Glass Matrix Ceramics

These nonmetallic inorganic multiphase ceramics are usually composed of a residual glass phase with finely dispersed crystalline phases. ^, The glassy ceramics are usually fabricated by precise crystallization of the glass nucleated evenly throughout the glass phase or by embedding 1 or more crystals in the structure. ^, The crystallin phase can occupy 0.5% to 99.0% of the composition, but usually contains 30% to 70% of the final composition. The type, size, and volume fraction of the crystallin phase along with its distribution in the glass matrix are among the important factors controlling the mechanical and aesthetic properties, such as the toughness and translucency of the final material. Moreover, the presence of crystallin phase improves the mechanical strength of the final material by inhibiting crack propagation and growth. The glassy phase also contributes to the improved mechanical properties by filling the grain boundaries. The advantage of glass matrix ceramics over traditional ceramics include ease of synthesis processes, less shrinkage, improved translucency owing to the decreased internal light scattering. ^, The glass matrix ceramics are generally divided into 2 subcategories: natural materials such as Feldspathic ceramics and synthetic materials such as lithium disilicate.

Feldspathic ceramics

The feldspar-based ceramics are known as the traditional dental ceramics based on a ternary system composed of natural feldspars (potassium/sodium aluminosilicate), Kaolin (Al ₂ O ₃ .SiO ₂ .2H ₂ O), quartz (SiO ₂ ), and some metal oxides as additives. During the fabrication process, the melting feldspar provides a glassy matrix for distribution of a disordered network of silica tetrahedra. The improved strength of potassium containing feldspars known as leucite (K ₂ Al ₂ Si ₆ O ₁₆ ) turn them into a suitable option for veneering material on metal and ceramic substrates or as reinforced resin-bonded glass-ceramic cores. This class of glass ceramic dental restorative material can be produced in different tooth shades with a range of opacity and translucency by adding different metal oxide pigments. In routine dentistry, the opaque feldspar ceramics are used as a first layer shielding the underlying metal followed by a layer of enamel ceramics to develop the natural color of the tooth. Despite the advantageous properties, feldspathic porcelains are limited to low load-bearing anterior applications owing to lesser flexural strength and increased brittleness. The perceived color of natural teeth is the result of the reflectance from the dentin modified by the absorption, scattering, and thickness of the enamel. One of the primary factors that influences the appearance of layered ceramic restorations is the layering effect. Translucent porcelain may have a greater effect on the color of the veneered restoration than the opaque substrate. There is a significant correlation between the thickness ratio of core and veneer ceramics and the color of the restoration. Even when adequate ceramic thickness exists, clinical shade matches are difficult to achieve, because there is a wide range of translucency among the core materials of all-ceramic systems at clinically relevant core thicknesses. Clinicians claim that porcelain of high opacity should be used to decrease the perceived color difference of porcelain veneers after bonding to dark or discolored tooth substances. Different manufacturers have introduced porcelain systems with increased opacity and claim superior color stability over different backgrounds. Layered feldspathic veneers are indicated when restoring tetracycline tooth or discolored preparations owing to the fact that layered porcelain powders gives more flexibility with respect to choice of opacity and translucent areas. Further, layered feldspathic veneers are indicated when undertaking more conservative cases in younger population with large pulps owing to the fact that less tooth structure needs to be removed. Many dentists choose feldspathic veneers because they can be made thinner than pressed ceramic with more than 1 color of porcelain throughout the veneer. Vitablocs (Vita Zahnfabrik, Bad Säckingen, Germany) are among the most used feldspar-based computer-aided drafting/computer-aided manufacturing ceramics with an average flexural strength of 154 MPa.

Lithium disilicate

The synthetic glass ceramics have emerged as an alternative to become independent of the natural resources and their inherent variations with higher volume fraction of crystallin phase distributed in a glassy matrix. Currently, lithium disilicate glass ceramics are among the popular restorative dental materials that are available as heat-pressable ingot and a partially crystallized machinable block. Lithium disilicate glass-ceramics are one of the well-known synthetic glass-ceramics based on SiO ₂ –Li ₂ O system composed of randomly oriented fine platelet-like or rod-like entangled lithium disilicate crystals at higher concentrations of up to 70% and a lower concentration of lithium orthophosphate (Li ₃ PO ₄ ) crystals heterogeneously dispersed in the glass matrix. ^, The higher concentration of crystallin phase and the tighter interlocking matrix of this synthetic glass ceramic poses significantly higher strength of approximately 350 MPa and fracture toughness of 2.5 MPa m ^1/2 compared with the naturally occurring feldspathic porcelain. ^, Secondary to the improved mechanical properties, this restorative material can have a wide range of applications including resin-bonded veneers, crowns, and 3-unit bridges up to the second premolar. IPS e.max (Ivoclar Vivadent, Schaan, Liechtenstein), a lithium disilicate ceramic, was developed in part by Prof. Wolfram Holland at Ivoclar Vivadent. After the development of clinical applications, the material was released to the dental community about 12 years ago. This ceramic material is well-researched, and many authors have described its physical properties. The effect that flaws in IPS e.max lithium disilicate or luting agent spaces have on fracture potential and tensile strength have been tested, as well as the effects of physiologic aging in a water environment, abrasiveness, wear, and surface roughness. It has met, or exceeded, almost all of the clinical requirements considered ideal for a dental ceramic used in clinical practice.

Celtra Duo (Dentsply Sirona, York, PA) is one of the newly available lithium silicate-based ceramics available in the market. It is a zirconia-reinforced lithium silicate with the amount of Zr inclusion is around 10% zirconium oxide. The manufacturer claims that the smaller ceramic crystal size and ultrafine microstructure material lead to increased mechanical strength. Celtra Duo lithium silicate-based ceramics are available in pressable or milled formats and can be used crowns and bonded partial crowns (inlays, onlays, and veneers).

The other lithium silicate-based ceramics is GC Initial LiSi Press (GC America, Alsip, IL), which is a high-strength and high-density lithium disilicate-based ceramic. This new lithium silicate ceramic offers equal distribution of microcrystals, leading to excellent mechanical and optical properties. The recommended indications for this pressable ceramics are crowns, inlays, onlays, veneers and 3-unit fixed partial dentures up to the second premolar.

Resin Matrix Ceramics

A generalized definition of a resin matrix ceramic is a hybrid material composed of an organic matrix highly filled with inorganic particles such as ceramics, glasses, and glass ceramics. The polymer matric provides a supportive network reinforcing the inorganic network of the hybrid material. The American Dental Association Code on Dental Procedures and Nomenclature used to exclude the resin matrix material from being classified as ceramic materials; however, its 2013 version defined the term ceramic as “pressed, fired, polished, or milled materials containing predominantly inorganic refractory compounds – including porcelains, glasses, ceramics, and glass-ceramics.” Therefore, the resin matrix hybrids fall into dental ceramics category because they contain more than 50% inorganic particles. This class of dental materials are a suitable choice for computer-aided drafting/computer-aided manufacturing, providing superior properties compared with the traditional ceramics and glass ceramics such as better mimicking the elasticity of dentin and ease of milling and adjusting.

Polycrystalline Ceramics

This class of dental restoratives are nonmetallic inorganic all ceramic materials without any glassy phase. The fine grain crystals tightly arranged into regular arrays by directly sintering the crystals in the absence of any intervening material that promotes the strength of the material by reducing crack propagation. Despite improved mechanical properties, the polycrystalline ceramics have limited translucency. One approach to enhance the translucency is to reduce the light scattering through decreasing number of grain boundaries by increasing the grain size. However, increasing the crystallin grain size can result in reduction of mechanical properties. In addition, etching the polycrystalline ceramics with hydrofluoric acid seems to be very difficult owing to absence of the glass phase. Aluminum oxide (Alumina) and Zirconia are among the popular polycrystalline ceramics that will be discussed briefly in the following subsections.

Alumina

Alumina (Al ₂ O ₃ ) is a natural metal oxide with a broad range of industrial applications such as abrasive materials secondary to its high hardness. Besides higher hardness, the superior wear and corrosion resistance along with biocompatibility have turned this material into a popular candidate in dentistry. The very fine grain size of medical-grade alumina ceramics hinders static fatigue and deflects cracks while under load. All these properties have turned Alumina into a desirable substrate for a wide range of medical applications, including dental restoratives and orthopedic applications. However, the Alumina ceramics are prone to bulk fractures owing to higher elasticity. Procera AllCeram from Nobel Biocare (Zürich, Switzerland) (the first fully dense polycrystalline ceramic) and In-Ceram AL, a product of VITA Zahnfabrik, are representatives of this type of ceramic.

Zirconia

Zirconia (zirconium dioxide) is a polycrystalline ceramic with excellent toughness, strength, and fatigue resistance. Depending on the temperature, this polymorphic ceramic can be formed in 3 different crystallographic phases as cubic forming at temperatures of more than 2300°C, tetragonal occurring at temperatures between 1100°C and 2300°C, and monoclinic phase occurring at room temperatures to 1100°C. ^, Zirconia has an elasticity similar to stainless steel, but superior biocompatibility. In addition, the decreased plaque retention results in healthier gums after application of zirconia. Zirconia restorations continue to be successful (from the standpoint of fit, retention, wear, and fracture resistance), and yet controversial in some clinical leader and academic circles, for use in simple single unit and more complex restorations. There are perhaps 50 different zirconia ceramics on the market and, often, the laboratory technician or clinician have no idea what the manufacturing standards were for a given zirconia block or disk used to fabricate restorations coming from different sources. The physical strength and ability to resist fatigue fracture depends on the manufacturing standard that is applied by a specific manufacturer. Yttria-stabilized zirconia (with flexural strength of >900 MPa) is indicated for clinical situations including anterior and posterior crowns, implant abutments/crowns, 3-unit inlay and onlay bridges, cantilever bridges with a minimum of 2 abutment teeth and a maximum of 1 pontic of no more than 1 premolar width, and multiunit long-span (up to 14 units) (DC-Zircon). Some of the available zirconia dental ceramics such as Lava Plus High Translucency Zirconia (3M ESPE, St Paul, MN) are indicated for clinical circumstances with limited interocclusal space, in addition to situations when the tooth-preserving preparation is needed where minimum of 0.5 mm occlusal wall thickness is available. The flexural strength of widely used zirconia dental ceramics range from 1.0 to 1.4 GPa (Glidewell [Newport Beach, CA] BruxZir Solid Zirconia, 1–1.4 GPa; Ivoclar Vivadent Zenostar T, 1.2 GPa; Katana Zirconia [Kuraray Noritake Dental Inc., Okayama, Japan]).

Owing to their high opacity, several studies have tried to enhance the translucency of the zirconia materials by modification of their microstructure, including decreasing the alumina content, increasing the density, decreasing the grain size, adding cubic phase zirconia, and decreasing the amount of impurities and structural defects. The size of the crystalline grain is the microstructural feature that is more closely related to the adjustment of the translucency of polycrystalline ceramics. The creation of ceramic materials with high translucency has been done in the past by means of increasing the grain size during sintering. Larger grains lead to a smaller number of grain boundaries, thereby decreasing light scattering. For Y-TZP, it has been shown that larger grains are detrimental for both the mechanical properties and the stability of the tetragonal phase. Therefore, the translucency of zirconia cannot be achieved by means of increasing its grain size. The increased translucency made the zirconia ceramic materials slightly more esthetic, but it was challenging for laboratory technician to achieve esthetic outcomes consistently.

An alternative method to fabricate high translucent Y-TZP is achieved by decreasing the grain size significantly. Nevertheless, the grain size needs to be decreased until reaching a critical value that results in mitigation of the so-called birefringence phenomenon. Birefringence occurs in Y-TZP owing to the large amount of tetragonal crystal phase (>90%), which is a crystal that has different refractive indexes according to its crystallographic orientation in the microstructure. Such anisotropic behavior related to the variation in the refractive index causes significant light scattering. Another way to overcome these scattering effects is the use of cubic zirconia, which offers optical isotropic behavior, increasing the translucency. These newer, highly translucent zirconia materials are certainly more translucent than the original monolithic zirconia dental ceramic materials. However, increased translucency (owing to increase in cubic structure content) causes a decrease in the flexural strength. The flexural strength of highly translucent zirconia ranges from 500 to 800 MPa (Glidewell BruxZir Anterior and Ivoclar Weiland Zenostar MT, and Katana [Noritake] STML, 750 MPa and UTML 560 MPa). It has been reported that the translucency of zirconia is variable depending on the processing method, manufacturer formulation, and laboratory sintering times and temperatures.