Surface Treatment And Bonding Of Zirconia

Dental zirconia ceramics have good physical and chemical properties and are widely used in the oral field. However, the long-term effects of zirconia restorations are not as good as those of metal-ceramic restorations. Complications often appear as poor retention.  This is especially true in cases where the preparation has a short abutment. The zirconia structure is stable and lacks chemical bonding with the binder. Conventional bonding methods for silicon-based ceramics do not achieve the desired bond strength, thus increasing zirconia and resin.  The bonding strength has become a hot topic of research this year.

Characteristics of zirconia ceramics

A META analysis showed that in the all-ceramic restoration, the 5-year incidence rate of the tempered glass-ceramic core-factor fracture was 8.0%, and the glass-insulating alumina ceramics had a higher fracture rate of 12.9%, zirconia nucleus.  The stability is the best, with a 5-year failure rate of 1.9%.  With the clinical application and development of aesthetic restoration, in the past 10-15 years, research on all-ceramic materials has gradually focused on improving its mechanical properties.  Zirconium oxide ceramics are favored for their strong mechanical strength and good biocompatibility.

Zirconium oxide has three crystal forms: a monoclinic phase at low temperatures, a tetragonal phase at temperatures above 1170 C, and a cubic phase at over 2370 C.  As the temperature decreases, the zirconia will have a volume expansion of 3% to 4%. This volume expansion is accompanied by a large internal stress, which eventually leads to cracking.  In the yttrium-stabilized tetragonal phase zirconia (Y-TZP), a metastable tetragonal phase can be formed by adding 2-3 mol% of yttrium oxide, thereby ensuring the relative stability of zirconia.  When stress is applied to zirconia and cracks are generated, crystals around and near the crack are converted from the t phase to the m phase, and the volume is expanded while generating stress, which is offset by the stress generated by the crack, thereby increasing the toughness of the zirconia.  Studies have shown that Y-TZP has a fracture toughness of 5-10 MPa/m/2 and a flexural strength of 900-1400 MPa, which is equivalent to twice the alumina-based material and three times the lithium disilicate-based material. Static load  Can withstand 2000N force.  Moreover, Y--TZP does not contain a glass component, and does not cause decomposition and crack protection of the glass structure due to the reaction between moisture and glass in the saliva.

zirconia surface treatment method and principle

Zirconium oxide surface treatment methods are classified into mechanical methods and chemical methods.  Mechanical treatment refers to roughening the bonding surface by physical means, increasing the bonding surface area and mechanical fitting force. The chemical method refers to changing the properties of the zirconium surface by using some chemical agents to enhance the bonding.

1.Selective Permeation Etching Technology

       It is a new technology to increase the surface roughness of zirconium porcelain. The principle is to coat a special silicate glass on the surface of zirconium, and then heat it to above 750C to melt the glass coating and follow the grain boundary of zirconia.  Diffusion in the region promotes the sliding and splitting of grains on the surface of zirconia.  Then, it is further etched with hydrofluoric acid to form a three-dimensional network structure of intergranular pores, thereby facilitating the mechanical inclusion of the adhesive into the voids and increasing the bonding strength of the ceramic resin.

       Studies by Casucci et al. show that the surface roughness of zirconia treated by this technique is greater than that of sandblasted and hydrofluoric acid treated surfaces.

2. acid etching

    2.1 hydrofluoric acid etching

       Hydrofluoric acid is a commonly used ceramic acid etchant to enhance the mechanical fitting force between resin and porcelain by dissolving the glass matrix in the ceramic material.  Since zirconia ceramic does not contain a glass matrix, it is considered that hydrofluoric acid is ineffective for zirconia.  However, some scholars have found that hydrofluoric acid etching makes the surface particles of porcelain smaller and the particle gap increases, but the adhesive does not enter the grain gap.

   2.2 hot acid solution acid etching

       The principle of this technology is to selectively etch and dissolve the irregular high-energy atoms on the surface of zirconia after heating with strong acid, and form a three-dimensional surface structure of a large number of pores, which provides a good mechanical retention force for zirconium-ceramic resin bonding.  Casucci et al. used HCL and Fe2CI3 as acid etchants and etched at 100C for 30 min. The results showed that the bond strength was significantly higher than that of the control group.  Some studies have used HF and HNO3 mixture, H2SO4 and HF and HNO3 mixture, H2SO4 and (NH4) 2SO4 mixture to heat to 100C acid zirconia for 30 min. The comparison results show that the bonding strength of the sandblasting treatment group is significantly improved.  There was no significant difference between the different acids (P>0.05).  It can be seen that the surface treatment method of the hot acid solution acid etching can effectively roughen the surface of the zirconium porcelain and significantly improve the bonding strength of the porcelain-resin

3 mechanical treatment

3.1 mechanical polishing

       Mechanical grinding is an operation often performed during the all-ceramic crown fitting process.  Some scholars believe that the clinical grinding process will form residual tensile stress, accelerate the aging of the restoration, and thus affect the life of the restoration.  Chen Yingying and other studies have found that grinding makes ceramic stability decline, while polishing and glazing have the effect of inhibiting ceramic aging.

  3.2 Alumina blasting technology

       Alumina particle blasting can increase the roughness and cleanliness of the zirconia ceramic surface, thereby increasing the mechanical retention between the ceramic block and the tooth, and can be combined with 10-methacryloyloxyphosphazyl phosphate (MDP).  The resin bonding material of the phosphoric acid monomer chemically bonds to increase the adhesion between the zirconia and the tooth.  Guazzato et al. found that air blasting has the least defects on the zirconia surface compared with grinding wheels and burs, and it has the best effect on the long-term use of zirconia restorations.  On the selection of alumina particle size, 120, 80, 40 pm Al2O3 particles were used. The results of zirconia blasting at 0.4 MPa for 20 s showed no significant difference in the ceramic surface of the 120 and 80 μm particle treatment groups.  And all are below the 40 μm group.

       The results of a few researchers are not the same. Yan Haixin and other studies have found that although the sandblasting treatment increases the surface roughness, it does not enhance the bonding effect. The reason for this remains to be confirmed.

       3.3 laser etching technology

       Laser etching refers to the irradiation of a zirconia ceramic with a high-energy laser to cause melting and re-quenching of the surface to form scattered small pits to increase the mechanical locking force of the zirconia and the resin.  Commonly used lasers are Er: YAG laser, Nd:YAG laser and carbon dioxide (CO2) laser.

       Ma Yonggang and other studies confirmed that the shear strength of these three laser-treated ceramics was significantly higher than that of the control group, and the difference between the three was not statistically significant.  Laser etching has a significant effect on improving the bonding strength between ceramic and resin.  However, this technique has no significant effect on improving the bonding durability. The adhesion of the laser-etched zirconia ceramic and the resin-bonded test piece after aging for 6 months is significantly reduced.

3.4 NobelBond surface treatment

       NobelBond is a new ceramic surface treatment technology that has been used for bonding zirconia surfaces in recent years.  The principle is that the surface of the pre-sintered or fully sintered zirconia scaffold after cutting is coated with a slurry containing zirconia powder and a pore former, and after sintering, the pore formation decomposes to form pores on the surface of the zirconium.

       Phark et al. compared the shear strength of zirconia after NobelBond and grit blasting. The results show that the former has high shear strength immediately after aging and the latter, and the latter has shear strength after artificial thermal cycle aging.  Dropped significantly.  At the same time, the surface of the zirconium porcelain treated by NobelBond does not need to be sandblasted.  As the technology is newer, the effect evaluation needs further verification.