Basic Research—Technology
Influence of Acidic Environment on Properties of Biodentine and White Mineral Trioxide Aggregate: A Comparative Study Amr M. Elnaghy, BDS, MSc, PhD Abstract Introduction: The purpose of this study was to evaluate the surface microhardness, compressive strength, bond strength, and morphologic microstructures of Biodentine (BD; Septodont, Saint Maur des Fosses, France) and white mineral trioxide aggregate (WMTA) after exposure to a range of acidic pH levels. Methods: For each test, 4 groups of each material were exposed to pH values of 7.4, 6.4, 5.4, and 4.4, respectively, for 7 days. The surface hardness was determined using Vickers microhardness. The compressive strength and micro–push-out bond strength were determined using the universal testing machine at a crosshead speed of 0.5 mm/min. The morphologic microstructures of specimens were evaluated using scanning electron microscopy. Results: BD showed higher surface hardness, compressive strength, and bond strength to root dentin compared with WMTA after exposure to different pH values. A substantial change in the microstructure of BD and WMTA occurred after exposure to different pH values. WMTA appeared to be more sensitive to acidic pH environments than BD. Conclusions: BD material seems more appropriate for use when exposed to an acidic environment compared with WMTA. (J Endod 2014;-:1–5)
Key Words Acidic, Biodentine, bond, compressive, hardness, microstructures
From the Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt. Address requests for reprints to Dr Amr M. Elnaghy, Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, PC 35516, Egypt. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.11.007
JOE — Volume -, Number -, - 2014
M
ineral trioxide aggregate (MTA) has been commonly used as a promising biomaterial for the repair of root and furcation perforations, root-end fillings, apical plugs, and root canal fillings (1, 2). MTA consists of a fine powder of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and bismuth oxide (3). There are 2 types of MTA: gray and white; the main difference between the 2 types is the absence of iron in white MTA (WMTA) (4). WMTA was developed for its application in esthetically sensitive areas (5). During clinical applications of MTA as root-end and perforation filling material or as an apical plug in necrotic teeth with open apices, MTA may be exposed to an acidic environment because of the presence of periradicular inflammation (3, 6). Changes in the pH of host tissues because of the presence of pre-existing disease (7) may influence the physical and chemical properties of the material (8). It has been reported that the hardness (9), diametric tensile strength (6), push-out bond strength to dentin (8), and sealing ability (10) of MTA were decreased after placement in an acidic environment. A variety of new calcium silicate–based materials have been developed recently (11, 12) to further improve MTA drawbacks such as the prolonged setting time, difficult handling characteristics, high cost, and potential of tooth discoloration (13). Recently, a new calcium silicate–based material Biodentine (BD) (Septodont, Saint Maur des Fosses, France) was introduced. BD is composed of tricalcium silicate, calcium carbonate, zirconium oxide, and a water-based liquid containing calcium chloride used as setting accelerator and water-reducing agent (12). BD is a fast-setting calcium silicate–based material that claimed to be used as a dentin restorative material as well as endodontic indications comparable with those of MTA (14). An acidic pH value, mostly as a result of bacterial-induced local metabolic acidosis or tissue inflammation, could probably affect the physical and chemical properties of calcium silicate–based material used as repair of root and furcation perforations, root-end fillings, and apical plugs (3, 6, 9, 10, 15). The effect of an acidic environment on the properties of BD has not been studied. Consequently, the purpose of this study was to evaluate and compare the properties of BD and WMTA after exposure to a range of acidic pH levels.
Materials and Methods Microhardness Measurement A total of 120 disc-shaped specimens of BD and WMTA were prepared in a split Teflon mold (DuPont, Tokyo, Japan) (diameter = 5 mm and height = 1.5 mm) using a nonsurgical manual MTA carrier (Dentsply Tulsa Dental, Tulsa, OK). BD and WMTA Branco (Angelus Soluc¸~oes Odontologicas, Londrina, Parana, Brazil) were mixed according to their manufacturers’ instructions. Sixty specimens of each material were divided into 4 groups (n = 15/group) according to storage media: group 1: specimens were wrapped in pieces of gauze soaked in sterile distilled water at a pH of 7.4 and for groups 2, 3, and 4, specimens were wrapped in pieces of gauze soaked in butyric acid buffered at pH values of 6.4, 5.4, and 4.4, respectively, and then incubated for 7 days at 37 C. After 7 days, the specimens were washed and dried with air spray. The surface microhardness of the specimens was measured using a digital microhardness tester (FM-7; Future Tech Corp, Tokyo, Japan). A diamond indenter with a 50-g load and a dwell time of 10 seconds were used. The Vickers microhardness (VHN) was calcuL , where L is the applied lated using the following formula (9): VHN ¼ 1:854 d2 load (kg) and d is the mean indentation diagonal length (mm).
Influence of pH on Biodentine
1
2
Elnaghy
58.9 3.5 47.6 3.0 43.7 2.1 26.1 1.9 95.2 9.3 81.4 7.7 73.6 6.6 58.8 4.8 9.1 1.8 7.2 1.1 5.3 0.9 4.3 0.7C,a 44.4 3.9A,b 35.9 2.9B,b 31.2 1.7C,b 16.3 1.4D,b 71.0 6.9A,b 60.1 5.1B,b 53.4 4.7C,b 31.6 2.4D,b 7.0 1.2A,b 5.2 0.7B b 3.4 0.6C,b 2.5 0.4C,b C,a
5.4
B a
6.4
BD, Biodentine; MPa, megpascals; VHN, Vickers microhardness; WMTA, white mineral trioxide aggregate. Mean values for each property represented with different superscript uppercase letter (row) or lowercase letter (column) are significantly different (P < .05).
BD WMTA
7.4
A,a
6.4
B,a
5.4
C,a
4.4
D,a
7.4
A,a
6.4
B,a
5.4
C,a
pH pH
4.4
D,a
7.4
A,a
Micro–push-out (MPa)
pH Material
Statistical Analysis Statistical analyses (SPSS 13.0; SPSS Inc, Chicago, IL) of the tested properties data were analyzed using 2-way analysis of variance considering 2 factors, the type of cement and storage solution, and Tukey post hoc tests. Statistical significance level was set at P < .05.
Compressive strength (MPa)
Scanning Electron Microscopy A total of 24 disc-shaped specimens of BD and WMTA, 12 specimens of each material (n = 3/group), were prepared and grouped as mentioned before according to the microhardness test. A scanning electron microscope (JEOL JXA-840A; JEOL Ltd, Tokyo, Japan) was used to characterize the microstructural surface morphology of the specimens. The specimens were sputtered (Sputter Coater S 150A; Edwards, Crawley, England) with gold and imaged using a scanning electron microscope at magnifications of 1000 and 5000.
Microhardness (VHN)
Micro–push-out Test Single-rooted human teeth with straight root canals were selected for this study. Midroot dentin was sectioned horizontally into slices with a thickness of 1.0 mm using a low-speed diamond saw (Isomet 1000; Beuhler Ltd, Lake Bluff, IL) to obtain 120 root dentin slices, 60 specimens for each material (n = 15). The lumens of the root slices were drilled with #2–#5 Gates-Glidden burs (Dentsply Maillefer, Ballaigues, Switzerland) to obtain 1.3-mm diameter standardized cavities. BD and WMTA were mixed according to their manufacturers’ instructions and placed inside the lumens of the root slices. Saline-moistened Gelatamp (Roeko-Coltene/Whaledent, Langenau, Germany) was used as a matrix to prevent extrusion of the mixed materials. In group 1, the specimens were wrapped in pieces of gauze soaked in sterile distilled water at a pH of 7.4. In groups 2, 3, and 4, specimens were wrapped in pieces of gauze soaked in butyric acid buffered at pH values of 6.4, 5.4, and 4.4, respectively, and then incubated for 7 days at 37 C (8). The slice was loaded with a cylindrical plunger of a 1.00-mm diameter at a crosshead speed of 0.5 mm/min using a universal testing machine. The maximum load applied to filling material at the time of dislodgement was recorded in newtons and divided by the adhesion area (mm2) of root canal filling to calculate the bond strength in MPa. The bonding area was calculated using the formula 2pr h, where p is the constant 3.14, r is the root canal radius, and h is the thickness of the root slice in millimeters (18). Slices were then examined under a stereomicroscope (Olympus SZX-ILLB100; Olympus Optical, Tokyo, Japan) at 40 magnification to determine the mode of the bond failure. The modes of failure were classified into 3 categories as follows: (1) adhesive failure that occurred at the filling material and dentin interface, (2) cohesive failure that happened within the filling material, and (3) mixed failure mode.
TABLE 1. Mean Standard Deviation of Microhardness (VHN), Compressive Strength (MPa), Micro–push-out (MPa) of Different Calcium Silicate-Based Materials, and Tukey Analysis
Compressive Strength A total of 120 cylindrical specimens of BD and WMTA, 60 specimens of each material, were prepared in a stainless steel split mold (diameter = 4 mm and height = 6 mm) and grouped (n = 15/group) as mentioned before according to the microhardness test. The compressive strength (megapascals [MPa]), Cs, of the specimens was performed using the universal testing machine (Model TT-B; Instron Co, Canton, MA) at a crosshead speed of 0.5 mm/min, and it was calcu4Pf lated using the following formula (16, 17): Cs ¼ pD 2 , where Pf is the load (N) at fracture and D is the diameter of specimen (mm).
4.4
Basic Research—Technology
JOE — Volume -, Number -, - 2014
Basic Research—Technology Results Two-way analysis of variance of the microhardness, compressive strength, and micro–push-out testing data revealed that they were significantly affected by the type of cement and storage solution (P < .001). The mean and standard deviations of the tested properties are presented in Table 1. The highest mean surface hardness (58.9 3.5 VHN), compressive strength (95.2 9.3 MPa), and bond strength values (9.1 1.8 MPa) were presented for BD after exposure to a pH value of 7.4 among the groups. BD showed a statistically significant difference in microhardness, compressive strength, and bond strength values compared with WMTA in all groups (P < .001). Most failure modes were adhesive type of failures between the filling material and dentin interface (73.4%) followed by cohesive failure within the filling material (25.8%). In addition, mixed failure was also observed (0.8%). The microstructure of BD and WMTA surfaces exposed to a pH value of 7.4 showed an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels (Figs. 1A–H and 2A–H). These structures disappeared when the cement hardened in an acidic environment with a selective loss of matrix. BD exposed to pH values of 6.4, 5.4, and 4.4 revealed needle-shaped crystals, microchannels, cubic crystals, and honeycomb-shaped structures (Fig. 1). However, WMTA showed needle structures with microchannels appeared after exposure to a pH value of 6.4 (Figs. 2C and D). The surface became more eroded with laminated cross-stratified structures after exposure to a pH value of 5.4 (Figs. 2E and F). Black areas interpreted as pores and microchannels with laminated cross-stratified structures can be observed for specimens exposed to a higher acidic pH value of 4.4 (Figs. 2G and H).
Discussion In the human body, a slight alteration in pH under normal physiologic conditions is regulated by the carbonic acid–bicarbonate buffer system and the other pH regulatory systems active in connective and periodontal tissues (19, 20). On the other hand, under certain clinical applications, calcium silicate–based materials used for the repair of root and furcation perforations, root-end fillings, and apical plugs are placed in an environment in which inflammation may be present and the surface of the unset material will be exposed to a low pH environment (1, 9). The placement of calcium silicate–based materials in an inflamed low pH environment may affect its physical and chemical properties (9, 21). In the present study, the properties of BD and WMTA were evaluated and compared after exposure to a range of acidic pH levels. The butyric acid, a byproduct of anaerobic bacteria metabolism, was used to simulate the clinical environmental conditions of periradicular infection (9, 21). The results of present study indicated that the surface microhardness of BD was significantly higher compared with WMTA in all groups. At pH 7.4, the surface microhardness of BD was 58.9 VHN; however, this value decreased significantly after exposure to pH 6.4, 5.4, and 4.4. This finding was also observed for WMTA material. This finding is in agreement with Namazikhah et al (9) where they found the surface microhardness of MTA was impaired in an acidic environment. It could be explained as the material could not harden as well in low pH environment (9, 21). A low pH could potentially inhibit the setting reaction, affect adhesion, or increase the solubility of calcium silicate–based materials (1, 5, 9, 22), which could affect the mechanical properties of the material including the surface microhardness. The results of compressive strength of BD and WMTA after exposure to different pH values were in accordance with the findings of microhardness values. The compressive strength of the tested filling materials was significantly decreased after exposure to low pH values JOE — Volume -, Number -, - 2014
Figure 1. Scanning electron micrographs of BD specimens at 2 different magnifications (1000 and 5000) exposed to the following pH levels: (A and B) pH = 7.4: an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels can be seen; (C and D) pH = 6.4: needle-shaped crystals and cubic crystals are notable; (E and F) pH = 5.4: a selective loss of matrix with a needle-like structure; and (G and H) pH = 4.4: a needle-like structure with honeycomb-shaped structures and microchannels can be seen.
(Table 1). This finding is in agreement with Watts et al (5). As in microhardness, BD showed the highest compressive strength and more resistance to the acidic environment compared with WMTA. In the present study, the micro–push-out test method was used to test the bond strength between BD and WMTA and dentin while exposed to butyric acid solutions with different pH values. The micro–push-out test has been shown to be efficient and reliable to assess the bond strength of calcium silicate–based materials used as perforation repair materials, root-end filling materials, and materials used for apical barrier formation (15, 18). The results revealed that the mean micro–push-out
Influence of pH on Biodentine
3
Basic Research—Technology
Figure 2. Scanning electron micrographs of WMTA specimens at 2 different magnifications (1000 and 5000) exposed to the following pH levels: (A and B) pH = 7.4: an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels can be seen; (C and D) pH = 6.4: needle-shaped crystals with microchannels are notable; (E and F) pH = 5.4: the surface became more eroded with laminated cross-stratified structures; and (G and H) pH = 4.4: black areas interpreted as pores and microchannels with laminated cross-stratified structures can be observed.
bond strength of BD and WMTA to intraradicular dentin decreased significantly after exposure to pH levels of 4.4, 5.4, and 6.4 compared with a pH level of 7.4. The highest and lowest bond strength values were at pH levels of 7.4 and 4.4, respectively. This finding is in agreement with Shokouhinejad et al (8). These results could be attributed to the changes in the physical and chemical properties of BD and WMTA in such a low pH environment (8, 9, 21). In the presence of tissue fluid, the hydration of calcium silicate–based materials results in the formation of hydroxyapatite crystals and the development of a hybrid layer between dentin and calcium silicate–based materials 4
Elnaghy
(8, 23). The development of hydroxyapatite crystals and consequently a hybrid layer at the calcium silicate–based materials–dentin interfacial gap are likely to be disturbed in an acidic environment (8). Different calcium silicate–based materials could present differences in push-out strength (15, 24). The analysis of failure modes showed that most of the failures were adhesive failure between the filling material and dentin interface. This finding is in agreement with previous studies (8, 25). A substantial change in the microstructure of BD and WMTA occurred after exposure to different pH values. Even though the precise elucidation for the morphologic differences is unknown, the acid etching effect most likely results in morphologic changes of the BD in a manner that differed from the WMTA. A needle-shaped structure and cubic crystals with honeycomb-shaped structures of BD were observed (Fig. 1). The honeycomb structure has been reported previously by Kayahan et al (16) and Lee et al (21). On the other hand, the microstructures of WMTA appeared more eroded with laminated crossstratified structures and pore formation after exposure to different pH values (Fig. 2). This finding is in accordance with Namazikhah et al (9) and Kayahan et al (16), even though in the Kayahan et al study, the MTA specimens were exposed to phosphoric acid and not the butyric acid of the present study. WMTA might be more sensitive to acidic pH environments than BD as revealed by the results of the present study. Namazikhah et al (9) reported that a higher porosity of WMTA was found after soaking in butyric acid solution that was buffered to a pH of 4.4, 5.4, 6.4, and 7.4. The high acidic environment leads to more degradation and dissolution of calcium silicate–based materials, which may reveal the environment associated with bacterial colonization that could facilitate the passage of microorganisms or their metabolic products into the periapical tissues (1, 6). Lee et al (21) reported that the MTA crystals may dissolve in an acidic environment of pH 5, which results in an unstable cohesive structure. It has been recommended that treating the inflammation with an alkaline medication, such as calcium hydroxide (Ca[OH]2), may neutralize the environmental pH before applying MTA on an inflamed area (21). In addition, Ca(OH)2 has a denaturing effect on proinflammatory mediators (26). However, the influence of pretreatment with Ca(OH)2 on the properties of MTA is debatable (18). Also, it might be useful to provide pretreatment with antibiotic paste to disinfect the root canal dentin (27) before the application of calcium silicate–based materials. Future studies should be performed to evaluate the effect of these treatments modalities on the behavior of BD and WMTA. Under the conditions of this study, BD material seems more appropriate for use when exposed to an acidic environment compared with WMTA. Surface microhardness, compressive strength, cement-dentin bond strength, and microstructures of BD and WMTA were impaired after exposure to a more acidic solution.
Acknowledgments The author denies any conflicts of interest related to this study.
References 1. Torabinejad M, Chivian N. Clinical applications of mineral trioxide aggregate. J Endod 1999;25:197–205. 2. Felippe WT, Felippe MC, Rocha MJ. The effect of mineral trioxide aggregate on the apexification and periapical healing of teeth with incomplete root formation. Int Endod J 2006;39:2–9. 3. Giuliani V, Nieri M, Pace R, Pagavino G. Effects of pH on surface hardness and microstructure of mineral trioxide aggregate and aureoseal: an in vitro study. J Endod 2010;36:1883–6. 4. Asgary S, Parirokh M, Eghbal MJ, et al. A qualitative X-ray analysis of white and grey mineral trioxide aggregate using compositional imaging. J Mater Sci Mater Med 2006;17:187–91.
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Basic Research—Technology 5. Watts JD, Holt DM, Beeson TJ, et al. Effects of pH and mixing agents on the temporal setting of tooth-colored and gray mineral trioxide aggregate. J Endod 2007;33:970–3. 6. Shie MY, Huang TH, Kao CT, et al. The effect of a physiologic solution pH on properties of white mineral trioxide aggregate. J Endod 2009;35:98–101. 7. Nekoofar MH, Namazikhah MS, Sheykhrezae MS, et al. pH of pus collected from periapical abscesses. Int Endod J 2009;42:534–8. 8. Shokouhinejad N, Nekoofar MH, Iravani A, et al. Effect of acidic environment on the push-out bond strength of mineral trioxide aggregate. J Endod 2010;36:871–4. 9. Namazikhah MS, Nekoofar MH, Sheykhrezae MS, et al. The effect of pH on surface hardness and microstructure of mineral trioxide aggregate. Int Endod J 2008;41:108–16. 10. Saghiri MA, Lotfi M, Saghiri AM, et al. Effect of pH on sealing ability of white mineral trioxide aggregate as a root-end filling material. J Endod 2008;34:1226–9. 11. Asgary S, Shahabi S, Jafarzadeh T, et al. The properties of a new endodontic material. J Endod 2008;34:990–3. 12. Han L, Okiji T. Uptake of calcium and silicon released from calcium silicate-based endodontic mayerials into root canal dentine. Int Endod J 2011;44:1081–7. 13. Parirokh M, Torabinejad M. Mineral trioxide aggregate: a comprehensive literature review-part I: chemical, physical, and antibacterial properties. J Endod 2010;36:16–27. 14. Laurent P, Camps J, De Meo M, et al. Induction of specific cell responses to a Ca3SiO5-based posterior restorative material. Dent Mater 2008;24:1486–94. 15. Hashem AA, Amin SA. The effect of acidity on dislodgment resistance of mineral trioxide aggregate and bioaggregate in furcation perforations: an in vitro comparative study. J Endod 2012;38:245–9. 16. Kayahan MB, Nekoofar MH, Kazandag M, et al. Effect of acid-etching procedure on selected physical properties of mineral trioxide aggregate. Int Endod J 2009;42: 1004–14.
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17. Basturk FB, Nekoofar MH, G€unday M, et al. The effect of various mixing and placement techniques on the compressive strength of mineral trioxide aggregate. J Endod 2013;39:111–4. 18. Saghiri MA, Shokouhinejad N, Lotfi M, et al. Push-out bond strength of mineral trioxide aggregate in the presence of alkaline pH. J Endod 2010;36:1856–9. 19. Azuma M. Fundamental mechanisms of host immune responses to infection. J Periodontal Res 2006;41:361–73. 20. Wray S. Smooth muscle intracellular pH: measurement, regulation, and function. Am J Physiol 1988;254:213–25. 21. Lee YL, Lee BS, Lin FH, et al. Effects of physiological environments on the hydration behavior of mineral trioxide aggregate. Biomaterials 2004;25:787–93. 22. Roy CO, Jeansonne BG, Gerrets TF. Effect of an acid environment on leakage of rootend filling materials. J Endod 2001;27:7–8. 23. Sarkar NK, Caicedo R, Ritwik P, et al. Physicochemical basis of the biologic properties of mineral trioxide aggregate. J Endod 2005;31:97–100. 24. Reyes-Carmona JF, Felippe MS, Felippe WT. The biomineralization ability of mineral trioxide aggregate and portland cement on dentin enhances the push-out strength. J Endod 2010;36:286–91. 25. Vanderweele RA, Schwartz SA, Beeson TJ. Effect of blood contamination on retention characteristics of MTA when mixed with different liquids. J Endod 2006;32: 421–4. 26. Khan AA, Sun X, Hargreaves KM. Effect of calcium hydroxide on proinflammatory cytokines and neuropeptides. J Endod 2008;34:1360–3. 27. Sato I, Ando-Kurihara N, Kota K, et al. Sterilization of infected root-canal dentine by topical application of a mixture of ciprofloxacin, metronidazole and minocycline in situ. Int Endod J 1996;29:118–24.
Influence of pH on Biodentine
5
Influence of Acidic Environment on Properties of Biodentine and White Mineral Trioxide Aggregate: A Comparative Study Amr M. Elnaghy, BDS, MSc, PhD Abstract Introduction: The purpose of this study was to evaluate the surface microhardness, compressive strength, bond strength, and morphologic microstructures of Biodentine (BD; Septodont, Saint Maur des Fosses, France) and white mineral trioxide aggregate (WMTA) after exposure to a range of acidic pH levels. Methods: For each test, 4 groups of each material were exposed to pH values of 7.4, 6.4, 5.4, and 4.4, respectively, for 7 days. The surface hardness was determined using Vickers microhardness. The compressive strength and micro–push-out bond strength were determined using the universal testing machine at a crosshead speed of 0.5 mm/min. The morphologic microstructures of specimens were evaluated using scanning electron microscopy. Results: BD showed higher surface hardness, compressive strength, and bond strength to root dentin compared with WMTA after exposure to different pH values. A substantial change in the microstructure of BD and WMTA occurred after exposure to different pH values. WMTA appeared to be more sensitive to acidic pH environments than BD. Conclusions: BD material seems more appropriate for use when exposed to an acidic environment compared with WMTA. (J Endod 2014;-:1–5)
Key Words Acidic, Biodentine, bond, compressive, hardness, microstructures
From the Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt. Address requests for reprints to Dr Amr M. Elnaghy, Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, PC 35516, Egypt. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2013.11.007
JOE — Volume -, Number -, - 2014
M
ineral trioxide aggregate (MTA) has been commonly used as a promising biomaterial for the repair of root and furcation perforations, root-end fillings, apical plugs, and root canal fillings (1, 2). MTA consists of a fine powder of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and bismuth oxide (3). There are 2 types of MTA: gray and white; the main difference between the 2 types is the absence of iron in white MTA (WMTA) (4). WMTA was developed for its application in esthetically sensitive areas (5). During clinical applications of MTA as root-end and perforation filling material or as an apical plug in necrotic teeth with open apices, MTA may be exposed to an acidic environment because of the presence of periradicular inflammation (3, 6). Changes in the pH of host tissues because of the presence of pre-existing disease (7) may influence the physical and chemical properties of the material (8). It has been reported that the hardness (9), diametric tensile strength (6), push-out bond strength to dentin (8), and sealing ability (10) of MTA were decreased after placement in an acidic environment. A variety of new calcium silicate–based materials have been developed recently (11, 12) to further improve MTA drawbacks such as the prolonged setting time, difficult handling characteristics, high cost, and potential of tooth discoloration (13). Recently, a new calcium silicate–based material Biodentine (BD) (Septodont, Saint Maur des Fosses, France) was introduced. BD is composed of tricalcium silicate, calcium carbonate, zirconium oxide, and a water-based liquid containing calcium chloride used as setting accelerator and water-reducing agent (12). BD is a fast-setting calcium silicate–based material that claimed to be used as a dentin restorative material as well as endodontic indications comparable with those of MTA (14). An acidic pH value, mostly as a result of bacterial-induced local metabolic acidosis or tissue inflammation, could probably affect the physical and chemical properties of calcium silicate–based material used as repair of root and furcation perforations, root-end fillings, and apical plugs (3, 6, 9, 10, 15). The effect of an acidic environment on the properties of BD has not been studied. Consequently, the purpose of this study was to evaluate and compare the properties of BD and WMTA after exposure to a range of acidic pH levels.
Materials and Methods Microhardness Measurement A total of 120 disc-shaped specimens of BD and WMTA were prepared in a split Teflon mold (DuPont, Tokyo, Japan) (diameter = 5 mm and height = 1.5 mm) using a nonsurgical manual MTA carrier (Dentsply Tulsa Dental, Tulsa, OK). BD and WMTA Branco (Angelus Soluc¸~oes Odontologicas, Londrina, Parana, Brazil) were mixed according to their manufacturers’ instructions. Sixty specimens of each material were divided into 4 groups (n = 15/group) according to storage media: group 1: specimens were wrapped in pieces of gauze soaked in sterile distilled water at a pH of 7.4 and for groups 2, 3, and 4, specimens were wrapped in pieces of gauze soaked in butyric acid buffered at pH values of 6.4, 5.4, and 4.4, respectively, and then incubated for 7 days at 37 C. After 7 days, the specimens were washed and dried with air spray. The surface microhardness of the specimens was measured using a digital microhardness tester (FM-7; Future Tech Corp, Tokyo, Japan). A diamond indenter with a 50-g load and a dwell time of 10 seconds were used. The Vickers microhardness (VHN) was calcuL , where L is the applied lated using the following formula (9): VHN ¼ 1:854 d2 load (kg) and d is the mean indentation diagonal length (mm).
Influence of pH on Biodentine
1
2
Elnaghy
58.9 3.5 47.6 3.0 43.7 2.1 26.1 1.9 95.2 9.3 81.4 7.7 73.6 6.6 58.8 4.8 9.1 1.8 7.2 1.1 5.3 0.9 4.3 0.7C,a 44.4 3.9A,b 35.9 2.9B,b 31.2 1.7C,b 16.3 1.4D,b 71.0 6.9A,b 60.1 5.1B,b 53.4 4.7C,b 31.6 2.4D,b 7.0 1.2A,b 5.2 0.7B b 3.4 0.6C,b 2.5 0.4C,b C,a
5.4
B a
6.4
BD, Biodentine; MPa, megpascals; VHN, Vickers microhardness; WMTA, white mineral trioxide aggregate. Mean values for each property represented with different superscript uppercase letter (row) or lowercase letter (column) are significantly different (P < .05).
BD WMTA
7.4
A,a
6.4
B,a
5.4
C,a
4.4
D,a
7.4
A,a
6.4
B,a
5.4
C,a
pH pH
4.4
D,a
7.4
A,a
Micro–push-out (MPa)
pH Material
Statistical Analysis Statistical analyses (SPSS 13.0; SPSS Inc, Chicago, IL) of the tested properties data were analyzed using 2-way analysis of variance considering 2 factors, the type of cement and storage solution, and Tukey post hoc tests. Statistical significance level was set at P < .05.
Compressive strength (MPa)
Scanning Electron Microscopy A total of 24 disc-shaped specimens of BD and WMTA, 12 specimens of each material (n = 3/group), were prepared and grouped as mentioned before according to the microhardness test. A scanning electron microscope (JEOL JXA-840A; JEOL Ltd, Tokyo, Japan) was used to characterize the microstructural surface morphology of the specimens. The specimens were sputtered (Sputter Coater S 150A; Edwards, Crawley, England) with gold and imaged using a scanning electron microscope at magnifications of 1000 and 5000.
Microhardness (VHN)
Micro–push-out Test Single-rooted human teeth with straight root canals were selected for this study. Midroot dentin was sectioned horizontally into slices with a thickness of 1.0 mm using a low-speed diamond saw (Isomet 1000; Beuhler Ltd, Lake Bluff, IL) to obtain 120 root dentin slices, 60 specimens for each material (n = 15). The lumens of the root slices were drilled with #2–#5 Gates-Glidden burs (Dentsply Maillefer, Ballaigues, Switzerland) to obtain 1.3-mm diameter standardized cavities. BD and WMTA were mixed according to their manufacturers’ instructions and placed inside the lumens of the root slices. Saline-moistened Gelatamp (Roeko-Coltene/Whaledent, Langenau, Germany) was used as a matrix to prevent extrusion of the mixed materials. In group 1, the specimens were wrapped in pieces of gauze soaked in sterile distilled water at a pH of 7.4. In groups 2, 3, and 4, specimens were wrapped in pieces of gauze soaked in butyric acid buffered at pH values of 6.4, 5.4, and 4.4, respectively, and then incubated for 7 days at 37 C (8). The slice was loaded with a cylindrical plunger of a 1.00-mm diameter at a crosshead speed of 0.5 mm/min using a universal testing machine. The maximum load applied to filling material at the time of dislodgement was recorded in newtons and divided by the adhesion area (mm2) of root canal filling to calculate the bond strength in MPa. The bonding area was calculated using the formula 2pr h, where p is the constant 3.14, r is the root canal radius, and h is the thickness of the root slice in millimeters (18). Slices were then examined under a stereomicroscope (Olympus SZX-ILLB100; Olympus Optical, Tokyo, Japan) at 40 magnification to determine the mode of the bond failure. The modes of failure were classified into 3 categories as follows: (1) adhesive failure that occurred at the filling material and dentin interface, (2) cohesive failure that happened within the filling material, and (3) mixed failure mode.
TABLE 1. Mean Standard Deviation of Microhardness (VHN), Compressive Strength (MPa), Micro–push-out (MPa) of Different Calcium Silicate-Based Materials, and Tukey Analysis
Compressive Strength A total of 120 cylindrical specimens of BD and WMTA, 60 specimens of each material, were prepared in a stainless steel split mold (diameter = 4 mm and height = 6 mm) and grouped (n = 15/group) as mentioned before according to the microhardness test. The compressive strength (megapascals [MPa]), Cs, of the specimens was performed using the universal testing machine (Model TT-B; Instron Co, Canton, MA) at a crosshead speed of 0.5 mm/min, and it was calcu4Pf lated using the following formula (16, 17): Cs ¼ pD 2 , where Pf is the load (N) at fracture and D is the diameter of specimen (mm).
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Basic Research—Technology Results Two-way analysis of variance of the microhardness, compressive strength, and micro–push-out testing data revealed that they were significantly affected by the type of cement and storage solution (P < .001). The mean and standard deviations of the tested properties are presented in Table 1. The highest mean surface hardness (58.9 3.5 VHN), compressive strength (95.2 9.3 MPa), and bond strength values (9.1 1.8 MPa) were presented for BD after exposure to a pH value of 7.4 among the groups. BD showed a statistically significant difference in microhardness, compressive strength, and bond strength values compared with WMTA in all groups (P < .001). Most failure modes were adhesive type of failures between the filling material and dentin interface (73.4%) followed by cohesive failure within the filling material (25.8%). In addition, mixed failure was also observed (0.8%). The microstructure of BD and WMTA surfaces exposed to a pH value of 7.4 showed an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels (Figs. 1A–H and 2A–H). These structures disappeared when the cement hardened in an acidic environment with a selective loss of matrix. BD exposed to pH values of 6.4, 5.4, and 4.4 revealed needle-shaped crystals, microchannels, cubic crystals, and honeycomb-shaped structures (Fig. 1). However, WMTA showed needle structures with microchannels appeared after exposure to a pH value of 6.4 (Figs. 2C and D). The surface became more eroded with laminated cross-stratified structures after exposure to a pH value of 5.4 (Figs. 2E and F). Black areas interpreted as pores and microchannels with laminated cross-stratified structures can be observed for specimens exposed to a higher acidic pH value of 4.4 (Figs. 2G and H).
Discussion In the human body, a slight alteration in pH under normal physiologic conditions is regulated by the carbonic acid–bicarbonate buffer system and the other pH regulatory systems active in connective and periodontal tissues (19, 20). On the other hand, under certain clinical applications, calcium silicate–based materials used for the repair of root and furcation perforations, root-end fillings, and apical plugs are placed in an environment in which inflammation may be present and the surface of the unset material will be exposed to a low pH environment (1, 9). The placement of calcium silicate–based materials in an inflamed low pH environment may affect its physical and chemical properties (9, 21). In the present study, the properties of BD and WMTA were evaluated and compared after exposure to a range of acidic pH levels. The butyric acid, a byproduct of anaerobic bacteria metabolism, was used to simulate the clinical environmental conditions of periradicular infection (9, 21). The results of present study indicated that the surface microhardness of BD was significantly higher compared with WMTA in all groups. At pH 7.4, the surface microhardness of BD was 58.9 VHN; however, this value decreased significantly after exposure to pH 6.4, 5.4, and 4.4. This finding was also observed for WMTA material. This finding is in agreement with Namazikhah et al (9) where they found the surface microhardness of MTA was impaired in an acidic environment. It could be explained as the material could not harden as well in low pH environment (9, 21). A low pH could potentially inhibit the setting reaction, affect adhesion, or increase the solubility of calcium silicate–based materials (1, 5, 9, 22), which could affect the mechanical properties of the material including the surface microhardness. The results of compressive strength of BD and WMTA after exposure to different pH values were in accordance with the findings of microhardness values. The compressive strength of the tested filling materials was significantly decreased after exposure to low pH values JOE — Volume -, Number -, - 2014
Figure 1. Scanning electron micrographs of BD specimens at 2 different magnifications (1000 and 5000) exposed to the following pH levels: (A and B) pH = 7.4: an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels can be seen; (C and D) pH = 6.4: needle-shaped crystals and cubic crystals are notable; (E and F) pH = 5.4: a selective loss of matrix with a needle-like structure; and (G and H) pH = 4.4: a needle-like structure with honeycomb-shaped structures and microchannels can be seen.
(Table 1). This finding is in agreement with Watts et al (5). As in microhardness, BD showed the highest compressive strength and more resistance to the acidic environment compared with WMTA. In the present study, the micro–push-out test method was used to test the bond strength between BD and WMTA and dentin while exposed to butyric acid solutions with different pH values. The micro–push-out test has been shown to be efficient and reliable to assess the bond strength of calcium silicate–based materials used as perforation repair materials, root-end filling materials, and materials used for apical barrier formation (15, 18). The results revealed that the mean micro–push-out
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Figure 2. Scanning electron micrographs of WMTA specimens at 2 different magnifications (1000 and 5000) exposed to the following pH levels: (A and B) pH = 7.4: an amorphous poorly crystallized superficial gel structure containing globular aggregate particles and microchannels can be seen; (C and D) pH = 6.4: needle-shaped crystals with microchannels are notable; (E and F) pH = 5.4: the surface became more eroded with laminated cross-stratified structures; and (G and H) pH = 4.4: black areas interpreted as pores and microchannels with laminated cross-stratified structures can be observed.
bond strength of BD and WMTA to intraradicular dentin decreased significantly after exposure to pH levels of 4.4, 5.4, and 6.4 compared with a pH level of 7.4. The highest and lowest bond strength values were at pH levels of 7.4 and 4.4, respectively. This finding is in agreement with Shokouhinejad et al (8). These results could be attributed to the changes in the physical and chemical properties of BD and WMTA in such a low pH environment (8, 9, 21). In the presence of tissue fluid, the hydration of calcium silicate–based materials results in the formation of hydroxyapatite crystals and the development of a hybrid layer between dentin and calcium silicate–based materials 4
Elnaghy
(8, 23). The development of hydroxyapatite crystals and consequently a hybrid layer at the calcium silicate–based materials–dentin interfacial gap are likely to be disturbed in an acidic environment (8). Different calcium silicate–based materials could present differences in push-out strength (15, 24). The analysis of failure modes showed that most of the failures were adhesive failure between the filling material and dentin interface. This finding is in agreement with previous studies (8, 25). A substantial change in the microstructure of BD and WMTA occurred after exposure to different pH values. Even though the precise elucidation for the morphologic differences is unknown, the acid etching effect most likely results in morphologic changes of the BD in a manner that differed from the WMTA. A needle-shaped structure and cubic crystals with honeycomb-shaped structures of BD were observed (Fig. 1). The honeycomb structure has been reported previously by Kayahan et al (16) and Lee et al (21). On the other hand, the microstructures of WMTA appeared more eroded with laminated crossstratified structures and pore formation after exposure to different pH values (Fig. 2). This finding is in accordance with Namazikhah et al (9) and Kayahan et al (16), even though in the Kayahan et al study, the MTA specimens were exposed to phosphoric acid and not the butyric acid of the present study. WMTA might be more sensitive to acidic pH environments than BD as revealed by the results of the present study. Namazikhah et al (9) reported that a higher porosity of WMTA was found after soaking in butyric acid solution that was buffered to a pH of 4.4, 5.4, 6.4, and 7.4. The high acidic environment leads to more degradation and dissolution of calcium silicate–based materials, which may reveal the environment associated with bacterial colonization that could facilitate the passage of microorganisms or their metabolic products into the periapical tissues (1, 6). Lee et al (21) reported that the MTA crystals may dissolve in an acidic environment of pH 5, which results in an unstable cohesive structure. It has been recommended that treating the inflammation with an alkaline medication, such as calcium hydroxide (Ca[OH]2), may neutralize the environmental pH before applying MTA on an inflamed area (21). In addition, Ca(OH)2 has a denaturing effect on proinflammatory mediators (26). However, the influence of pretreatment with Ca(OH)2 on the properties of MTA is debatable (18). Also, it might be useful to provide pretreatment with antibiotic paste to disinfect the root canal dentin (27) before the application of calcium silicate–based materials. Future studies should be performed to evaluate the effect of these treatments modalities on the behavior of BD and WMTA. Under the conditions of this study, BD material seems more appropriate for use when exposed to an acidic environment compared with WMTA. Surface microhardness, compressive strength, cement-dentin bond strength, and microstructures of BD and WMTA were impaired after exposure to a more acidic solution.
Acknowledgments The author denies any conflicts of interest related to this study.
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