full Veener Crown

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METAL – CERAMIC RESTORATIONS

Introduction :

A metal ceramic restoration is composed of a metal casting or coping

which fits over a tooth preparation and ceramic that is fused to the coping.The coping

may be a little more than a thimble or it may be clearly recognizable as a cast crown

with some portion cut away. These cut away contours will be replaced by porcelain

that will mask / hide the metal underneath. Therefore metal ceramic restorations

combine the strength and accuracy of the metal with esthetics of porcelain.

History :

It was introduced to dentistry nearly 4 decades ago. The different names used were :

• eramco crown !one of the first brands of porcelain for fabricating the crown".

• #orcelain veneer crown !#$"

• #orcelain fused to gold !#%&"

• #orcelain fused to metal !#%'" !()*+s , -+s"

Ceramo metal restoration – the state of science:

evelopments in the field of metal ceramics have led to numerous

improvements some answers but as in most scientific endeavours more 0uestions.

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1nowledge of the science and appreciation of the art are necessary to realize the full

potential of the constantly evolving restoration.

#hysics chemistry engineering and material science have all helped to rationalize our

approach to ceramo2metal restorations.

The scientific base of the technical procedures can be divided into smaller topics as

dealt below:

The substructure

The ceramic

The 3ond

THE S!"STRCTRE

e0uirements to be fulfilled:

• The substructure should strengthen the restoration to resist failure which could

result from poor tensile shear and impact strength of the veneer !i.e." porcelain.

• It should provide good marginal fit

• 5hould not interfere with esthetics

• 6ption available: metal alloys

Metallur#ical $ro%erties of an ideal alloy for a ceramic su&"structure:

'( Hi#h modulus of elasticity:

'odulus of elasticity reflects the rigidity of the material within its elastic

range. &reater the '67 the less a given thickness of material will fle8 when loaded.

The brittle ceramic demands a rigid substructure. Any deformation even if elastic

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generates destructive stresses in the ceramic veneer. 9owever as the restoration cools

the alloy should be able to deform a small amount to relieve the stress produced by

thermal contraction of porcelain. 9igh '67 does not allow alloy to relieve stress

which would remain in porcelain and cause crazing.

)( Hi#h yield stren#th:

The yield strength reflects the resistance of the material to permanent

deformation. If the sub2structure permanently deforms the restoration will fail. In

addition yield strength is critical to the ability of a material to about energy

especially in materials which have a high '67.

*( +ine #rain structure:

Important to the mechanical stability of the marginal area corrosion

resistance and hardness.

,( Sa# resistance:

Alloy should resist deformation at firing temp of ceramic

-( Casta&ility:

Alloy should be easy to handle and cast. Accuracy of fit of a casting is a

must. 6ne can compensate for poor mech. prep by intelligent structural design but

there is no compensation for ill2fitting restorations.

.( !ond %otential:

The alloy must allow good wetting provide a suitable bond and be

thermally compatible with the veneer material.Alloys vary in techni0ue sensitivity

because of difference in casting temp casting shrinkage specific gravity surface

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tension investment compatibility soldering capability resistance to overheating and

gas absorption.

#redictability with respect to alloy choice therefore is still very much dependent on

individual e8perience e8pectation and e8pertise.

/esi#n of the Metal Su&"structure:

egardless of alloy selected intelligent design is critical to success. The

sub2structure should satisfy both biologic and mechanical demands. 'echanical

demands dictate that it should be as thick as possible esthetics dictate that it should be

as thin as possible. %ramework design in sinple words should be so that there is

ade0uate embrasure space to allow gingival health and ade0uate thickness of

porcelain to allow for esthetics and yet maintain resistance to deformation.

$rinci%les in /esi#n:

(. igidity of support

. ontrol of tensile and compressive

forces

;. 5hear resistance

4. 'arginal integrity

<. 7sthetics form and function

=. Access for maintenance

'0 Ri#idity of Su%%ort:

The metal substrate must be firm and unyielding to preserve precise abutment metal

form when the porcelain is being fired !sintered".

istortion of this form will lead to:

a" Tensile strains within porcelain and

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b" #otential porcelain fracture while seating restorations compromising marginal

integrity and fit.

In addition to sintering forces the metal substrate for bridges should

withstand functional and parafunctional forces of occlusion without fatigue or

deflection beyond the limits of porcelain or metal.

'echanical engineering principles govern basic design concepts. The la1

of &eams applies here ie. Assuming a vertical vector of force metal substructures

will alter their resistance in the following manner:

a( oubling the height of metal produces - times the resistance of deflection as it

is inversely proportional to the cube of the change in size.

&( oubling the width of the metal produces twice the resistance , it is directly

proportional to change.

c( oubling the length of the pontic space increases the deflection by eight times

because the change in dimension is inversely proportional to the cube.

5tructural integrity is most efficiently controlled by manipulation of vertical

dimension in connectors.

5ound conceptual geometric forms still re0uire skill and >udgmental

alterations for diverse situations.

) 0Control of com%ressi2e and tensile forces:

ental porcelain is a brittle material with low fracture toughness. Tensile

and shear forces are easily achieved under many circumstances which may fracture

porcelain? though porcelain@s resistance to breakage from compression is great.

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Therefore design forms endeavor to emphasize metal support wherever force is

applied to the porcelain , preferably perpendicular to the ma>or force vector.

The thickness of porcelain veneer demands careful control , another

problem in tensile and compressive forces. orrectly applied thin porcelain is

stronger near the metal2porcelain interface due to the mismatched coefficient of therm

e8pansion designed by manufacturers to produce compression of porcelain at the

interface. The temptation to keep metal thin and to thicken the porcelain for optimal

optical 0uality is a constant threat to the integrity of the dual system.

#orcelain that e8ceeds (.<2.+ mm thickness develops more fracture when shear

forces are applied. These fractures probably follow the flaw mechanisms enhanced by

tension or lack of compression in the thickened porcelain. Another way of viewing

this problem is to provide metal designs that permit porcelain to shrink on metal

rather than on itself for it will surely produce flaws. Thick porcelain ! mm" violates

the basic concept of manufactured metal , porcelain composite.

*0Shear resistance:

#roper metal substructures enhance the inherent shear resistance of the

fused porcelain and help withstand intraoral forces.

Three ma>or areas deserve attention.

(. Incisal cosness the metal support limits porcelain to ≤ .+ mm in this area.

. #osterior porcelain cusps2 the metal support maintains porcelain cusp integrity

by limiting shearing forces and increasing compressive resistance.

;. #ro8imal posterior porcelain margins

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• The pro8imal metal structure provides superior support to preclude

fracture in this crucial area while using porcelain occlusals.

4. 'arginal Integrity:

The finished restoration must:

(. 5eal the denuded tooth from bacterial invasion.

. Allow minimal cement dissolution

;. eproduce a normal emergence profile

%ull metal margins permit superior marginal adaptation. osmetic veneering materials

fused !porcelain" introduce problems in controlling marginal integrity.

) as%ects here re3uire e4amination:

(. 5eating of finished restoration during cementation

. Avoiding metal creep !lifting of metal margin during porcelain sintering and

shrinkage"

These problems could be limited if the need to hide the metal were less

demanding.

'etal collar enhances control of gingival contour and the emergence

profile dictated by the wa8 , up is preserved in its original form when metal collars

are used.

Mar#in /esi#n and castin# seatin#:

3evel is considered as a geometric form that enhances marginal seal

!1ashani et al ()-(". 3urnishable ductile gold alloys are used over marginal bevels

of !slip2>oint" inlay and onlays. The stiffness of metal2ceramic alloys precludes that

possibility. 3evelled cutting tool permits considerable vertical variation as it is guided

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around the tooth while still preserving horizontal linearity of margin? this is an

important 0uality of the beveled margin.

osner !()=;" notes four functions of the bevel in castings. These are:

(. eduction of inherent defects in casting and cementation.

. #rotection of enamel rods at the margins

;. Allowance for burnishing

4. evelopment of circumferential retention.

The 0uestion of shoulder/bevel centers around the open margin 0uality and

0uantity of the cemented cast restoration.!3est seal is from featheredge and parallel

bevel preparations consistent with geometric considerations". !&avelis et al ()-(".

The best seating during cementation was produced with a )+° full shoulder

and poorer was with )+° shoulders with parallel bevel. This is e8plained on the basis

that shoulder preparations have poor seal prior to cementation facilitating cement

escape marginally.!To clarify controversy 'c Bean and Cilson !()-+" presented

mathematical evidence indicating that levels must be in the region of *+°2-+° for an

improvement in marginal seal and cement dissolution. 6vere8tended margins can be

kept till final polishing and then trimmed prior to cementing. The full shoulder does

not allow this liberty". The casting should be milled to a knife2edge at the margin

after which porcelain is fi8ed to the e8act line. If metal over e8tension is left for

reduction after firing the reduction of metal may produce a metal collar and opa0ue

line.

Cree% %ro&lem durin# sinterin#:

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hamfer produces the most distortion during shrinkage of porcelain ,

5hilling burg et al.5upported by Taucher D Eicholls !()-+"

It was concluded that %orcelain a%%lication and firin# does not

mechanically distort the facial mar#in as the layered porcelain precludes metal

creep. 5intering shrinkage and potential creep is countered by incremental layering of

porcelain.

6pa0ue , layers , for good wetting with min. thickness.

(st 3ody build up , away from the margin.

nd , complete the undercontoured cervical area.

Avoiding e8tremely long firing cycles can control creep. $arying the alloy

comp. also helps in controlling creep by inducing a dispersion strengthening effect.

These principles of design are incorporated in various stages of fabricating

metal ceramic restorations in the form of the following features. !5hillingburg"

'0 Thic5ness of metal :

igid copings give strength and longevity to restorations. If restorations fle8 under

occlusal loads tensile stresses occur in porcelain which would lead to its shearing.

igidity is re0uired to avoid fle8ure.

Eoble metal copings should be ;2< mm thick and base metal compings? mm !as

their yield strength and melting temp are high".

The coping should have an evenly flowing conve8 contour of veneering area. This

helps stress distribution unlike sharp angles and undercuts. Chich increase stress.

The outer >unc. of porcelain to metal should be at )+ °2(;<°. If acute the metal2

porcelain interface may produce porcelain crazing and if it is beveled the porcelain

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will and in a feather edge through which the o8idized metal or opa0ue will show. The

veneered porcelain should be kept at a minimum thickness and should still be

compatible with good esthetics.

1elatively thin porcelain of uniform thickness and supported by rigid metal is the

strongest. The absolute min thickness of porcelain is .* mm and the desirable

thickness is (.+ mm.

)0 Occlusal and $ro4imal contacts :

ontacts should be placed on metal wherever possible because:

• ontacts can be precisely located.

• The area covered by ceramic can be more precisely controlled.

• 'etal causes less wear on opposing teeth than ceramic.

&lazed porcelain removes 4+ times as much opposing tooth structure as

gold. !Facobi et al" The contacts should occur well away from the porcelain , metal

>unction line.

ontact near the >unction can cause metal flow and subse0uent porcelain

fracture. The porcelain , metal >unc. should be (.+ mm from occlusal contacts at the

position of ma8imum intercuspation.

In anteriors :

To avoid / minimize stress from the lower incisor contacts on the lingual

surface of ma8. ant. estorations the metal ceramic >unc. should be placed away from

the vicinity of the contacts.

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Chen the vertical overlap is inade0uate the >unc. is placed far enough

gingivally for the contact to occur on porcelain.

6ne more thing worth mentioning is the placement f porcelain , metal >unc. Too close

to incisal edge. This destroys the incisal transluscency and fracture of porcelain

increases.

E4tent of 2eneered area :

As the occlusal contacts occur in metal the porcelain coverage is as follows:

6n ma8 premolars and molars , #orcelain e8tends over the facial cusp tip

and about half way down the lingual incline of facial cusp. A rounded ledge of metal

under the facial cusp support the porcelain so that the ceramic metal >unc. is kept

away from the occlusal contacts. This design is more resistant to fracture than when

porcelain e8tends to central groove or covers the entire surface.

$ariants for ma8illary teeth :

(. #orcelain coverage of mesial marginal ridge upto the middle of the triangular

ridge.

. omplete coverage for those who demand absolute esthetics.

6n mand first premolars : complete porcelain coverage on occlusal

surface. 'and. 5econd premolars and molars 3ru8ism occlusal restorations on

opp arch and patients whishes dictate e8tent of ceramic.

The distal half of premolars and molars can be uneven to allow more occlusal contacts

in metal.

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'etal collar of mm on facial and ; mm on lingual can be used so that the

tooth preparation is more conservative through the greater portion of crown is to be

covered with porcelain it should be wa8ed to full contour and then cut back to insure

a uniform thickness of porcelain and correct contours.

GThimble@ copings result in unsupported fracture prone porcelain. Anterior

metal ceramic restorations with guidance in lateral e8cursions and protrusion on

porcelain teeth will abrade opposing teeth eventually will re0uire restorations.

The collar of e8posed metal on the lingual should be at least ;.+ mm wide

incisogingivally. #ro8imal contacts for anteriors should be in porcelain which should

be facilitated by ade0uate tooth reduction.

The porcelain metal >unc is placed lingual to the pro8imal contact areas so that:

• #ro8imal porcelain has greater depth and translucency and hence better esthetics.

• 6ptimumstress distribution

+acial mar#ins :

%or many years the conventional facial margin for metal , ceramic crown was a

narrow metal collar.

It had the following disadvantages :

• unesthetic display of metal

• The need to create subgingival finish lines which in turn caused gingival

inflammation and recession.

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• Chen porcelain was veneered right upto the metal collar to avoid metal display

the restorations would get overcontoured at the margin or would give a thin

fracture , prone porcelain or an undetected open margin.

These unesthetic facial margins led to the thought of an all porcelain margin facially.

This can be either even with the gingival or slightly supragingival which had a

positive influence on gingival health. $arious techni0ues were devised for fabrication

of all2porcelain facial margins.

(. Hse of patient fail to support the margin while firing

. Hse of refractory die to support margins while firing

;. direct2lift tech ii" correction porcelain was added to the margin after a full2

contour buildup of the crown. The porcelain was condensed by compression and

fired to produce the final margin.

4. In ()*) $ryonis described a techni0ue which re0uired tooth preparation with

)+° shoulder and a metal coping that terminated at the gingivo2a8ial line angle.

6pa0ue porcelain was applied to the metal coping and the shoulder on the

die. After obtaining a satisfactory margin dentin and enamel were added to complete

the crown. #reviously dentin and enamel blends were used to create margins.

9owever the margins of conventional porcelain tend to round or slump

during subse0uent firings because the fusion temp are identical.

To overcome this manufacturers created special shoulder porcelains

containing aluminous porcelain that fuse at temp ;+2-+° higher than dentin or

enamel porcelains.

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Therefore the shoulder porcelains allow repeated firings with no effect on

completed margins and are also stronger in fle8ure than conventional porcelains

making margins more resistant to fracture.

5tudies with shoulder porcelains and direct lift tech have shown consistent

level of marginal adaptation with mean marginal openings of (<2; µm and -2((µm.

emonstration of acceptable margins with a wide assortment of techni0ues

porcelains and binders indicate that the 0uality of margins is directly related to the

skill of the ceramist.

Chen talented ceramists are absent Gall2porcelain@ facial margins are

contra2indicated.

Alloys used for #%'s : !AA classification"

(. 9igh Eoble

. Eoble

;. #redominantly base metal alloys

(. 9igh Eoble : ! =+ Eoble metal content with atleast 4+ &old"

&old , #t #d.

&old , #t Ag

&old , #d

. Eoble : !at < Eoble metal"

#d , Ag

9igh #alladium

;. #redominantly base metal !J < Eoble metal content"

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Ei2r

Ei2r23e

o2r

It is important that metal has a thermal coefficient of e8pansion and

melting temp that is compatible with the porcelain when used in restorations.

egarding coefficient of thermal e8pansion !α" a diff of (.* 8 (+2=°c can

shear the band between metal and ceramic. The optimum diff between the coefficients

of metal and ceramic should be no greater than ( 8 (+ 2=°. onventional gold alloys

have a coefficient of therm e8p e0ual to (4 8 (+2=° and conventional porcelain? 24

8 (+2=°.

Therefore the coefficient of thermal e8pn of porcelain was increased to *2-

8 (+2= by addition of alkaline like Bi arbonate and at the same time the Gα@ of

metal was lowered to *2- 8 (+2=° by adding #t or #d.

egarding melting temp the melting range of alloy should be (*+2(-+ °

!;++ , <++°%" higher than the porcelain.

A similar melting range would obviously result in distortional or melting

of coping during firing.

The greater the diff the fewer the problems that are encountered.

'elting temp of noble alloys K (2=+° !;++*"

And they are sub>ected to creep when temp K )-+°%

#orcelain 5 fuse at a temp K )-+°%

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Therefore the alloys are modified to possess high fusion temp.

evelopment of Alloys :

As gold became very e8pensive in late ()*+s alloys with little or no gold were

developed.

A logical transition was the use of # alloys which had advantages like : Bow cost

Increased strength and hardness

9igh fusion temp

esistance to distortion during firing

3ut they had certain disadvantages are well? when used in a metal2cer.system :

• 78cessive o8ide for motion

• ifficulty in finishing and polishing

• Luestionable biocompatibility

Ei 3e Ei , ontact dermatitis

3e , ust is carcinogenic

9azardous to personnel

Another alternative in cost cutting was to use Gu@ or cobalt to modify

traditional alloys. 3ut these elements caused dark o8ide formation and poor high

temperature strength.

5ubse0uently small amounts of gold and silver replaced cobalt or copper.

5ilver containing alloys have disadvantage of Ggreening@.

hoice of an alloy :

epends on a variety of factors :

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ost igidity

astability 7ase of finishing and polishing

orrosion resistance ompatibility with specific porcelains

#ersonal preference

Eo alloy system is without disadvantages , financial or technical and so no system is

without disadvantages , financial or technical and so no system is superior in all

aspects.

The Ceramic:

ental #orcelain is generally categorized into ; classes:

9igh fusing

'edium

Bow2fusing

9igh D 'ed fusing are nearly same in composition and microstructure but

are considerably different from low fusing porcelains.

9igh fusing porcelains are used primarily for denture teeth and medium

fusing porcelains for pontics !tru2pontics" enture teeth are built up by layering

several different shades of porcelain. 7ach layer is chemically and microstructurally

similar. These porcelains are true porcelains as defined by a ceramic engineer. They

are composed of mi8tures of raw minerals mined from earth such as natural feldspars

!clay like minerals composed of o8ides of 5i A( Ea D 1" and 0uartz !5io ". The are

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powdered mi8ed together and heated. Above +++° multiple chemical reaction

between the o8ides result in the formation of a molten o8ide li0uid which glues the

reacting and unreacted particles together. ooling this solid li0uid composite results in

a solid piece of true porcelain.

'icroscopic e8amination reveals a composite physical structure made up

of a host of small crystalline particles within an amorphous matri8. This structure is

physically and chemically stable at low temp but the reactions will begin again if

reheated. As the internal chemical reactions can be started and stopped by cycling the

temperature the physical properties of true porcelains are unstable upon repeated

episodes of heating and cooling.

'edium fusing porcelains differ from high2fusing porcelains only in their

proportion of reactive o8ides. They re0uire less heat to fuse the particles together and

can be more easily self2glazed at a lower temp than the high2fusing porcelains.

ue to instability of high and med. %using porcelains upon repeated episodes of re2

heating they are used for situations where they need only be ground and may be

glazed once and mounted in metal or plastic bases.

Lo1"fusin# %orcelains:

In veneering a metal sub2structure for a metal2ceramic restoration

porcelain powder is fused at relatively high temp. directly onto the metal.

The porcelain must be chemically and physically stable through the numerous bakes

until the desired form is established.

This is accomplished by producing a glassy material that is chemically

similar but microstructurally different from the high and med fusing porcelains.

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Bow2fusing porcelains are produced by first mi8ing raw minerals similar

to those used in high2fusing porcelains but with a relatively high proportion of

sodium and potassium 1 and Ea help break down the 5i26 network !and are therefore

called glass modifiers". This results in favourable conse0uences:

(" 5oftening temp of glass is reduced

" The coefficient of thermal e8pn is increased.

These o8ides readily react with 5i6 D Al 6; at high temp to produce

li0uid glass. Hnlike high fusing porcelains the components are nearly completely

dissolved via chemical reaction so that the material when cooled shows a nearly

homogeneous microstructure of amorphous glass which can be powdered remi8ed

and refi8ed without further chemical !and conse0uently physical" change.

Bayering D %using to metal simply involve a coalescence of particles at

elevated temp of the particles. This homogeneous glass can be modified by

manufacturers by small additions of o8ides that will impart colour to glass without

significantly changing its properties. &lass can be made opa0ue by adding small

percentages of nearly insoluble o8ides Tio Mro. As they are white or yellow white

they create colour shades. ue to their insolubility they interrupt light transmission

and have no appreciable effect on fusion temp. range of glass.

ifferent blends of low2fusing porcelains have been used for metal ,

ceramic restorations. They play different roles in the fabrication.

6pa0ue porcelains: omposed of low fusing glass and insoluble o8ides.

The density of o8ides is greater than that of the glass matri8 eg. 68ides of Ti 5n Mr

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In e. They cause incident light to scatter and reflect rather than transmit through the

porcelain.

Therefore the opa0ue porcelain applied as a first ceramic coat serves functions:

(" 'ask the colour fo alloy

" esponsible for metal2ceramic bond

!ody %orcelains : 3asic low fusing glass with various concentrations of colorant

o8ides.

There are generally ; body porcelains:

Those with no colouring o8ides 2 Incisal !enamel" porcelains

Those with small amount 2 &ingival !entin" shades

6f colourants esp yellow.

Those with colorants ranging 2 'odifiers

Across the color spectrum including white and gray

These ; body porcelains have the same chemical and physical properties

and can be intermi8ed freely. They are layered and fused over the opa0ue porcelain to

build tooth contour and esthetics by diffusing and softening the opa0ue colour.

Stains and 6la7es: omposed of glasses with a lower proportion of silica and

alumina than body porcelains.

The relatively higher content of o8ides of Ea D 1 plus colorant o8ides

gives these glasses a considerable fluidity at temp around (=++2(*++°. These

porcelains are balanced for nearly e0ual thermal e8pn. with body and opa0ue

porcelains but they should be intermi8ed with caution due to their high fluidity.

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They are used to create a glassy veneer and to impart superficial

characterization to a fused ceramo2metal restoration >ust prior to cementation.

To achieve consistently successful ceramo metal restorations the strengths

and weaknesses of the veneer member must be understood inorder to minimize

potential ceramic failure.

#robably the most imp mech property in terms of potential failure is the

relative weakness of ceramic under tensile stress.

Though the theoretical strength value of vitreous materials based on

intermolecular bonds is in the order of ( 8 (+* psi these values are approached only in

very fine glass fibres free of flaws and protected from conditions that would enhance

flaw generation.

%laws in ceramics are basically microcracks !(+

2=

m" predictability with

respect to ceramic hinges on an understanding of the implication generation

prevention and control of these e8tremely small flaws.

The common causes of these microcracks are :

(" 'ismatch in the co2efficient of thermal e8pn of the sub2structure and the

veneer. This was common during earlier stages of development when

interchange of metal2ceramic systems caused failure. Eow that the ceramics

are better matched to metals some interchange of one manufacturers metal

with another@s porcelain is possible.

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" Another potential cause is the heat generated while grinding and ad>usting

porcelain. This can cause differences in e8pansion in diff. areas of porcelain

which lead to micrcracks. 5taticfatigueN

!;" The inevitable abrasion and corrosion in the oral environment.

!4" 5tatic loads :

If the load places ceramic under continuous tensile stress the potential solvent

effect of the mouth is greatly enhanced.

6ne risks static load failure of this type whenever a ceramometal restoration is

forced place.

A forced2fit places a constant stress on the ceramic which given time and

moisture can fail even though the stress generated is below the reported

tensile stress of the porcelain used.

Therefore the ceramo2metal restoration should have a passive fit that

minimizes the risk of development of static load on cementation and thus the

time2delay failures.

!<" 3ubbles and voids in ceramic decrease strength and translucency

Appliacation of opa0ue proper condensation and strict adherence to

manufactures instructions regarding firing cycles help in producing a dense

porosity free restoration.

The !ond:

3ond between metal and ceramic has been e8plained by 4 mechanisms.

(. 'echanical entrapment

. ompressive forces

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;. $an der Caal@s forces

4. hemical bonding

Mech0 Entra%ment: is ceramic interlocking within incroabrasions on the metal

surface. %inishing the surface of metal coping with noncontaminating discs or stones

and air abrasion has proved to give a better bond.

Air abrasion increases wettability of metal with ceramic provide

mechanical interlocking and increases surface area for chemical bonding. #latinum

spheres !;2= µm diameter" used as bonding agent increase bond strength significantly.

ompressive forces :

The slight difference in the coefficient of thermal e8pansion cause

porcelain to draw toward the metal coping when the restoration cools after firing.

$ander waals forces : omprise an affinity based on mutual attraction of changed

molecules. They are a minor force and not as significant as once thought. Though it is

only a minor contribution to overall bond strength it is significant in the initiation of

the most imp. 'echanism the chemical bond.

Chemical &ond:

Indicated by the formation of an o8ide layer on the metal and by bond

strength that is increased by fi8ing in an o8idizing atmosphere.

Chen fi8ed in air the trace elements in gold alloy 2 such as %e 5n &a In

migrate to the surface and form o8ides. They subse0uently bond to similar o8ides in

the opa0ue layer of porcelain gold alloy containing significant amount of %e and 5n

creates a significantly stronger bond than a pure gold alloy.

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The bond strength of true adhesion is such that failure or fracture will

occur in porcelain rather than at the porcelain , metal interface.

3ase metal alloys readily form chromium o8ides that bond to porcelain

without the addition of any trace elements.

The intervening link between the metal alloy and the glass is a layer of

metal o8ide which need be only a monolayer in thickness to be effective.

Eono8idisable purely noble alloys do not bond chemically eg. Au2#t2#d

will cleanly lift from the surface of porelain with no evidence of attachment.

Addition of o8idisable elements eg. In 5n establishes the potential for o8idation and

conse0uent adhesion to glass. &lass is a powerful solvent for metal o8ides and

digestion of relatively large conc. of interfacial metal o8ides can lead to changes

!generally lowering" in coefficient of therm e8pn of the interfacial glass. 5uch a

change can cause internal tensile stresses upon thermal cycling and facilitate fracture.

ie" 78cessive metal o8ide diminishes bond strength by interposing an o8ide layer. 5o

thick that fracture can easily proceed through it.

Also the diffusion of dissolved o8ides through porcelain can also lead to

graying or show2through of darkened glass. 7g. &reening in silver alloys.

Therefore care should taken to minimize o8idation.

%actors affecting the bond :

As the factors contributing to the bond are better comprehended than before the

factors affecting the bond are now clearer.

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As the factors contributing to the bond are better comprehended than before the

factors affecting the bond are now clearer.

(" 'ost metal alloys need an initial o8idation prior to ceramic firing !e8ception ,

#d2u2&a" high palladium alloy , freedom plus".

This was also called conditioning bake or degassing.

!egassing , wrong term" &ases come out of metal only at higher temp ie"

from molten metal and not during firing ceramic.

An optimum thickness of o8ide layer is essential for a strong metal2ceramic

interfacial bond. G3e@ has been added to some Ei2r systems to control the

thickness of o8ide layer.

5ome systems are provided with a bonding agent that contains some elements

contained in porcelain !eg. Al 5n 5i" bonding agents may increase or decrease

the width of interaction zone between metal and ceramic. 'anufacturer will

indicate whether a bonding agent is necessary or beneficial.

" Air borne particle abrasion with Al6; is routinely performed to provide

mechanical bond.

ecent research shows that controlled amounts of mech. 5urface

roughening that yields greater notch depth for irregularities increases the metal

ceramic bond strength than coarse roughening.

eramic bond2strength was analysed using finite element analysis !eg:

tests like pull2shear three point bending fo8u point bending". 3ut the problem was

that the stress varied with position along the metal ceramic interface and simulation of

loads that could cause clinical failure was difficult.

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To avoid these problems 6@3rien proposed a different approach focusing

on the mode of failure of metal ceramic specimens or restorations though the method

of microscopic measurement. Ii" Adhesive / cohesive failure can occur at si8 is not

specified. #ossible sites or combinations of those sites.

Adhesi2e failure :

(. #orcelain2metal interface.

. 'etal o8ide2metal

;. #orcelain2metal o8ide

Cohesi2e failure :

(. #orcelain porelain

. 'etal o8ide , metal o8ide

;. 'etal2metal !very rare"

6ther techni0ues being used to evaluate the metal ceramic interfacial bond were:

• O2ray spectrometry

• Three point bending test in I56 standard )=);.

• Benz et al analysis etc.

The fracture mechanics approach is the most recent.

#redictability with respect to bond strength still needs research.

There are many treatment variables numerous alloy and little standardization with

respect to testing.

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esearch efforts continue so that eventually the techni0ues will become predictable by

having a scientific than an empiric base.

The Art : !Techni0ue of ceramo2metal"

esign of coping 2 5ingle unit

3ridge

5plint

Metal %re%aration 8 Alloy treatment:

The investment on the cast framework is removed by air borne particle abrasion steam

and ultrasonic cleaning prior to treatment.

The surfaces of a coping that are to porcelain must be properly finished to assure a

strong bond and an esthetic restoration.

Metal finishin# :

After sprue removal the veneering area is finished using clean non2

contaminating stones and discs. Instruments used on other types of metal will

contaminate the veneering area.

ough finishing can be done using Al6; stones.

%inishing the surface in one direction and using light pressure will help

avoid trapping debris between folds of metal which is a problem when using high

content alloys with high elongation values.

The thickness of the sub2structure should be checked from time to time

and it should be atleast .;mm thick for noble alloys and . mm for basemetal. ervical

collars can be thinned to a knife edge !.(mm".

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The demarcation between the surfaces to be and those not to be veneered

should be distinct with an e8ternal angle of )+° and rounded internal angle.

The surfaces not to be veneered are brought to a rubber wheel finish prior

to ceramic firing.

The metal2ceramic >unction is delineated with stone or tungsten2carbide

bur and the veneering surface is air abraded with alumina !fine2grit".

The ne8t step is to clean the alloy before o8idation.

Cleanin# :

&rindin debris oil and finger grease are some common contaminants that need to be

removed as they might interfere with wetting of metal by porcelain.

Hltrasonic cleansing !< min" insing with ) alcohol and steam are the

various methods out of which steam has been found to be very effective.

68idising :

'etal surface treatments are uni0ue for each porcelain , alloy combination

and manufacturers recommendations should be followed.

9eat treatment of noble alloys causes the trace 0uantities of n &a In and Mn in the

alloys to form o8ides that enhance bonding with porcelain.

3ase metal alloys on the other hand o8idize readily and so o8idation has

to be carefully controlled. %ollowing o8idation most of these alloys are needed to be

air2abraded with alumina to minimize the thickness of o8ide layer.

5ome systems do not need the work to be held in the furnace for o8idation

as they undergo continuous o8ide formation.

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Although tech differ base metal alloys are now not sub>ected to o8idation.

%or noble alloys the o8idation procedure includes inserting the

substructure into the furnace and raising temp above the firing temp of porcelain.

68idation is only one of the fune of this initial firing. It also helps to

release the 9 gas trapped on the alloy surface during casting which would otherwise

weaken the metal2ceramic bond.

6btaining an optimum thickness of o8ide layer is the final step in alloy preparation to

receive a ceramic veneer.

• 3uild up is a skill that re0uires a great deal practice to develop. This is a brief

description for familiarization.

$orcelain Addition :

The casting is now ready for the actual placement of porcelain.

ental porcelain is usually received from the manufacturer in powder form which is

mi8ed either with water or a water based glycerin containing li0uid to form a paste of

a workable consitency.

'anufacturers specify the alloy systems with which their porcelain is compatible.

Hsually compatibility refers to the relative coefficients of thermal e8pansion.

The shade selected clinically determines which powders to combine.

O%a3ue %orcelain a%%lication:

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6pa0ue porcelain is applied first to mask the metal to give the restoration

its basic shade and to initiate the porcelain2 metal bond.

A small amount of opa0ue powder is dispensed on a glass slab or palette

and mi8ed with some modeling li0uid.

The coping / framework is held with tweezers and moistened with some of

the li0uid.

A small bead of opa0ue is picked with the tip of brush and applied to the

coping as a wash. Bight vibration is used to spread it thinly and evenly. 'oving a

serrated instrument back and forth over the handle of the tweezer will create the

necessary vibration. 78cess moisture that comes to the surface can be blotted off with

a clean tissue. These steps help in proper condensation of porcelain. Eo attempt is

made to thoroughly mask the metal with this initial application.

It is intended to completely wet the metal and penetrate the striations created by

finishing.

Any e8cess opa0ue on the unveneered surface removed with a short stiff dry brush.

The coping is placed on the saggertray dried and fi8ed under vaccum to a specific

temp. The vaccum is then broken and the coping is held at the temp under air for (

min.

The second application of opa0ue is then applied and this should mask the metal.

After firing the second layer the total thickness of opa0ue should not

e8ceed .; mm thick and should fulfil the following:

• 5hould be a relatively smooth layer masking the colour of the framework.

• 9ave an eggshell appearance

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• Eo e8cess on any e8ternal or internal surfaces of the restoration.

!ody and Incisal $orcelains :

Chen a satisfactory opa0ue layer has been fired the body and incisal porcelains are

applied.

5everal porcelains are used in a restoration.

7g. 6pacious entins are used where less transluscency is re0uired !eg. &ingival area

of pontic incisal mamelons etc."

5pecial neck powders can be applied on the cervical (/; rd and incisal

powders to the incisal edge to simulate natural enamel.

&enerally the restoration is built to the anatomic contour when it is

acceptable a cut2back similar to that made during the wa8ing stage will allow for a

veneer of the more transluscent incisal porcelain.

/is%ense the re3uired %o1ders on a #lass sla& or %alette0

Cet the previously fi8ed opa0ue with a small amount of li0uid. #lace a

small bead of neck powder on the cervical portion of the neneering surface &entle

patting with a brush and light tapping in the cast will produce ade0uate vibration

during preliminary stage of condensation. A tissue is held close for removal of e8cess

surface moisture.

3lotting consistently from lingual aspect is recommended and will result in

superior esthetics.

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%acial surface should not be blotted because the small pigment particles might be

removed.

After placing the neck powder and sculpting it build the veneer to

anatomic contour with body porcelain using ad>acent and opposing teeth as guide.

Chen contact is anticipated between the wet build2up and stone2cast the cast can be

coated with a small amount of cyanoacrylate resin to seal the surface and prevent

absorption of moisture from the build up.

To compensate for firing shrinkage slightly overbuild the porcelain.

A typical meta2ceram ant crown will shrink +.= mm at the incisal edge and

< mm midfacially.

epending on the desired appearance make a cut2back for the more

translucent incisal powder.

5ome manufactures recommend carrying the incisal veneer all the way to

the cervical portion while others limit it to the incisal third.

Almost an infinite variety of possibilities e8ist and only with e8perience

can the dentist predict the finished products appearance.

The cut2back should be made from incisal to cervical to minimize the

chance of damaging the incisal portion of the build up.

Apply the incisal powder and overbuild the restoration in the same manner

as for the body porcelain.

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The opposing cast should be marked with green or red markers and the

articulator is closed to check for occlusal contacts. The marks transferred onto

ceramic bun off without residue.

6 E6T H57 3BH7 3BA1 pigments as markers as they contain metal

o8ides and carbon that discolour porcelain.

'oisten the pro8imal contact areas before removing the completed build

up from the cast to reduce the risk of fracturing that portion of the buildup.

#lace the work on a sagger tray inspect for any e8cess on the unveneered

area and remove if any and dry it for =2(+ min in front of the open muffle and then

proceed with the firing.

After the bake allow the cooling rate recommended by manufacture

otherwise residual stresses result in porcelain fracture during function.

ritically evaluate this first bake !low bis0uebake". If any fissures are

found grind before adding the second layer.

emove all e8cess with ceramic bound stones and use a fle8ible diamond

disk to shape the embrasures. !'oist disk lasts longer".

#rior to the second corrective bake !patch bake" ultrasonically clean the

restoration to remove grinding debris.

#lace the second body and incisal layers on the slightly moistened first

bake. 'ultiple bakes may be needed for an e8tensive prosthesis.

The number of bakes should be as low as possible as multiple fi8ing leads

to devitrification which causes loss of transluscency and decrease in the restoration

fracture resistance.

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#orcelain 5urface treatment :

6nce the desired contours and occlusion have been achieved the

restoration must receive a surface treatment. Three commonly used treatments

include:

(" Eatural or autoglaze

" Applied overglaze

;" #olishing

Auto#la7e :

#orcelain has the ability to glaze itself when held at its fusing range under

air for (24 min. 'any ceramists prefer this treatment as they feel that it preserves the

surface te8ture and character of the porcelain.

A%%lied o2er – #la7e :

It is a low2fusing clear porcelain that is painted on the surface and fired at

a temp. lower than the fusing temp of dentin and enamel porcelains.

#orcelain loses its ability to form a natural glaze after multiple firings an

applied overglaze may be indicated on large restorations that have re0uired numerous

corrections.

$olishin# :

Traditionally polished porcelain has been regarded as a rougher surface

than the glazed porcelain.

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ecent 0ualitative and 0uantitative studies indicate that an acceptable surface

can be obtained by using a commercially available polishing system.

Facobi et al showed polished porcelain to be less destructive of tooth structure in the

opposing arch than glazed porcelain.

#olishing lends itself to use on relatively small areas of ad>ustments such

as pro8imal contacts and limited areas of occlusal contact.

All $orcelain mar#ins :

'ost patients ob>ect to the grayness at the margin associated with metal

ceramic restorations. 5ub2gingival margins are best avoided.

Chen esthetics is of prime importance a collarless metal2ceramic crown

should be considered.

ollarless crowns have a facial margin of porcelain and lingual and

pro8imal margins of metal.

Ad2anta#es :

o &ood esthetics

o 7asy pla0ue removal

isadvantages :

• Inferior marginal adaptation as compared to cast restoration

• 5usceptibility to fracture during handling

• Time consuming and therefore e8pensive

Indications :

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Cherever superior esthetics are needed.

Contraindications :

(. onditions where an e8tremely smooth ( mm wide , shoulder cannot be

prepared.

. 5kill of the technician

Arame"1or5 desi#n for la&ial mar#in :

$arious designs with different facial framework reduction have been suggested.

In general the more the metal reduce the better the esthetics.

emoval of upto mm of labial framework has been shown not to decrease the

fracture resistance of restoration

Methods of fa&rication :

(" #latinum foil matri8

" irect lift or cyanoacrylate resin

;" #orcelain wa8

$atient foil matri4 tech :

#atient foil is burnished onto the facial portion where the porcelain margin

is to be placed. #lace the coping onto the casting and stabilize the foil with stickywa8.

Bift the coping along with the foil and spot weld them. After the porcelain build2up

staining and glazing the foil is removed prior to cementation.

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/irect"Lift : ie is coated with cyanoacylate resin and special shoulder porcelain is

condensed onto the die at the facial margin of the opa0ued coping. The coping along

with the shoulder porcelain is teared out of the die and fired.

$orcelain – 1a4 Techni3ue :

A mi8ture of body porcelain and wa8 != : ( by weight" is applied to the die

for the final adaptation of the porcelain labial margin.

The techni0ue involves application of the porcelain wa8 to the cervical

shoulder of the opa0ued coping and firing in the conventional manner.

'etal2ceramic restorations with e8cellent appearance and good mech prop

are obtainable if the techni0ues of metal preparation framework design porcelain

manipulation drying and fi8ing are carefully followed.

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+ull 2eener cro1n

/E+INITION

• Accordin# to the 6lossary of $rosthodontic terms – 9

%ull veneer crown is described as “A restoration that covers all the coronal tooth

surfaces (Mesial, Distal, Facial, Lingual and Occlusal)”

• $ORCELAIN +SE/ TO METAL CRON

Indications

6n teeth that re0uire complete coverage and where significant aesthetic demands

are placed

If all ceramic crown is contraindicated . . . . . . . .

Ad2anta#es

• 5uperior esthetics as compared to cast metal crowns

• #osterior porcelain fused to metal crown preparation

• Bab knife with no. < blade

• 5ilicone putty with accelerator

• 9andpiece

• %lat ended tapered diamond

• 5hort needle diamond

• Torpedo diamond

• Torpedo bur

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• adial fissure bur

• 3iangle chisel

#7#AATI6E

5ilicone putty is adapted to the facial lingual and occlusal surfaces of the tooth to be

prepared.

Occlusal reduction

• ound end tapered diamond is used to give depth orientation grooves. In the

areas of ceramic coverage reduction should be (.< to mm

epth orientation grooves for functional cusp bevel is provided on the lingual

inclines of the ma8illary lingual cusps and facial inclines of the mandibular

facial cusps. they provide a uniform bulk of restorative material. the depth is (.<

mm if covered by metal and mm if veneered by porcelain.

A4ial reduction

ut three vertical grooves in the occlusal portion of the facial surface.

These are placed with full diameter of the instrument fading out in the area where the

facial surface is most curved. Eow align the bur along the gingival component and

place atleast more grooves near the line angles of the tooth. the tip of diamond

should be supragingival at this time. Eow remove the facial surface with the flat end

tapered diamond. If facial reduction is less than(.mm for a base metal crown or

(.4mm for a noble metal ceramic crown . . . . . . . . .

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At the lingual most e8tension of the pro8imal contact the transition from a

deeper facial reduction to the relative shallower lingual reduction results in vertical

wall or PwingQ of tooth structure.

• The facial silicone and midsaggital silicone putty inde8 shows uniform

reduction on the facial surface and occlusogingivally.

All Ceramic Cro1n

IN/ICATIONS

• In areas with a high esthetic re0uirement where a more conservative restoration

will be inade0uate.

CONTRAIN/ICATIONS

• Chere a more conservative restoration can be used

• Eot indicated in molar tooth because of increased occlusal load and reduced

aesthetic demand.

A/;ANTA6ES

• 5uperior aesthetics

/ISA/;ANTA6ES

• educed strength

• #ro8imal and lingual reduction are less conservative

• Eot effective as retainers for fi8ed partial dentures

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• Cear has been observed on the natural teeth opposing all ceramic restoration

• If inade0uate tooth preparation goes uncorrected can result in fracture.

All ceramic cro1n tooth %re%aration

• A combination of facial and lingual inde8 is made by adapting silicone putty to the

facial lingual and occlusal surface of the posterior teeth.

• This will provide an accurate reference for both facial and lingual reduction

6BH5AB 7HTI6E

• Hse large round end tapered diamond to place depth orientation grooves on the

occlusal surface.

• The final occlusal preparation should be between (.<mm to mm.

•

• emove the tooth structure remaining between the grooves following inclined

planes of occlusal surface.

•

• Hse the same round ended tapered diamond to produce depth orientation grooves

for the functional cusp bevel

• reate the functional cusp bevel to insure that the facial incline of the facial cusp

will have the same porcelain thickness as the lingual incline..

• heck the occlusal reduction by asking the patient to bite on a (.<mm leaf of

thickness gauge

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• 'ake depth orientation grooves on the facial and lingual surfaces to insure that

ade0uate reduction of tooth with a minimum thickness of (mm at gingival finish

line

• (.<mm at slightly less at mid2crown

• emove the remaining tooth structure between the grooves with the help of large

round end tapered diamond so that shoulder with a rounded internal line angle

can be formed.

• Eow use this short needle diamond to begin the pro8imal a8ial reduction.

• As more space is created the needle diamond can be brushed across to produce

more reduction.

• Hse round end tapered diamond to blend the pro8imal a8ial reduction and

shoulder of facial and lingual surfaces.

• %inishing of preparation2 round end tapered diamond bur..

COMMON ERRORS IN TOOTH $RE$ARATION

• Insufficient occlusal reduction

• Back of uniform reduction on buccal surface comprising esthetics

• 'inimal a8ial reduction on the buccal and lingual surfaces

• 6ver reduction of teeth and violation of biologic width

• 78cessive taper of pro8imal surfaces

• $ariable shoulder width

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• Hndercuts on the distolingual surface of the preparation ough tooth preparations.

CONCLSIONCONCLSION

The removal of all morphologic form of the tooth is a radical treatment

and restoring it properly can be difficult.

The full veneer crown is a restoration that replaces lost tooth structure and

imparts some measure of structural support to the tooth.

9ence one must be able to >udge correctly the type of restoration re0uired

for each individual tooth and try to follow the guidelines for the respective tooth

preparation.

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References"

osensteil !++(" ontemporary %i8ed #rosthodontics ;rd edition 'osby

5hillingburg!()-(" %undamentals of %i8ed #rosthodontics nd edition

Luintessence

Tylman !()-)" Theory and #ractice of %i8ed #rosthodontics. -th edition

Ishiyaku 7uroAmerica Inc

5hillingburg !()-*" %undamentals of Tooth #reparation for ast metal and

#orcelain restoration. Luintessence #ub. o.

.F.&oodacre ,designing tooth preparation for optimal success.EA

++4?4-:;<)2;-<.

%.'.3lair .C.Cassell F.&.5teele , #reparation of full veneer crowns. 3F

++?():<=(2<*(

9erbert T. 5hillingburg 5umiya 9obo onald C. %isher. #reparation design

and margin distortion in #orcelain2fused2to2metal restorations. F# ()*;? ):

*=2-4

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C.A.1ent 9.T.5hillinberg '.&.uncanson. Taper of clinical preparation for

cast restoration. Luintessence international ()--?():;;)2;4<

avid B. 1oth , %ull crown restoration and gingival inflamation in a

controlled population.F# ()-?4-:=-(2=-<

#. F. 3. Beempoel B. '. Beemens #. A. 5noek '.A.$ant 9off , The

onvergence Angle 6f Tooth #reparation %or omplete rowns. F#

()-*?<-:4(424(=

F..&avelis F..'orency 7..iley .3.5ozio2 The effect of various finish

line preparations on the marginal seal and occlusal seat of full crown

preparation. F# ()-(?4<:(;-2(4<

full Veener Crown

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