How to firmly bond the porcelain restoration to the tooth body? This is one of the important factors for the success of all-ceramic restorations. This paper will introduce the mechanism and method of bonding technology in detail.
The all-ceramic crown is an ideal restoration because of its stable and natural color and good biocompatibility, and its clinical application is increasing day by day. Porcelain restorations currently used clinically have good mechanical properties. Therefore, the bonding strength between porcelain restorations and teeth is an important factor for the success of restorations. The following is a review of the research status of bonding systems.
Understanding the formation mechanism of the adhesive force is the basis for correctly mastering the bonding technology. Let’s learn together the key and necessary conditions for bonding:
1. The formation mechanism of cohesion
The force that the adhesive and the surface of the adherend attract each other through the interface and produce continuous action is called cohesive force. When the adhesive joins two adherends, the bond site forms a bond head. In most cases, the bonding head is a multiphase system consisting of three homogeneous phases, including the binder and two adherends, and two interfaces. Adhesive forces usually include the following four types: chemical bond force, intermolecular force, electrostatic attraction, and mechanical force.
Many scholars believe that the bonding effect is caused by the mutual attraction between the binder and the adherend molecules on the interface area, which includes physical adsorption force and chemical adsorption force, that is, the bonding force is produced by the force between molecules and the force between atoms. This is the more recognized adsorption theory today to explain the bonding phenomenon. The closer the distance between the binder and the bonded molecules or atoms, the greater the interaction force between the two, and the higher the bonding strength. Only when the adhesive liquid can fully wet the surface of the adherend, the distance between the two can reach the range of effective valence bond force. This is the key and necessary condition for bonding.
What are the commonly used adhesives in clinical practice?
2.1 Cement Adhesive
Cement adhesives mainly include zinc phosphate cement, polycarboxylate zinc cement, glass ionomer cement
This type of adhesive is mainly bonded to the tooth and has no bond to the porcelain restoration.
2.1.1 Zinc phosphate cement
Zinc phosphate cement is a paste with a certain fluidity before solidification, which can penetrate into the fine structures on the surface of teeth and restorations to form a certain mechanical fit. However, zinc phosphate cement will release free phosphoric acid during and after solidification, which has a certain stimulating effect on dental pulp and gums. Because of its low bonding strength and strong irritation, its clinical application range is small. The study by Gu Xinhua found that the cement layer of the all-ceramic crown cemented with zinc phosphate cement had obvious defects, and the defects increased after the chewing simulation test. This also limits the clinical application of zinc phosphate cement.
2.1.2 Zinc polycarboxylate cement
Zinc polycarboxylate cement can not only form a mechanical fit with teeth and restorations, but unreacted -COOH can also form hydrogen bonds with -OH on the tooth surface, and have a certain degree of complexation reaction with Ca2+ in teeth, and dissociated COO- can produce a certain degree of opposite ion attraction with Ca2+ in teeth. Therefore, its cohesive force is higher than that of zinc phosphate cement.
2.1.3 Glass ionomer cement
Glass ionomer cement is currently widely used because of its good adhesion, biological safety, anti-caries and dissolution resistance.
2.2 Composite resin binder
It has similar composition and properties to traditional restorative resins. It consists of organic matrix (such as Bis-GMA, TEGDMA, UDMA) and baseless filler. According to the initiation form, it can be divided into three categories: self-curing type (chemical curing type), light curing type, and dual curing type. Compared with the commonly used clinical adhesives, the resin adhesive used for all-ceramic restoration has the advantages of good translucency, high bonding strength, not easy to dissolve in the oral environment, and good color matching. Resin binders commonly used in porcelain restorations include BIS-GMA and modified composite resins containing phosphoric acid monomers.
2.3 Hybrid binder
This kind of binder is made of composite resin and glass ionomer, which has the advantages of both. It is easy to use, has small volume shrinkage during solidification, good sealing effect of cement, can reduce the occurrence of micro-leakage and secondary caries, and has good color stability, durability, biocompatibility and high strength. The study of hybrid binders has become the focus of binder research.
How does the surface preparation work? What are the techniques commonly involved in clinical practice?
3. Surface treatment technology
Composite resin binders are mainly bonded with ceramics and are widely used in clinical practice. Its bonding with porcelain restorations is closely related to various surface treatment methods. Different all-ceramic materials require different surface treatment methods to obtain the best bonding effect. There are usually two mechanisms for the bonding between porcelain and resin: one is to produce micro-mechanical interlocking between porcelain and resin by HF acid etching or sandblasting on the surface of the porcelain; the other is to chemically bond porcelain and resin by using silane coupling agent.
3.1 Etching technology
Hydrofluoric acid is commonly used to treat the surface. Although the acid concentration and time used are different, satisfactory bond strength can be obtained. Etching with 2.5%-10% hydrofluoric acid for 2-3 minutes can achieve better results. The feldspar ceramic IPS Empress with more leucite content can be etched with 9% hydrofluoric acid for 60 seconds. Acid etching can form microscopic grooves, clean and increase the bonding surface, obtain a rough surface structure and reduce surface tension. Kato et al. compared the effects of different etching agents with sandblasting and found that hydrofluoric acid and sulfated hydrofluoric acid provided the highest and most durable bond strength.
Sand blasting can increase the roughness of the porcelain surface and increase the bonding area, and the microscopic unevenness formed can generate mechanical fitting force, thereby improving the bonding strength. Some scholars do not advocate sandblasting on feldspar ceramics, because sandblasting may damage the bonding layer on the surface and reduce the bonding strength instead. Porcelain materials based on alumina (In-Ceram Alumina, GI-II type infiltrated ceramics, etc.) are suitable for sandblasting. Because this type of material has less feldspar components, higher strength, less impact on the surface structure, and less loss of surface components. Sand blasting is commonly used clinically with 110μm Al2O3 and 0.24MPa pressure. After comparing HF acid etching and alumina sand blasting, Madani et al. found that a higher bond strength can be obtained between sand blasted In-Ceram ceramics and resin binders.
3.3 Coating coupling agent
Silane coupling agent is easy to combine with carbon dioxide, alumina and other glass ions as the main components to form stable siloxane. The organic functional group at the other end of silane can co-polymerize with organic matter (resin) or interweave with resin molecules, so that ceramics and composite resin binders are tightly combined. The study by Lacy et al. showed that sandblasting of traditional silicate-based ceramics did not enhance the retentive force of the restoration unless a coupling agent was added. Sorensen et al. believe that acid etching and silane treatment of ceramics can significantly reduce microleakage, but silane treatment alone cannot achieve it. Some scholars suggest the combined application of sandblasting (50μmAl2O3), hydrofluoric acid etching and silane coupling agent.
4. Surface modification treatment
Silica coating, through the silane coupling agent can make ceramics and resins chemically bonded. Silica is currently available in two ways:
①Chemical friction method, taking the Rocatec system as an example: first use 110μm alumina to sandblast the bonding surface, and then use a special Rocatec-plus powder for the second sandblasting. Rocatec powder is composed of irregular alumina powder with a size of 110 μm and regular silica. After sandblasting, the silica is bonded to the treated surface by friction.
②Heat treatment method, such as the Silicoater system, the basic process is: first use 110μm alumina sandblasting, and then coat the silica particles containing Cr2O3, and bond the silica to the treated surface by sintering. PyrosilPen technology is a simpler and more effective technology for treating the surface of alumina and zirconia ceramics, which is produced by the improvement of Silicoater technology. Silicoater technology has been proven effective on a variety of dental alloys. Although the two methods are different, their purpose is to increase the content of silica on the surface of the infiltrated ceramics, so that the silicon dioxide coating can be stably combined with the infiltrated ceramics.
In summary, the current all-ceramic restoration materials are based on alumina, zirconia, and feldspar. Due to their different surface properties, the bonding strength obtained by using the same adhesive and surface treatment technology is not the same. The ideal adhesive for all-ceramic restorations should have properties such as low viscosity, good fluidity, no effect on the placement of the restoration due to the thickness of the cement, high strength, and wear resistance. The treatment methods used to improve the surface of the restoration should also be simpler and more effective.