Ceramic materials have been used as oral restoration materials for over 200 years. Since Land produced a feldspar all ceramic crown in 1886, all ceramic materials have gradually become a research hotspot. Since the 1960s, scholars have successively added garnet and alumina to feldspar ceramics, improving the mechanical and physical properties of all ceramic materials. In the early 1990s, zirconia ceramics were introduced into the field of dentistry. Due to their excellent flexural strength and fracture toughness, they were quickly applied in various fields, including post core crowns, implants and abutments, orthodontic brackets, fixed bridge brackets, etc.
Oral ceramic materials have excellent color stability, wear resistance, biocompatibility, as well as excellent optical properties and aesthetics, and occupy an increasingly important position in clinical dental restoration. At the same time, a large number of all ceramic products have emerged on the market, including ceramic like materials launched in recent years. The numerous materials and brands often cause confusion for clinical doctors in their choices. This article categorizes the current mainstream oral ceramics and ceramic like restorative materials based on their microstructure, and provides a review from the aspects of material properties, main products, clinical indications, surface treatment, and bonding. The aim is to elucidate their clinical applications, help clinical doctors understand the characteristics of different ceramic ceramics and ceramic like materials, and make reasonable choices.
1、 Classification of all ceramic materials
All ceramic materials are usually composed of two or more phases. According to the different contents of glass phase and crystal phase in the microstructure of ceramic materials, all ceramic materials can be divided into three categories: ① feldspar ceramics mainly composed of glass phase; ② Glass ceramics containing both glass and crystal phases; ③ Polycrystalline ceramics without glass phase.
1. Feldspar porcelain
Feldspar porcelain is a type of amorphous glass made by high-temperature sintering of natural feldspar, quartz, and kaolin. Feldspar porcelain is a ceramic material that has long been used in dentistry, and its optical properties are very close to enamel and dentin. However, due to its poor mechanical performance, the flexural strength is usually only 60-70MPa, so it is commonly used as a porcelain fused to metal restoration (PFMS), fused to ceramic restoration (PFCS), or veneer.
2. Glass ceramics
Glass ceramics, also known as microcrystalline glass, are a type of composite material made by high-temperature melting, forming, and heat treatment that combines crystal phases with glass. Compared with amorphous glass, the addition or growth of crystal fillers in the glass phase significantly changes the mechanical and optical properties of glass based ceramics, such as increasing the coefficient of thermal expansion and toughness, and changing the color, milkiness, and transparency of the material. According to the different crystal composition, there are several types of glass ceramics commonly used in clinical practice. Leucite reinforced glass ceramics: Leucite (mass fraction 35%~55%) is used as a reinforcing phase in all ceramic materials. Due to its aesthetic similarity to feldspar ceramics, its strength is improved (flexural strength can reach 120-180MPa), and it is more conducive to resin bonding, making it more widely used.
Although clinical studies have shown that the fracture rate of garnet reinforced glass ceramics is relatively low, the success rate of using it for posterior teeth is significantly lower than that for anterior teeth. Therefore, it is mainly used for single crown restoration with inlays, high inlays, veneers, and anterior teeth. Lithium disilicate glass ceramic: SiO2 Li2O glass ceramic has a high crystal phase content (about 70%), so its flexural strength can reach over 300-400 MPa. Although the crystal phase content is high, the transparency of the material is still high due to the low refractive index of lithium disilicate. It can be directly used as an all ceramic restoration or decorated with fluoroapatite glass ceramics on its surface.
Except for IPSEpress2, which has lower strength, all other products can be used for three unit fixed bridges with veneer, inlay, high inlay, anterior and posterior single crown, and anterior/premolar area. However, as a fixed bridge, its clinical success rate significantly decreases, mainly due to the fracture at the connection of the fixed bridge. In recent years, improved materials based on this material have begun to emerge. Vita Supernity (VITA) is a zirconia reinforced lithium silicate glass ceramic (ZLS) formed by CAD/CAM in 2013. By using a special process to add zirconia (about 10% specific gravity) to glass ceramics, compared with traditional lithium disilicate glass ceramics, it not only improves the flexural strength (494.5 MPa), but also has good transparency, fluorescence, milky effect, and higher edge stability and accuracy. Moreover, studies have shown that the adhesive strength of ZLS is stronger than that of traditional lithium disilicate glass ceramics.
Fluorapatite glass ceramics: Glass ceramics containing fluoroapatite crystals [Ca5 (PO4) 3F] are similar to hydroxyapatite crystals in enamel, thus improving the optical properties and thermal expansion coefficient of traditional feldspar ceramics. They are commonly used as decorative ceramics for metal scaffolds, lithium disilicate glass ceramics, and zirconia crowns.
In Ceram glass infiltrated ceramics: Currently, glass infiltrated ceramics specifically refer to the In Ceram series. In CeramAlumina is an all ceramic system that can be used for single crown and three unit fixed bridge restoration of anterior teeth, with moderate strength (350-450MPa) and transparency. The second generation product, In Ceram Spinel, has increased transparency but decreased strength and is only used for making front dental crowns. In Ceram Zirconia added 35% partially stabilized zirconia, achieving a flexural strength of over 650MPa but almost opaque, making it mainly used for single crowns and three unit fixed bridges in posterior teeth. Although the In Ceram series has a high clinical success rate, its clinical use has gradually decreased with the development of zirconia and other new materials in recent years.
3. Polycrystalline ceramics
Polycrystalline ceramics are dense ceramic materials that are directly fired from crystals and do not contain glass or gas phases. They have high strength and hardness and are processed using CAD/CAM equipment. This type of material, due to the lack of glass phase, usually has low transparency and needs to be decorated with decorative ceramics, including alumina and zirconia. Procera All Ceram (Nobel Biocare) is a polycrystalline ceramic early used in the dental field, containing 99% to 99.5% high-purity dense alumina ceramics, with a strength of approximately 600Mpa, second only to zirconia ceramics. However, due to its high elastic modulus, it is prone to material fragmentation, so its clinical use has gradually been replaced by zirconia. The zirconia used for dental materials is tetragonal zirconia polycrystalline (Y-TZP) stabilized with yttrium oxide. It has excellent stability, wear resistance, and biocompatibility, with a flexural strength of 900-1200Mpa and a fracture toughness of 9-10MPa/m1/2, which is twice that of dense alumina and three times that of lithium disilicate glass ceramics. It is commonly used for posterior crown and multi unit fixed bridge repairs.
The main reason for the failure of zirconia restoration in clinical practice is the cracking of decorative porcelain. Although processing methods that slow down heating and cooling rates can improve the rate of ceramic failure, there are currently few clinical literature reports on such methods. Compared with traditional zirconia crown with decorative porcelain, all zirconia restorations have the following advantages: ① No need for decorative porcelain, avoiding porcelain collapse, and achieving significant results in fixed bridge restoration for posterior teeth. Ramos et al.'s in vitro fatigue tests showed that under continuous action of the piston (100N, 3Hz) × After 106 cycles, the failure rate of all zirconia restorations is much lower than that of traditional zirconia A smaller amount of tooth preparation preserves more dental tissue, making it suitable for cases with insufficient facial space. Taking the all zirconia restoration VitaYZT (VITA) as an example, its posterior teeth have a full crown reserve of face ≥ 0.7mm, adjacent face ≥ 0.5mm, and shoulder ≥ 0.2mm, which is much smaller than the traditional zirconia plus porcelain full crown reserve (face ≥ 1.4mm, adjacent face ≥ 1.2mm, shoulder ≥ 0.5mm) With the development of materials, new types of high permeability all zirconia restorations continue to emerge, improving the transparency of zirconia materials. As introduced in 2014, PRETTUANTERIOR (Zirkonzahn) has the same transparency as lithium disilicate glass ceramics, and its strength is much higher than that of glass ceramics (>670MPa), which can greatly replace glass ceramics as an aesthetic restoration for anterior teeth Less wear on natural teeth in the opposite jaw. Studies have shown that although the hardness of zirconia is much higher than that of glass ceramics, highly polished all zirconia restorations have a lower degree of wear on natural teeth in the opposite jaw compared to glass ceramics. 3M Monolith and 3M Translucent (3MESPE) can also be used for patients with night molars (single crown or three unit fixed bridge). Although all zirconia restorations have many advantages, due to the lack of decorative porcelain, zirconia materials have low-temperature aging phenomenon when used in hydrothermal environments for a long time. The impact of whether the surface of the restorations is highly polished in clinical operations on friction performance and the strength reduction brought about by aesthetic performance improvement still require long-term clinical practice to confirm and improve.
2、 Resin ceramic composite materials/ceramic like materials
With the continuous development of aesthetic restoration, new types of glass ceramics and polycrystalline ceramics are constantly emerging, but these materials usually require a time-consuming heat treatment process before clinical application. Moreover, the elastic modulus of materials currently used for repairing dental defects is much lower than (polymer composite materials) or much higher than (ceramic materials) the elastic modulus of enamel and dentin. Therefore, in recent years, a new type of CAD/CAM resin ceramic composite material has begun to appear in the market, which is highly filled with ceramic particles (>50% weight) in organic scaffolds. Some scholars believe that according to the definition of ceramics in the 2013 ADA Dental Regulations and Naming Rules, composite materials composed mainly of ceramic materials and supplemented by organic polymers can be classified as ceramic materials. However, there is still controversy surrounding this issue internationally. Therefore, this article will separately introduce this type of material as a type of ceramic material.
Compared with traditional ceramic materials, it has special properties due to the presence of organic scaffolds, which have the following advantages: ① closer elastic modulus to dentin; ② Reduced the brittleness and hardness of the material, making it easier to cut; ③ More convenient to use resin for repair; ④ After adjustment, it does not affect the intensity and is easy to operate clinically; ⑤ The wear on natural teeth is much smaller than that on glass ceramics; ⑥ No heat treatment is required, and its design and production can be completed next to the chair. VitaEnamic (VITA), launched in 2013, is a resin ceramic composite material in the world, consisting of a dual network structure of 86% weight feldspar glass ceramic and resin polymer. Its strength is about 150-160MPa, and its elastic modulus is about 30GPa, very close to dentin. Some scholars classify it as glass infiltrated ceramics. Compared with traditional dental ceramic materials, its toughness and elasticity are enhanced, and its wear resistance, strength, and color change resistance are superior to existing resin materials. The degree of wear is similar to that of natural enamel. Simultaneously possessing high transparency, it can effectively complete the aesthetic restoration of front teeth. In addition, the machinability and edge stability of the material are superior to other CAD/CAM ceramics. Its small tooth preparation amount is also smaller than that of glass ceramics, making it suitable for veneers, inlays, high inlays, single crowns, and minimally invasive restorations, such as in cases where the preparation amount is significantly insufficient and the repair space is very limited.
Lava Ultimate (3MESPE) is a resin nanoceramic formed by filling approximately 80% weight of SiO2, ZrO2, and polymer SiO2/ZrO2 nanoceramic fillers in processed resin scaffolds. It has good aesthetic performance and durability, with a high strength of 200Mpa, and can be used as a restoration for posterior teeth. In clinical application, due to the possibility of debonding as a single crown, its application is limited compared to VitaEnamic. It is only used for inlays, high inlays, and veneers, and the preparation of teeth for inlays and high inlays must ensure maximum internal fixation design.
At present, this type of material still has some defects, such as inferior wear resistance and light transmittance compared to glass ceramics, and research is still limited to in vitro experiments, with limited clinical literature reporting.
3、 Surface treatment and bonding
In clinical practice, a very important factor affecting the indications and success rate of ceramics and ceramic like restorative materials is the adhesion of the materials. The recommended bonding methods vary for ceramic materials with different compositions. Currently, ceramic adhesive resin adhesives are used in clinical practice. The commonly used methods in clinical practice to increase the adhesive strength of ceramic materials containing glass matrix (feldspar ceramics, glass ceramics) are hydrofluoric acid etching and surface silanization. Due to the presence of a large number of Si-O bonds in the structure, micro grooves and small pores can be formed by hydrofluoric acid etching, which creates a mechanical locking effect between the ceramic surface and the resin, thereby improving the bonding strength. The best etching effect is achieved by using a hydrofluoric acid solution with a concentration of 2.5% to 10% for 2.0 to 3.0 minutes. However, it is worth noting that glass infiltrated ceramics have a low content of glass components, and acid etching is not sufficient to make their surface rough enough. Therefore, acid etching treatment is usually not used in clinical practice. Silane coupling agents can form covalent and hydrogen bonds with SiO2 on the surface of ceramics, while copolymerizing with resin, significantly improving the bonding strength. Therefore, ceramic materials containing glass matrix have good adhesive strength, and for restorations with poor retention, such as ceramic veneers, inlays, and high inlays, their reliability is high. Clinical evaluation shows a 5-year success rate of 93% to 98%, and a 10-year success rate of 64% to 95%.
Polycrystalline ceramics cannot be etched by traditional acid etchants because they do not contain glass components, so their adhesive strength is not as strong as glass ceramics, especially zirconia ceramics. Their application is greatly controlled by their adhesive properties, so it is not recommended to use them as veneers. Inokoshi et al. reviewed a large number of literature and found that the surface modification of zirconia (chemical friction silicon coating or Al2O3 sandblasting) using treatment agents and adhesives containing phosphate esters (MDP) has high and long-lasting bonding strength. The surface treatment method of VitaEnamic is similar to that of traditional glass ceramics, as the glass component on its surface can be selectively removed by hydrofluoric acid, exposing the resin support and increasing the roughness. It is recommended to use hydrofluoric acid etching and surface silanization. Research has shown that its adhesive performance is reliable, with a strength higher than Lava Ultimate, equal to Vitalocs MarkII, and slightly lower than lithium disilicate glass-ceramic (IPSe. maxCAD) and ZLS (VitaSuprinity, CellraDuo). LavaUltimate cannot be treated with hydrofluoric acid or phosphoric acid, and adhesives containing clove oil cannot be used as temporary restorations. It should be noted that resin ceramic composite materials can only use resin adhesives as restorations at present.
In summary, oral ceramics and ceramic like restorative materials have developed rapidly in recent decades, and there will be more developments and breakthroughs in the near future. Faced with an increasing number of choices, clinical doctors should have a better understanding of the classification and characteristics of materials, and make reasonable choices based on their strength, transparency, adhesive properties, etc., while considering the patient's economic ability and abutment conditions.
