Purpose: This study evaluated the shear bond strength (SBS) of resin cement to dentin after applying two adhesive (A) systems with a combination of four different immediate dentin sealing (IDS) strategies, and two surface conditioning (SC) methods.
Material and methods: Human third molars (n = 140) were collected and randomly split (n = 70 each) between the two A systems (Clearfil SE Bond; Kuraray [AC] and Optibond FL; Kerr [AO]). The A groups were further divided into four IDS strategies (2 x one adhesive layer (IDS-1L); 2 x two adhesive layers (IDS-2L); 2 x one adhesive layer and one flowable layer (IDS-F); 2 x no adhesive layer (delayed dentin sealing [DDS]). Finally, each strategy group was categorized into one of the two SC methods (only pumice [SC-P] or pumice and silica coating [SC-PS]), except the DDS group, where only SC-P was used. This resulted in 14 groups of 10 specimens each. The occlusal coronal third was removed from each molar crown with a diamond saw (Isomet 1000), and IDS was applied, followed by temporary restorations. These were removed after 2 weeks of water storage, and the IDS surfaces were subsequently conditioned. The standard adhesive procedure (Syntac Primer and Adhesive, Heliobond; Ivoclar Vivadent) was executed, followed by the application of a resin cement (Variolink II; Ivoclar Vivadent) and photopolymerization. All specimens were subjected to thermocyclic aging (10,000 cycles, 5°C to 55°C). Shear force was applied to the adhesive interface in a universal testing machine (1 mm/min). Fracture types and locations after loading were classified. The data were analyzed using analysis of variance (ANOVA) and independent samples t tests.
Results: AO groups exhibited higher mean SBS values (14.4 ± 6.43) than AC groups (12.85 ± 4.97) (P = 0.03). ANOVA showed the main effect of the applications on the SBS in the different groups (P = 0.00). Both DDS groups showed significantly lower SBS values compared with all the IDS groups (IDS-1L, IDS-2L, IDS-F). No significant differences in SBS results were found between the IDS groups (P = 0.43) and between the SC methods (P = 0.76). Dentin–cement interface failures diminished with the application of IDS.
Conclusion: IDS improves the SBS compared with DDS. No significant differences were found between the tested conditioning methods. (Int J Esthet Dent 2019;(14); 3; 52–63)
Authors: Carline RG van den Breemer, Mutlu Özcan, Margot RE Pols, Anique R Postema, Marco S Cune and Marco MM Gresnigt
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The use of glass-ceramics in combination with micromechanical and chemical adhesion to dentin facilitates minimally invasive preparation procedures. Good adhesion to dentin and enamel is especially important when bonding partial ceramic restorations. A component of the overall strength of the tooth–restoration complex relies on the quantity and quality of the remaining enamel,1 and the quality of the adhesive procedure.2
Pashley et al3 postulated that sealing the dentin with a dentin bonding agent immediately after preparation reduces the permeability of the dentin, both in the short and long term. This technique has evolved into what is now known as immediate dentin sealing (IDS).3,4 It improves bond strength as well as the marginal and internal adaptation of the restoration and reduces postoperative sensitivity.2,4,5-11 In vitro studies obtained higher bond strength to dentin using IDS (16.34 to 19.04 MPa) compared with delayed dentin sealing (DDS) (0.26 to 14.90 MPa).12 With the IDS method, maturation of the adhesive interface is possible between the two patient visits (visit 1: tooth preparation/impression; visit 2: restoration delivery). Therefore, the tensile stress on the hybrid layer is postponed for several weeks.2,5,10 This is different from the DDS method, where the hybrid layer is applied in the second patient visit, and is then immediately loaded on the adhesive surface, possibly resulting in shrinkage that negatively influences the tensile stress. Polymerization of the dentin bonding agent prior to cementation ensures a hybrid layer that is not influenced by stress exerted during cementation.5,7,9,10 The hybrid layer discourages contamination and denaturation of the dentin until the indirect restoration is seated.7
The three-step etch-and-rinse system is seen as the gold standard among adhesive systems, but there is a quest for simpler and less time-consuming techniques.13 The etching step is omitted with self-etch adhesives, which is considered more user-friendly and less technique-sensitive, and also has a good clinical track record.13-15 The quality of the bond and the bond strength to dentin can be increased by applying more than one adhesive layer.1-19 The application of a flowable layer on the adhesive layer also improves adhesive strength (20.8 MPa and 27.2 MPa, compared with 10.5 MPa and 17.7 MPa without flowable composite).20
Different surface conditioning methods can be used to reactivate the IDS layer prior to bonding the indirect restoration, which can influence the IDS bond strength.21-23 Polishing and airborne particle abrasion with silica-coated aluminum oxide or glycine proved to be equally efficient.24 Airborne particle abrasion with both aluminum oxide and fluoride-free pumice paste systems6,8,25,26 also yielded good results with respect to bond strength. However, it is unknown which method is most suitable for conditioning the sealed dentin surface.
The objective of this study, therefore, was to compare the effect of different adhesive systems, different IDS application methods, and different surface conditioning methods on the shear bond strength (SBS) to dentin. Three hypotheses were tested: 1) There is no significant difference in effect between the different adhesive systems on SBS; 2) There is no significant difference in the outcome of the IDS strategies regarding SBS; and 3) SBS is not significantly affected by different surface conditioning methods.
Materials and Methods
Three independent variables were tested in this study: Adhesive (A) systems, IDS strategies, and surface conditioning (SC) methods. A total of 140 sound human molars were randomly divided into 14 groups of 10 teeth each. These were subjected to the following experimental protocols:
- Two adhesive (A) systems (AC: Clearfil SEBond [Kuraray], and AO: Optibond FL [Kerr]).
- Four different IDS strategies (one adhesive layer [IDS-1L]; two adhesive layers [IDS-2L]; one adhesive layer and one flowable layer [IDS-F]; no adhesive layer [DDS]).
- Two different SC methods (only pumice [SC-P]; pumice and silica coating [SC-PS]).
Only SC-P was used in the DDS group because the IDS did not have to be activated; only the temporary cement had to be removed (leading to 14 groups instead of 16).
A flowchart showing the experimental group distribution is presented in Figure 1.
Recently extracted, sound human molars (n = 140) were collected, stored in water, and used a maximum of 1 month postextraction. Each specimen was embedded in polymethylmethacrylate (PMMA) in a polyvinylchloride (PVC) ring to facilitate handling and for the seating in the universal testing machine. The occlusal coronal third of the crown was removed with a diamond saw (Isomet 1000; Buehler), thereby exposing a flat dentin surface (Fig 2). The dentin surfaces were polished using Sof-Lex discs (course and medium) (3M ESPE), and verified for the absence of enamel and/or pulp tissue exposition using a stereomicroscope (magnification x35; Wild M5A).
The brands, types, main chemical compositions, manufacturers, and batch numbers of products used in this study are shown in Table 1.
Table 1 Brands, types, manufacturers, main chemical composition, and batch numbers of products used in this study
In the AC + IDS-1L groups, primer (Clearfil SE Bond) was applied onto the dentin for 20 s and air dried. A thin layer of adhesive (Clearfil SE Bond) was applied by a light brushing motion, gently air dried, and photopolymerized (Bluephase 20i; Ivoclar Vivadent) for 10 s (1000 mW/cm2) (Fig 3). The AC + IDS-2L groups received the same initial procedure as the AC + IDS-1L groups, except that an additional layer of adhesive was applied, which was photopolymerized separately. The AC + IDS-F groups also received the same initial procedure as the AC + IDS-1L groups, but then a flowable composite (Grandio Flow; Voco) was administered after adhesive application, followed by photopolymerization. To prevent the formation of an oxygen inhibition layer, glycerin gel (Liquid Strip; Ivoclar Vivadent) was applied after the last photopolymerized layer, and this was finally photopolymerized for another 40 s in all groups. The dentin was not sealed in the AC + DDS group.
Regarding the AO + IDS-1L groups, the dentin was etched for 15 s with 37% phosphoric acid (Total Etch; Ivoclar Vivadent), then rinsed thoroughly with water and air for 15 s. The surface was air dried, but not desiccated, for 3 s, and primer (Optibond FL Primer) was applied with a light brushing motion for 15 s, withdrawn for 10 s, and suction dried for 15 s. A thin layer of adhesive (Optibond FL Adhesive) was applied onto the surface using a light brushing motion for 15 s, and photopolymerized for 10 s (1000 mW/cm2). The AO + IDS-2L groups were subjected to the same procedure as the AO + IDS-1L groups, except that a second layer of adhesive was applied, which was photopolymerized separately. The AO + IDS-F groups also underwent the same initial procedure as the AO + IDS-1L groups, but then a flowable composite (Grandio Flow) was applied after adhesive application, followed by photopolymerization. To prevent the formation of an oxygen inhibition layer, glycerin gel (Liquid Strip) was applied after the last photopolymerized layer, and this was finally photopolymerized for another 40 s in all groups. The dentin was not sealed in the AO + DDS group.
After the IDS application, a temporary restoration (Protemp 4; 3M ESPE) was luted onto the flat dentin surface using a temporary zinc-oxide luting cement (Temp-Bond NE; Kerr) (Fig 4). The specimens were stored in water at room temperature for 2 weeks.
Surface conditioning and build up
The temporary restorations were removed after 2 weeks.
The teeth in the SC-P groups were cleaned using pumice (Pumice Sand; Denteck). The pumice was manually constituted with two small scoops of pumice into a dappen glass with a small amount of water. Any redundant water was removed from the dappen glass with a cotton roll. The pumice was then applied with a rotary brush for 10 s. The teeth in the SC-PS groups were conditioned using pumice and silica coating (distance 10 mm, angle 45 degrees, 2 bar, CoJet Sand, SiO2; 3M ESPE). In the DDS groups, SC-P was used to remove the temporary cement.
All specimens were rinsed thoroughly with water and air dried for 15 s. In the SC-PS group, silane (silane coupling agent, ESPE Sil; 3M ESPE) was applied (according to the Özcan et al36 method) onto the IDS surfaces and left to dry for 5 min.
Primer (Syntac Primer; Ivoclar Vivadent) was then brushed lightly onto all specimens for 15 s and slightly air dried. A thin layer of adhesive (Syntac Adhesive; Ivoclar Vivadent) was applied onto the surface with light brushing motions for 10 s and slightly air dried. Another layer of adhesive (Heliobond; Ivoclar Vivadent) was brushed onto the dentin and not light cured. Two plastic tubes (diameter 3 mm, height 5 mm) filled with composite cement (Variolink II; Ivoclar Vivadent) were placed onto the dentin, and glycerin gel (Liquid Strip) was applied around the tubes to prevent the formation of an oxygen inhibited layer, after which the composite was photopolymerized from all angles for 40 s (1000mW/cm2) (Fig 5).
All the specimens were artificially aged by thermocycling (Willitec): x10,000 cycles between 5°C to 55°C with a dwell time of 30 s. The specimens were subsequently mounted in a universal testing machine (1 mm/min). The maximum shear force to produce a fracture was recorded (MPa). Specimens that failed before actual testing (pretesting failure) were counted and explicitly noted, which meant they were taken into account when calculating the mean SBS.
Failure sites were initially observed using an optical microscope (OPMI pico; Zeiss) at x24 magnification and classified as follows: D (fracture in dentin), DC (fracture interface dentin and cement), DI (fracture interface dentin and IDS), IC (fracture interface IDS and cement), and C (fracture in the cement). Additionally, representative specimens from each group were sputter-coated with a 3-nm layer of gold (80%)/palladium (20%) (90 s, 45 mA; Balzers SCD 030; Balzars) and analyzed using cold field emission scanning electron microscope (SEM) (LEO 440; Electron Microscopy Ltd).
Data were analyzed using the SPSS 22 (PASW statistics 18.0.3; Quarry Bay) statistical software package. As the data were normally distributed, parametrical tests were applied to find possible differences between the groups in terms of A (AC; AO) systems (independent-samples t test), IDS strategies (IDS-1L; IDS-2L; IDS-F; DDS) (ANOVA, Student-Newman-Keuls), and SC methods (SC-P; SC-PS) (independent-samples t test) on SBS results.
Disregarding the subgroups, the AC specimens (M = 12.85, SD = 4.97) exhibited lower mean SBS values than the AO specimens (M = 14.4, SD = 6.43, independent-samples t test, t (256) = 2.23, P = 0.03). Analysis of variance (ANOVA) showed the main effect of the different applications on the SBS F(13, 261) = 14.02, P = 0.00). Group AC + DDS + SC-P resulted in the lowest SBS, followed by group AO + DDS + SC-P (Student-Newman-Keuls tests). The DDS groups exhibited significantly lower mean SBS values compared with the IDS groups (IDS-1L, IDS-2L, IDS-F). The difference in SBS values among the IDS groups were not statistically significant (Student-Newman-Keuls tests, P = 0.43) (Fig 6; Table 2). No significant differences were observed between the SC-P (M = 15.15, SD = 4.99) and SC-PC specimens (M = 14.97, SD = 4.43, independent-samples t test, t (233) = 0.30, P = 0.76).
Table 2: Mean shear bond strength (MPa) with standard deviation (SD) for different groups
IFig 7: SEM image of adhesive fracture surface between dentin and the IDS layer interface. (D: dentin; I: immediate dentin sealing).
Dentin–cement interface fractures were seen less frequently with the application of IDS (Table 3). Mainly cohesive failures occurred with AC (Fig 7), but there were hardly any failures in the cement. Regarding AO, the failures were mostly of an adhesive nature in the dentin–IDS interface. All the pretesting failures were in the DDS group.
The survival rate of glass-ceramic posterior restorations relies heavily on the strength of the adhesive interface. The weakest link of the interface is the connection of the adhesive to dentin.1,27,28 The application of an IDS layer onto freshly exposed dentin increases the bond strength to dentin,29,30 especially when large dentin surfaces are exposed.31
Table 3 Summary of failures (%)
Based on the results of the present study, the hypotheses suggesting that there is no significant difference in effect between the IDS application methods on SBS, and that there is no significant difference in the outcome of the different bonding systems regarding SBS, can both be rejected. The hypothesis that SBS is not statistically significantly affected by different surface conditioning methods can be accepted.
In general, it is very difficult to perform a ‘true’ SBS test; therefore, SBS is not very reliable.32 Yet SBS is often used to describe differences between groups, and caution can be taken in the methodology to increase reliability. To avoid adhesive area modification during resin cement pouring in this study, tubes filled with resin cement were attached to the dentin and then photopolymerized. This was thought to overcome resin cement pouring.
The application of IDS in any form improved the SBS of composite cement to dentin. This result was also found in other studies.2,4,5-9,10,11 Higher bond strength can be explained due to a better adhesion to freshly cut dentin2-4 compared with dentin that is contaminated by temporary cement.33 Polymerization of the IDS layer before impression taking prevents the hybrid layer from degradation2,4,5 and allows it to mature without any tensile forces.2 Other studies have demonstrated that the use of multiple adhesive layers17,18 or the use of an extra layer of flowable composite16,20 results in higher bond strengths. This contrasts with the results of the present study, which is perhaps due to the fact that filled adhesives were used in our study because unfilled adhesives need more layers to completely cover the dentin.11,18 However, the bond strength results are better when specimens are not aged.17 Most previous studies have refrained from thermocycling or have performed thermocycling for only a minimum number of cycles.2,5,6,9,10 There is a difference between the bond strength in the short and long term. The adhesive strength in the long term is significantly lower because degradation occurs within the adhesive interface.34 Micromechanical retention is reduced by 30% to 40% in 6 to 12 months.35 Since the results of the present study prove that the application of an IDS layer (in any form) results in better bond strength than with the use of DDS, our clinical recommendation is to use an extra adhesive layer or flowable composite to create a thick adhesive layer. In clinical practice, a thin IDS layer is more vulnerable when using silica coating, and the dentin may become re-exposed. This, in turn, will be detrimental to the bond strength. A thick IDS layer provides a smooth preparation in little chair time. It is also easier to eliminate undercuts.
The present study could not prove that one conditioning method was superior to another. Looking at the clinical application, the use of silica coating is recommended over the use of pumice. Cement residues are easier to remove using silica coating compared with pumice because, with sandblasting, it is easier to reach difficult parts of the preparation than it is with the use of a rotary brush in the application of pumice. Therefore, we recommend creating a thick IDS layer that is conditioned with silica coating because the clinical application is easier, not because of a higher bond strength. In the literature, silica coating in combination with silanization is often described as a better alternative to sandblasting only. Silica coating enlarges the adhesive surface area by depositing silica particles onto the composite surface, which enables better mechanical retention.36 This is in contrast to sandblasting with alumina, where loss of filling particles may occur and thereby reduce the interaction with silane. This, in turn, reduces the composite-to-composite bond strength.37
Optibond FL resulted in a significantly higher bond strength compared with Clearfil SE Bond; however, the standard deviation (SD) of Optibond FL is much higher. Clinically, this means that the consistency of Clearfil SE Bond is slightly better. Although less time-consuming techniques are popular,13 the three-step etch-and-rinse system is seen as the gold standard in the literature38-40 and in fact attained the highest bond strengths in the present study. Optibond FL is a filled adhesive resin with a uniform film thickness of around 88 µm.11
Fewer dentin–cement interface failures were seen with the application of IDS, but more failures were seen with the application of IDS in the dentin, the dentin–IDS interface, and the cement–IDS interface. The presence of cohesive failures in the dentin could indicate that the actual bond strength to dentin surpasses the maximal dentin strength and does not provide actual strength at the interface. Cohesive failures were not excluded from the failure analysis, and this may have influenced the results of our study. Failures in the substrate are seen more often in SBS studies because this test creates a non-homogenous stress distribution on the surface. This may lead to non-valid (worse) results.41,42 In some of the control groups, the tubes detached spontaneously during thermal cycling. This pretest failure could have been caused by insufficient dentin adhesion or technical malfunction. No pretest failures were described by studies on adhesion of resin cement to an IDS layer.2,6-10
The following can be concluded from this study:
- Applying Optibond FL yields the highest SBS; however, Clearfil SE Bond showed a smaller SD.
- IDS improves SBS, compared with the DDS strategy.
- No significant differences were found on conditioning the IDS layer with pumice or silica coating.
When bonding a glass-ceramic partial indirect restoration, using an IDS layer improves the bond strength to exposed dentin. From the several methods tested to reactivate the IDS layer, no single procedure obtained superior SBS values.
This study was fully supported by the authors’ institution.
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