1

Introduction

Zirconia ceramic is an attractive core material for fabrication of all-ceramic restorations due to its outstanding mechanical properties . However compared to silica ceramics, which can be bonded using hydrofluoric acid etching and silanation, zirconia ceramic requires alternative techniques for long-term durable resin bonding . Various surface treatments such as air-borne particle abrasion, silica coating and selective infiltration etching technique have been used to improve bonding to zirconia . However, the results were conflicting especially regarding long-term bond durability. Indeed, the maintenance of a durable bond under the influence of fatigue conditions, in presence of saliva, and temperature changes is of outermost importance , so silanes and ceramic primers are used to enhance chemical bonding to these ceramics .

However the most often used silane coupling agent for silica-based ceramics, 3-methacryloxypropyltrimethoxysilane (MPS), might help in surface wetting of oxide ceramics but it does not promote adequate bonding to zirconia ceramics . Recently several new ceramic primers have been introduced into the dental market to enhance chemical bonding to zirconia ceramic , e.g., primers containing organophosphates, carboxylic acids, silanes, other adhesive monomers and combinations of monomers. A newly developed universal primer (Monobond Plus, Ivoclar Vivadent, Schaan, Liechtenstein) containing an alcohol solution of 3-methacryloxypropyl-trimethoxysilane, phosphoric acid methacrylate and sulphide methacrylate is claimed by its manufacturer to bond effectively to zirconia ceramic. However, no independent data whether this primer promotes durable bonding to zirconia ceramic has been published yet.

Moreover after surface conditioning and prior to primer application the bonding surfaces are cleaned from dust resulting from air-borne particle abrasion either by cleaning with a stream of oil-free air or by ultrasonic cleaning. Some studies reported long-term bond durability to alumina and zirconia ceramics after silica coating and ultrasonic cleaning, while Nishigawa et al. reported a decreased bond strength after ultrasonic cleaning of silica-coated zirconia ceramic.

Therefore the purpose of this laboratory study was to investigate the influence of different surface conditioning and cleaning methods on the tensile bond strength of luting resin to zirconia ceramic. In addition, the effect of the application of a silane or a universal primer containing a silane and a phosphate monomer was tested.

The hypotheses of the study were that (1) using a new universal primer will increase resin bond strength to zirconia ceramic regardless of surface conditioning, (2) ultrasonic cleaning in alcohol will improve bonding durability to zirconia ceramic regardless of surface conditioning.

2

Material and methods

A total of 96 disc-shaped specimens were fabricated from zirconia ceramic (e.max ZirCAD, Ivoclar Vivadent, Schaan, Liechtenstein). Specimens were divided into 6 test groups ( n = 16) according to surface conditioning, cleaning methods and ceramic primers used as follows (materials and manufacturers are listed in Table 1 ):

Group ROC-A-S: air-borne particle abrasion with 110 μm alumina particles according to manufacturer instructions for 15 s at 0.28 MPa (Rocatec Pre), followed by air-borne particle abrasion with 110-μm grain sized aluminum trioxide particles surface-modified with silica (tribochemical silica coating, Rocatec Plus), at 0.28 MPa, from a perpendicular distance of 10 mm for 15 s , cleaning with oil-free air stream for 15 s in the Rocatec device and application of one coat of a silane (Espe Sil) with a clean disposable brush (Ivoclar Vivadent) .

Group ROC-U-S: as group ROC-A-S but 3 min ultrasonic cleaning in 99% isopropanol after tribochemical silica coating. Then specimens were dried using oil-free air stream for 15 s.

Group ROC-A-P: as group ROC-A-S but a universal primer containing a silane and a phosphate monomer (Monobond Plus) was used instead of the silane. The primer was applied in excess to the pre-treated ceramic with a disposable brush and was allowed to react for 60 s, and then it was dispersed with oil-free air stream for 5 s.

Group ROC-U-P: as group ROC-U-S but the universal primer was used instead of the silane.

Group ABR-A-P: air-borne particle abrasion with 110 μm alumina particles (Rocatec Pre) for 15 s at 0.28 MPa, followed by cleaning with oil-free air stream for 15 s in the Rocatec device and application of the universal primer.

Group ABR-U-P: as group ABR-A-P but 3 min ultrasonic cleaning in 99% isopropanol after air-borne particle abrasion. Then specimens were dried using oil-free air stream for 15 s.

Transparent plastic tubes (Plexiglas, Rohn, Darmstadt, Germany) with 3.2 mm inner diameter were filled with freshly mixed composite resin (Multicore flow) and allowed to autopolymerize for 6 min before bonding . Then, a dual curing adhesive luting resin (MultiLink Automix) was used according to the manufacturer’s instructions for bonding the filled plastic tubes to the zirconia ceramic discs using an alignment apparatus under a load of 750 g. Bonded specimens were light-cured from two opposite sides for 20 s at 5 mm distance and light intensity of 650 mW/cm ² with a handheld light-curing device (UniXS, Heraeus Kulzer, Wehrheim, Germany). Furthermore specimens were light-cured in light-curing device (Dentacolor XS, Heraeus Kulzer) for 90 s and kept in an oven at 37 °C for 5 min. Each main group was divided into 2 subgroups ( n = 8). Eight specimens were stored in distilled water bath at 37 °C for 3 days without thermal cycling, while the other 8 specimens were stored for 30 days in the same water bath at 37 °C interrupted by thermal cycling between 5 °C and 55 °C in distilled water with a dwell time of 30 s (Willytec, Munich, Germany) for 7500 cycles. Tensile bond strength (TBS) was measured in a universal testing machine (Zwick Z010, Zwick, Ulm, Germany) at a crosshead speed of 2 mm/min using a chain loop alignment which provided a moment-free axial application . Statistical analyses were performed with three-way analyses of variance (ANOVA) followed by serial two-way ANOVAs and serial one-way (ANOVAs) at each level of the study followed by Post Hoc Tukey-HSD test at ( α = 0.05).

The fractured interfaces of the zirconia ceramic specimens were examined with a light microscope (Wild Makroskop M 420; Heerbrug, Germany) at 20× magnification to assess the failure modes. Debonded surfaces were assigned to cohesive failure within luting resin or composite resin, adhesive at ceramic/cement interface or mixed adhesive/cohesive modes . Representative specimens for each failure mode were examined using a scanning electron microscope (SEM; XL 30 CP; Philips, Eindhoven, Netherlands) with an acceleration voltage of 15 kV and a working distance of 10 mm.

2

Material and methods

A total of 96 disc-shaped specimens were fabricated from zirconia ceramic (e.max ZirCAD, Ivoclar Vivadent, Schaan, Liechtenstein). Specimens were divided into 6 test groups ( n = 16) according to surface conditioning, cleaning methods and ceramic primers used as follows (materials and manufacturers are listed in Table 1 ):

Group ROC-A-S: air-borne particle abrasion with 110 μm alumina particles according to manufacturer instructions for 15 s at 0.28 MPa (Rocatec Pre), followed by air-borne particle abrasion with 110-μm grain sized aluminum trioxide particles surface-modified with silica (tribochemical silica coating, Rocatec Plus), at 0.28 MPa, from a perpendicular distance of 10 mm for 15 s , cleaning with oil-free air stream for 15 s in the Rocatec device and application of one coat of a silane (Espe Sil) with a clean disposable brush (Ivoclar Vivadent) .

Group ROC-U-S: as group ROC-A-S but 3 min ultrasonic cleaning in 99% isopropanol after tribochemical silica coating. Then specimens were dried using oil-free air stream for 15 s.

Group ROC-A-P: as group ROC-A-S but a universal primer containing a silane and a phosphate monomer (Monobond Plus) was used instead of the silane. The primer was applied in excess to the pre-treated ceramic with a disposable brush and was allowed to react for 60 s, and then it was dispersed with oil-free air stream for 5 s.

Group ROC-U-P: as group ROC-U-S but the universal primer was used instead of the silane.

Group ABR-A-P: air-borne particle abrasion with 110 μm alumina particles (Rocatec Pre) for 15 s at 0.28 MPa, followed by cleaning with oil-free air stream for 15 s in the Rocatec device and application of the universal primer.

Group ABR-U-P: as group ABR-A-P but 3 min ultrasonic cleaning in 99% isopropanol after air-borne particle abrasion. Then specimens were dried using oil-free air stream for 15 s.

Transparent plastic tubes (Plexiglas, Rohn, Darmstadt, Germany) with 3.2 mm inner diameter were filled with freshly mixed composite resin (Multicore flow) and allowed to autopolymerize for 6 min before bonding . Then, a dual curing adhesive luting resin (MultiLink Automix) was used according to the manufacturer’s instructions for bonding the filled plastic tubes to the zirconia ceramic discs using an alignment apparatus under a load of 750 g. Bonded specimens were light-cured from two opposite sides for 20 s at 5 mm distance and light intensity of 650 mW/cm ² with a handheld light-curing device (UniXS, Heraeus Kulzer, Wehrheim, Germany). Furthermore specimens were light-cured in light-curing device (Dentacolor XS, Heraeus Kulzer) for 90 s and kept in an oven at 37 °C for 5 min. Each main group was divided into 2 subgroups ( n = 8). Eight specimens were stored in distilled water bath at 37 °C for 3 days without thermal cycling, while the other 8 specimens were stored for 30 days in the same water bath at 37 °C interrupted by thermal cycling between 5 °C and 55 °C in distilled water with a dwell time of 30 s (Willytec, Munich, Germany) for 7500 cycles. Tensile bond strength (TBS) was measured in a universal testing machine (Zwick Z010, Zwick, Ulm, Germany) at a crosshead speed of 2 mm/min using a chain loop alignment which provided a moment-free axial application . Statistical analyses were performed with three-way analyses of variance (ANOVA) followed by serial two-way ANOVAs and serial one-way (ANOVAs) at each level of the study followed by Post Hoc Tukey-HSD test at ( α = 0.05).

The fractured interfaces of the zirconia ceramic specimens were examined with a light microscope (Wild Makroskop M 420; Heerbrug, Germany) at 20× magnification to assess the failure modes. Debonded surfaces were assigned to cohesive failure within luting resin or composite resin, adhesive at ceramic/cement interface or mixed adhesive/cohesive modes . Representative specimens for each failure mode were examined using a scanning electron microscope (SEM; XL 30 CP; Philips, Eindhoven, Netherlands) with an acceleration voltage of 15 kV and a working distance of 10 mm.

3

Results

Means TBS were compared across test groups at two times with a three-factor ANOVA model, including the following factors: ceramic primer, cleaning methods, storage condition and their interactions. The overall ANOVA F -test ( Table 2 ) was highly significant ( P < .0001), indicating differences in mean TBS across at least one of the factors. Ceramic primers and time factors were both significant ( P < .0001), while the cleaning factor was not significant ( P = .343). However, significant interactions between ceramic primers and the other factors were detected ( P < .01).

Table 2

Summary of overall 3-way ANOVA and serial 2-way ANOVAs.

By level	Sum of squares	Df	Mean square	F	P -Values
Overall
Cleaning	65.3	1	65.3	.911	.343
Priming	5237.5	1	5237.5	73	<.0001
Storage time	54,607.96	1	54,607.96	64.2	<.0001
Cleaning × Priming	453.9	1	453.9	6.3	.01
Priming × Time	937.2	1	937.2	13	.001
Cleaning × Time	19.4	1	19.4	0.27	.605
Cleaning × Priming × Time	5.1	1	5.14	0.72	.790
Error	6313.7	88	71.75
Total	132,202.6	96
Total (Corr.)	16,720.95	95
Priming	5237.5	1	5237.5	70.8	<.0001
Storage time	4608	1	4608	62.3	<.0001
Time × Priming	937.2	1	937.2	12.7	.001
Error	6803.7	92	73.953
Total	132,202.6	96
Total (Corr.)	16721	95
Priming	5237.5	1	5237.5	43.7	<.0001
Cleaning	65.3	1	65.3	.545	.462
Cleaning × Priming	453.9	1	453.9	3.8	.06
Error	11,028.5	92	119.9
Total	132,202.6	96
Total (Corr.)	16721	95
Cleaning	1	1	1	.008	.9
Storage time	3742.5	1	3742.5	26.6	<.0001
Cleaning × time	30	1	30	.2	.646
Error	12947.5	92	140.7
Total	132202.6	96
Total (Corr.)	16721	95

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