Bonding to Zirconia
The use of ceramics in dentistry has created a tremendous transformation; In the meantime, zirconia has found a special place due to its unique structural characteristics. Today, zirconia has replaced metal copings and has opened its place in implant systems.
The introduction of zirconia frameworks has removed the limitations of design and application of all-ceramic restorations and has brought more success and credibility. Today, long-span and complex all-ceramic restorations are possible due to the unique and excellent mechanical properties of zirconia; On the other hand, the surface stability of zirconia has led to other problems; Because it is difficult to create a durable chemical or mechanical bond with it. Unlike glass-containing ceramic systems, which have a glass structure and can be etched with hydrofluoric acid and chemically bonded with silane, zirconia is not able to create a micromechanical and chemical bond. The bond strength reported to dental ceramics varies widely, making it difficult to assess their clinical significance. The minimum strength of the band required clinically is 10-13 Mpa.
Structure and physical and mechanical properties: Zirconia is used in many dental and medical cases due to its tissue compatibility. One of the most common all-ceramic blind materials is zirconia, whose bending strength is 1000 MPa; While feldspathic porcelains have a bending strength of 100-300 MPa. Zirconia and alumina are ceramics with metal oxide structure.
Zirconia has different crystal forms: 1- Monoclinic 2- Tetragonal 3- Cubic among which yttrium tetragonal zirconia polycrystal (Y-TZP) has good properties compared to other crystal forms. The tetragonal form alone is unstable and may change into other forms with low properties; For this reason, 5% of yttrium is added to zirconium oxide to obtain its stable form.
Zirconia has 2-3 times the strength of alumina, its stiffness is less and creates a smoother surface than alumina, and thus it has replaced alumina. Zirconia is chalky white in color and does not have any structure for etching and bonding.
The fracture strength and fracture toughness of zirconia originates from its suitable physical properties. The lack of stability of conventional ceramics in the long term is related to the development of cracks and kerogen stress caused by the water in the saliva, which reacts with the glass structure and leads to the decomposition of the glass structure. Zirconia blinds do not have glass and have a polycrystalline microstructure, so they show excellent long-term stability. The chemical stability of zirconia makes it a desirable material, especially in environments prone to kerogen. In addition, zirconia has a hard and dense surface, which makes it ideal for resistance to wear and contact damage, and in general, zirconia is introduced as a material of choice in cases where there are high functional needs., LAVA (3M ESPE), Procera LLZirkon (NobleBiocare) Cercon (Dentsply) are among the systems with zirconia base.
Resin or conventional cements (conventional):
Considering the fact that the most important factor in zirconia restorations is correct and basic cutting, but knowing the type of cement also has a significant impact on its long-term durability. Most reference books recommend the use of resin and ordinary cements such as glass ionomer as if there is not much difference between them; While recent studies have shown that resin cements are more effective and durable; For example, several studies have shown that glass ionomer is not effective and shows little bond strength. Glass ionomer has a small bond strength of about 4 MPa and is susceptible to water absorption.
A study also showed that the bond strength of glass ionomer with zirconia is equivalent to the bond of zirconia with self-adhesive resin systems.
Common resin cements (Conventional resin cement) or self-adhesive resin cements (adhesive resin cement):
Studies have shown that self-adhesive resin cements are not durable due to low physical and mechanical properties and can be hydrolyzed in humid environments; While ordinary resin cements, which are individually etched and bonded, have more and more stable bonds. One of the reasons for the stability of the zirconia bond is the suitable physical properties of the resin cement. Resin cements that have functional groups are often acidic and hydrophilic and can absorb water even after polymerization.
The degree of polymerization of resin cement is one of the most important factors of the bond of zirconia to dentin; Theoretically, a hydrophobic cement that polymerizes in the form of self-cure within 5 to 12 minutes is considered the best case, but to control the hardening time and to increase the amount of polymerization, dual-cure resin cements are used. Cement is an ideal resin that is well polymerized in self-cure and light-cure conditions, is not affected by aging, and sets in a suitable period of time. If Dual Cure cement self-cures and fully polymerizes within 6 minutes, it allows the interproximal areas to be pulled and cleaned; While there are cements that need 10 to 12 times to clean without disrupting the bonding.
Therefore, the choice of resin cement and the amount of restoration is important; In this way, the characteristics of self-adhesive cements in terms of viscosity, self-curing and hydrophilicity are different from ordinary resin cements and they are not recommended for lathes that do not have sufficient and suitable adhesion.
Phosphate monomers or silane?
Unlike normal ceramics that have a structure
They bond with silane, zirconia not only with silane
There is no reaction; It is also effective in reducing their bandwidth.
Phosphate and carboxylate monomers can interact with zirconium metal oxides
react; In the meantime, phosphate monomers such as MDP create a stronger bond
they do In addition, the use of resins containing phosphate monomers after
Thermocycling is more durable and shows less band reduction; At
While other bonding cements with zirconia, their bond strength decreases
MDP monomers have been introduced as another chemical agent to increase the bonding properties of these ceramics; Because the MDP functional phosphate ester group combines directly with metal oxides. It is expected that the combination of air abrasion with alumina and MDP will create a stable bonding between the YPSZ ceramic surface and the adhesive resin. Instability of MDP bonding has been reported especially after aging, while higher bond strength of silica coating is provided in dry conditions.
One of the most widely used cement resins used in dentistry is Panavia F 2.0. This cement consists of a bifunctional monomer 10-methacryloxydecyl dihydrogen phosphate (MPD). A recent study showed that a modified application of Panavia improved the adhesion of this cement to zirconia; But in practice, when only the manufacturer’s instructions were followed (which did not suggest any preparation method), no adhesion was obtained.
The effect of sandblasting on increasing the bond with zirconia:
One of the common methods to increase surface roughness and micromechanical grip is sandblasting. Sandblasting increases the mechanical strength of zirconia in the short term, but increases the monoclinic phase at the same time. Aging is also one of the factors that increases the monoclinic phase. Due to the increase of surface stress and monoclinic phase (decrease of tetragonal phase) due to sandblasting or roughening, the use of this method is not recommended for the long-term stability of the zirconia band.
However, mechanical treatments on zirconia should be done with caution; Because it has been shown that thermal treatments, sandblasting and grinding can be effective on mechanical properties. In a study conducted by Sundh and Sjogren, it was shown that the effect of resistance to fracture of zirconia depended on the length of time the samples were exposed to sandblasting, among other factors. The possible reason is that the sandblasting or grinding treatment can stimulate compressive stresses or phase transformation on the surface, which increases the strength on the one hand, and at the same time can create other flaws and defects that reduce the strength. Gives.
Covering the inner surface of zirconia with silica can effectively help in increasing the bond; For this purpose, various methods such as
Tribochemical coating and pyrochemical coating can be used.
Tribochemical silica coating method is usually performed in dental laboratories using aluminum oxide particles with a particle size of 110 µm and coated with silica. Airflow pressure causes the silica-coated aluminum oxide particles to be buried in the ceramic surface, making the silica-modified surface more chemically reactive to the resin. Laboratory air abrasion or chair side with aluminum particles coated with 110 and 30 micron silica (tribochemical silica coating) provides surface preparation for acid resistant ceramics. After this operation, the silica surface is covered with a layer of silane.
An alternative to airborne particle abrasion in the laboratory is the application of sand particles using air abrasion tools in the office. This process increases the working area and surface energy for the adhesion of resin cements and increases the micromechanical grip. Also, surface tension is reduced and optimal wetting is provided for silane or adhesive agents.
It has been reported that the chairside version of the silicoater technology (silicoater MD, Heraeus-Kuzler) that can create acceptable adhesion by means of a portable tool (silane-Pen or PyrosilPen) and the use of a flame preparation approach. In this flame system, there is a reaction zone in which tetraethoxysilane is decomposed into pieces of organic silicon (SiOx-C). These fragments cover the surface of the substrate with an adhesive boundary layer through van der Waals forces. This very thin layer (approximately 0.1 µm) has quasi-glass characteristics and can be silanized with MPS silane. Like metal primers, this method was originally developed for the preparation of metals, but more recently it is also indicated for ceramics.
using metal primer :
To increase the bonding, it has recently been suggested that the use of metal primer after air abrasion acts as an adhesion enhancer. Most metal primers are in liquid form and include monomers in their structure for polymerization. In addition to repairing broken metal-ceramic fixed prostheses, metal primers are also indicated for conditioning and strengthening zirconia ceramics. The results with these materials are not always favorable; Because they create high bond strength in dry conditions, but there are negative reports about their hydrolytic stability.
Application of special primers:
Primers that meet the special needs of non-silicone oxides (zirconia and alumina) and metals and are useful for restorations that are formed and resistant to danger. Zirconia bonding agents create a soft bond with hydroxyl groups on the surface of zirconia. Reports indicate that the shear bond strength improves before thermocycling, but a significant decrease is observed after thermal cycles, which depends on the concentration of silane.
As a new approach to increase the strength of the zirconia-to-resin bond, selective infiltration etching (SIE) of zirconia-based materials has been introduced to create a holding surface where the adhesive resin can be infiltrated and locked. In one study, 5 types of engineered zirconia primers were used in combination with SIE in an attempt to improve bond. The results of this study showed that there is a significant difference in the bonding ability between these primers despite the fact that they were used on the same surface of zirconia. It was concluded that different primers can react differently with zirconia etched surfaces. This issue was probably due to the chemical differences between the tested active agents. Most commercial primers contain phosphate or phosphonate monomers that bond to zirconia; But they are different based on the formulation and acid level (which is important for compatibility with self-curing and dual-curing resin cements). Phosphate monomers form a covalent bond with the zirconia surface and have resin ends that can be copolymerized with resin cements.
Examples of primers are briefly introduced:
۱-Clearfil Ceramic Primer(Kurrary) : This primer contains MDP, silane and ethanol.
Its application method is to apply a layer with a brush and then gently rub it until the solvent evaporates and the reaction takes place.
۲- Metal/Zirconia primer(Ivoclar Vivadent) : This primer contains phosphonic acid acrylate, benzoyl peroxide, T-butyl alcohol and methyl isobutyl ketone.
Its application method is that one of it is applied and left for 180 seconds to fully react with the surface. This primer is provided to create a chemical bond with metals, zirconia and alumina.
۳- AZ-Primer(Shofu) : This primer contains phosphonic acid monomer and acetone. Its application method is that a uniform layer of it is applied and it remains on the surface for 10 seconds until it reacts completely.
۴- Z primer Plus(Bisco) : Z-primer plus is a special formula containing both functional phosphate and carboxylate monomers. The synergistic binding of these tested monomers results in a stronger bond with improved durability compared to other commercial primers. This primer contains phosphate monomer, carboxylic acid monomer and ethanol.
The method of its application is as follows: a layer is applied and a gentle poar is taken to evaporate the solvent. According to the manufacturer’s claim, Z-Primer Plus is the only commercial primer that does not need to be kept in the refrigerator.
Bisco Dental Products’ internal research has shown that Z-Primer Plus not only provides high bond strength to zirconia with various surface treatments (such as sandblasting or polishing), but is also compatible with dual-cure or light-cure cements.
Z-Primer Plus cannot be used for feldspathic ceramics, normal pressable porcelains and lithium disilicate type pressable porcelains.
Short and accurate sandblasting along with the use of special primers can prepare the zirconia surface. A very important point is that the surface of the dentin must be coated with dentin bonding without any etching. Studies have shown that leaving a smear layer on the dentine surface creates a better and more effective bond in these systems. It is recommended to use a resin with hydrophobic properties and dual cure.
Relying on a chemical bond alone results in a weak bond between the zirconia and the resin cement, which is responsible for rapid hydrolysis under moist conditions. Therefore, providing a mechanically receptive zirconia surface is a critical prerequisite for strong bonding.
Creating a strong bond with zirconia is only part of the problem. Its more critical aspect is maintaining this band under fatigue conditions and in the presence of saliva and thermal changes for a clinically acceptable period of time. Several studies have investigated the effect of accelerated artificial aging by using storage in water, thermocycling or fatigue, and a decrease in zirconia-resin bond strength has been observed.
In clinical conditions, fatigue is the dominant factor in causing failure, and the reduction of the expected bond will lead to marginal destruction and debonding of zirconia restorations bonded with resin. The analysis of broken samples shows interfacial failure, which shows that the zirconia-resin interface is the weakest. The connection is in the structure. This finding can be related to two important factors:
The first factor is the hydrolytic effect of water on adhesive joints. Active agents in zirconia experimental primers are silane coupling agent. They are characterized by their structural metamorphosis that the unactivated primary monomers are initially very hydrophobic, but during the activation period (hydrolysis) they change into hydrophilic silanol type monomers, which initiate oligomerization reactions, which are formed by the polymerization reaction with followed by zirconia and cement resin.
The second factor is the phenomenon of limitation by water, which can lead to the thickening of the cement layer and as a result of the bond destruction. Relatively hot water is absorbed in composite resin cements and heat treatment also leads to some post-polymerization of MDP-containing cements.
Therefore, in general, it seems that the long-term durability of the band with zirconia is a challenge in cosmetic dentistry that requires more studies and research. In addition, the long-term stability of the resin-zirconia bond is directly related to the chemistry of the materials used, including the primers. Further research is needed to develop more hydrophobic compounds that can resist the damaging effect of hydrolysis.
Dr. Kasri Tabari – restorative and cosmetic specialist
Dr. Sodeh Jabari-Resident and Aesthetics