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