Metal Ceramic Restorations Flashcards
Porcelain Fused to Metal (PFM) crown consists of…
a metal casting or coping which fits over the prepared tooth and has ceramic fused to the metal
How thick should the metal coping be for PFM?
-0.3 – 0.5mm thick with Noble metal
-0.2mm thick with Base metal
-Where Porcelain is not overlayed:-0.8– 1.0mm thick
How thick should the opaquing porcelain be for PFM?
-0.2– 0.3mm thick normally
How thick should the body porcelain be for PFM?
- done in layers (Dentin, Enamel, Translucent)
- Modifiers-0.8 – 2.0mm thick
What is the purpose of the metal coping in a PFM?
-Designed with thickness to support the porcelain
-Design to support occlusal and proximal contacts
-Designed with extensions to support porcelain
-Margin design to adapt perfectly to margins and support the porcelain if porcelain margins are used
What should the design of the coping look like for a PFM?
- Convex, rounded angles
- Ledge of metal to support a 90 degree Metal-Ceramic finish
Assuming Metal coping is at least 0.3mm – 0.5mm thick, the metal coping should provide:
-an even porcelain thickness needed to provide optimum strength
-From 0.5 to 2mm of porcelain thickness
-Provide support in stress bearing areas like cusp tips, incisal edges, marginal ridges
If porcelain is more than ___ mm thick, a fracture of the porcelain becomes a higher risk
2
Ideal of ________ mm thick porcelain means that the metal design will vary and change
to allow for this uniform thickness
1–1.5
Metal Ceramic bridge designs need adequate thickness in both the pontic and connectors for rigidity to prevent porcelain ________ while also trying to optimize _________
fracture
esthetics
What is the first step to manufacture a PFM?
Starting with a wax up to create the ideal contour and esthetics, a metal coping is then designed based on the desired porcelain thickness.
The metal for a PFM is cast at ___________ degrees or milled
2300-2600
What are the steps to manufacture a PFM?
- start with a wax up
- metal coping is cast
- porcelain is stacked onto coping in layers
- stain and glaze
What is the purpose of opaque porcelain?
- Masks the metal
- Established the metal-ceramic bond
What is the purpose of body porcelain?
Colors include dentin, enamel, translucent, shoulder porcelains
What is the purpose of a stain/glaze for a PFM?
- Final color match
- Color modification once crown is complete
- Lower fusing temp than body porcelains
What are the three types of metal substructures for PFM?
High Noble, Noble, and Base metal
What are the desirable characteristics of metal ceramic alloys?
-Biocompatibility
-Accuracy of fit
-High Modulus of elasticity and high yield strength
— High resistance to deformation
-Easily soldered
High Noble metal content greater than or equal to ____% with at least ____% of content being Gold
60%
40%
What are the different type of high noble metal?
Gold-Platinum-Palladium
Gold-Palladium-Silver
Gold-Palladium
What are the characteristics of Gold-Platinum-Palladium?
- This formulation is up to as much as 88% Gold
- This much Gold makes this formulation softer and susceptible to sag
What are the characteristics of Gold-Palladium-Silver?
- The silver used to discolor some porcelains in a process called Greening
- That problem is mostly gone and overcome with new formulations
What are the characteristics of Gold-Palladium?
- Gold 44% - 55% – Palladium 35% - 45%
- Resistant to sag with excellent working properties
- This formulation is what we use in our PFM’s here at UMKC
What is sag?
a measure of the resistance to deformation at high temperatures
What are the characteristics of noble metals?
- Noble metals are a compromise between the high noble metals and the base metals in terms of cost and in density
- The workability and fabrication can be similar to high noble
- Because of less Gold, noble metals tend to have a higher strength and greater restistance to deformation
For a noble metal at least ____% noble metals but doesn’t have to be Gold
25%
What are the types of noble metals?
Palladium – Silver
Palladium – Copper
Palladium – Cobalt
Palladium – Gallium
What are the types of base metal?
Nickel-Chromium
Nickel-Chromium-Beryllium
Cobalt-Chromium
What are the characteristics of base metal?
- Stiffer metal
- Harder metal
- Greater Sag resistance
- Lowest Density
- Often the lowest cost and lowest quality of materials.
- Higher likelihood of reaction if patient has metal allergy.
What does the low density of base metal do to a PFM?
- more difficult to cast and therefore more difficult to finish in preparation for the porcelain and not able to create strong solder joints
- This can also lead to high oxide formation and therefore problems with bonding
What are the two main issues with base metals with nickel and beryllium?
- Allergic reaction: Similar to reaction to cheap jewelry due to the Nickel
- Toxicity: Lab tech handling the Beryllium is at risk due to carcinogenicity
PFM crowns are layered with ________ porcelain
feldspathic
What are the properties of ceramic?
-Excellent Esthetic properties
-Biocompatible
-Excellent insulator and protection against heat and electricity
-Shrinks as its fired
-Brittle
What does feldspathic porcelain being brittle cause?
- Crack propagation: Crack will propagate through material until it meets something to stop it
- Porosities in the porcelain act as crack initiators
- There is a stress dependent chemical reaction between water vapor and crack tips in porcelain that cause growth and further fracture even with comparatively low occlusal loading over long periods.
What are the different types of ceramics?
- predominately glass (etchable)
- particle-filled glass ceramic (etchable)
- polycrystalline ceramic (not etchable)
What are the properties of predominately glass ceramics?
Feldspathic porcelain
(etchable)
- Optical properties include most translucent
- Weakest strength
- Most Esthetic
What are the properties of particle-filled glass ceramics?
Lithium Disilicate
(etchable)
- Increased strength
- Less translucent than Feldspathic
What are the properties of polycrystalline ceramics?
Zirconia
(not etchable)
- Highest strength possibilities
- Lowest translucency
What are the components of feldspathic porcelain?
- feldspar (75-85%)
- quartz (silica)
- alumina
- glass modifiers
- leucite (10-20%)
What does feldspar do in feldspathic porcelain?
Responsible for forming the glass matrix
What does quartz do in feldspathic porcelain?
framework around which other components flow
What does alumina do in feldspathic porcelain?
- Hard strong oxide which increases strength
- Increases the viscosity of the melt when firing
What do glass modifiers do in feldspathic porcelain?
- Alkali fluxes (mostly potassium and sodium oxides)
- Increases the Coefficient of Thermal Expansion
- Includes opacifiers and coloran
What does leucite do in feldspathic porcelain?
Key ingredient to increase Coefficient of Thermal Expansion to bring ceramic closer to the Coefficient of Thermal Expansion of the metal
Where does the strength of the PFM come from?
-The bond between the ceramic and the metal substructure
-The compatibility of the metal and the porcelain
-Design and rigidity of the metal coping or framework
The metal-porcelain bond is accomplished with…
- Mechanical interlocking of materials
- Vander Waal’s forces
- Chemical bonding (at the molecular level)
- Compressive force
What is the type of bond between metal-porcelain?
Mechanical Interlocking
______________ from Metal finishing or from Air Abrasion create a surface that allows for the interlocking of porcelain and metal
Microabrasions
What do microabrasions allow for in the metal-porcelain bond?
- create a surface that allows for the interlocking of porcelain and metal
- clean the surface
- increase the surface area of the metal
- increase the “Wettability” of the metal
What does an increase in wettability lead to?
the porcelain will flow over the metal material better and therefore flow into the abraded surface
What is the process of the metal-porcelain bond?
- An Oxide layer is formed as the metal coping is fired in the heating treatment
- Traces of Gallium, Tin, Indium, and Iron move to the surface-Base metal alloys readily oxidize
- Gasses are released from the metal during this process
- The oxide layer of the metal bonds to similar oxides in the opaque layer of the porcelain
How is opaque porcelain involved in the metal-porcelain bond?
- Forms oxide bond with oxide layer of metal coping
— Silica oxide from Ceramic to oxide layer of metal
—Also some Vander Waal’s forces here
Porcelain coefficient of thermal expansion is ________ than metal
less
- Coefficient of Thermal Expansion is matched with dental porcelain to reduce ceramic fracture in response to heating and cooling.
The melting temperature of metal is considerably ___________ than the fusing range of the veneering dental porcelain (1600-1900 degrees)
higher
The melting temperature of metal must be high enough so as to NOT sag or melt when porcelain is applied. Usually need a difference of at least ________ degrees F.
300
Higher the amount of Gold in the alloy, the ________ the melting temperature
lower
Adding Platinum and Palladium ________ the melting temperature
raises
Adding Base metals to the alloy ______ the melting temperature
raises
What are the three margin types?
What do you need to consider when picking which porcelain margin?
-Patients concerns for esthetics
-Clinical crown height
-Opposing occlusal surfaces
-Bruxism
Where should the porcelain-metal finish line stop in anteriors?
Perferable to be at least 1.5mm away
Why should the porcelain-metal finish line be at least 1.5 mm away from occlusion on an anterior?
- Need adequate porcelain overlap from facial onto the lingual
- This provides the incisal translucency desired
- Metal flexure could cause porcelain to fracture
- Occlusion should not be on the finish line
What is the problem with lingual porcelain on a PFM anterior?
- WILL CAUSE WEAR ON OPPOSING TEETH
- Contrainidcated in deep vertical overlap
What should you be aware of if you choose a lingual porcelain on a PFM anterior?
- A lingual metal collar is desired to support porcelain
- More tooth reduction required as well if porcelain in covering the lingual
- WILL CAUSE WEAR ON OPPOSING TEETH
What is the advantage of a metal lingual on maxillary anterior?
-Easier to form anatomy
-Easily polished
-LESS WEAR ON OPPOSING TEETH
-Less tooth reduction required
For optimum esthetics on a PFM, wrap porcelain through the…
contacts
Where should the porcelain stop in a PFM posterior?
Occlusal surface
What are the options for the occlusal surface on a posterior PFM?
-Full Porcelain
-Full Metal
-Combination
What needs to be considered with the proximal contacts on a posterior PFM?
- Consider esthetics
- A ceramic contact is much easier to add porcelain to if a contact is too light or is missing
- Highly suggest all contacts are in porcelain
In a posterior PFM, Finish line is best placed at least _____mm away from occlusal contacts in MI, or porcelain will tend to fracture
1.5 mm
Suggest for Mandibular Posterior PFM to either go with ___________ or __________ to prevent fracturing of porcelain
full coverage porcelain veneering or full metal occlusal
Second molars almost always get ____________ occlusal if patient can be convinced
full metal
For a maxillary posterior PFM, If porcelain is on the occlusal, ___ mm of reduction on the occlusal is necessary
2
Ideal to have 3mm metal collar on the lingual
For a mandibular posterior PFM, If porcelain is on the occlusal, ___ mm of reduction on the occlusal is necessary
2
Metal support under lingual cusps and marginal ridges of 3 mm
What are the characteristics of the porcelain shoulder margin?
-Slight subgingival placement for best esthetics
-Great margin for esthetic zones
-Requires special shoulder porcelain and additional technique
What special technique is needed to make the procelain shoulder margin on a PFM?
-Uses high fusing aluminous porcelain
-Need a good ceramist for this
-Need 2 additional firings at a higher temperature
-90 degree angle for strength and sealed finish line
What are the characteristics of a metal collar margin?
-This design is a more conservative tooth preparation in cervical area
-Restoration design is easier for the lab to fabricate
-Great for molars that have margins deep sub-gingival
-Allows for multiple types of preparation finish line designs
What are the characteristics of a disappearing margin?
-Indicated for areas that the margin cannot be hidden under the gingiva
-Often metal copings are over contoured and thick at gingival area
What is a cohesive fracture?
a bond failure due to traumatic incident or heavy occlusion
What is an adhesive fracture?
the de-lamination of the porcelain from the metal
- This is typically a failure of the manufacture
process where there occurs an incorrect
oxidation process
- Contamination of oxide surface
The CTE of metal is slightly _________ than the CTE of porcelain
CTE = coefficient of thermal expansion
greater
If the cooling rate after firing is too rapid, the porcelain will…
compress too rapidly and fracture
Where are areas of poor placement of metal ceramic finish line?
- Lingual for mandibular finish line too close to centric occlusion points or incisal edge
- Finish line too close to incisal edge.
What is the biggest drawback to the PFM crown?
Fracture of the porcelain
Can you repair fractured porcelain on a PFM?
- Sometimes the fracture porcelain can be repaired with composite or amalgam
- Sometimes if the fractured portion of porcelain is intact and fits well back together, the porcelain piece can be bonded back together
— This repair is reasonable, but still very weak and likely to break again
What is the #1 reason for porcelain on a PFM to fracture?
Poor design
What is the #2 reason for porcelain on a PFM to fracture?
occlusion
How can you be successful with PFM crowns?
- The type of porcelain and metal also enhance or detract from the bond
- Design of the metal coping for proper support of the more fragile porcelain
- A preparation design that allows for good resistance and retention
- Know what kind of margin you desire
