What Are Dental Onlays?

Dental porcelain refers to glass-ceramic materials and products used for dental treatment and restoration. Belongs to the medical device category.

Dental porcelain is divided into type I and type II. Type I dental ceramics are products provided in powder form, and type II dental ceramics are products provided in various forms other than powder. To distinguish the category of porcelain powder and add colors to it, it is recommended to use the colors in the table. Type type 1 dental ceramic materials are used to make the support structure of crowns, veneers, inlays, and onlays, and the surface will be covered with one or more type I dental ceramics of type 2-8. Type type 2 ceramics are used to directly make veneers, inlays and onlays.

Dental metal porcelain restorations have both the strength of metal and the aesthetics of ceramics, and are well received by people. They are increasingly used in the field of oral orthopedic restorations. However, the porcelain powder used in China depends on foreign imports, which is very expensive, costing tens of thousands of yuan per kilogram. Research and development to replace imported ceramic materials for porcelain, and produce ceramic material powders for baking porcelain crowns on alloys, including shading porcelain and body porcelain. Its manufacturing methods all use chemical reagents as raw materials. The raw materials are easy to obtain and the ingredients are accurate. They are calcined in one step using a step cold crystallization method. The method is simple and saves labor and energy. Bismuth powder, and adding homemade coloring material to the bismuth powder, and then re-milled for series color matching. Masking porcelain and body porcelain are each made into A, B, C, and O series with 17 different colors. According to the different color of human natural teeth, the color is realistic and beautiful. Its strength is high, it is firmly combined with the metal substrate, its expansion coefficient matches that of the alloy substrate, and its chemical stability is good. It can be directly produced for clinical dental porcelain application in the dental department. The research technology is mature, the production equipment is simple, and the production is easy. ^[1]

High-speed periodic pulsed laser treatment of dental ceramic materials can change the surface structure of nanocrystalline materials on the surface and increase the flexural strength by more than 50%. Untreated ceramic materials can form gaps with residual teeth or metal fixtures, which gradually expand and cause dental damage.

This proprietary technology controls shape and microstructure, and treated composites are a bit like biomaterials with complex layered structures. ^[2]

Dental ceramic materials have good biocompatibility, abrasion resistance, and corrosion resistance. Their unique aesthetic properties are also unmatched by metal materials, polymer materials, and other materials. The restoration made of ceramic is fine, safe and non-toxic. Its surface is smooth and wear-resistant, and bacterial plaque is not easy to attach. Ceramic materials are widely used as dental restoration materials, and are widely used by patients and dental workers. Nevertheless, the inherent brittleness and low strength of ceramic materials limit their further application in dental restorations. Until the development of porcelain that can be fused to metal surfaces in the 1950s, the use of ceramic materials in dentistry was promoted. Dental ceramic alloys (dentalceramicalloys, DCA) are used to make the metal bottom layer of porcelain-fused-metal restorations (PFM), which have the characteristics of high strength, strong durability. At the same time, the porcelain layer fused on the metal bottom layer is corrosion-resistant and beautiful in color. The combination of the two makes the whole restoration beautiful and durable. Therefore, PFM is very common in developed countries. In the subsequent clinical application, it was found that there are some shortcomings in the gold porcelain restoration, such as the poor combination of the porcelain layer and the metal layer, which leads to the collapse of the porcelain, and the unsightly color of the metal bottom tire at the edge of the restoration. In order to make up for the shortcomings of PFM, people began to devote to the research of all-ceramic restorations, and the existing all-ceramic materials generally have the fatal weakness of high brittleness and low strength. Therefore, materials workers have done a lot of related research and made many ceramic toughening and reinforcement attempts. It is hoped that by improving the strength and toughness of ceramic materials, it will not only maintain superior biocompatibility and aesthetic properties, but also It can meet the clinical requirements for the mechanical properties of dental restorations. Toughening and strengthening methods of several ceramic materials are introduced, as well as several toughening and strengthening dental ceramic materials that are applied to practice on the basis of their theory.

Method for improving strength and toughness of dental ceramic material by using dental porcelain

(1) Internal enhancement

Internal enhancement of dental ceramics is achieved by dispersing second-phase crystals, particles or fibers in the matrix. These second phase materials are dispersed in the matrix, which can cause the generated cracks to shift, branch, dull or stop, thereby increasing the strength and toughness of the material. Commonly used second-phase materials are Al ₂ O ₃ particles, ZrO ₂ particles, titanium particles, tetrasilicofluorite mother crystals, garnet crystals, MgO crystals, magnesia-alumina crystals, apatite crystals, wollastonite crystals, Fluorite mother crystal, stainless steel fiber, etc. The study found that Al ₂ O ₃ can increase the strength and toughness of the matrix by inhibiting the growth of ZrO ₂ particles. When the Al ₂ O ₃ content reaches 30% (mass fraction), the composite ceramic has a flexural strength of 986 MPa and a fracture toughness. It is 13.7Mpa · m ^1/2 .

(2) Surface treatment

The brittle fracture of porcelain restorations is often caused by the generation and expansion of surface microcracks. Therefore, a certain treatment of the surface of the restorations can bridge the surface microcracks produced during the preparation. Surface treatments include polishing, glazing, chemical strengthening and thermal strengthening. There are two methods of glazing porcelain glazing and self-glazing. Glaze porcelain glazing is to apply a sintered glaze porcelain on the surface of a suitable porcelain restoration to form a uniform thin glass layer. For self-glazing, the porcelain restoration is put into the porcelain furnace, and the temperature rises above the glass transition temperature, so that a glassy fluid layer is generated on the ceramic surface, and the surface micro-cracks are repaired. In addition, the surface of ceramics can be treated with a laser to increase its strength. After the dental ceramic surface was treated with 308rm XeCl laser, the roughness was significantly reduced. Compared with the same ceramic material, the power of 6.28J / cm ² and 1.57J / cm ² and 3.14J / cm ^{2 were} irradiated on the surface for the same time. Lower roughness. However, due to the existence of microcracks and bubbles on the surface, dental ceramics treated with XeCl laser need further processing. Studying the effects of surface treatment and heat treatment on the strength of dental ceramics, they found that the compressive stress layer due to phase transformation, the heat-treated polished and sandblasted samples have higher strength. In addition, a specimen whose grinding direction is parallel to the specimen bending axis is stronger than a specimen whose grinding direction is perpendicular to the specimen bending axis.

Chemical enhancement mainly uses ion exchange technology. Ion exchange is also called ion packing. Sodium ion exchange feldspar porcelain with a smaller diameter is usually used. The mechanism of ion exchange toughening mainly has the following two points: Replace the smaller radius ions with the larger radius ions at a temperature lower than the glass softening temperature. The rigidity of the material prevents the introduced stress from being released and forms a pressure layer on the surface Replacing sodium ions with lithium ions reduces the thermal expansion coefficient of the surface layer of the material, which makes the surface layer in a compressed state during the cooling process, increasing the energy required for crack growth. The effect of ion exchange is affected by factors such as exchange time, temperature, and ion concentration. A commonly used paste is K ₂ HPO ₄ or potassium nitrate, which is coated on the surface of porcelain and heated in a standard dental laboratory furnace to complete the ion exchange reaction.

Conclusions of dental porcelain research

Since ceramic materials entered the field of dental restorations, they have been widely used because of their good biocompatibility and aesthetically pleasing effects. However, their inherent shortcomings such as insufficient strength and large brittleness have greatly limited their use in dental restorations. Application. Therefore, materials workers have done a lot of work to strengthen and strengthen dental ceramics, such as internal reinforcement, surface treatment, particle toughening, phase change toughening, etc., and developed a series of Toughened and reinforced ceramic materials, and the related products developed therefrom have also achieved good clinical application results, such as the InCeram system launched by the German VITA company and the GLtype Al ₂ O ₃ glass infiltration ceramics developed by the Fourth Military Medical University in China. Among them, the In-Ceram system has better near-term and long-term clinical effects, and its clinical application has expanded from the anterior crown to the posterior crown and bridge.

As high-strength crown and bridge repair materials, people have high hopes for Al ₂ O ₃ ceramics, ZrO ₂ ceramics, Al ₂ O ₃ -ZrO ₂ composite ceramics, and hydroxyapatite-coated ceramics. As we all know, the structure of a material determines its performance. How to increase the energy absorption mechanism in the ceramic microstructure and increase the path of crack propagation are the core issues to improve the toughness of ceramics. With the improvement of the mechanical properties and the reliability of the use of ceramic materials, the application of ceramic materials in dental prosthetics will definitely make great progress and development. ^[3]