Dental porcelain (also known as dental ceramic) is a dental material used by dental technicians to create biocompatible lifelike dental restorations, such as crowns, bridges, and veneers. Evidence suggests they are an effective material as they are biocompatible, aesthetic, insoluble and have a hardness of 7 on the Mohs scale. For certain dental prostheses, such as three-unit molars porcelain fused to metal or in complete porcelain group, zirconia-based restorations are recommended.
The word "ceramic" is derived from the Greek word keramos, meaning "potter's clay". It came from the ancient art of fabricating pottery where mostly clay was fired to form a hard, brittle object; a more modern definition is a material that contains metallic and non-metallic elements (usually oxygen). These materials can be defined by their inherent properties including their hard, stiff, and brittle nature due to the structure of their inter-atomic bonding, which is both ionic and covalent. In contrast, metals are non-brittle (display elastic behavior), and ductile (display plastic behaviour) due to the nature of their inter-atomic metallic bond. These bonds are defined by a cloud of shared electrons with the ability to move easily when energy is applied. Ceramics can vary in opacity from very translucent to very opaque. In general, the more glassy the microstructure (i.e. noncrystalline) the more translucent it will appear, and the more crystalline, the more opaque.
Composition
Ceramic used in dental application differs in composition from conventional ceramic to achieve optimum aesthetic components such as translucency.
As example the composition of dental feldspathic porcelain is as follows: Kaolin 3-5% Quartz (silica) 12-25% Feldspar 70-85% Metallic colourants 1% Glass up to 15%
Classification
Ceramics can be classified based on the following:
Classification by Microstructure
At the microstructural level, ceramics can be defined by the nature of their composition of amorphous-to-crystalline ratio. There can be an infinite variability of the microstructures of materials, but they can be broken down into four basic compositional categories, with a few subgroups: Composition category 1 – glass-based systems (mainly silica), example is the feldspathic porcelain. Composition category 2 – glass-based systems (mainly silica) with fillers, usually crystalline (typically leucite or, more recently, lithium disilicate) Composition category 3 – crystalline-based systems with glass fillers (mainly alumina) Composition category 4 – polycrystalline solids (alumina and zirconia).
Dental ceramic is generally regarded as biologically inert. However, other toxicities may exist from depleted uranium as well as some of the other accessory materials; in addition, the restoration may increase wear on opposing teeth.
Classification by Processing Technique
Powder/liquid, glass-based systems
Machinable or pressable blocks of glass-based systems
CAD/CAM or slurry, die-processed, mostly crystalline systems
Classification of crystalline ceramics
Types of dental ceramics
The range of dental ceramics determined by their respective firing temperatures are: Ultra-low Fired below 850 °C - mainly used for shoulder ceramics (aims to combat the problem of shrinkage, specifically at the margins of the prep, when the early sintered ceramic state is fired to produce the final restoration), to correct minor defects and to add colour/shading to restorations Low fusing Fired between 850 and 950 °C - to prevent the occurrence of distortion, this type of ceramic should not be subjected to multiple firings Higher fusing This type is used mainly for denture teeth
Laboratory procedure
The dentist will usually specify a shade or combination of shades for different parts of the restoration, which in turn corresponds to a set of samples containing the porcelain powder. There are two types of porcelain restorations:
Porcelain fused to metal
Complete porcelain
Ceramic restorations can be built on a refractory die, which is a reproduction of a prepared tooth made of a strong material with the ability to withstand high temperatures, or it can be constructed on a metal coping or core.