Dental Porcelain
Porcelain
1.
Composition and Properties of Dental Porcelain
2.
Denture Teeth
5.
First use of porcelain: for denture teeth
Porcelain-jacket crowns: all-porcelain jacket crowns specialized for restoring
anterior teeth that bore little to no occlusion
Porcelain-fused-to-metal crowns: biggest role for porcelain, most common application of
porcelain
Definition of porcelain: a ceramic material and is by natural brittle, transluscent and has low corrosion and
wear, and these properties are in its favor
brittleness of the procelain limits the porcelain's usefulness, kaya its required to
be fused to a metal substructure to reinforce it
Composition
used in powder form in dental
laboratory
Manufactured from three ingredients: Silica (SiO₂), Feldspat (K₂O - Al₂O₃ - 6SiO₂),
Alumina (Al₂O₃)
Classified according to their fusing
temperatures:
Heated together with fluxes (Sodium Carbonate or Lithium Carbonate) making the raw ingredients to form a non-cystalline glass, that melts at relatively low temperature,
with Leucite (forms in the glass under certain conditions)
therefore, Dental Porcelains is a matrix of low-melting glass with leucite
crystals embedded in it
Refired with metal oxides to add color necessary to match
the teeth
After cooling, it is ground to fine
powder
Manufacturing process of Powdered porcelain
High Fusing: 1288-1371°C
Low Fusing: 871-1066°C
majority of dental restoration are made
Porcelain-fused-to-metal restoration
3.
Properties
Most important properties of dental
porcelains
Transverse Strength
Coefficient of thermal expansion
Color
Ability of a porcelain bar to resist fracture
Compressive strength and tensile strength combined
High Transverse Strength: Porcelain will have better
resistance to fracture
Range: 56 - 446 MpA
amount of expansion of porcelain when its heated/shrinkage when cooled
high coefficient of thermal expansion = expands a large amount when heated
most porcelains: about 12 x 10⁻⁶ /°C
important when the porcelain is bonded to metal or another porcelain
expansion of both metal and porcelain must be properly matched or the porcelain
will fracture when the crown cools from the temperature
for matching the tooth structure
can be expressed in terms of:
Hue
Value
Chroma
basic color
intensity of the color, measure of its purity (blue to intense blue)
amount of
grayness
Matching porcelain color: critical to a successful porcelain restoration
primarily involves matching the value, since hue and chroma are similar mostly on teeth
accomplished by shade tabs constructed for available shades of porcelain, placed adjacent to the
tooth to be matched
Light source: major factor that influences how a color appear to an observer; therefore critical for
proper matching
therefore, experienced dentist will take the shade under lighting conditions that are
as close as possible to those in which the patient will reside
Translucency, surface reflectance, and fluorescence: other color-related properties of porcelain; important to the esthetics of a porcelain in dental
restoration
most common type of porcelain dental restoration
consist of several layers of different types of porcelain fused chemically to a metal substructure, which supports the porcelain and
makes it more durable under the load of oral forces
have sufficient strength to be used in long-span bridges or in restorations for posterion teeth
Porcelain-metal Bonding
Fundamental property of porcelain that makes the PFM resto possible: Chemical Bond between the
porcelain and the metal
Without the bonding, the porcelain would rapidly fail in the mouth because its brittle (would fracture in
service)
Alloys for PFM restorations are often specifically formulated so that an oxide will form (oxide is needed for the porcelain and
metal to bond)
Fracture, debonding, or porcelain from a PFM resto: big clinical problem
creates unesthetic blemish in an area where esthetics are generally important, and repair often requires
replacement of an expensive restoration / significant trauma
Failure of porcelain-metal bond can result from:
chemical reaction occurs between metal and porcelain's oxide layer when porcelain is fired
onto the alloy surface
no distinct boundaries between metal, metal oxide, and the porcelain, but rather a gradual
blend of the three phases
metal has inadequte oxide layer, and metal will only bond weakly
brittle oxide can be too thick, and thickness will increase the risk of failure
Crack formation at the interface of metal and porcelain
most common failure
Most desirable situation to match porcelain and metal's thermal expansion: porcelain having lesser coefficient of thermal expansion than metal
metal will expand more during heating and contract during
cooling
Fabrication of PFM restorations
Fabrication process
esthetic restoration of porcelain: an art, requiring many years of practice
Casting and recontouring the metal sufaces from a dental casting alloy
Degassing: removes impurities from the alloy surface and promotes formation of an oxide layer
on the metal
Apply Opaquing porcelain to hide the color of the metal substructure
opaque powder + water > paste mixture > paste applied to metal and condensed
condensed opaque paste is fired in a porcelain oven
sintering: particles of porcelain melt together at their edges
during firing
gingival and incisal porcelains are applied
gingival porcelains: gingival one half of the crown
incisal porcelains: incisal thirds of the crown
glaze: final porcelain layer
stains are sometimes applied to provide individual characteristic of the crown to improve matching
All-ceramic restorations
4.
All-ceramic crowns
made for teeth since early 1900s
advantage of an all ceramic crown: its excellent esthetics
porcelain jacket crowns: earliest types of crowns that were made of low fusing
porcelain
esthetic but were too weak to be used in any load-bearing situations
used primarily on small anterior restorations that had little occlusion as on maxillary lateral insisors (newer all-ceramic crowns are
used for posterior applications)
Strategies
aluminous, magnesia-based porcelains, glass-infiltrated aluminous porcelains
cannot be used for the entire restoration- lacks adequate esthetics for this purpose
use a very strong core porcelain underneath traditional porcelains
use castable or pressed ceramics
how cast ceramics are made: wax pattern was fabricated, invested > casting to ceramic material > ceramming ( casting is crystallized) in an oven for high
strength > apply surface glaze
Have excellence transverse strength
In anoother system: ceramic is injected under pressure into the mold > press-fit to a die of the tooth
Veneers
made from traditional/newer porcelains
to replace only the facial and incisal portions of anterior teeth
Ceramic veneers: used to mask permanent stains or other defects in the natural teeth, generaally fabricated on refactory dies (heat-resistant dies),
bonded to the tooth using resin cement
Inlays
similar to those used for gold-based inlays
intracoronal restoration
used primarily for small restorations of esthetic importance in posterior teeth
Ceramic inlays: fabricated either on a refactory die, or may be fabricated by computer technology, cemented and bonded to the tooth by means of acid etching and resin cements
porcelain has been used for it in many years
it is made by a factory and are not custom made for patients, but molds (variety of standard shades and shapes) are available
the use of porcelain denture teeth has been replaced by
plastic teeth
Plastic teeth are softer and do not tend to wear away opposing
natural teeth
Porcelain teeth > transmit occlusal forces directly to the bony ridge supporting the denture, and the bone tends to resorb in response to the load
Plastic teeth > absorb some of the occlusal force and spare the bone
Other advantages: less breakage, better bonding to the denture base,
easier adjustment
Color Matching: an art and requires a great deal of practice
Subtopic
Medium Fusing: 1093-1260°C
