|Year : 2016 | Volume
| Issue : 3 | Page : 86-92
Bonding effectiveness of contemporary composite cements to dentin after 6-month water storage
Mouhamed Sarr, Babacar Faye, Fatou Leye-Benoist, Khaly Bane, Adjaratou Wakha Aidara, Babacar Touré
Department of Conservative Dentistry and Endodontics, Cheikh Anta Diop University, Dakar-Liberté, Sénégal
|Date of Web Publication||10-Aug-2016|
Department of Conservative Dentistry and Endodontics, Cheikh Anta Diop University, BP 17256, Dakar-Liberté
Source of Support: None, Conflict of Interest: None
Purpose: To evaluate the bonding effectiveness to dentin of eight dual-cure composite cements after 6-month water storage. Materials and Methods: This study is a follow-up of a recent study that investigated the 1-week bonding effectiveness of feldspathic ceramic blocks luted to dentin using the same composite cements and experimental protocol. The microtensile bond strength (μTBS) of different composite cements, including two etch-and-rinse cements (Calibra, Dentsply; Variolink II, Ivoclar-Vivadent), two self-etch cements (Panavia F2.0, Kuraray; Clearfil Esthetic Cement, Kuraray), and four self-adhesive cements (Unicem, 3M ESPE; Maxcem, Kerr; Monocem, Shofu; G-Cem, GC), was measured using a standardized μTBS protocol after 6-month water storage. As control, a two-step self-etch adhesive (Clearfil SE, Kuraray) combined with a microhybrid restorative composite (Clearfil AP-X, Kuraray) was used. Twenty-seven human third molars were used with specific preparation, and after 6-month water storage, microspecimens were prepared and subjected to a μTBS test. Results: The mean μTBS varied from 0 to 26.1 MPa, the latter being measured for the control adhesive composite combination. All specimens prepared using the self-adhesive composite cements Maxcem and Monocem failed during specimen processing. Most specimens failed at the dentin-cement interface, except the self-etch composite cement Panavia F2.0 that failed in 53% of the cases at the cement-ceramic interface and the control of which all specimens failed in the resin part of the microspecimens. Conclusion: The largely varying bonding effectiveness recorded for the different composite cements highlights the need for material specifications. Such specifications should also include a bond durability test as the specimens in the present study that were subjected to 6-month water storage. To lute ceramic restorations that allow light transmission to a sufficient degree, a conventional light-curable adhesive and composite should be considered.
Keywords: Etch-and-rinse cement, luting cement, self-adhesive cement, self-etch cement
|How to cite this article:|
Sarr M, Faye B, Leye-Benoist F, Bane K, Aidara AW, Touré B. Bonding effectiveness of contemporary composite cements to dentin after 6-month water storage. J Res Dent 2016;4:86-92
|How to cite this URL:|
Sarr M, Faye B, Leye-Benoist F, Bane K, Aidara AW, Touré B. Bonding effectiveness of contemporary composite cements to dentin after 6-month water storage. J Res Dent [serial online] 2016 [cited 2017 Aug 17];4:86-92. Available from: http://www.jresdent.org/text.asp?2016/4/3/86/188231
| Introduction|| |
Recent developments in adhesive dentistry enable teeth to be preserved longer when they are restored minimal-invasively.  Following such an approach, ceramics can be used since they can be bonded to the remaining sound, especially for large restorations where conventional composites are not indicated. The clinical success of bonded ceramic restorations depends primarily on a good bond to tooth/dentin and an optimal marginal fit after cementation. Further, adhesive resin type and the resin cement affected the mean bond strength. 
Today's composite cements can be classified into three groups according to the adhesive approach employed to bond the cement to the tooth structure. Etch-and-rinse composite cements utilize an etch-and-rinse adhesive, including a phosphoric acid etching step, followed by a rinse phase. "Self-etch" composite cements utilize a self-etch adhesive that does not need to be rinsed off while finally, self-adhesive cements as per manufacturer's instructions do not require any pretreatment at all, simplifying the clinical procedure considerably.  At the restoration side, glass-containing ceramics are best etched with hydrofluoric acid, silanized, and coated with an unfilled resin  while oxide-ceramics require specific primers though currently there is no consensus which luting protocol is most effective. Many of the multi-step composite cements have been documented to perform satisfactory.  Nonetheless, little information is available in the literature about their long-term bonding effectiveness, this to even a much larger extent for self-adhesive composite cements.
Therefore, the aim of this study was to evaluate the bond durability of different composite cements that were used to lute feldspathic ceramic blocks onto dentin. The hypothesis tested was that after 6-month water storage, no difference in microtensile bond strength (μTBS) to dentin exists for the different cements tested.
| Materials and Methods|| |
This study follows a previous study  that investigated the short-term or "immediate" bonding effectiveness of composite cements that were employed to lute feldspathic ceramic blocks onto dentin. All materials presently employed were exactly the same (same batch numbers) as in the previous study, with the tests executed by the same operators following the same experimental protocol. Eight composite cements were used that include two etch-and-rinse, two self-etch, and four self-adhesive composite cements [Table 1]. As control, the "mild," two-step self-etch adhesive Clearfil SE (Kuraray, Tokyo, Japan) that can be regarded as the "gold standard" self-etch adhesive was used in combination with the microhybrid restorative composite Clearfil AP-X (Kuraray).
|Table 1: List of composite cements tested with their application procedure |
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Noncarious human molars gathered following informed consent approved by the Commission for Medical Ethics of the K.U. Leuven were stored in 0.5% chloramine/water at 4°C and were used within 3 months after extraction. Flat dentin surfaces were obtained by removing coronal enamel with an IsoMet low-speed diamond saw (IsoMet 1000, Buehler, Lake Bluff, IL, USA). A standardized smear layer was produced first grinding with a SiC-paper (600-grit). Dentin surfaces were verified for the absence of enamel and/or pulp tissue using a stereomicroscope (Wild M5A, Heerbrugg, Switzerland). Subsequently, they were cleaned with pumice paste, rinsed with water, and air-dried (without desiccating dentin), unless mentioned differently, as for the self-adhesive composite cement MonoCem as per manufacturers' instructions.
Ceramic block preparation
The feldspathic ceramic computer-aided design/computer-aided manufacturing (CAD/CAM) blocs (VITA Mark II for Cerec/inlab, Vita, Bad Sδckingen, Germany) were sectioned in 3 smaller blocs of approximately 8 mm × 8 mm and a thickness of 4.15 mm. The ceramic surface was etched with hydrofluoric acid (IPS Ceramic Etching Gel, Ivoclar-Vivadent, Schaan, Liechtenstein) for 60 s after which the ceramic surface was thoroughly rinsed with water and air-dried. Next, silane (Monobond S, Ivoclar-Vivadent) was applied and left undisturbed for 60 s after which the surface was again air-dried. Finally, to improve resin infiltration into the etch-pits in the ceramic block, fluid, unfilled bonding resin, Heliobond (Ivoclar-Vivadent) was applied but not light-cured.
The ceramic blocks were luted to dentin using the different cements, each strictly following the respective manufacturer's recommendations [Table 1]. Each block was luted under a fixed pressure of 1 kg and light-cured for 20 s from each side (5 s × 20 s) (Demetron Optilux 500, Kerr, Orange, USA), with a light output of not less than 550 mW/cm 2 ( 100 s of light emission in total).
Microtensile bond strength testing
After 6 months of water storage at 37°C, the specimens were sectioned with a water-cooled diamond saw (IsoMet 1000) in both x and y directions, perpendicular to the adhesive interface to obtain about 16 sticks with a cross-sectional area of approximately 1 mm 2 . The dimensions of the sticks were measured by means of a digital caliper (Mitutoyo, Kanagawa, Japan). The sticks originating from the outer area, where the ceramic block was bonded to enamel, were excluded from the study. After examination under a light microscope for the absence of enamel, the specimens were fixed to a BIOMAT jig  with cyanoacrylate glue (Model Repair II, Dentsply-Sankin, Ohtawara, Japan). This jig was then fixed in an LRX testing machine (Lloyd, Hampshire, UK) with a load cell of 100 N after which the specimens were stressed in tension at a crosshead speed of 1.0 mm/min. The bond strength was calculated in MPa, derived from dividing the imposed force (in N) at the time of fracture by the bond area (in mm 2 ). Specimens that failed prior to testing (so-called "pretesting failure" [ptf]) were explicitly noted and assigned 0 MPa in further analysis. The mode of failure was determined with a stereomicroscope (Wild M5A, Heerbrugg, Switzerland) at magnification of × 50 and recorded as "cohesive failure in dentin," "failure at the resin-dentin interface," "cohesive failure in the cement," "failure at the cement-ceramic interface," "cohesive failure in the ceramic," or as "mixed failure" for any combination of the previous.
The bond strength values were subjected to a Kruskal-Wallis test at a level of significance of 5%. To assess reduction in bonding effectiveness, the "immediate" and "aged" bond strengths were pair-wise compared to the results of the former study using an unpaired nonparametric Mann-Whitney U-test.
| Results|| |
The μTBS results are shown in [Table 2] and graphically presented in [Figure 1]. After 6-month water storage, significant differences in μTBS were observed among the cements tested (Kruskal-Wallis, P < 0.0001). The multi-step cements revealed a μTBS that was not different among each other, except for Variolink II, for which almost all specimens failed before testing; thus, a very low μTBS was recorded. The control self-etch adhesive/restorative composite combination exhibited the highest bond strength after 6-month water storage, which was not significantly different from all multi-step cements, except Variolink II [Table 2]. For the self-adhesive cements MonoCem and Maxcem, on the other hand, 100% pretesting failures were recorded after 6-month water storage. In addition, almost 40% of ptfs were recorded for G-Cem while Rely X Unicem was the only self-adhesive cement to exhibit almost no ptfs (4%) after 6-month water storage. Failure analysis revealed that after 6-month water storage, most specimens failed at the dentin/cement interface, except for Panavia F2.0, of which 53% of the specimens failed at the cement/ceramic interface. The specimens of the control self-etch adhesive/restorative composite combination failed entirely in the resin part of the μTBS stick.
|Figure 1: Box-Whisker plots of the microtensile bond strength (MPa) after 6-month water storage. The central horizontal line represents the median and the dot the mean value. The whiskers denote minimum and maximum values. As a reference, the mean microtensile bond strength measured after 1-week water storage is indicated by an asterisk|
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|Table 2: Mean microtensile bond strength and standard deviation after 6-month water storage (pretesting failure included as 0 MPa) |
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Comparing the "aged" 6-month μTBS data to the "immediate" 1-week μTBS data, a significant reduction in μTBS was observed for all cements except the self-etch multi-step composite cements. A relatively small μTBS reduction was also observed for the control self-etch adhesive/restorative composite combination, of which the μTBS remained the highest of all groups after 6-month water storage. The highest bond strength reductions were recorded for the self-adhesive composite cements Maxcem and MonoCem, with also a reduction in μTBS of 92% measured for the etch-and-rinse composite cement Variolink II.
| Discussion|| |
The study hypothesis that after 6-month water storage, no difference in μTBS to dentin exists for the different composite cements tested was rejected.
In this study, the bonding effectiveness of eight contemporary luting agents to bond ceramic blocks to dentin was compared. As control, the "gold standard" two-step self-etch adhesive Clearfil SE in combination with the microhybrid restorative composite Clearfil APX was chosen. Since the composition of composite cement resembles very much the composition of a restorative composite, the latter can also be used as luting agent on the condition that it is sufficiently fluid to reach a sufficiently thin film thickness and that light can be sufficiently transmitted through the ceramic restoration to be adequately cured.  In this study, the ceramic blocks were made of feldspathic ceramic (VITA Mark II) which transmits light relatively well, also considering the small dimensions of the blocks. To ensure sufficient polymerization, every specimen was light-cured for in total 100 s. All composite cements tested in this study were dual-curing. Their polymerization can be initiated by activation of light and chemical initiators due to which they remain the materials of choice to lute indirect tooth-colored restorations with a thickness of 4 mm or more. 
As recommended by many other researchers, ,,, the cements in this study were applied under constant pressure (1 kg) before being light-cured for 20 s from each side. Doing so, we ensured that the cement intimately adapted to the dentin surface and simulated the pressure applied clinically.  To ensure a maximal mechanical strength, all composite cements were light-cured in this study, indicating that their actual polymerization was initiated both chemically and by light. The use of dual-curing luting composites was shown to optimize the dentin bond under thick ceramic layers.  Further, the curing mode was decisive for the bonding effectiveness of adhesively luted composite CAD/CAM restorations to dentin. Light-curing of adhesive and cement revealed the highest μTBS which decreased significantly for all other curing modes. 
The ceramic blocks used in this study were made of feldspathic ceramic (VITA Mark II) and were prepared using a well-established technique, , consisting of hydrofluoric acid etching to produce a microretentive surface and subsequent silane treatment to achieve an additional chemical bond to the ceramic block. This treatment protocol ensured a favorable and consistent bonding performance and is essential for bonding self-adhesive resin cement to a feldspathic ceramic. 
The same μTBS testing was conducted for all specimens to avoid bias. 
As a result, bonding effectiveness to the ceramic block appeared very effective, and only for Panavia F2.0, some failures at this ceramic composite interface were observed after 6-month water storage. Little differences were also found between different ceramic groups, where failure type was primarily adhesive between cement and ceramic. 
Therefore, all differences in bonding effectiveness observed in the present study can be largely related to differences in bonding performance of the composite cement. Moreover, as some of the better performing cements showed no bond strength reduction at all, the bond to the ceramic block is considered as very durable.
According to Suyama, when luted to "smear-covered" dentine, all self-adhesive composite cements appeared equally effective and durable as the "etch-and-rinse" and "self-etch" multi-step composite cements. 
Our results showed that bonding effectiveness of all self-adhesive cements decreased upon aging in water [Table 2]. For two of them (Maxcem and MonoCem), the reduction in μTBS was so severe that the bond strength could not be measured anymore after 6-month water storage. The best-performing self-adhesive cement (Rely X Unicem), on the other hand, had only a limited amount of pretesting failures (4%) and did not differ significantly from the most better-performing multi-step cements. Composition-wise, this is the only cement that comes in a powder-liquid formulation. For all other self-adhesive cements, both components are pastes that are mixed by application through an automix tip. This delivery may appear more user-friendly as no mixing step is needed; however, as both types require neither any hand-mixing nor pretreatment of the surface, both can be regarded very easy to use. The powder-liquid formulation may have the advantage of dealing more effectively with different smear layer types, as suggested in a former study.  All these self-adhesive cements appear relatively well-balanced cocktails that have to fulfill different tasks. As a result, small differences in composition may have a significant effect on the bonding performance as was also observed in the current study. In addition, the physical integrity of the luting material itself is in part reflected in the bond strength as this test measures the strength of the whole ceramic-cement-dentin assembly, to which the material strength also contributes. These cements must also age differently as was proven before. 
When luted to "smear-covered" dentine, all self-adhesive composite cements appeared equally effective and durable as the "etch-and-rinse" and "self-etch" multi-step composite cements. 
Both self-etch composite cements tested (Panavia F2.0 and Clearfil Esthetic Cement) employ the same primer. This primer is very similar to the primer employed in the control group and is based on the same functional monomer: 10-methacryloyloxydecyldihydrogenphosphate (10-MDP). This monomer is known for its chemical bonding capacity to hydroxyapatite.  The differences in μTBS observed between both cements, though not significant, must therefore be attributed to the specific composition of the cement itself, differences in mechanical properties, and/or the presence of material defects (voids, cracks,…). Regarding the composition, Clearfil esthetic cement does not contain MDP which renders the cement more hydrophobic. This increased hydrophobicity may have affected surface wetting but may on the other hand has made the cement more resistant against hydrolysis. Panavia F2.0 was mixed manually with a spatula in contrast to the automix syringe provided with Clearfil esthetic cement. However, this did not seem to have affected the bond durability. Nevertheless, bond strength of all MDP-based cementation methodologies appeared relatively stable and performed among the best after 6-month water storage.
The bonding effectiveness of composite cements depends largely on the adhesive approach and the luting agents have influence on bond strength between restorative materials and dentin. 
Both etch-and-rinse composite cements tested employ a two-step etch-and-rinse adhesive. Two-step etch-and-rinse adhesives are however not so reputed when it comes to bond durability.  This may explain the large reduction in bond strength, and the increased amount of interfacial failures observed for these two cements upon 6-month water storage.
In this study, the adhesion to the ceramic part of the interface complex was not a variable. Therefore, we opted for an optimized protocol comprising a micromechanical and chemical pretreatment of the surface by air abrasion and silane treatment.  This protocol proofed to be effective, as no failures at this location were observed, despite the 6 months water storage.
| Conclusion|| |
The following conclusions could be retained:
- The long-term bonding effectiveness of some self-adhesive cements is very questionable
- The restorative composite Clearfil AP-X is suitable when used as cement for luting feldspathic ceramics.
The authors would like to thank the manufacturers for supplying materials for this study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]