Effectiveness of protecting a zirconia bonding surface against contaminations using a newly developed protective lacquer

Abstract

Objectives

The purpose of this study was to test the effectiveness of a newly developed lacquer and its ability to protect pre-conditioned bonding surfaces of zirconia ceramic against contamination with saliva or silicone remnants.

Methods

Disk-shaped zirconia ceramic specimens were conditioned and cleaned using air-abrasion. Before contamination with saliva or silicone, a newly developed protective lacquer (1% ethyl cellulose in ethanol) was applied to the bonding surface. After contamination, all specimens of the test groups were cleaned in an ultrasonic bath filled with 99% ethanol for 3 min and then air-dried. A universal primer (Monobond Plus) was applied to the surfaces and then the specimens were bonded to composite resin filled acrylic tubes using a luting resin (Multilink Automix). Each group ( n = 16) was divided into 2 subgroups ( n = 8). One subgroup was stored for 3 days in 37 °C tap water and the other subgroup was stored for 150 days in 37 °C tap water interrupted by 37,500 thermal cycles between 5 °C and 55 °C. After the storage, the bond strength was measured using a material testing machine.

Results

The specimens of the test groups showed comparable bond strengths to the positive control group after short-term storage. After artificial aging, bond strengths of the test groups were statistically significantly lower compared to the positive control and were statistically significantly higher compared to the negative control groups.

Significance

Overall, the use of the newly developed protective lacquer appears to be a promising approach to protect pre-conditioned surfaces of zirconia ceramics against contamination.

Introduction

Adhesive cementation of all-ceramic zirconia based restorations is widely accepted for clinical use. Various clinical studies document the long-term success of bonded ceramic restorations, such as resin-bonded all-ceramic fixed dental prostheses and all-ceramic crowns .

For durable adhesive cementation, zirconia ceramic restorations need to be air-abraded ( Fig. 1 ) . When delivered already air-abraded by the dental technician, the ceramic surface might be contaminated by saliva, blood or silicone fit-indicators during clinical try-in procedures and the resulting residual organic and silicone contaminants may have a negative influence on the long-term bond strength and the longevity of the restoration, and need to be removed from the surface before cementation by using air-abrasion again . Due to economic and practical considerations, using air-abrasion devices may not be widely accepted and therefore widespread in dental practices. Hence alternative cleaning methods like ethanol-irrigation or cleaning in an ultrasonic bath are often used by dentists. However, these alternative cleaning methods are not sufficient in removing the contaminations . Therefore, the development of a new method to protect air-abraded zirconia ceramic bonding surfaces from contaminations using a protective coating appears to be an eligible attempt.

Fig. 1
Detailed magnification of a zirconia ceramic bonding surface after air-abrasion with 50 μm Al 2 O 3 . SEM: 1000× magnification.

Ethyl cellulose is a derivative of cellulose in which some of the hydroxyl groups on the repeating glucose units are converted into ethyl ether groups. It is mainly used as a thin film coating material. Due to its chemical, biological and mechanical properties (insoluble in water, soluble in organic solvents, good biocompatibility and abrasion resistance), ethyl cellulose may be used as a protective coating for air-abraded zirconia ceramic bonding surfaces . To the knowledge of the authors, no scientific data on the effectiveness of a protective coating for pre-conditioned ceramic bonding surfaces is available yet.

Therefore, the purpose of this in vitro study was to test the effectiveness of a newly developed lacquer and its ability to protect pre-conditioned bonding surfaces of zirconia ceramic from contamination by saliva or silicone. The study was designed to test the hypothesis, that an ethyl cellulose based protective coating has a positive influence on the bond strength of a bonding resin to zirconia ceramic after contamination.

Materials and methods

Specimen preparation

Disk-shaped specimens ( n = 256) with a diameter of 7 mm and thickness of 3.4 mm were made of zirconia ceramic (Cerconbase, Degudent). All specimens were wet polished with rotating silicon carbide paper down to 600 grit and then air-abraded with 50 μm Al 2 O 3 at 0.25 MPa pressure ( Fig. 1 ). After air-abrasion, the average roughness was R a = 0.458 μm as measured with a laser scanning microscope (VK-X100, Keyence).

Lacquer application

A protective lacquer using a solution of 1% ethyl cellulose in 99% ethanol was applied to the bonding surfaces of the specimens of the three test groups directly after air-abrasion, while negative controls and positive controls were not coated with the lacquer. After the ethanol had completely evaporated after 24 h of storage, the lacquer formed a thin and hard continuous coating on the bonding surface with an average thickness of approximately 20 μm as determined by scanning electron microscopy ( Figs. 2 and 3 ).

Fig. 2
Bonding surface of a zirconia ceramic coated with an ethyl cellulose-based protection lacquer. SEM: 2000× magnification.

Fig. 3
Lacquer-coated specimen in cross section. Top of the figure: zirconia; center of the figure: ethyl cellulose based protection lacquer with an average thickness of approximately 20 μm. SEM: 1000× magnification.

Contamination

Directly after air-abrading the specimens of the protected test groups and the unprotected negative control groups ( n = 4) were contaminated in two different ways. Contamination with saliva: The specimens were immersed in saliva for 1 min. Saliva was collected from one of the authors who had refrained from eating and drinking 1.5 h prior to the collection procedure. All experiments were performed using fresh saliva collected at the same occasion. Then the excess of saliva was removed by water-spraying for 15 s followed by air-drying for another 15 s. Contamination with silicone: The specimens were pressed into the freshly mixed silicone disclosing medium (Fit Checker black, GC Corporation, Tokyo, Japan) with light force for 3 min. Because no remnants were visible after removing the black silicone, the specimens were not cleaned with water spray again. One test group was not contaminated after lacquer application.

Cleaning, bonding and storage conditions

All contaminated groups were ultrasonically cleaned in 99% ethanol for 3 min. A new bath was used for any specimen. The specimens of the test group that were not contaminated after lacquer application were ultrasonically cleaned as well. Afterwards, a universal primer (Monobond Plus, Ivoclar Vivadent, Schaan, FL) was applied to the ceramic bonding surface.

Plexiglas tubes with an inner diameter of 3.2 mm were filled with dual-curing composite resin (Multicore Flow, Ivoclar Vivadent). 5 min after composite application, the filled tubes were bonded with a luting composite resin (Multilink Automix, transparent, Ivoclar Vivadent) to the ceramic surface using an alignment apparatus under a load of 750 g. This apparatus ensured that the tube axis was perpendicular to the surface ( Fig. 4 ). After excess resin was removed, an air blocking gel (Liquid Strip, Ivoclar Vivadent) was applied around the bonding margins. The bonded specimens were light-cured for 20 s from two opposite sides with a dental curing light at a light intensity of 650 mW/cm 2 (Demetron Optilux 501, Kerr, Danbury, USA), then further cured in a laboratory light-curing unit (Heraflash, Heraeus Kulzer, Hanau, D) for 90 s, placed at room temperature for 10 min, and then stored in 37 °C water.

Fig. 4
Test configuration. Alignment apparatus (left), thermal cycling apparatus (middle) and self-aligning debonding jig with upper and lower rings for chain suspension (right). TC, thermal cycling.

For each test and control group 16 specimens were bonded each. Subgroups of eight bonded specimens were stored in tap water (37 °C) either for 3 days without artificial aging or for 150 days interrupted by 37,500 thermal cycles (5–55 °C) with a dwell time of 30 s . Composition and batch numbers of the materials used are shown in Table 1 .

Table 1
List of used materials and their characteristics.
Material Main composition a Manufacturer Batch No.
Cerconbase 38 >92% ZrO 2 Y 2 O 3 , HfO 2 , Al 2 O 3 , SiO 2 and other oxides Dentsply Detrey 20017175
Ethylcellulosis Base for protection lacquer Dow Chemical Company 5245KA
Monobond Plus Ethanol, water, silane methaycrylate, phosphoric acid methacrylate, sulphide methacrylate Ivoclar Vivadent M35022
MultiCore Flow Dimethacrylates, inorganic fillers, ytterbiumtrifluoride, initiators, stabilizers, pigments cont. composite Ivoclar Vivadent N55968
Multilink Automix Dimethacrylates, hydroxyethyl methacrylate, inorganic fillers, ytterbiumtrifluoride, initiators, stabilizers, pigments cont. dental luting material Ivoclar Vivadent M55053
Liquid Strip >90% Glycerin, silicondioxide, aluminum oxide cont. gel Ivoclar Vivadent M32317
Fit Cecker Flowable C-Silicone GC Corporation 911021

a According to the information provided by the manufacturers.

Debonding and analysis

At the end of the storage period, tensile bond strength (TBS) was measured in a universal testing apparatus (Z010, Zwick, Ulm, D) at a crosshead speed of 2 mm/min using a chain loop alignment which provided a moment-free axial load. The fractured interfaces of the debonded specimens were examined using a light microscope (LM, Zeiss S7, Carl Zeiss AG, Oberkochen, D) at 30× magnification to calculate the debonded area and to assign failure modes as either adhesive or cohesive (failure in tube composite or bonding resin). After sputtering (Leica EM QSG 100, Wetzlar, Germany) a conductive gold layer with a thickness of approximately 15 nm (the thickness was measured using a quartz crystal film thickness monitor), representative samples were examined in a scanning electron microscope (SEM, XL 30 CP, Philips, Kassel, Germany) with an acceleration voltage of 15 keV.

Statistical analysis using the Shapiro–Wilk test showed that not all groups were distributed normally. Therefore statistical analysis of the was performed using the Kruskal–Wallis test ( p ≤ 0.05) followed by multiple pair-wise comparisons of the groups using the Wilcoxon rank sum test, corrected with the procedure of Bonferroni–Holm for multiple comparisons. See Fig. 4 for an overview of the study design.

Materials and methods

Specimen preparation

Disk-shaped specimens ( n = 256) with a diameter of 7 mm and thickness of 3.4 mm were made of zirconia ceramic (Cerconbase, Degudent). All specimens were wet polished with rotating silicon carbide paper down to 600 grit and then air-abraded with 50 μm Al 2 O 3 at 0.25 MPa pressure ( Fig. 1 ). After air-abrasion, the average roughness was R a = 0.458 μm as measured with a laser scanning microscope (VK-X100, Keyence).

Lacquer application

A protective lacquer using a solution of 1% ethyl cellulose in 99% ethanol was applied to the bonding surfaces of the specimens of the three test groups directly after air-abrasion, while negative controls and positive controls were not coated with the lacquer. After the ethanol had completely evaporated after 24 h of storage, the lacquer formed a thin and hard continuous coating on the bonding surface with an average thickness of approximately 20 μm as determined by scanning electron microscopy ( Figs. 2 and 3 ).

Fig. 2
Bonding surface of a zirconia ceramic coated with an ethyl cellulose-based protection lacquer. SEM: 2000× magnification.

Fig. 3
Lacquer-coated specimen in cross section. Top of the figure: zirconia; center of the figure: ethyl cellulose based protection lacquer with an average thickness of approximately 20 μm. SEM: 1000× magnification.

Contamination

Directly after air-abrading the specimens of the protected test groups and the unprotected negative control groups ( n = 4) were contaminated in two different ways. Contamination with saliva: The specimens were immersed in saliva for 1 min. Saliva was collected from one of the authors who had refrained from eating and drinking 1.5 h prior to the collection procedure. All experiments were performed using fresh saliva collected at the same occasion. Then the excess of saliva was removed by water-spraying for 15 s followed by air-drying for another 15 s. Contamination with silicone: The specimens were pressed into the freshly mixed silicone disclosing medium (Fit Checker black, GC Corporation, Tokyo, Japan) with light force for 3 min. Because no remnants were visible after removing the black silicone, the specimens were not cleaned with water spray again. One test group was not contaminated after lacquer application.

Cleaning, bonding and storage conditions

All contaminated groups were ultrasonically cleaned in 99% ethanol for 3 min. A new bath was used for any specimen. The specimens of the test group that were not contaminated after lacquer application were ultrasonically cleaned as well. Afterwards, a universal primer (Monobond Plus, Ivoclar Vivadent, Schaan, FL) was applied to the ceramic bonding surface.

Plexiglas tubes with an inner diameter of 3.2 mm were filled with dual-curing composite resin (Multicore Flow, Ivoclar Vivadent). 5 min after composite application, the filled tubes were bonded with a luting composite resin (Multilink Automix, transparent, Ivoclar Vivadent) to the ceramic surface using an alignment apparatus under a load of 750 g. This apparatus ensured that the tube axis was perpendicular to the surface ( Fig. 4 ). After excess resin was removed, an air blocking gel (Liquid Strip, Ivoclar Vivadent) was applied around the bonding margins. The bonded specimens were light-cured for 20 s from two opposite sides with a dental curing light at a light intensity of 650 mW/cm 2 (Demetron Optilux 501, Kerr, Danbury, USA), then further cured in a laboratory light-curing unit (Heraflash, Heraeus Kulzer, Hanau, D) for 90 s, placed at room temperature for 10 min, and then stored in 37 °C water.

Fig. 4
Test configuration. Alignment apparatus (left), thermal cycling apparatus (middle) and self-aligning debonding jig with upper and lower rings for chain suspension (right). TC, thermal cycling.

For each test and control group 16 specimens were bonded each. Subgroups of eight bonded specimens were stored in tap water (37 °C) either for 3 days without artificial aging or for 150 days interrupted by 37,500 thermal cycles (5–55 °C) with a dwell time of 30 s . Composition and batch numbers of the materials used are shown in Table 1 .

Table 1
List of used materials and their characteristics.
Material Main composition a Manufacturer Batch No.
Cerconbase 38 >92% ZrO 2 Y 2 O 3 , HfO 2 , Al 2 O 3 , SiO 2 and other oxides Dentsply Detrey 20017175
Ethylcellulosis Base for protection lacquer Dow Chemical Company 5245KA
Monobond Plus Ethanol, water, silane methaycrylate, phosphoric acid methacrylate, sulphide methacrylate Ivoclar Vivadent M35022
MultiCore Flow Dimethacrylates, inorganic fillers, ytterbiumtrifluoride, initiators, stabilizers, pigments cont. composite Ivoclar Vivadent N55968
Multilink Automix Dimethacrylates, hydroxyethyl methacrylate, inorganic fillers, ytterbiumtrifluoride, initiators, stabilizers, pigments cont. dental luting material Ivoclar Vivadent M55053
Liquid Strip >90% Glycerin, silicondioxide, aluminum oxide cont. gel Ivoclar Vivadent M32317
Fit Cecker Flowable C-Silicone GC Corporation 911021

a According to the information provided by the manufacturers.

Debonding and analysis

At the end of the storage period, tensile bond strength (TBS) was measured in a universal testing apparatus (Z010, Zwick, Ulm, D) at a crosshead speed of 2 mm/min using a chain loop alignment which provided a moment-free axial load. The fractured interfaces of the debonded specimens were examined using a light microscope (LM, Zeiss S7, Carl Zeiss AG, Oberkochen, D) at 30× magnification to calculate the debonded area and to assign failure modes as either adhesive or cohesive (failure in tube composite or bonding resin). After sputtering (Leica EM QSG 100, Wetzlar, Germany) a conductive gold layer with a thickness of approximately 15 nm (the thickness was measured using a quartz crystal film thickness monitor), representative samples were examined in a scanning electron microscope (SEM, XL 30 CP, Philips, Kassel, Germany) with an acceleration voltage of 15 keV.

Statistical analysis using the Shapiro–Wilk test showed that not all groups were distributed normally. Therefore statistical analysis of the was performed using the Kruskal–Wallis test ( p ≤ 0.05) followed by multiple pair-wise comparisons of the groups using the Wilcoxon rank sum test, corrected with the procedure of Bonferroni–Holm for multiple comparisons. See Fig. 4 for an overview of the study design.

Results

Median TBS of all groups are shown in Table 2 and boxplots of all groups are shown in Fig. 5 .

Table 2
Median TBS in MPa of all test and control groups.
Protected by lacquer Contamination Ultrasonic cleaning in ethanol TBS after
3 days
(MPa)
TBS after
150 days
(MPa)
No No No 36.6Aα
36.6 α A
31.3Aα
31.3 α A
No Saliva No 0Bα
0 α B
0Bα
0 α B
No Saliva Yes 0Bα
0 α B
0Bα
0 α B
No Silicone No 16.5Cα
16.5 α C
0Bβ
0 β B
No Silicone Yes 11.7Cα
11.7 α C
1.5Bα
1.5 α B
Yes No Yes 42.1Aα
42.1 α A
27Cα
27 α C
Yes Saliva Yes 30.5Aα
30.5 α A
22.7Cα
22.7 α C
Yes Silicone Yes 40.8Aα
40.8 α A
22.2Cβ
22.2 β C
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Effectiveness of protecting a zirconia bonding surface against contaminations using a newly developed protective lacquer
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