Porcelain-Fused-to-Metal
Porcelain-fused-to-metal crowns are also called metal-ceramic crowns. Due to the fact that they possess both the mechanical strength of metal and the aesthetics of porcelain, they have a high flexural resistance, a stable color, and a certain level of corrosion resistance. There are many applications in the field of dental prosthesis. However, because the metal base of the metal-ceramic crown is opaque, the restoration lacks translucency. Additionally, non-precious metals have poor corrosion resistance, and the precipitated metal ions can irritate the gums, leading to staining and allergies on the gumline. These factors restrict the clinical use of metal-ceramic crowns.
All Ceramic Materials
All-ceramic materials are gradually becoming recognized by doctors and patients due to their attractive color, good biocompatibility, stable chemical properties, exceptional corrosion resistance, and non-conductivity. According to the manufacturing process, all-ceramic materials for clinical use currently are divided into the following categories:
Pressure Casting Ceramic
This type of ceramics is made of enhanced leucite ceramics. The production method involves first creating wax patterns, then selecting porcelain blocks based on clinical colorimetry, and finally heating the mold cavity and porcelain blocks in a special casting furnace to 1180°C. Under a certain pressure (0.5MPa), alumina rods are die-casted to form stent porcelain, and then a layer of veneer porcelain is sintered on the surface of the stent using conventional methods.
Cast ceramic restorations have a high translucency and a stunning appearance, but their strength is low (only 180MPa), and in general, they are exclusively utilized to manufacture veneers, inlays, and single crowns for anterior teeth.
Infiltrated Ceramic
In-ceram is the representative product of this type of ceramics. Using an alumina powder slurry containing 99.56 percent Al2O3 to form the prototype of the base crown by the plastic coating method, then sintering it in a special furnace to form a porous structure, and finally coating it with a layer of special glass frit and sintering it. The glass frit is melted and infiltrated into the alumina voids to form a cross-linked and interpenetrated composite structure of alumina and glass, and the restoration is then formed by stacking veneering porcelain as is customary. In-ceram was found to have a high bending resistance that ranges from 370 to 600 MPa.
CAD/CAM Zirconia Cutting Ceramics
The representative products are the all-ceramic WIELAND and LAVA systems. The production process begins with scanning the restoration model with computer-aided programs, followed by the coarse and fine machining of the pre-sintered high-strength zirconia ceramic block and then sintering it to form the base crown. Finally, in order to form the restoration, veneering porcelain is stacked on the base crown.
Application of Zirconia All-Ceramic Materials
Zirconia ceramics are a brand-new category of ceramic material that has excellent biological safety, is non-cytotoxic, and is an inert substance. The most significant aspect is that under induced stress, zirconia ceramics produce dual effects of phase transformation toughening and microcrack toughening, which significantly increase their toughness. It is necessary to add stabilizers with various contents, such as yttrium oxide, magnesium oxide, and cerium oxide, in order to prevent cracking brought on by phase transition. It is possible to achieve stable performance of zirconia by replacing the zirconia ions with the cations in the stabilizer, which will prevent excessive transformation of the crystal phase.
Tetragonal polycrystalline zirconia with a small amount of yttrium oxide stabilizer is currently the most common type of zirconia all-ceramic material used in the creation of dental restorations. Tetragonal zirconia (3Y-TZP), which has been shown in studies to have sufficient strength and toughness when the content of yttrium oxide is 3 mol%, is best suited for the manufacture of dental restorations. Restorations that are made of high-strength and high-toughness industrial preformed zirconia ceramics that are processed by CAD/CAM have become the development direction in the field of dental all-ceramic restorations at the present time, and figuring out how to improve the bonding strength of zirconia base crowns and veneering porcelain has become the key to successful prosthetic restorations.
Factors Affecting Bonding Strength Between Base Crown and Veneering Porcelain
The bonding area between the base crown and the veneer porcelain, the thermal expansion coefficient of the two, the wettability of the interface between the two, the application of the sintered bonded substrate porcelain, and the surface treatment methods of the base crown are all factors that may have an impact on the bonding strength. The sintered bonded substrate porcelain stands out among them for its effective enhancement effect of the porcelain-to-porcelain bond. The principle behind it is that the fusion bonding of the bottom porcelain and thin-layer bonded porcelain can produce a greater intermolecular van der Waals' force than directly sintered thicker veneer porcelain. Since the bonding porcelain and the veneering porcelain have similar chemical compositions, some of the components of the two are fused together when the veneering porcelain is sintered, which can result in a powerful chemical bonding force.
Surface Treatment Methods of Zirconia Base Crown
Currently, the most prevalent surface treatment methods for clinical use include hydrofluoric acid etching, silane coupling agent treatment, silicon coating, sandblasting prior to zirconia sintering, etc. Among them, zirconia sandblasting is a widely used technique for material surface roughening. Through the impact grinding of abrasives, sand blasting technology removes impurities from the surface of the material, increases the roughness of the surface layer, and decreases the surface contact angle. Additionally, it can enhance the wettability, improve the affinity, increase the interface area, increase the contact area and mechanical fit between the zirconia bottom porcelain and the veneering porcelain, thereby enhancing the bonding strength of their interface.
In this article, the effects of two kinds of surface treatment methods and conventional treatment method on the bonding strength between zirconia base crowns and veneering porcelain are compared. The results are shown below:
Shear Strength Test Results
The shear strength of the sand blasting group was (18.06±0.59) MPa,the treatment agent group was (21.04±1.23) MPa, and the control group was (13.80±1.54) MPa. The differences between the results of each group all had a statistical significance(P<0.05).
SEM Observation Results
Sand blasting group and treatment agent group: On the surface of zirconia, irregular depressions and protrusions formed.
Control group: The surface of the zirconia has regular, distinct ridges and grooves, but the depth is not evident.
Bonding Interface Observation Results
Sand blasting and treatment agent groups: the zirconia base crown and the veneering porcelain are closely combined, with no gaps, and there is clear mosaic fusion.
Control group: there is no obvious mutual penetration or chimerism between the base crown and veneering porcelain; and the binding interface is straight, and their combination is relatively tight.
Spectrum Analysis Results
Sand blasting group: Zr element concentration falls dramatically at the bonding interface, and traces of Zr element can be detected on the side of the veneering porcelain; Si element rapidly decreases at the interface, and a trace amount of Si element is detected on the side of bottom porcelain; Al element has a slow decline at the bonding interface.
Treatment agent group: Zr element content decreases sharply at the bonding interface, but increases on the side of the veneering porcelain; the content of Si element drops more quickly at the interface, and a trace amount of Si element is detected on the side of bottom porcelain; a t the bonding interface, the content of Al element drops more slowly.
Control group: Zr element declined significantly at the bonding interface, and no Zr element was detected on the side of the veneering porcelain; Al element decreased slowly at the bonding interface while Si element decreased quickly, and Si element still can be detected on the side of bottom porcelain.
Conclusion
The following conclusion can be drawn by comparing the effects of various surface treatment methods on the bonding strength between the zirconia base material and the veneering porcelain and by analyzing the microscopic morphology and element distribution of the bonding interface: Both sandblasting and the use of sintered bonded substrate porcelain can enhance the bonding strength between zirconia base crowns and veneering porcelain.
If the opposite conclusion is reached, it may be influenced by the following aspects: varying zirconia ceramic manufacturers, as well as variations in sandblasting duration, particle size, and direction; the thermal expansion coefficients of bottom porcelain and veneer porcelain produced by various manufacturers vary, causing different stresses on the bonding interface after sintering; additionally, during the sandblasting process of zirconia bottom porcelain, zirconia may change its crystal phase and mechanical properties due to the variations in the granularity or pressure of sandblasting.