Composite cements benefit from light-curing

Abstract

Objective

To investigate the effect of curing of composite cements and a new ceramic silanization pre-treatment on the micro-tensile bond strength (μTBS).

Methods

Feldspathic ceramic blocks were luted onto dentin using either Optibond XTR/Nexus 3 (XTR/NX3; Kerr), the silane-incorporated ‘universal’ adhesive Scotchbond Universal/RelyX Ultimate (SBU/RXU; 3M ESPE), or ED Primer II/Panavia F2.0 (ED/PAF; Kuraray Noritake). Besides ‘composite cement’, experimental variables were ‘curing mode’ (‘AA’: complete auto-cure at 21 °C; ‘AA*’: complete auto-cure at 37 °C; ‘LA’: light-curing of adhesive and auto-cure of cement; ‘LL’: complete light-curing) and ‘ceramic surface pre-treatment’ (‘HF/S/HB’: hydrofluoric acid (‘HF’: IPS Ceramic Etching Gel, Ivoclar-Vivadent), silanization (‘S’: Monobond Plus, Ivoclar-Vivadent) and application of an adhesive resin (‘HB’: Heliobond, Ivoclar-Vivadent); ‘HF/SBU’: ‘HF’ and application of the ‘universal’ adhesive Scotchbond Universal (‘SBU’; 3M ESPE, only for SBU/RXU)). After water storage (7 days at 37 °C), ceramic–dentin sticks were subjected to μTBS testing.

Results

Regarding the ‘composite cement’, the significantly lowest μTBSs were measured for ED/PAF. Regarding ‘curing mode’, the significantly highest μTBS was recorded when at least the adhesive was light-cured (‘LA’ and ‘LL’). Complete auto-cure (‘AA’) revealed the significantly lowest μTBS. The higher auto-curing temperature (‘AA*’) increased the μTBS only for ED/PAF. Regarding ‘ceramic surface pre-treatment’, only for ‘LA’ the μTBS was significantly higher for ‘HF/S/HB’ than for ‘HF/SBU’.

Significance

Complete auto-cure led to inferior μTBS than when either the adhesive (on dentin) or both adhesive and composite cement were light-cured. The use of a silane-incorporated adhesive did not decrease luting effectiveness when also the composite cement was light-cured.

Introduction

Indirect ceramic restorations became more popular during the last years because of their superior esthetics, biocompatibility and long-term stability. So-called ‘etchable’ feldspathic (and glass-) ceramic restorations should be adhesively luted; it improves the fracture resistance and consequently enhances the survival rate . Possible curing modes of composite cements are ‘dual-cure’, involving also light-curing, and ‘auto-cure’. The beforehand applied adhesive, if any (not in case of self-adhesive composite cements), can be separately light-cured on the condition that its film thickness is sufficiently thin and does not impair the restoration fit. When adhesively luted ceramic restorations are light-cured, both the adhesive and cement may however not cure completely due to light attenuation, caused by the opacity of the restoration, its shade, and/or simply its thickness, the latter obviously related to cavity depth . A ceramic thickness of 2–3 mm is considered to be the threshold to still effectively light-cure adhesively luted ceramic restorations . Up to a thickness of 2 mm, there was according to Akgungor et al. no effect of both ceramic thickness and polymerization mode on bond strength. For thicker restorations of 4 mm, a lower micro-tensile bond strength (μTBS) was detected . Light attenuation already starts at a composite thickness of 1 mm, as the light intensity was reduced about 85%, and restorations of 4 mm thickness blocked the light almost completely . It was also reported that light-curing the adhesive and the composite cement separately improved the bond strength to dentin . Furthermore, auto-curing was found to influence the degree of conversion (DC) of composite cements and their mechanical properties . Depending on the material, the bond strength of composite cements that were allowed to auto-cure was inferior to when they were light-cured .

Apart from the curing mode, the μTBS of composite cements may also depend on how the ceramic surface is pre-treated. Besides the strength of the dentin–cement interface also that of the cement–ceramic interface will contribute to the overall strength of the bond of the indirect restoration to the tooth . Well accepted and most reliable is the etching of (feldspathic) ceramic with hydrofluoric acid (HF) followed by silanization. This pre-treatment provided the highest shear bond strength for four different cements and appeared also to remain stable six months after cementation . Silanization after etching with HF appeared also to be the decisive step in a study by Filho et al. , because HF-etching only, without separate silanization, resulted in a significantly lower μTBS. Ikemura et al. developed different experimental ‘multi-purpose’ adhesives with an incorporated silane; they were claimed to bond to various dental materials, including ceramics and metal alloys. The 30 wt.% silane-incorporated formulations bonded more effectively than those without silane . Likewise, a so-called ‘universal’ adhesive with incorporated silane was recently introduced (Scotchbond Universal, 3M ESPE, Seefeld, Germany; SBU); it is claimed to be effective for bonding to both tooth surfaces and ceramics, the latter without the need of an additional and separate silane primer.

Although numerous studies reported on luting effectiveness, interactions of different factors involved in adhesive luting and their relative importance remain unclear. Therefore, the purpose of this study was to determine the effect of curing mode on the μTBS of composite cements to dentin and to evaluate a new ceramic surface pre-treatment using a silane-incorporated, so-called ‘universal’ adhesive. Therefore, the first hypothesis tested was that there was no significant difference in μTBS among the different experimental groups that varied for ‘composite cement’ and ‘curing mode’ (1). In addition, the second hypothesis tested was that the auto-cure temperature (room vs. body temperature) did not influence the μTBS (2). Finally, the third hypothesis tested was that the μTBS was not influenced by the ceramic surface pre-treatment (3: HF-etching followed by silanization and application of an unfilled adhesive, vs. HF-etching followed by application of a silane-incorporated adhesive).

Materials and methods

Dentin surfaces of 88 human 3rd molars were prepared as described by De Munck et al. and randomly assigned to one of the 15 experimental groups. For the luting procedure, three different ‘self-etch’ composite cements were used: Nexus 3 combined with Optibond XTR (XTR/NX3; Kerr, Orange, USA), RelyX Ultimate with Scotchbond Universal (SBU/RXU; 3M ESPE) and Panavia F2.0 with ED Primer II (ED/PAF; Kuraray Noritake, Tokyo, Japan) ( Table 1 ).

Table 1
List of all materials used and their application instructions (following the respective manufacturer).
Adhesive/composite cement Manufacturer Manufacturer’s instructions Lot number
Optibond XTR (XTR) Kerr, Orange, USA PRIMER: Application to enamel and dentin for 20 s using a scrubbing motion; air-thin for 5 s.
ADHESIVE: Shake; apply for 15 s using light brushing motion; air-thin for 5 s before light curing; light-cure for 10 s or air-thin for 15 s (maximum air-pressure) when left to auto-cure.
PRIMER: 3611504
ADHESIVE: 3594445
Nexus third generation (NX3) Kerr Apply auto-mixed cement and cure following the respective curing mode: auto-cure for minimum 2–3 min; light-cure for 20 s. Shade: Yellow 3653758, 3551229
ED Primer II (ED) Kuraray Noritake, Tokyo, Japan Mix one drop of liquid A and B (use within 5 min); apply and leave for 30 s; gently air-dry. Liquid A: 00299A
Liquid B: 00173A
Panavia F2.0 (PAF) Kuraray Noritake Remove cement from the fridge 15 min before use; use paste within 3 min after mixing; use equal amounts of paste (half turn); mix for 20 s; apply on ceramic; apply Oxyguard II onto the margins and auto-cure for minimum 3 min; light-cure for 20 s. Paste A: 00488A
Paste B: 00248A
Oxyguard II Kuraray Noritake Auto-cure Panavia F2.0 by applying Oxyguard onto the margins and wait for 3 min. 00643A
Scotchbond Universal (SBU) 3M ESPE, Seefeld, Germany SELF-ETCH MODE: Do not overdry dentin (dry with cotton pellet); rub for 20 s; blow with gentle air for 5 s; light-cure for 10 s. 451145
RelyX Ultimate (RXU) 3M ESPE Apply auto-mixed cement on pre-treated ceramic; auto-cure for minimum 3 min; light-cure for 20 s or wait for 6 min (auto-cure). TR447059

Ceramic surface pre-treatment ( Table 2 )

Feldspathic ceramic blocks (10.3 mm × 9 mm with a thickness of 3 mm; Vitablocs Mark II for CEREC/inLab, Vita, Bad Säckingen, Germany) were pre-treated with hydrofluoric acid (‘HF’: IPS Ceramic etching gel, Ivoclar-Vivadent, Schaan, Liechtenstein) for 60 s. Next, two different ceramic pre-treatment protocols were applied. For the XTR/NX3 and ED/PAF, a silane primer (‘S’: Monobond Plus, Ivoclar-Vivadent) was applied and left untouched for 60 s, followed by the application of an unfilled bonding agent (‘HB’: Heliobond, Ivoclar-Vivadent) that was not light-cured. This ceramic surface pre-treatment is further being referred to as ‘HF/S/HB’. For SBU/RXU, the ceramic surface of each block was divided into two parts (5.15 mm × 9 mm) by cutting a shallow, approximately 1 mm deep groove using a diamond saw. A razorblade was inserted into the groove in order to separate the two surfaces from each other and to allow different surface pre-treatments to be applied to each part. Then, one side of the surface was treated with the silane primer and adhesive resin following the protocol described above and being referred to as ‘HF/S/HB’, while the other side received the silane-incorporated adhesive Scotchbond Universal (3M ESPE) only, thus without the application of a separate silane primer. The latter ceramic surface pre-treatment is further being referred to as ‘HF/SBU’. In order to directly compare the two different pre-treatment methods (‘HF/S/HB’ vs. ‘HF/SBU’) per tooth, each ceramic block of the SBU/RXU group received two treatments (separated by the groove and temporarily by the razorblade) and was then luted to one tooth.

Table 2
Ceramic and materials for ceramic surface pre-treatment.
Material Manufacturer Manufacturer’s instructions LOT
Vitablocs Mark II for CEREC VITA, Bad Säckingen, Germany Clean the inner surfaces with alcohol; etch for 60 s with hydrofluoric acid (5%); rinse for 60 s with water spray; apply silane primer and adhesive (see below). 1M2C I10: 12601
IPS Ceramic Etching Gel Ivoclar Vivadent Etch the restoration’s inner surfaces for 60 s; rinse for 60 s. P25429
Monobond Plus Ivoclar Vivadent, Schaan, Liechtenstein Apply a thin coat of the material with a micro-brush; allow to react for 60 s; remove excess by strong air-blowing. P20536
Heliobond (HB) Ivoclar Vivadent Apply a thin layer onto the ceramic surface; air-thin. N75960
Scotchbond Universal (SBU) 3M ESPE Apply the adhesive to the entire surface of the restoration to be cemented and allow it to react for 20 s, air-thin for 5 s. 451145

Curing modes ( Table 3 )

The ceramic blocks were luted onto the dentin surfaces following four different curing modes. Following curing mode ‘AA’, the adhesive applied on dentin was not light-cured after application, but air-thinned according to the manufacturer’s instructions. The ceramic blocks were luted using the composite cements under a constant seating force of 1 kg for 1 min for groups which included light-curing of the composite cement, or for 7 min for auto-curing at room temeprature (21 °C) in full darkness. Afterwards, the teeth were stored for 7 days in water (37 °C) in an incubator, until being further processed and subjected to micro-tensile bond strength testing (μTBS). Following curing mode ‘AA*’, the more severe ‘AA’ protocol was slightly changed in order to resemble better the clinical situation. The teeth were warmed up to 37 °C before cementation and the specimens were left in darkness to auto-cure for 10 min at 37 °C. Then, the teeth were stored in pre-warmed water (37 °C), and further processed as described for ‘AA’. Following curing mode ‘LA’, the respective adhesive was light-cured for 20 s using a polywave LED light-curing unit (Bluephase, Ivoclar-Vivadent) with a light intensity above 1000 mW/cm 2 (‘high power’ mode); the latter was checked regularly throughout the procedure. The cement was allowed to auto-cure in full darkness for 7 min at 21 °C. For ED/PAF, this curing mode was not applicable and therefore omitted. Following curing mode ‘LL’, the respective adhesive was light-cured for 20 s, as well as the cement was light-cured from each side and from the top surface for 20 s (total curing time of 100 s). For ED/PAF, also this protocol was not applicable as the primer cannot be light-cured; it was dried only. The cement was nevertheless light-cured (this adapted curing mode is being referred to ‘AL’). For ED/PAF, Oxyguard II (Kuraray Noritake) was applied onto the margin between the ceramic specimen and the dentin surface for both auto-cure modes AA and AA*.

Table 3
Overview of the different curing modes employed.
Curing mode Adhesive Composite cement
AA No cure Auto-cure for 7 min at 21 °C (room temperature)
AA* No cure Auto-cure for 10 min at 37 °C, tooth pre-warmed (37 °C)
LA Light-cure Auto-cure for 7 min at 21 °C (room temperature)
LL Light-cure Light-cure
AL a No cure Light-cure

a The curing mode ‘AL’ was only applied for ED/PAF.

μTBS testing

After storage in water for 7 days (37 °C), the specimens were sectioned in x and y direction using a semi-automatic high-precision diamond saw (Accutom 50, Struers, Ballerup, Denmark) in order to obtain μTBS-specimens with a cross-sectional area of approximately 1 mm 2 . Only sticks from the central part of the dentin surface were used to avoid regional variability. The μTBS was measured with a crosshead speed of 1.0 mm/min, as described by De Munck et al. .

Statistical analysis

To statistically assess the dentin-bond strength data, one-way ANOVA with the different groups as factors and Tukey multiple comparisons were first performed. Additional two-way ANOVA and Tukey multiple comparisons were used to evaluate the effect of the curing mode ‘AA*’ versus that of ‘AA’, and the effect of the new ceramic pre-treatment involving the application of the silane-incorporated adhesive (‘HF/SBU’) versus the conventional ceramic surface pre-treatment ‘HF/S/HB’ (R 2.13.1, R Foundation for Statistical Computing, Vienna, Austria). All tests were performed at a significance level of α = 0.05. Sticks that fractured during sectioning (pre-testing failures, ptf’s) were included as zero MPa into the statistical analyses.

Failure analysis and SEM

After testing, the failure mode was determined with a stereomicroscope (Wild M5A, Heerbrugg, Switzerland) at 25-/50-fold magnification and recorded as ‘cohesive in dentin’, ‘adhesive at the interface dentin–cement’, ‘cohesive in cement’, ‘adhesive at the interface cement–ceramic, ‘cohesive in ceramic’, or ‘mixed’. Representative specimens with characteristic failure modes were also examined by means of scanning electron microscopy (JEOL JSM-6610/6610LV, Tokyo, Japan) using common standard processing techniques including fixation, dehydration, chemical drying, and gold-sputter coating.

Materials and methods

Dentin surfaces of 88 human 3rd molars were prepared as described by De Munck et al. and randomly assigned to one of the 15 experimental groups. For the luting procedure, three different ‘self-etch’ composite cements were used: Nexus 3 combined with Optibond XTR (XTR/NX3; Kerr, Orange, USA), RelyX Ultimate with Scotchbond Universal (SBU/RXU; 3M ESPE) and Panavia F2.0 with ED Primer II (ED/PAF; Kuraray Noritake, Tokyo, Japan) ( Table 1 ).

Table 1
List of all materials used and their application instructions (following the respective manufacturer).
Adhesive/composite cement Manufacturer Manufacturer’s instructions Lot number
Optibond XTR (XTR) Kerr, Orange, USA PRIMER: Application to enamel and dentin for 20 s using a scrubbing motion; air-thin for 5 s.
ADHESIVE: Shake; apply for 15 s using light brushing motion; air-thin for 5 s before light curing; light-cure for 10 s or air-thin for 15 s (maximum air-pressure) when left to auto-cure.
PRIMER: 3611504
ADHESIVE: 3594445
Nexus third generation (NX3) Kerr Apply auto-mixed cement and cure following the respective curing mode: auto-cure for minimum 2–3 min; light-cure for 20 s. Shade: Yellow 3653758, 3551229
ED Primer II (ED) Kuraray Noritake, Tokyo, Japan Mix one drop of liquid A and B (use within 5 min); apply and leave for 30 s; gently air-dry. Liquid A: 00299A
Liquid B: 00173A
Panavia F2.0 (PAF) Kuraray Noritake Remove cement from the fridge 15 min before use; use paste within 3 min after mixing; use equal amounts of paste (half turn); mix for 20 s; apply on ceramic; apply Oxyguard II onto the margins and auto-cure for minimum 3 min; light-cure for 20 s. Paste A: 00488A
Paste B: 00248A
Oxyguard II Kuraray Noritake Auto-cure Panavia F2.0 by applying Oxyguard onto the margins and wait for 3 min. 00643A
Scotchbond Universal (SBU) 3M ESPE, Seefeld, Germany SELF-ETCH MODE: Do not overdry dentin (dry with cotton pellet); rub for 20 s; blow with gentle air for 5 s; light-cure for 10 s. 451145
RelyX Ultimate (RXU) 3M ESPE Apply auto-mixed cement on pre-treated ceramic; auto-cure for minimum 3 min; light-cure for 20 s or wait for 6 min (auto-cure). TR447059

Ceramic surface pre-treatment ( Table 2 )

Feldspathic ceramic blocks (10.3 mm × 9 mm with a thickness of 3 mm; Vitablocs Mark II for CEREC/inLab, Vita, Bad Säckingen, Germany) were pre-treated with hydrofluoric acid (‘HF’: IPS Ceramic etching gel, Ivoclar-Vivadent, Schaan, Liechtenstein) for 60 s. Next, two different ceramic pre-treatment protocols were applied. For the XTR/NX3 and ED/PAF, a silane primer (‘S’: Monobond Plus, Ivoclar-Vivadent) was applied and left untouched for 60 s, followed by the application of an unfilled bonding agent (‘HB’: Heliobond, Ivoclar-Vivadent) that was not light-cured. This ceramic surface pre-treatment is further being referred to as ‘HF/S/HB’. For SBU/RXU, the ceramic surface of each block was divided into two parts (5.15 mm × 9 mm) by cutting a shallow, approximately 1 mm deep groove using a diamond saw. A razorblade was inserted into the groove in order to separate the two surfaces from each other and to allow different surface pre-treatments to be applied to each part. Then, one side of the surface was treated with the silane primer and adhesive resin following the protocol described above and being referred to as ‘HF/S/HB’, while the other side received the silane-incorporated adhesive Scotchbond Universal (3M ESPE) only, thus without the application of a separate silane primer. The latter ceramic surface pre-treatment is further being referred to as ‘HF/SBU’. In order to directly compare the two different pre-treatment methods (‘HF/S/HB’ vs. ‘HF/SBU’) per tooth, each ceramic block of the SBU/RXU group received two treatments (separated by the groove and temporarily by the razorblade) and was then luted to one tooth.

Table 2
Ceramic and materials for ceramic surface pre-treatment.
Material Manufacturer Manufacturer’s instructions LOT
Vitablocs Mark II for CEREC VITA, Bad Säckingen, Germany Clean the inner surfaces with alcohol; etch for 60 s with hydrofluoric acid (5%); rinse for 60 s with water spray; apply silane primer and adhesive (see below). 1M2C I10: 12601
IPS Ceramic Etching Gel Ivoclar Vivadent Etch the restoration’s inner surfaces for 60 s; rinse for 60 s. P25429
Monobond Plus Ivoclar Vivadent, Schaan, Liechtenstein Apply a thin coat of the material with a micro-brush; allow to react for 60 s; remove excess by strong air-blowing. P20536
Heliobond (HB) Ivoclar Vivadent Apply a thin layer onto the ceramic surface; air-thin. N75960
Scotchbond Universal (SBU) 3M ESPE Apply the adhesive to the entire surface of the restoration to be cemented and allow it to react for 20 s, air-thin for 5 s. 451145
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Composite cements benefit from light-curing
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