Articles of Interest
Here are a selection of articles written by Dr M K
Vasant, which were published in professional dental journals.
PORCELAINS
- NEW TRICK FOR AN OLD GAME!
CPD Dentistry 2003; 4(2):27-34 |
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Dr Manny
Vasant MBE
BDS MGDS RCS (Eng) MGDS RCS(Ed) FFGDP(UK)
FDS RCS(Ed) Specialist in Prosthodontics
Dr Sandip Popat
BDS (Lond) LDS RCS (Eng) FDS RCPS (Glasg)
MClinDent (prosthodontics) MRD RCS (Eng)
Specialist in Prosthodontics
After reading this paper you should:
- Be familiar with
the different porcelain system used in
dentistry.
- Understand which
system may be the most appropriate far
given clinical situations.
- Be better informed
when communicating with the technical
members of the dental team.
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Abstract
Dentists today have a plethora of porcelain systems
to choose from. New systems are being introduced all
the time and the discerning practitioner should be able
to make an informed choice in conjunction with his/her
dental technician.
This article aims to discuss the concept behind the
available porcelain systems very simply, by dividing
extracoronal restorations into the core material (which
has a high degree of variation amongst the systems available),
and the surface or veneer (which varies little between
the systems). It aims to explain the chemical composition
of the material in question since many of the systems
available on the market today are merely slight variations
of one another. These differences, however, may influence
their suitability in a particular clinical situation.
An understanding of these subtle differences will improve
the clinical care and may impact upon the cost of a
given restoration, both biological and financial!
It should therefore form a part of clinical governance.
Needless to say, the choice should also be based on
the degree of technical support available locally.
Keywords
Core, surface layer, porcelains, feldspathic
porcelains, leucite, low fusing ceramics.
Introduction
In essence a full coronal tooth - coloured
restoration is composed of a sub-structure, or 'core',
and a surface layer or veneer. In a given restoration,
an attempt is made to mimic the function and appearance
of dentine in the 'core', and the function and appearance
of enamel in the surface layer. Of all the available
restorative materials, porcelain has over the years
led the way in mimicking natural tooth aesthetics, albeit
with considerable limitations.
Porcelain is a specific type of ceramic. It is made
up of white clay (kaolin), quartz and feldspar. The
ingredients are pulverised, blended, shaped and finally
baked. It is essentially the same material as "white
ware" used in industry for the construction of
tiles, sanitary ware etc. Historically, Pierre Fauchard
was credited with recognising the potential of porcelain
enamels and initiating research into porcelains in order
to imitate the colour of teeth and gingivae.1
Evolution of Ceramics
Aluminous Porcelains
In the 1950s and 1960s great improvements were made
in the strength of porcelain, following the addition
of alumina. These became known as Aluminous Porcelains.
Much early work on prefritting alumina-glass composites
was described by John McLean. However, whilst the addition
of aluminium oxide increased the strength, the presence
of this second phase reduced the translucency of the
porcelains, and thus reduced the aesthetics. This therefore
limits the use of the alumina-reinforcement to the construction
of the internal core to support the more translucent
feldspathic porcelain.
Vacuum Fired Porcelain
The introduction of vacuum-fired porcelains
in the 1960's led to a great improvement in the aesthetics
due to the reduction in internal porosity. Much of the
research and developments over the next 20 years or
so were directed at improving the strength and marginal
integrity.
Leucite Reinforcement (Figure
1)
In the 1950's the addition of leucite (crystals
of a potash-alumina-silica complex) increased the
strength of porcelain further by limiting crack
propagation. Unfortunately, the leucite increases
the potential for wear of opposing teeth and also
reduces opalescence. The proportion of the crystals
varies with each system and to counteract the reduction
in opalescence with the increased leucite content,
blue pigments are added to the system. A new wave
of ceramics has appeared since the 1980s aiming
to further improve the aesthetics and address one
of the most important drawbacks of porcelains -
the potential for catastrophic failure. |
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Figure
1:
Leucite crystals |
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The increase in coefficient of thermal expansion also
enabled these porcelains to be fused to certain gold
alloys to form complete crowns and metal ceramic bridges.
Figure
2:
Illustration of a “self-healing”
effect of hydrothermal ceramic due to the
build up of Si-OH layer on the surface which
seals micro flaws continuously.
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Figure
3:
This surface structure of regular dental ceramics
before and after hydrolytic testing showing
increased surface flaws. (Figures 2 and 3
reproduced from DUCERA dental GmhH page 20) |
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Chemical surface treatments to help reduce
catastrophic failure
Ionic exchange strengthening (ion stuffing)
is a process that creates a layer of surface compressive
stress by exchanging the smaller glass modifying ions
with large ones. During this process, the smaller sodium
ions are replaced with larger potassium ions. Potassium
ions are squeezed into the spaces formerly occupied
by sodium ions. The larger ions enter the glass or porcelain
by diffusional exchange at an elevated temperature from
a molten salt bath. These larger ions produce atomic
crowding thus surface compression. Modification of surface
chemistry also causes a reduction in thermal contraction,
which results in surface compression.
Commercially available pastes such as Ceramicoat and
Tuffcoat have eliminated the use of hazardous salt baths
and simplified the process.
These porcelains have hydro thermally introduced OH-groups
and fuse at lower temperatures hence called low fusing
ceramics (LFC) (Figures 2 and 3). As explained conventional
porcelains contain surface micro flaws or develop them
after exposure to the oral environment. These flaws
can increase over a periods of time, resulting in surface
discolorations and flexural strength reduction. In contrast,
it is claimed that hydrothermal porcelain (which are
low fusing Ceramics) have a potential for "self
healing" and are more resistant to fracture due
to their continuing ability to form the hydrolytic layer
(Si-OH) in the mouth!
2,3,4
This increase in strength results from a surface layer
with many OH-groups, built through alkali and hydroxyl
exchange. The surface layer is thus more flexible and
is claimed to "heal" surface flaws.
Thermal treatments
Thermal tempering has also been explored to
strengthen dental porcelains. This extends much deeper
than surface chemical treatments, but controlling cooling
rates for complex objects such as crowns makes it impractical
to use.
Despite all the latest developments to improve strength,
most porcelain systems rely on the conventional feldspathic
porcelains to reproduce the outer surface for good aesthetics.
For the purposes of this article, it is convenient to
discuss the core materials and the surface layer materials
separately.
Core materials
A veneer of surface feldspathic porcelain
is bonded to a core. As discussed, feldspathic porcelain
is a very brital material. The use of a 'stronger' core
will help minimise catastrophic failure.
- Natural Tooth as a core e.g. dentine in the case
of resin bonded crowns or enamel in porcelain veneers
- Metals e.g. porcelain fused to metal crowns
- Metal free or ceramic cores e.g. alumina oxide,
lithium disilicate, tetrasilicate glass or infused
ceramics etc
We shall discuss each of these in turn
1. Natural Tooth Core
Bonding to dentine has been shown to increase the strength
of the porcelains.5
Effectively, natural dentine core is used as the inner
core to which feldspathic porcelain is bonded instead
of artificial substitutes. This enables the operator
to prepare the tooth more conservatively as no space
is required for a synthetic core (Figures 4,5,6,7).
In-vitro testing of natural teeth restored with dentine-bonded
crowns has yielded fracture strengths similar to those
of intact teeth.5 In a retrospective study of 25 posterior
inlays, a much higher failure rate was noted when light
cured resin, rather than dual cured resincements were
used.6
Figure
4:
Pre-operative
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Figure
5:
Preparations Dentine Bonded Crns (mirror image) |
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Figure
6:
Dentine bonded (feldspathic porcelain) crowns |
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Figure
7:
Pos- operative view |
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2. Metal Core
This is the most commonly used method in clinical practice
whereby feldspathic porcelain is bonded onto a cast metal
framework. For production of multiple units, this is still
the most reliable choice.7
The metal ceramic crown remains the workhorse in restorative
dentistry and has rightfully earned the nickname of-"
a dental brick".
Grey Metal Cores
A metal framework may be chosen from precious,
semi-precious or non-precious alloys. Each has its advantages.
The mechanism of bonding is facilitated by matched thermal
expansions of the two materials, Van der Waal forces and
a chemical union between various metallic oxides. Despite
the advent of other systems described, the strength, longevity
and performance of this material is undisputed. The grey
metal base however, affects the light transmission through
the porcelain and creates metal ion discolouration.
Yellow Gold (and Hydrothermal Bonding Dental Ceramics)
More recently yellow alloys have been introduced
which are capable of bonding to hydrothermal bonding ceramic
eg Duceragold. The ceramic used here is claimed to have
better opposing tooth wear properties due to reduced size
of leucite crystals and more homogeneous material structure
which is more transparent. Furthermore it is claimed that
"healing" hydroxyl layer in the mouth leads
to strength increase e.g Duceram LFC (low fusing ceramic)
discussed later on.
The yellow gold provides a better colour match in certain
situations, particularly where yellow (Vita A and B) shades
are to be reproduced. The yellow gold also seems to have
more acceptable appearance against gingival tissues.
MacLean Sced and Renaissance Crown
These techniques are an alternative to the traditional
metal ceramic crown.
The McLean Sced Crown involves the use of twin platinum
foils in the construction of the crown using the conventional
Aluminous Jacket Crown technique. The inner platinum matrix,
which eventually forms part of the crown, is tin plated
and oxidised to facilitate bonding to the porcelain. This
procedure appears to eliminate open surface defects from
which tensile failures may originate. Minassian (1978)
tested porcelain crowns using the twinfoil platinum bonded
technique and found that there was a statistically significant
increase in the fracture strength from 184N for standard
aluminous porcelain crowns to 286N for the bonded platinum
crowns.15
The second technique is similar but the grey platinum
foil is replaced with gold (Captek System, Shottlander).
This involves the adaptation of a wax strip impregnated
with gold-platinum-palladium powdered alloy, on to a refractory
die. Firing produces a rigid porous layer, which then
is in-filled with gold from a second wax strip by capillary
action. This is then veneered with porcelain. It is claimed
that this improves marginal fit, gives better aesthetics
and better biocompatability compared to metal ceramic
crowns. The improved biocompatability may be due to relatively
non-oxidising alloy that is used. Juntavee et al have
reported shear bond strength values similar to that of
metal ceramic crowns.8
Procera-Alltitan
The biocomatability of titanium makes it a very desirable
material for restorations. Computer Aided Design - Computer
Aided Manufacture (CAD-CAM) technology is used in the
laboratory (as opposed to scanning the tooth at the chairside)
so that the clinician only needs to supply a conventional
impression. A die made from this impression is scanned
by the local laboratory and transmitted to a central laboratory
in Sweden via a modem for the machining of the core. The
external contours of the individual titanium cores for
Procera bridges are milled and graphite rods create the
fitting surface by a spark erosion process. Individual
components of the bridge are then welded by laser before
the addition of special porcelains to veneer on to the
outer surface. In this way the application of the CAD-CAM
technology, coupled with traditional spark erosion, is
said to enable the production of accurately fitting titanium
based restorations, whilst eliminating the need for costly
and technically difficult casting procedures. The deficiencies
of this system appear to be those inherent with processing
titanium at elevated temperatures. The low fusing ceramics
alleviate this problem to a certain extent, but do not
eliminate it entirely.9
3. Metal Free (Ceramic Cores)
Metal free ceramics can be achieved in various ways. The
conventional technique of "hydroplastic forms”
whereby the powder and the liquid are mixed and fired
several times still has a place. In fact, many of the
newer techniques only produce the cores in a different
way and the surface porcelain is still added in the conventional
way. The newer techniques include thermal spraying, castable
ceramics, pressable, infused and machinable ceramics.
These are considered in more detail below.
Conventional Powder Slurry
Optec HSP (JenericfPentron)
Due to the increased leucite content, these porcelain
have a higher compressive strength (146 MPa) compared
to conventional feldspathic porcelain. As would be expected,
in vitro studies have shown a greater potential for wear
of the opposing tooth. A special semi-permeable die material
is necessary for the processing but no other special laboratory
equipment is necessary.
Fortress (Mirage Dental System)
It is claimed that this reinforced ceramic has a refractive
index of the surrounding glass matrix virtually identical
to that of the leucite crystals. This is thought to result
in better aesthetics as it enhances the natural look of
the finished restoration.
Hi-Ceram (Vita Zahnfabrik)
This is a dispersion-strengthened dental porcelain
il which over 50% alumina crystals are employed as a reinforcing
phase. The volume of the reinforcing phase is increased
by means of a specific particle size distribution without
sacrificing the aesthetics of the restoration or the ease
of manipulation of the powder slurry.16
Mirage II ( Mirage Dental Systems)
The increased strength of this material is attributed
to the zirconia fibres present.
Ducera Metal Ceramic
Ducera MC is a high leucite containing core material which
is used on a refractory die and baked at 9300 C. This
is then layered with Ducera LFC, discussed below, which
has better tooth wear properties.
This material must not be confused with the third type
of Duceragold which is a ceramic layered onto a yellow"
gold alloy. Duceragold is a 2-phased hydrothermal bonding
ceramic. The homogeneity of Ducergold lies between Ducera
LFC and Ducera MC.
Thermal Spray Technique
Techceram, a British development, uses a thin (0.1-1mm)
alumina core base produced by thermal spray technique.
This results in a very dense core (of 80-90 %) which is
then subjected to sintering to 11700 C. The surface is
then constructed using the conventional porcelain build
up technique with specifically developed porcelains. The
inner fitting surface has microscopic roughness conducive
to bonding and according to the manufacturers does not
require silane coupling or etching.
Castable Ceramics:
Dicor is a tetrasilicic micaglass ceramic. As the
name suggests it is the product of a marriage between
Dentsply International (D1) and Corning ware New York
(COR). The material is translucent and is claimed to have
similar wear properties to that of the tooth. The laboratory
technique required is similar to that of producing a crown
using the lost wax process (Figures 8 and 9).
Figure
8:
Wax pattern for Dicor |
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Figure
9:
Cast Dicor glass |
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Surface glaze and colourants are necessary to reproduce
the colour. Unfortunately the wear of the glaze
poses problems. Hence Dicor Plus- feldspathic porcelain
is recommended on the surface after cutting back
the surface layer. This unfortunately negates the
alleged beneficial wear characteristics of the core
material. Furthermore, although the flexural strengths
yielded values of 240 MPa compared to 116 MPa for
Alumina reinforced core porcelain, the force required
to break DICOR crowns is not significantly different
to the latter.8 The core can be etched for bonding
to tooth structure. However, theoretically such
treatments can affect the strength of the materials,
either by weakening the ceramic or by enlarging
the surface flaws. However, a recent study by Yen
et al by using ammonium bifluoride etchant showed
no significant change in the strength.10
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Figure
10:
Empress furnace |
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Figure
11:
Empress completed crowns |
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Figure
12:
Empress 2 Lithium disilicate core |
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Pressable Ceramics:
Empress (Ivoclar Vivadent) and Optec OPC (Jeneric/Pentron)
utilises a technique where by a wax pattern of the proposed
restoration is invested in a phosphate-bonded material.
Following the burnout procedure, this leucite reinforced
material is pressed into the mould using a special furnace
(Figure 10) Final shading of the restoration is done by
surface stains or by cutting back to apply Empress leucite
reinforced porcelain in powder and slurry form depending
on the aesthetic requirements (Figure 11). The core material
is shaded, translucent and etchable for bonding to the
tooth. Flexural strength of the material ranges from 126-165
MPa. These materials are suitable for individual crowns.
It is proposed that the marginal fit of these restorations
will be superior due to the technique used.
More recendy IPS Empress 2 layering ceramic has been developed
with flexural strength greater than 350 MPa which is suitable
for 3 unit bridges up to second premolar as the final
retainer. This material is quite different to the original
Empress. Empress 2 consists of lithium disilicate glass
ceramic as framework material (Figure 12) which is then
coated with sintered glass ceramic. Their crystalline
structure consists of only apatite crystals (fluoroapatite)
unlike the conventional layering materials whose crystalline
phase consists of leucite. The antagonist abrasion is
claimed to be superior due to the apatite crystalS.11
Infiltrated Ceramics:
In-Ceram: The core is formed from slurries of the fine
alumina powder and water ("slip") and is applied
to an absorbent refractory matrix, dried and lightly sintered
to produce a porous core (Figure 13). The residual pores
are then infused with molten glass by capillary action-
hence the name In-Ceram (Figure 14). In-Ceram is now generally
considered to be stronger than conventional porcelains.10
The resultant structure is the most efficient strengthening
technique and gives a flexural strength value of up to
450 MPa. Unfortunately, the high degree of the alumina
content makes it difficult to etch with hydrofluoric acid
for bonding to tooth structure. The restoration can be
bonded with Panavia 21TC after sandblasting. It is also
more opaque than natural tooth.
Figure
13:
In-Ceram Slip |
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Figure
14:
In-ceram glass infusion |
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In-Ceram Spinel: This modification substitutes magnesium
aluminosilicate for the aluminium oxide to provide for
better translucency. It also avoids the yellow opacity
typical of the In-Ceram. The yellow opacity may hinder
the reproduction of the grey shades (e.g. Vita C and D).
The abrasiveness of this material is the same as that
of conventional feldspathic porcelains.
In-Ceram Zirconia: Compared with In-Ceram Alumina, this
AL2 O3-ZrO2 ceramic material has improved mechanical characteristics.
Due to its increased opacity, however, there are aesthetic
limitations. It may be a suitable material for posterior
bridges and posts and cores. As this Zirconia technique
is still at its developmental stages, it is not yet available
for extensive clinical study and application.
Machinable Ceramics:
CAD CAM - Computer assisted design and computer assisted
manufacturing systems are available using various techniques.
The Cerec System (Siemens) is the longest established
CAD CAM system. In this method the preparation is scanned
and the restoration milled from a block of ceramic material.
Cerec have now introduced finer grained porcelain blocks
to reduce opposing tooth wear as well as a wider range
of tooth shades. They have also converted to an electric
turbine to give better cutting control. Despite these
improvements Siervo et al (1994) have shown that the marginal
gap is two or three times at approximal margins and occlusal
interfaces compared to Celay or sintered inlays.13
The inlay can be characterised with surface stains.
Celay (Mikrona Switzerland):
The original system used a pre-formed porcelain block
similar to above system except that the diamond cutting
wheel is steered by a pantographic arm with an attached
probe which is guided by a preformed acrylic or wax pattern.
The technique is similar to the established key cutting
principle in industry. The modified technique uses an
In-Ceram block manufactured for the purpose to produce
a porous structure. The latter is then infused with glass
akin to In-Ceram restorations discussed above.
Figure
15:
Crown preparations for Procera |
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Figure
16:
Procera crowns |
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Figure
17:
Procera core 3 unit bridge |
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Procera (Nobel Biocare):
These are all ceramic individual restorations and
comprise o f a densely sintered alumina core. A
die is first created by a computer controlled milling
process. Aluminium oxide is compacted onto the die
to form the inner surface. The outer coping is then
milled before the sintering is carried out. Feldspathic
porcelain is added to complete the anatomic form
and final appearance. This material is not indicated
for inlays, onlays or veneers at the present time. |
Surface or veneering
porcelains
It can be seen that vast majority of the systems still
rely on the feldspathic porcelains to complete the outer
layer of the crowns. This is true of Dicor, Procera, Inceram,
Empress (but not Empress 2). Posterior Dicor and Empress.
Restorations could, in theory, be completed and simply
stained without the need to layer with feldspathic porcelains
if one is able to compromise the colour match slightly
and accept the wear of the glaze in the long term. The
newer layering or veneer porcelains are discussed below.
It should be noted that manufacturers insist that in order
to use these porcelains, the recommended core material
for the system be used.
Duceram LFG (Degussa) or Finesse (Dentsply)
This is relatively new porcelain referred to as a "hydrothermal
low fusing ceramic" (LFC). It is composed of an amorphous
glass containing hydroxyl ions. It is claimed to have
higher flexural strengths, greater fracture resistance
and lower hardness than feldspathic porcelain (hence less
abrasion). The inner core for the crowns is made from
Duceram Metal Cerarnic- a leucite containing porcelain
placed on a refractory dye and baked at 9300C. The Duceram
LFC on the surface is subsequently baked at 6600C and
can be surface characterised. There are no clinical studies
substantiating these claims but in theory the material
sounds promising. It has been suggested that the material
is "self healing" as the potential cracks self-repair
within the material. The wear rate is similar to that
of the natural tooth. There are also some reports that
the polishing of the surface with rubber wheels (e.g.
Brassler polishing wheels) generates enough heat to "heal"
the micro-cracks thus reducing the potential for crack
propagation.
Finesse
This is also a low fusing ceramic which can be used with
many high-gold alloys as well. In addition to the standard
shades, it is available in two new shades AO and BO to
match bleached white teeth.
LFCs do not etch very well and therefore cannot be used
alone for bonded restorations. For this application, a
thin coping of conventional porcelain must first be fired.
Duceragold (Degussa)
This porcelain system is designed to veneer specially
fabricated yellow gold metal sub-structure made by the
same manufacturers. It is claimed to have better wear
characteristics due to reduced size of leucite crystals
and the more homogeneous material structure which is more
transparent.3 Furthermore it is claimed that "healing"
hydroxyl layer in oral situation leads to strength increase
as with Duceram LFC (low fusing ceramic) discussed above.
IPS Empress 2- sintered glass ceramic
The crystalline structure consists of only apatit
crystals (fluoroapatite) unlike the conventional layerin
materials whose crystalline phase consists of leucite.
From a materials point of view, it is quite different
from the conventional IPS Empress layering materials,
whOse crystalline phase consists of leucite. It is thought
to have superior wear properties due to its apatite crystals.
Summary
The choice of the ceramic system is dictated
by the presenting clinical situation.
- A knowledge of the properties of the various porcelains
is essential in making an informed choice and providing
a detailed laboratory prescription.
- In choosing the material, the clinician should
select the most appropriate material with function,
aesthetics and longevity in mind. It is essential
to remember to assess adjacent restorations to those
being replaced and to use similar materials that are
in harmony. For example, if one crown is all ceramic
and a porcelain fused to metal placed adjacent to
it, the aesthetics may be extremely difficult to match.
References
- Kelly R. Ccramics in Dentistry: Historical roots
and current perspectives Journal of Prosthetic Dentistry
1996; 75: 18-29
- Seghi RR, Crispin BC. Mito W. The effect of ion
exchange on flexural strength of feldspathic porcelains.
International Journal of Prosthodontics 1990; 4: 130-134
- Ottrnar Koma .Documentation Duccram Dental GmbH.Rodheimer
Strasse 7. 611191 Rosbach
- Anusavice KJ. Shen C. Vermost B,Chow B Strengthening
of porcelain by ion exchange subsequent to thermal
tempering. Dental Materials 1992; 8: 149-152
- Burke FJT, Watts DC. Fracture resistance of teeth
restored with dentine-bonded crowns. Quintessence
International 1994; 25: 335-340
- Isidor F, Brondum K. A clinical evaluation of porcelain
inlays. Journal of Prosthetic Dentistry 1995; 74:
40-44
- CRA Newsletter Volume 25 Issue 3
- Juntavee N, Giordano R, Nathanson D. Porcelain shear
bond strength to a new ceramo-metal system. Journal
of Dental Research 1995; 74: 159
- Walter M, Boenig K, Reppel P-D. Clinical performance
of machined titanium restorations. Journal of Dentistry
1994; 22: 346-348
- Yen TW, Blackman RB, Baez FJ. Effect of acid etching
on the flexur; strength of a feldspathic porcelain
and a castable glass ceramic. . journal. Prosthetic
Dentistry 1993; 70: 224-233
- Seghi RR, Denry IL, Rosentiel SF. Relative &acture
toughness and hardness
of new dental ceramics. Journal of Prosthetic Dentistry
1995; 74 145-150
- Scientific Documentation IPS Empress Meridian South
Liecester LE3 2WY
- Siervo S,Parnpalone A, Siervo P, Siervo R. Where
is the gap? Machinable ceramic systems and conventional
laboratory restorations at a glancc Quintessence International
1994; 25: 773-779
- Mclean JW and Sced, I R. The Bonded Alumina Corwn.
The bondin of platinum to aluminous dental porcelain
using tin-oxide coatings. Australian Dentaljournal.
1976; 21:119
- Minassian, R(1978): An investigation into some
factors affecting the strength of porcelain jacket
crowns. MDS Thesis, University of Bristol.
- Hondrum SO. A review of strength properties of
dental ceramics.Journal of Prosthetic Dentistry 1992;
67: 859-864
Further reading
- Qualtrough A and Piddock V Recent Advances in Ceramic
Materials and Systems for Dental Restorations March
Dental Update 1999; March: 65-72
- Rosenblum M A review of all ceramic restorations
Journal of the American Dental Association 1997; 128:
- Dickinson AJG. A comparison of the Cerestore and
Dicor systems. In:Proceedings of International Symposium.
Alternatives to the use of traditional Porcelain.
Amsterdam, The Netherlands 1986:1-24
- Dental Ceramics - Proceedings of the First International
Symposium 01 Ceramics John McLean Quintessence Books
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