Highlights
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Etching or thermocycling could have a wakening effect on IPS e.max ® Press glass ceramic, but the resin cement bonding to appropriately etched surface would strengthen the dental ceramic.
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In the present investigation, no significant differences before and after thermocycling on the flexural strengths were evident.
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It is possible that the etching decrease the strength of glass ceramic, but the strength will increase when the etched ceramic was bonded using the resin cement.
Abstract
Objective
To evaluate the effect of hydrofluoric acid (HFA) etching time and resin cement bond on the flexural strength of IPS e.max ® Press glass ceramic.
Methods
Two hundred and ten bars, 25 mm × 3 mm × 2 mm, were made from IPS e.max ® Press ingots through lost-wax, hot-pressed ceramic fabrication technology and randomly divided into five groups with forty-two per group after polishing. The ceramic surfaces of different groups were etched by 9.5% hydrofluoric acid gel for 0, 20, 40, 60 and 120 s respectively. Two specimens of each group were selected randomly to examine the surface roughness and 3-dimensional topography with atomic force microscope (AFM), and microstructure was analyzed by the field emission scanning electron microscope (FE-SEM). Then each group were subdivided into two subgroups ( n = 20). One subgroup of this material was selected to receive a thin (approximately 0.1 mm) layer of resin luting agent (Variolink N) whereas the other subgroup remained unaltered. Half of subgroup’s specimens were thermocycled 10,000 times before a 3-point bending test in order to determine the flexural strength. Interface between resin cement and ceramic was examined with field emission scanning electronic microscope.
Results
Roughness values increased with increasing etching time. The mean flexural strength values of group 0 s, 20 s, 40 s, 60 s and 120 s were 384 ± 33, 347 ± 43, 330 ± 53, 327 ± 67 and 317 ± 41 MPa respectively. Increasing HF etching times reduced the mean flexural strength ( p < 0.05). However, the mean flexural strength of each group, except group 0 s, increased significantly to 420 ± 31, 435 ± 50, 400 ± 39 and 412 ± 58 MPa after the application of dual-curing resin cement. In the present investigation, no significant differences after thermocycling on the flexural strengths were evident.
Significance
Overtime HF etching could have a wakening effect on IPS e.max ® Press glass ceramic, but resin cement bonding to appropriately etched surface would strengthen the dental ceramic.
1
Introduction
Dental ceramics are appreciated as highly esthetic restorative materials that can simulate the appearance of natural dentition better compared to other material. The preference is directly related to the success of ceramic resin bond that contributes to the restoration longevity . Additionally, long-term retention of the restoration depends primarily on the strength and durability of the bond of luting composite resin to the tooth and porcelain substrates to prevent fracture, marginal discoloration, and secondary caries . To achieve successful bonding, the ceramic surface may be modified chemically by hydrofluoric (HF) acid etching or mechanically by grit blasting to promote the roughness of ceramic surface and/or reactivity of the ceramic to luting agent, and chemical bonding by a silane coupling agent ; however, dental ceramic is fragile under tensile strain. The weakness can be attributed to the presence and propagation of microflaws presented on the surface of the material, making dental ceramic susceptible to fracture during the luting procedure and under occlusal force . Therefore a major concern existing about the use of HF acid etching is due to its possible deleterious effects on ceramic strength while obtaining excellent bonding strength to tooth structure.
Since HF acid etching was first suggested as a ceramic surface pretreatment for resin bonding, many different etching periods have been advocated and used. The manufacturer’s recommended etching time for cementation of the IPS e.max Press glass ceramic restorations with a luting resin is about 20 s. However, clinically, the optimal HF acid etching time and concentration to treat the glass ceramic restoration is not clear. Therefore, it is important to know the adequate HF etching time for resin cement bonding without weakening the ceramic. At the same time, ceramic surface etched by HF would finally be cemented to the underlying tooth structure utilizing a resin luting cement. Hence, the flexural strength of the ceramic etched by HF may not exactly reflect the actual strength of all-ceramic restoration. Therefore, the purpose of this study was to evaluate the effect of HF acid etching time and resin cement bond on flexural strength of a novel lithium disilicate-based glass ceramic (IPS e.max Press), and then test the hypothesis that different HF etching times and resin cement bonding procedures produce different flexural strength values for this ceramic.
2
Materials and methods
2.1
Preparation of specimens
A total of 210 bars (2 mm × 3 mm × 25 mm) were fabricated from the lithium disilicate-based core ceramic (IPS e.max Press HT ingots, Ivoclar-Vivaden AG, Schaan, Liechtenstein) using a lost wax, hot press technique according to the manufacturer’s recommended instructions for this study. The ceramic surface was ground (with 400-, 600-, and 800-grit silicon carbide paper on a grinding device (Mecatech 234, Presi, Grenoble, France)), and highly was polished down to 0.5 μm by using progressively finer diamond polishing paste. Finally these specimens were cleaned ultrasonically for 5 min with acetone before and after using 9.5% hydrofluoric acid gel (Bisco Inc., Schaumburg IL, USA), as recommended for clinical practice. The specimens were randomly divided into 5 groups ( n = 42) according to the etching time of 0, 20, 40, 60 and 120 s respectively.
2.2
Examination of the roughness and topography of etched lithium disilicate glass ceramic
The treated surfaces of two ceramic bars from each group were measured by a multimode atomic force microscope (Autoprobe CP Research System, Veeco Instruments, Sunnyvale, CA, USA) operating in tapping mode. All samples were scanned over an area of 20 μm × 20 μm, Ra (roughness average) measured from a mean line within the sampling length, which is the most commonly reported roughness parameter in the dental literature, was recorded . Two other parameters (Rp-v and Rq) were also recorded in addition to Ra to quantify the amount of etching induced by the treatments being examined. The Rp-v value is the maximum peak-to-valley height measured parallel to the traverse and Rq (root mean square) value is the geometric average of roughness component irregularities measured from the mean line within the sampling length. Similarly, the topography of the treated samples surfaces of each group were observed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Ltd., Tokyo, Japan).
2.3
Preparation of resin bonding specimens
Other samples were divided into 2 subgroups within each group. Silane coupling agent (Monobond N Ivoclar-Vivadent AG, Schaan, Liechtenstein) was then applied to etched surfaces of subgroup A for 60 s and lightly air thinned. Then the silanated ceramic surfaces were received a layer of unfilled resin bonding agent (Heliobond, Ivoclar-Vivadent AG, Schaan, Liechtenstein). A small amount of dual-polymerized composite resin (Variolink N Base, Ivoclar-Vivadent AG, Schaan, Liechtenstein) was applied to the surfaces and placed with bonding agent-side down on a polyethylene film-covered microscope slide. A constant load of 9.8 N was applied to the ceramic for 3 min in order to create a relatively uniform resin cement of approximately 100 μm to simulate the range of resin luting film thickness for all-ceramic crowns . Excess resin cement was removed with a brush and then polymerized (1200 mW/cm −2 , Ivoclar-Vivadent AG, Schaan, Liechtenstein) for 40 s, according to the manufacturer’s instructions. Samples of subgroup B remained untreated. Half of every subgroup’s specimens were immersed in distilled water at 37° for 24 h which was defined as thermocycle 0. The other half were thermo-cycled between 5° and 55° for 10,000 times with 60 s dwell time after stored in distilled water at 37° for 24 h (TC501, Weier Inc., Suzhou, China).
2.4
Flexural strength test and bonding interface
The flexural strength of the ceramic bars was measured using the 3-point bending test on a Universal Testing Machine (Synergie 100, MTS Co., Eden Prairie, MN, USA). A load was applied on the ceramic surface with the resin cement side down at a crosshead speed of 0.5 mm/min until failure. The following equation was used for flexural strength ( F ) calculation: F = 3 PL /2 bh 2 , P was the load at fracture; L was the test span (20 mm); b was the thickness of the sample; h was the height of the sample. b and h of the fracture surfaces were recorded using the digital indicator after the failure.
The representative samples of ceramic-resin cement were selected and embedded with epoxy resin from each group for secondary electron imaging under FE-SEM. Each specimen surface was ground with 400, 600, 800, 1200 grit silicon carbide paper on a grinding device (Mecatech 234, Presi, Grenoble. France), and highly polished using progressively finer diamond polishing paste down to 0.5 μm.
2.5
Statistical analysis
Statistical analysis was performed using statistical software (SPSS 11.5 Inc., Chicago, IL USA). One-way analysis of variance was applied in order to compared Ra, Rq, and Rp-v of all groups, with p < 0.05 to establish significance. A three-way factorial design was utilized with the factors identified as: hydrofluoric acid etching time at five levels (0 s, 20 s, 40 s, 60 s or 120 s); resin cement bond at two levels (no bonding, or resin bond with a dual-curing resin cement); thermocycling at two levels (thermocycle 0 time, or thermocycle 10,000 times). The general liner model was utilized to identify which of the three factors and interactions between factors were significant influencing the strength of the glass ceramic. And then post hoc all paired Tukey tests were employed at a significance level of p < 0.05 to identify statistically significant differences between paired group with the corrected p -value stated.
2
Materials and methods
2.1
Preparation of specimens
A total of 210 bars (2 mm × 3 mm × 25 mm) were fabricated from the lithium disilicate-based core ceramic (IPS e.max Press HT ingots, Ivoclar-Vivaden AG, Schaan, Liechtenstein) using a lost wax, hot press technique according to the manufacturer’s recommended instructions for this study. The ceramic surface was ground (with 400-, 600-, and 800-grit silicon carbide paper on a grinding device (Mecatech 234, Presi, Grenoble, France)), and highly was polished down to 0.5 μm by using progressively finer diamond polishing paste. Finally these specimens were cleaned ultrasonically for 5 min with acetone before and after using 9.5% hydrofluoric acid gel (Bisco Inc., Schaumburg IL, USA), as recommended for clinical practice. The specimens were randomly divided into 5 groups ( n = 42) according to the etching time of 0, 20, 40, 60 and 120 s respectively.
2.2
Examination of the roughness and topography of etched lithium disilicate glass ceramic
The treated surfaces of two ceramic bars from each group were measured by a multimode atomic force microscope (Autoprobe CP Research System, Veeco Instruments, Sunnyvale, CA, USA) operating in tapping mode. All samples were scanned over an area of 20 μm × 20 μm, Ra (roughness average) measured from a mean line within the sampling length, which is the most commonly reported roughness parameter in the dental literature, was recorded . Two other parameters (Rp-v and Rq) were also recorded in addition to Ra to quantify the amount of etching induced by the treatments being examined. The Rp-v value is the maximum peak-to-valley height measured parallel to the traverse and Rq (root mean square) value is the geometric average of roughness component irregularities measured from the mean line within the sampling length. Similarly, the topography of the treated samples surfaces of each group were observed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Ltd., Tokyo, Japan).
2.3
Preparation of resin bonding specimens
Other samples were divided into 2 subgroups within each group. Silane coupling agent (Monobond N Ivoclar-Vivadent AG, Schaan, Liechtenstein) was then applied to etched surfaces of subgroup A for 60 s and lightly air thinned. Then the silanated ceramic surfaces were received a layer of unfilled resin bonding agent (Heliobond, Ivoclar-Vivadent AG, Schaan, Liechtenstein). A small amount of dual-polymerized composite resin (Variolink N Base, Ivoclar-Vivadent AG, Schaan, Liechtenstein) was applied to the surfaces and placed with bonding agent-side down on a polyethylene film-covered microscope slide. A constant load of 9.8 N was applied to the ceramic for 3 min in order to create a relatively uniform resin cement of approximately 100 μm to simulate the range of resin luting film thickness for all-ceramic crowns . Excess resin cement was removed with a brush and then polymerized (1200 mW/cm −2 , Ivoclar-Vivadent AG, Schaan, Liechtenstein) for 40 s, according to the manufacturer’s instructions. Samples of subgroup B remained untreated. Half of every subgroup’s specimens were immersed in distilled water at 37° for 24 h which was defined as thermocycle 0. The other half were thermo-cycled between 5° and 55° for 10,000 times with 60 s dwell time after stored in distilled water at 37° for 24 h (TC501, Weier Inc., Suzhou, China).
2.4
Flexural strength test and bonding interface
The flexural strength of the ceramic bars was measured using the 3-point bending test on a Universal Testing Machine (Synergie 100, MTS Co., Eden Prairie, MN, USA). A load was applied on the ceramic surface with the resin cement side down at a crosshead speed of 0.5 mm/min until failure. The following equation was used for flexural strength ( F ) calculation: F = 3 PL /2 bh 2 , P was the load at fracture; L was the test span (20 mm); b was the thickness of the sample; h was the height of the sample. b and h of the fracture surfaces were recorded using the digital indicator after the failure.
The representative samples of ceramic-resin cement were selected and embedded with epoxy resin from each group for secondary electron imaging under FE-SEM. Each specimen surface was ground with 400, 600, 800, 1200 grit silicon carbide paper on a grinding device (Mecatech 234, Presi, Grenoble. France), and highly polished using progressively finer diamond polishing paste down to 0.5 μm.
2.5
Statistical analysis
Statistical analysis was performed using statistical software (SPSS 11.5 Inc., Chicago, IL USA). One-way analysis of variance was applied in order to compared Ra, Rq, and Rp-v of all groups, with p < 0.05 to establish significance. A three-way factorial design was utilized with the factors identified as: hydrofluoric acid etching time at five levels (0 s, 20 s, 40 s, 60 s or 120 s); resin cement bond at two levels (no bonding, or resin bond with a dual-curing resin cement); thermocycling at two levels (thermocycle 0 time, or thermocycle 10,000 times). The general liner model was utilized to identify which of the three factors and interactions between factors were significant influencing the strength of the glass ceramic. And then post hoc all paired Tukey tests were employed at a significance level of p < 0.05 to identify statistically significant differences between paired group with the corrected p -value stated.
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