Download - Lab Fab PFM-11
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FABRICATION OF THE 3-UNIT METAL-CERMAIC FIXED PARTIAL DENTURE
Andrea D. Jackson, DDS, MS, FACPHoward University College of Dentistry
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GOAL:Expose the student to the laboratory fabrication of a posterior three-unit
metal – ceramic fixed partial denture. The student will also be exposed to the
soldering procedure for gold and ceramo - metal alloys and the uses of solder.
OBJECTIVES:The student upon completion of the instruction session and suggested
reading assignments should be able to:
A. Understand the requirements for the wax pattern of a three-unit metal – ceramic fixed partial
denture.
B. Understand the concept of cut-back for porcelain application in the wax pattern
C. Understand the various pontic designs.
D. Understand the indirect spruing procedure and the reason for it.
E. Understand and select the proper investment for metal – ceramic alloys and
the investing procedures
F. Understand the burnout, casting, and divesting procedure and how it differs from gold alloys.
H. Understand the finishing procedure and the preparation of the surface for porcelain application.
I. Understand the reasons for uses of solder
J. Understand the process involved in soldering
K. Understand the difference between pre and post solder
L. Understand the classification and requirements of a solder
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Clinical Considerations
Clinical execution of tooth preparation
Finish-line design
Proper axial reduction
Proper occlusal/proximal reduction
Reduction appropriate for selected restorative materials
Clinical procedures necessary for proper laboratory fabrication
Impression making
Placement of retraction cord
Capturing details of preparation
Inter-occlusal registration
Articulation of cast
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TERMS/DEFINITIONS
Framework – Fixed partial denture or metal occlusal single unit
Coping – single tooth thimble
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Sequence of Procedures and Clinical Considerations:
Tooth Preparation/Design
Combination
Shoulder
Chamfer
Location
Proper Taper
Adequate tooth reduction
parallelism
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Metal-ceramic restoration design
Metal-Ceramic Restoration
Ceramic layer
Metal understructure
Framework
Copings
Esthetics
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LABORATORY PROCEDURES
Waxing
Wax add-on technique
Full-cast crown
PFM crown
Pontic
Full anatomic wax-up
Specifications for FPD
Tooth #19 – full cast metal retainer
Tooth #20 – PFM pontic
Tooth #21 - PFM
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LABORATORY PROCEDURES
Wax cutback of pontic and retainer unit
Well-rounded
No undercuts
No concavities
No sharp corners
No angles
Finished Wax Patterns
No inner surface defects
Smooth, polished outer surfaces
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COPING/FRAMEWORK DESIGN
There are four features of importance to be considered when designing the metal coping for a metal-ceramic restoration:
1.Thickness of metal underlying and adjoining the
porcelain
2.Placement of occlusal and proximal contacts
3.Extensions of the area to be veneered for porcelain
4.Design of the facial margin
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LABORATORY PROCEDURES
Coping design
Thin porcelain of uniform thickness
Rigid metal
Convex surfaces
Noble vs. base metals
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Dental porcelains (which are, chemically
speaking, glasses) resist compressive
loading but tend to succumb to tensile
stress.
Therefore, the metal substructure must
be designed so that any tensile stresses
in the porcelain are minimized.
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Ideally, an esthetic restoration should wear at approximately the same
rate as the enamel it replaces (about 10 µm per year5.) In addition, the
restoration should not increase the wear rate of an opposing enamel
surface.
Dental porcelain is more abrasive of enamel than of other restorative
materials (e.g., gold or amalgam, and has been implicated in severe
occlusal wear, particularly when the porcelain is not glazed or highly
polished.
This factor should be considered whenever a metal-ceramic restoration
is being designed.
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Restorations with porcelain occlusal surfaces must be planned carefully. Although they are esthetically very acceptable, these restorations have disadvantages, especially wear of the
opposing enamel.
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Types of Facial Margins
1. Metal margins
1. – unesthetic
2. Porcelain margins on metal collar
1. – over contoured
3. All porcelain margins
1. – most esthetic
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To avoid fracture, the thickness of a ceramic veneer must not exceed 2
mm; however, a To avoid fracture, the thickness of a ceramic veneer
must not exceed 2 mm; however, a minimum thickness of 1 mm is
needed for an esthetically pleasing restoration.
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Coping Design - Thickness of Metal Thin uniform thickness of porcelain supported by
rigid metal is strongest
Absolute minimum thickness of porcelain is .7mm
Desirable thickness for porcelain is 1mm
Minimum thickness with good esthetics
Always compensate with extra thickness of metal when the tooth preparation allows for more clearance
Maximum restoration strength and longevity is achieved by coping rigidity
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Coping design continued
Surface should be convex
Occlusal contact must be 1mm from porcelain-metal junction
Minimum thickness is .3 -.5 mm for noble metals
Higher yield strength base metals copings can be as thin as .2mm
The ultimate goal of 1mm of uniform porcelain thickness will determine thickness of coping
Proximal contacts on anterior teeth should be in porcelain for esthetic reasons
Porcelain opposing natural teeth can cause wear of the opposing teeth-must make patient aware that a restoration may be needed in the future
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Full contour wax- up and cut-back
The correct steps in the fabrication of a metal-
ceramic restoration: A, full-contour wax pattern;
B, coping wax pattern cut back; C, porcelain
addition to metal coping.
If the coping pattern (A) is the first step in
fabrication, the porcelain veneer on the final
restoration may have contours that are not
continuous with those of the unveneered coping
(B).
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Full contour > > > Cut - Back
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METAL – PORCELAIN DESIGN
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CUT BACK >>>COPING DESIGN
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Alloys
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Classification for Dental Casting Alloys
Noble metal content: Au, Pt, Pd
(minimum percentage by weight)
High-noble metal
60% (>40% gold)
Noble metal
25% (no gold requirement)
Predominantly base metal
<25% (no gold requirement)
Au, gold; Pd, palladium; Pt, platinum.
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Alloys Used for Fabricating Metal-Ceramic Restorations
High noble
Gold-platinum-palladium
Gold-palladium-silver
Gold-palladium
Noble
Palladium-silver
High palladium
Predominantly base
Nickel-chromium
Nickel-chromium-beryllium
Cobalt-chromium
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Dental Casting Alloys
Classifications High-Noble metal content
Noble-noble metal content of 60% or greater. At least 40% of the alloy must be gold.
Noble metals must be at least 25% noble metal.
Predominately Base a noble metal content of less than 25%.
Cost Precious Semi-precious nonprecious
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Titanium Alloys
Titanium based alloys have been studied since the late 1970’s as potential casting alloys.
Advantages of titanium:
Excellent bio-compatibility and corrosion resistance
Titanium oxide surface
Low density in comparison to gold or palladium resulting in lighter and possibly less expensive restorations
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Titanium Disadvantages
The dental casting of titanium and titanium
alloys poses special problems because of the
high melting point of titanium (1668° C)
Its strong tendency to oxidize and react with
other materials.
Special casting machines needed that provide either a
vacuum environment or an argon atmosphere
must be used.
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Dental Casting Alloys
Predominantly base alloys
Noble metal < 25%
Challenge to dentists and technicians
Physical properties
Handling characteristics
Fabrication techniques
Higher fusing elements
higher melting temperature 2300-2600 degrees Fahrenheit
Necessitates use of gas-oxygen torch and phosphate-bonded investment with high-heat burnout
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Titanium disadvantages
Selecting a dental laboratory experienced in fabricating these castings is essential, and such dental laboratories are not common in the United States. Further research is needed to optimize the metallurgical structure and casting technology for titanium alloys,
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Base Metal Alloys
The potential health problems associated with beryllium- and nickel-containing alloys have led to the development of another alternative base metal alloy system: cobalt-chromium.
The representative Co-Cr alloys have higher hardness than the Ni-Cr alloys suggests that finishing restorations made with the former alloys may be more difficult.
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Clinical Case
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS and die
CONSTRUCTION OF A WAX PATTERN
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS
CONSTRUCTION OF A WAX PATTERN
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LABORATORY PROCEDURES
Pontic Design Modified ridge lap pontic
Other pontic designs
Saddle
Hygienic
Ovate
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PONTIC DESIGN
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Size and shape of connectors
Posterior Anterior compared to posterior
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METAL – CERAMIC RELATIONSHIP
The melting range of the alloy used in the coping must be 170 to
280°C (300 to 500°F) higher than the fusing temperature of the
porcelain applied to it.
The metal coping is an important part of the metal-ceramic
restoration, and one that unfortunately is often overlooked. Its
design can have an important effect on the success or failure of the
restoration.
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LABORATORY PROCEDURES
Spruing Indirect (Runner Bar
Technique) Feeder sprues
Transverse bar
Manifold sprues
Direct Main sprues
6 mm from wax pattern to end of ring
6 mm from top of crucible former
Patterns for metal-ceramic fixed partial dentures must be sprued by an indirect method because the alloys used fuse and solidify at much higher temperatures.
Because the ambient air is much colder than the molten metal, the exposed button is likely to solidify while the metal at the center of the ring is still liquid.
This means that the button cannot serve as a reservoir to prevent shrink-spot porosity. Instead, a bulky horizontal runner bar is placed between crucible former and pattern.
runner bar – stabilizes pattern; serves as reservoir for solidification and contraction,
equalizes flow of metal through all parts of mold,
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Pattern for a metal-ceramic fixed
partial denture is sprued indirectly.
The feeder sprues and the horizontal
runner are 8 gauge, and the
manifold sprues are 10 gauge.
Molten alloy swirls through the manifold system,
raising the temperature of the surrounding invest-
ment (shaded area).
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LABORATORY PROCEDURES
Surrounding wax pattern with material that duplicates its shape and anatomic features
Requirements Reproduce precisely
Sufficient strength to withstand burn-out/casting
Expand to counter solidification shrinkage
Solidification shrinkage of metal: multi-metal shrinks as cooling occurs
Smaller casting as a result
Expansion of Mold (> original pattern) 4 mechanisms
1.Setting expansion (silica particles interfere with forming crystalline structure of gypsum-bonding)
2.Hygroscopic expansion (hydration process)
3.Wax pattern expansion
4.Thermal expansion of the investment
INVESTING
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LABORATORY PROCEDURES
Investing De-bubblizer
Reducing surface tension
Liner Cellulose ***
Unrestricted thermal expansion
Ceramic
Mixture 60 grams powder : 9.5 cc liquid
Mix under vacuum (vac-u-spat)
Bench set for one hour after investing
Scrape top of casting ring after setting Allow gases to escape during
burn-out
It may be necessary to paint phosphate-bonded
investment into the wax pattern with a small brush.
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LABORATORY PROCEDURES
Phosphate bonded investment
Higher fusing alloys
Stronger & withstands higher temperatures
Used for investing casting alloys with higher temperatures
Silver palladium
Gold platinum
Gypsum bonded investment
fuse <1975 degrees Fahrenheit
Gold alloys
Type I: small inlays
Type II: larger inlays and onlays
Type III: onlays, crowns, short-span FPD
Type IV: long-span FPD; thin veneer crown; RPD
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS
CONSTRUCTION OF A WAX PATTERN
INVESTING AND CASTING
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INVESTED WAX PATTERN
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LABORATORY PROCEDURESBURNOUT
Removal of wax pattern to create mold into which molten alloy is placed
Thermal expansion of the mold Investment allowed to harden, Scrape top to allow gasses to escape Place in oven at 600 degrees F for 30 minutes Then to 1300 degrees Fahrenheit for1 hour If longer, investment may start to break down Investment and ring expand to compensate for shrinkage
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LABORATORY PROCEDURES
Casting -Crucibles
-Gas-Oxygen systems
-Induction casting machine
-Electric melting machines
Gas-air torches for lower temperatures
As the alloy begins to solidify, the heat around the
manifold (dark shading) keeps it molten longer,
preventing porosity in the bridge.
)>
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LABORATORY PROCEDURES
Divesting
Bench cool for 1 hour
Break investment with pointed instrument
Aluminum oxide abrasive to clean casting
Ultrasonic cleaning
Finishing Remove sprue and button
from casting
Use stones, sandpaper, and burs to finish
Sprue immediately adjacent to the
casting is removed with a separating
disc.
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LABORATORY PROCEDURES
Polishing
Silicone wheels and cones
Abrasive paste or compound
Common shapes of abrasive stones are: cone (CN), flame (FL), cylinder (CY), barrel (BA), wheel (WH), inverted cone (IC), knife edge (KN), round (RD), round edge (RE).
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CASTING DEFECTS
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS
CONSTRUCTION OF A WAX PATTERN
INVESTING AND CASTING
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PREPARATION FOR PORCELAIN APPLICATION – preparing the metal
The veneering area is
prepared with aluminum
oxide stones.
The coping thickness is checked
with an Iwanson thickness gauge.
Do not use any polishing compounds,
as they may contaminate the surface
of the metal to be veneered later.
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PREPARING THE METAL
Must be cleaned and uncontaminated from oils from the skin or debris from finishing with steam
The final step in metal preparation is reduction
of the oxide layer on the part of the coping to be
veneered with porcelain by air abrading with
50 μm aluminum oxide.
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Metal framework finishing
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS
CONSTRUCTION OF A WAX PATTERN
INVESTING AND CASTING
PORCELAIN APPLICATION
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Heat Treatment – Oxidation cycle
Coping is placed in furnace and the temperature raised from 300 – 400 oC
Directions are alloy specific
Removal of hydrogen gases that was incorporated in the casting process
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Porcelain buildup - Opaque application
The veneering surface of the coping
is wetted with distilled water or special
liquid recommended by the manufacturer.
Two layers of opaque are baked
individually
Done to mask the metal, supply the base shade, and
creates the initial bond
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Application of Dentin porcelain/ Body porcelain
Made with a brush
Tissue is used to absorb water
Over build contour to compensate for shrinkage
A sable brush is used for condensing and final shaping
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Cut back and apply Incisal/ enamel porcelain Create translucency and esthetics
Done with a spatula on the inciisal edges and proximal contact areas cutback
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Cut back and apply Incisal/ enamel porcelain
Incisal porcelain ready for
firing
Adding small amount to proximal
Contact before firing
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Firing / Baking porcelain
Requires more than one bake to make adjustments in the contours
A clean green stone is used for contouring
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Finishing Porcelain
Shade modification done prior to final finish if necessary
Auto glaze – glazes itself during firing process
Must avoid over firing
Applied glaze - a low fusing clear porcelain is applied and baked to the surface
Polishing – done on small areas that may have been adjusted in the mouth
May be less destructive to opposing tooth structure
*Finishing the metal is the same as any other metal
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LABORATORY INSTRUMENTS FOR FABRICATION OF CAST RESTORATIONS
CONSTRUCTION OF WORKING CASTS
CONSTRUCTION OF A WAX PATTERN
INVESTING AND CASTING
PORCELAIN APPLICATION
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Bisque bake try-in
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Cementation of definitive restoration
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JOINING METALS/ SOLDERING
The addition of filler
Uses: Soldering/joining units of
conventional gold alloy (FPD’s, splints)
Adding proximal contacts
Repairing casting voids
Soldering porcelain fused to metal alloys
Soldering
Classification
Fineness (gold content)
Characteristics
Corrosion resistant
Lower fusing
Non-pitting
Strength
Free-flowing
Welding (melting)
.
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Pre and Post Soldering for metal ceramic restorations
Pre-Soldering Done prior to placing the
porcelain
Post – soldering Done after the porcelain is
placed
Wax patterns for metal-ceramic fixed partial dentures are
invested and cast as one unit whenever possible because of the
difficulty encountered in soldering the alloys used for this type of
restoration
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Gold Solder Fineness
Refers to parts per thousand of the solder that is gold
Ex:
600 fine solder is 600 parts of gold per 1,000 or
60% gold. (minimum that should be used is 580 fine)
The higher the fineness, the higher the melting range and the more corrosion resistance
A casting alloy is designated by carat (refers to parts per 24
ex: 18K = 75% gold
A solder designated 18K means it should be used with 18K casting alloy
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Soldering
Solder should possess a fusion temperature that is about 60 degrees C (100 – 150 F) below that of the metal being soldered
Four- and five-unit fixed partial dentures joined by soldering have better-fitting margins than do one-piece castings of the same length.
Any fixed partial denture larger than three units should still be cast in two pieces and soldered.
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The SOLDERING procedureAn index is made in the mouth, the restoration is invested and soldered.
1. Applying anti-flux 2. Heating the area to be joined
3. Adding solder 4. Completed solder joint
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