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Micro-shear bond strength of different composites and glass-ionomers used to reinforce root dentin
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ScienceDirect Tanta Dental Journal 10 (2013) 58e66 www.elsevier.com/locate/tdj
Micro-shear bond strength of different composites and glass-ionomers used to reinforce root dentin D.A. Abo Al-Hana*, A.A. El-Messairy, F.H. Shohayb, H.A. Alhadainy Conservative Dentistry Dep., Faculty of Dentistry, Tanta University, Egypt
Abstract Purpose: Comparison of microshear bonding strength of reinforcing root dentin materials (microhybrid composite, nanocomposite, conventional glass ionomer, nano-glass ionomer). Materials & methods: A total of 20 roots of similar-sized undamaged one rooted lower premolars were used, 40 root halves were embedded in the acrylic block and were randomly divided into four equal groups (10 each) according to the reinforcing materials. Group I: microhybrid composite, group II: nano-composite, group III: conventional glass ionomer was used and group IV: light cured nano-ionomer was used. Teeth were sliced and loaded to a testing machine. A shearing load with tensile mode of force was applied via materials. The mode of failure was observed under stereomicroscope. The results were statistically analyzed using one way ANOVA and t-test. Results: Nano-composite group recorded the highest m-shear bond strength mean value (23.52 3.997 MPa) followed by m-Hybrid composite group (16.88 3.356 MPa) then nano glass-ionomer group (8.77 1.341 MPa) while conventional glass-ionomer group showed the lowest m-shear bond strength mean value (4.062 0.9623 MPa). The difference was statistically significant between all groups (P < 0.05). The failure modes indicated that high bond strength showed cohesive or mixed modes, while low bond strength groups tended to exhibit adhesive, cohesive or mixed modes. Conclusion: Nano-composite recorded the highest microshear bond strength close to that required to resist the polymerization contraction stress. Ó 2013, Production and Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University. Keywords: Composite resin; Glass ionomer; Microshear bond strength; Mode of failure; Reinforcing materials
1. Introduction
* Corresponding author. E-mail address: [email protected] (D.A. Abo Al-Hana). Peer Review under the responsibility of the Faculty of Dentistry, Tanta University
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The restoration of endodontically treated teeth commonly presents a challenge to dentists, especially in cases of extensive crown-root destruction [1,2]. Excessive root canal flaring can result in a large, conical and insufficiently retentive post [3,4]. On the other hand, the use of posts with a smaller diameter than that of the post-space leads to the formation of voids that, even if filled with cement, represent potentially weakened areas [3].
1687-8574/$ - see front matter Ó 2013, Production and Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University. http://dx.doi.org/10.1016/j.tdj.2013.08.004
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A recent technique has been proposed for restoring a compromised endodontically treated teeth involves the use of reinforcing materials to replace lost root dentin and possibly strengthen the root [5,6]. The thickness of dentin remaining after tooth preparation is the most relevant factor in determining tooth strength [7,8]. It is necessary to retain as much tooth tissue as possible during restorative procedures, as roots with little remaining dentin for structural support are less able to withstand functional and impact stresses [8]. In certain situations, the destruction of the tooth structure extends to the internal region of the root as a result of dental caries, fractures, removal of previously placed posts, previous endodontic treatment, internal reabsorption or idiopathic causes [9,10]. A thin peripheral dentinal wall may be prone to fracture after cementation of a cast post and core as a cast post has been shown to improve the retention of the restoration rather strengthen the root [11]. Especially if a cast post is cemented into a flared canal, the post may exert a wedging effect on the canal walls [12]. The structure of dentin is an important factor that should be taken into account in terms of bonding [13]. Bonding of adhesive restorations can be influenced by the anatomical and histological characteristics of the root canal, including the orientation of the dentin tubules. Dentinal tubules in the root are straighter, less divergent [14], and not as numerous as in the crown [15]. Moreover, as the number of dentinal tubules decreases from the cervical to the apical part of the root [16], dentin bonding. To avoid extraction of weakened roots, filling of the radicular defects with restorative materials has been suggested [10,17]Several materials have been used to fill radicular defects with the aim of increasing the resistance of the weakened roots, such as different types of glass-ionomer cements and composite resins and hybrids of glass-ionomers cement and composite resin [10,17,18]. In a recent extensive discussion about a framework for definitions presented to the European Commission, the nanoscale was defined as being of the order of 100 nm or less. Similarly, nanomaterial has been defined as any form of a material that is composed of discrete functional parts, many of which have one or more dimensions of the order of 100 nm or less. There are two major characteristics conferring the special properties of any nanosized material, nanophase materials have unique surface properties, such as an increased number of grain boundaries and defects at the surface, a huge surface area and an altered electronic structure, when compared to conventional
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micron-sized materials [19,20]. Finally, nanosized and nanocrystalline materials have different mechanical and optical properties compared to larger grained materials of the same chemical composition [21]. Therefore, nanotechnology is of great interest glass ionomer. The manufacturer calls the new restorative cement a ‘nano-ionomer’ because the formulation is ‘based on bonded nanofiller technology [22], which allows a highly packed filler composition (69%), of which approximately two-thirds are nano-fillers [23]. To test the bond strength of these materials the ideal bond-strength test should be in the first place easy (meaning low technique-sensitivity) and relatively fast [24]. The microshear bond strength (m-SBS) test was introduced as an alternative to the microtensile bond test. Advantages of the microshear bond test include less demanding specimen collection and easier control of the bond test area by means of microbore (polyvinyl chloride) tubes. Shimida et al., 2002 [25] modified the microshear bond test by replacing the blade with a looped orthodontic wire. 2. Materials & methods Twenty similar-sized one-rooted lower premolars with fully developed apices and were extracted for orthodontic purpose the patient’s age was ranging from (20e35) years. All teeth were radiographed; any tooth with root caries, internal calcification or resorption was excluded. Teeth were cleaned from any attached soft or hard tissue and stored in 10% natural buffered formalin [26]. Crown was sectioned transversally at the cemento-enamel junction (CEJ) with double-faced diamond disc at low speed. The twenty roots were then split longitudinally in a buccolingual direction giving a total 40 halves. The inner surface of each root was ground until it was smooth and flat using 1000-grit silicon carbide paper. The cut surface was pretreated with 1% NaOCL and 17% EDTA solutions for 5 min to remove the smear layer [27]. Each prepared root- half was placed onto a glass cover slab (inner surface of root half touches the flat surface). To imbed the root half in an acrylic block a plastic tube with a diameter of 2 cm and a height of 2 cm was painted with separating medium and the opening of the tube was placed over the tooth specimen and filled with acrylic material around the outer surface of the root. After acrylic setting, the tubes were removed from the samples producing 40 root halves embedded in the 40 acrylic blocks with a diameter of 2 cm and a height of 2 cm. The entire specimen unit
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Table 1 Materials classification used for sample preparation. Group
Reinforced material
I II III IV
Esthet X Flow Flowable composite FiltekÔ Z350 flowable Nanocomposite KetacÔ Fil Plus conventional glass ionomer KetacÔn100 Light cured nano-ionomer
was placed in a 37 C incubator with 95% humidity for 24 h before testing. The specimens were randomly divided into four equal groups as shown in Table 1 The root dentin surface was prepared with selfetching adhesive or dentin conditioning according to the material used. Reinforced material was filled into iris cut from micro-bore polyvinyl chloride tubing with an internal diameter approximately 1 mm and a height of 2 mm, composite was delivered directly from the dispensing tip. The tube was caught with twizer during application of the material and held over the prepared dentin surface. The material was carefully condensed within the tube by using very small condenser to avoid formation of air bubbles. The material was cured. The light guide tip was held as close to the opening of the polyvinyl chloride tube as possible during light exposure for 40 s Fig. 1. The polyvinyl chloride tube was removed, by carefully making two parallel cuts with blade for easy
Fig. 2. Tube removals.
removal of the tube without breaking material microcylinder. Micro-cylinder of materials was obtained which is approximately 1 mm in diameter and 2 mm in height that was bonded to the dentin surface. Figs. 1 and 2. Each acrylic block with the embedded root slice and its own bonded reinforcing material microcylinders was secured with tightening screws to the lower fixed compartment of the testing machine with a load cell of 5 kN. A loop prepared from an orthodontic wire (0.01400 in diameter) was wrapped around the bonded microcylinder assembly as close as possible to the base of the microcylinder and aligned with the loading axis of the upper movable compartment of the testing machine, a shearing load with tensile mode of force was applied via materials testing machine1 at a crosshead speed of 0.5 mm/min Fig. 3. The relatively slow crosshead speed was selected in order to produce a shearing force that resulted in debonding of the microcylinder along the substrateeadhesive interface. The load required to debonding was recorded in Newton. Data were recorded using computer software. Modes of bond failure After m-shear bond strength test, all the failed specimens were examined using USB digital microscope at 30 magnification and photographed using image analysis software (Scope Capture 1.1.1.1. Ltd Co.), to determine the nature of their fractures that were classified as: A) Adhesive failure, between dentin and material. B) Cohesive failure, (within material or within dentin) C) Mixed failure.
Fig. 1. Material microcylinder sample preparation.
1
Nexygen-MT Lloyd Instruments.
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Fig. 4. A column chart of m-shear bond strength mean values for all the tested materials. Fig. 3. Measuring of microshear bond strength.
Statistical analysis The data were converted to Mega Pascal (Mp) and tabulated for statistical analysis using SPSS computer software. A descriptive analysis was computed as means and standard deviation for each group. Statistical analysis using one way analysis of variance (ANOVA) to detect a significant difference between groups and Student t-test for the difference between each two groups were performed.
3. Results Descriptive statistics of m-shear bond strength results for all groups including minimum, maximum, mean, standard deviation and median values are summarized in Table 2 and mean values are represented graphically in Figs. 4 and 5. Nano-composite group recorded the highest m-shear bond strength mean value (23.52 3.997 MPa) followed by m-Hybrid composite group (16.88 3.356 MPa) then nano glass-ionomer group (8.77 1.341 MPa) while conventional glass-ionomer group showed the lowest statistically significant mshear bond strength mean value (4.062 0.9623 MPa). The difference in m-shear bond mean values between groups was statistically significant (P < 0.05) as revealed by ANOVA and pair-wise NewmaneKeuls post-hoc tests.
Effect of nano-filler on m-shear bond strength a) Composite restoration Nano-composite group recorded a higher m-shear bond strength mean value than m-hybrid composite. The difference in m-shear bond mean values between nano-composite and m-hybrid composite was statistically significant as revealed by t-test (t ¼ 6.6, P < 0.05) (Table 3). b) Glass ionomer restoration Nano glass-ionomer group recorded higher m-shear bond strength mean value than conventional glass-ionomer. The difference in mshear bond means values between nano-glassionomer and conventional glass-ionomer was statistically significant using t-test (Fig. 6). In Comparing composite vs. glass ionomer restoration it was found that groups I, II record higher m-shear bond strength compared to groups III, IV mean value (20.36 1.083 MPa), (6.013 0.6330 MPa). The difference in m-shear bond mean values between composite and glass-ionomer was statistically significant as indicated by t-test. (t ¼ 10.55, P < 0.05) Tables 4 and 5, Fig. 7. Failure type was noted as adhesive, cohesive, or mixed. Evaluation of failure modes after m-shear testing indicated that high bond strength showed cohesive or mixed modes, while low bond strength groups tended to exhibit adhesive, cohesive or mixed modes as seen in Figs. 8e11.
Table 2 Descriptive statistics of m-shear bond strength results for all groups. Mean Std. deviation Median Maximum Minimum
m-hybrid composite
Nano composite
Conventional glass-ionomer
Nanoglass-ionomer
16.88 3.356 16.18 23.82 13.47
23.52 3.997 24.67 29.96 17.85
4.062 0.9623 3.63 5.735 3.227
8.77 1.341 7.137 10.18 6.468
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Fig. 5. A column chart of m-shear bond strength mean values for mhybrid composite and nano-composite.
4. Discussion Many dentists prefer to extract badly destructed teeth with flared root canals, because roots with thin walls are prone to fracture upon post cementation or during functional and para-functional activities [10]. Several materials have been investigated for restoration of weakened roots such as glass-ionomer cements, composite resins and hybrids of glass-ionomers cement and composite resin [4,17]. New restorative materials were also proposed for such cases like nano-particles, therefore, the microshear bond strength for nanocomposite, micro-hybrid, nano-glass ionomer and conventional glass-ionomer used for reinforcement of weakened root was measured. Dentin cut surface was pretreated with 1% NaOCl and 17% EDTA solutions for 5 min to remove the smear layer. Violich and Chandler 2010 [28] concluded that the smear layer covers the instrumented walls interfere with the close adaptation of root filling materials to canal walls; therefore smear layer was removed for better adaptation of materials to the canal walls. NaOCL has the ability to dissolve organic tissue combined with saponification, deodorization, and lubrication capacity [29]. EDTA acts on the mineral matrix hence increasing the contact surface of the filling material with the dentin [30]. Therefore, if the smear layer is to be removed, the method of choice is the alternate use of EDTA and sodium hypochlorite solutions [28]. The reinforcement materials were applied in to micro-bore tube for less demanding specimen
Table 3 Comparison of m-Shear bond strength results (Mean values SDs) between m-hybrid composite and nano-composite. Composite group
Mean SD
t-test
m-Hybrid composite Nano-composite
16.88 3.356 23.52 3.997
t-value 4.1
*Significant (p < 0.05).
P value 0.0006*
Fig. 6. A column chart of m-shear bond strength mean values for conventional glass-ionomer and nano-glass-ionomer.
collection and easier control of the bond test area [24]. Sano et al., 1994; Phrukkanon et al., 1998 [31,32]reported that higher mean bond strengths have been obtained using smaller bonded surface areas. This is attributed to fewer interfacial defects and variations within the tooth. The choice of light-polymerized composite resin as the restorative material was based on research that recommended light-polymerized resins for restorative purposes in conjunction with endodontic therapy [10,33,34]. Microshear bond strength has been claimed to be more reliable, because the stress distribution in the bonding surface of specimens in microshear tests is more homogenous due to lower dimensions of specimens, thus, the statistical analysis would be more accurate. The size of contact surface stress concentration is decreased so that failure pattern shifts to adhesive instead of cohesive failure and this will decrease the errors [35]. In the current study the wire microshear was used instead of using blade, Shimida et al., 2002 [25] modified the microshear bond test by replacing the blade with a looped orthodontic wire. Foong et al., 2006 [24] claimed that the wire microshear test was shown to be easier and more reliable compared to the blade microshear test. Braga et al., 2010 [36] reported that, the use of the chisel as a loading device causes the most severe stress concentration, which is also supported by experimental findings showing lower bond strengths compared to the wire loop. In the present study, nano-composite group recorded higher m-shear bond strength mean value than m-hybrid composite, although both materials were almost within the accepted value, nano composite was higher than mTable 4 Comparison of m-Shear bond strength results (Mean values SDs) between conventional glass-ionomer and nano-glass-ionomer. Glass-ionomer group
Mean SD
t-test
Conventional glass-ionomer Nano glass ionomer
4.062 0.9623 8.77 1.341
t-value 5.7
*Significant (p < 0.05).
P value 0.0001*
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Table 5 Comparison of m-Shear bond strength results (Mean values SDs) between composite and glass-ionomer. Group
Mean SD
t-test
Composite Glass-ionomer
20.36 1.083 6.013 0.6330
t value 0.55
P value <0.0001*
hybrid composite. The difference in m-shear bond mean values between m-hybrid composite and nanocomposite was statistically significant. These findings agree with many authors who suggested that the increase in mechanical resistance results from increasing the filler content of a composite material and decreasing the filler size which influence the resin properties [37e39]. The micro-filler composite resins present generally inferior physical and mechanical properties, since 40e80% (by volume) of the material is constituted of organic matrix [40]. Moszner and Salz, 2001 [18] mentioned that the reduction in particle dimensions and their increased quantity (nano-fillers development principle), could reduce polymerization shrinkage and enhance the mechanical properties of the resins. Meguid and Sun, 2004 [41] reported that the tensile and shear bond strength depend on the filler particles size. The nanosized fillers are characterized by large surface areas per unit gram. As the number of adhesively joined points increases, the cohesive strength of the epoxy increases leading to a higher mechanical strength of the interface. Heshmat et al., 2009 concluded [42] that using composites with a higher filler content and modulus of elasticity resulted in higher shear bond strength in the dentin bonding system. In the present study nano glass-ionomer group recorded higher m-shear bond strength mean value than conventional glass-ionomer. The difference in m-shear bond mean values between conventional glass-ionomer and nano-glass-ionomer was statistically significant.
Fig. 7. A column chart of m-shear bond strength mean values for between composite and glass-ionomer.
Fig. 8. A stacked column chart of different failure modes for all groups.
Glass-ionomer bond chemically to tooth substrates, and bond strengths have been studied extensively in sound and carious dentin using conventional shear testing methods [43,44]. The bond strengths of GIC have been reported in the range of 3e4 MPa [45,46]. Previous studies of the bond strength of GIC to dentin using tensile or shear tests indicated that bond strengths above 5 MPa were seldom achieved [47,48]. The findings of this study agree with some authors who explained the chemical bonding of GIC by ionic exchange between resin modified glass-ionomer and dentin substrate, the penetration and further lightcuring of the resin modified glass-ionomer through the smear layer into the dentinal tubules provide an additional mechanical interlocking of the polymer to dentin [49,50] also working time is improved in resinmodified glass ionomer [53]. Franckenberger et al., 1997 [51]; suggested that decrease in the size of glass particles has significantly
Fig. 9. Microhybrid composite, mixed failure (adhesive and cohesive within microhybrid composite). A) Tube’s border, B) Dentin surface treated with self-etch bonding, C) Microhybrid composite, D) Acrylic block.
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Fig. 10. Conventional glass-ionomer, adhesive failure; A) Tube’s border, B) Dentin inside tube, C) Dentin outside tube, D) Acrylic block.
improved the mechanical properties, this is thought to be due to better distribution of particles within the matrix. El-Askary and Nassif, 2011 [52]Concluded that the pre-conditioning step effectively improves bonding of the nano-filled RMGI to dentin. As general comparison, the results of this study revealed that composite groups recorded significantly
higher m-shear bond strength mean value than glassionomer. These results agree with Toledano et al., 2003 [53] who concluded that the mean bond strength of the light cure composite resin is clinically acceptable, while the GIC showed the lowest shear bond strength. The lower strength of glass ionomer was due to their brittleness, initial sensitivity to moisture contamination, sensitivity to dehydration [54,55]. Moisture leads the material to be chalky and porous, resulting in a loss of surface hardness [56]. Shear bond strength value obtained for conventional glass ionomer is below minimal recommended values for clinical purposes [53]. Nicholson, 1998 [50] and other studies (Mitchell et al., 1995; Fruits et al., 1996; Nicholson, 1998) [50,57,58] concluded that glass-ionomers as well as resin modified glass-ionomers have lower shear bond strengths when compared with composite adhesive systems, also Prabhakar, 2003 concluded [59] that the composite showed higher mean shear bond strength in permanent teeth as compared to compomer and resin modified glass ionomer. Failure mode in the current study was noted as adhesive, cohesive, or mixed. Evaluation of the failure modes after m-shear testing indicated that high bond strength showed cohesive or mixed modes. These results agree with Schneider et al., 2000 [60] who found that the better the bond between dentin and composite the higher the percentage of cohesive failure within each sample. The amount of substrate fracture is often indicative of the retentive strength of the adhesive, increasing the tendency of mixed failure as the bond strength increases. In the current study the lowest bond strength group (convential glass-ionomer) tended to exhibit adhesive failure mode. These results agree with Chung et al., 2009 [61]who reported that failures in conventional glass were adhesive along the dentin interface, indicating a weak bond between material and dentin. Vargas, 1995 [62] suggested that the cohesive fracture indicated that the shear bond strength between dentin and this material is higher than the cohesive strength of the material itself. Armstrong et al., 1998 [63] stated that cohesive substrate fractures, in either the tensile or shear mode of strength testing, cannot be attributed to the bond strength of the adhesive being greater than the respective tensile or shear strength of the adherent. 5. Conclusion
Fig. 11. Nano composite, cohesive failure within dentin; A) Dentin inside the tube, B) Tube’s border, C) Dentin outside the tube, D) Acrylic block.
Nano-composite recorded the highest value of microshear bond strength which is close to the
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accepted value of bond strength that required to resist the polymerization contraction forces of composite. The microshear bond strength of either type of flowable composite resin higher than the both type of GICs. From the present study it might be concluded that for higher bond strength to root dentin, it is more favorable to use flowable composite resin rather than GICs for reinforcement of weakened roots. References [1] Schwartz RS, Robbins JW. Post placement and restoration of endodontically treated teeth: a literature review. J Endod 2004;30:289e301. [2] Dietschi D, Duc O, Krejci I, Sadan A. Biomechanical considerations for the restoration of endodontically treated teeth: a systematic review of the literature. Part 1. Composition and micro- and macrostructure alterations. Quintessence Int 2007;38:733e43. [3] Boschian-Pest L, Cavalli G, Bertani P, Gagliani M. Adhesive post-endodontic restorations with fiber posts: push-out tests and SEM observations. Dent Mater 2002;18:596e602. [4] Macari PCA, Conceicao EN, Nunes MF. Fracture resistance of endodontically treated teeth restored with three different prefabricated esthetic posts. J Esthet Restor Dent 2003;15:25e30. [5] Glassman G, Serota K, Soll J. A new method for the restoration of the endodontically treated tooth with the Luscent Anchor System. Oral Health 1999;89:23e6. [6] Martelli R. Fourth generation intraradicular posts for the aesthetic restoration of anterior teeth. Pract Periodont Aesthetic Dent 2000;12:579e84. [7] Wu X, Chan AT, Chen YM. Effectiveness and dentin bond strengths of two materials for reinforcing thin-walled roots. Dent Mater 2007;23:479e85. [8] Marchi GM, Mitsui FH, Cavalcanti AN. Effect of remaining dentin structure and thermal-mechanical aging on the fracture resistance of bovine roots with different post and core systems. Int Endod J 2008;41:969e76. [9] Linde LA. The use of composite resins in combination with anchorage posts as core material in endodontically treated teeth: clinical aspects of the technique. Quintessence Int 1993;24:115e22. [10] Lui JL. Composite resin reinforcement of flared canals using light transmitting plastic posts. Quintessence Int 1994;25:313e9. [11] Sorensen JA, Engelman MJ. Effect of post adaptation on fracture resistance of endodontically treated teeth. J Prosthet Dent 1990;64:419e24. [12] Lui JL. Enhanced post crown retention in resin composite reinforced compromised root filled teeth: a case report. Quintessence Int 1999;30:601e6. [13] Bitter K, Kielbassa AM. Post-endodontic restorations with adhesively luted fiber-reinforced composite post systems: a review. Am J Dent 2007;20:353e60. [14] Lertchirakarn V, Palamra JEA, Messer HH. Anisotropy of tensile strength of root dentin. J Dent Res 2001;80:453e6. [15] Mannocci F, Pileck P, Bertelli E, Watson TF. Density of dentinal tubules affects the tensile strength of root dentin. Dent Mater 2004;20:293e6.
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[50] Nicholson JW. Chemistry of glass-ionomer cements: a review. Biomaterials 1998;19:485e94. [51] Franckenberger HJE, Sindlel J, kra¨mer N. Viscous glassionomer cements: a new alternative to amalgam in the primary dentition? Quintessence Int 1997;28:667e75. [52] El-Askary FS, Nassif MS. The effect of the pre-conditioning step on the shear bond strength of nano-filled resin-modified glass-ionomer to dentin. Eur J Dent 2011;5:150e6. [53] Toledano M, Osorio R, Osorio E, Romeo A, Higuera B, Garcı´a-Godoy F. Bond strength of orthodontic brackets using different light and self-curing cements. Angle Orthod 2003;73:56e63. [54] Trimpeneers LM, Verbeeck RMH, Dermaut LR, Moors MG. Comparative shear bond strength of some orthodontic bonding resins to enamel. Eur J Orthod 1996;18:89e95. [55] Cook PA. Direct bonding with glass ionomer cement. J Clin Orthod 1990;24:509e11. [56] Swift EJ. An update on glass ionomer cements. Quintessence Int 1988;19:125e30. [57] Mitchell CA, O’Hagan E, Walker JM. Probability of failure of orthodontic brackets bonded with different cementing agents. Dent Mater 1995;11:317e22. [58] Fruits TJ, Duncanson Jr MG, Miller RC. Bond strengths of fluoride releasing restorative materials. Am J Dent 1996;9:219e22. [59] Prabhakar AR, Raj S, Raju OS. Comparison of shear bond strength of composite, compomer and resin modified glass ionomer in primary and permanent teeth: an in vitro study. J Indian Soc Pedo Prev Dent 2003;21:86e94. [60] Schneidera BT, Baumanna MA, Watanabeb LG, Marshall GW. Dentin shear bond strength of compomers and composites. Dent Mater 2000;16:15e9. [61] Chung CW, Yiu CK, King NM, Hiraishi N, Tay FR. Effect of saliva contamination on bond strength of resin luting cements to dentin. J Dent 2009;37:923e31. [62] Vargas MA, Fortin D, Swift Jr EJ. Bond strengths of glass ionomers using a dentin adhesive. Am J Dent 1995;8:197e200. [63] Armstrong S, Boyer D, Keller J. Microtensile bond strength testing and failure analysis of two dentin adhesives. Dent Mater 1998;14:44e50.
ScienceDirect Tanta Dental Journal 10 (2013) 58e66 www.elsevier.com/locate/tdj
Micro-shear bond strength of different composites and glass-ionomers used to reinforce root dentin D.A. Abo Al-Hana*, A.A. El-Messairy, F.H. Shohayb, H.A. Alhadainy Conservative Dentistry Dep., Faculty of Dentistry, Tanta University, Egypt
Abstract Purpose: Comparison of microshear bonding strength of reinforcing root dentin materials (microhybrid composite, nanocomposite, conventional glass ionomer, nano-glass ionomer). Materials & methods: A total of 20 roots of similar-sized undamaged one rooted lower premolars were used, 40 root halves were embedded in the acrylic block and were randomly divided into four equal groups (10 each) according to the reinforcing materials. Group I: microhybrid composite, group II: nano-composite, group III: conventional glass ionomer was used and group IV: light cured nano-ionomer was used. Teeth were sliced and loaded to a testing machine. A shearing load with tensile mode of force was applied via materials. The mode of failure was observed under stereomicroscope. The results were statistically analyzed using one way ANOVA and t-test. Results: Nano-composite group recorded the highest m-shear bond strength mean value (23.52 3.997 MPa) followed by m-Hybrid composite group (16.88 3.356 MPa) then nano glass-ionomer group (8.77 1.341 MPa) while conventional glass-ionomer group showed the lowest m-shear bond strength mean value (4.062 0.9623 MPa). The difference was statistically significant between all groups (P < 0.05). The failure modes indicated that high bond strength showed cohesive or mixed modes, while low bond strength groups tended to exhibit adhesive, cohesive or mixed modes. Conclusion: Nano-composite recorded the highest microshear bond strength close to that required to resist the polymerization contraction stress. Ó 2013, Production and Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University. Keywords: Composite resin; Glass ionomer; Microshear bond strength; Mode of failure; Reinforcing materials
1. Introduction
* Corresponding author. E-mail address: [email protected] (D.A. Abo Al-Hana). Peer Review under the responsibility of the Faculty of Dentistry, Tanta University
Production and hosting by Elsevier
The restoration of endodontically treated teeth commonly presents a challenge to dentists, especially in cases of extensive crown-root destruction [1,2]. Excessive root canal flaring can result in a large, conical and insufficiently retentive post [3,4]. On the other hand, the use of posts with a smaller diameter than that of the post-space leads to the formation of voids that, even if filled with cement, represent potentially weakened areas [3].
1687-8574/$ - see front matter Ó 2013, Production and Hosting by Elsevier B.V. on behalf of the Faculty of Dentistry, Tanta University. http://dx.doi.org/10.1016/j.tdj.2013.08.004
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A recent technique has been proposed for restoring a compromised endodontically treated teeth involves the use of reinforcing materials to replace lost root dentin and possibly strengthen the root [5,6]. The thickness of dentin remaining after tooth preparation is the most relevant factor in determining tooth strength [7,8]. It is necessary to retain as much tooth tissue as possible during restorative procedures, as roots with little remaining dentin for structural support are less able to withstand functional and impact stresses [8]. In certain situations, the destruction of the tooth structure extends to the internal region of the root as a result of dental caries, fractures, removal of previously placed posts, previous endodontic treatment, internal reabsorption or idiopathic causes [9,10]. A thin peripheral dentinal wall may be prone to fracture after cementation of a cast post and core as a cast post has been shown to improve the retention of the restoration rather strengthen the root [11]. Especially if a cast post is cemented into a flared canal, the post may exert a wedging effect on the canal walls [12]. The structure of dentin is an important factor that should be taken into account in terms of bonding [13]. Bonding of adhesive restorations can be influenced by the anatomical and histological characteristics of the root canal, including the orientation of the dentin tubules. Dentinal tubules in the root are straighter, less divergent [14], and not as numerous as in the crown [15]. Moreover, as the number of dentinal tubules decreases from the cervical to the apical part of the root [16], dentin bonding. To avoid extraction of weakened roots, filling of the radicular defects with restorative materials has been suggested [10,17]Several materials have been used to fill radicular defects with the aim of increasing the resistance of the weakened roots, such as different types of glass-ionomer cements and composite resins and hybrids of glass-ionomers cement and composite resin [10,17,18]. In a recent extensive discussion about a framework for definitions presented to the European Commission, the nanoscale was defined as being of the order of 100 nm or less. Similarly, nanomaterial has been defined as any form of a material that is composed of discrete functional parts, many of which have one or more dimensions of the order of 100 nm or less. There are two major characteristics conferring the special properties of any nanosized material, nanophase materials have unique surface properties, such as an increased number of grain boundaries and defects at the surface, a huge surface area and an altered electronic structure, when compared to conventional
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micron-sized materials [19,20]. Finally, nanosized and nanocrystalline materials have different mechanical and optical properties compared to larger grained materials of the same chemical composition [21]. Therefore, nanotechnology is of great interest glass ionomer. The manufacturer calls the new restorative cement a ‘nano-ionomer’ because the formulation is ‘based on bonded nanofiller technology [22], which allows a highly packed filler composition (69%), of which approximately two-thirds are nano-fillers [23]. To test the bond strength of these materials the ideal bond-strength test should be in the first place easy (meaning low technique-sensitivity) and relatively fast [24]. The microshear bond strength (m-SBS) test was introduced as an alternative to the microtensile bond test. Advantages of the microshear bond test include less demanding specimen collection and easier control of the bond test area by means of microbore (polyvinyl chloride) tubes. Shimida et al., 2002 [25] modified the microshear bond test by replacing the blade with a looped orthodontic wire. 2. Materials & methods Twenty similar-sized one-rooted lower premolars with fully developed apices and were extracted for orthodontic purpose the patient’s age was ranging from (20e35) years. All teeth were radiographed; any tooth with root caries, internal calcification or resorption was excluded. Teeth were cleaned from any attached soft or hard tissue and stored in 10% natural buffered formalin [26]. Crown was sectioned transversally at the cemento-enamel junction (CEJ) with double-faced diamond disc at low speed. The twenty roots were then split longitudinally in a buccolingual direction giving a total 40 halves. The inner surface of each root was ground until it was smooth and flat using 1000-grit silicon carbide paper. The cut surface was pretreated with 1% NaOCL and 17% EDTA solutions for 5 min to remove the smear layer [27]. Each prepared root- half was placed onto a glass cover slab (inner surface of root half touches the flat surface). To imbed the root half in an acrylic block a plastic tube with a diameter of 2 cm and a height of 2 cm was painted with separating medium and the opening of the tube was placed over the tooth specimen and filled with acrylic material around the outer surface of the root. After acrylic setting, the tubes were removed from the samples producing 40 root halves embedded in the 40 acrylic blocks with a diameter of 2 cm and a height of 2 cm. The entire specimen unit
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Table 1 Materials classification used for sample preparation. Group
Reinforced material
I II III IV
Esthet X Flow Flowable composite FiltekÔ Z350 flowable Nanocomposite KetacÔ Fil Plus conventional glass ionomer KetacÔn100 Light cured nano-ionomer
was placed in a 37 C incubator with 95% humidity for 24 h before testing. The specimens were randomly divided into four equal groups as shown in Table 1 The root dentin surface was prepared with selfetching adhesive or dentin conditioning according to the material used. Reinforced material was filled into iris cut from micro-bore polyvinyl chloride tubing with an internal diameter approximately 1 mm and a height of 2 mm, composite was delivered directly from the dispensing tip. The tube was caught with twizer during application of the material and held over the prepared dentin surface. The material was carefully condensed within the tube by using very small condenser to avoid formation of air bubbles. The material was cured. The light guide tip was held as close to the opening of the polyvinyl chloride tube as possible during light exposure for 40 s Fig. 1. The polyvinyl chloride tube was removed, by carefully making two parallel cuts with blade for easy
Fig. 2. Tube removals.
removal of the tube without breaking material microcylinder. Micro-cylinder of materials was obtained which is approximately 1 mm in diameter and 2 mm in height that was bonded to the dentin surface. Figs. 1 and 2. Each acrylic block with the embedded root slice and its own bonded reinforcing material microcylinders was secured with tightening screws to the lower fixed compartment of the testing machine with a load cell of 5 kN. A loop prepared from an orthodontic wire (0.01400 in diameter) was wrapped around the bonded microcylinder assembly as close as possible to the base of the microcylinder and aligned with the loading axis of the upper movable compartment of the testing machine, a shearing load with tensile mode of force was applied via materials testing machine1 at a crosshead speed of 0.5 mm/min Fig. 3. The relatively slow crosshead speed was selected in order to produce a shearing force that resulted in debonding of the microcylinder along the substrateeadhesive interface. The load required to debonding was recorded in Newton. Data were recorded using computer software. Modes of bond failure After m-shear bond strength test, all the failed specimens were examined using USB digital microscope at 30 magnification and photographed using image analysis software (Scope Capture 1.1.1.1. Ltd Co.), to determine the nature of their fractures that were classified as: A) Adhesive failure, between dentin and material. B) Cohesive failure, (within material or within dentin) C) Mixed failure.
Fig. 1. Material microcylinder sample preparation.
1
Nexygen-MT Lloyd Instruments.
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Fig. 4. A column chart of m-shear bond strength mean values for all the tested materials. Fig. 3. Measuring of microshear bond strength.
Statistical analysis The data were converted to Mega Pascal (Mp) and tabulated for statistical analysis using SPSS computer software. A descriptive analysis was computed as means and standard deviation for each group. Statistical analysis using one way analysis of variance (ANOVA) to detect a significant difference between groups and Student t-test for the difference between each two groups were performed.
3. Results Descriptive statistics of m-shear bond strength results for all groups including minimum, maximum, mean, standard deviation and median values are summarized in Table 2 and mean values are represented graphically in Figs. 4 and 5. Nano-composite group recorded the highest m-shear bond strength mean value (23.52 3.997 MPa) followed by m-Hybrid composite group (16.88 3.356 MPa) then nano glass-ionomer group (8.77 1.341 MPa) while conventional glass-ionomer group showed the lowest statistically significant mshear bond strength mean value (4.062 0.9623 MPa). The difference in m-shear bond mean values between groups was statistically significant (P < 0.05) as revealed by ANOVA and pair-wise NewmaneKeuls post-hoc tests.
Effect of nano-filler on m-shear bond strength a) Composite restoration Nano-composite group recorded a higher m-shear bond strength mean value than m-hybrid composite. The difference in m-shear bond mean values between nano-composite and m-hybrid composite was statistically significant as revealed by t-test (t ¼ 6.6, P < 0.05) (Table 3). b) Glass ionomer restoration Nano glass-ionomer group recorded higher m-shear bond strength mean value than conventional glass-ionomer. The difference in mshear bond means values between nano-glassionomer and conventional glass-ionomer was statistically significant using t-test (Fig. 6). In Comparing composite vs. glass ionomer restoration it was found that groups I, II record higher m-shear bond strength compared to groups III, IV mean value (20.36 1.083 MPa), (6.013 0.6330 MPa). The difference in m-shear bond mean values between composite and glass-ionomer was statistically significant as indicated by t-test. (t ¼ 10.55, P < 0.05) Tables 4 and 5, Fig. 7. Failure type was noted as adhesive, cohesive, or mixed. Evaluation of failure modes after m-shear testing indicated that high bond strength showed cohesive or mixed modes, while low bond strength groups tended to exhibit adhesive, cohesive or mixed modes as seen in Figs. 8e11.
Table 2 Descriptive statistics of m-shear bond strength results for all groups. Mean Std. deviation Median Maximum Minimum
m-hybrid composite
Nano composite
Conventional glass-ionomer
Nanoglass-ionomer
16.88 3.356 16.18 23.82 13.47
23.52 3.997 24.67 29.96 17.85
4.062 0.9623 3.63 5.735 3.227
8.77 1.341 7.137 10.18 6.468
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Fig. 5. A column chart of m-shear bond strength mean values for mhybrid composite and nano-composite.
4. Discussion Many dentists prefer to extract badly destructed teeth with flared root canals, because roots with thin walls are prone to fracture upon post cementation or during functional and para-functional activities [10]. Several materials have been investigated for restoration of weakened roots such as glass-ionomer cements, composite resins and hybrids of glass-ionomers cement and composite resin [4,17]. New restorative materials were also proposed for such cases like nano-particles, therefore, the microshear bond strength for nanocomposite, micro-hybrid, nano-glass ionomer and conventional glass-ionomer used for reinforcement of weakened root was measured. Dentin cut surface was pretreated with 1% NaOCl and 17% EDTA solutions for 5 min to remove the smear layer. Violich and Chandler 2010 [28] concluded that the smear layer covers the instrumented walls interfere with the close adaptation of root filling materials to canal walls; therefore smear layer was removed for better adaptation of materials to the canal walls. NaOCL has the ability to dissolve organic tissue combined with saponification, deodorization, and lubrication capacity [29]. EDTA acts on the mineral matrix hence increasing the contact surface of the filling material with the dentin [30]. Therefore, if the smear layer is to be removed, the method of choice is the alternate use of EDTA and sodium hypochlorite solutions [28]. The reinforcement materials were applied in to micro-bore tube for less demanding specimen
Table 3 Comparison of m-Shear bond strength results (Mean values SDs) between m-hybrid composite and nano-composite. Composite group
Mean SD
t-test
m-Hybrid composite Nano-composite
16.88 3.356 23.52 3.997
t-value 4.1
*Significant (p < 0.05).
P value 0.0006*
Fig. 6. A column chart of m-shear bond strength mean values for conventional glass-ionomer and nano-glass-ionomer.
collection and easier control of the bond test area [24]. Sano et al., 1994; Phrukkanon et al., 1998 [31,32]reported that higher mean bond strengths have been obtained using smaller bonded surface areas. This is attributed to fewer interfacial defects and variations within the tooth. The choice of light-polymerized composite resin as the restorative material was based on research that recommended light-polymerized resins for restorative purposes in conjunction with endodontic therapy [10,33,34]. Microshear bond strength has been claimed to be more reliable, because the stress distribution in the bonding surface of specimens in microshear tests is more homogenous due to lower dimensions of specimens, thus, the statistical analysis would be more accurate. The size of contact surface stress concentration is decreased so that failure pattern shifts to adhesive instead of cohesive failure and this will decrease the errors [35]. In the current study the wire microshear was used instead of using blade, Shimida et al., 2002 [25] modified the microshear bond test by replacing the blade with a looped orthodontic wire. Foong et al., 2006 [24] claimed that the wire microshear test was shown to be easier and more reliable compared to the blade microshear test. Braga et al., 2010 [36] reported that, the use of the chisel as a loading device causes the most severe stress concentration, which is also supported by experimental findings showing lower bond strengths compared to the wire loop. In the present study, nano-composite group recorded higher m-shear bond strength mean value than m-hybrid composite, although both materials were almost within the accepted value, nano composite was higher than mTable 4 Comparison of m-Shear bond strength results (Mean values SDs) between conventional glass-ionomer and nano-glass-ionomer. Glass-ionomer group
Mean SD
t-test
Conventional glass-ionomer Nano glass ionomer
4.062 0.9623 8.77 1.341
t-value 5.7
*Significant (p < 0.05).
P value 0.0001*
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Table 5 Comparison of m-Shear bond strength results (Mean values SDs) between composite and glass-ionomer. Group
Mean SD
t-test
Composite Glass-ionomer
20.36 1.083 6.013 0.6330
t value 0.55
P value <0.0001*
hybrid composite. The difference in m-shear bond mean values between m-hybrid composite and nanocomposite was statistically significant. These findings agree with many authors who suggested that the increase in mechanical resistance results from increasing the filler content of a composite material and decreasing the filler size which influence the resin properties [37e39]. The micro-filler composite resins present generally inferior physical and mechanical properties, since 40e80% (by volume) of the material is constituted of organic matrix [40]. Moszner and Salz, 2001 [18] mentioned that the reduction in particle dimensions and their increased quantity (nano-fillers development principle), could reduce polymerization shrinkage and enhance the mechanical properties of the resins. Meguid and Sun, 2004 [41] reported that the tensile and shear bond strength depend on the filler particles size. The nanosized fillers are characterized by large surface areas per unit gram. As the number of adhesively joined points increases, the cohesive strength of the epoxy increases leading to a higher mechanical strength of the interface. Heshmat et al., 2009 concluded [42] that using composites with a higher filler content and modulus of elasticity resulted in higher shear bond strength in the dentin bonding system. In the present study nano glass-ionomer group recorded higher m-shear bond strength mean value than conventional glass-ionomer. The difference in m-shear bond mean values between conventional glass-ionomer and nano-glass-ionomer was statistically significant.
Fig. 7. A column chart of m-shear bond strength mean values for between composite and glass-ionomer.
Fig. 8. A stacked column chart of different failure modes for all groups.
Glass-ionomer bond chemically to tooth substrates, and bond strengths have been studied extensively in sound and carious dentin using conventional shear testing methods [43,44]. The bond strengths of GIC have been reported in the range of 3e4 MPa [45,46]. Previous studies of the bond strength of GIC to dentin using tensile or shear tests indicated that bond strengths above 5 MPa were seldom achieved [47,48]. The findings of this study agree with some authors who explained the chemical bonding of GIC by ionic exchange between resin modified glass-ionomer and dentin substrate, the penetration and further lightcuring of the resin modified glass-ionomer through the smear layer into the dentinal tubules provide an additional mechanical interlocking of the polymer to dentin [49,50] also working time is improved in resinmodified glass ionomer [53]. Franckenberger et al., 1997 [51]; suggested that decrease in the size of glass particles has significantly
Fig. 9. Microhybrid composite, mixed failure (adhesive and cohesive within microhybrid composite). A) Tube’s border, B) Dentin surface treated with self-etch bonding, C) Microhybrid composite, D) Acrylic block.
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Fig. 10. Conventional glass-ionomer, adhesive failure; A) Tube’s border, B) Dentin inside tube, C) Dentin outside tube, D) Acrylic block.
improved the mechanical properties, this is thought to be due to better distribution of particles within the matrix. El-Askary and Nassif, 2011 [52]Concluded that the pre-conditioning step effectively improves bonding of the nano-filled RMGI to dentin. As general comparison, the results of this study revealed that composite groups recorded significantly
higher m-shear bond strength mean value than glassionomer. These results agree with Toledano et al., 2003 [53] who concluded that the mean bond strength of the light cure composite resin is clinically acceptable, while the GIC showed the lowest shear bond strength. The lower strength of glass ionomer was due to their brittleness, initial sensitivity to moisture contamination, sensitivity to dehydration [54,55]. Moisture leads the material to be chalky and porous, resulting in a loss of surface hardness [56]. Shear bond strength value obtained for conventional glass ionomer is below minimal recommended values for clinical purposes [53]. Nicholson, 1998 [50] and other studies (Mitchell et al., 1995; Fruits et al., 1996; Nicholson, 1998) [50,57,58] concluded that glass-ionomers as well as resin modified glass-ionomers have lower shear bond strengths when compared with composite adhesive systems, also Prabhakar, 2003 concluded [59] that the composite showed higher mean shear bond strength in permanent teeth as compared to compomer and resin modified glass ionomer. Failure mode in the current study was noted as adhesive, cohesive, or mixed. Evaluation of the failure modes after m-shear testing indicated that high bond strength showed cohesive or mixed modes. These results agree with Schneider et al., 2000 [60] who found that the better the bond between dentin and composite the higher the percentage of cohesive failure within each sample. The amount of substrate fracture is often indicative of the retentive strength of the adhesive, increasing the tendency of mixed failure as the bond strength increases. In the current study the lowest bond strength group (convential glass-ionomer) tended to exhibit adhesive failure mode. These results agree with Chung et al., 2009 [61]who reported that failures in conventional glass were adhesive along the dentin interface, indicating a weak bond between material and dentin. Vargas, 1995 [62] suggested that the cohesive fracture indicated that the shear bond strength between dentin and this material is higher than the cohesive strength of the material itself. Armstrong et al., 1998 [63] stated that cohesive substrate fractures, in either the tensile or shear mode of strength testing, cannot be attributed to the bond strength of the adhesive being greater than the respective tensile or shear strength of the adherent. 5. Conclusion
Fig. 11. Nano composite, cohesive failure within dentin; A) Dentin inside the tube, B) Tube’s border, C) Dentin outside the tube, D) Acrylic block.
Nano-composite recorded the highest value of microshear bond strength which is close to the
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