March 2014, Volume 10, Issue 3
Published by AEGIS Communications
Understanding the Mechanics of Ceramics
How physical properties affect strength and esthetics
Understanding the properties of ceramic and porcelain materials, which includes but is not limited to their composition, microstructure, translucency, and fracture resistance, can help clinicians choose the proper materials when planning treatment. Generally, all-ceramic restorations have been confined to the anterior region until recently with the introduction of monolithic lithium dioxide and zirconia restorations. These types of restorations have no limit in terms of where they can be used in the dental arch. All other ceramic systems (when used in a monolithic form) should be limited from the canine forward because of the lower flexural strength. These same ceramics can be used in the posterior regions only when supported by a high-strength core (metal or ceramic).
Ceramics that are composed mostly of glass have the highest esthetics. Manufacturers sometimes add small amounts of filler particles to control the optical effects that mimic natural enamel and dentin. Generally, the more filler particles that are added to a ceramic, the greater the increase in the mechanical properties but the greater the decrease in its esthetic properties. Polycrystalline ceramics contain no glass at all. The crystalline arrangement lends these ceramic materials the highest strength, but they are generally less esthetic. With composite resins, filler particles are added to a resin matrix; greater filler content means greater mechanical properties, but results in lower translucency. With ceramics, the glass is the matrix and the fillers are crystalline particles that melt at high temperatures. Nonglass-containing polycrystalline ceramics comprise an aluminum oxide or zirconium oxide matrix and fillers that are not particles but elements that alter optical properties. These added elements are called dopants.1
Conventional dental ceramics are based on a silica network and potash feldspar, soda feldspar, or both.2 To control the coefficient of thermal expansion, solubility, and fusing and sintering temperatures, different elements are added, such as pigments, opacifiers, and glasses.
Porcelains have two different phases: the glass phase (responsible for the esthetics) and the crystalline phase (associated with mechanical strength). In the case of feldspathic porcelain, a crystalline mineral called leucite (potassium-aluminum-silicate) forms when feldspar is melted. The leucite crystalline phase has a diffraction index similar to the glassy matrix that, in this case, contributes to the overall esthetics of the porcelain.3 The leucite content of a porcelain is also associated with the crack propagation strength. Greater leucite content means a greater decrease in the propagation of a crack.4 During the sintering process of all-ceramic restorations, microporosities are formed on the surface that lead to crack initiation and propagation and that ultimately result in failure.5-7
Hot-pressed ceramics have high amounts of leucite crystals and are considered leucite-reinforced. During the heated injection molding cycle, the sintering process is avoided8 and the leucite crystals act as barriers that counteract the increase in tensile stresses that can lead to the formation of microcracks.7 This type of ceramic can be used to press as an all-ceramic restoration or to a metal coping.
Alumina increases the strength of feldspathic porcelain more than leucite, which increases the fracture resistance.9 The particle size of the alumina may be responsible for the increase in the mechanical properties by decreasing agglomeration.10 When ceramics are sintered, the particle size is critical. Finer powder yields a greater reduction in surface area. Fine powders tend to form clusters of irregular shape and uncontrolled size and are referred to as “agglomerates,” which hinder flow properties.11
Lithium disilicate was the second generation of hot-pressed ceramic materials. These ceramic restorations contain 70% lithium disilicate crystals, which results in an increased flexural strength of approximately 360 MPa (milled version) to 400 MPa (hot-pressed version).12 The increase in strength is found in the unique microstructure of lithium disilicate, which consists of any small interlocking platelike crystals that are randomly oriented. The lithium disilicate crystals cause cracks to deflect, branch, or blunt, which arrests the propagation of cracks.13
Zirconia is widely used in medicine and dentistry because of its mechanical strength as well as its chemical and dimensional stability and elastic modulus similar to stainless steel.14 Zirconia has a normal density of 6 g/cm2. The theoretical density (ie, 100% dense) of zirconium oxide is 6.51 g/cm2. The closer these two density values are, the less space between the particles, resulting in greater strength and a smoother surface.15
Translucency is the relative amount of light transmitted through a material.16 The shade of a human tooth is determined by the shade of the dentin because the enamel is more translucent. This translucency becomes more apparent in the interproximal and incisal portions of the tooth because of the lack of underlying dentin.
There are several factors that affect the translucency of dental ceramics. Thickness of the material has a great effect, but translucency can also be affected by the number of firings, the shade of the substrate, and the type of light source or illuminant. Because clinical settings can vary so widely, specimens should be compared at the recommended minimum thickness.17
The chemical nature, size, and amount of crystals in a ceramic matrix will determine the amount of light that is absorbed, reflected, and transmitted compared with the wavelength of the source light.18 Therefore, the greater the number of crystals in the glassy matrix, the less translucent the ceramic. The greater the amount of unfilled glassy matrix (as in feldspathic porcelains), the more light that can travel through unobstructed, producing more translucency. Zirconia dioxide, which is absent of any glass matrix, has the highest opacity.
Fracture toughness is the ability to resist crack growth. If a material has a large value of fracture toughness, it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with a low fracture toughness value.19 Flexural strength is defined as a material’s ability to resist deformation under load. Flexural strength represents the highest stress experienced within the material at its moment of rupture and is measured in terms of stress.2 For example, zirconia’s reported flexural strength values range between 900 MPa and 1,100 MPa,20,21 and fracture has been reported between 8 MPa and 10 MPa.20
As manufacturers continue to introduce new materials and formulations, clinicians should be cognizant of changes in material selection and continue to base their choices on the clinical needs of the patient. In addition to esthetic concerns, which are often top of mind for patients, general practitioners will certainly consider longevity of the restoration and its ability to handle forces and wear when making a sound clinical decision.
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About the Author
Gregg A. Helvey, DDS, MAGD
Adjunct Associate Professor
Virginia Commonwealth University School of Dentistry