Metal- Ceramic Restorations Part I FILEminimizer FILEminimizer

8/3/2019 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

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Metal- Ceramic Restorations Part I FILEminimizer FILEminimizer