In 1962, Weinstein et al [1] invented the technique of porcelain with fused metal, which organically combines the strength of the metal base structure and the aesthetic properties of porcelain to produce a certain aesthetic The technique combines the strength of the metal base structure and the aesthetic properties of porcelain to produce a gold-ceramic crown and bridge restoration with certain aesthetic effects. This technology is now mature and widely used in the production of dental prostheses. However, due to the base metal (nickel-chromium alloy) there is a tendency to be oxidized and the phenomenon of gingival gray line, on the other hand, the gold and porcelain edge of the denture in the metal and porcelain while contacting the gingival margin, resulting in the restoration edge coloring, affecting the long-term effectiveness of the denture disadvantages. With the improvement of material culture, people are not satisfied with the oral prosthesis only to restore its physiological function, but require a better aesthetic effect. These factors have promoted the development and application of all-ceramic crowns with higher aesthetic effects. All-ceramic crowns because of the transparency and refractive index close to the enamel, the appearance of its color and natural teeth, can achieve the best aesthetic effect, coupled with its good biocompatibility, increasing strength and toughness, more and more popular. Now all-ceramic restorations have become an important restorative means in fixed restorations, all-ceramic materials are widely used in the production of full crowns, veneers, inlays, pile cores and fixed bridges. At the same time, the success rate of clinical all-ceramic restorations is also rising, research shows that the average failure rate of all-ceramic restorations is 5%, the following discussion of the causes of common problems. A porcelain crack High strength, good fit and acceptable aesthetic results are necessary to fix the restoration. Despite the high strength of all-ceramic materials in the test, but in the tensile stress due to its surface cracks and microfractures make it easy to fracture. Porcelain fracture is also one of the main causes of failure of all-ceramic restorations, as Charles J [2] found from 22 clinical studies on porcelain fracture: 338 porcelain fractures out of a total of 4,277 all-ceramic crowns, or about 7% of the total. The firing rate of porcelain cracks differed depending on the duration of the study, with 18 cases in studies of 1 to 4 years and a mean porcelain crack rate of 5%; and 4 cases in studies of 5 years and more with a mean porcelain crack rate of 13%. 1, the effect of porcelain layer thickness All-ceramic restorations are composed of two parts: body porcelain and veneer porcelain. LiuYH et al [3] conducted a study on the flexural strength of IPSEmpress2 nuclear porcelain and veneer porcelain with different thickness ratios, and the results showed that the flexural strength of veneer porcelain was significantly lower than that of nuclear porcelain, and the effect of different thickness ratios of nuclear porcelain and veneer porcelain on the flexural strength of the restorations was not significant, but the large thickness ratio of nuclear porcelain The restorations with a large proportion of nuclear porcelain thickness are less likely to show the phenomenon of separation between porcelain layers. Domestic scholars believe that no matter what kind of restoration, cast porcelain layer thickness shall not be less than 0.8mm, otherwise it is very easy to make the bending deformation of more than 0.1%, resulting in the fragmentation of the restoration. 2, the impact of porcelain surface treatment nuclear porcelain crown and the combination of external coating materials directly affect the strength of the entire porcelain restoration, the cleanliness and roughness of the inner crown and the combination of external coating materials is essential. Different treatments are available for different internal crown materials, and AlbakryM [5] concluded that sandblasting increases the roughness of the nuclear porcelain surface and facilitates the bond between the two layers of porcelain. He also concluded that glazing did not increase the strength of the cast porcelain material, while polishing significantly improved the strength of the material and sandblasting and grinding had no significant effect on the strength of the restoration. This may be due to the fact that polishing produces a highly polished surface and reduces defective cracks, and the compressive layer formed on the surface of the material during polishing also helps to prevent the expansion of microcracks, while sandblasting and grinding can also form a similar compressive stress layer on the surface of the material, and the strength reduction caused by the internal compressive stress release due to heating during the glazing process and the improvement of strength due to the reduction of sharp cracks cancel each other out. 3, the impact of dental preparation In comparison with metal porcelain restorations, the amount of dental preparation needs to be more in order to ensure the strength of the all-ceramic system. According to Feng Hailan [6], the cervical margin of all-ceramic veneers is generally 0.5-0.6 mm without an angular shoulder, the amount of lip preparation is generally 0.7-0.8 mm, and the amount of cut-end preparation is generally 1.0 mm. If excessive shading is not required, the cervical margin is generally prepared on the gingiva. All-ceramic inlays or high inlays have an apical and facial preparation of 2 mm and 1.5 mm, respectively; the width of the isthmus is at least 1.5 mm; the degree of abduction needs to be greater than that of metal inlays, about 12 to 15°; the facial and gingival margins should avoid forming a bevel. The preparation of the posterior surface of the all-ceramic crown restoration is generally 1.5 mm, the posterior cusp and anterior incisal end is generally 2 mm, the labial surface, buccal surface is generally 1 to 1.5 mm; edge for the right-angle shoulder (inner line angle rounded) or deep angular shoulder, the width of at least 1 mm. TroedsonM et al [7] used a two-dimensional finite element method to study the effect of edge design on the stress distribution of porcelain veneers, the results found that ProosKA et al [8] investigated the effect of cervical margin design and abutment polymerization angle on the stress distribution of anterior molar all-ceramic crowns by applying the finite element method. It is generally believed that all-ceramic restorations need to be protected to the maximum extent without affecting the restorative morphology and strength of the restoration. 4, the selection of all-ceramic materials SiogrenG et al [9]. A clinical evaluation of 98 Di2cor all-ceramic crowns in 46 patients in a general practice after an average of 6 years showed a satisfaction rate of 82% among the 98 all-ceramic crowns. Fifty-one percent of the all-ceramic crowns had good edge-to-edge fit. Fourteen Dicor crowns were fractured, and the best feature of Dicor crowns is their good light transmission and color. However, the long-term effect of Dicor, especially the shatter resistance, needs further improvement. DeniG [10] showed that 37 IPS2Empress all-porcelain crowns in 20 patients followed for 2 years showed a satisfaction of 94.6%, only 1 case of crown fracture, and no significant difference in the health of the gingival tissue regardless of whether the crown edge is located on the gingival or flush with the gingival margin, indicating that IPS2Empress The recent restorative results of all-ceramic crowns are good and the failure rate is low. Zirconia is a polycrystalline material with three forms of existence. At its melting point (2680°C), it exists as a cubic structure, while at 2370°C, it deforms into a tetragonal phase with strengths of up to gigapascals. Fixed partial denture is recommended to choose zirconia, because zirconia has the highest damage load compared with alumina-based and lithium disilicate based ceramics. 5, porcelain full crowns in the oral cavity position in the oral cavity, because the anterior teeth are susceptible to shear forces, while the posterior teeth withstand greater occlusal forces, these factors, will affect the effect of all-ceramic crowns in the oral cavity. probsterL.[11] on 96 porcelain full crowns for up to 56 weeks saw only one case of molar crowns with porcelain cracks. segal[12] on 546 cases of In -ceram all-ceramic crowns over 6 years comparing successful and failed cases and showed 541 successful cases and 5 failed cases. The overall success rate was 99.1% (n=541) and the failure rate was 0.9% (n=5). The success and failure rates for anterior crowns were 98.9% and 1.1%, respectively; the success and failure rates for posterior crowns were 99.2% and 0.8%, respectively. With the development of all-ceramic bonding materials, the retention of all-ceramic restorations has been greatly improved. Charles J [2] found that the rate of restoration loss is about 2%. The choice of bonding agent and the operation during bonding are the key factors affecting the retention of all-ceramic restorations. ChaoYongLie [13] advocated that bonding should be done with resin bonding agents, and that the treatment of the bonding interface before bonding varies with the all-ceramic material, with sandblasting, hydrofluoric acid etching, silanization treatment, and silicon spraying of the bonding surface being the measures often used. For silicate ceramics, due to the glass phase structure, hydrogen fluoride etching can obtain a rough bonding surface; silane coupling agent and bonding surface of silicon oxide combined with the formation of siloxane, coupled with resin binder, you can get good bond strength and edge closure effect. For alumina ceramics or magnesium oxide ceramics, some contain a small amount of silicon oxide, some do not contain silicon oxide, so sandblasting can form a rough surface, you can use resin binder or zinc phosphate, glass ionomer adhesives, to obtain a better bonding effect. The bonding effect can also be enhanced if silicon spraying plus coupling agents are used. For zirconia ceramic restorations, due to its own high strength, if the restoration has a good retention type, a resin bonding agent or any of the zinc phosphate or glass ionomer adhesives can be used to obtain good results. Excess bonding agent should be thoroughly removed, otherwise it is very irritating to the gums and can lead to gingivitis and periodontitis. For the high transparency of all-ceramic restorations, should be used in advance to test the color paste to choose a different color bonding agent, in order to achieve the aesthetic effect after bonding. In summary, problems in any part of the all-ceramic restoration may lead to the failure of the final all-ceramic restoration. Ceramic materials and their production methods are now in a process of development, to improve the handling of each possible problem in the restoration process requires continuous research and exploration.