Clinical application of zirconia ceramic materials

Ceramics have been used as oral restorative materials for more than 200 years. Ceramic materials have the characteristics of good aesthetics, high mechanical strength (hardness, wear resistance, compressive strength, flexural strength), high stability, and strong permeability. It is widely used in dental restoration.

Classification of all-ceramic dental restoration materials

All-ceramic materials are divided into three categories according to the content of glass phase and crystal phase in the microstructure of materials:

① Feldspar porcelain. It is mainly a glass phase, which is sintered at high temperature by three components of natural feldspar, quartz and kaolin. Feldspar porcelain is the earliest ceramic material used in dentistry, and its optical properties are very close to those of enamel and dentin. However, due to its poor mechanical properties, the flexural strength is usually only 60-70MPa, so it is often used as a porcelain-fused metal restoration or a fusion-bonded ceramic restoration.

② Glass ceramics. Contains glass phase and crystal phase at the same time, also known as glass-ceramic, is a kind of composite material combined with crystal phase and glass made through high temperature melting, molding and heat treatment. Compared with amorphous glass, the addition or growth of crystalline fillers in the glass phase greatly changes the mechanical and optical properties of glass-based ceramics, such as increasing the thermal expansion coefficient and toughness, changing the color of the material, opalescence sex and transparency.

③ Polycrystalline ceramics. It is a kind of dense ceramic material directly sintered by crystal, without glass phase and gas phase. It has high strength and hardness, and is processed by CAD/CAM equipment. Due to the lack of a glassy phase, these materials usually have low transparency and need to be decorated with veneer porcelain. Glass-ceramic with glass phase as the main part has good aesthetic properties, but with the increase of the number of crystals, its strength becomes higher and higher, but its transparency becomes worse.

High transparency zirconia denture

This classification method implies the relationship between ceramic components and indications. However, the advent of more translucent zirconia and stronger glass-ceramics with reduced transparency with the development of current polycrystalline ceramic microstructures challenges this concept. Fundamental development of ceramic technology in industry: The manufacturing process of these materials has changed from natural components (i.e. feldspar) to synthetic ceramics.

● According to the chemical composition and microstructure of all-ceramic materials, all-ceramic materials are classified into the following three categories: glass-based ceramics, polycrystalline ceramics and resin-based ceramic materials. Compared with traditional ceramic materials, resin-based ceramic materials have special properties because they contain organic scaffolds. It has the following advantages: the modulus of elasticity is closer to that of dentin; the brittleness and hardness of the material are reduced, and it is easier to cut; Resin repair is used; the strength is not affected after modification, and the clinical operation is simple; the abrasion of natural teeth is much less than that of glass ceramics; no thermal processing is required, and its design and production can be completed by the chair.

With the progress of research on the structure and processing methods of zirconia materials, the performance of zirconia has gradually improved, and its clinical applications have become more extensive, such as artificial hip joints and oral restorations that we are more familiar with.

The structure and characteristics of zirconia

Zirconia is a polycrystalline material with three forms: monoclinic (m), tetragonal (t) and cubic (c), which can be converted into each other under certain temperature conditions. When the sintered zirconia is cooled to room temperature, due to the transformation of the crystal structure (from tetragonal phase to monoclinic phase), and the unit cell volume of the monoclinic crystal is about 4% larger than that of the tetragonal crystal, cracks will appear inside the zirconia. Reduces the mechanical strength of zirconia. Adding stable oxides such as CaO, MgO, CeO2, Y2O3 can stabilize this process. Zirconia ceramics added with yttrium oxide have a unique stress-induced phase transformation toughening effect, making it have excellent mechanical properties, and the bending strength can reach 900 ~1200Mpa. Another way to stabilize tetragonal zirconia at room temperature is to reduce the grain size (average critical grain size <0.3 μm).

Phase transition of pure zirconia crystals with temperature

In practical applications, in order to obtain the required crystal form and performance, different types of stabilizers are usually added to make different types of zirconia ceramics. Zirconia ceramics can be divided into three types according to their microstructure: fully stabilized zirconia ( FSZ), partially stabilized zirconia (PSZ), tetragonal zirconia polycrystal (TZP). For example, when the stabilizer is CaO, MgO, Y2O3, it is expressed as Ca-PSZ, Mg-PSZ, Y-PSZ, etc. respectively. Zirconia for dental materials is Yttria Stabilized Tetragonal Polycrystalline Zirconia (Y-TZP)

Zirconia commonly used crystal form stabilizer-rare earth oxide

Zirconia ceramic materials have good aesthetic properties, good biocompatibility, and excellent toughness, strength, and fatigue resistance, in addition to excellent wear resistance. The main disadvantage of zirconia is the wear of the coating material during the bonding process, which affects the strength of the ceramic and the tightness of the interface. The chemical inertness of zirconia also affects the bonding effect and thus the function of the restoration. Full-contour zirconia restorations are opaque and degrade in vivo at low temperatures. Surface treatment of zirconia

At present, the commonly used ceramic adhesives in clinic can be mainly divided into four categories: resin adhesives, glass ionomer adhesives, resin plus glass ionomer adhesives, and phosphate adhesives. Among them, resin adhesives mainly rely on chemical adhesion and mechanical fitting, glass ionomer adhesives are physical and mechanical bonding, and phosphate adhesives mainly rely on mechanical fitting and retention. Among them, resin adhesives dominate.

Ceramic surface treatment can improve the bonding force with resin, common ceramic surface treatment is mainly divided into mechanical method and chemical method. Generally speaking, sand blasting, etching technology and silane coupling agent are the most common methods. However, since zirconia ceramics are polycrystalline ceramics without glass matrix, the acid etching effect is limited. Scholars changed the surface roughness of zirconia, Composition, etc. to improve its mechanical locking and chemical bonding performance.

● Sandblasting: Grinding or sandblasting will cause the surface to change from four-way monoclinic, so that the content of monoclinic zirconia crystals will increase sharply. The high-speed movement of alumina particles strongly impacts the surface of zirconia to form a rough and wet bonding surface. A number of experiments have proved that using 50μm alumina particles and sandblasting under a pressure of less than 0.25MPa is the most suitable choice, which can improve the strength and durability of the bond between zirconia all-ceramic and resin adhesive.

● Coupling agent: through covalent bonds to achieve a firm bond between the interfaces, currently there are mainly two types of primers containing 10-methacryloyloxydecyl phosphate (10-MDP) and silane primers .

● Laser etching: improve the micro-mechanical properties of the zirconia all-ceramic surface, which is beneficial to form a micro-mechanical connection between the zirconia surface and the resin, and improve the bonding effect of the zirconia all-ceramic.

Clinical application of zirconia

① Zirconia bottom crown with veneer porcelain

The study found that the survival rate of zirconia restorations was 95.3% after 1 year of implantation and 80.2% after 2 years, which is the best result among known materials. Clinically, the main reason for the failure of zirconia restoration is the cracking of porcelain veneer. Although porcelain decoration has a good aesthetic effect, the probability of porcelain cracking of zirconia-based porcelain veneer restorations is higher (6%-25% after three years), which is higher than that of glass all-ceramic restorations or metal-ceramic restorations . Mismatches in fracture toughness, flexural strength, coefficient of thermal expansion, and modulus of elasticity, among others, can affect the bonding of porcelain veneer to zirconia.

PRETTAUANTERIOR (Zirkonzahn) material characteristics

With the development of materials, new high-permeability all-zirconium restorations are emerging, which improves the transparency of zirconia materials. For example, PRETTAUANTERIOR (Zirkonzahn) was launched in 2014, which has the same light transmittance as lithium disilicate glass-ceramic, and its strength is much higher than glass-ceramic (>670MPa), which can largely replace glass-ceramic as an aesthetic restoration of anterior teeth.

Compared with the traditional zirconia bottom crown and veneer porcelain, the all-zirconia restoration has a smaller amount of tooth preparation, retains more tooth tissue, and avoids restoration failure caused by porcelain collapse, further improving the success rate of restoration.

② Zirconia post-core crown

Zirconia ceramic materials have better biocompatibility and radiopaque properties, as well as better elasticity and hardness. Metal materials have good stability and mechanical strength, but they are easy to break and corrode, and there are artifacts in clinical MRI. The use of zirconia post-core for restoration has better long-term effects in terms of tooth integrity and color, and the post-core crown after restoration has less damage.

Fiber posts and cores are translucent, have good corrosion resistance, and are highly similar to autogenous teeth. They are often used in anterior tooth restoration in recent years. When there is a large area of tooth defect, the occlusal force is required to be high so that the unique metal mechanical advantages of the zirconia post and core are reflected. Studies have found that zirconia post-core crowns are superior to fiber post resin cores in repairing large-area tooth defects, and fiber post resin cores can be selected for small-area defects with low occlusal strength, such as maxillary anterior teeth.

③ Zirconia abutment

Zirconia abutments have lower surface free energy and surface wettability compared to metals, thus reducing bacterial adhesion and reducing the risk of peri-implant disease. The zirconia abutment is more in line with the aesthetic requirements of patients and has better biocompatibility. Titanium and metal abutments may show through the soft tissues surrounding the implant, resulting in graying of the marginal tissue and less esthetic results.