Abstract
Objective
To compare long-term microtensile bond strength of zirconia, surface-modified via a novel treatment, to current surface conditioning methods for zirconia, when resin bonded to dental composite.
Methods
Two ProCAD (porcelain) and 10 sintered ZirCAD (ZrO 2 ) blocks (18 mm × 14 mm × 12 mm) were obtained from manufacturers. Twelve Herculite XRV composite blocks were fabricated (18 mm × 14 mm × 12 mm). Bonding surface of blocks was polished through 1200-grit SiC and air-abraded (50 μm alumina, 0.28 MPa, 20 s). Blocks were then separated into six groups: (1) porcelain (control), HF-etched/silane-treated, (2) ZrO 2 , tribochemical-coated/silane-treated, (3) ZrO 2 , primer-treated, (4) ZrO 2 , modified via novel 3.2 nm silica layer/silane-treated, (5) ZrO 2 , modified via novel 5.8 nm silica layer/silane-treated, and (6) ZrO 2 , modified via novel 30.4 nm silica layer/silane-treated. Blocks were bonded to composite using Clearfil Esthetic cement. Blocks were stored in distilled water (37 °C, 24 h), then cut into microtensile bars ( n = 8/gp), then bond strengths were measured using a universal testing machine at 0, 1, 3, and 6 months. All groups were statistically analyzed (ANOVA, Tukey’s, p < 0.05).
Results
At 6 months (aging), all silica seed layer specimens displayed microtensile bond strength similar to CoJet specimens but less than that of silane-modified dental porcelain.
Conclusion
The deposition of a silica layer on zirconia resulted in similar or superior long-term resin bond strength when compared to traditional silanation and bonding techniques for zirconia but lower than that for silane-treated dental porcelain.
1
Introduction
Advances in ceramic materials over the years have resulted in the introduction of zirconia (ZrO 2 ) as a viable material for use in dental prosthetics. Zirconia provides superior strength and fracture toughness when compared to porcelain and other silica-based materials while presenting improved esthetic properties compared to metallic-based prosthetics. Although these properties have led to the use of zirconia in a variety of dental applications, there exists a problem with zirconia.
Bonding of zirconia to tooth structure or other synthetic materials is difficult when compared to silica-based materials. The bonding of traditional dental ceramics, i.e., silica-based ceramics, utilizes mechanical and adhesive bonding . Mechanical bonding is dependent on the micromechanical interlocking between the resin cement and silica-based ceramics caused by surface roughening. Phosphoric acid (H 3 PO 4 ) or hydrofluoric acid (HF) etching is a common method used to roughen silica-based ceramic surfaces .
Chemical adhesion between resin cement and a silica-based ceramic is achieved with the use of silanes. Silanes are bi-functional compounds that promote chemical bonding between dissimilar organic (i.e., resin cements) and inorganic (i.e., silica-based) materials . This is achieved by functional alkoxy groups on the silane molecule bonding to the silica (SiO 2 ) phase hydroxyl groups (–OH) on the surface of the dental ceramic. Organo-silanes also have a degradable functional group that copolymerizes with the organic matrix of resin cement . These processes create the chemical bonds necessary for the successful bonding of resin cement to dental ceramics. Silanes are also responsible for increasing surface energy and wettability of ceramic surfaces, which enhances both mechanical and chemical bonding.
These traditional methods of mechanical and adhesive bonding used on silica-based ceramics are not applicable for use with high-strength ceramics, i.e., zirconia. The absence of silica or any substantial glassy phase in the microstructure of zirconia eliminates the viability of acid etching as a method to roughen the surface for significant mechanical bonding, and nullifies the use of traditional silanes since there is no silica present to readily form surface hydroxyls for chemical bonding . The difficulty of bonding to zirconia has resulted in alternative methods of adhesion being developed. Current techniques used to facilitate mechanical bonding of zirconia include grinding, particle air-abrasion using alumina or other abrasive particles, and rotary abrasion using diamond burs. However, surface grinding and abrasion can introduce surface flaws that can decrease the fracture strength of zirconia .
Chemical adhesion of resin cements to zirconia has been addressed using several techniques. Application of a silica coating through tribochemical and plasma spray techniques has yielded varying results with respect to bond strength. The tribochemical technique, a commonly used method of silica coating high strength ceramics, embeds silica on a ceramic surface by air-abrading the surface with silica-coated alumina particles . It has been demonstrated that applying a tribochemical coating, followed by traditional silanation, does improve the resin adhesion of zirconia . However, bond strengths are not as high as those reported for resin bonded to silane-treated porcelain, and there is still the issue of creating surface flaws from air-abrasion that could decrease the fracture strength of zirconia.
Improvements in resin cements and silane coupling agents have been shown to increase bond strength. The combination of silane primers and resin cements, that contain phosphoric acid in the form of a phosphate monomer (MDP), have demonstrated improvements in adhesion to zirconia ceramics . However, the use of these phosphate containing primers and resin cements alone produce lower bond strength when compared to tribochemical coating coupled with silanation and resin bonding .
Recently, Piascik et al. reported on a method for improving adhesion to zirconia ceramics via a unique vapor-phase deposition technique, whereby silicon tetrachloride is combined with water vapor to form an ultra-thin silicate layer on the zirconia surface. The study confirmed that this modification improved adhesion of zirconia substrates to resin cement when used in conjunction with traditional silanation and bonding techniques. Application of the chloro-silane pretreatment resulted in a resin bond strength comparable to that of silane-treated dental porcelain. With the confirmation of the effectiveness of the chloro-silane pretreatment, the long-term usage of the vapor-phase deposition technique needs to be established. Therefore, the present study evaluates the long-term microtensile bond strength of the zirconia-composite interface modified using this novel chemical surface treatment and the effect of silicate layer thickness on long-term microtensile bond strength. It is hypothesizes that the use of the chloro-silane surface treatment to zirconia will result in a long-term resin bond strength similar to that of silane-treated dental porcelain and that increasing silicate layer thickness will result in a decrease in long-term bond strength.
2
Materials and methods
For this study, two porcelain blocks (ProCAD, Ivoclar Vivadent, Schaan, Liechtenstein) and 10 sintered zirconia blocks (Lava, 3M-ESPE, St. Paul, MN, USA), all measuring 12 mm × 14 mm × 18 mm, were obtained from the manufacturers. Following the procedures implemented by Plascik et al. , the bonding surfaces of the blocks were polished through 1200 grit using silicon carbide (SiC) abrasive paper (CarbiMet 2, Buehler, Lake Bluff, IL) to ensure equal starting surface roughness. Additionally, all surfaces were air abraded (100-μm alumina abrasive, 0.28 MPa, 20 s) and ultrasonically cleaned (distilled water, 300 s) prior to chemical surface treatments or bonding procedures. Twelve composite blocks (Herculite XRV, Kerr Corporation, Orange, CA, USA) were fabricated by condensing material into a Teflon mold (12 mm × 14 mm × 18 mm) in 2 mm increments and light-cured (700 mW/cm 2 for 40 s per increment, Optilux 501, Kerr Corporation, Orange, CA). The bonding surfaces of these blocks were polished through 1200 grit using SiC abrasive paper. Specimens for microtensile testing were fabricated with minor variations for each surface treatment/ceramic substrate group. Two specimen blocks for each of the six different groups were fabricated as follows, with Groups 2 and 3 representing current clinical procedures used with zirconia restorations:
- •
Group 1 (control): Porcelain, acid etched with 5% HF gel (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s (rinsed and air-dried), treated with silane (Monobond-S, Ivoclar Vivadent, Schaan, Liechtenstein), and bonded to corresponding dental composite block using dual-cure, MDP-containing (phosphoric acid modified) resin cement (Clearfil Esthetic Cement and DC Bond, Kuraray, Okayama, Japan).
- •
Group 2 : Zirconia (no novel surface treatment), silica coated with 30-μm Al 2 O 3 particles modified with salicylic acid (CoJet, 3M-ESPE, St. Paul, MN – 0.28 MPa, 10-mm working distance, 15 s), treated with silane, and bonded to dental composite block using dual-cure resin cement.
- •
Group 3 : Zirconia (no novel surface treatment), treated with phosphoric acid modified primer (Metal/Zirconia Primer, Ivoclar Vivadent, Schaan, Liechtenstein), and bonded to dental composite block using dual-cure resin cement.
- •
Group 4 : Zirconia (novel surface treatment yielding silica layer with thickness of 3.2 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
- •
Group 5 : Zirconia (novel surface treatment yielding silica layer with thickness of 5.8 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
- •
Group 6 : Zirconia (novel surface treatment yielding silica layer with thickness of 30.4 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
All ceramic/composite specimens were bonded under a compressive load of 7 N for 10 min using a universal testing machine (Instron Model 8841, Canton, MA, USA). After bonding, the interface was light-cured on all four sides for 20 s each and stored in distilled water at 37 °C for 24 h. After 24 h, the specimens were sectioned using a diamond wafering blade (15 LC IsoMet Diamond Wafering Blade, Buehler, Lake Bluff, IL, USA) with a low-speed diamond saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA) into wafers (cross-section 14 mm × 1.5 mm) and then into microtensile bars (cross-section 1.5 mm × 1.5 mm). The outer material from each of the larger blocks was discarded, as these surfaces were contaminated with the wax (Type I Impression Compound, Kerr Corp., Romulus, MI, USA) used to bond to the specimen holder during sectioning. After fabricating the microtensile specimens, each group was randomly divided into 4 sub-groups, 0 (baseline), 1, 3, and 6 months, and stored in distilled water at 37 °C for the designated time period. The cohesive tensile strength of the dental composite was also tested at baseline after storage in water at 37 °C for 24 h.
For microtensile testing, an outer edge of each specimen was bonded to a microtensile fixture using cyanoacrylate glue (Rocket Heavy and Accelerator, Dental Ventures of America, Corona, CA, USA). The microtensile specimens were tested using a universal testing machine (Instron Model 8841, Canton, MA, USA) at a crosshead speed of 1.0 mm/min. The microtensile bond strength, σ , was calculated using the equation:
σ = P A
where P is the load at the moment of failure (N) and A is the bonding area of the specimen (mm 2 ).
The effect of the various surface treatments on microtensile bond strength was evaluated using a one-way analysis of variance (ANOVA), with a level of significance set at 0.05. A post hoc Tukey’s test was performed if the microtensile bond strengths were determined to be significantly different ( p < 0.05).
Fractured surfaces of the samples were analyzed using a stereomicroscope (SMZ-140, VWR International, West Chester, PA, USA) and scanning electron microscope (SEM – Quanta 200, FEI, Hillsboro, OR, USA) to assess whether the failure modes were adhesive (partial or complete failure in the adhesive or debonding of the adhesive from the surface), cohesive (partial or complete failure in the ceramic or composite), or mixed (combination of adhesive/cohesive failure). All fracture surfaces were gold sputter coated before SEM analysis. Energy-dispersive X-ray spectroscopy (EDS – INCA x-sight, Oxford Instruments, Tubney Woods, Oxfordshire, UK) was used to determine the elemental composition of the features on the fracture surface of the specimens and to aid in determining the failure mode.
2
Materials and methods
For this study, two porcelain blocks (ProCAD, Ivoclar Vivadent, Schaan, Liechtenstein) and 10 sintered zirconia blocks (Lava, 3M-ESPE, St. Paul, MN, USA), all measuring 12 mm × 14 mm × 18 mm, were obtained from the manufacturers. Following the procedures implemented by Plascik et al. , the bonding surfaces of the blocks were polished through 1200 grit using silicon carbide (SiC) abrasive paper (CarbiMet 2, Buehler, Lake Bluff, IL) to ensure equal starting surface roughness. Additionally, all surfaces were air abraded (100-μm alumina abrasive, 0.28 MPa, 20 s) and ultrasonically cleaned (distilled water, 300 s) prior to chemical surface treatments or bonding procedures. Twelve composite blocks (Herculite XRV, Kerr Corporation, Orange, CA, USA) were fabricated by condensing material into a Teflon mold (12 mm × 14 mm × 18 mm) in 2 mm increments and light-cured (700 mW/cm 2 for 40 s per increment, Optilux 501, Kerr Corporation, Orange, CA). The bonding surfaces of these blocks were polished through 1200 grit using SiC abrasive paper. Specimens for microtensile testing were fabricated with minor variations for each surface treatment/ceramic substrate group. Two specimen blocks for each of the six different groups were fabricated as follows, with Groups 2 and 3 representing current clinical procedures used with zirconia restorations:
- •
Group 1 (control): Porcelain, acid etched with 5% HF gel (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Liechtenstein) for 60 s (rinsed and air-dried), treated with silane (Monobond-S, Ivoclar Vivadent, Schaan, Liechtenstein), and bonded to corresponding dental composite block using dual-cure, MDP-containing (phosphoric acid modified) resin cement (Clearfil Esthetic Cement and DC Bond, Kuraray, Okayama, Japan).
- •
Group 2 : Zirconia (no novel surface treatment), silica coated with 30-μm Al 2 O 3 particles modified with salicylic acid (CoJet, 3M-ESPE, St. Paul, MN – 0.28 MPa, 10-mm working distance, 15 s), treated with silane, and bonded to dental composite block using dual-cure resin cement.
- •
Group 3 : Zirconia (no novel surface treatment), treated with phosphoric acid modified primer (Metal/Zirconia Primer, Ivoclar Vivadent, Schaan, Liechtenstein), and bonded to dental composite block using dual-cure resin cement.
- •
Group 4 : Zirconia (novel surface treatment yielding silica layer with thickness of 3.2 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
- •
Group 5 : Zirconia (novel surface treatment yielding silica layer with thickness of 5.8 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
- •
Group 6 : Zirconia (novel surface treatment yielding silica layer with thickness of 30.4 nm) treated with silane and bonded to dental composite block using dual-cure resin cement.
All ceramic/composite specimens were bonded under a compressive load of 7 N for 10 min using a universal testing machine (Instron Model 8841, Canton, MA, USA). After bonding, the interface was light-cured on all four sides for 20 s each and stored in distilled water at 37 °C for 24 h. After 24 h, the specimens were sectioned using a diamond wafering blade (15 LC IsoMet Diamond Wafering Blade, Buehler, Lake Bluff, IL, USA) with a low-speed diamond saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA) into wafers (cross-section 14 mm × 1.5 mm) and then into microtensile bars (cross-section 1.5 mm × 1.5 mm). The outer material from each of the larger blocks was discarded, as these surfaces were contaminated with the wax (Type I Impression Compound, Kerr Corp., Romulus, MI, USA) used to bond to the specimen holder during sectioning. After fabricating the microtensile specimens, each group was randomly divided into 4 sub-groups, 0 (baseline), 1, 3, and 6 months, and stored in distilled water at 37 °C for the designated time period. The cohesive tensile strength of the dental composite was also tested at baseline after storage in water at 37 °C for 24 h.
For microtensile testing, an outer edge of each specimen was bonded to a microtensile fixture using cyanoacrylate glue (Rocket Heavy and Accelerator, Dental Ventures of America, Corona, CA, USA). The microtensile specimens were tested using a universal testing machine (Instron Model 8841, Canton, MA, USA) at a crosshead speed of 1.0 mm/min. The microtensile bond strength, σ , was calculated using the equation:
σ = P A
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