Metal- Ceramic Restorations Part I FILEminimizer FILEminimizer
Transcript of Metal- Ceramic Restorations Part I FILEminimizer FILEminimizer
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Reference:
- Shillingberg, Fundamentals
of fixed prosthodontics3rd edition
Chapter 25
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What are Metal-Ceramic restorations??
Advantages??
Bonding porcelain to metal
Alloys for metal-ceramic restorationsMetal framework design
Metal surface treatment
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Esthetic
Superior casting fit
Not subjected to discoloration
Withstand loading under function
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Four mechanisms:
1- mechanical entrapment
2- compressive forces
3- Van der Waals forces4- chemical bonding
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Finishing stones or disks
Air abrasion
Also increase surface area for chemicalbonding
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Coefficient of thermal expansion for the metalcoping should be slightly higher than that ofporcelain
Porcelain will draw toward the metal whencools after firing
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Affinity based on attraction of chargedmolecules
Minor forces
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Formation of oxide layer on the surface ofmetal
Gold alloys have trace elements like iron, tin
and indium that form oxides when fired in airThese oxides bonds to similar oxides in the
porcelain
Base metal alloys readily form chromium
oxides that bonds to porcelain
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Properties:Coefficient of thermal expansion:
- Should be slightly higher than that of porcelain but notby more than 1 x 10-6 0 c.
Melting range:
- Should be 170-280 0 c higher than that of porcelain
Modulus of elasticity
- Measure of rigidity- Alloys with higher modulus of elasticity have higher
rigidity and are more suitable for long span bridges- High noble alloys have lower rigidity than base metal
alloys
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Yield strength- Alloy used should have adequate Yield
strength not less than stresses caused bybiting forces
Hardness- Alloy must be hard enough in order to resist
wear- Alloys with high hardness values are difficult
to adjust and finished- Alloys with hardness values exceeding that for
enamel (around 350) will cause abrasion for
opposing teeth
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Corrosion resistance:
- Corrosion can lead to roughening of thesurface of metal, weakening of therestoration, and discoloration of the adjacentsoft tissues
- Noble metals are added to an alloy to reduceits tendency to corrode
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Strain (cm/cm)
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Gold
Platinum
Palladium
RutheniumRhodium
Osmium
iridium
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Contains more than 60% noble metal
Contains more than 40% gold
Excellent corrosion resistance
Low rigidity (inadequate for long span FPDs)Relatively low melting range
Poor sag deformation resistance upon firingof porcelain
High cost
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Corrosion resistance varies according to thecomposition
Relatively high strength and hardness
Contains higher % of silver which can causegreening discoloration of porcelain
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Harder
Stronger
More rigid (suitable for long span FPDs )
High melting rangeGreater casting shrinkage
Less burnishable than noble metals
Poor corrosion resistance
Nickel and beryllium can cause allergicreactions
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Titanium alloys
- Excellent biocompatibility and corrosionresistance due to TIO2 layer
- Less expensive than noble metals- Light weight
- High melting point and tendency to oxidizecreates problems in technique
- Special casting machines should be used
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Metal coping must allow the porcelain toremain in compression
Any tensile stresses in the porcelain must beminimized
- Thickness of metal
- Occlusal and proximal contacts
- Extension of veneered area- Design of the facial margin
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Uniform thicknessof porcelainsupported by metal
for better stressdistribution andappearance
- Deficiencies or extra
space should becompensated by themetal coping
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- Minimum thicknessof porcelain is 0.7mm for strengthand esthetic
- Thickness ofporcelain shouldnot exceed 2 mms
Sharp angles andundercuts shouldbe avoided
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Outer junction between metaland porcelain should be at rightangle
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Metal coping should be rigid
- Noble metal coping should be at least 0.3-0.5mm thick
- Base metal alloys should be at least 0.2 mmthick
- The thickness of metal coping may varyaccording to the preparation in order to
achieve uniform thickness of porcelain
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Occlusal contactsshould occur on metal ifpossible
-Porcelain cause moreabrasion for opposingenamel than metal
Porcelain metal junction
should be placed 1-1.5mm from occlusalcontacts
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Porcelain metal junction should not beplaced on metal-ceramic junction
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Proximal contacts should not be placedon junction between metal and porcelain toavoid plaque accumulation.
Usually proximal contacts are placed onporcelain
- Esthetic
- Better stress distribution
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Depends on:
Patient wishes
Opposing occlusal restorations
Presence or absence of bruxismRetention requirements
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Metal margins:
- Better accuracy but
- Esthetic??
- Extending preparation subgingivally??
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Porcelain margins:
- Esthetic
-No need to extend preparation subgingivally- Accuracy??
- Technique sensitive??
- Shoulder porcelains:
Stronger aluminous porcelain with 30-80 0 chigher fusing temperature than dentinporcelain
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Wax pattern should be made to the fullanatomic contour of the restoration
The area to be veneered with porcelain is cutback
Wax should not be thinned to less than 0.5mm
This will ensure:
- Uniform thickness of porcelain- Smooth continuation of contours between
metal and porcelain
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Wax Pattern should fulfill the followingcriteria:
1- conform to the normal anatomic form
2- centric stops should be at least 1.5 mm fromjunction between porcelain and metal
3- surfaces of veneered areas should be smoothand rounded
4- pontics and connectors of adequate size anddesign
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1- Checking the metal framework on thedie
2- Checking the metal framework in thepatient mouth
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Check the internal surface of the metal fordefects and the die for any damage
Fitting: ideally the casting should touch the
die only in the marginal region
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Temporary restoration is removed andpreparation is cleaned from any cements
Try-in procedure usually does not require
anasthesia
The metal framework should be seatedwithout force
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Precaution!!
- Upright position
- Dry crown and dry fingers- Gauze
- Finger behind the crown
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Checkthe internal surface for nodules orcasting blebs
Checkfor retained temporary cement
Checkfor overextended margins
Checkthe internal surface for internaldiscrepancies
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Internal discrepancies can be detected usingdisclosing waxes, aerosol sprays, chloroformand rouge, fit checker or by sandblasting
- Areas of metal-tooth contact will appear inside
the crown as shiny spots devoid of thematerial used
- These areas can be relieved using carbideburs
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Acceptable margin should not beoverextended, underextended, too thick oropen
Margin is considered open if a sharp explorertip50 micrometer can be inserted inbetween
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The crown should be checked for tendency topivot when rocked
Causes:
- High spot in the fitting surface
- Too much die spacer