Current All-Ceramic Systems in Dentistry: An Updated Review

Dental ceramics have been widely utilized to restore anterior and posterior teeth due to several qualities, most notably their optical properties, color stability, wear resistance, biocompatibility, and excellent esthetics.^1-3 Since the 1960s, when leucite content was added to the existing feldspathic ceramic formulation to increase its coefficient of thermal expansion to enable use with dental casting alloys, metal frameworks have been veneered with dental ceramic in an effort to ally the esthetic features of ceramic with the fracture resistance of the metal substructure.^4,5

Although porcelain fused to metal (PFM) has shown good results with long-term clinical success rates (survival rates at 5 years above 94.4%),^2,4,5 it presents some disadvantages, mostly related to the presence of metal, which can create esthetic challenges and, on rare occasions, provoke allergic reactions. Previous studies have reported the presence of metal allergy to various metals, such as gold, silver, cobalt, tin, palladium, chromium, and nickel, among different populations.^6-9 Limited esthetic results are a consequence of the presence of a metal framework, which decreases light transmission through ceramic, especially when insufficient space is available for the veneering material.^1,2 The gray metal framework has also been attributed to the bluish appearance of the surrounding soft tissues. This problem was partially resolved in the 1970s by the introduction of collarless metal-ceramics that proposed the use of a reduced framework design with shoulder ceramics.^10,11

In recent decades, the increasing demand for esthetic restorations allied to the desire to eliminate the metal coping has driven the development of new types of dental ceramic materials.^1,12 In the 1980s, the introduction of low-shrinkage ceramics and a castable glass-ceramic system (Dicor, Dentsply) marked the introduction of advanced ceramics with innovative processing methods.¹² Later, Mormann and Brandestini developed the first operational computer-aided design/computer-aided manufacturing (CAD/CAM) system to fabricate inlays and onlays from solid ceramic blocks (CEREC I, Siemens Dental/now Sirona Dental). Since then, CAD/CAM technology has been pursued worldwide with the introduction of different ceramic systems that have adopted the CAD/CAM technique.^2,13 In the early 1990s, the lost-wax press technique was introduced as an innovative processing method for all-ceramic restorations.¹⁴

Because of the worldwide acceptance of all-ceramic restorations, ceramics with high flexural strength were developed in order to extend their indication for anterior and posterior fixed dental prostheses (FDPs). The flexural strength and fracture toughness of zirconia are the highest ever reported for any dental ceramic, and its use is rapidly growing, especially for FDPs.^12,15

The improvements achieved in ceramic materials have resulted in greater quality control and have simplified the work of dental technicians through various processing methods.^12,16,17 The array of ceramic compositions and different types of manufacturing techniques has afforded clinicians numerous systems from which to choose; thus, a more comprehensive understanding of each system is needed in order to make wise choices. This article, therefore, aims to describe different all-ceramic systems available in dentistry according to the ceramic composition and fabrication technique. Clinical indications and survival rates will also be discussed.

Manufacturing Techniques

All-ceramic restorations may be fabricated by different methods: powder condensation (conventional powder slurry ceramics), heat-pressed (pressable ceramics), slip casting (infiltrated ceramics), and milled (machinable or CAD/CAM ceramics). (Note: Commercial names are identified in Table 1, which can be accessed online at compendiumce.com/go/1502.)

Powder Condensation (conventional powder slurry ceramics)

Powder condensation is a traditional method to fabricate feldspathic ceramic restorations. It involves the use of powders, available in various shades and translucencies, and de-ionized water to produce a slurry.^1,2 The moist porcelain powder is applied over a refractory die, copings, or frameworks with a brush, and vibrated and compacted to remove excess moisture. The ceramic restoration is fired under vacuum, which helps to remove remaining air and improve the density and esthetics.^17,18

This handmade technique may result in a large amount of residual porosity, which can affect the final strength.^1,3,17 The number and size of voids remaining will depend on the particle size distribution, sintering time, temperature, ceramic chemical composition, and viscosity of the melt.¹⁸

Ceramics fabricated by powder condensation have good translucency and are typically applied as the esthetic veneer layers on metal or all-ceramic frameworks. Other applications include their use for anterior veneers, inlays, and onlays restorations.^1,18

Heat-Pressed (pressable ceramics)

Prefabricated pressable ceramics are available in monochromatic ingots made of crystalline particles distributed throughout a glassy material.^1,14,16 Although the microstructure of the ingots is similar to that of conventional powder ceramics, it presents lower porosity and higher crystalline content. The ingots are manufactured from nonporous glass by applying a heat treatment that transforms some of the glass into crystals, producing a well-controlled and homogeneous material.¹⁷

The lost-wax method is used in combination with the heat-pressed technique.^13,14,17 A desired monochromatic ingot is heated to a temperature at which it becomes a highly viscous liquid to allow the material to flow under pressure into the lost-wax mold. One of the advantages of the hot-pressing technique is that dental technicians have experience working with the lost-wax method to cast metal alloys.^16,17,19,20 The final restoration is subsequently stained and glazed to achieve the final esthetic result.

The first generation of heat-pressed dental ceramics uses leucite as the reinforcing crystalline phase (eg, IPS Empress^®, Ivoclar Vivadent, www.ivoclarvivadent.com), while the second generation consists of lithium-disilicate–based ceramics (IPS Empress 2). Feldspathic leucite-reinforced and lithium-disilicate compositions of pressed ceramic have been used as inlays, onlays, veneers, single-unit crowns, and limited FDPs.^1,16,19,20 They can also be used as substrate, usually as core and coping or framework materials for conventional powder feldspathic porcelain buildup to achieve the restorations’ final shape and shade.^16,20

Slip Casting (infiltrated ceramics)

The slip-casting technique consists of preparing stable suspensions, the slip, and fabricating structures by building a solid layer on the surface of a porous mold that absorbs the liquid phase by means of capillary forces.^1,3 The slip is a homogeneous mixture of ceramic powder particles suspended in a fluid, usually water, applied over a gypsum die that absorbs some water from the slip through capillary action, forming the underling framework.^17,20 The alumina, very porous framework is partially sintered to increase its strength to a point where it can be removed from the die and infiltrated with a molten lanthanum glass, which flows into the pores by capillary action. The final core is fully sintered to yield a ceramic coping of high density and strength before the veneering porcelain can be applied. This technique has been widely used in dentistry on In-Ceram® products (Vita Zahnfabrik, www.vita-zahnfabrik.com).^2,16,17

Ceramics fabricated by slip casting can have higher fracture resistance than those produced by powder condensation, because the strengthening crystalline particles form a continuous network throughout the framework.^16,17 The disadvantage of this method, however, is the number of complicated steps involved, which may result in internal defects that weaken the material due to incomplete glass infiltration.^1,17 Glass-infiltrated ingots can be used with CAD/CAM technology, eliminating some of the steps and simplifying the technique.

Milled (machinable ceramics)

Dental CAD/CAM systems use a scanning device, design software, and a milling machine to fabricate copings, frameworks, and restorations from industrially prefabricated ceramic blocks.^13,21 Two methods are available to process the ceramic blocks. The first method was developed with the intention to machine fully sintered ceramic (hard machining); however, machining of fully sintered ceramic blocks can result in significant tool wear and residual flaws at the ceramic surface, which can reduce the survival of the ceramic restorations.²⁰

More recently, CAD/CAM technology has been used with partially sintered ceramics (soft machining or green machining), which are subsequently fully sintered to eliminate porosity. In this case, the computer software takes into account the shrinkage that is generated during sintering to promote accuracy of fit of the final restoration.^16,17

The increasing popularity and variety of CAD/CAM systems has extended and consolidated the use of this manufacturing technique.^13,16,21 The machinable ceramic blocks for CAD/CAM restorations are available in feldspathic porcelain-based ceramic, leucite-reinforced glass ceramic, lithium-disilicate glass ceramics, glass-infiltrated ceramics, and polycrystalline alumina and zirconia materials.^13,15,22

Dentists and laboratories can use CAD/CAM technology in many different ways. Conventional impressions can be sent to a laboratory to begin the CAD process using a stone model, or dentists can take a digital impression with a handheld scanner, using a compact chairside CAD/CAM machine, and either send it to a laboratory for fabrication of the restorations or use their own CAD and do the milling in-house.^13,20,22 Design work is done on the monitor and the instructions are sent to a computer-assisted processing machine for milling the restorations from the prefabricated blocks.^21,23,24

The CAD/CAM blocks are fabricated under optimum conditions, and during the CAM process the computer-controlled fabrication reduces the potential for errors, creating restorations without the variations and inaccuracies that might be found in conventional laboratory-fabricated restorations.^16,20,21 Individual blocks are bar-coded with the actual density of each block for shrinkage calculations, and the milling machines can automatically change milling tools according to need.² Additionally, time and cost involved in labor-intensive waxing, casting, and soldering of frameworks can be reduced with the application of CAD/CAM technology.^13,20,21 CAD/CAM can be used to fabricate inlays, onlays, veneers, crowns, FDPs, and implant abutments.

Composition Classification and Clinical Application

Ceramic materials are formed via two or more distinct phases, usually based on a glass matrix and crystalline filler particles.^2,17 The development of higher strength ceramics is resultant of the increased use of crystalline material and filler particles that are added to the glass matrix aimed at improving the ceramic’s mechanical properties by using decreasingly less glass phase and, finally, no glass content. However, highly esthetic dental ceramics are predominantly glass, while higher strength substructure ceramics are generally crystalline.²

Based at the microstructural level, dental ceramics can be defined by their composition of glass-to-crystalline ratio in three main classes^2,17:

predominantly glassy materials with high glass content

particle-filled glasses with variable amounts of glass content

polycrystalline ceramics without glass content

(Note: Information about ceramic systems’ classification, composition, fabrication process, manufacturers, and clinical indication are summarized in Table 1, which can be accessed online at compendiumce.com/go/1502.)

Predominantly Glassy Ceramics / Feldspathic Amorphous Glass

A noncrystalline-containing material is classified as a glass.¹⁷ Glasses are 3-dimensional (3-D) networks of atoms that lack a regular pattern to the spacing (distance and angle), resulting in a structure that is “amorphous” or without form.² The glass-based systems used in dental ceramics originate from feldspar minerals that contain mainly silicon dioxide (silica or quartz), which have various amounts of alumina (aluminum oxide); they are also called aluminosilicate glasses.^1,2,16,17 Feldspathic glasses contain sodium and potassium, which are able to modify important properties of the glass, such as lowering firing temperatures or increasing thermal expansion and contraction behavior.²

Dental ceramics that best mimic the optical properties of enamel and dentin are predominantly glassy materials.^1,2 However, these ceramics present low mechanical properties, with flexural strength usually ranging from 60 MPa to 70 MPa.^1,17,24 They were first used in dentistry to make porcelain dentures, and later were also used as veneering materials for metal or all-ceramic frameworks, as well as for veneers, using either a refractory die technique or platinum foil.^16,17

Particle-Filled Glass with Variable Amount of Glass Content

Although the glass composition is similar to the glassy ceramics previously described, this category of particle-filled glasses presents varying amounts of crystal types that have either been added to or grown in the glassy matrix.¹⁸ Filler particles are added to the glassy ceramics in order to improve their mechanical properties—for example, to alter the coefficient of thermal expansion and inhibit crack propagation—and to control optical effects, such as opalescence, color, and opacity.^1,23,24 These fillers are usually crystalline that can be added mechanically to the glass, or, as per a more recent approach, the crystallites can grow within the glass by a special heat treatment.^1, This category presents a large range of glass-crystalline ratios and crystal types that includes four main subgroups: low-to-moderate leucite-containing feldspathic glass; high leucite-containing glass; lithium-disilicate glass; and glass-infiltrated ceramics with glass fillers (mainly alumina).^17,20

Low-to-moderate leucite (17 to 25 vol %) glass ceramic—Leucite was the first filler used in dental ceramics enriched with a crystalline mineral in order to increase its coefficient of thermal expansion (CTE) to enable it to be fired onto metal substructures.²³ Leucite is a reaction product of potassium feldspar and glass that may modify the CTE, alter the optical properties, and inhibit crack propagation, thereby improving the material’s strength.^2,3,23 The amount of leucite present in the glass base may be adjusted depending on the type of core and required CTE. Usually 17 to 25 mass % filler is added to the base dental glass to create ceramics that are thermally compatible during firing with dental alloys.^1,2,17 The relative amounts of crystal and glass depend on the ceramic system in question. Commercial ceramic systems incorporating leucite fillers consist of powder ceramics to be used as veneering material for metal-ceramic and all-ceramic substructures, and can also be employed for fabricating porcelain veneers, inlays, and onlays.^1,17,20

High-leucite (35 to 55 vol %) glass ceramic—In this category, leucite is used as a reinforcing crystalline at a concentration of 35 to 55 vol %.^2,20 The microstructure of this class of ceramic materials consists of a glass matrix surrounding individual crystals.^17,25 The material starts as a homogeneous glass, and a special heat treatment nucleates and grows leucite crystals, which results in improved mechanical properties such as increased fracture resistance and improved thermal shock resistance.²⁵ The strengthening effect added by the incorporation of crystal depends on the interaction between the crystals and glassy matrix, as well as on the crystal size and amount. Finer crystals generally produce stronger materials.^17,25 The crystals strengthen the ceramic material by acting as a barrier to cracks. A crack growing from a defect must go around the crystal, which may modify the propagating crack direction and could completely stop it.¹⁷ Additionally, because of the difference in the CTE between the leucite crystals and the glassy matrix, tangential compressive stresses are developed around the crystals on cooling, which may contribute to crack deflection and improved mechanical performance.^16,17,20,25

Commercial ceramics incorporating leucite fillers include different groups, according to the fabrication technique. In the first group the ceramic is pressed at high temperature. Examples include: OPC (Jeneric/Pentron, www.jeneric-pentron.de); IPS Empress® and IPS Empress® Esthetic (Ivoclar Vivadent, www.ivoclarvivadent.com); Finesse® All-Ceramic (DENTSPLY Prosthetics, www.dentsply.com); Authentic® (Jensen Dental, www.jensendental.com); and VITA PM®9 (VIDENT, www.vident.com). In the second group the ceramic is provided as a powder for traditional porcelain build-up. Examples include: OPC Plus (Jeneric/Pentron); Fortress™ (Mirage Dental Systems, www.miragecdp.com); and VITA VMK 68 (VITA Zahnfabrik). The third group comprises materials that have been developed into fine-grain machinable blocks for CAD/CAM systems. These include: VITABLOCS® Mark II (VIDENT); IPS ProCAD and IPS Empress® CAD (Ivoclar Vivadent); and Paradigm™ C porcelain block (3M ESPE, www.3MESPE.com).^12,17,21 Multicolored blocks were developed to reproduce color transitions and shading as well as different levels of translucency.²¹

High-leucite (35 to 55 vol %) glass ceramic materials are indicated for fabrication of inlays, onlays, anterior veneers, and crowns.^24,25 Pressable ceramics with high-leucite content present flexural strength around 130 MPa.²⁴

Lithium-disilicate glass ceramic—Glass ceramics enriched with lithium-disilicate crystals (SiO2-Li2O) were developed by Ivoclar Vivadent to be used with the lost-wax/heat pressed technique (IPS Empress® 2 and IPS e.max® Press) and later on with the CAD/CAM technology (IPS e.max® CAD). The high crystal content (70 vol %) is considerably higher than that of leucite materials. The ceramic microstructure consists of highly interlocked lithium-disilicate crystals, 5 mm in length and 0.8 mm in diameter.^17,26

IPS Empress 2 has improved flexural strength (360 MPa) that is more than two times that of leucite-based IPS Empress, and is indicated for anterior and premolars crowns, as well as three-unit FDPs in the anterior region.^17,26 The framework is veneered with fluoroapatite-based veneering porcelain (IPS Eris, Ivoclar Vivadent), which presents similar optical properties and CTE, thus it matches the lithium-disilicate material resulting in an esthetic restoration with enhanced light transmission.¹²

In 2005, IPS e.max Press was introduced with improved physical properties (flexural strength 400 MPa) compared to the former IPS Empress 2.^12,20 It also consists of a lithium-disilicate glass ceramic, but with refined crystal size, presenting improved physical properties and translucency acquired through a different firing process.²⁷ Due to the relatively low refractive index of the lithium-disilicate crystals, this material presents high translucency despite its high crystalline content.² The IPS e.max Press pressable lithium-disilicate ceramic can be used in monolithic application for inlays, onlays, and posterior crowns or as a core material for crowns and three-unit FDPs in the anterior region.²⁰

Machinable lithium-disilicate blocks (IPS e.max CAD) were recently developed to be used with CAD/CAM processing technology. These blocks are subjected to a two-stage crystallization process. In the first stage, 40 vol % of lithium-metasilicate crystals are formed, resulting in a flexural strength of 130 MPa to 150 MPa, which permits easier machining and intraoral occlusal adjustment. During the final crystallization process, 70% of crystal volume is incorporated in a glass matrix, increasing its final resistance. Additionally, in this stage the blue shade of the precrystallized block is changed to the selected tooth shade. The final restoration presents a flexural strength of 360 MPa; these are indicated for anterior or posterior crowns, implant crowns, inlays, onlays, and veneers.^20,24,26

Glass-infiltrated alumina-based ceramics (slip-cast ceramics)—Glass-infiltrated alumina-based ceramics have been limited to one series of In-Ceram products (Vita Zahnfabrik).¹⁶ In-Ceram ceramic consists of a high-strength core based on alumina particles (70 to 80 wt % aluminum oxide) and a lathanum-aluminosilicate glass, generally fabricated with the slip-casting technique.^2,17 A slip composed of densely packed aluminum oxide (Al2O3) and water is applied over a gypsum die and baked at 1120°C for 10 hours to create a porous matrix, which will be filled by a second-phase material, the lanthanum-aluminosilicate glass.^1,28 The capillary action draws a liquid or molten glass into all the pores during a second firing at 1100°C for 4 hours to produce the dense interpenetrating material. The lanthanum decreases the viscosity of the glass to assist infiltration and increases its index of refraction to improve translucency of In-Ceram ceramic.^1,2,17,33

The slip-casting technique may be used to fabricate the ceramic core, or as has been done more recently, it can be milled from a pre-sintered block.^12,17 Glass-infiltrated CAD/CAM blocks have similar composition to slip-cast ceramics while eliminating the complicated steps of slip casting. Partially sintered blocks are initially milled, and the porosity is eliminated by molten glass infiltration. Afterwards, the coping is veneered with feldspathic porcelain.^12,16,20 The In-Ceram products include different compositions and fabrication techniques designed to cover the wide scope of all-ceramic restorations, including veneers, inlays, onlays, and anterior/posterior crowns and bridges.^17,28,33 Flexural strengths range from 350 MPa for spinel to 450 MPa for alumina, and up to 650 MPa for zirconia.^17,28

In-Ceram Alumina (alumina matrix) was the first all-ceramic system available for anterior and posterior single-unit restorations and three-unit anterior bridges with a high-strength ceramic core and lower translucency.^12,17 In-Ceram Spinell (alumina and magnesia matrix, MgAl2O4) is a modification of the original In-Ceram Alumina system. Substituting magnesium aluminate spinel for the aluminum oxide improved translucency, partly because of the crystalline spinel, which provides isotropic optical properties, and partly because of a lower index of refraction compared with alumina.^1,12,17 In-Ceram Spinel is the most translucent, with moderately high strength, though not as strong as the alumina-based material, and is used for anterior crowns.^2,17 In-Ceram Zirconia (alumina and zirconia matrix, 12 Ce-TZP-Al2O3) presents higher strength and lower translucency.¹⁷ It is also a modification of the original In-Ceram Alumina system, with an addition of 35% partially stabilized zirconia oxide (ZrO2) added to the slip or block composition to strengthen the ceramic material. Since the core is very opaque and lacks translucency, this material is recommended for posterior crown copings and FDP frameworks, primarily for three-unit posterior bridges.^12,17

Fully Polycrystalline or Polycrystalline Ceramics / No Glass Content

The development of high-strength polycrystalline ceramics came about as a result of the increased use of crystalline material with subsequent decreased amounts of the glass phase, until no glass was present in the material microstructure.² These ceramics are formed by directly sintering crystals together, resulting in a dense, glass-free polycrystalline structure without any intermediary matrix.¹⁷ These solid-sintered monophase ceramics are characterized by regular arrays in which the atoms are densely packed, resulting in a very strong and tough material that is difficult to crack compared to a less dense and irregular network found in glasses.^2,15

Polycrystalline ceramics are more difficult to process than glassy ceramics. The availability of computer-aided manufacturing allowed the fabrication of either solid-sintered aluminous oxide (alumina, AlO) or zirconium oxide (ZrO) dental cores and frameworks.^2,28 These higher strength ceramics tend to be relatively opaque compared to glassy ceramics, and some polycrystalline ceramics present opacity similar to cast alloys. They are usually indicated as substructure materials upon which glassy ceramics are veneered to achieve pleasing esthetics.²⁸

Alumina—The first fully dense polycrystalline material used for dental applications was developed in 1983. With Procera® AllCeram alumina Nobel Biocare (www.nobelbiocare.com) embraced the concept of CAD/CAM technology to fabricate all-ceramic crowns composed of a densely sintered, high-purity aluminum-oxide coping combined with a low-fusing all-ceramic veneering porcelain. Copings contain 99.5% to 99.9% high-purity aluminum oxide and a strength of approximately 600 MPa.³⁴

The working die is scanned using a scanner probe that contacts the surface of the die, enabling visualization of a defined 3-D shape of the preparation. The data is sent electronically to a manufacturing facility where a 20% enlarged model is copy-milled and fabricated, taking into account the ceramic shrinkage. A high-purity aluminum-oxide powder is mechanically compacted and milled on the enlarged die. The coping is then sintered at about 1600°C to full density eliminating porosity and returning the dense coping to the dimensions of the working die.^13,34 The coping is mailed to the dental laboratory and the crown is completed by the addition of a low-fusing feldspathic porcelain that matches the CTE of the aluminum oxide.^12,34 Procera has the highest strength of the alumina-based materials; however, its strength is lower than zirconia.¹² Alumina systems have been replaced by zirconia and lithium dislocate due to the increase risk of failure in the molar region.³⁵ 

Zirconia—In its pure form zirconia oxide (ZrO2) is a polymorphic material that occurs in three crystalline forms that are temperature-dependent: monoclinic (room temperature to 1170°C), tetragonal (1170°C to 2370°C), and cubic (2370°C to melting point).²² During firing zirconium oxide is transformed from one crystalline state to another. At firing temperature zirconia is tetragonal and at room temperature monoclinic. The tetragonal-to-monoclinic phase transformation occurs below 1170°C and is accompanied by a 3% to 5% volume expansion that causes high compressive stresses, which can generate crack propagation and failure.^12,22 In order to stabilize the tetragonal phase at room temperature, yttrium-oxide (Y2O 3% mol) is added in small amounts to pure zirconia to control the volume expansion.^20,22

Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) is a high-strength ceramic, introduced for dental use as a core or framework material for FDPs and crowns. Although hard machining of fully sintered zirconia ceramics is possible, it may compromise the microstructure and strength of the material and would require extensive milling to produce the framework. Soft machining of partially sintered zirconia blocks by CAD/CAM technology is used to produce enlarged frameworks in a so-called green state.^12,20,22 Processing with this softer pre-sintered material not only shortens the milling time but also reduces the wear on the milling tools.¹² The disadvantage of this process is the need for subsequent sintering treatment to eliminate the ceramic porosity. In these cases the software used for restoration design must compensate the 20% to 25% of shrinkage that occurs during the sintering procedure to provide accuracy of fit to the final restoration.^12,16 Currently available zirconia ceramics for soft machining of dental frameworks require sintering temperatures varying from 1350°C to 1550°C and durations from 2 to 6 hours, depending on the manufacturer.¹⁶

The zirconia ceramics are characterized by a dense, monocrystalline homogeneity, and possess low thermal conductivity, low corrosion potential, good radiopacity, high biocompatibility, low bacterial surface adhesion, and favorable optical properties.^12,16,20,22 Compared with high-strength alumina ceramic, zirconia has twice the flexural strength (900 MPa to 1200 MPa).^17,20 Possible problems with zirconia ceramics may involve long-term instability in the presence of water, veneering porcelain compatibility issues, esthetic limitations due to their opacity, and no adequate bond with resin-based luting cements.²

Previous reports revealed that the most common clinical problems have not been associated with cracking of the zirconia framework, but with chipping and cracking of the veneering porcelain.^2,12,17,22 Although these failures may be associated with non-anatomic framework designs or with poor bonding between zirconia and veneer, other hypotheses include problems related to the material itself and are often associated with low degradation phenomenon at mouth temperature, auto-catalytic transformation during porcelain firing, and residual stresses resultant from thermomechanical parameters.² Recent research supports residual stresses developed as a result of rapid cooling during the porcelain firing procedure and suggest a slow-cooling protocol to equalize the heat dissipation from zirconia and veneering porcelain, increasing the fracture resistance of the veneer.^2,17 The need for a reduced cooling rate after final firing or glazing has been reinforced by other studies.^2,20

Dental zirconia presents properties to be used in single- and multiple-unit anterior and posterior FDPs.^16,17,22

The development of more translucent monolithic restorations was a huge advancement in dentistry. The simplified procedure achieved with full-contour restorations reduced problems associated with complex bi-layered restorations and resulted in faster, cost-saving restorations. This material became an alternative for lithium disilicate monolithic restoration, and most importantly, solved the problem of porcelain veneer chipping off the zirconia core.^36,39

Because of the high resistance to fracture and the high flexural strength of zirconia, reduced thickness of monolithic zirconia restorations has been suggested. Manufacturers recommend minimal occlusal thickness, varying between 0.5 and 1.5 mm. Several studies have tested different occlusal thicknesses and have stated that the flexural strength significantly increases with thickness.^40-44 Although some studies have reported that thickness of 0.6 mm monolithic zirconia displayed adequate fracture strength for posterior restorations,^40,41,44 more laboratory studies and randomized controlled clinical trials are needed to determine the exact thickness needed for successful 3Y-TZP and 5Y-TZP monolithic restorations.

Survival and Failure

The variability of all-ceramic systems available presents a challenge when combining data from several studies. Survival rates change dramatically when comparing different ceramic systems among different periods of evaluation.

Clinical evaluations of glass-ceramic materials used for inlays, onlays, and veneers present survival rates ranging from 93% to 98% at 5 years, to 64% to 95% after 10 years.^45-49 The high success rate is associated with the capacity of these glass-ceramic systems to be etched and bonded to the tooth structure with resin-based cements.^45-50 Although several studies have reported higher fracture resistance for machinable and pressable systems when compared to powder/liquid conventional ceramics,^17,50 other studies have reported similar survival for both ceramic systems.^348,49

Clinical reports on leucite-reinforced glass-ceramic IPS Empress crowns have presented low overall fracture rates.^51-54 However, a statistically significant higher survival rate has been reported for anterior crowns.³⁷⁵²

Glass-infiltrated ceramic crowns placed in the anterior and posterior segments presented a high survival rate, which ranged from 91% to 100% at 5 years for In-Ceram Alumina and Zirconia.⁵⁵ Another study reported a cumulative survival rate of 96.9% for anterior teeth and 87.7% for posterior teeth after 5 years.⁵⁶ These data are comparable to those of Procera AllCeram alumina crowns, presenting survival rates ranging from 90.9% to 95.2% after 6 years, and at 93.5% after 10 years.^57,58 When the In-Ceram system was used for three- and four-unit FDPs, the survival rate ranged from 85% to 96% at a 5-year period.⁵⁵

Lithium-disilicate–based glass-ceramic restorations have also achieved high survival rates. The IPS Empress 2 crowns showed survival rates of 95% to 100% at 5 years, and 95.5% after 10 years.^59,60 Reich and Schierz (2013)⁶¹ reported a survival rate of 96.3% after a period of 4.6 years for chairside-generated e.max CAD crowns. Another study showed 100% survival rate for crowns after 5 years, but the survival rate dropped to 70% when this material was used on three- and four-unit FDPs.44 The main cause of clinical failure was associated with connector fracture.^59-62 Kern et al (2013)⁶² reported survival rates of 100% for the IPS e.max Press three-unit FDPs after 5 years and 87.9% after 10 years.

Zirconia-supported crowns and three- to five-unit FDPs presented survival rates from 73.9% to 100% at 5 years.^63-65 Clinical evaluations of zirconia restorations have reported no problems related to the zirconia framework. The most frequent causes of clinical failure are related to chipping and cracking of veneering porcelain.^20,63-66 Veneer chipping rates are reported at 2% to 9% for single crowns at 3 years and at 3% to 36% for FDPs at 5 years.^20,63,65 Rinke et al (2013)⁶⁶ reported 83.4% overall survival of zirconia FDPs at 7 years and attributed the major failures to fractures of the veneering ceramic and decementations.

The use of all-ceramic systems for FDPs has limitations. Correct diagnosis and patient selection are important factors for success. The connector height for each system should be created according to the guidelines proposed for each material. Several factors should be analyzed, some of which may restrict the use of all-ceramic restorations, such as reduced interocclusal distance, deep vertical overlap without horizontal overlap, cantilevers, periodontal problems, severe bruxism, and parafunctional activity.¹² The selection of the all-ceramic material depends on the indication, and the dentist should be especially careful with preparation guidelines, meticulous with occlusal adjustment, and aware of parafunctional habits.

Conclusion

With the introduction of innovative fabrication techniques, such as heat-pressed, slip-cast, and CAD/CAM, the development of several ceramic systems marked the last four decades. The gradual improvement of ceramic materials for fixed prosthodontics led to the development of fully polycrystalline materials that present increased flexural strength and fracture toughness. The manufacturing of polycrystalline ceramics became possible due to recent advances in ceramic milling by CAD/CAM technology, especially with the soft machining processing of partially sintered ceramics. As a consequence of the consolidation of CAD/CAM technology, different ceramic systems became available in blocks for CAD/CAM processing, including feldspathic porcelain-based ceramic, leucite-reinforced glass ceramic, lithium-disilicate glass ceramics, glass-infiltrated ceramics, and polycrystalline alumina and zirconia.

Disclosure

The authors report no affiliation with any of the companies mentioned in this article.

About the Author

Maria Jacinta M.C. Santos, DDS, MSc, PhD
Assistant Professor, Schulich School of Medicine & Dentistry, Western University, London, Ontario, Canada

Max Dorea Costa, DDS, MSc
PhD Student, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil

José H. Rubo, DDS, MSc, PhD
Associate Professor, Department of Prosthodontics, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil

Luis Fernando Pegoraro, DDS, MSc, PhD
Associate Professor, Department of Prosthodontics, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil

Gildo C. Santos Jr., DDS, MSc, PhD
Associate Professor, Chair of the Division of Restorative Dentistry, Schulich School of Medicine & Dentistry, Western University, London, Ontario, Canada

Queries to the author regarding this course may be submitted to authorqueries@aegiscomm.com.

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