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Effect of thermomechanical aging on bond strength and interface morphology of glass fiber and zirconia posts bonded with a self-etch adhesive and a self-adhesive resin cement to natural teeth
Effect of thermomechanical aging on bond strength and interface morphology of glass fiber and zirconia posts bonded with a self-etch adhesive and a self-adhesive resin cement to natural teeth Batu Can Yaman, DDS, PhD,a Fusun Ozer, DMD, PhD,b Takuro Takeichi, DDS, PhD,c Bekir Karabucak, DMD, PhD,d Fatma Koray, DDS, PhD,e and Markus B. Blatz, DMD, PhDf Istanbul University, Faculty of Dentistry, Istanbul, Turkey; University of Pennsylvania School of Dental Medicine, Philadelphia, Pa; Aichi Gakuin University School of Dentistry, Nagoya, Japan Statement of problem. Information regarding the effect of thermomechanical aging (TMA) on the bond strength of luting cements to root canal dentin and endodontic posts is limited. Purpose. The purpose of this study was to investigate the effect of TMA on the bond strength of fiber and zirconia posts bonded to root canal dentin with 2 different resin cements with microtensile and scanning electron microscopic evaluation. Material and methods. Eighty extracted single-rooted human premolars were endodontically treated and restored with either a glass fiber post (FP) or a zirconia post (ZP) with 2 commercially available resin luting cements. The teeth were divided into 2 main groups. In the first group, posts (n¼40) were bonded with a self-etch adhesive cement (SEAC). In the second group (n¼40), posts were bonded using a self-adhesive cement (SAC). During the first aging phase, all specimens in each group were stored in distilled water for 30 days at 37 C. During the second phase, half of the specimens in each group were subjected to the TMA. The test groups were as follows: FP/SEAC, FP/SEACþTMA, ZP/SEAC, ZP/SEACþTMA, FP/SAC, FP/ SACþTMA, ZP/SAC, and ZP/SACþTMA. The bond strength was measured with a microtensile test. Data were analyzed by 3way analysis of variance and the Tukey honest significant different test (a¼.05). Results. FP/SEAC at 30 days was higher than in the other groups. However, bond strength values were significantly reduced in this group after TMA (P<.001). Conclusions. Bond strength values and physical properties of SEAC with higher filler content were more affected by the TMA than those of SALC. According to scanning electron microscopic observation, TMA also affected the micromorphologic interface between the posts and the resin cements as well as between the resin cements and the root canal dentin. (J Prosthet Dent 2014;-:---)
Clinical Implications Self-etch adhesive luting cement with higher filler content was more adversely affected by artificial aging applications than self-adhesive luting cement. Prefabricated post systems have become popular because of satisfactory clinical results and an associated reduction in treatment time and cost.1,2 They are usually luted with a resin cement to a
increase their retention and improve the mechanical performance of the restored teeth, reducing the risk of root fracture.3-6 Zirconia has become a widely used material because of its chemical
stability, high mechanical strength, and rigidity.7 Pfeiffer et al8 showed that zirconia posts have a significantly higher yield strength compared to titanium and glass fiber posts. However, the high
Associate Professor, Istanbul University, Faculty of Dentistry, Department of Operative Dentistry. Professor, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine. c Associate Professor, Department of Fixed Prosthodontics, Aichi Gakuin University School of Dentistry. d Associate Professor, Department of Endodontics, University of Pennsylvania School of Dental Medicine. e Professor, Department of Operative Dentistry, Istanbul University, Faculty of Dentistry. f Professor, Chairman of Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine. b
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Volume elastic modulus of the zirconia post of 200 GPa may cause stress which is transferred to the less rigid dentin, resulting in root fractures.9-12 Various luting resin cements and corresponding adhesive systems have been proposed for bonding different types of posts to root canal dentin.13-16 However, technique sensitivity is a significant factor in adhesive luting procedures.17-19 Therefore, resin-based self-adhesive cements have recently been developed to eliminate this problem. These systems do not require the pretreatment of tooth substrates and are designed to combine the favorable characteristics of different cements into a single product.17-20 The retention of posts depends on adequate bond strength between the post and the resin luting cement and between the resin luting cement and the dentin.21,22 Teeth are continuously subjected to stress and fatigue load during mastication, swallowing, and parafunctional habits.23 Little is known regarding the long-term clinical bonding behavior of adhesively luted posts.24 Clinical conditions are often simulated in vitro by thermal applications.25 It is important to determine whether mechanical aging by masticatory simulation might affect the fracture resistance of restored roots and the bond strength of luting cements to root canal walls.26,27 The objectives of this study were to evaluate the effect of thermomechanical aging (TMA) and the type of luting cement on the microtensile bond strength (mTBS) of zirconium and glass fiber posts to the coronal, middle, and apical thirds of post space dentin and to investigate the micromorphology of the interface between 2 resin cements and the root canal dentin as well as the changes between the posts and cement materials with scanning electron microscopy (SEM). The null hypotheses tested were that the 2 resin cements would not exhibit different bond strengths with the study posts; that TMA would not affect the bond strengths of the posts in both cement groups; and that the micromorphology
of the interfaces between the posts and resin cements and between the resin cements and the root canal dentin would not change after aging procedures.
MATERIAL AND METHODS Tooth selection Eighty extracted caries-free permanent human premolars with single and straight root canals were collected for this study. The use of human teeth in the study protocol was reviewed and approved by the Eskis¸ehir Osmangazi University, Faculty of Dentistry, Department of Academic Ethics. After removing residual tissues with a scaler, the teeth were examined stereomicroscopically for defects in the enamel and root surface. The teeth were stored in 1% chloramine T solution until testing. A sample size calculation in power analysis showed that at least 10 teeth per group were required to detect at least a single effect size difference with a Type I error of 5% and Type II errors of 10%. This sample size was similar to that in other studies.25,28
Tooth preparation for post application The crowns of the teeth were removed with a diamond-coated separating disk (IsoMet Wafering Blades15LC; Buehler) under running water. The root canals were instrumented with Hedstrom files after removal of the pulp tissue. During instrumentation, the canals were irrigated with 1 mL of 5.25% NaOCl. The canals were then obturated with a cold lateral compaction technique, with gutta percha cones and a root canal sealer (AH Plus; Dentsply DeTrey GmbH). The cervical root canal opening was then filled with a provisional restorative material (Cavit-G; 3M ESPE). The endodontically treated roots were stored in deionized water for 24 hours at 37 C before the preparation of the post spaces. Canal drills were used, following the recommendations by the
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manufacturers of the glass fiber (Radix; Dentsply-Maillefer) and zirconia posts (B&L Zirconia Post; B&L Biotech Co). The coronal gutta percha was removed from the root canal with a reamer (Largo Peeso Reamer; DentsplyMaillefer), and a post space was prepared, leaving 4 mm of gutta percha to preserve the apical seal. The post space was drilled in each root with a calibrated drill (Radix Precision Drill; Dentsply-Maillefer) corresponding to the Radix glass fiber post (FP) size 2 and B&L zirconia post (ZP) size 2. Before application of the resin cement systems and posts, the root canals were irrigated with a 0.5% sodium hypochlorite solution for 1 minute, then rinsed with distilled water and dried with paper points. Table I lists the materials used in the study.
Post application and cementation The teeth were equally divided into 2 main groups. In the self-etch adhesive cement group (SEAC) (n¼40), FP (n¼20) and ZP (n¼20) were bonded with dual-polymerizing (light- and/or self-polymerizing) self-etch resin cement Panavia F2.0 (Kuraray Medical Inc). In the self-adhesive cement group (SAC) (n¼40), FP (n¼20) and ZP (n¼20) were bonded with SAC RelyX Unicem (3M ESPE).
SEAC group The FPs and ZPs were cleaned with alcohol and air dried. Equal amounts of the ED primer liquids A and B (Kuraray Medical Inc) were mixed and applied to the post space with a microbrush for 30 seconds, gently air dried, and the excess removed with paper points. The Panavia F2.0 pastes A and B were mixed for 20 seconds and applied in the post spaces with a lentulo spiral instrument (DentsplyMaillefer). The posts were placed in the post space, and the cement was polymerized for 40 seconds with a lightpolymerization unit (Demetron A2; Kerr Dental). Silicone molds were used as a supporting structure for the roots, and
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Table I.
3 Material composition
Material Luting cement
Product (Manufacturer), Batch Number
Bonding System
Panavia F2.0 (Kuraray Medical Inc), 041113
Base
Hydrophobic aromatic and aliphatic dimethacrylate, sodium aromatic sulfinate, N,N diethanol-p-toluidine, sodium fluoride, silanized barium glass sodium benzene sulfinate
Catalyst
MDP, hydrophobic aromatic and aliphatic dimethacrylate, photoinitiator, dibenzoyl peroxide, hydrophilic dimethacrylate, silanized silica
RelyX Unicem (3M ESPE), 419029
Post
ED Primer II
HEMA, MDP, 5-NMSA, dimethacrylate, sodium benzene sulfinate, water, accelerator
Powder
Glass fillers, silica, calcium hydroxide, substituted pyrimidine, peroxy compound, pigments, self-polymerized initiators
Liquid
Methacrylated phosphoric esters, dimethacrylates, acetate, stabilizers, self-polymerized initiators, light-polymerized initiators
Radix Fiber (DentsplyMaillefer), 6846221
Zirconium-enriched glass fiber (60%), epoxy resin matrix (40%)
B&L Zirconia (B&L Biotech Co), 222022
Zirconium oxide
the light source was placed directly on the flat cervical tooth surfaces in all post groups. After post insertion, a composite resin core foundation restoration was made with Clearfil SE (Kuraray Medical Inc)/Clearfil Majesty Posterior (Kuraray Medical Inc).
SAC group FPs and ZPs were cleaned with alcohol and air dried. The post spaces were cleaned and gently air dried with an air syringe and paper points. A RelyX Unicem capsule was activated and mixed with a mixing device for 10 to 15 seconds. The cement was applied into the post space and onto the post surface. The posts were then inserted into the canals, and excess cement was removed with a small brush. The cement was polymerized with a polymerizing light for 40 seconds. After post insertion, a composite resin foundation was prepared in the same manner described for the SEAC group.
TMA procedure During a first aging phase, all specimens in each group were stored for 30 days in distilled water at 37 C. During the second phase, half of the
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Composition
specimens in each group were subjected to the TMA procedure. The SEAC and SAC groups were divided into 4 subgroups (n¼10): FP/SEAC, FP/SEACþTMA, ZP/ SEAC, ZP/SEACþTMA, FP/SAC, FP/ SACþTMA, ZP/SAC, and ZP/SACþTMA. In the TMA groups, the specimens were subjected to a thermocycling regimen of 20 000 cycles (THE-1100; SD Mechatronik) between 5 C and 55 C with a dwell time of 60 seconds. After thermocycling, the roots were covered with a polyvinyl siloxane impression material (Aquasil Ultra Extra; Caulk-Dentsply) to imitate artificial periodontal ligament in the chewing simulator. The thickness of the impression material was adjusted to 200 mm, according to the average thickness of the original human periodontal ligament.29,30 The roots were then embedded in a transparent resin material to allow for adequate specimen placement in the artificial chewing simulator (CS-4.2; SD Mechatronik). TMA groups were submitted to 50 000 cycles with a load of 50 N at a rate of 1 Hz.
mTBS testing The roots were attached to the arm of a low-speed diamond saw (Isomet
1000; Buehler). Six 1-mm-thick slabs were obtained from each root (2 apical, 2 middle, and 2 cervical). The slices were then held with finger pressure and trimmed, starting from the mesial and distal surfaces until a diamond rotary instrument (NTI N889; Axis Dental Corp) touched the post28 under water cooling (Fig. 1). This procedure was performed under a light magnifier (Carson CL-65 MagniFlex; Carson Optical Inc). The dumbbell-shaped sections were attached to a special device designed for the mTBS test (Bisco Microtensile Tester; Bisco) with cyanoacrylate glue (Zapit; Dental Venture of America) at room temperature and were kept moist throughout. The specimens were subjected to a tensile force at a crosshead speed of 0.5 mm/min until fracture. Failure loads were recorded in N and mTBS was calculated (MPa) as follows. The bonded surface area of the specimens was calculated by the equation proposed by Mallmann et al28:
A ¼ ðPC=2ÞDIWDt; where A is the surface area (mm2), PC is the post circumference (2pr) (where p¼3.14 and r is the post radius), DIWD is the diamond rotatory cutting instrument working diameter
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The slabs were dehydrated through an ascending series of ethanol (25% to 100%) and stored in a freon solution (Freon 123 solution; Sigma-Aldrich Corp). The surface morphology of the specimens was examined with a SEM (JSM2300; JEOL) at 350 and 1500 magnification.
RESULTS 1 Preparations of test materials. A, Cutting sequence followed to obtain slices. B, Interproximal slice trimming. C, Dumbbell-shaped specimen. D, Dentin. P, fiber or zirconia post; LC, luting cement.
Table II.
Microtensile bond strength results of all experimental groups
Mean Bond Strength (MPa) and Standard Deviation at: Experimental Group SEACþFP
30 d
30 dDTCDLC
13.9 3.8d
4.7 1.5a
SACþFP
9.9 2.9bcd
9.6 2.5bcd
SEACþZP
7.2 2.1
6.1 2.1ab
SACþZP
11.5 4.0cd
abc
7.4 2.4abc
Same superscript letters demonstrate no significant differences (P<.05). SEAC, self-etch adhesive cement; SAC, self-adhesive cement; TC, thermocycles; LC, load cycles.
(0.6 mm-NTI N889, Axis Dental Corp), and t is the thickness of the slice. The load at failure (N) was divided by the bonded surface area (mm2) to calculate bond strength (MPa). Statistical analyses were performed with commercially available software (SigmaPlot 12; Systat Software Inc). The mTBS data were analyzed with 3-way analysis of variance (ANOVA) and the Tukey honestly significant different tests.
Failure analysis After microtensile testing, the specimens were examined with a stereomicroscope (Olympus ZS 61; Olympus Corp) to determine the mode of fracture. Failure modes were classified as adhesive when between the resin cement and root dentin (RC-D), adhesive when between the resin cement and post (RC-P), mixed when the fracture was partially at the resin cement-root
dentin interface, or cohesive when within the resin cement, post, and/or root dentin.27
SEM specimen preparation Two roots were selected from each test group and prepared for SEM analysis. Three 2-mm-thick slabs were obtained from each root (1 apical, 1 middle, and 1 cervical). All of the slabs were embedded in epoxy resin (Leco Corp) and polished with 600-, 800-, and 1200-grit silicon carbide abrasive papers. For the high polish, they were submitted to 1 mm, 0.3 mm, and 0.05 mm alumina powder on a cloth. The specimens were immersed in a 10% phosphoric acid solution for 15 seconds, rinsed with water for 15 seconds, and then air dried. The specimens were treated with a 10% sodium hypochlorite solution for 30 seconds, rinsed thoroughly with water, and fixed in glutaraldehyde solution (pH 7.4) for 2 hours.
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The mean mTBS values and standard deviations of tested resin cements/ posts are listed in Table II. The post hoc Tukey test revealed that both the 30 days in distilled water and the TMA did not significantly affect the bonding of either FPs or ZPs to the root canal dentin, except for the FPs bonded with SEAC (P<.001). The FPs bonded with SEAC, group FP/SEAC, showed the highest bond strength values after 30 days in distilled water. However, bond strength values decreased dramatically after TMA application. When the ZP system was evaluated, the SAC exhibited higher bond strength values than the SEAC in the 30-day distilled water group. The results of the 3-way ANOVA of mTBS data are listed in Table III. Statistical analysis indicated that the type of resin cement, the post material, and aging significantly all adversely affected bond strength values (P<.001). A significant post material and resin cement interaction occurred after 30 days’ storage in distilled water (P<.001). However, no significant post material and resin cement interaction occurred after 30 days’ storage in distilled water and TMA (P¼.092). The regional bond strength values of the test groups are shown in Table IV. In the cervical and apical root regions, the best bonding performance was obtained with the FP group with SEAC before TMA (group FP/SEAC). However, the ZP group with ZP/SAC exhibited the best bonding performance in the middle region of root dentin. After the TMA procedure, the FPs with FP/ SEACþTMA presented the lowest bond strength values in both the cervical and apical root dentin regions.
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5 Three-way ANOVA results of microtensile bond strength test (split-
Table III. plot model)
Source of Variation
DF
SS
MS
F
P
Post material
1
26.772
26.772
2.862
.096
Cement
1
31.678
31.678
3.386
.071
TMA
1
193.813
193.813
20.717
<.001
Post materialCement
1
29.259
29.259
3.127
.082
Post materialTMA
1
11.794
11.794
1.261
.266
CementTMA
1
44.378
44.378
4.744
.034
Post materialCementTMA
1
164.331
164.331
17.565
<.001
ANOVA, analysis of variance; DF, degrees of freedom; SS, sum of squares; MS, mean of squares; F, F test statistic; P, probability; TMA, thermomechanical aging.
Table IV.
Mean microtensile bond strength of root dentin regions
Mean Bond Strength (MPa) and Standard Deviation for Root Dentin Region: Material
Cervical
SEACþFP
15.7 1.5de
SEACþFP (TCþLC)
3.7 1.1ab
6.8 2.3bcde
3.5 1.4a
SACþFP
11.9 1.7cde
9.8 3.8cde
7.9 3.3bcde
SACþFP (TCþLC)
9.1 5.1cde
8.9 4.2cde
6.3 1.7
6.2 2.1
SEACþZP
Middle
Apical
13.1 4.9cde
12.9 4.9cde
bcd
6.3 2.1bcde
SEACþZP (TCþLC) SACþZP
13.9 6.7de
SACþZP (TCþLC)
10.4 1.5cde
10.9 4.2cde 9.3 2.1cde
bcd
6.1 1.8bcde
6.8 3.1bc
15.9 4.9e
6.9 2.8bcde
8.5 1.2bcde
4.7 2.9abc
Same superscript letters demonstrate no significant differences (P<.05). SEAC, self-etch adhesive cement; SAC, self-adhesive cement; TC, thermocycle; LC, load cycle.
Table V.
Percentage of fracture modes of all experimental groups
30 d AF Experimental RC-P Group (%)
TCDLC CF
MF
RC-D C M (%) (%) (%)
AF RC-P (%)
CF MF
RC-D C M (%) (%) (%)
SEACþFP
91.75
8.25
X
X
80.35
5.88
X
11.76
SACþFP
90.25
9.75
X
X
81.48
11.25
3.12
6.25
SEACþZP
81.55
11.45
X
5
78.33
16.11
X
5.55
SACþZP
83.13
10.34
X
6.52
80.61
11.69
X
7.69
AF, adhesive failure; CF, cohesive failure; MF, mixed failure; RC-P, resin cement post; RC-D, resin cement-dentin; TC, thermocycle; LC, load cycle; SEAC, self-etch adhesive cement; SAC, self-adhesive cement; FP, fiber post; ZP, zirconia post.
Table V presents the percentage of failure modes in each group. The distribution of failures for each group was homogeneous, and failures were predominantly at the interface between the resin cement and the post.
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The formation of different resin tags and a clear hybrid layer were found in all test groups. After 30 days’ storage in water, the FP with SEAC (Fig. 2A, B) and the ZP with SEAC (Fig. 3A, B) groups exhibited uniform and different
size resin tags at the root dentin regions. However, the same type of aging in the FP with SAC (Fig. 2E, F) and ZP with SAC (Fig. 3E and 3F) groups exhibited a reduced number of short and thin resin tags in the root dentin. After TMA, the FP with SEAC (Fig. 2C, D) and the ZP with SEAC (Fig. 3C, D) groups showed broken and diverged resin tags as well as a deformation between the FPs and luting resin cements.
DISCUSSION This study investigated the mTBS of FPs and ZPs bonded with 2 different resin luting cements to root canal dentin with and without TMA. In light of the mTBS test results, the first and second null hypotheses of the study were partially accepted after TMA, because no significant differences in bond strength values were observed between the different luting resin cements, except for the groups with FP luted with SEAC. The lowest bond strengths were observed for FP luted with SEAC after TMA. The SEM results also confirmed that the last null hypothesis was rejected. Several studies have reported the retention of endodontic posts shortly after cementation without any simulation of oral conditions.13,14,16,31 However, clinically, the dislodgement of post-retained restorations occurs after some years of function as a result of such factors as temperature change and dynamic mechanical fatigue loading.1,26,27 Laurell and Lundgren32 reported that in prosthetically restored dentitions, the largest total force of masticatory strokes approached 80 N. To simulate the severe effects of the oral environment and to gain better knowledge of the materials’ behavior under adverse conditions, a storage time of 30 days at 37 C, thermal cycling, and cycling loading with 50 N parallel to the long axis of the posts were applied in this study. In an attempt to further simulate clinically relevant intraoral biomechanical conditions in a laboratory environment, the periodontal ligament was simulated
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2 Representative SEM images of FPs with SEAC and SAC (original magnification, 350 and 1500) A, B, Thick, long, and dense resin tags in SEAC group. C, D, After TMA procedure; broken resin tags, cracks in cement and dentin and separation between SEAC and FP. E, F, Hybrid zone is identified without visible resin tags with SAC group. G, H, Short, thin, and inconsistent resin tags and some cracks within SAC and also separation between SAC and FP after TMA. with a polyether-based molding material during cyclic loading. This artificial periodontal ligament functions as a less rigid and resilient liner, which absorbs part of the compressive forces, attenuating stress on the post and resin cement.30 Bond strength values reported in the current study are comparable to those found by other researchers. Zaitter et al25 evaluated and compared
2 different glass fiber posts and 4 different resin luting cements. Mean microtensile values for fiber posts luted with Panavia F2.0 were 10.3 MPa for Exacto and 25.9 MPa for everStick. For fiber posts luted with RelyX Unicem after 1000 thermocycles and then stored in distilled water at 37 C for 30 days, those values were 19.8 MPa for Exacto and 30.5 MPa for everStick. For
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FPs stored for 30 days in distilled water at 37 C, bond strength values were 13.9 MPa for FPs used with Panavia F2.0 and 9.9 MPa for FPs used with RelyX Unicem. The lower bond strength values can be attributed to the differences in both the post materials and number of thermal cycles. The greater number of thermal cycles (20 000 vs 1000) in the present study likely caused
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2 (continued) Representative SEM images of FPs with SEAC and SAC (original magnification, 350 and 1500) A, B, Thick, long, and dense resin tags in SEAC group. C, D, After TMA procedure; broken resin tags, cracks in cement and dentin and separation between SEAC and FP. E, F, Hybrid zone is identified without visible resin tags with SAC group. G, H, Short, thin, and inconsistent resin tags and some cracks within SAC and also separation between SAC and FP after TMA.
greater stress on the resin-bonding interface and therefore resulted in lower bond strength values. In this study, the bond strength values of FPs and ZPs to root canal dentin were different depending on which luting cement was used. The bonding performance of FPs with SEAC was better than that of ZPs with SEAC without TMA application. Similarly, another study evaluated the bond strengths of FPs and ZPs luted with various resin luting agents into the post spaces of extracted teeth. The ZP showed lower bond strength values when compared to the FP luted with SEAC, which is in agreement with the results of the current study.33 The authors found that SEAC had a lower bond strength than SAC with FP after TMA, which is consistent with another report by Bitter et al.27 These authors reported higher bond strengths with SAC (RelyX Unicem) compared to SEAC (Panavia F2.0) after the thermomechanical loading. The bonding mechanism of this self-adhesive cement is claimed to be based on micromechanical retention and chemical adhesion to hydroxapatite. The secondary chemical interaction may also be responsible for a higher tolerance to moisture and thermomechanical stress.19 It has also been confirmed that SAC (RelyX Unicem) has a high tolerance to moisture because water
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was formed during the neutralization reaction of phosphoric acid, methacrylate basic fillers, and hydroxyapatite.34 The authors also speculated that the performance of SEAC was influenced by the priming agent used together with the resin luting agent. The self-etching ED primer used with Panavia F2.0 is a 1-step adhesive. One-step self-etch adhesives are hydrophilic, as they contain high concentrations of both ionic and hydrophilic monomers. A large amount of water is added to these 1-step adhesives. It is therefore difficult to evaporate water from the adhesive solution. Even if evaporation is successful, water will rapidly diffuse back from the bonded dentin into the adhesive resin, creating permeable membranes and water movement across the adhesive layer, contributing to the degradation of the resin/dentin bond.35 The current study confirmed that although ZPs had lower bond strengths with SEAC and SAC after TMA application, the difference was not significant; it was quite significant with FPs and SEAC. This change in bond strength values can be explained by the fact that the outer surface of the ZPs of this study had a microporous surface, whereas the FPs have a factory smooth surface (according to the manufacturers’ product profiles). The increased surface roughness and surface area of the ZPs could have influenced the mechanical
retention of the luting cements, which contributes to the adhesive strength between the post and the cement after TMA. The adhesive luting systems demonstrated measurable adhesive properties in this study, with the highest values for the cervical third in the FPs with SEAC without the TMA, and the lowest values for the apical third in the FPs with SEAC after the TMA. These results can be associated with TMA as well as various other factors, including the less dense dentinal tubule configuration in the apical portion of the root canal system,36 apical sclerosis,37 the cavity configuration factor,38 the difficulty of visualization and access to the apical part of the root canal system, and the restrictions in the flow of the cement to this part of the root canal.39 In this study, 2 types of predominant failures were found: resin cementpost and resin cement-dentin adhesive failures. However, the predominant failure was between the resin cement and post for all experimental groups with or without TMA application. The weakest link seems to be the bond of the cements to the posts. Different studies support these observations. They showed significant debonding between ceramic posts and luting resin cement and observed failures at the resin cement-post interfaces.40,41
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3 Representative SEM images of ZPs with SEAC and SAC (original magnification, 350 and 1500). A, B, Distinctive hybrid zone and equal size resin tags. Separation between SEAC and ZP. C, D, Acceptable adaptation between SEAC and ZP after TMA application. However, separations were observed with broken resin tags between root dentin and SEAC. E, F, Various sized resin tags and hybrid zone were observed with SAC. Separation between SAC and ZP. G, H, Distinctive hybrid layer and few resin tags. Separation between SAC and ZP after TMA application. SEM analysis demonstrated that without TMA, the SEAC group showed different and long resin tag formations. Also, no inner cracks were observed in the luting cement for both types of posts. However, without TMA, inadequate resin penetration into the dentin tubules was observed in the SAC group. After TMA, some typical large
cracks were observed between the SEAC and root dentin with FPs. Specifically, these cracks extended from the root dentin to the fiber post surface, which may be associated with the reduced bond strength of SEAC in the FP group (Fig. 4). Walker et al42 investigated the mechanical property characterization of resin cement Panavia F2.0 after
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aqueous aging, with and without cyclic loading. The authors concluded that after aqueous aging with cyclic loading to simulate the clinical function of the resin cement, initial degradation may be related to the breakdown of the filler/resin interface bond. Such a breakdown was potentially reflective of a slow crack propagation that may
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3 (continued). Representative SEM images of ZPs with SEAC and SAC (original magnification, 350 and 1500). A, B, Distinctive hybrid zone and equal size resin tags. Separation between SEAC and ZP. C, D, Acceptable adaptation between SEAC and ZP after TMA application. However, separations were observed with broken resin tags between root dentin and SEAC. E, F, Various sized resin tags and hybrid zone were observed with SAC. Separation between SAC and ZP. G, H, Distinctive hybrid layer and few resin tags. Separation between SAC and ZP after TMA application.
4 Typical SEM images of FP with SEAC after TMA procedure (original magnification, 500 and 1000). A, Large separations were observed between luting cement and FP and root dentin. B, Some large cracks and gaps were found in cement and dentin. contribute to in vivo resin cement cohesive failure. Additionally, resin cements with greater filler content exhibit higher contraction stress, lower bond strength, and a more defective bonding interface.43 Panavia F2.0 has a higher filler content (59 vol% with a filler size of 19 mm) compared to RelyX Unicem (50 vol% with a filler size of 12 mm). Therefore, on the basis of the results of this study, increased and larger filler content may increase contraction stress within the material and cause cracks.
CONCLUSIONS The bond strength values and physical properties of SEAC with higher filler content were improved by the TMA
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application than those of SAC. TMA significantly reduced the bond strength values of FPs with SEAC. TMA also adversely affected the micromorphologic interface between the posts and the resin cements as well as between the resin cements and the root canal dentin.
REFERENCES 1. Balbosh A, Kern M. Effect of surface treatment on retention of glass-fiber endodontic posts. J Prosthet Dent 2006;95: 218-23. 2. Stockton LW. Factors affecting retention of post systems: a literature review. J Prosthet Dent 1999;81:380-5. 3. Ferrari M, Vichi A, Grandini S, Goracci C. Efficacy of a self-curing adhesive-resin cement system on luting glass-fiber posts into root canals: an SEM investigation. Int J Prosthodont 2001;14:543-9.
4. Chang JW, Soo I, Cheung GS. Evaluation of fiber post-supported restorations under simulated occlusal loading. J Prosthet Dent 2012;108:158-64. 5. Ferrari M, Vichi A, Mannocci F, Mason PN. Retrospective study of the clinical performance of fiber posts. Am J Dent 2000;13:9-13. 6. Schiavetti R, García-Godoy F, Toledano M, Mazzitelli C, Barlattani A, Ferrari M, Osorio R. Comparison of fracture resistance of bonded glass fiber posts at different lengths. Am J Dent 2010;23:227-30. 7. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 8. Pfeiffer P, Schulz A, Nergiz I, Schmage P. Yield strength of zirconia and glass fibre-reinforced posts. J Oral Rehabil 2006;33:70-4. 9. Guazzato M, Albakry M, Ringer SP, Swain MV. Strength, fracture toughness and microsructure of a selection of allceramic materials. Part II. Zirconiabased dental ceramics. Dent Mater 2004;20: 449-56.
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Volume 10. Bateman G, Ricketts DN, Saunders WP. Fibre-based post systems: a review. Br Dent J 2003;195:43-8. 11. Ozkurt Z, Is¸eri U, Kazazoglu E. Zirconia ceramic post systems: a literature review and a case report. Dent Mater J 2010;29:233-45. 12. Bottino MA, Baldissara P, Valandro LF, Galhano GA, Scotti R. Effects of mechanical cycling on the bonding of zirconia and fiber posts to human root dentin. J Adhes Dent 2007;9:327-31. 13. Bouillaguet S, Troesch S, Wataha JC, Krejci I, Meyer JM, Pashley DH. Microtensile bond strength between adhesive cements and root canal dentin. Dent Mater 2003;19:199-205. 14. Kalkan M, Usumez A, Ozturk AN, Belli S, Eskitascioglu G. Bond strength between root dentin and three glass-fiber post systems. J Prosthet Dent 2006;9:41-6. 15. Radovic I, Mazzitelli C, Chieffi N, Ferrari M. Evaluation of the adhesion of fiber posts cemented using different adhesive approaches. Eur J Oral Sci 2008;116:557-63. 16. Monticelli F, Osorio R, Albaladejo A, Aguilera FS, Ferrari M, Tay FR, Toledano M. Effects of adhesive systems and luting agents on bonding of fiber posts to root canal dentin. J Biomed Mater Res B Appl Biomater 2006;77:195-200. 17. Ibarra G, Johnson GH, Geurtsen W, Vargas MA. Microleakage of porcelain veneer restorations bonded to enamel and dentin with a new self-adhesive resin-based dental cement. Dent Mater 2007;23:218-25. 18. Behr M, Rosentritt M, Regnet T, Lang R, Handel G. Marginal adaptation in dentin of a self-adhesive universal resin cement compared with well-tried systems. Dent Mater 2004;20:191-7. 19. Abo-Hamar SE, Hiller KA, Jung H, Federlin M, Friedl KH, Schmalz G. Bond strength of a new universal self-adhesive resin luting cement to dentin and enamel. Clin Oral Investig 2005;9:161-7. 20. Castellan CS, Santos-Filho PC, Soares PV, Soares CJ, Cardoso PE. Measuring bond strength between fiber post and root dentin: a comparison of different tests. J Adhes Dent 2010;12:477-85. 21. Bitter K, Paris S, Pfuertner C, Neumann K, Kielbassa AM. Morphological and bond strength evaluation of different resin cements to root dentin. Eur J Oral Sci 2009;117:326-33. 22. Farina AP, Cecchin D, Garcia Lda F, Naves LZ, Sobrinho LC, Pires-de-Souza Fde C. Bond strength of fiber posts in different root thirds using resin cement. J Adhes Dent 2011;13:179-86.
23. d’Incau E, Couture C, Maureille B. Human tooth wear in the past and the present: tribological mechanisms, scoring systems, dental and skeletal compensations. Arch Oral Biol 2012;57:214-29. 24. Goracci C, Ferrari M. Current perspectives on post systems: a literature review. Aust Dent J 2011;56:77-83. 25. Zaitter S, Sousa-Neto MD, Roperto RC, SilvaSousa YT, El-Mowafy O. Microtensile bond strength of glass fiber posts cemented with self-adhesive and self-etching resin cements. J Adhes Dent 2011;13:55-9. 26. Albaladejo A, Osorio R, Aguilera FS, Toledano M. Effect of cyclic loading on bonding of fiber posts to root canal dentin. J Biomed Mater Res B Appl Biomater 2008;8: 264-9. 27. Bitter K, Perdigão J, Exner M, Neumann K, Kielbassa A, Sterzenbach G. Reliability of fiber post bonding to root canal dentin after simulated clinical function in vitro. Oper Dent 2012;4:397-405. 28. Mallmann A, Jacques LB, Valandro LF, Muench A. Microtensile bond strength of photoactivated and autopolymerized adhesive systems to root dentin using translucent and opaque fiber-reinforced composite posts. J Prosthet Dent 2007;97: 165-72. 29. Nanci A, Bosshardt DD. Structure of periodontal tissues in health and disease. Periodontol 2000 2006;40:11-28. 30. Feitosa VP, Medina AD, Puppin-Rontani RM, Correr-Sobrinho L, Sinhoreti MA. Effect of resin coat technique on bond strength of indirect restorations after thermal and load cycling. Bull Tokyo Dent Coll 2010;51:111-8. 31. Aksornmuang J, Nakajima M, Foxton RM, Panyayong W, Tagami J. Regional bond strengths and failure analysis of fiber posts bonded to root canal dentin. Oper Dent 2008;33:636-43. 32. Laurell L, Lundgren D. A standardized programme for studying the occlusal force pattern during chewing and biting in prosthetically restored dentitions. J Oral Rehabil 1984;11:39-44. 33. Perdigao J, Geraldeli S, Lee IK. Push-out bond strengths of tooth-colored posts bonded with different adhesive systems. Am J Dent 2004;17:422-6. 34. Bitter K, Meyer-Lueckel H, Priehn K, Kanjuparambil JP, Neumann K, Kielbassa AM. Effects of luting agent and thermocycling on bond strengths to root canal dentine. Int Endod J 2006;39: 809-18.
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35. Van Landuyt KL, De Munck J, Snauwaert J, Coutinho E, Poitevin A, Yoshida Y, et al. Monomer-solvent phase separation in onestep self-etch adhesives. J Dent Res 2005;84: 183-8. 36. Shemesh H, Wu MK, Wesselink PR. Leakage along apical root fillings with and without smear layer using two different leakage models: a two-month longitudinal ex vivo study. Int Endod J 2006;39:968-76. 37. Paque F, Luder HU, Sener B, Zehnder M. Tubular sclerosis rather than the smear layer impedes dye penetration into the dentine of endodontically instrumented root canals. Int Endod J 2006;39:18-25. 38. Tay FR, Loushine RJ, Lambrechts P, Weller RN, Pashley DH. Geometric factors affecting dentin bonding in rootcanals: a theoretical modeling approach. J Endod 2005;31:584-9. 39. de Durâo Mauricio PJ, González-López S, Aguilar-Mendoza JA, Félix S, GonzálezRodríguez MP. Comparison of regional bond strength in root thirds among fiberreinforced posts luted with different cements. J Biomed Mater Res B Appl Biomater 2007;83:364-72. 40. Dietschi D, Romelli M, Goretti A. Adaptation of adhesive posts and cores to dentin after fatigue testing. Int J Prosthodont 1997;10: 498-507. 41. Galhano GA, Melo RM, Pavanelli CA, Baldissara P, Scotti R, Valandro LF, Bottino MA. Adhesive cementation of zirconia posts to root dentin: evaluation of the mechanical cycling effect. Braz Oral Res 2008;22:264-9. 42. Walker MP, Spencer P, David Eick J. Mechanical property characterization of resin cement after aqueous aging with and without cyclic loading. Dent Mater 2003;19:645-52. 43. Goracci C, Ferrari M. Current prospectives on bond systems: a literature review. Aust Dent J 2011;56:77-83.
Corresponding author: Dr Batu Can Yaman Eskis¸ehir Osmangazi University Faculty of Dentistry Department of Operative Dentistry Meselik Kampus, Odunpazarı, 26480, Eskisehir TURKEY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications Self-etch adhesive luting cement with higher filler content was more adversely affected by artificial aging applications than self-adhesive luting cement. Prefabricated post systems have become popular because of satisfactory clinical results and an associated reduction in treatment time and cost.1,2 They are usually luted with a resin cement to a
increase their retention and improve the mechanical performance of the restored teeth, reducing the risk of root fracture.3-6 Zirconia has become a widely used material because of its chemical
stability, high mechanical strength, and rigidity.7 Pfeiffer et al8 showed that zirconia posts have a significantly higher yield strength compared to titanium and glass fiber posts. However, the high
Associate Professor, Istanbul University, Faculty of Dentistry, Department of Operative Dentistry. Professor, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine. c Associate Professor, Department of Fixed Prosthodontics, Aichi Gakuin University School of Dentistry. d Associate Professor, Department of Endodontics, University of Pennsylvania School of Dental Medicine. e Professor, Department of Operative Dentistry, Istanbul University, Faculty of Dentistry. f Professor, Chairman of Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine. b
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Volume elastic modulus of the zirconia post of 200 GPa may cause stress which is transferred to the less rigid dentin, resulting in root fractures.9-12 Various luting resin cements and corresponding adhesive systems have been proposed for bonding different types of posts to root canal dentin.13-16 However, technique sensitivity is a significant factor in adhesive luting procedures.17-19 Therefore, resin-based self-adhesive cements have recently been developed to eliminate this problem. These systems do not require the pretreatment of tooth substrates and are designed to combine the favorable characteristics of different cements into a single product.17-20 The retention of posts depends on adequate bond strength between the post and the resin luting cement and between the resin luting cement and the dentin.21,22 Teeth are continuously subjected to stress and fatigue load during mastication, swallowing, and parafunctional habits.23 Little is known regarding the long-term clinical bonding behavior of adhesively luted posts.24 Clinical conditions are often simulated in vitro by thermal applications.25 It is important to determine whether mechanical aging by masticatory simulation might affect the fracture resistance of restored roots and the bond strength of luting cements to root canal walls.26,27 The objectives of this study were to evaluate the effect of thermomechanical aging (TMA) and the type of luting cement on the microtensile bond strength (mTBS) of zirconium and glass fiber posts to the coronal, middle, and apical thirds of post space dentin and to investigate the micromorphology of the interface between 2 resin cements and the root canal dentin as well as the changes between the posts and cement materials with scanning electron microscopy (SEM). The null hypotheses tested were that the 2 resin cements would not exhibit different bond strengths with the study posts; that TMA would not affect the bond strengths of the posts in both cement groups; and that the micromorphology
of the interfaces between the posts and resin cements and between the resin cements and the root canal dentin would not change after aging procedures.
MATERIAL AND METHODS Tooth selection Eighty extracted caries-free permanent human premolars with single and straight root canals were collected for this study. The use of human teeth in the study protocol was reviewed and approved by the Eskis¸ehir Osmangazi University, Faculty of Dentistry, Department of Academic Ethics. After removing residual tissues with a scaler, the teeth were examined stereomicroscopically for defects in the enamel and root surface. The teeth were stored in 1% chloramine T solution until testing. A sample size calculation in power analysis showed that at least 10 teeth per group were required to detect at least a single effect size difference with a Type I error of 5% and Type II errors of 10%. This sample size was similar to that in other studies.25,28
Tooth preparation for post application The crowns of the teeth were removed with a diamond-coated separating disk (IsoMet Wafering Blades15LC; Buehler) under running water. The root canals were instrumented with Hedstrom files after removal of the pulp tissue. During instrumentation, the canals were irrigated with 1 mL of 5.25% NaOCl. The canals were then obturated with a cold lateral compaction technique, with gutta percha cones and a root canal sealer (AH Plus; Dentsply DeTrey GmbH). The cervical root canal opening was then filled with a provisional restorative material (Cavit-G; 3M ESPE). The endodontically treated roots were stored in deionized water for 24 hours at 37 C before the preparation of the post spaces. Canal drills were used, following the recommendations by the
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manufacturers of the glass fiber (Radix; Dentsply-Maillefer) and zirconia posts (B&L Zirconia Post; B&L Biotech Co). The coronal gutta percha was removed from the root canal with a reamer (Largo Peeso Reamer; DentsplyMaillefer), and a post space was prepared, leaving 4 mm of gutta percha to preserve the apical seal. The post space was drilled in each root with a calibrated drill (Radix Precision Drill; Dentsply-Maillefer) corresponding to the Radix glass fiber post (FP) size 2 and B&L zirconia post (ZP) size 2. Before application of the resin cement systems and posts, the root canals were irrigated with a 0.5% sodium hypochlorite solution for 1 minute, then rinsed with distilled water and dried with paper points. Table I lists the materials used in the study.
Post application and cementation The teeth were equally divided into 2 main groups. In the self-etch adhesive cement group (SEAC) (n¼40), FP (n¼20) and ZP (n¼20) were bonded with dual-polymerizing (light- and/or self-polymerizing) self-etch resin cement Panavia F2.0 (Kuraray Medical Inc). In the self-adhesive cement group (SAC) (n¼40), FP (n¼20) and ZP (n¼20) were bonded with SAC RelyX Unicem (3M ESPE).
SEAC group The FPs and ZPs were cleaned with alcohol and air dried. Equal amounts of the ED primer liquids A and B (Kuraray Medical Inc) were mixed and applied to the post space with a microbrush for 30 seconds, gently air dried, and the excess removed with paper points. The Panavia F2.0 pastes A and B were mixed for 20 seconds and applied in the post spaces with a lentulo spiral instrument (DentsplyMaillefer). The posts were placed in the post space, and the cement was polymerized for 40 seconds with a lightpolymerization unit (Demetron A2; Kerr Dental). Silicone molds were used as a supporting structure for the roots, and
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Table I.
3 Material composition
Material Luting cement
Product (Manufacturer), Batch Number
Bonding System
Panavia F2.0 (Kuraray Medical Inc), 041113
Base
Hydrophobic aromatic and aliphatic dimethacrylate, sodium aromatic sulfinate, N,N diethanol-p-toluidine, sodium fluoride, silanized barium glass sodium benzene sulfinate
Catalyst
MDP, hydrophobic aromatic and aliphatic dimethacrylate, photoinitiator, dibenzoyl peroxide, hydrophilic dimethacrylate, silanized silica
RelyX Unicem (3M ESPE), 419029
Post
ED Primer II
HEMA, MDP, 5-NMSA, dimethacrylate, sodium benzene sulfinate, water, accelerator
Powder
Glass fillers, silica, calcium hydroxide, substituted pyrimidine, peroxy compound, pigments, self-polymerized initiators
Liquid
Methacrylated phosphoric esters, dimethacrylates, acetate, stabilizers, self-polymerized initiators, light-polymerized initiators
Radix Fiber (DentsplyMaillefer), 6846221
Zirconium-enriched glass fiber (60%), epoxy resin matrix (40%)
B&L Zirconia (B&L Biotech Co), 222022
Zirconium oxide
the light source was placed directly on the flat cervical tooth surfaces in all post groups. After post insertion, a composite resin core foundation restoration was made with Clearfil SE (Kuraray Medical Inc)/Clearfil Majesty Posterior (Kuraray Medical Inc).
SAC group FPs and ZPs were cleaned with alcohol and air dried. The post spaces were cleaned and gently air dried with an air syringe and paper points. A RelyX Unicem capsule was activated and mixed with a mixing device for 10 to 15 seconds. The cement was applied into the post space and onto the post surface. The posts were then inserted into the canals, and excess cement was removed with a small brush. The cement was polymerized with a polymerizing light for 40 seconds. After post insertion, a composite resin foundation was prepared in the same manner described for the SEAC group.
TMA procedure During a first aging phase, all specimens in each group were stored for 30 days in distilled water at 37 C. During the second phase, half of the
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specimens in each group were subjected to the TMA procedure. The SEAC and SAC groups were divided into 4 subgroups (n¼10): FP/SEAC, FP/SEACþTMA, ZP/ SEAC, ZP/SEACþTMA, FP/SAC, FP/ SACþTMA, ZP/SAC, and ZP/SACþTMA. In the TMA groups, the specimens were subjected to a thermocycling regimen of 20 000 cycles (THE-1100; SD Mechatronik) between 5 C and 55 C with a dwell time of 60 seconds. After thermocycling, the roots were covered with a polyvinyl siloxane impression material (Aquasil Ultra Extra; Caulk-Dentsply) to imitate artificial periodontal ligament in the chewing simulator. The thickness of the impression material was adjusted to 200 mm, according to the average thickness of the original human periodontal ligament.29,30 The roots were then embedded in a transparent resin material to allow for adequate specimen placement in the artificial chewing simulator (CS-4.2; SD Mechatronik). TMA groups were submitted to 50 000 cycles with a load of 50 N at a rate of 1 Hz.
mTBS testing The roots were attached to the arm of a low-speed diamond saw (Isomet
1000; Buehler). Six 1-mm-thick slabs were obtained from each root (2 apical, 2 middle, and 2 cervical). The slices were then held with finger pressure and trimmed, starting from the mesial and distal surfaces until a diamond rotary instrument (NTI N889; Axis Dental Corp) touched the post28 under water cooling (Fig. 1). This procedure was performed under a light magnifier (Carson CL-65 MagniFlex; Carson Optical Inc). The dumbbell-shaped sections were attached to a special device designed for the mTBS test (Bisco Microtensile Tester; Bisco) with cyanoacrylate glue (Zapit; Dental Venture of America) at room temperature and were kept moist throughout. The specimens were subjected to a tensile force at a crosshead speed of 0.5 mm/min until fracture. Failure loads were recorded in N and mTBS was calculated (MPa) as follows. The bonded surface area of the specimens was calculated by the equation proposed by Mallmann et al28:
A ¼ ðPC=2ÞDIWDt; where A is the surface area (mm2), PC is the post circumference (2pr) (where p¼3.14 and r is the post radius), DIWD is the diamond rotatory cutting instrument working diameter
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The slabs were dehydrated through an ascending series of ethanol (25% to 100%) and stored in a freon solution (Freon 123 solution; Sigma-Aldrich Corp). The surface morphology of the specimens was examined with a SEM (JSM2300; JEOL) at 350 and 1500 magnification.
RESULTS 1 Preparations of test materials. A, Cutting sequence followed to obtain slices. B, Interproximal slice trimming. C, Dumbbell-shaped specimen. D, Dentin. P, fiber or zirconia post; LC, luting cement.
Table II.
Microtensile bond strength results of all experimental groups
Mean Bond Strength (MPa) and Standard Deviation at: Experimental Group SEACþFP
30 d
30 dDTCDLC
13.9 3.8d
4.7 1.5a
SACþFP
9.9 2.9bcd
9.6 2.5bcd
SEACþZP
7.2 2.1
6.1 2.1ab
SACþZP
11.5 4.0cd
abc
7.4 2.4abc
Same superscript letters demonstrate no significant differences (P<.05). SEAC, self-etch adhesive cement; SAC, self-adhesive cement; TC, thermocycles; LC, load cycles.
(0.6 mm-NTI N889, Axis Dental Corp), and t is the thickness of the slice. The load at failure (N) was divided by the bonded surface area (mm2) to calculate bond strength (MPa). Statistical analyses were performed with commercially available software (SigmaPlot 12; Systat Software Inc). The mTBS data were analyzed with 3-way analysis of variance (ANOVA) and the Tukey honestly significant different tests.
Failure analysis After microtensile testing, the specimens were examined with a stereomicroscope (Olympus ZS 61; Olympus Corp) to determine the mode of fracture. Failure modes were classified as adhesive when between the resin cement and root dentin (RC-D), adhesive when between the resin cement and post (RC-P), mixed when the fracture was partially at the resin cement-root
dentin interface, or cohesive when within the resin cement, post, and/or root dentin.27
SEM specimen preparation Two roots were selected from each test group and prepared for SEM analysis. Three 2-mm-thick slabs were obtained from each root (1 apical, 1 middle, and 1 cervical). All of the slabs were embedded in epoxy resin (Leco Corp) and polished with 600-, 800-, and 1200-grit silicon carbide abrasive papers. For the high polish, they were submitted to 1 mm, 0.3 mm, and 0.05 mm alumina powder on a cloth. The specimens were immersed in a 10% phosphoric acid solution for 15 seconds, rinsed with water for 15 seconds, and then air dried. The specimens were treated with a 10% sodium hypochlorite solution for 30 seconds, rinsed thoroughly with water, and fixed in glutaraldehyde solution (pH 7.4) for 2 hours.
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The mean mTBS values and standard deviations of tested resin cements/ posts are listed in Table II. The post hoc Tukey test revealed that both the 30 days in distilled water and the TMA did not significantly affect the bonding of either FPs or ZPs to the root canal dentin, except for the FPs bonded with SEAC (P<.001). The FPs bonded with SEAC, group FP/SEAC, showed the highest bond strength values after 30 days in distilled water. However, bond strength values decreased dramatically after TMA application. When the ZP system was evaluated, the SAC exhibited higher bond strength values than the SEAC in the 30-day distilled water group. The results of the 3-way ANOVA of mTBS data are listed in Table III. Statistical analysis indicated that the type of resin cement, the post material, and aging significantly all adversely affected bond strength values (P<.001). A significant post material and resin cement interaction occurred after 30 days’ storage in distilled water (P<.001). However, no significant post material and resin cement interaction occurred after 30 days’ storage in distilled water and TMA (P¼.092). The regional bond strength values of the test groups are shown in Table IV. In the cervical and apical root regions, the best bonding performance was obtained with the FP group with SEAC before TMA (group FP/SEAC). However, the ZP group with ZP/SAC exhibited the best bonding performance in the middle region of root dentin. After the TMA procedure, the FPs with FP/ SEACþTMA presented the lowest bond strength values in both the cervical and apical root dentin regions.
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5 Three-way ANOVA results of microtensile bond strength test (split-
Table III. plot model)
Source of Variation
DF
SS
MS
F
P
Post material
1
26.772
26.772
2.862
.096
Cement
1
31.678
31.678
3.386
.071
TMA
1
193.813
193.813
20.717
<.001
Post materialCement
1
29.259
29.259
3.127
.082
Post materialTMA
1
11.794
11.794
1.261
.266
CementTMA
1
44.378
44.378
4.744
.034
Post materialCementTMA
1
164.331
164.331
17.565
<.001
ANOVA, analysis of variance; DF, degrees of freedom; SS, sum of squares; MS, mean of squares; F, F test statistic; P, probability; TMA, thermomechanical aging.
Table IV.
Mean microtensile bond strength of root dentin regions
Mean Bond Strength (MPa) and Standard Deviation for Root Dentin Region: Material
Cervical
SEACþFP
15.7 1.5de
SEACþFP (TCþLC)
3.7 1.1ab
6.8 2.3bcde
3.5 1.4a
SACþFP
11.9 1.7cde
9.8 3.8cde
7.9 3.3bcde
SACþFP (TCþLC)
9.1 5.1cde
8.9 4.2cde
6.3 1.7
6.2 2.1
SEACþZP
Middle
Apical
13.1 4.9cde
12.9 4.9cde
bcd
6.3 2.1bcde
SEACþZP (TCþLC) SACþZP
13.9 6.7de
SACþZP (TCþLC)
10.4 1.5cde
10.9 4.2cde 9.3 2.1cde
bcd
6.1 1.8bcde
6.8 3.1bc
15.9 4.9e
6.9 2.8bcde
8.5 1.2bcde
4.7 2.9abc
Same superscript letters demonstrate no significant differences (P<.05). SEAC, self-etch adhesive cement; SAC, self-adhesive cement; TC, thermocycle; LC, load cycle.
Table V.
Percentage of fracture modes of all experimental groups
30 d AF Experimental RC-P Group (%)
TCDLC CF
MF
RC-D C M (%) (%) (%)
AF RC-P (%)
CF MF
RC-D C M (%) (%) (%)
SEACþFP
91.75
8.25
X
X
80.35
5.88
X
11.76
SACþFP
90.25
9.75
X
X
81.48
11.25
3.12
6.25
SEACþZP
81.55
11.45
X
5
78.33
16.11
X
5.55
SACþZP
83.13
10.34
X
6.52
80.61
11.69
X
7.69
AF, adhesive failure; CF, cohesive failure; MF, mixed failure; RC-P, resin cement post; RC-D, resin cement-dentin; TC, thermocycle; LC, load cycle; SEAC, self-etch adhesive cement; SAC, self-adhesive cement; FP, fiber post; ZP, zirconia post.
Table V presents the percentage of failure modes in each group. The distribution of failures for each group was homogeneous, and failures were predominantly at the interface between the resin cement and the post.
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The formation of different resin tags and a clear hybrid layer were found in all test groups. After 30 days’ storage in water, the FP with SEAC (Fig. 2A, B) and the ZP with SEAC (Fig. 3A, B) groups exhibited uniform and different
size resin tags at the root dentin regions. However, the same type of aging in the FP with SAC (Fig. 2E, F) and ZP with SAC (Fig. 3E and 3F) groups exhibited a reduced number of short and thin resin tags in the root dentin. After TMA, the FP with SEAC (Fig. 2C, D) and the ZP with SEAC (Fig. 3C, D) groups showed broken and diverged resin tags as well as a deformation between the FPs and luting resin cements.
DISCUSSION This study investigated the mTBS of FPs and ZPs bonded with 2 different resin luting cements to root canal dentin with and without TMA. In light of the mTBS test results, the first and second null hypotheses of the study were partially accepted after TMA, because no significant differences in bond strength values were observed between the different luting resin cements, except for the groups with FP luted with SEAC. The lowest bond strengths were observed for FP luted with SEAC after TMA. The SEM results also confirmed that the last null hypothesis was rejected. Several studies have reported the retention of endodontic posts shortly after cementation without any simulation of oral conditions.13,14,16,31 However, clinically, the dislodgement of post-retained restorations occurs after some years of function as a result of such factors as temperature change and dynamic mechanical fatigue loading.1,26,27 Laurell and Lundgren32 reported that in prosthetically restored dentitions, the largest total force of masticatory strokes approached 80 N. To simulate the severe effects of the oral environment and to gain better knowledge of the materials’ behavior under adverse conditions, a storage time of 30 days at 37 C, thermal cycling, and cycling loading with 50 N parallel to the long axis of the posts were applied in this study. In an attempt to further simulate clinically relevant intraoral biomechanical conditions in a laboratory environment, the periodontal ligament was simulated
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2 Representative SEM images of FPs with SEAC and SAC (original magnification, 350 and 1500) A, B, Thick, long, and dense resin tags in SEAC group. C, D, After TMA procedure; broken resin tags, cracks in cement and dentin and separation between SEAC and FP. E, F, Hybrid zone is identified without visible resin tags with SAC group. G, H, Short, thin, and inconsistent resin tags and some cracks within SAC and also separation between SAC and FP after TMA. with a polyether-based molding material during cyclic loading. This artificial periodontal ligament functions as a less rigid and resilient liner, which absorbs part of the compressive forces, attenuating stress on the post and resin cement.30 Bond strength values reported in the current study are comparable to those found by other researchers. Zaitter et al25 evaluated and compared
2 different glass fiber posts and 4 different resin luting cements. Mean microtensile values for fiber posts luted with Panavia F2.0 were 10.3 MPa for Exacto and 25.9 MPa for everStick. For fiber posts luted with RelyX Unicem after 1000 thermocycles and then stored in distilled water at 37 C for 30 days, those values were 19.8 MPa for Exacto and 30.5 MPa for everStick. For
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FPs stored for 30 days in distilled water at 37 C, bond strength values were 13.9 MPa for FPs used with Panavia F2.0 and 9.9 MPa for FPs used with RelyX Unicem. The lower bond strength values can be attributed to the differences in both the post materials and number of thermal cycles. The greater number of thermal cycles (20 000 vs 1000) in the present study likely caused
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2 (continued) Representative SEM images of FPs with SEAC and SAC (original magnification, 350 and 1500) A, B, Thick, long, and dense resin tags in SEAC group. C, D, After TMA procedure; broken resin tags, cracks in cement and dentin and separation between SEAC and FP. E, F, Hybrid zone is identified without visible resin tags with SAC group. G, H, Short, thin, and inconsistent resin tags and some cracks within SAC and also separation between SAC and FP after TMA.
greater stress on the resin-bonding interface and therefore resulted in lower bond strength values. In this study, the bond strength values of FPs and ZPs to root canal dentin were different depending on which luting cement was used. The bonding performance of FPs with SEAC was better than that of ZPs with SEAC without TMA application. Similarly, another study evaluated the bond strengths of FPs and ZPs luted with various resin luting agents into the post spaces of extracted teeth. The ZP showed lower bond strength values when compared to the FP luted with SEAC, which is in agreement with the results of the current study.33 The authors found that SEAC had a lower bond strength than SAC with FP after TMA, which is consistent with another report by Bitter et al.27 These authors reported higher bond strengths with SAC (RelyX Unicem) compared to SEAC (Panavia F2.0) after the thermomechanical loading. The bonding mechanism of this self-adhesive cement is claimed to be based on micromechanical retention and chemical adhesion to hydroxapatite. The secondary chemical interaction may also be responsible for a higher tolerance to moisture and thermomechanical stress.19 It has also been confirmed that SAC (RelyX Unicem) has a high tolerance to moisture because water
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was formed during the neutralization reaction of phosphoric acid, methacrylate basic fillers, and hydroxyapatite.34 The authors also speculated that the performance of SEAC was influenced by the priming agent used together with the resin luting agent. The self-etching ED primer used with Panavia F2.0 is a 1-step adhesive. One-step self-etch adhesives are hydrophilic, as they contain high concentrations of both ionic and hydrophilic monomers. A large amount of water is added to these 1-step adhesives. It is therefore difficult to evaporate water from the adhesive solution. Even if evaporation is successful, water will rapidly diffuse back from the bonded dentin into the adhesive resin, creating permeable membranes and water movement across the adhesive layer, contributing to the degradation of the resin/dentin bond.35 The current study confirmed that although ZPs had lower bond strengths with SEAC and SAC after TMA application, the difference was not significant; it was quite significant with FPs and SEAC. This change in bond strength values can be explained by the fact that the outer surface of the ZPs of this study had a microporous surface, whereas the FPs have a factory smooth surface (according to the manufacturers’ product profiles). The increased surface roughness and surface area of the ZPs could have influenced the mechanical
retention of the luting cements, which contributes to the adhesive strength between the post and the cement after TMA. The adhesive luting systems demonstrated measurable adhesive properties in this study, with the highest values for the cervical third in the FPs with SEAC without the TMA, and the lowest values for the apical third in the FPs with SEAC after the TMA. These results can be associated with TMA as well as various other factors, including the less dense dentinal tubule configuration in the apical portion of the root canal system,36 apical sclerosis,37 the cavity configuration factor,38 the difficulty of visualization and access to the apical part of the root canal system, and the restrictions in the flow of the cement to this part of the root canal.39 In this study, 2 types of predominant failures were found: resin cementpost and resin cement-dentin adhesive failures. However, the predominant failure was between the resin cement and post for all experimental groups with or without TMA application. The weakest link seems to be the bond of the cements to the posts. Different studies support these observations. They showed significant debonding between ceramic posts and luting resin cement and observed failures at the resin cement-post interfaces.40,41
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3 Representative SEM images of ZPs with SEAC and SAC (original magnification, 350 and 1500). A, B, Distinctive hybrid zone and equal size resin tags. Separation between SEAC and ZP. C, D, Acceptable adaptation between SEAC and ZP after TMA application. However, separations were observed with broken resin tags between root dentin and SEAC. E, F, Various sized resin tags and hybrid zone were observed with SAC. Separation between SAC and ZP. G, H, Distinctive hybrid layer and few resin tags. Separation between SAC and ZP after TMA application. SEM analysis demonstrated that without TMA, the SEAC group showed different and long resin tag formations. Also, no inner cracks were observed in the luting cement for both types of posts. However, without TMA, inadequate resin penetration into the dentin tubules was observed in the SAC group. After TMA, some typical large
cracks were observed between the SEAC and root dentin with FPs. Specifically, these cracks extended from the root dentin to the fiber post surface, which may be associated with the reduced bond strength of SEAC in the FP group (Fig. 4). Walker et al42 investigated the mechanical property characterization of resin cement Panavia F2.0 after
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aqueous aging, with and without cyclic loading. The authors concluded that after aqueous aging with cyclic loading to simulate the clinical function of the resin cement, initial degradation may be related to the breakdown of the filler/resin interface bond. Such a breakdown was potentially reflective of a slow crack propagation that may
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3 (continued). Representative SEM images of ZPs with SEAC and SAC (original magnification, 350 and 1500). A, B, Distinctive hybrid zone and equal size resin tags. Separation between SEAC and ZP. C, D, Acceptable adaptation between SEAC and ZP after TMA application. However, separations were observed with broken resin tags between root dentin and SEAC. E, F, Various sized resin tags and hybrid zone were observed with SAC. Separation between SAC and ZP. G, H, Distinctive hybrid layer and few resin tags. Separation between SAC and ZP after TMA application.
4 Typical SEM images of FP with SEAC after TMA procedure (original magnification, 500 and 1000). A, Large separations were observed between luting cement and FP and root dentin. B, Some large cracks and gaps were found in cement and dentin. contribute to in vivo resin cement cohesive failure. Additionally, resin cements with greater filler content exhibit higher contraction stress, lower bond strength, and a more defective bonding interface.43 Panavia F2.0 has a higher filler content (59 vol% with a filler size of 19 mm) compared to RelyX Unicem (50 vol% with a filler size of 12 mm). Therefore, on the basis of the results of this study, increased and larger filler content may increase contraction stress within the material and cause cracks.
CONCLUSIONS The bond strength values and physical properties of SEAC with higher filler content were improved by the TMA
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application than those of SAC. TMA significantly reduced the bond strength values of FPs with SEAC. TMA also adversely affected the micromorphologic interface between the posts and the resin cements as well as between the resin cements and the root canal dentin.
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Corresponding author: Dr Batu Can Yaman Eskis¸ehir Osmangazi University Faculty of Dentistry Department of Operative Dentistry Meselik Kampus, Odunpazarı, 26480, Eskisehir TURKEY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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