First ever man made materials are believed to be ceramics. They are among the earliest group of inorganic materials to be structurally modified by man.1 The desire for the use of durable and esthetic material was always preferred and ceramics are the recent addition to it.2 These are the most natural appearing replacement material for missing tooth and is available in a different translucencies and shades so as to achieve good results.3 However, certain drawbacks have been seen in ceramics in relation to manufacturing technique, mechanical properties and physical properties i.e. crack propagation, sintering shrinkage, restoration fracture, excessive brittleness, low tensile strength and wear of antagonists.4

Dentists today can choose from a variety of metal ceramic and all-ceramic materials in dentistry, available for fabrication of ceramic restorations.5 This review outlines the developments in the evolution of dental ceramics over the last century and considers the state of the art in the several extended and innovative applications of dental ceramics.

Ceramics are compounds of metallic and non-metallic elements such as oxides, nitrides and silicates. ‘Ceramic, is an earthy material which silicate in nature nature and may be defined as: A combination of one or more metals, with a non-metallic element, usually oxygen (Gilman, 1987).6

In dentistry, three different types of porcelain compositions are used (depending on their applications) Table 1 7, 8

Structure7, 9

Ceramics can appear as either crystalline or amorphous solids (also called glasses). Thus, ceramics can be broadly classified as:

Figure 0

Composition7, 8

Ingredients used for various formulations of ceramics are:

1.Silica (Quartz or Flint) – Filler

2. Kaolin (China clay) – Binder

3. Feldspar – Basic glass former

4. Nepheline, Syenite & Leucite

5. Water – Important glass modifier

6. Fluxes – Glass modifiers

7. Colour pigments

8. Opacifying agents

9. Stains and colour modifiers

10. Fluorescent agents

11. Glazes and Add-on porcelain

12. Alumina

13.Alternative Additives

Classification of ceramic materials

I. According to application10

II. Classes of Dental Ceramics for Fixed Prosthetics6

Various methods for fabricating ceramic restorations are : pressure molding and sintering, condensation and sintering, casting and ceraming, slip casting, sintering and glass-infiltration, computer controlled mining.

III. Dental porcelains are classified according to the firing temperatures6, 11

High fusing 1300°C (2072°F)

Medium fusing 1101 – 1300°C (2013 – 2072° F)

Low fusing 850 – 1100°C (1962 – 2012°F)

Ultra-low fusing <850°C (1562°F)

IV. According To Use6

A) Metal – ceramic systems

B) All – ceramic systems

V. Classified according to method of fabrication12 13

Conventional Powder & Slurry Ceramics: Using condensing & sintering.

1) Alumina reinforced Porcelain e.g. : Hi-Ceram

2) Magnesia reinforced Porcelain e.g. : Magnesia cores

3) Leucite reinforced (High strength porcelain) e.g.: Optec HSP

4) Zirconia whisker – fibre reinforced e.g.: Mirage II (Myron Int)

5) Low fusing ceramics : (a) Hydrothermal LFC e.g. : Duceram LFC

(b) Finesse (Ceramco Inc)

a) Mechanical e.g. : Celay

b) Automatic e.g.: Ceramatic II, DCP

b) Spark-erosion e.g: DFE, Procera

Direct e.g: Cerec 1 & Cerec 2

1) Alumina based e.g.: In-Ceram Alumina

2) Spinel based e.g.: In-Ceram Spinel

3) Zirconia based e.g.: In-Ceram Zirconia

VI. According to microstructure12

1) Non-Crystalline Ceramics e.g.: Feldspathic porcelain

2) Crystalline Ceramics e.g.: Aluminous porcelain, Glass-Ceramics

Manufacture & Dispensing

Manufacture of porcelain14

Pyrochemical reactions during manufacture of porcelain

When the ceramic raw materials are mixed together in a refractory crucible and heated to a temperature well above their ultimate maturing (fusion) temperature, a series of reactions occur.

After the water of crystallization is lost, the flux reacts with the outer layers of the grains of silica (filler), kaolin (binder) and feldspar (basic glass former) and partly combines them together

Figure 0

Fritting is the process of blending, melting and quenching the glass components is termed.

Dispensing of dental porcelain14

The conventional dental porcelain material may be generally supplied as a kit containing

Fine ceramic powders in different shades of enamel, dentin, core/opaque

Steps in fabrication6

Mixing ceramic powders of selected shades with distilled water or a special liquid (binder)

Figure 0

Various methods of fabricating ceramic restorations vary according to different formulations available:15, 16, 17, 18

For the fabrication of conventional porcelain restoration, there are following stages: Condensation – Sintering – Glazing – Cooling

Condensation (Compaction)

Porcelain powder is built into shape using a liquid binder to hold the particles together. The process of packing the particles together and removing the liquid binder is known as condensation. It is a 2-part process – Agitation of the particles & Removal of excess moisture. It is repetitious and the two components are carried out alternatively until no further moisture comes to the surface. The movement of the particles is generated by a number of standard methods such as

A working model; die of the prepared tooth is used for condensation of porcelain. A matrix is used to support the unfired porcelain both during condensation and firing.

Binder

It helps to hold the particles together, as the porcelain material is extremely fragile in the ‘green’ state.

Types of binder used:

Sintering or Firing of Dental Porcelain

It is defined as a process of heating closely packed particles to achieve inter particle bonding and sufficient diffusion to decrease the surface area or increase density of the structure. The partial fusion or compaction of glass is often referred to as sintering.

During the process of sintering, the points at which the individual particles are in contact (grain boundaries) soften and fuse at sufficiently high temperatures. This process relies on diffusion, which is greatly accelerated by elevated temperatures.

Glaze

Surface porcelain would undergo pyroplastic flow i.e. the matte surface would disappear and a smooth shiny surface would result (self-glaze) if the porcelain was held in the furnance for a greater length of time at the end of high bisque stage.

Stages of maturity of porcelain has been describes in 19 Table 2

Bonding to porcelain20, 21

The bonding of resins and ceramics introduced new restorative techniques and aroused considerable interest. Bonding composite resins (organic substance) to a porcelain surface (inorganic substrate) requires the modification of the porcelain surface to enhance the compatibility of resin and achieve high bond strength.

For the bonding between porcelain (inorganic substrate) and composite resin (organic substrate) the silane primer is essential.

Silanization20, 21

This refers to silane coating of an etched glass surface to increase its surface affinity to polymers. The silanes, often called Coupling agents react and bond to the silica crystals in the glass matrix through the ethoxy-, chloro-or amino- groups leaving the vinyl group to react and form a bond with the resin. When applied and subsequently dried, the resultant condensation forms a strong chemical bond.

Repair of ceramic restorations22, 23, 24, 25, 26

Fracture is totally in porcelain (Simplest repair)

Preparation of porcelain surface for repair by bonding:

Mixed (Porcelain/ Metal) repair (More complicated)

It involves exposed metal

Remaining porcelain:

Preparation of metal surface for repair by bonding:

Metal repair (Most difficult)

It involves exposed metal with minimal or no porcelain

Two methods :

Porcelain Repair Systems

These pocelain-silane-acrylic adhesive systems are basically composite resins utilizing silane-coupling agents for clinical repair of porcelain (minor fractures) in sites E.g. Fusion (George Taub Products, N. Jersy), Enamelite 500 (Lee Pharmaceuticals), Ultra Bond (Den-Mat Corp).

Sandblasting (Grit blasting/ Microetching)

Gritblasting refers to the process of air abrading a material with alumina particles (25- 250μm) or glass beads at a specific pneumatic pressure. Microetching also refers to the use of sandblasting to prepare all types of surfaces for bonding restorations.This technique is used for :

Welding of ceramic materials

Laser induced surface homogenization of dental ceramics can be used to remove local surface defects and polishing marks without the need to reconstruct a complete construction firing.

The surface characteristics of Dental Ceramics following firing:

Laser-induced modification of ceramic materials can be done by a process of heat induction using a suitable laser such as CO₂ laser (its emission wavelength is almost totally absorbed by ceramics).During focused CO₂ laser beam, a local glaze firing occurs on the ceramic surface.

Properties of dental ceramics Table 3

Color stability

Ceramics are the most stable tooth colored materials. The metallic oxides used as colorants do not undergo any change in shade after firing is complete. Adherence of exogenous stains is resisted by the smooth glossy surface. In fact over a period of years, a porcelain restoration may develop a mismatch with adjacent teeth caused by changes in color of the adjacent natural teeth with age.27

Brittleness

It is the relative inability of a material to sustain plastic deformation before fracture of the material occurs. Ceramics are brittle at oral temperatures (5⁰ to 55⁰ C). In other words it fractures at or near its proportional limit.28

Strength29

Abrasion Resistance27

When not glazed properly, it causes wearing of natural tooth and metal restorations.

Dimensional Stability

Porcelain has a coefficient of thermal expansion, slightly less than that of the tooth structure. It does not exhibit microleakage and is comparable to a cemented metal restoration. It also does not imbibe or synergize water.30

Shrinkage

Shrinkage can be minimized by using proper condensation, lesser binder, build-up of restoration 1/3^rd larger than original size and firing in successive stages.27

Degradability

Porcelain is generally resistant to degradation in the oral environment and is susceptible to mechanical degradation by brittle fracture (chipping), and chemical degradation by fluoride attack.31

Abrasion resistance and wear

The hardest dental material which is commonly used is fused porcelain. Wear resistance by opposing restoration or natural teeth is better in fused porcelain than other dental materials. On the other hand, it will cause metal restorations and tooth structure to wear more rapidly; particularly when not adequately glazed or when glaze is removed during occlusal adjustment (should be smoothened by polishing).27

Coefficient of thermal expansion (CTE)

Coefficient of expansion of porcelain is slightly less than that of tooth structure. When different porcelain formulations are veneered together (all-ceramic) and over metal copings (metal – ceramics) ,coefficient of thermal expansion should be matched to prevent development of interfacial stresses leading to separation or fracture.32

Strength properties of dental ceramics33

Dental ceramics are inherently fragile in tension. While the theoretical strength of porcelain is dependent upon the silicon – oxygen bond, the practical strength is 10 to 1000 times less than the nominal strengths.

In ceramics, microcracks are caused by:

Method of strengthening ceramics20

Methods of Strengthening brittle materials6

Development of residual compressive stresses

Interruption of crack propagation

Methods of designing components to minimize stress concentrations and tensile stresses:

Minimizing tensile stress Reducing stress raisers.

Esthetic Properties of Dental Ceramics

The principal reason for the choice of porcelain as a restorative material is its aesthetic qualities in matching the adjacent tooth structure in translucency, colour and chroma.

Color reproduction

Perfect color matching is extremely difficult, if not impossible. Correct color matching of natural teeth by the observer (clinician / ceramic technician) is dependent upon his subjective assessment and even with the use of the most modern types of shade guide and colour corrected lighting, he will experience difficulty in producing consistent shade matchings.6

Colour production in natural teeth27

The structure of tooth influences its colour. The bulk of tooth structure is comprised of 2 layers of calcified tissues; enamel and dentin, surrounding a central core or pulp chamber.

Variation of tooth colour is also apparent in different regions of the tooth such as the incisal, middle and cervical/gingival third Table 4

Specimens of each shade (collectively called a shade guide) are provided for the dentist, who in turn, attempts to match the tooth colour as nearly as possible. Shade guides made of solid porcelain are used most often by dentists to describe a desired appearance of a natural tooth or ceramic prosthesis.

Colour reproductivity6

Without intrinsic and extrinsic colorants, dental porcelain match the color of its respective shade tab. To produce an acceptable match with corresponding shade guides, several factors plays important role which include porcelain type, batch, underlying metal, manufacturers, thickness and perceptible differences in color imparted by extrinsic colorants before and after firing whereas varied firing temperatures condensation techniques, repeated firings and firing cycles do not affect the color of dental porcelain.

Problems in reproducing natural teeth colors in ceramics restorations:

Side effect of dental ceramics34, 35, 36

Ceramics are inert; nevertheless they do pose certain problems:

Extended & innovative applications of ceramics in dentistry37, 38

· Implant supported ceramic restorations

· Silanized ceramic fibres in Ceromers (Eg: Targis)

Ceramic materials have been used in dentistry for well over 200 years. They are the most biocompatible dental restorative materials, because they are chemically very stable. A desirable feature of ceramics is that their appearance can be customized to simulate the colour, translucency and flourescence of natural teeth. A major problem with the use of ceramics as tooth replacement materials is that they exhibits a very low flexibility before fracture. and also they exhibit large firing shrinkage.

Presently there is no restorative system which can ideally replace the natural tooth structure. In the last few years, ceramic research has gained attention for restorative use. Ceramics will play an important role in restorative dentistry. Improvements in fracture resistance and wear properties enhance their restorative use.

Table 1

Denture Tooth Porcelain	Feldspathic Porcelain	Aluminous porcelain
Begins as a mixture of powders of feldspar, clay and quartz. (This is referred to as high fusing porcelain in some dental material text books)	Used for ceramo-metal restorations; begins as a mixture of powders of potassium feldspar and glass. It can also be used for fabricating porcelain veneers and inlays.	Used in PJS’s. It is composed of mixture similar to that of feldspathic porcelain with increased amounts of aluminium oxide

Table 2

	Low Bisque	Medium Bisque	High Bisque
Characteristic features	Grains of porcelain start to soften and coalesce at the contact points	Flow of glass grains increase and the residual entrapped furnace air become sphere shaped.	Firing shrinkage is complete, and has adequate strength, for any corrections by grinding prior to glazing.
Particle cohesion	Incomplete	Considerable	Complete
Porosity	Highly porous, and easily absorbs water soluble dye	Reduced although still porous	Slight / Absent depending on material used
Shrinkage	Minimal	Majority or definite	Complete
Strength	Weak & friable	Moderate	High
Surface texture	Porous	Still porous & matte surface	Completely sealed (egg shell appearance) i.e., relatively smooth surface with a light sheen
Colour & Translucency	Opaque	Less opaque and colour developed	Colour & translucency developed

Table 3

Properties of ceramics
Desirable properties	Principal deficiencies
Good esthetic qualitiesHigh hardnessHigh compressive strengthGood chemical durabilityExcellent biocompatibility	BrittlenessLow fracture toughnessLow tensile strength-susceptible to fracture during placement, mastication and trauma

Table 4

Incisal third	Middle third	Cervical/gingival third
Enamel covering with little or no dentin underneath produces a wrap around effect which results in increased translucency in the incisal third and approximal areas.	This region consists predominantly of dentin, hence the overlying enamel takes on some of the dentinal hue (yellow-orange) which is modified by the translucent blue grey enamel resulting in a composite colour.	Enamel thins down towards the cervical line, hence the underlying dentinal hue results in a deep hue ranging from orange-yellow to often a distinct brown depending on the degree of calcification of dentin.

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