Abstract
Objective
To assess by means of “thin-slice” push-out test, light and scanning electron microscopy, the interfacial strength and sealing ability of new self-adhesive resin cements when used to lute fiber posts into endodontically treated teeth.
Methods
RelyX Unicem 2 Automix (3M ESPE, RUA), Maxcem Elite (Kerr, ME) and seT (SDI, S) were utilized in combination with RelyX Fiber Posts (3M ESPE). In each group 5 posted roots were used for push-out testing and 5 were processed for observations of interfacial morphology and nanoleakage under light microscope and scanning electron microscope. Five to six slices were obtained from each posted root. The statistical significance of the influence on post push-out strength of luting agent, dowel space level, and between-factor interaction was assessed using Two-Way Analysis of Variance (ANOVA) and Tukey test as needed. Differences in nanoleakage scores were also statistically evaluated (Kruskal–Wallis ANOVA) ( p < 0.05).
Results
Luting agent was a significant factor for post push-out strength. The interfacial strength achieved by RUA (9.3 ± 2.6 MPa) was significantly higher than that of ME (6.7 ± 2.7 MPa) and that of S (5.4 ± 3.1 MPa), which were comparable to each other. Dowel space level and between factor interaction were not statistically significant. Statistically comparable interfacial nanoleakage was observed in all the groups.
Significance
The three new self-adhesive resin cements exhibited similar sealing properties, although the post retentive ability was superior with RUA.
1
Introduction
Fiber posts are widely used to restore endodontically treated teeth in alternative to prefabricated posts or metal dowels . One of the advantages of fiber posts is the modulus of elasticity similar to dentin . This allows to reduce stress transmission to root canal walls and the risk of vertical fractures . Fiber posts are relatively easy to remove by boring through the middle of the post with an ultrasonic or a rotary instrument . Moreover, quartz or glass fiber posts offer the most favorable optical properties for reproducing the natural aspect of the restored tooth . In addition, glass fiber posts are biocompatible and do not corrode .
Notwithstanding the satisfactory clinical performance of fiber post-composite core systems , in vitro and in vivo studies indicate that post debonding is the most common failure mode of post-retained restorations . Endodontic irrigants and sealers , the presence of a thick smear layer, limited moisture control, the unfavorable C-factor , light transmission through the post have been reported to negatively affect bonding to radicular dentin .
In addition to post debonding, leakage within the root is another cause of failure . It is recognized that tooth structure, post luting agent and core build-up material must be a sealed system . If the coronal seal is disrupted, inadequate marginal adaptation of the luting agent may allow for recontamination of the root . Microorganisms proliferate and take few days to pass through the remaining apical filling .
Among resin cements recommended for fiber posts luting, self-adhesive resin cements have been introduced with the advantage that no pretreatment of dental and restorative substrate is required . This leads to a simplified cementation procedure and chair-time saving. RelyX Unicem was the first self-adhesive resin cement to be introduced in the market, and its use in fiber post luting has been largely investigated . Lately, an evolution of the original RelyX Unicem was launched into the market as RelyX Unicem 2 Automix. Overall, the variety of commercially available auto-adhesive resin cements is large. Conversely, the information present in the literature concerning their bonding and sealing ability to root dentin is limited. For some lately marketed self-adhering luting agents scientific data are actually lacking.
The present study was indeed directed at evaluating by means of “thin-slice” push-out test, light and scanning electron microscopy (SEM), the interfacial strength and marginal integrity of recently introduced self-adhesive resin cements, when used to lute fiber posts into endodontically treated teeth.
The tested null hypothesis was that the self-adhesive resin cements expressed comparable sealing ability and bond strength to root dentin when used to lute endodontic fiber posts.
2
Materials and methods
Twenty-four single-rooted, single-canal premolars that were extracted due to periodontal reasons were selected for the study. Patients’ informed consent was received under a protocol approved by the Institutional Review Board of the University of Siena, Italy. All teeth were stored in 1% chloramine T at 4 °C until use. External debris were removed with a scaler and teeth were decoronated using a low-speed diamond saw (Isomet, Buelher, Lake Bluff, IL) under water cooling. Each tooth was endodontically instrumented at a working length of 0.5 mm from the apex. Canals were treated using a crown-down preparation technique with Protaper rotary instruments (Dentsply-Maillefer, Ballaigues, Switzerland) to size F3. Irrigation was performed using 5.25% sodium hypochlorite between changes of instrument size and 17% EDTA as the final rinse. Root canal spaces were dried with paper points and laterally obturated with gutta-percha cones and AH-26 (Dentsply DeTrey; Konstanz, Germany). Each filled root was coronally sealed using Fuji VII (GC; Tokyo, Japan). Specimens were randomly divided into 3 groups of 8 teeth each and were stored for 24 h (37 °C and 100% humidity) to ensure complete setting of the sealer.
The root canal of each tooth was enlarged with low-speed drills provided by the post manufacturer, and a 8 mm-deep post space was prepared. RelyX Fiber Posts (3M ESPE, Seefeld, Germany) were inserted with the following self-adhesive resin cements: (i) RelyX Unicem 2 Automix (3M ESPE), (ii) Maxcem Elite (Kerr, Orange, CA, USA), and (iii) seT (SDI, Bayswater, Australia). Cements were handled according to the manufacturers’ instructions that are reported along with chemical composition and batch numbers in Table 1 .
| Cement | Composition | Instructions for use |
|---|---|---|
| RelyX Unicem 2 Automix (MP0030) | Silane treated glass powder, TEGDMA, substituted dimethacrylate, sodium p-toluenesulfinate, 1,12-dodecane dimethacrylate, 1-benzil-5-phenil-barbic-acid, calcium salt, silane treated silica, calcium hydroxide, 2-(phosphonooxy)-1,3-propanediyl bismethacrylate, 2-propenoic acid, 2-methyl-, phosphinicobis(oxy-2,1,3-propan), mixture of other methacrylated glycerol, glycerol-1,3-dimethacrylate, 2-propenoic acid, 2-methil-, 1,1′-[1-[(phosphonooxy)methyl]-1,2-ethanediyl]ester, sodium persulfate | Extrude the cement in the root canal. Place the post in the root canal and rotate the post slightly during insertion. Light-curing for 40 s |
| Maxcem Elite (3283155) | GPDM, methacrylate ester monomers, HEMA, 4 methoxyphenol, cumene hydroperoxide, titanium dioxide and pigments | Dispense the cement onto post or directly into canal space. Seat post and vibrate slightly. Allow the cement to slowly flow from canal space. Light-curing for 40 s |
| seT (S0904142) | UDMA, fluoroaluminosilicate glass, camphorquinone, acidic monomer | Activate the capsule and mix it for 10 s. Apply the cement onto post and in root canal. Vibrate post while inserting into root canal. Seat post, and leave for 30 s before light-curing for 40 s |
2.1
Push-out bond strength test
Five posted roots per group were destined to thin-slice push-out bond strength testing. Each root was sectioned into five or six 1-mm-thick slices under water cooling using the Isomet saw. None of the slices failed during the cutting procedure. The push-out load was applied using a cylindrical plunger attached to a universal testing machine (Controls; Milano, Italy). The punch diameter (0.65 mm, 0.98 mm, and 1.25 mm) was selected up to the luted fiber post cross-section for each slice, introducing shear stresses along the bonded interface. The loading force was extended in the apical-coronal direction in order to move the post toward the larger part of the root slice (crosshead speed 0.5 mm/min) until failure. The latter was manifested by the extrusion of the post segment from the root slice. The retentive strength of the post segment was expressed in MPa, by dividing the load at failure in Newtons by the interfacial area ( A ) of the post fragment, which corresponded to the bonded area, in mm 2 . The interfacial area was calculated as the lateral surface of a truncated cone using the following formula: A = π ( R + r )[ h 2 + ( R − r )2]0.5, where π = 3.14, R = coronal post radius, r = apical post radius, and h = root slice thickness. The measurements were made using a digital caliper with 0.01 mm accuracy. The level of the post space from which each slice was derived was noted in order to discriminate among the retentive conditions provided by the luting agents at the coronal, middle and apical thirds of the dowel space.
All fractured specimens were analyzed using a stereomicroscope (Nikon type 102; Tokyo, Japan) at 60× magnification and classified as adhesive (between dentin/cement or fiber post/cement), cohesive (within the post or the luting agent), or mixed.
After analyzing the bond strength data for normality of data distribution (Kolmogorov–Smirnov test) and homogeneity of variances, the Two-Way Analysis of Variance was applied with post push-out strength in MPa as the dependent variable, luting agent and post space level as factors. The Tukey test was then used for post hoc comparisons as needed. In all the analyses the level of significance was set at α = 0.05.
2.2
Interfacial nanoleakage analysis
Three teeth from each experimental group were processed for interfacial nanoleakage evaluation. The root segment containing the post was sectioned into 1-mm-thick serial slabs using the Isomet saw. Both coronal and apical luted interfaces were included in the study. Undemineralized and unembedded specimens were immersed in a 50 wt% ammoniacal silver nitrate solution for 24 h, then in a photo-developing solution for 8 h . Sections were polished under wet condition with increasingly finer grits of SiC papers (Buelher, #600, #1000, #1200, and #2400), and then observed under a stereomicroscope (Nikon type 102; Tokyo, Japan). On each root slice images of both the apical and the coronal side were acquired at a 30× magnification. Two examiners independently scored the amount of tracer along the interface with reference to the method described by Saboia et al. . In case of disagreement between the two examiners, the worse score was considered for statistical calculations.
The Kruskall–Wallis Analysis of Variance on Ranks was applied to assess the statistical significance of between group differences in nanoleakage scores. The level of significance was set at α = 0.05.
2.3
SEM analysis
The same slabs used for interfacial nanoleakage analysis were processed for scanning electron microscope observations of the dentin–cement–post interfaces. The coronal interface was brought into relief by etching with 32% silica-free phosphoric acid gel (Uni-Etch, Bisco, Schaumburg, IL, USA) for 30 s, followed by brief deproteinization with a 2% sodium hypochlorite solution for 120 s. The deproteinized interface was rinsed with deionized water and air-dried for 10 s. Finally, the specimens were mounted on aluminum stubs and directly inspected with scanning electron microscope (JSM-6060LV, JEOL, Tokyo, Japan) at different magnifications and under low-vacuum conditions.
2
Materials and methods
Twenty-four single-rooted, single-canal premolars that were extracted due to periodontal reasons were selected for the study. Patients’ informed consent was received under a protocol approved by the Institutional Review Board of the University of Siena, Italy. All teeth were stored in 1% chloramine T at 4 °C until use. External debris were removed with a scaler and teeth were decoronated using a low-speed diamond saw (Isomet, Buelher, Lake Bluff, IL) under water cooling. Each tooth was endodontically instrumented at a working length of 0.5 mm from the apex. Canals were treated using a crown-down preparation technique with Protaper rotary instruments (Dentsply-Maillefer, Ballaigues, Switzerland) to size F3. Irrigation was performed using 5.25% sodium hypochlorite between changes of instrument size and 17% EDTA as the final rinse. Root canal spaces were dried with paper points and laterally obturated with gutta-percha cones and AH-26 (Dentsply DeTrey; Konstanz, Germany). Each filled root was coronally sealed using Fuji VII (GC; Tokyo, Japan). Specimens were randomly divided into 3 groups of 8 teeth each and were stored for 24 h (37 °C and 100% humidity) to ensure complete setting of the sealer.
The root canal of each tooth was enlarged with low-speed drills provided by the post manufacturer, and a 8 mm-deep post space was prepared. RelyX Fiber Posts (3M ESPE, Seefeld, Germany) were inserted with the following self-adhesive resin cements: (i) RelyX Unicem 2 Automix (3M ESPE), (ii) Maxcem Elite (Kerr, Orange, CA, USA), and (iii) seT (SDI, Bayswater, Australia). Cements were handled according to the manufacturers’ instructions that are reported along with chemical composition and batch numbers in Table 1 .
| Cement | Composition | Instructions for use |
|---|---|---|
| RelyX Unicem 2 Automix (MP0030) | Silane treated glass powder, TEGDMA, substituted dimethacrylate, sodium p-toluenesulfinate, 1,12-dodecane dimethacrylate, 1-benzil-5-phenil-barbic-acid, calcium salt, silane treated silica, calcium hydroxide, 2-(phosphonooxy)-1,3-propanediyl bismethacrylate, 2-propenoic acid, 2-methyl-, phosphinicobis(oxy-2,1,3-propan), mixture of other methacrylated glycerol, glycerol-1,3-dimethacrylate, 2-propenoic acid, 2-methil-, 1,1′-[1-[(phosphonooxy)methyl]-1,2-ethanediyl]ester, sodium persulfate | Extrude the cement in the root canal. Place the post in the root canal and rotate the post slightly during insertion. Light-curing for 40 s |
| Maxcem Elite (3283155) | GPDM, methacrylate ester monomers, HEMA, 4 methoxyphenol, cumene hydroperoxide, titanium dioxide and pigments | Dispense the cement onto post or directly into canal space. Seat post and vibrate slightly. Allow the cement to slowly flow from canal space. Light-curing for 40 s |
| seT (S0904142) | UDMA, fluoroaluminosilicate glass, camphorquinone, acidic monomer | Activate the capsule and mix it for 10 s. Apply the cement onto post and in root canal. Vibrate post while inserting into root canal. Seat post, and leave for 30 s before light-curing for 40 s |
2.1
Push-out bond strength test
Five posted roots per group were destined to thin-slice push-out bond strength testing. Each root was sectioned into five or six 1-mm-thick slices under water cooling using the Isomet saw. None of the slices failed during the cutting procedure. The push-out load was applied using a cylindrical plunger attached to a universal testing machine (Controls; Milano, Italy). The punch diameter (0.65 mm, 0.98 mm, and 1.25 mm) was selected up to the luted fiber post cross-section for each slice, introducing shear stresses along the bonded interface. The loading force was extended in the apical-coronal direction in order to move the post toward the larger part of the root slice (crosshead speed 0.5 mm/min) until failure. The latter was manifested by the extrusion of the post segment from the root slice. The retentive strength of the post segment was expressed in MPa, by dividing the load at failure in Newtons by the interfacial area ( A ) of the post fragment, which corresponded to the bonded area, in mm 2 . The interfacial area was calculated as the lateral surface of a truncated cone using the following formula: A = π ( R + r )[ h 2 + ( R − r )2]0.5, where π = 3.14, R = coronal post radius, r = apical post radius, and h = root slice thickness. The measurements were made using a digital caliper with 0.01 mm accuracy. The level of the post space from which each slice was derived was noted in order to discriminate among the retentive conditions provided by the luting agents at the coronal, middle and apical thirds of the dowel space.
All fractured specimens were analyzed using a stereomicroscope (Nikon type 102; Tokyo, Japan) at 60× magnification and classified as adhesive (between dentin/cement or fiber post/cement), cohesive (within the post or the luting agent), or mixed.
After analyzing the bond strength data for normality of data distribution (Kolmogorov–Smirnov test) and homogeneity of variances, the Two-Way Analysis of Variance was applied with post push-out strength in MPa as the dependent variable, luting agent and post space level as factors. The Tukey test was then used for post hoc comparisons as needed. In all the analyses the level of significance was set at α = 0.05.
2.2
Interfacial nanoleakage analysis
Three teeth from each experimental group were processed for interfacial nanoleakage evaluation. The root segment containing the post was sectioned into 1-mm-thick serial slabs using the Isomet saw. Both coronal and apical luted interfaces were included in the study. Undemineralized and unembedded specimens were immersed in a 50 wt% ammoniacal silver nitrate solution for 24 h, then in a photo-developing solution for 8 h . Sections were polished under wet condition with increasingly finer grits of SiC papers (Buelher, #600, #1000, #1200, and #2400), and then observed under a stereomicroscope (Nikon type 102; Tokyo, Japan). On each root slice images of both the apical and the coronal side were acquired at a 30× magnification. Two examiners independently scored the amount of tracer along the interface with reference to the method described by Saboia et al. . In case of disagreement between the two examiners, the worse score was considered for statistical calculations.
The Kruskall–Wallis Analysis of Variance on Ranks was applied to assess the statistical significance of between group differences in nanoleakage scores. The level of significance was set at α = 0.05.
2.3
SEM analysis
The same slabs used for interfacial nanoleakage analysis were processed for scanning electron microscope observations of the dentin–cement–post interfaces. The coronal interface was brought into relief by etching with 32% silica-free phosphoric acid gel (Uni-Etch, Bisco, Schaumburg, IL, USA) for 30 s, followed by brief deproteinization with a 2% sodium hypochlorite solution for 120 s. The deproteinized interface was rinsed with deionized water and air-dried for 10 s. Finally, the specimens were mounted on aluminum stubs and directly inspected with scanning electron microscope (JSM-6060LV, JEOL, Tokyo, Japan) at different magnifications and under low-vacuum conditions.
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