Abstract
Objective
To evaluate the effect of different chemo-mechanical surface treatments on the morphology of three recently marketed dental zirconia ceramics.
Materials and methods
Ceramic discs (Ø 10 mm × 1 mm height) were obtained from three sintered zirconia ceramics (Lava™, Cercon ® , and Aadva Zr) and treated with: (1) airborne particle abrasion with 125 μm Al 2 O 3 particles (S); (2) selective infiltration etching (SIE); (3) experimental hot etching solution applied for 30 min (ST); (4) no treatment (C). Five discs per group were used for surface roughness analysis using an atomic force microscope (AFM). Data were statistically analyzed by Kruskall–Wallis analysis of variance and Mann–Whitney tests ( α < 0.05). The same discs were evaluated under SEM for surface topography analysis. Three discs per group were cemented to a composite overlay (Paradigm MZ100) with a total-etch resin luting agent (Calibra) and cross-section interfacial analysis was performed under SEM.
Results and Significance
Airborne particle abrasion improved the average surface roughness of Cercon ® and Lava ( p < 0.05) while SIE induced significant changes on Lava and Adava Zr. Statistical differences in surface roughness were recorded after selective infiltration etching when compared to no treated group, independently from the zirconia type. SEM and AFM analyses revealed changes in surface topography for all the tested ceramics and intergrain spaces opening, especially after ST.
The effectiveness of the tested chemo-mechanical surface treatments depends on the zirconia type. The hot experimental etching solution increased the surface roughness of all the tested ceramics creating retentive micro-spaces that may potentially improve zirconia/resin cement interfacial strengths.
1
Introduction
The recent introduction of partially stabilized zirconium dioxide ceramic (ZrO 2 ) allowed clinicians to develop single core restorations as well as multiunit prosthetic rehabilitations thanks to its superior chemical stability, optimal mechanical properties and enhanced esthetics .
Different luting strategies (i.e. conventional cements, glass-ionomer cements, self-adhesive cements) have been proposed for luting zirconia frameworks in the attempt to ensure retentive and sealed restorations . Resin cements containing 10-MDP have been considered the materials of choice because of the chemical interaction established between the hydroxyl groups of the ZrO 2 ceramic and the phosphate ester monomers of the MDP-containing cements . This chemical affinity could be beneficial in preventing crown dislodgement, debonding and consequent microleakage. However, this could limit clinicians’ choice to a unique procedure for luting ceramic frameworks, despite of the wide range of cements and conditioning procedures available on the market.
Previous investigations have been focused on different chemo-mechanical surface treatments in order to optimize the cement/zirconia bonding mechanism. The rationale of these conditioning processes relies on increasing the surface area available for bonding and establishing stronger and durable restorations .
Zirconia has specific structural characteristics that provide high chemical and mechanical resistance to conventional conditioning methods usually employed on traditional ceramics . The combination of airborne particle abrasion and 10-MDP-based resin cements achieved in vitro satisfactory bonding performances .
Novel surface treatments have been proposed in order to improve zirconia/resin cement bonds, such as the selective infiltration etching (SIE) . This procedure is based on the application of a heated glass-infiltrating agent on zirconia creating abraded and porous surfaces and achieving promising results in terms of bond strength to resin cements.
An experimental hot chemical etching solution, previously used for conditioning metal and/or alloys, has been recently applied on zirconia modifying the surface topography . However, only one type of zirconia was evaluated (Lava) and no information is available in the literature regarding the responsiveness of other zirconia ceramics to this experimental conditioning method . Several types of dental zirconia ceramics are available in the market. Although manufacturers do not reveal their exact chemical composition, research in material science reports that slight differences in the sintering temperature may influence the final polycrystal structure arrangement, grain sizes and mechanical properties. As a consequence, different ceramics reactions to specific surface treatments may be expected.
Therefore, the aim of this study was to evaluate the effect of different chemo-mechanical treatments on the surface topography and roughness of three commercially available partially-stabilized zirconia ceramics and on the ceramic/resin cement interfacial aspect. The null hypotheses tested are: (1) neither the ceramic material nor the surface treatment influence the zirconia surface topography and roughness, and (2) no signs of micro-mechanical interlocks are detectable at the zirconia ceramic/resin cement interfaces, independently from the surface pre-treatment performed.
2
Materials and methods
Three types of partially-stabilized ZrO 2 ceramics were evaluated in the study: (1) Lava™ (3 M ESPE, Seefeld, Germany); (2) Cercon ® (DETREY DENTSPLY Ceramco, York, USA); (3) Aadva Zr (GC corp., Tokyo, Japan). Ninety-six cylinder-shaped zirconia discs (Ø 10 mm × 1 mm height) were obtained from each material. Samples were polished with SiC abrasive papers (grit #600, 1000, 1200 and 2000) and a final polishing was carried out with nylon cloths in combination with 1- and 0.50-μm grit size diamond pastes. Discs were then sonicated in deionized water for 5 min. Specimens were divided into four subgroups ( n = 8) according to the surface treatment performed:
Group 1 . Airborne particle abrasion (S): the Al 2 O 3 particles (125 μm) were applied perpendicularly on the entire zirconia surface for 10 s at 0.41–0.68 MPa and a distance of 20 mm.
Group 2 . Selective infiltration etching (SIE). A modified version of the original technique has been proposed . Specimens were coated with a thin layer of an infiltrating agent containing low temperature melting glass and additives ( Table 1 ) and consecutively heated up and cooled. Infiltrating agent remnants were dissolved by immersing ceramic discs in an ultrasonic bath with 5% hydrofluoric acid solution (30 min).
| Group | Treatment | Chemical composition |
|---|---|---|
| 1 | Airborne particle abrasion (S) | 125 μm Al 2 O 3 particles |
| 2 | Selective Infiltration Etching (SIE) | SiO 2 (65 wt%); Na 2 O (15 wt%); Al 2 O 3 (8 wt%); Li 2 O (3 wt%); B 2 O 3 (4 wt%); CaF 2 (5 wt%) |
| 3 | Exp. hot etching solution 30 min (ST) | Methanol (800 ml); 37% HCl (200 ml); and ferric chloride (2 g) |
| 4 | No treatment (C) |
Group 3 . Experimental hot etching solution (ST): a hot acidic solution containing HCl and FeCl 3 (100° C) was applied for 30 min .
Group 4 . No treatment was performed (C).
The experimental groups and the chemical composition of the different conditioning agents are summarized in Table 1 .
After each surface treatment, specimens were rinsed with tap water for 1 min, ultrasonically cleaned in deionized water for 30 min and gently air-dried.
2.1
Atomic force microscope (AFM) evaluation
Eight zirconia discs for each subgroup were evaluated under an Atomic force microscope (AFM, Multimode Nanoscope IIIa, Digital Instruments, Veeco Metrology group, Santa Barbara, CA, USA). Images were taken in air. The tapping mode was performed using a 1–10 Ohm cm phosphorus ( n ) dopes Si tip (at 50 μ). Changes in vertical position provide the height of the images, registered as bright and dark regions. The tip-specimens were maintained stable through constant oscillation amplitude (set-point amplitude). Fields of view at 5 μm × 5 μm scan size were considered and recorded with a slow scan rate (0.1 Hz). A single operator analyzed the average surface roughness ( R a ) of the pre-treated ceramics, expressing it as a numeric value (in nanometers) using a specific software (Nanoscope V530R35R). Four measurements were performed for each pre-treated ceramic disc using a standardized rectangular spot (1.5 μm × 1.5 μm).
Average surface roughness values were checked for normal and equal distribution (Kolmogorov–Smirnov and Levene’s tests, respectively). Data were analyzed with Kruskal–Wallis test in order to compare roughness values between different materials and surface treatments ( α = 0.001). Multiple post hoc comparisons were computed using the Mann–Witney test ( p < 0.05). The statistical analysis was performed with SPSS 15.0 for Windows (SPSS Inc., Chicago, IL, USA).
2.2
Scanning electron microscopy (SEM) surface and interfacial evaluation
Conditioned zirconia discs from each experimental subgroup were rinsed with 96% ethanol and gently air-dried, mounted on metallic stubs, gold sputter-coated (Polaron Range SC 7620, Quorum Technology, Newhaven, UK) and evaluated under a scanning electron microscope (SEM, JSM-6060LV, Jeol, Tokyo, Japan) at 10,000× magnification to assess changes in surface topography.
The remaining three conditioned discs from each subgroup were used for SEM cement/ceramics interfacial analysis. Composite discs (Paradigm MZ100, 3 M ESPE, LOT: 20060213; Ø 10 mm × 2 mm-thick) were ground with 180-grit SiC paper, cleaned with 100% ethanol and gently air-dried. A dual-cure, total-etch resin cement (Calibra™ DeTrey DENTSPLY; batch no.: 080910) was used for luting the composite discs to the conditioned ceramic surfaces in combination with its specific etch-and-rinse priming-adhesive (XP Bond, DENTSPLY, batch no.: 0810003096) following manifacturers’ instructions. Bonded specimens were stored in a laboratory oven for 24 h (37 °C and 100% relative humidity) before being sectioned perpendicularly to the interface into 2 mm-thick slabs with a slow-speed diamond saw under constant water cooling (Isomet 1000). Each slice was polished with wet abrasive SiC papers (grit #600, 1000 and 1200), ultrasonicated in 96% ethanol for 2 min and dried with an oil-free air stream before being processed for SEM evaluation. Specimens were sputter-coated with gold and evaluated at 5000× magnifications to assess zirconia/resin cement interfacial properties. Measurements of surface topography were performed using a specific software (J Image 1.40g National Institute of Health, USA).
2
Materials and methods
Three types of partially-stabilized ZrO 2 ceramics were evaluated in the study: (1) Lava™ (3 M ESPE, Seefeld, Germany); (2) Cercon ® (DETREY DENTSPLY Ceramco, York, USA); (3) Aadva Zr (GC corp., Tokyo, Japan). Ninety-six cylinder-shaped zirconia discs (Ø 10 mm × 1 mm height) were obtained from each material. Samples were polished with SiC abrasive papers (grit #600, 1000, 1200 and 2000) and a final polishing was carried out with nylon cloths in combination with 1- and 0.50-μm grit size diamond pastes. Discs were then sonicated in deionized water for 5 min. Specimens were divided into four subgroups ( n = 8) according to the surface treatment performed:
Group 1 . Airborne particle abrasion (S): the Al 2 O 3 particles (125 μm) were applied perpendicularly on the entire zirconia surface for 10 s at 0.41–0.68 MPa and a distance of 20 mm.
Group 2 . Selective infiltration etching (SIE). A modified version of the original technique has been proposed . Specimens were coated with a thin layer of an infiltrating agent containing low temperature melting glass and additives ( Table 1 ) and consecutively heated up and cooled. Infiltrating agent remnants were dissolved by immersing ceramic discs in an ultrasonic bath with 5% hydrofluoric acid solution (30 min).
| Group | Treatment | Chemical composition |
|---|---|---|
| 1 | Airborne particle abrasion (S) | 125 μm Al 2 O 3 particles |
| 2 | Selective Infiltration Etching (SIE) | SiO 2 (65 wt%); Na 2 O (15 wt%); Al 2 O 3 (8 wt%); Li 2 O (3 wt%); B 2 O 3 (4 wt%); CaF 2 (5 wt%) |
| 3 | Exp. hot etching solution 30 min (ST) | Methanol (800 ml); 37% HCl (200 ml); and ferric chloride (2 g) |
| 4 | No treatment (C) |
Group 3 . Experimental hot etching solution (ST): a hot acidic solution containing HCl and FeCl 3 (100° C) was applied for 30 min .
Group 4 . No treatment was performed (C).
The experimental groups and the chemical composition of the different conditioning agents are summarized in Table 1 .
After each surface treatment, specimens were rinsed with tap water for 1 min, ultrasonically cleaned in deionized water for 30 min and gently air-dried.
2.1
Atomic force microscope (AFM) evaluation
Eight zirconia discs for each subgroup were evaluated under an Atomic force microscope (AFM, Multimode Nanoscope IIIa, Digital Instruments, Veeco Metrology group, Santa Barbara, CA, USA). Images were taken in air. The tapping mode was performed using a 1–10 Ohm cm phosphorus ( n ) dopes Si tip (at 50 μ). Changes in vertical position provide the height of the images, registered as bright and dark regions. The tip-specimens were maintained stable through constant oscillation amplitude (set-point amplitude). Fields of view at 5 μm × 5 μm scan size were considered and recorded with a slow scan rate (0.1 Hz). A single operator analyzed the average surface roughness ( R a ) of the pre-treated ceramics, expressing it as a numeric value (in nanometers) using a specific software (Nanoscope V530R35R). Four measurements were performed for each pre-treated ceramic disc using a standardized rectangular spot (1.5 μm × 1.5 μm).
Average surface roughness values were checked for normal and equal distribution (Kolmogorov–Smirnov and Levene’s tests, respectively). Data were analyzed with Kruskal–Wallis test in order to compare roughness values between different materials and surface treatments ( α = 0.001). Multiple post hoc comparisons were computed using the Mann–Witney test ( p < 0.05). The statistical analysis was performed with SPSS 15.0 for Windows (SPSS Inc., Chicago, IL, USA).
2.2
Scanning electron microscopy (SEM) surface and interfacial evaluation
Conditioned zirconia discs from each experimental subgroup were rinsed with 96% ethanol and gently air-dried, mounted on metallic stubs, gold sputter-coated (Polaron Range SC 7620, Quorum Technology, Newhaven, UK) and evaluated under a scanning electron microscope (SEM, JSM-6060LV, Jeol, Tokyo, Japan) at 10,000× magnification to assess changes in surface topography.
The remaining three conditioned discs from each subgroup were used for SEM cement/ceramics interfacial analysis. Composite discs (Paradigm MZ100, 3 M ESPE, LOT: 20060213; Ø 10 mm × 2 mm-thick) were ground with 180-grit SiC paper, cleaned with 100% ethanol and gently air-dried. A dual-cure, total-etch resin cement (Calibra™ DeTrey DENTSPLY; batch no.: 080910) was used for luting the composite discs to the conditioned ceramic surfaces in combination with its specific etch-and-rinse priming-adhesive (XP Bond, DENTSPLY, batch no.: 0810003096) following manifacturers’ instructions. Bonded specimens were stored in a laboratory oven for 24 h (37 °C and 100% relative humidity) before being sectioned perpendicularly to the interface into 2 mm-thick slabs with a slow-speed diamond saw under constant water cooling (Isomet 1000). Each slice was polished with wet abrasive SiC papers (grit #600, 1000 and 1200), ultrasonicated in 96% ethanol for 2 min and dried with an oil-free air stream before being processed for SEM evaluation. Specimens were sputter-coated with gold and evaluated at 5000× magnifications to assess zirconia/resin cement interfacial properties. Measurements of surface topography were performed using a specific software (J Image 1.40g National Institute of Health, USA).
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