Classification and Selection Criteria for Dental Ceramics

Classification and Selection Criteria for Dental Ceramics

Classify ceramics. Enumerate its advantages and disadvantages. Explain the mechanism of bonding and describe the laboratory steps involved in fabrication of different ceramics.

Ceramics Classification

Depending on use

Type 1

For denture teeth

(Powders of feldspar, clay and quartz).

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Type 2

For ceramometal applications

(Potassium feldspar and glass).

Type 3

• For all-porcelain restorations
• Porcelain jacket crowns, veneers and inlays
  (Feldspathic dental porcelain with increased amounts of aluminum oxide).

Depending on fusion temperature

• High fusing (1288–1371°C)
• Medium fusing (1093–1260°C)
• Low-fusing (871–1066°C)
• Ultra-low fusing (Below 850°C).

Based on application

• Core porcelain: The basis of porcelain jacket crown must have good mechanical properties.
• Dentine or body porcelain: More translucent than the above, this largely governs the shapes and color of the restoration. Enamel porcelain forms the outer part of the crown and is translucent.

All ceramic restorations

Types

• Conventional powder
  Slurry ceramics
  For Example. Optec HSP, Duceram LFC.
• Castable ceramics
  For Example. Picon, and Dicor Plus.
• Machinable ceramics/Machined Densely Sintered Ceramics
  For Example. Cerec vitablocs Mark I, Mark II, Dicor MGC, Celay.
• Pressable ceramics
  For Example. IPS empress, OpTEC
• Infiltrated ceramics
  For Example. INCERAM.

Ceramics Advantages

• Highly esthetic, can match the tooth color in translucency, color, and intensity
• Special types of porcelain are available that can simulate all colors including gingival tones
• Life-like porcelain is achieved by glazing porcelain
• Glazed porcelain in contact with tissue surface is more biocompatible than metal
• All ceramic restoration has lower failure rates
• Porcelain fused to metal prepared from foil copings uses thin metal copings (0.2–0.3 mm), which can increase bulk of porcelain.

Ceramics Disadvantages

• If there is difference in thermal matching between metal and porcelain, the porcelain fails at the margin
• More reduction of tooth structure to accommodate bulk of porcelain
• Brittle in nature and if surface glaze is removed cracks propagate leading to failure of porcelain
• Porcelain restorations do not give as close a marginal adaptation as metal marginal finish
• All Porcelain has less tensile and shear strength, hence ideal to make a metal coping
• Porcelain shrinks during firing and if porosities are present the overall strength and translucency diminishes
• Ceramics and glasses have tensile strengths that are much lower than their compressive strengths.
• Strength of ceramics reduces to 50% by sandblasting or by etching with hydrofluoric acid solution for 1 minute
• If tensile stresses or sub-microcracks are present on the surface, in presence of moisture may lead to stress corrosion
• Presence of a chemical environment decreases strength of porcelain.

Enamel–Metal Bond

• Mechanical bond
  Porcelain wets the surface of metal.
• Compressive stresses
  Compressive stresses set up during cooling of the sintered porcelain veneer, aid in bonding of porcelain.
• Chemical Bonding.

Mechanical Bond

Microscopic irregularities on metal surface are filled with porcelain; retention of the porcelain veneer by mechanical interlocking is achieved.

Increasing mechanical bond strength

• Grinding or sand blasting
• Roughening by oxidation
• Electrochemical corrosion of the metal by molten glass
• Selective oxidation of grain boundaries
• Etching by acid in pre-treatment for fusing.

Compressive Stresses

• A very small degree of thermal mismatch between metal and porcelain leaves the porcelain in a state of compression
• This increases the bonding between metal and porcelain.

Chemical Bond

True chemical bonding results from electron transfer between the oxygen of the glassy phase of porcelain and an oxidised metal surface.

Increasing chemical bond

• By addition of tin, indium, iron to a noble metal casting alloy
• In nickel–chromium alloy there is an indirect bond between glass and metal through an undissolved oxide formed on the metal surface.

Fabrication of a Ceramic Restoration

Condensation

Fine porcelain powder is mixed with water and condensed into the desired form.

Dense condensation provides

• Lower firing shrinkage
• Less porosity in the fired porcelain.

Methods of condensation

• Vibration:
  • Mild vibration to densely pack the wet powder upon the underlying framework
  • The excess water is blotted with a clean tissue.
• Spatulation:
  • A small spatula is used to apply and smooth the wet porcelain.
• Brush technique:
  • Dry porcelain powder is added to the surface with brush to absorb the water
  • Porcelain must never be allowed to dry out until condensation is complete.

Firing

• The thermochemical reaction is complete during fritting
• Firing is done to fuse the particles of powder together, a process called sintering.

Method

• The condensed porcelain mass is placed in a preheated furnace (approximately 650°C) for 5 minutes
• This removes the remaining water.

Stages in Firing

A low bisque (low biscuit) firing
In which the glass grains have softened and have started to flow.

A medium bisque (medium biscuit) firing
The glass grains have flowed to the extent that the powder particles exhibit complete cohesion.

In high bisque (high biscuit) firing

• The shrinkage is complete and the mass exhibits a smoother surface
• Lesser the firing cycles higher will be the strength and esthetics.

Glazing

Air-fired porcelain cannot be polished.

Application of over glaze/self glaze

• Self-glazing, which was previously fired to a high bisque, is heated rapidly (10–15 minutes) to its fusion temperature
• It is maintained at that temperature for approximately 5 minutes before it is cooled
• Glass grains flow over the surface to form a vitreous layer
• Glazed porcelain is much stronger and reduces crack propagation than the unglazed variety.

Cooling

• Because of the low thermal conductivity of the porcelain, the differential between the thermal dimensional change of the outside and inside can introduce stresses, which embrittle the porcelain
• The ideal cooling method for a porcelain restoration from its firing temperature to room temperature is controversial.

Other Types of Porcelain Fabrications

Cast coping

Requirements

• To be fusible to alloys, the porcelain has to be sufficiently low fusing
• It has to have a coefficient of thermal expansion that is considerably higher than ordinary porcelain
• The alloy should be sag resistant at the firing temperatures of porcelain
• It should be rigid to prevent fracture of porcelain
• There should be no pigmentation reaction produced between the porcelain and the alloy
• Further oxide formed should be soluble in porcelain and provide good wetting of the metal or metal oxide for attachment.

Bonded platinum foil coping

• Makes use of tin oxide coatings on platinum foil
• The esthetics is improved with a thin platinum foil, which allows more room for porcelain
• The bonded foil reduces subsurface porosity and micro cracks in the porcelain and increases the strength of the unit.

Swaged gold alloy foil coping

• Renaissance, by the Williams Gold refining company, is a laminated gold-alloy foil that is delivered to the use in a fluted shape
• This foil shape is swaged onto the die and flame sintered to form a coping
• An interfacial alloy powder is applied and fired, and then coping is veneered with porcelain.

All-ceramic Materials

Conventional powder–slurry ceramics

Available as

• Powders to which water is added to produce a slurry
• Available in various shades and translucencies, with characterizing stains and glazes.

Slurry Ceramics Manufacturing

• Leucite crystals are dispersed in a glassy matrix by controlling their nucleation and crystal growth
• The leucite and glassy matrix fuse together during the baking process
• The build-up and contouring of the crown can be done by using the powder–slurry technique on semi permeable die material.

Slurry Ceramics Advantages

• Greater strength than conventional feldspathic porcelain due its leucite content
• Does not require a core as with aluminous porcelain jacket crowns
• The body and incisal porcelains can be pigmented to provide desired shade and translucency
• Does not require special processing equipment
• These restorations fit accurately.

Slurry Ceramics Disadvantage

Their high leucite content can cause increased wear to opposing teeth.

Other types

Duceram LF

• Hydrothermal low-fusing ceramic
• Composed of an amorphous glass containing hydroxyl ions.

Advantages

• Greater density, higher flexural strength, greater fracture resistance and lower hardness than feldspathic porcelain
• No special laboratory techniques or equipment required.

Uses
For fabrication of ceramic inlays, veneers and full-contour crowns.

Procedure

Base layer:
Duceram metal–ceramic (a leucite-containing porcelain) is placed on a refractory die using powder–slurry techniques and baked at 930°C.

Over base layer:
Duceram LFC is applied using the powder–slurry technique and baked at a relatively low temperature (660°C).

Castable ceramic systems

Available as

• Solid ceramic ingots, which are cast using lost-wax and centrifugal casting technique
• Only one shade is available
• Staining is done to obtain proper shading of the final restoration.

Picon

A polycrystalline glass–ceramic material, in which glass is heat-treated under controlled crystallization.

Procedure

• When solid ceramic ingots are cast at 1350°C, a transparent glass crown is obtained
• This crown is then heat-treated at 1075°C for 10 hours. “Ceramming” causes partial crystallization (55%) of tetra-silic-mica-like crystals.

Castable Ceramic Advantages

• Easier, as it is a lost wax technique
• The transparent crown after heat treatment at 1075°C for 10 hours forms an opaque crown, which increases fracture resistance and strength
• Less abrasive to opposing tooth structure
• For final restoration colorant, stains are baked on the surface of the glass–ceramic material.

Dicor Plus
It is a shaded feldspathic porcelain veneer applied to the Dicor substrate.

Castable Ceramic Disadvantages

• Abrasive to opposing teeth
• Requires a special high-temperature, electric-heated casting unit
• Fracture of the restorations is common.

Machinable ceramics

Available as

• Ceramic ingots in various shades
• Used in computer-aided design and computer-aided manufacturing (CAD-CAM)
• procedures
• The machined restoration are stained and glazed to the desired characterization.

Cerec vitablocs Mark 1

• This is a feldspathic porcelain, used with the Cerec system
• Composition, and properties are the same as feldspathic porcelain used for porcelain-fused-to-metal restorations.

Cerec vitablocs Mark 2

This is a feldspathic porcelain of increased strength with less abrasive wear of the opposing tooth structure.

Dicor MGC

• Contains fluoro-silicic mica crystals in a glass matrix
• It has greater flexural strength than the castable Dicor and Cerec
• Less abrasive wear of the opposing tooth structure than Cerec Mark I.

Celay

• Can be used for CAD-CAM and copy-milling technique
• Identical in physical properties to Cerec Vitablocs Mark II.

Disadvantage

Marginal fit is not good.

Pressable ceramics

Available as

• Ceramic ingots, which are melted at high temperatures and pressed into a mould created by lost-wax technique
• Can be made to full contour, or can be built up using feldspathic porcelain.

IPS Empress

• This is a feldspathic porcelain supplied in ingot form
• The ingots are heated and moulded under pressure to produce the restorations.

Procedure

• The ceramic ingot is placed under the plunger, and heated to 1150°C.
• The plunger presses the molten ceramic into the mould.
• The final shade of the crown is done by staining or veneering (cut back technique of wax pattern).

Optec Pressable Ceramic

This is a type of feldspathic porcelain with increased leucite content, processed by moulding under pressure and heat.

Advantages of IPS and OPC

• Produce strong, translucent, dense and etchable ceramic restorations
• Useful in fabricating ceramic veneers.

Disadvantage

Require special equipment to fabricate the restorations.

Infiltrated ceramics

Available as

• Powder (aluminum oxide or spinel), which is fabricated into a porous substrate, and a glass, which is infiltrated at high temperature into the porous substrate
• The infiltrated ceramic is then veneered using conventional feldspathic porcelain technique.

In-Ceram

• The spinel cores are more translucent than the aluminum oxide cores
• The core is made from fine-grained particles that are mixed with water to form a suspension referred to as a “slip”.

Procedure

• The slip is placed on a gypsum die and baked at 1120°C for 10 hours to produce the opaque, porous core
• An appropriate shade of glass powder is applied to the core, which is baked again at 1100°C for four hours
• During this process, the molten glass infiltrates the porous alumina core by capillary action.

Infiltered Ceramics Advantages

• Extremely high flexure strength
• The aluminum oxide or spinel crystals limit crack propagation and the glass infiltration reduces porosity
• Provide an accurate fit.

Infiltered Ceramics Disadvantages

• Cannot etch internal surface hence a resin cement, such as Panavia 21TC is recommended
• Not as esthetic as other systems
• Requires specialized equipment to fabricate a restoration.

Recent Advances in Metal–Ceramics

• Pure titanium can be used as a coping and framework metal for metal ceramic restorations because of its excellent biocompatibility
• Copy milling is used to prepare duplicate dies of graphite and to machine the outer form of a titanium crown. The graphite die is then used as a positive electrode in a spark erosion system to serve as the pattern for removal of interior portion of the crown
• Other titanium-based products, such as Tycast are melted in a specialized casting machine and cast using the conventional lost-wax technology. Ultra low fusing porcelain is used along with this.