Abstract
Objectives
For root canal fillings, a thin layer of sealer cement is generally recommended. However, with resin-based sealers, lower bond strength to dentin has been shown in thin layers compared to thick, contrary to typical behavior of adhesive layers between two adherents. The aim of this study was to evaluate tensile and shear bond strength of thin and thick films of three resin-based sealers (one epoxy-based and two methacrylate-based) materials and to investigate corner effects of one methacrylate-based resin sealer.
Methods
Freshly mixed sealer cements were placed between metal-to-metal surfaces of plano-parallel stainless steel aligned rods with diameter 4.7 mm. Ten samples were prepared for each type, thickness (0.1 and 1.0 mm) of sealer and test. Tensile and shear strengths were measured after 48 h for the methacrylate-based materials and after 7 days for the epoxy-based material using a universal testing machine at a crosshead speed of 1 mm/min. Corner effects were investigated using one methacrylate-based resin material.
Results
Film thickness had a highly significant influence on both tensile and shear strengths. For methacrylate resin-based sealers, thin films had higher bond strength than thick ( p < 0.001 for both tensile and shear bond strength). With the epoxy-based sealer either no difference (shear) or lower bond strength in thin films (tensile; p < 0.05) was found, and appeared to result from numerous voids created during mixing. The methacrylate based sealer demonstrated typical engineering behavior for an adhesive material, with corner effects shown as a material property and in good agreement with the tensile bond strength results.
Significance
The higher tensile and shear bond strength of resin-based sealer in thin films is the opposite of that previously reported for bonding to dentin. The substrate clearly has an important role in failure behavior.
1
Introduction
Most root canal sealer cements shrink during setting, leaving gaps that potentially serve as pathways for leakage . Sealers also gradually dissolve in tissue fluid. For these reasons, most root canal obturation techniques have been developed to minimize the thickness of the sealer, in order to enhance the sealing ability of the root filling. Thin layers leak less than thick in experimental studies of most types of sealers. However, some studies have shown that epoxy-based sealer leaked less in thick layers or did not show significant differences between thin and thick layers . Of the resin-based sealers, the epoxy-based type has been shown to produce higher bond strength to root dentin compared with both non resin-based and methacrylate resin-based sealers . The possibility of better sealing, bonding and root strengthening with the use of methacrylate-based sealers using adhesive technologies, however, has not resulted in the anticipated benefits .
Adhesives are also used in thin layers in luting cements (for posts, crowns and other indirect restorations). Characteristics of adhesion of thin layers have received little attention in dentistry. Resin-based endodontic sealers have been found to show higher bond strength to dentin in thick than in thin layers, using both push-out and micro-shear bond tests . The suggested explanation for the effect was based on the extensive penetration of sealers into dentinal tubules. Because filler particles tend to be too large to enter tubules, the resin matrix material is selectively drawn into the tubules, leaving a particle-enriched but resin-depleted layer on the canal wall when the cement layer is thin. In contrast, resin luting cements used for cementing fiber posts in root canals showed no difference in bond strength in relation to thickness or a variable affect of increasing, and then decreasing strength with increasing thickness .
The behavior of resin composite in different thicknesses bonded between two parallel impervious metal plates was studied by Alster et al. They found that tensile bond strength was inversely related to the thickness , in contrast to the finding with sealer cements when the dentin serves as the bonding surface . At about the same time, an engineering study of adhesively bonded butt joints also documented that bond strength was inversely related to thickness of the adhesive layer. The authors attributed the effect to interface corner stresses (‘corner effects’) which are influenced by the geometry of the test system. Neves et al. reported the same phenomenon using finite element modeling of adhesion of resin composites to dentin, although they modeled dentin as an impervious adherent surface .
This study was undertaken to investigate bond strength in relation to film thickness of endodontic sealer cements, when bonding occurred between impervious metal surfaces rather than in contact with dentin. Both epoxy resin-based and methacrylate resin-based sealers were evaluated in the first part of the study. The null hypothesis tested that tensile and shear bond strength of resin-based sealer cements were not affected by film thickness. In the second part of the study, the impact of corner effects was investigated more systematically using one methacrylate resin-based cement.
2
Materials and methods
2.1
Sealer cements
Three resin-based sealer cements were used in this study ( Table 1 ), one epoxy resin-based sealer (AH Plus ® ) and two methacrylate resin-based sealers (EndoREZ ® and RealSeal ® ). Each sealer cement was mixed according to the manufacturer’s instructions. AH Plus was mixed using AH Plus Jet ® mixing system, EndoREZ with an Ultra-Mixer ® and RealSeal was mixed with an auto-mix syringe. The mixed sealer was expressed onto a mixing pad, and the initially extruded material was discarded to ensure uniform mixing of the materials.
| Sealer cement | Manufacturer | Batch | Composition | |||||
|---|---|---|---|---|---|---|---|---|
| AH Plus | Dentsply/Maillefer DeTrey, Konstanz, Germany | LOT 0905004007 | Epoxide paste Bisphenol-A-epoxy resin Bisphenol-F-epoxy resin Calcium tungstate Iron oxide pigments Zirconium oxide Aerosil Pigment |
Amine Paste I-adamantane amine N,N′-dibenzyl-5-oxa-nonandiamine-1,9 Aerosil Tricyclodecane-diamine Calcium tungstate Zirconium oxide Silica Silicone oil |
||||
| EndoREZ | Ultradent Product, South Jordan, UT | LOT B47F2 | Part 1 | Part 2 | ||||
| Resin | Fillers | Dual-cured initiators | Resin | Fillers | Dual-cured initiators | |||
| TEGDMA | Bismuth oxychloride | benzoyl peroxide | TEGDMA | Bismuth oxychloride | p -tolyimino diethanol | |||
| Diurethane dimethylacrylate | Calcium lactate pentahydrate | Diureathane dimethacrylate | Calcium lactate pentahydrate | Phenyl bis (2,4,6-trimethyl benzoyl) phosphate oxide | ||||
| Bisglycerol dimethacrylate phosphate | Silica | Bisglycerol dimethacrylate phosphate | Silica | |||||
| RealSeal | Sybron Endo, Ormco Corporation, Orange, CA | LOT 172286 | Resins PEGDMA EBPADMA UDMA BisGMA |
Amines Silane-treated barium borosilicate glasses Barium sulphate Silica Calcium hydroxide Bismuth oxychloride with amines Peroxide Photoinitiator Stabilizers Pigments |
||||
2.2
Test system
Both tensile and shear bond strengths were measured using stainless steel rods in pairs, with sealer cement placed between the ends of concentrically mounted rods with a pre-set gap (0.1 or 1.0 mm). The setup consisted of 10 pairs of cylindrically shaped stainless steel rods, 5 cm long and with a diameter of 4.7 mm. The rods were milled to produce an accurate plano-parallel bonding surface perpendicular to the long axis of the rod. Before bonding procedures, the surfaces were sandblasted with alumina particles (average grain size 50 μm) for 3 s from a 3 cm distance perpendicular to the surface. All surfaces then were cleaned with acetone and dried for 10 min.
An adjustable alignment device was used to mount and secure the rods in a precisely aligned position. Spacers were used to establish a precise gap between the rods, which were then fixed in position. The rods were then separated, the bonding surface of each rod was coated with sealer cement and then the rods were repositioned at the pre-measured thickness. Excess sealer was carefully removed with a cement spatula. For the dual-cured methacrylate resin-based sealers, no light curing procedure was performed and all samples were kept in a nitrogen chamber for 2 h to prevent oxygen inhibition of polymerization. All samples were stored in a 37 °C incubator with 95% humidity for 48 h before testing, except for the epoxy-based sealer, for which all specimens were stored for 7 days before testing. A pilot study found that maximum strength was not achieved until this time.
Before the bond strength tests, any excess cement around the periphery was carefully removed with a scalpel blade. A magnified image of the material thickness was recorded at 16× magnification (Leica DMEP Microsystem, Wetzlar GmbH, Germany) and the thickness of the layer between the stainless steel rods was measured using Image Tool software (UTHSCSA Image Tool for Windows version 3.00) after calibration. Both tensile and shear testing were performed without removing the specimens from the alignment device to avoid premature failure.
For tensile tests, each sample was carefully mounted in a universal testing machine (Instron model 5544, Instron Corp, Canton, MA, USA) with one end held rigidly and the other attached to a universal joint to prevent any shear component during testing. For shear tests, one rod was precisely positioned (under magnification) in a metal holder at the junction between the sealer cement and rod surface, with the rod at right angles to the load direction. A shear load was applied using a blade with a circular aperture placed over the sealer–rod interface. Specimens were subjected to a tensile or shear load at a constant cross-head speed of 1 mm per minute. The force ( N ) required to fracture the bond was recorded and used to calculate the bond strength (MPa).
Modes of failure were evaluated visually and with light microscopy at 16× magnification. After the tests, impressions of the fractured surfaces were taken with polyvinyl siloxane impression material (Elite ® HD+, Zhermack Clinical, Zhermack SpA, Italy). Epoxy resin replicas (Epofix™, Struers, Copenhagen) were prepared, mounted on stubs, sputter coated with gold and examined using a scanning electron microscope (Quanta FEG SEM, FEI Co, Oregon, USA).
For data analysis, the level of significance was 5%. Two-way ANOVA was conducted separately for the tensile and shear bond data, with post hoc tests (Bonferroni test) comparing thick vs. thin films for each sealer type.
2.3
Investigation of corner effects
Additional samples using only one methacrylate resin-based material (EndoREZ ® ) were subjected to tensile testing, using a wider range of film thicknesses (0.05–1.3 mm). The experimental set-up was identical to that in the first part of the study, with the gap pre-set using spacers of appropriate thickness. The interface corner stress intensity factor ( K f ) and interface corner fracture toughness ( I – C K fc ) were calculated according to Reedy and Guess, using literature values for Young’s modulus and Poisson’s ratio for methacrylate resin .
To confirm whether interface corner stress occurred as predicted, a calculation was done based on Reedy and Guess . Firstly the I – C K fc was calculated with parameter Poisson’s ratio of 0.4 for polymethyl methacrylate resin material and based on the Poisson’s ratio value, the stress singularity ( λ − 1) was selected .
2
Materials and methods
2.1
Sealer cements
Three resin-based sealer cements were used in this study ( Table 1 ), one epoxy resin-based sealer (AH Plus ® ) and two methacrylate resin-based sealers (EndoREZ ® and RealSeal ® ). Each sealer cement was mixed according to the manufacturer’s instructions. AH Plus was mixed using AH Plus Jet ® mixing system, EndoREZ with an Ultra-Mixer ® and RealSeal was mixed with an auto-mix syringe. The mixed sealer was expressed onto a mixing pad, and the initially extruded material was discarded to ensure uniform mixing of the materials.
| Sealer cement | Manufacturer | Batch | Composition | |||||
|---|---|---|---|---|---|---|---|---|
| AH Plus | Dentsply/Maillefer DeTrey, Konstanz, Germany | LOT 0905004007 | Epoxide paste Bisphenol-A-epoxy resin Bisphenol-F-epoxy resin Calcium tungstate Iron oxide pigments Zirconium oxide Aerosil Pigment |
Amine Paste I-adamantane amine N,N′-dibenzyl-5-oxa-nonandiamine-1,9 Aerosil Tricyclodecane-diamine Calcium tungstate Zirconium oxide Silica Silicone oil |
||||
| EndoREZ | Ultradent Product, South Jordan, UT | LOT B47F2 | Part 1 | Part 2 | ||||
| Resin | Fillers | Dual-cured initiators | Resin | Fillers | Dual-cured initiators | |||
| TEGDMA | Bismuth oxychloride | benzoyl peroxide | TEGDMA | Bismuth oxychloride | p -tolyimino diethanol | |||
| Diurethane dimethylacrylate | Calcium lactate pentahydrate | Diureathane dimethacrylate | Calcium lactate pentahydrate | Phenyl bis (2,4,6-trimethyl benzoyl) phosphate oxide | ||||
| Bisglycerol dimethacrylate phosphate | Silica | Bisglycerol dimethacrylate phosphate | Silica | |||||
| RealSeal | Sybron Endo, Ormco Corporation, Orange, CA | LOT 172286 | Resins PEGDMA EBPADMA UDMA BisGMA |
Amines Silane-treated barium borosilicate glasses Barium sulphate Silica Calcium hydroxide Bismuth oxychloride with amines Peroxide Photoinitiator Stabilizers Pigments |
||||
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