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Aust Endod J 2015; 41: 128–134
ORIGINAL RESEARCH
Thin and thick layers of resin-based sealer cement bonded to root dentine compared: Adhesive behaviour Epita S. Pane, BDs PhD; Joseph E.A. Palamara, PhD; and Harold H. Messer, MDSc PhD Melbourne Dental School, The University of Melbourne, Melbourne, Victoria, Australia
Keywords root canal sealer, shear bond strength, tensile bond strength, thickness. Correspondence Professor Harold H. Messer, Melbourne Dental School, The University of Melbourne, 720 Swanston Street, Carlton, Melbourne, Vic. 3010, Australia. Email: [email protected] doi:10.1111/aej.12113
Abstract This study aims to evaluate tensile and shear bond strengths of one epoxy (AH) and two methacrylate resin-based sealers (EZ and RS) in thin and thick layers bonded to root dentine. An alignment device was prepared for accurate positioning of 20 root dentine cylinders in a predefined gap of 0.1 or 1 mm. Sealer was placed in the interface. Bond strength tests were conducted. Mode of failures and representative surfaces were evaluated. Data were analysed using ANOVA and post-hoc tests, with P < 0.05. The thick layer of sealer produced higher bond strength, except for the shear bond strength of EZ. Significant differences between thin and thick layers were found only in tensile bond strengths of AH and RS. Mixed type of failure was constantly found with all sealers. Bond strengths of thick layers of resin-based sealers to root dentine tended to be higher than with thin layers.
Introduction Sealer cement acts as a ‘joint’ between radicular dentine and root canal filling materials (1). Therefore sealer cement needs to have good adhesive properties (2) to eliminate any space that allows percolation of fluid into the canal system (3), to resist disruption during intraoral tooth flexure (4), to resist dislodgment of the filling during restoration manipulation (5) and to stabilise the apical seal during post-space preparation (6). Although bond strength testing may not be a strong predictor of the clinical behaviour of sealer cements (7), bond strength testing is the best measure of adhesion currently available (3). Among the different types of root canal sealer cements, adhesion of resin-based sealer cements to radicular dentine has been shown to be superior to that of other types of sealer (3,8,9). The epoxy resin-based sealers produced higher bond strength and less leakage compared with the non-resin-based sealers (5,8). The methacrylate resin-based sealer cements with dentine bonding technology, on the contrary, have been shown not to produce better sealing and bonding properties than epoxy resinbased sealers (10). 128
Clinically, the use of a thin layer of sealer in root canal filling is recommended to minimise dissolution when in contact with tissue (11). However, bond strength to dentine tends to be lower with a thin layer of sealer compared with a thick layer (12). Contradictory results have been reported with dentine bonding agents used with restorative materials. The bond strength increased significantly as the layer thickness increased (13,14), did not differ significantly (15) or was considered unrelated to sealer thickness (16). Although the influence of thickness on bond strength of resin-based sealer cements has been shown, the benefit of thin versus thick sealer layer in promoting adhesion to root dentine is still uncertain. The aim of this study was to evaluate tensile and shear bond strengths of resin-based sealer cements in different thicknesses bonded to root dentine. It was hypothesised that tensile and shear bond strength of resin-based sealer cements increase as the thickness of the materials increases. In a previous study, the bond strength between two impervious steel plates was greater with thin than with thick layers (17), which is a function of the geometry of the samples. In this study, bonding to dentine, with the presence of tubules rather than an impervious smooth surface, was evaluated.
© 2015 Australian Society of Endodontology
Resin-based sealer cements
E. S. Pane et al.
Materials and methods The sealer cements used were an epoxy resin-based material (AH Plus™, Dentsply/Maillefer DeTrey, Konstanz, Germany) (AH) and two methacrylate resin-based materials (EndoREZ®, Ultradent Products, South Jordan, UT, USA (EZ), and RealSeal™, SybronEndo, Orange, CA, USA (RS)).
Specimen preparation Sixty extracted single-rooted mandibular premolars were used. All teeth were collected with ethics approval by the institutional Human Ethics Advisory Group (Ethics ID: 1033306). Each tooth was embedded in resin (EpoFix™ Cold Setting, Batch No. 1123-2188, Struers, Ballerup, Denmark) and cut vertically in a bucco-lingual direction to produce two root halves, using a 0.3 mm thick sintered diamond wafering blade (Struers). A bonding area of 2 mm diameter centred over the canal space and an outer area for mounting of 8 mm diameter in the cervical coronal part of the root were made with water-cooled trephine bur (Hu-Friedy, Rockwell St, Chicago, IL, USA). The surface was cleaned with an ultrasonic cleaning system (L&R®, Kearny, NJ, USA) and polished with 800 grit silicon carbide paper and treated with 15% ethylenediaminetetraacetic acid (EDTA) and then 1% NaOCl for 5 min followed by distilled water to remove smear layer before testing. A special alignment device was prepared for accurate positioning of the dentine cylinder (Fig. 1). The prepared dentine cylinder was positioned concentrically with a 2 mm stainless steel rod fixed in a plano-parallel orientation. A predefined gap of 0.1 or 1.0 mm between the tooth surface and the rod was established. Freshly mixed sealer was placed in the interface and contained within the space with a 2 mm diameter silicone boot. Twenty samples of thin and thick layers were prepared for each of the sealer cement. For the methacrylate resin-based sealers, all samples were kept in a nitrogen chamber for 2 h to prevent oxygen inhibition of setting and all rods were stored in a 37°C incubator with 95% relative humidity for 48 h before testing. For the epoxy-based sealer, all samples were kept at 37°C and 95% relative humidity for 7 days before testing.
Tensile and shear bond tests Microtensile and shear bond tests were conducted using a universal testing machine (Mecmesin, Imperial 1000, West Sussex, UK) without removing the specimens from the alignment device to prevent premature failure. For the tensile test, a wire loop was placed around a prepared hook at the end of the stainless steel rod. For the shear
© 2015 Australian Society of Endodontology
Figure 1 (a) Alignment device for accurate positioning of dentine cylinder concentric with parallel stainless steel rod surface. The movable part can be positioned and fixed based on predetermined thickness. (b) Stainless steel rods and dentine cylinder in the slots. Sealer cement is placed between the rod and dentine surfaces. The silicone boot used to confine the sealer within the gap during setting has been removed.
test, a metal loop was placed around the specimen adjacent to the dentine surface. A ramped load was applied until fracture occurred. The force (N) required to fracture the bond was recorded and used to calculate the bond strength (MPa).
Mode of failure Mode of failure was evaluated visually and with light microscopy (Leica DML, Ernst-Leitz-Strasse, Wetzler, Germany) at 20× magnification. Mode of failure was classified as cohesive within the sealer, adhesive at the dentine–sealer interface, or mixed within the sealer and at the interfaces. Adhesive failure at the sealer–stainless steel interface was not observed.
Scanning electron microscopy After the tests, representative dentine surfaces from thin and thick specimens were examined. The samples were 129
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Figure 2 Tensile and shear bond strength of thin and thick layers of resin-based sealer cements. Mean tensile bond strengths of thin and thick layers of AH are 1.59 ± 0.9 and 4.73 ± 2.25 MPa; EZ 1.28 ± 0.7 and 2.18 ± 2.05 MPa, RS 1.94 ± 1.21 and 3.77 ± 1.88 MPa. Mean shear bond strengths of thin and thick layers of AH are 8.9 ± 4.7 and 11.67 ± 3.27 MPa, EZ 4.65 ± 3.6 and 2.87 ± 1.26 MPa, RS 7.21 ± 1.89 and 8.25 ± 3.84 MPa. The boxplots showed the outliers and distribution of data with 25%, 50% and 75% lower, median and upper quartiles.
mounted on stubs with the dentine–sealer interface uppermost, coated with gold and then examined using scanning electron microscopy (Quanta FEG SEM, FEI Co., Hillsboro, Oregon, USA). With the thick layers of sealer cement, it was also possible to examine the sealer side of the interface by removing the sealer from the stainless steel rod and mounting it on a stub.
Statistical evaluation The data were analysed separately for the tensile and shear data. A two-way ANOVA (SPSS Inc., Statistic 17.0, Armonk, New York, USA) was used to examine the effect of material and thickness on bond strengths with Tukey’s post-hoc test. The minimum level for statistical significance was P < 0.05.
Results Bond strengths are shown in Figure 2. Tensile strength was significantly influenced by both material (P = 0.000) and sealer thickness (P = 0.017), with no statistically significant interaction between materials and thickness. Shear bond strength was significantly influenced by material (P = 0.000) but not by thickness (P > 0.05), with no interaction between the two. In both tensile and shear tests, the epoxy resin-based sealer had the highest strength, followed by the two methacrylate resin-based sealers. 130
For each material, the thick layer of sealer produced a higher bond strength to root dentine compared with the thin layer, except for the shear bond strength of EZ which was the opposite (Fig. 2). Statistically, pairwise comparisons showed significant differences between the thin and thick layers of each sealer only in the tensile bond strength of AH and RS (P = 0.001 and P = 0.019). Mixed type of failure was consistently found for the three different sealer cements with both thicknesses (Fig. 3). Failure occurred adhesively at the dentine – sealer interface and cohesively within the sealer layer. Any area of adhesive failure to the metal surface was rarely found, and no evidence of cohesive failure within dentine was seen. Exclusively adhesive or cohesive failure throughout the entire interface was not observed. Higher magnification of the dentine–sealer interface showed a gritty layer at the material surface, consisting predominantly of filler particles with little surrounding resin matrix (Fig. 3). This gritty appearance of sealer layer was a typical finding in all materials and thickness bonded to root dentine. In Figure 4, the comparisons between images of fractured thick and thin materials bonded to root dentine are shown. In thick layer, the filler particles appeared to be completely surrounded by resin matrix, while the thin layer was characterised by loose filler particles with sparse supporting resin matrix. Areas of adhesive failure at the sealer–dentine interface showed pulled
© 2015 Australian Society of Endodontology
E. S. Pane et al.
Figure 3 Scanning electron microscope images of the thin layer of AH. (a) is a 100× original magnification of the fractured dentine surfaces and (b) is a 2000× original magnification of the surfaces. Apparently adhesive failure at light microscope level shows a thin coating of sealer on the dentine surface. Higher magnification of supposedly dentine–sealer adhesive failure showed a gritty thin layer of material with visible dentinal tubules.
and stretched resin tags emerging from dentinal tubule orifices (Fig. 5).
Discussion The compressive axial and non-axial occlusal stresses that occur during normal oral function are transmitted to the root area of a tooth and can generate both tensile and shear stresses at the interface between the root filling and
© 2015 Australian Society of Endodontology
Resin-based sealer cements
the canal wall. This effect of Poisson’s ratio at the interface between dentine and the root canal filling materials will produce complex stresses in the root-filled area, potentially resulting in bond failure (18).Tensile and shear bond testing methods have been used to evaluate adhesive bond strength of materials in root canals, in addition to the more ‘functional’ push-out test (19). The tensile test was used in this study because it allows better evaluation of bond failure (20) plus the use of a flat dentine surface with better accessibility to the working area (21). The tensile test has been accepted to produce more uniform stress (22) but is very sensitive and depends on correct axial alignment during specimen preparation and load application (23). The shear bond test was selected for the reason that it is considered a more reliable and reproducible method (5) and because of the simplicity of the test (24). The test is strongly influenced by bonding geometry, loading condition and differences in the elasticity of the materials (25). Only a few tensile bond studies (3,8,9,26) and shear test studies (5,27,28) have been conducted to evaluate root canal sealer cements bonded to root dentine. The test has been conducted with coronal dentine (3,5) using larger diameter specimens (3.7–6.4 mm) (8,9). Premature failure of the specimens was a problem encountered in previous tensile (21,26) and shear (28) studies. The method used in the present tensile and shear bond tests investigation was very successful in preventing premature failure since no manipulation of the specimens was required during the test set-up. However, it was difficult to eliminate occasional slight misalignment between the dentine cylinder and metal surfaces with the use of a small interface (2 mm). Failure mode assessment of fractured surfaces has been used to evaluate the nature and location of bond failure. However, categorising it as adhesive, cohesive and mixed oversimplifies what is actually a very complex fracture pattern (29). With the three different sealers used in this study, all fractured surfaces resulted in a complex pattern of failure (Fig. 3): both adhesive at the sealer–dentine interface and cohesive within the sealer cement. Failure predominantly at the dentine–sealer interface showed that the bonding property of the resin-based sealers was inferior to the dentine–metal substrate interface. A characteristic feature of adhesive failure to coronal dentine exposed to NaOCl and EDTA is a non-mineralised collapsed surface with numerous lateral branches of dentinal tubules, empty dentinal tubules with resin tags being pulled out and partially dissolved peritubular dentine (26). However, that study did not examine root dentine and only a thick layer of sealer was applied (26). With the same sealer, the present study investigating both thin and thick layers of sealer bonded to root dentine produced an 131
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Figure 4 (1a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of AH sealer cement. In thick layer, abundant resin matrix surrounded the filler particles. (1b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thin layer of AH sealer cement bonded to root dentine. In thin layer, much less resin surrounding the filler was noted. (2a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of EZ sealer cement. Abundant resin matrix surrounded the filler particles. (2b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces of EZ when bonded to root dentine. Much less resin surrounding the filler was noted. (3a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of RS sealer cement. Abundant resin matrix surrounded the filler particles. (3b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of RS bonded to root dentine. Much less resin surrounding the filler was noted.
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hollow rather than solid. Cohesive failure within the sealer cement often demonstrated only a sparse resin matrix surrounding the filler particles (Fig. 5). Previous studies have used a thin layer of epoxy resinbased sealer applied between dentine and gutta-percha to compare it with other conventional sealers. Higher tensile bond strength (8,9) and higher shear bond strength (5) were found with epoxy resin-based sealers (3,10–12). Shear bond testing has been performed with both thin (30) and thick layers (5,30), with higher values reported for thick layers (5,30). With push-out tests, it was found that a thick layer of resin-based sealers produced higher bond strength than thin layers (12). It was proposed (12) that the resin matrix of the sealer penetrated extensively into dentinal tubules in root dentine, leaving a resindepleted interface rich with filler particles, which caused poorer adherence. This present study consistently showed a layer consisting predominantly of loose filler particles not supported by resin matrix (Fig. 4b), with a very different appearance compared with fractured bulk material which showed filler particles embedded in abundant resin matrix (Fig. 4a). Although the bond strength of a sealer cement may not be as important as bacterial control in terms of the outcome of root canal treatment, intimate contact of the sealer with dentine and penetration into dentinal tubules is still important. Bond strength is a surrogate for assessing the quality of the lateral seal of a root canal filling. The tendency for thicker layers of sealer cement to create a stronger bond than thin layers is consistent with the results of push-out testing (19) and suggests that there may be some advantage to the increased thickness of the sealer cement.
Acknowledgements Figure 5 Scanning electron microscope (SEM) images of fractured surfaces of thick layer of sealer. (a) SEM image with 2000× original magnification showed numerous resin tags protruding from dentinal tubules. (b) SEM image with 5000× original magnification showed the hollow appearance of some tags. This feature was noticed in all sealers. The tags seemed to be partially pulled or stretched away from the dentinal tubules, explaining the apparently empty tubules in some areas. Some tags seemed to be not bonded to the tubule walls.
irregular surface (gritty) (Figs 3,4). Some dentinal tubules appeared empty, but many were filled or covered with various sizes and forms of filler (Fig. 4). On the dentine surface, resin tags seemed to be found only when a thick layer of sealer was applied, and tags were seen partially extruded from the tubules and were often
© 2015 Australian Society of Endodontology
This research was supported by Post-Graduate Research Grant, Australian Society of Endodontics and Postgraduate Research Fund of Melbourne Dental School, University of Melbourne. We would like to thank Mr. Roger Curtain for assisting in the use of scanning electron microscopy in Advanced Microscopy Facility, Bio21. We would like to thank Ms. Rachel Sore, Statistical Consulting Centre, University of Melbourne, for the statistical consultation.
References 1. Huffman BP, Mai S, Pinna L et al. Dislocation resistance of ProRoot Endo Sealer, a calcium silicate-based root canal sealer, from radicular dentine. Int Endod J 2009; 42: 34–46.
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2. Branstetter J, von Fraunhofer JA. The physical properties and sealing action of endodontic sealer cements: a review of the literature. J Endod 1982; 8: 312–16. 3. Orstavik D, Eriksen HM, Beyer-Olsen EM. Adhesive properties and leakage of four root canal sealers in vitro. Int Endod J 1983; 16: 59–63. 4. Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod 1995; 21: 57–61. 5. Tagger M, Tagger E, Tjan AHL, Bakland LK. Measurement of adhesion of endodontic sealers to dentin. J Endod 2002; 28: 351–4. 6. Saunders EM, Saunders WP, Rashid MY. The effect of post space preparation on the apical seal of root fillings using chemically adhesive materials. Int Endod J 1991; 24: 51–7. 7. Sudsangiam S, van Noort R. Do dentin bond strength tests serve a useful purpose? J Adhes Dent 1999; 1: 57–67. 8. Wennberg A, Orstavik D. Adhesion of root-canal sealers to bovine dentin and gutta-percha. Int Endod J 1990; 23: 13–19. 9. Saleh IM, Ruyter IE, Haapasalo M, Orstavik D. The effects of dentine pretreatment on the adhesion of rootcanal sealers. Int Endod J 2002; 35 (10): 859–66. 10. Schwartz RS. Adhesive dentistry and endodontics. Part 2: bonding in the root canal system – the promise and the problems: a review. J Endod 2006; 32 (12): 1125– 34. 11. Kontakiotis EG, Wu MK, Wesselink PR. Effect of sealer thickness on long term sealing ability: a 2 year follow up study. Int Endod J 1997; 30: 307–12. 12. Jainaen A, Palamara JEA, Messer HH. Push-out bond strengths of the dentine-sealer interface with and without a main cone. Int Endod J 2007; 40 (11): 882– 90. 13. Zheng L, Pereira PNR, Nakajima M, Sano H, Tagami J. Relationship between adhesive thickness and microtensile bond strength. Oper Dent 2001; 26: 97–104. 14. D’Arcangelo C, Vanini L, Prosperi GD et al. The influence of adhesive thickness on the microtensile bond strength of three adhesive systems. J Adhes Dent 2009; 11: 109– 15. 15. Coelho PG, Calamia JM, Harsono M, Thompson VP, Silva NRFA. Laboratory and FEA evaluation of dentinto-composite bonding as a function adhesive layer thickness. Dent Mater 2008; 24: 1297–303. 16. Pazinatto FB, Atta MT. Influence of differently oriented dentin surfaces and the regional variation of specimens
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on adhesive layer thickness and bond strength. J Esthet Restor Dent 2008; 20: 119–28. Pane ES, Palamara JEA, Messer HH. Behavior of resinbased endodontic sealer cements in thin and thick films. Dent Mater 2012; 28: e150–9. Kohn DH. Mechanical properties. In: Craig RG, Powers JM, eds. Restorative dental materials. 11th ed. St. Louis, MI: Mosby, Inc; 2002. p. 78. Pane ES, Palamara JEA, Messer HH. Critical evaluation of the push-out test for root filling materials. J Endod 2013; 39: 669–73. Pashley DH, Carvalho RM, Sano H et al. The microtensile bond test: a review. J Adhes Dent 1999; 1: 299–309. 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. Pashley DH, Sano H, Ciucchi B, Yoshiyama M, Carvalho RM. Adhesion testing of dentin bonding agents: a review. Dent Mater 1995; 11: 117–25. Pashley DH, Ciucchi B, Sano H, Carvalho RM, Russell CM. Bond strength versus dentine structure: a modelling approach. Arch Oral Biol 1995; 40 (12): 1109–18. Versluis A, Tantbirojn D, Douglas WH. Why do shear bond tests pull out dentin? J Dent Res 1997; 76: 1298– 307. Van Noort R, Cardew GE, Howard IC, Noroozi S. The effect of local interfacial geometry on the measurement of the tensile bond strength to dentin. J Dent Res 1991; 70: 889–93. Doyle MD, Loushine RJ, Agee KA et al. Improving the performance of EndoRez root canal sealer with a dualcured two-step self-etch adhesive. I. Adhesive strength to dentin. J Endod 2006; 32: 766–70. Gogos C, Economides N, Stavrianos C, Kolokouris L, Kokorikos I. Adhesion of a new methacrylate resinbased sealer to human dentin. J Endod 2004; 30: 238– 40. Hiraishi N, Lousbine RJ, Vano M et al. Is an oxygen inhibited layer required for bonding of resin-coated gutta-percha to a methacrylate-based root canal sealer? J Endod 2006; 32: 429–33. Dickens SH, Milos MF. Relationship of dentin shear bond strengths to different laboratory test designs. Am J Dent 2002; 15: 185–92. Rahimi M, Jainaen A, Parashos P, Messer HH. Bonding of resin-based sealers to root dentin. J Endod 2009; 35: 121–4.
© 2015 Australian Society of Endodontology
Aust Endod J 2015; 41: 128–134
ORIGINAL RESEARCH
Thin and thick layers of resin-based sealer cement bonded to root dentine compared: Adhesive behaviour Epita S. Pane, BDs PhD; Joseph E.A. Palamara, PhD; and Harold H. Messer, MDSc PhD Melbourne Dental School, The University of Melbourne, Melbourne, Victoria, Australia
Keywords root canal sealer, shear bond strength, tensile bond strength, thickness. Correspondence Professor Harold H. Messer, Melbourne Dental School, The University of Melbourne, 720 Swanston Street, Carlton, Melbourne, Vic. 3010, Australia. Email: [email protected] doi:10.1111/aej.12113
Abstract This study aims to evaluate tensile and shear bond strengths of one epoxy (AH) and two methacrylate resin-based sealers (EZ and RS) in thin and thick layers bonded to root dentine. An alignment device was prepared for accurate positioning of 20 root dentine cylinders in a predefined gap of 0.1 or 1 mm. Sealer was placed in the interface. Bond strength tests were conducted. Mode of failures and representative surfaces were evaluated. Data were analysed using ANOVA and post-hoc tests, with P < 0.05. The thick layer of sealer produced higher bond strength, except for the shear bond strength of EZ. Significant differences between thin and thick layers were found only in tensile bond strengths of AH and RS. Mixed type of failure was constantly found with all sealers. Bond strengths of thick layers of resin-based sealers to root dentine tended to be higher than with thin layers.
Introduction Sealer cement acts as a ‘joint’ between radicular dentine and root canal filling materials (1). Therefore sealer cement needs to have good adhesive properties (2) to eliminate any space that allows percolation of fluid into the canal system (3), to resist disruption during intraoral tooth flexure (4), to resist dislodgment of the filling during restoration manipulation (5) and to stabilise the apical seal during post-space preparation (6). Although bond strength testing may not be a strong predictor of the clinical behaviour of sealer cements (7), bond strength testing is the best measure of adhesion currently available (3). Among the different types of root canal sealer cements, adhesion of resin-based sealer cements to radicular dentine has been shown to be superior to that of other types of sealer (3,8,9). The epoxy resin-based sealers produced higher bond strength and less leakage compared with the non-resin-based sealers (5,8). The methacrylate resin-based sealer cements with dentine bonding technology, on the contrary, have been shown not to produce better sealing and bonding properties than epoxy resinbased sealers (10). 128
Clinically, the use of a thin layer of sealer in root canal filling is recommended to minimise dissolution when in contact with tissue (11). However, bond strength to dentine tends to be lower with a thin layer of sealer compared with a thick layer (12). Contradictory results have been reported with dentine bonding agents used with restorative materials. The bond strength increased significantly as the layer thickness increased (13,14), did not differ significantly (15) or was considered unrelated to sealer thickness (16). Although the influence of thickness on bond strength of resin-based sealer cements has been shown, the benefit of thin versus thick sealer layer in promoting adhesion to root dentine is still uncertain. The aim of this study was to evaluate tensile and shear bond strengths of resin-based sealer cements in different thicknesses bonded to root dentine. It was hypothesised that tensile and shear bond strength of resin-based sealer cements increase as the thickness of the materials increases. In a previous study, the bond strength between two impervious steel plates was greater with thin than with thick layers (17), which is a function of the geometry of the samples. In this study, bonding to dentine, with the presence of tubules rather than an impervious smooth surface, was evaluated.
© 2015 Australian Society of Endodontology
Resin-based sealer cements
E. S. Pane et al.
Materials and methods The sealer cements used were an epoxy resin-based material (AH Plus™, Dentsply/Maillefer DeTrey, Konstanz, Germany) (AH) and two methacrylate resin-based materials (EndoREZ®, Ultradent Products, South Jordan, UT, USA (EZ), and RealSeal™, SybronEndo, Orange, CA, USA (RS)).
Specimen preparation Sixty extracted single-rooted mandibular premolars were used. All teeth were collected with ethics approval by the institutional Human Ethics Advisory Group (Ethics ID: 1033306). Each tooth was embedded in resin (EpoFix™ Cold Setting, Batch No. 1123-2188, Struers, Ballerup, Denmark) and cut vertically in a bucco-lingual direction to produce two root halves, using a 0.3 mm thick sintered diamond wafering blade (Struers). A bonding area of 2 mm diameter centred over the canal space and an outer area for mounting of 8 mm diameter in the cervical coronal part of the root were made with water-cooled trephine bur (Hu-Friedy, Rockwell St, Chicago, IL, USA). The surface was cleaned with an ultrasonic cleaning system (L&R®, Kearny, NJ, USA) and polished with 800 grit silicon carbide paper and treated with 15% ethylenediaminetetraacetic acid (EDTA) and then 1% NaOCl for 5 min followed by distilled water to remove smear layer before testing. A special alignment device was prepared for accurate positioning of the dentine cylinder (Fig. 1). The prepared dentine cylinder was positioned concentrically with a 2 mm stainless steel rod fixed in a plano-parallel orientation. A predefined gap of 0.1 or 1.0 mm between the tooth surface and the rod was established. Freshly mixed sealer was placed in the interface and contained within the space with a 2 mm diameter silicone boot. Twenty samples of thin and thick layers were prepared for each of the sealer cement. For the methacrylate resin-based sealers, all samples were kept in a nitrogen chamber for 2 h to prevent oxygen inhibition of setting and all rods were stored in a 37°C incubator with 95% relative humidity for 48 h before testing. For the epoxy-based sealer, all samples were kept at 37°C and 95% relative humidity for 7 days before testing.
Tensile and shear bond tests Microtensile and shear bond tests were conducted using a universal testing machine (Mecmesin, Imperial 1000, West Sussex, UK) without removing the specimens from the alignment device to prevent premature failure. For the tensile test, a wire loop was placed around a prepared hook at the end of the stainless steel rod. For the shear
© 2015 Australian Society of Endodontology
Figure 1 (a) Alignment device for accurate positioning of dentine cylinder concentric with parallel stainless steel rod surface. The movable part can be positioned and fixed based on predetermined thickness. (b) Stainless steel rods and dentine cylinder in the slots. Sealer cement is placed between the rod and dentine surfaces. The silicone boot used to confine the sealer within the gap during setting has been removed.
test, a metal loop was placed around the specimen adjacent to the dentine surface. A ramped load was applied until fracture occurred. The force (N) required to fracture the bond was recorded and used to calculate the bond strength (MPa).
Mode of failure Mode of failure was evaluated visually and with light microscopy (Leica DML, Ernst-Leitz-Strasse, Wetzler, Germany) at 20× magnification. Mode of failure was classified as cohesive within the sealer, adhesive at the dentine–sealer interface, or mixed within the sealer and at the interfaces. Adhesive failure at the sealer–stainless steel interface was not observed.
Scanning electron microscopy After the tests, representative dentine surfaces from thin and thick specimens were examined. The samples were 129
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Figure 2 Tensile and shear bond strength of thin and thick layers of resin-based sealer cements. Mean tensile bond strengths of thin and thick layers of AH are 1.59 ± 0.9 and 4.73 ± 2.25 MPa; EZ 1.28 ± 0.7 and 2.18 ± 2.05 MPa, RS 1.94 ± 1.21 and 3.77 ± 1.88 MPa. Mean shear bond strengths of thin and thick layers of AH are 8.9 ± 4.7 and 11.67 ± 3.27 MPa, EZ 4.65 ± 3.6 and 2.87 ± 1.26 MPa, RS 7.21 ± 1.89 and 8.25 ± 3.84 MPa. The boxplots showed the outliers and distribution of data with 25%, 50% and 75% lower, median and upper quartiles.
mounted on stubs with the dentine–sealer interface uppermost, coated with gold and then examined using scanning electron microscopy (Quanta FEG SEM, FEI Co., Hillsboro, Oregon, USA). With the thick layers of sealer cement, it was also possible to examine the sealer side of the interface by removing the sealer from the stainless steel rod and mounting it on a stub.
Statistical evaluation The data were analysed separately for the tensile and shear data. A two-way ANOVA (SPSS Inc., Statistic 17.0, Armonk, New York, USA) was used to examine the effect of material and thickness on bond strengths with Tukey’s post-hoc test. The minimum level for statistical significance was P < 0.05.
Results Bond strengths are shown in Figure 2. Tensile strength was significantly influenced by both material (P = 0.000) and sealer thickness (P = 0.017), with no statistically significant interaction between materials and thickness. Shear bond strength was significantly influenced by material (P = 0.000) but not by thickness (P > 0.05), with no interaction between the two. In both tensile and shear tests, the epoxy resin-based sealer had the highest strength, followed by the two methacrylate resin-based sealers. 130
For each material, the thick layer of sealer produced a higher bond strength to root dentine compared with the thin layer, except for the shear bond strength of EZ which was the opposite (Fig. 2). Statistically, pairwise comparisons showed significant differences between the thin and thick layers of each sealer only in the tensile bond strength of AH and RS (P = 0.001 and P = 0.019). Mixed type of failure was consistently found for the three different sealer cements with both thicknesses (Fig. 3). Failure occurred adhesively at the dentine – sealer interface and cohesively within the sealer layer. Any area of adhesive failure to the metal surface was rarely found, and no evidence of cohesive failure within dentine was seen. Exclusively adhesive or cohesive failure throughout the entire interface was not observed. Higher magnification of the dentine–sealer interface showed a gritty layer at the material surface, consisting predominantly of filler particles with little surrounding resin matrix (Fig. 3). This gritty appearance of sealer layer was a typical finding in all materials and thickness bonded to root dentine. In Figure 4, the comparisons between images of fractured thick and thin materials bonded to root dentine are shown. In thick layer, the filler particles appeared to be completely surrounded by resin matrix, while the thin layer was characterised by loose filler particles with sparse supporting resin matrix. Areas of adhesive failure at the sealer–dentine interface showed pulled
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Figure 3 Scanning electron microscope images of the thin layer of AH. (a) is a 100× original magnification of the fractured dentine surfaces and (b) is a 2000× original magnification of the surfaces. Apparently adhesive failure at light microscope level shows a thin coating of sealer on the dentine surface. Higher magnification of supposedly dentine–sealer adhesive failure showed a gritty thin layer of material with visible dentinal tubules.
and stretched resin tags emerging from dentinal tubule orifices (Fig. 5).
Discussion The compressive axial and non-axial occlusal stresses that occur during normal oral function are transmitted to the root area of a tooth and can generate both tensile and shear stresses at the interface between the root filling and
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the canal wall. This effect of Poisson’s ratio at the interface between dentine and the root canal filling materials will produce complex stresses in the root-filled area, potentially resulting in bond failure (18).Tensile and shear bond testing methods have been used to evaluate adhesive bond strength of materials in root canals, in addition to the more ‘functional’ push-out test (19). The tensile test was used in this study because it allows better evaluation of bond failure (20) plus the use of a flat dentine surface with better accessibility to the working area (21). The tensile test has been accepted to produce more uniform stress (22) but is very sensitive and depends on correct axial alignment during specimen preparation and load application (23). The shear bond test was selected for the reason that it is considered a more reliable and reproducible method (5) and because of the simplicity of the test (24). The test is strongly influenced by bonding geometry, loading condition and differences in the elasticity of the materials (25). Only a few tensile bond studies (3,8,9,26) and shear test studies (5,27,28) have been conducted to evaluate root canal sealer cements bonded to root dentine. The test has been conducted with coronal dentine (3,5) using larger diameter specimens (3.7–6.4 mm) (8,9). Premature failure of the specimens was a problem encountered in previous tensile (21,26) and shear (28) studies. The method used in the present tensile and shear bond tests investigation was very successful in preventing premature failure since no manipulation of the specimens was required during the test set-up. However, it was difficult to eliminate occasional slight misalignment between the dentine cylinder and metal surfaces with the use of a small interface (2 mm). Failure mode assessment of fractured surfaces has been used to evaluate the nature and location of bond failure. However, categorising it as adhesive, cohesive and mixed oversimplifies what is actually a very complex fracture pattern (29). With the three different sealers used in this study, all fractured surfaces resulted in a complex pattern of failure (Fig. 3): both adhesive at the sealer–dentine interface and cohesive within the sealer cement. Failure predominantly at the dentine–sealer interface showed that the bonding property of the resin-based sealers was inferior to the dentine–metal substrate interface. A characteristic feature of adhesive failure to coronal dentine exposed to NaOCl and EDTA is a non-mineralised collapsed surface with numerous lateral branches of dentinal tubules, empty dentinal tubules with resin tags being pulled out and partially dissolved peritubular dentine (26). However, that study did not examine root dentine and only a thick layer of sealer was applied (26). With the same sealer, the present study investigating both thin and thick layers of sealer bonded to root dentine produced an 131
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Figure 4 (1a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of AH sealer cement. In thick layer, abundant resin matrix surrounded the filler particles. (1b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thin layer of AH sealer cement bonded to root dentine. In thin layer, much less resin surrounding the filler was noted. (2a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of EZ sealer cement. Abundant resin matrix surrounded the filler particles. (2b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces of EZ when bonded to root dentine. Much less resin surrounding the filler was noted. (3a) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of RS sealer cement. Abundant resin matrix surrounded the filler particles. (3b) Scanning electron microscope images (5000× original) of cohesively fractured surfaces in a thick layer of RS bonded to root dentine. Much less resin surrounding the filler was noted.
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hollow rather than solid. Cohesive failure within the sealer cement often demonstrated only a sparse resin matrix surrounding the filler particles (Fig. 5). Previous studies have used a thin layer of epoxy resinbased sealer applied between dentine and gutta-percha to compare it with other conventional sealers. Higher tensile bond strength (8,9) and higher shear bond strength (5) were found with epoxy resin-based sealers (3,10–12). Shear bond testing has been performed with both thin (30) and thick layers (5,30), with higher values reported for thick layers (5,30). With push-out tests, it was found that a thick layer of resin-based sealers produced higher bond strength than thin layers (12). It was proposed (12) that the resin matrix of the sealer penetrated extensively into dentinal tubules in root dentine, leaving a resindepleted interface rich with filler particles, which caused poorer adherence. This present study consistently showed a layer consisting predominantly of loose filler particles not supported by resin matrix (Fig. 4b), with a very different appearance compared with fractured bulk material which showed filler particles embedded in abundant resin matrix (Fig. 4a). Although the bond strength of a sealer cement may not be as important as bacterial control in terms of the outcome of root canal treatment, intimate contact of the sealer with dentine and penetration into dentinal tubules is still important. Bond strength is a surrogate for assessing the quality of the lateral seal of a root canal filling. The tendency for thicker layers of sealer cement to create a stronger bond than thin layers is consistent with the results of push-out testing (19) and suggests that there may be some advantage to the increased thickness of the sealer cement.
Acknowledgements Figure 5 Scanning electron microscope (SEM) images of fractured surfaces of thick layer of sealer. (a) SEM image with 2000× original magnification showed numerous resin tags protruding from dentinal tubules. (b) SEM image with 5000× original magnification showed the hollow appearance of some tags. This feature was noticed in all sealers. The tags seemed to be partially pulled or stretched away from the dentinal tubules, explaining the apparently empty tubules in some areas. Some tags seemed to be not bonded to the tubule walls.
irregular surface (gritty) (Figs 3,4). Some dentinal tubules appeared empty, but many were filled or covered with various sizes and forms of filler (Fig. 4). On the dentine surface, resin tags seemed to be found only when a thick layer of sealer was applied, and tags were seen partially extruded from the tubules and were often
© 2015 Australian Society of Endodontology
This research was supported by Post-Graduate Research Grant, Australian Society of Endodontics and Postgraduate Research Fund of Melbourne Dental School, University of Melbourne. We would like to thank Mr. Roger Curtain for assisting in the use of scanning electron microscopy in Advanced Microscopy Facility, Bio21. We would like to thank Ms. Rachel Sore, Statistical Consulting Centre, University of Melbourne, for the statistical consultation.
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