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Ceramics in dentistry 

All-ceramic dental materials can be very different in their chemical composition as well as in their structure and therefore demonstrate very different material properties. Veneer ceramics are feldspathic porcelains which consist almost entirely of an amorphous glass phase and therefore deliver ideal optical characteristics for the veneering. In dentistry there are three different groups of ceramics: polycrystalline ceramics, glass infiltrated ceramics and glass ceramics

Polycrystalline ceramic (glassfree),
e.g. LavaTM

Glass ceramic Empress® I/II (contains glass),e.g. Empress® I/II

Infiltrated ceramic (contains glass),
e.g. In-CeramTM

Glass ceramics and glass infiltrated ceramics are multi-phase materials and contain crystalline constituents (e.g. leucite crystallites in the glass ceramic Empress® II, Al2O3-crystals in infiltrated ceramics etc.) in addition to an amorphous glass phase. Aluminia and zirconia are the only two polycrystalline ceramics suitable for use in dentistry as framework materials able to withstand large stresses. These materials are shown to provide both necessary esthetics (tooth color) and material properties required of a modern tooth restoration (Ref.1).

A dental material needs to adjust to the different influences and conditions of the oral environment. It should have high stability in order to spontaneously withstand extreme stresses and high fracture toughness in order to show the optimal tolerance level towards defects.


Various examinations prove higher stability of infiltrated ceramics than of glass ceramics (Ref.2).
The highest stability, however, has been measured in polycrystalline ceramics (Ref.3) .

Next to the initial stability, especially the long-term stability is the deciding factor in the clinical success of the different systems. Therefore, the question of long-term stability which is highly dependent on subcritical crack growth and fatigue is an exceptionally important aspect in the assessment of new all-ceramic systems. An after-treatment of all-ceramic can induce micro defects, which can grow by subcritical crack growth until a critical crack length leads to fracture. The subcritical crack growth velocity is an essential parameter of ceramic material which can greatly differ from material to material. It indicates the speed at which an existing defect in the oral environment can grow subject to static and/or dynamic stress, until it results in a complete failure. The speed of crack growth also depends on the surrounding medium as well as the previously mentioned fracture toughness. H2O in the salvia leads to so-called stress corrosion in systems containing glass (glass ceramic and infiltrated ceramic). The water (salvia) reacts with the glass causing corrosion of the latter, leading to increased crack propagation velocities and consequently to long-term strength issues. On the other hand, systems having a polycrystalline micro-structure, such as ZrO2 or Al2O3 are to a greater extent glassfree and display excellent long-term stability (Ref.4) .

Zirconia used in demanding environments is usually a tetragonal polycrystalline zirconia partially stabilized with yttria (Y-TZP = yttria tetragonal zirconia polycrystals) (addition of about 3mol %). This material is referred to as a transformation toughened material and it has the special property of a certain fracture inhibiting function. Tensile stresses acting at the ‘crack tip’ induce a transformation of the metastable tetragonal zirconia phase into the thermodynamically more favorable form. This transformation is associated with a local increase in volume, resulting in localized compressive stresses being generated at the ‘crack tip’, which counteract the external stresses acting on the crack tip. The result is a high initial strength and fracture toughness and, in combination with a low susceptibility to stress fatigue, an excellent life-time expectancy for zirconia frameworks.

Afterwards, restorations made from ceramic frameworks have to be esthetically veneered. Thereby the coefficients of thermal expansion (CTE) of both ceramics have to be checked against each other, especially for zirconia which shows a relatively low CTE (approx. 10 ppm). Special veneer ceramics with the same or lower CTE have been developed during the last few years. These veneer ceramics bond very well to the zirconia (see Material Characteristics LavaTM Ceram ).

Various in-vitro trials confirm the enormously high fracture strength of veneered 3-unit zirconia posterior bridges (Ref.5). Values greater than 2000 N have been achieved, which exceeds the  maximum masticatory load by a factor of 3 - 4. With this strength, zirconia bridges demonstrate markedly better values than other all-ceramic bridges. Consequently, zirconia can now be considered a suitable framework material for multi-unit posterior bridges. The strength values and high fracture toughness (resistance to crack propagation, KIC around 5 to 10 MPa m1/2 also enable a lower framework wall thickness than other all-ceramic systems previously available. Instead of a coping thickness of 1 mm, a Lava™ framework/coping wall thickness of 0.5 mm or 0.3 mm (anterior crowns) are considered adequate. This allows preparations requiring less aggressive tooth reduction than with most systems currently on the market. The excellent esthetics of the zirconia framework (ideal translucency and shading, see below) also enables the thickness of the veneer layer to be minimized leading to a conservative preparation technique similar to porcelain fused to metal.

Ref.1
R. Marx,Weber, Jungwirth
Vollkeramische Kronen- und Brückenmaterialien, Restaurationsmaterialien,
CC&A 2002, ISBN 3-00-002643-6

Ref.2
Wagner and Chu (1996)
Biaxial flexural strength and indentation fracture toughness of three new dental core
ceramics, The Journal of Prost Dentistry, 76, 2, 140-144

Tinschert et al. (2000)
Structural reliability of alumina-, feldspar-, leucite-, mica- and zirkonia-based
ceramics J Dent 28, 7, 529-535

Tinschert et al. (2000)
Belastbarkeit vollkeramischer Seitenzahnbrücken aus neuen Hartkernkeramiken
DZZ, 55, 9, 610-616

J.Tinschert,A. Schimmang, H. Fischer, R. Marx
Belastbarkeit von zirkonoxidverstärkter In-Ceram Alumina-Keramik
DZZ 54, 11, 1999, S. 695 – 699.

Bienieck K.W., Marx R. (1994)
The mechanical loading capacity of new all-ceramic crown and bridge material
Schweiz Monatsschr Zahnmed 104, 3, 284-289

Ref.3
R. Marx,Weber, Jungwirth
Vollkeramische Kronen- und Brückenmaterialien, Restaurationsmaterialien,
CC&A 2002, ISBN 3-00-002643-6

Wagner and Chu (1996)
Biaxial flexural strength and indentation fracture toughness of three new dental core
ceramics, The Journal of Prost Dentistry, 76, 2, 140-144

Tinschert et al. (2000)
Structural reliability of alumina-, feldspar-, leucite-, mica- and zirkonia-based
ceramics J Dent 28, 7, 529-535

J.Tinschert, G. Natt, B. Doose, H. Fischer, R. Marx
Seitenzahnbrücken aus hochfester Strukturkeramik
DZZ 54, 9, 1999, S. 545 – 550.

A. R. Curtis,A. J. Wright and G. J.P. Fleming,
The influence of simulated masticatory loading regimes on the biaxial flexure
strength and reliability of a Y-TZP dental ceramic, 2005, submitted

A. R. Curtis,A. J. Wright and G. J.P. Fleming,
The influence of surface modification techniques on the biaxial flexure strength and
reliability of a Y-TZP dental ceramic, 2005, submitted

Ref.4
R. Marx,Weber, Jungwirth
Vollkeramische Kronen- und Brückenmaterialien, Restaurationsmaterialien,
CC&A 2002, ISBN 3-00-002643-6

A. R. Curtis,A. J. Wright and G. J.P. Fleming,
The influence of simulated masticatory loading regimes on the biaxial flexure
strength and reliability of a Y-TZP dental ceramic, 2005, submitted

Ref.5
J.Tinschert, G. Natt, B. Doose, H. Fischer, R. Marx
Seitenzahnbrücken aus hochfester Strukturkeramik
DZZ 54, 9, 1999, S. 545 – 550.


 

Translucency