Cast

Scan of Cast & Scan of Wax Pattern

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

Fabrication Sequence

Scan of cast Scan of wax pattern

Milled green-state crown

Fired zirconia crown Surface colors applied

3Shape scanner was used to scan the cast

Katana CAD/CAM system was used to design and mill the zirconia copings

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.


•  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


•  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.

Impression Materials •  Aqueous elastomers

Irreversible hydrocolloid (alginate) Reversible hydrocolloid

•  Nonaqueous elastomers

Polysulfide Condensation reaction silicone Additional reaction silicone Polyether Alginate substitutes

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

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

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.

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.

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

Condensation Silicone

•  Mainly used today for duplication procedures and templates for fabrication of provisional crowns

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

Polysulfide •  It used to be the most commonly used

material (1970’s & 1980’s) but is not commonly used today in fixed prosthodontics.

Polysulfide •  It It is still used in complete denture

prosthodontics today by some clinicians

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

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

Sample impressions

Custom Trays & Their Fabrication

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

Types of Impression Trays

Stock trays 1. Plastic 2. Metal

Custom trays 1. Autopolymerizing resin 2. Light-polymerized resin

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

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

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.

Custom Tray Fabrication Technique Autopolymerizing Resin

•  Diagnostic cast with extension marked

•  Diagnostic cast with extension marked •  Covered with two layers of denture

baseplate wax or beeswax


•  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




who wants stops •  Cover the wax with aluminum foil




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


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

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

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

Blocking out spaces between teeth

Ultradent LC Block-Out Resin

Wax can be used but there is a better solution

The Continuum of Treatment: Examples Of Good Tooth

Preparation Followed By A Good Impression

Two Metal Ceramic Crowns

Ten All-Ceramic Crowns

Diagnostic Wax Patterns

Vacuum-Formed Template

Flowable composite

resin

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

Esthetic evaluation of resin veneers

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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secondard impression materials

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