I. Aesthetic properties of all-ceramic restorations All-ceramic crowns and bridges have no metal structure, avoiding the possible allergy, toxicity, light blocking and many other adverse effects of metal. The optical properties of the restoration are similar to those of natural teeth, with good semi-permeability and hierarchy, natural color, and good corrosion resistance and biocompatibility. For patients with a large range of dental defects, high aesthetic requirements, or metal allergies, MRI and radiotherapy cases in which metal is not suitable for the oral cavity, all-ceramic restorations can be used to obtain a natural and beautiful restoration effect. Compared with porcelain, the aesthetic performance of all-ceramic restorations is excellent, mainly in the sense of hierarchy, semi-permeability and the natural color of the gingival margin area. The sense of hierarchy: the sense of hierarchy comes from the reflection of light from different depths or interfaces of semi-permeable materials, and the reflected light is integrated by the brain to produce a “sense of depth” and “hierarchy” of semi-permeable structures. Porcelain crown lip and buccal tooth preparation volume and all-porcelain crown is not much difference, but the thickness of the metal base layer of porcelain about 0.3-0.5mm, plus about 0.2mm thick color shading porcelain, in fact, the space left for decorative porcelain is only about 0.7-1.0mm, which is the dental technician can color and semi-permeable performance space, the reflection of light and scattering levels; and all-porcelain base layer is also a certain semi-permeable Material, so the thickness of the entire tooth preparation belongs to even if the color and semi-permeable reproduction of space, so the aesthetic performance of all-ceramic than porcelain and not difficult to understand. Even if the dental preparation is conservative thin veneers, the preparation of 0.7mm is all porcelain layer performance space, coupled with the veneers are not opaque metal layer, so although very thin, its aesthetic performance is far better than metal porcelain. In addition, all-ceramic restorations generally use the same light-transmitting properties of resin-like materials bonded, the optical properties of the restoration bonded can be extremely similar to natural teeth, which is also one of the necessary conditions to reflect the hierarchy of all-ceramic crowns. ② semi-permeability: all-ceramic base layer is semi-permeable material, can allow part of the light through into the interior of the tooth. If the transmission and scattering off the light increases, the reflected light decreases, and all-porcelain light transmission and scattering are greater than porcelain (the greater the thickness of the porcelain layer, scattering is also more obvious). Therefore, porcelain restorations give a high brightness value of the dead white color, all-porcelain restorations give a softer feeling, and because of the thickness of the semi-permeable porcelain layer is larger, the restoration can show a certain “sense of depth”, the whole restoration to give a very natural feeling. The semi-translucent effect of all-ceramic restorations is shown in Figure 1. Figure 1 Left: The translucency of the upper left lateral incisor restored with an all-ceramic crown is very similar to that of a natural tooth. The right figure shows the semi-permeability of the restored upper left central incisor after all-ceramic crown restoration, but the pile core is metal, and the restoration is different from the natural tooth. The natural color of the gingival margin area: all-ceramic restorations because there is no metal base layer of light shielding blocking effect, the incident light looking to illuminate the restoration, due to complex scattering and refraction, the restoration is illuminated at the same time itself is equivalent to a luminous body, the cervical margin line of the root side of the tooth tissue and the cervical margin area of the gingival group is also illuminated, the light path in the tooth is very similar to the natural tooth, so the restoration cervical margin area and The cervical margin area and the surrounding tissues of the restoration behave in the same way as natural teeth, showing a vibrant natural appearance. Porcelain due to the limited thickness of the cervical margin porcelain layer, the lack of the corresponding refraction scattering light path, coupled with the metal base layer of the shoulder blocking light, so that the residual dentition lack of luminous body effect, restoration cervical margin area and the surrounding tissue dull, lack of vitality, or even gray. Second, the classification of dental all-ceramic restorative materials There are more types of all-ceramic restorative materials, since Laud produced the first porcelain nail crown in 1886, there have been a variety of all-ceramic crown restorative materials. However, it is only in the last 20 years that all-ceramic crown systems have really been used clinically and have been able to achieve high success rates. Because of the strength and brittleness issues, it is required that the proper thickness of porcelain layer be reserved for the final restoration when preparing the tooth to ensure the strength of the restoration, and therefore the amount of tooth preparation is the largest of all crown restorations. However, with the emergence of high-strength materials such as infiltrated alumina, dense pure alumina and dense pure zirconia ceramics, the thickness of the base crown can be as little as 0.5 mm, and precious metal porcelain metal base layer thickness is very close, so the concept of all-ceramic tooth preparation than porcelain is also gradually changing. Currently, the commonly used high-strength all-ceramic materials by material components are mainly silicon oxide-based ceramics and non-silicon oxide-based ceramics two categories. The former representative materials include feldspathic ceramics, white garnet crystal-reinforced casting ceramics, lithium disilicate whisker-reinforced casting ceramics; the latter representative materials include glass-permeable oxide ceramic series, densely sintered pure alumina ceramics, and densely sintered yttrium oxide partially stabilized tetragonal zirconia polycrystalline ceramics. The flexural strength of zirconia ceramics can reach 1000 MPa to, is the highest in all-ceramic materials, but also more than the strength of most dental alloys, so known as “porcelain steel”. But the fracture toughness value in 10 Mpa・m1/2 below, dental porcelain alloy generally in 40 Mpa・m1/2 above, that is to say, all-ceramic materials have a high bending strength, but brittle larger. The general rule of all-ceramic materials is that the higher the strength and toughness of the material, its semi-permeability and aesthetic properties are decreasing trend (see Figure 2). Therefore, when restoration should weigh the strength and aesthetics of these two elements before deciding on the choice of material. Materials with high strength and low permeability are now generally used in a layered fabrication technique to obtain good aesthetic results; materials with low strength and high permeability are generally used only for the fabrication of defective restorations and require a resin-based adhesive to bond the restoration, and are therefore also classified as resin-bonded restorations. Figure 2 Comparison of the semi-permeability of restorations made of different materials. The uppermost is a dense zirconia four-unit bridge, which is less permeable; the middle is a glass-permeable alumina crown, which is moderately permeable; and the lowermost is a glass-permeable spinel crown and veneer, which has the best semi-permeability. All-ceramic base layer in accordance with the process of production can be divided into refractory model stacking sintering technology, lost wax method of casting all-ceramic, powder slurry coating glass infiltration all-ceramic, computer-aided design and production CAD/CAM computer production all-ceramic (such as machined infiltration ceramic, dense alumina, dense polycrystalline zirconia material system), electrophoretic porcelain deposition all-ceramic. The same material such as infiltration ceramics can be hand-coated molding, electrophoretic porcelain deposition, computer-aided cutting technology to process the restoration, the latter two processing methods to obtain a more uniform texture of the restoration, mechanical properties and transparency than the hand to a higher degree, of course, the price is more expensive. The use of all-ceramic restorations ranges from inlays, veneers, partial crowns, full crowns to short and long bridges for front and back teeth. Because of the different mechanical and optical properties of materials, each all-ceramic materials have their own specific range of application, in the clinical selection must understand the relevant characteristics of materials and indications, in order to obtain good aesthetic results and the strength of the restoration. Third, the current commonly used all-ceramic material restorations ① casting ceramic full crown casting glass ceramic is composed of silicon oxide, potassium oxide, magnesium oxide based ceramics, containing a small amount of alumina. The representative of this type of ceramic is Dicor system, the basic principle is to make the wax type first according to the method of metal restoration system, embedding, casting, the casting of the glassy crown porcelain after becoming a ceramic crown with improved physical properties, and finally in the surface of the crown color grill, complete the restoration body. Due to the unsatisfactory mechanical properties of the casting material, the cumbersome fabrication system and the poor aesthetics, it is rarely used in clinical practice. However, the development and application of this type of ceramics and its systems laid the foundation for the development of the current commonly used all-ceramic systems. The IPS-Empress all-ceramic crown system, introduced in 1990 by Ivoclar in Liechtenstein, is a representative of hot-press cast ceramics. The basic principle is to make the bottom crown wax type, embedding, and then according to the clinical colorimetric selection of porcelain casting, the use of white garnet crystal to enhance, after heat treatment can make the flexural strength of more than 300Mpa, and finally according to the way of all-ceramic restorations stacked plastic decorative porcelain. IPS-Empress Ⅰ type is mainly used for the production of single crowns, inlays, veneers; IPS-Empress 2 can be used for three units of anterior bridges The IPS-Empress 2 can be used for the fabrication of three unit anterior bridges. This system produces full crowns with high light transmission, aesthetic appearance, shorter operation time, good thermal stability and high strength. Since this system does not offer porcelain blocks in special colors, it is not suitable for restorations in patients who choose tetracycline and fluorosis colors. In addition, the actual strength values of commonly used ceramic materials are lower than those under experimental ideal conditions, and porcelain cracks have occurred during clinical application. ②Infiltrating ceramic full crown Infiltrating ceramic is a ceramic with alumina as the main component. 1988 French scholar Sadoun proposed a powder slurry coating (slip casting) all-ceramic restoration technology, later improved by the Vita company launched under the trade name In-Ceram. The basic principle is to form a core crown with alumina powder coating on the replica of the special substitute, and then coated with glass material after grilling, the glass material melted and penetrated into the alumina particles to enhance the strength of the material, and finally in the core crown surface according to the gold porcelain crown method of stacking decorative porcelain, to complete the restoration body. The infiltrated ceramic has a high flexural strength of more than 300Mpa, which is 3~4 times higher than the Dicor system, and can be applied not only for the fabrication of single crowns of anterior and posterior teeth, but also for the fabrication of three-unit bridges. In terms of marginal suitability and aesthetics, infiltrated ceramic full crowns are ideal and have been widely used at home and abroad with a high short-term success rate. The disadvantage of infiltrated ceramic fabrication of full crowns is that the alumina sintering and infiltration grilling is long and time-consuming, and requires a high degree of operational skill. The earliest material representative is ProceraAllceram, where the dental preparation is scanned, a three-dimensional image is formed, the sintering shrinkage of the surrogate and alumina powder is calculated, an enlarged surrogate is processed with CAD/CAM technology, and a fine pure alumina powder is pressed onto the surrogate using isostatic pressure technology to form the restoration, and then the alumina is densified and sintered, and the restoration The body shrinks to the final size together with the generation type, sandblasting to remove the restoration body, and then the conventional finish porcelain finish. The technology now commonly used by manufacturers is to pre-sinter the alumina to form a pre-block for CAD/CAM processing, then CAD/CAM processing of the pre-amplified restorations, and then densification sintering shrinkage to form the final base layer, similar to the current zirconia material. The dense alumina material has a flexural strength of up to 600 MPa and can be used for the fabrication of all-ceramic restorations including bridges, and improves the reliability of clinical restorative results. Zirconia-toughened ceramic: Zirconia-toughened ceramic (ZTC) has greatly expanded the range of previous all-ceramic restorations due to the excellent toughness of the tetragonal-phase zirconia base crown. The earliest of these ceramic restorative systems, Cercon, has a very high fracture strength (over 900 MPa) comparable to that of dental high-strength alloys, allowing the fabrication of multiple anterior bridges and 4- to 5-unit posterior bridges. The basic principle of making restorations is to make a wax model on a plaster model, fix it on a Cercon special wax holder, spread Cercon light scanning powder evenly on it, then place the wax model on the Cercon scanning and milling machine, and install the preformed zirconia porcelain block according to the program, the machine automatically scans the wax model, cut and mill the porcelain block, and finally the base embryo completed by cutting and milling in the Cercon special The porcelain is baked in the Cercon furnace to make the base crown, and the restoration is finished by grilling the porcelain for the finish according to the procedure. In the early days of Cercon, the restorations were technically called CAM because of the need to produce a prototype restoration for scanning, but with the advances in CAD/CAM technology, the model can be scanned, a 3D image can be generated in the computer, and the design of the restoration can be completed through human-computer dialogue, followed by CAM cutting and molding, and the zirconia restorations are truly CAD/CAM. The fracture strength of zirconia toughened ceramic full crowns is satisfactory, and the fabrication process is simpler and less time-consuming than that of gold porcelain restorations. However, the expensive set of special equipment and special porcelain blocks make the production cost very high, limiting its application. The strength of all-porcelain materials is based on ① Strength requirements: the order of strength of all-porcelain materials commonly used in the market is dense Y-TZP-ZrO2, infiltration mixed ZrO2/Al2O3, dense pure Al2O3, infiltration Al2O3, hot die-cast Empress 2/e.max, infiltration spinel, hot die-cast Empress. Whether the restoration will be used for inlays/veneers, crowns or bridges, anterior or posterior areas. The above materials can be used for full crown restorations, but considering the difference between the front and back teeth and the occlusal force, it is better to use materials with higher strength in the posterior area; veneers and inlays require relatively low strength, but in order to meet the aesthetic requirements, generally choose materials with low strength but good translucency; 3 units of short bridge restorations for anterior teeth require materials with a strength of 300 MPa or more, so the materials before hot die-cast Empress 2 can meet the requirements. materials are available to meet the requirements. Posterior bridges currently only infiltration mixed ZrO2/Al2O3, dense pure Al2O3, dense Y-TZP can meet the application requirements. ② Transparency requirements: a general trend of all-ceramic materials is that the higher the strength of the material permeability is also lower, the lower the aesthetic properties. So high strength all-ceramic materials are generally used only for the production of the bottom layer, the surface also needs to be covered with decorative porcelain. The order of translucent type from high to low: hot die-cast ceramic Empress, In-Ceram spinel, In-Ceram alumina, Y-TZP dense zirconia, dense pure alumina, infiltration of mixed ZrO2/Al2O3 basically does not have light transmission. The strength of current high-strength all-ceramic materials can generally meet the needs of anterior crowns. The most important thing in the selection of materials for anterior crown restorations is the choice of material semi-permeability. Good permeability of the material can obtain good aesthetic results, the choice of all-ceramic base material transparency should be consistent with the transparency of natural teeth. Too high or too low translucency will affect the aesthetic effect of the restoration. Figure 3 The left figure shows a case with a fiber resin pile core restoration and a patient with heavy tetracycline staining after root canal treatment for periapical inflammation. The right figure shows the result after restoration with a glass-permeable alumina all-ceramic crown. The glass-permeable alumina base layer has moderate strength and semi-permeability among all all-ceramic materials. Figure 4 The left image shows the patient’s upper left central incisor with fractured pulp, discolored tooth after root canal treatment, and low semi-permeability of the patient’s other natural teeth. The right figure shows that after restoration with infiltrated alumina all-ceramic crown, the semi-permeability of the restoration is significantly too high due to the thick enamel porcelain layer, and the restoration has poor aesthetic results. Fig. 5 The left figure shows the discoloration of the patient’s mesial incisor after root canal treatment for proximal to mesial caries penetrating the pulp. The patient’s natural tooth has high semi-permeability. The right figure shows the appearance of the restoration after restoration with a cast metal pile core with infiltrated alumina all-ceramic crown, which is not sufficiently semi-permeable because of the lower semi-permeability of the infiltrated alumina and the color masking effect of the metal pile core. The choice of base material for full crowns should also take into account the clinical color masking requirements. The clinical application should take into account the semi-permeability and color masking requirements to determine the choice of base material. The figure below shows the different colors of the preparations in the clinic, and the semi-permeability of the base layer may have different requirements to achieve good color shading. Fig. 6 Dental preparations with different degrees of discoloration. The color of the preparation is normal in living pulp teeth; after restoration of a residual root with a fiber resin pile core, the color is uneven and the neck stains darker; after restoration of a dead pulp tooth with a fiber resin pile reinforcement, the preparation stains darker; after restoration of a residual root with a cast metal pile core, the preparation is metallic silver. ③ Restoration suitability: there are some differences in the suitability of different all-ceramic systems, but they can meet the requirements of clinical applications. Cast ceramic precision is mainly affected by the casting process (including wax shrinkage, embedding material expansion, porcelain material solidification shrinkage); using CAD/CAM technology dense pure alumina and zirconia precision by the model scanning quality (dental preparation quality, model accuracy, scanning accuracy) and processing grinding accuracy. Therefore, for other all-ceramic systems, the marginal fit values are basically similar in different studies, while the CAD/CAM system varies greatly between different researchers, and in addition to the differences in CAD/CAM systems, the way and quality of the dental preparation is another major reason for the differences. V. Success rate of all-ceramic restorations Early products such as Dicor all-ceramic crowns 5-year success rate of about 55% (Anusavice, 1993; Ellison , 1992); IPS Empress crowns 4-year success rate of 98.1% (D. Edelhoff , Dtsch Zahnarztl Z, 2000; 55); IPS Empress 2 bridge 10 months~1 year success rate 90-97% (Sorensen, 1998; Postpiech 2000). The currently used In-Ceram alumina crowns have a 6-year success rate of 98.9% for anterior crowns and 99.2% for posterior crowns (Castellani D, Int J Prosthodont 1994;7:149-54); 100% for anterior bridges at 3 years, 89% for bicuspids (1998, John A. Sorensen); Procera AllCeram crowns 5-10.5 years 97.7% (Int J Prosthodont 2001;14:504C509); Cercon zirconia two years, posterior bridges, 100% success rate (G. Bornemann, 2003). From the above values, the success rates of the mainstream all-ceramic systems are currently above 95%, exceeding the success rates of metal porcelain restorations. All-ceramic restorations failure causes: ① porcelain crack and restoration fracture: ceramic materials are brittle materials, high compressive strength, good wear resistance, but low flexural strength, low toughness, so it is generally believed that the possibility of fracture than gold porcelain restorations are high, prone to porcelain fracture. But because the consideration, gold porcelain restoration body surface is also covered with low strength decorative porcelain, cut gold – porcelain bonding strength is much lower than the porcelain – porcelain bonding, therefore, in the clinical situation observed in the all porcelain restoration body porcelain fracture is not higher than the gold porcelain restoration body, or even some lower. The causes of chipped porcelain mainly include improper selection of indications (each all-porcelain system has its clinical scope), improper tooth preparation (inadequate preparation, the presence of sharp angles), production defects, bonding is not bright, improper use or external forces. ② Pulp problems: In general, the amount of all-ceramic crowns on the tooth preparation is greater than gold porcelain, thus leading to increased risk of pulp problems. But high-strength all-porcelain material on the bottom crown thickness requirements are also 0.5mm, and precious metal porcelain bottom crown thickness requirements are the same, so the all-porcelain dental preparation amount of the concept is also changing with the progress of the material.