Requirements and expectations placed on increasingly higher standards regarding the aesthetic results of restorative treatments in dentistry have led to the particular qualitative development of the materials used for this purpose, especially as regards entirely ceramic restorations. This applies both to morphofunctional odontal (uni- or pluri-dental) restorations and/or remodeling by indirect techniques, as well as for fixed prosthetic works (dental bridges), where they are capable of replacing metal-ceramic systems, that have already become “classical”.
In this regard, this paper is a review of contemporary ceramic materials, with specific references to their chemical composition and its consecutive properties, correlated with a series of recommendations for their clinical use.
We have approached hereby issues of recent vitreous ceramics with second phase/ crystalline filler based on lithium disilicate (becoming more widely used in clinical practice), as well as those related to ceramic masses with resin matrix, leading to favorable properties, such as their elastic modulus similar to dentin’s, shock absorption, high resilience and fracture resistance.
Also, specific properties of zirconium dioxide are evaluated, with respect to its crystallographic transformations and stabilization, its degradation at low temperatures and the factors affecting its “aging” given the conditions in the oral cavity.
Dental restorative materials; Dental ceramics; Leucite; Lithium disilicate; Aluminum oxide; Zirconium dioxide.
1. Dental ceramics
Dental ceramics have long been recognized for their similarity to tooth enamel. They can be made to closely mimic the color and texture of the natural tooth. Ceramic provides the doctor with the clinic ability to restore a patient’s teeth to look and function as the patient’s original teeth. Developments in the strength and consistency of manufactured ceramic have increased the spread of ceramic restorations in modern dentistry.
The biocompatibility and quality of the result of the use of dental ceramics are unmatched by any other restorative material. Over the past two decades, the use of glue-bound ceramic has expanded exponentially. Previously, the main weak link in restorative procedures with ceramics were the cements used for fixation, designed to occupy the space between preparation and restoration, created unintentionally during therapeutic maneuvers. These materials did not confer homogeneity through chemical or physical integration within the general restoration treatment. The latest composite resin cements not only occupy the spaces that exist between preparation and restoration, but also integrate the assembly in a homogeneous manner, both mechanically and chemically.
1. 1. Structure of ceramic masses
Ceramic is an inorganic material composed of metals (Al, Ca, Mg, K) and non-metals (Si, O, B, F), forming oxides, nitrites, borates or silicates, as well as complex mixtures of these materials. Intermolecular links in the ceramic mass may be of the ionic or covalent type, the different proportions of which change their mechanical properties.
Most ceramics (not just dental porcelain) are obtained by heating a powder or liquid (in which powder particles are dissolved) to a temperature at which the particles will merge to form a solid mass. This results in materials with specific intrinsic properties: high melting temperatures, rigid materials, but at the same time brittle, hard, abrasion-resistant, low-tensile resistance, higher compressive strength, good electrical and thermal insulators, which can be modified by adding a virtually endless range of colors.
The practical importance of the molecular structure is reflected in the optical properties of the ceramic mass, so that the more amorphous (noncrystalline) vitreous phase predominates, the more translucent ceramic is, and the more crystalline the ceramic is, the opaquer it will become.
When the word “ceramics” is used, an erroneous limited reference is made to the materials used for dental prosthetic restorations. It should be noted that, based on the structure and properties, there are other examples of “non-standard” ceramics used in dentistry, such as zinc oxide ceramics used in the composition of packaging materials, such as an inorganic filler in composite resins or in the CIS.
1.2. Composition of dental ceramics
Ceramics for general use are clayey ceramics that have three basic elements in their composition: feldspar (main component), quartz and kaolin, in varying percentages.
Dental porcelain initially contained very small amounts of kaolin (4-5%), but it was completely removed in modern ceramics.
Like most types of glass, dental ceramics generally contain minerals such as feldspar (K2O Al2O3 6SiO2) (80%), important amounts of silica, the most widely spread mineral on earth (SiO2) (14- 18%), small amounts of alumina: aluminum oxide (Al2O3) (2%) and small amounts of other metal oxides, which significantly influence the properties.
Along with the basic components, dental ceramics also contain other substances:
– inorganic metallic pigments that provide the color of the ceramic mass: titanium oxide for yellowish-brown tones; manganese oxides for purple, iron oxides for brown, copper oxide for green, cobalt oxides for blue;
– in the past, uranium oxides were added to obtain the fluorescence of the ceramic mass, but due to its reactivity, they were replaced by lanthanide oxides.
Thus, modern dental ceramics have two different phases in their composition: an amorphous vitreous matrix made up of a network of silicon pyramid structures in which the crystalline phase is dispersed, which determines the mechanical, physical, chemical and optical properties of dental ceramics. The ratio of the two components varies, and if there is more vitreous amorphous phase, the resistance of fractures to propagation is lower and the translucence is higher. Generally, ceramics used for entirely ceramic systems will contain between 35 and 90% crystalline phase to increase mechanical properties.
Classical feldspar ceramic masses had initially a number of drawbacks: high shrinkage (30%) of the sintering, very tough and rigid, brittle, with a very low resistance to flexion, high risk of fracture, used only on front teeth.
New techniques and materials have been tried with increased resistance to traction and flexion. In general, research has focused in two directions:
a. Some authors have proposed a technique similar to the metal-ceramic one, based on a ceramic core, more resistant, even if less aesthetic (opaque), over which more aesthetic feldspar ceramic layers are laid;
b. Others have proposed to obtain ceramics with high mechanical properties, which do not require additional space to mask the core (whether metal or ceramic).
1.3. Properties of dental ceramics
1.3.1. Mechanical properties
– very high elasticity (alumina = 380 MPa): rigid material;
– high compressive strength (150-900 MPa): a wear-resistant material;
– very low tensile strength (20-60 MPa);
– extremely low fracture resistance, practically non-existent: a brittle material;
– maximum plastic deformation glass can withstand is 0,1: a rigid material;
– material very sensitive to the presence of surface micro-fissures;
– classical ceramic hardness was 460-660 VHN, so higher than dental enamel, resulting in the wear (abrasion/wear) of antagonist teeth. New ceramics with low sintering temperatures have 380 VHN, thus close to the enamel’s. Ceramics used for infrastructures (based on Al2O3) have a greater hardness, i.e. 1200 VHN, and therefore are always coated (covered) with feldspar or vitreous ceramic.
1.3.2. Other physical properties
– average density = 1.0-3.8 g/cm3: the weight of a ceramic work is lighter than a metal work;
– very high melting temperature: a refractory material;
– reduced thermal expansion coefficient (CTE) (1-15 x 10-6/°C): a good insulator material (advantage for entirely ceramic works, but for metal-ceramic works this is an inconvenient; in this case, ceramics with a lower sintering temperature should be used: 100-200°C lower than the solidification temperature of the metal alloy);
– the optical properties of the ceramic masses are of the highest interest because they are directly correlated to the aesthetic result. As we mentioned in the section about properties, translucency decreases if there is more crystalline phase, so that vitreous and feldspar ceramics will have the best optical properties (Empress, Dicor), then translucency is lower, so the opacity is increasingly higher for more resistant ceramics, with more crystalline phase (infiltrated ceramic: Spinell, Alumina, Zirconia).
1.3.3. Chemical properties
– reduced reactivity: a material basically inert chemically, in the oral cavity conditions (not influenced by wide variations of the oral pH, it is not attacked by acids in the mouth cavity).
1.3.4. Biological properties
– very good biocompatibility: an almost totally inert material in relation to the tissues of the oral cavity;
– coated ceramic is not prone to dental plaque: a material used in periodontitis prophylactic.
2. Classification of variants of dental ceramic masses
There are currently many types of dental ceramics (entirely ceramic systems) and a series of classifications have emerged for their systematization. The main criteria used in the classification are: sintering temperature (melting) of ceramics, composition and microstructure of ceramic masses, laboratory manufacturing technology and field applicability in dentistry.
2. 1. Classification by sintering temperature
• high sintering temperature (over 1300°C)
• average sintering temperature (1100-1300°C)
• low sintering temperature (850-1100°C)
• very low sintering temperature (680-780°C)
2. 2. Classification according to composites and microstructure
• Category 1: amorphous vitreous ceramic (primarily composed of silicon dioxide)
• Category 2: vitreous ceramics with secondary phase / crystalline filling (usually leucite or another type of glass with high fusion point)
• Category 3: crystalline ceramics (mainly aluminum oxide) with glass matrix
• Category 4: polycrystalline ceramics (aluminum oxide and zirconium oxide)
2.3. Classification by technology
A. Additive systems
• successive layers (Optec HSP, Vitadur, Duceram LFC)
• casting (Cerapearl, Dicor)
• infiltration and sintering (In-Ceram)
• pressing (PS Empress, Cerestore, Optec OPC, Cerapress)
B. Subtractive systems
• mechanical milling
• computerized milling (CAD / CAM)
2. 4. Classification by use of dental ceramics
• coating ceramic (generally laid in layers over a metallic frame or a core of oxide ceramic)
• ceramics for full ceramic crowns
• ceramics for the frame of partially fixed or mobile dentures
• ceramics for superstructures on implants
We will discuss below a series of additional matters as regards two of these classifications (after McLaren, Figueira, Giordano and Whiteman) on dental ceramic in order to understand the differences between the most commonly used ceramic systems, namely the classification based on composition and microstructure and respectively, based on the lab manufacturing technology.
2.2. Classification by composition and microstructure
2.2.1. Category 1: Amorphous vitreous ceramics
They mainly contain silicon dioxide (known as quartz or silica), with varying amounts of aluminum oxide – aluminosilicates. Their mechanical properties are weak, the flexural strength is 60-70 MPa, and are therefore generally used as a coating material (traditional feldspar ceramic) or other ceramic sublayers (on a high strength ceramic core), and to make dental veneers on refractory mass or on platinum sheet.
2.2.2. Category 2: Vitreous ceramics with secondary phase/crystalline filler
The glass matrix is the same as in the previous category; the difference is in the percentage and type of crystals, which either are introduced into the ceramic or are grown in the glass phase. Currently, the most commonly used crystals are leucite, lithium disilicate and fluoroapatite. These materials are used to make blocks for the CAD/CAM technique, for the CEREC system (Sirona Siemens): Vitablocks Mark II (Vita Zahnfabrik), they also have the lowest failure rate (about 1% / year) in the manufacture of ceramic inlays and onlays.
A. With low/average content of leucite
Improperly referred to as feldspathic ceramic, they are used as coating material over metallic frames, ceramic cores or for ceramic veneers on refractory mass. The first materials in this category had a very variable size and distribution of leucite particles, so they had a very low flexural strength. Generally, these ceramics are in the form of a powder/liquid. New materials contain fine particles with a diameter of 10-20 μm and with a homogeneous distribution of leucite crystals, being less abrasive and with a higher flexural strength (for example, Vita VM13 ceramics), these materials are especially used for new metallic-ceramic systems.
B. With high content (about 50%) of leucite
Initially, it looks as a vitreous amorphous matrix around leucite crystals, and following the second heating process new crystals are formed and grow, which block the propagation of fissures and generate compression forces around the crystals, which gives improved mechanical and physical properties: high fracture strength (120 MPa) and abrasion resistance. They may be opaque or translucent depending on the proportion of the crystalline phase. The most common materials of this class are: Empress (Ivoclar Vivadent), Finesse (Dentsply), PM 9 (Vita) and Empress CAD/CAM (Ivoclar) milling blocks used for the CEREC and E4D systems.
C. Based onlithium disilicate
Vitreous ceramic (glassy) with 70% crystalline phase and high flexural strength (about 360 MPa) was first introduced as Empress II, and then as IPS e.max (Ivoclar), both versions for CAD/CAM pressing and milling. Although it has a high proportion of crystalline phase, the material is translucent enough to be used in entirely ceramic restorations (veneers, crowns) due to the small refractive index of the lithium disilicate crystals. On the other hand, due to the high mechanical resistance, it can be used as a single ceramic core over which special ceramic with fluorapatite crystals will be layered, which will restore the morphology and the color of the final restoration.
2.2.3. Category 3: crystalline ceramics with interpenetrating phases
They have been in use since 1988, in the form of the InCeram (Vita) system; it contains at least two phases intersecting and extending from the depth to the surface. First a porous matrix is obtained, which will then be “saturated” by capillarity with a second vitreous phase (lanthanum aluminosilicate glass). The system has emerged as an alternative to metal-ceramics, since it starts from making a core with a very high strength (350-650 MPa) and can be obtained by sintering the alumina deposited on the tooth or by milling presinterized blocks.
2.2.4. Category 4: Monophasic polycrystalline ceramics
They contain a single crystalline phase and are obtained by sintering crystals, without a vitreous matrix, examples: aluminum oxide (alumina: Al2O3) and zirconium oxide (ZrO2). The first system was Procera (Nobel Biocare), wherein the ceramic in the form of aluminum oxide is pressed on the mold, then milled and sintered at 1600°C, resulting in a very dense core, highly resistant (fracture resistance: 600 MPa), but with a 20% contraction after burning, which makes marginal adaptation deficient. Another monophasic ceramic is zirconium, which is not used in its pure form, but partially stabilized by the addition of various oxides; most zirconium-based ceramics used in dentistry contain a 3% of yttrium (Y) in their mass: Y-TZP. Zirconium has a flexural strength of 900-1100 MPa, higher than any other ceramics, so one can make extensive restorations up to total bridges, infrastructures for complex implantation cases or movable partial denture even in areas of occlusal functional stress.
2.2.5. Clinical implications of classification based on ceramic composition and microstructure
A. Possibility of etching and adhesive fixing: thus, ceramics from categories 1 and 2 can be etched with hydrofluoric acid, so it can create a good adhesive bond, while those in categories 3 and 4 cannot be etched with acids, so it is harder to fix them (they require special procedures).
B. The content of a glass matrix ceramic makes it more transparent, therefore with applications in the aesthetic area, but with less flexural strength. In contrast, monophasic crystalline ceramics have the best mechanical properties, but are more opaque, so they can be used as core/infrastructure, covered with veneers.
2.3. Classification according to the laboratory manufacturing technology
From a clinical and practical point of view, the most relevant classification of today’s ceramic systems is based on the manufacturing technology, with 4 major classes of systems:
1. The processing of ceramics by manual stratification and sintering on refractory molds or on a platinum matrix / sheet
2. The processing of ceramics by plastic molding / injection / pressing
3. The processing of ceramics by glass infiltration of porous structures and sintering
4. Mechanical processing of prefabricated ceramic blocks (ingots)
2.3.1. Sintered ceramic on refractory mold or platinum sheet
It is the oldest method of manufacturing ceramics (dating from 1886) and was at one time the most widespread due to solid advantages:
• it does not require a special laboratory equipment, uses classical feldspar ceramic, similar to metal-ceramic coating (MC) and the same combustion oven;
• required preparation is conservative, ceramic veneers of 0.3-0.4 mm thick can be obtained by this process.
• the aesthetic results are excellent, because the feldspar ceramic is purely vitreous and has the best translucency, the stratification is carried out on the entire thickness of the restoration (it allows to results from the depth), it perfectly imitates the enamel of natural teeth;
• it can be etched with hydrofluoric acid, obtaining microretention for a very strong adhesive bond to dental enamel.
The problems of the technique entail a few difficult issues to solve:
• the technique is very sensitive and inconvenient for the doctor (very fragile before fixation) and for the technician (no further corrections are possible after the test in the oral cavity);
• it requires three models for a clinical case, so the technique is laborious;
• flexural resistance of feldspar ceramics is the smallest of all current systems (<100 MPa), so they are vulnerable to this type of stress.
Therefore, this technique is reserved in recent years only to feldspar ceramic veneers, especially in the no-prep technique or in ultra-conservative preparations, where the optical properties of the enamel are to be reproduced in a lower layer of ceramics (< 0.5 mm). For cases with severe intrinsic discoloration with extensive coronary destruction, with traumatic occlusions, there are currently other high strength ceramic systems that give a good aesthetic result.
2.3.2. Ceramics processed in a plastic state by casting / injection / compression
The method is widely used today in all countries due to the “lost wax” technique (which all technicians master), inexpensive laboratory equipment, versatility of current systems covering all types of prosthetic works.
The first generation contained vitreous ceramic enriched with leucite crystals (35-40% crystalline phase), flexural strength, and double fracture compared to the feldspathic ceramic, a degree of porosity of 9%, thus being suitable for unitary dental restoration: inlays, onlays, total/partial crown, veneers.
The materials from this class’ current generation have a higher percentage of crystalline phase (65%) and a reduced porosity (1%). The resulting material is an interconnected network of crystals in a vitreous matrix; due to the differences between the thermal expansion coefficients of the two phases, tangential compressive forces develop around the crystals, resulting in increased mechanical strength and fracture deviation (the resistance of the second generation is double compared to the first generation, thus 4 times higher than classical ceramics).
The most well-known example of pressed ceramics is IPS Empress (Ivoclar Vivadent), introduced over 20 years ago, with all subsequent variations, of which over 37 million restorations were made by 2010 (according to statistics).
All ceramic pressure systems use the same technology principle: they are molded from wax, restored to the final size or smaller one (core), injection rods are applied, wrapped, wax is removed, then the ceramics are heated at high temperature and cast / injected / pressed into the mold.
If the first generations of Empress I were suited only for inlays, onlays, veneers, frontal crowns, the latest generations, e.max Press, have almost generalized applications. They can be used for mini-veneers, occlusal veneers, table-top, inlays, onlays, frontal and lateral crowns, bridges with a single proxy in the frontal and lateral sides (save for the molar area), overpressing on unidental heads or on metallic precast abutments, frontal and lateral unidental implants up to the second premolar, primary heads for telescopic crowns.
The contraindications of this system are: bridges in the molar area, bridges with more than two proxies, profound subgingival preparations, subtotal edentulous patients, bruxism, extension bridges, Maryland bridges. For lateral bridges with more than three elements, up to twelve elements, the only ceramic resistant enough is the one based on zirconium dioxide, but here as well the vitreous ceramic e.max Press can be used because these ingots can be pressed over the zirconium frame (infrastructure) in order to obtain improved aesthetic results.
2.3.3. Ceramics with crystal core obtained by glass infiltration of someporous structures
The best known ceramic in this class is InCeram (Vita Zahnfabrik), introduced in the 1985. Starting from the principle of metal-ceramic technique, it has discovered the core of aluminum oxide with a flexural strength three times higher than other systems existing at that moment.
The InCeram system uses a high resistance ceramic head (from aluminum oxide or magnesium), which is then infiltrated with a lanthanum aluminasilicate glass. Layers of feldspar ceramic are laid over this core, which is sintered the same way as the metal-ceramic technique, which is well-known and widespread.
That is why the system has been well received by technicians since 1990, as it is successfully suited for all aesthetic all-ceramic restorations: veneers, inlays, onlays, unitary crowns, both for front and side teeth, and even for small bridges, with a rate of success of 98% for three years in the frontal area and 94% for premolars and molars.
The different variants of the system have allowed broad applications, such as:
A. The first system, InCeram Alumina (with a crystalline matrix of Al2O3), has high flexural resistance (450-500 MPa) and average translucency, so it can be used for frontal and side teeth as unitary crowns;
B. InCeram Spinell (with magnesium and aluminum oxide matrix) is the most translucent system and with an average mechanical resistance (350 MPa), so it can only be used for crowns and frontal veneers;
C. InCeram Zirconia (with an aluminum oxide matrix of 30% and 70% zirconium) is the most resistant system (650 MPa), but at the same time the opaquest, being mainly used in three-component bridges in the lateral area.
D. InCeram Celay (the 4th variant) is a subtractive system, which obtains the ceramic head by mechanical milling with the Celay (Vita) device; after infiltration with aluminasilicate, ceramic coating is applied for morphology purposes and coloring.
2.3.4. Mechanical processing (CAD/CAM systems) of prefabricated ceramic blocks
There are many CAD/CAM systems currently in use in dentistry, with two components: CAD (Computer Aided Design) and CAM (Computer Aided Manufacturing), all of which have four basic components:
A. an image acquisition unit;
B. software that will make the restoration design on a virtual mold;
C. a milling device that will produce restoration from ceramic blocks: in the doctor’s office (InOffice systems) or in the laboratory (InLab systems);
D. a communication system between the scanner and the milling device.
The systems can scan: either in the doctor’s office, the oral cavity directly with an intraoral scanner (Lava COS, E4D, CEREC BlueBam AC), or in the laboratory, where a plaster mold or the mold sent by the dentist will be scanned (CEREC InLab, Cercon).
Then the scanner’s built-in software will design a 3D virtual model, which will be used for the intended restauration in the doctor’s office with the help of InOffice systems (CEREC InOffice, E4D Dentist), by the dentist or their staff, or in the laboratory for the InLab systems (CEREC InLab, E4D Labworks).
Next is the milling stage, which can also be performed in the doctor’s office, using small milling devices, with fewer milling blades, or in the laboratory, using milling devices with more milling blades.
At present, the CAD/CAM technology is often “synonymous” with zirconium, but it is a misconception of the term. The confusion stems from the fact that this material could not be processed by any of the other methods of ceramic manufacturing, so its applications in dentistry have appeared only with the occurrence of CAD/CAM systems, hence the erroneous assumption that these computer systems mill all the zirconium oxide.
In general, a pre-sintered zirconium dioxide ceramic block will be milled, which requires a new sintering following milling, that is why it is oversized to compensate for further contraction. A pre-sintered zirconium frame will be designed, which will be sintered again and, in the end, we will have a zirconium frame of the size of the master mold, perfectly adapted to lateral sides.
The zirconium frame can be milled in different ways:
– Restoration to the final size (full contour Zr crowns) – occurred in the test phase in 2010, due to numerous failures by delamination of the ceramic coating on the Zr frame (Chipping);
– To a smaller form than the final restoration, leaving space for coating (generally, a vitreous ceramic with crystalline phase: e.max ZirPress (Ivoclar), Vita VM 9 or Vita VMP9 (VITA);
– Mixed: zirconium palatal veneers, a zirconium molar and the vestibular areas with a space for ceramic stratification or pressing.
At present there are several types of ceramic masses processed with this technology. In general, there are 3 categories of ceramics that can be milled with CAD/CAM systems (according to the classification based on content and microstructure):
1. Category 2 – vitreous ceramics with the addition of lithium disilicate: used for conservative restoration (inlays, onlays, unitary front crowns, veneers). There are two types of ceramic blocks in this category for CAD/CAM: Vitablocks Mark II and Esthetic Line (VITA) and ProCAD and IPS e.max CAD (Ivoclar Vivadent), both designed to be processed with the CEREC InLab (Sirona) device.
2. Category 3 – crystalline ceramics with interpenetrating phases: the blocks contain aluminum oxide or aluminum oxide and magnesium, being milled only in the CEREC InLab system and then infiltrated with lanthanum aluminosilicate glass, with a technique similar to the InCeram system. Aluminum oxide based materials are ideal for crowns on the front teeth, as the final product is sufficiently translucent, the rest have shown excellent results in front and side unitary crowns, even in 3-element bridges, suited only for the frontal area.
3. Category 4 – polycrystalline monophasic ceramics (zirconium oxide): there are multiple systems that fall into one of the following classes:
• CAD/CAM systems that mill the restoration directly into the intended final shape, using a block of fully sintered zirconium oxide (it does not require further sintering): very expensive, generates high temperatures during milling, long milling periods due to the hardness of the ceramic block, but research in the field keep improving it constantly (DCS Preciscan);
• The Procera (Nobel Biocare) system, which uses an oversized model over which aluminum oxide or zirconium oxide is used; it is sintered and will contract enough to adjust to the original mold per a 1: 1 ratio. The system is used to make abutments in laboratory for Bränemark implants;
• The third system, the most current at present, mills an oversized infrastructure from a pre-sintered zirconium oxide block, which will then be sintered again and will undergo a lower contraction. This will result into a head for a crown or a small frame for bridges, which will be adjusted to the master mold. To achieve this method there are several options in practice: CEREC InLab (with VITA YZ or Ivoclar e.max CAD ceramic blocks), Lava (3M ESPE), Cercon (Degudent), Everest (KaVo).
Dental ceramics and their processing technologies have evolved significantly over the last 10 years, with the most important part of the evolution being correlated with the new microstructures of the interpenetrating phases and the extensive use of CAD/CAM methods.
There is also an increasing trend for the use of monolithic ceramic restorations (for example, those based on lithium disilicate), as those made by multilayer coating of a resistance infrastructure (e.g. aluminum oxide or zirconium dioxide), although more aesthetically, are still vulnerable, being exposed to fissures and/or fracture, the coating ceramic being prone to chipping.
Although there is an opinion, increasingly common among practitioners, that entirely ceramic restorations using current materials can “cover” all situations where there is indication for restoration and/or (for one or more teeth) odontal morpho-functional remodeling through indirect techniques, as well as the one for fixed dentures (dental bridges), it is necessary to take into account the fact that there are still a number of limitations to be taken seriously.
Entirely ceramic restorations (including monolithic ones) are not recommended for patients experiencing the following situations: insufficient hard dental substance for the affected teeth (clinical crowns/short dentures), cases of subgingival expansion (mostly because of issues related to the final adhesive fixation), poor oral hygiene (which cannot be controlled and improved), cases of clinical manifestations due to masticatory/occlusal parafunctions (especially bruxism).
However, future materials for entirely ceramic restorations are very promising. New improvements in chemical composition, internal structure and architecture, modification of crystalline phase components to nanodimensions, and improvement of protocols for the industrial manufacture of precast ceramic blocks, as well as for ceramic mass processing in dental laboratories will lead to significantly superior properties of these materials in order to meet the most rigorous aesthetic, mechanical and biocompatibility requirements.
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