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Chemical Modification of ProRoot MTA to Improve Handling Characteristics and Decrease Setting Time
Basic Research—Technology
Chemical Modification of ProRoot MTA to Improve Handling Characteristics and Decrease Setting Time Benjamin S. Ber, DDS, MS,*† John F. Hatton, DMD,†‡ and Gregory P. Stewart, PhD§ Abstract Mineral trioxide aggregate (MTA) fulfills many of the ideal properties of a root-end filling material. However, the composition of this material often makes MTA difficult to use, a direct result of its granular consistency, slow setting time, and initial looseness. Additives used by the Portland cement (PC) industry to increase PC’s plasticity and decrease its setting time were added first to PC and then to gray MTA in an attempt to improve MTA’s handling characteristics, with the combination providing the best handling characteristics tested for its effect on compressive strength (for changes in the original material’s properties) and decrease in setting time. An admix of 1% methylcellulose and 2% calcium chloride resulted in a mix of chemically modified MTA that, when compared with unmodified MTA, (1) handled similarly to a reinforced zinc oxide– eugenol cement, (2) gave an approximately equal compressive strength, and (3) set one third faster (57 ⫾ 3 minutes). (J Endod 2007;33:1231–1234)
Key Words Compressive strength, handling characteristics, MTA, setting time
From *Private Practice, Houma, Louisiana; †Department of Endodontics, Saint Louis University Center for Advanced Dental Education, St Louis, Missouri; and ‡Section of Endodontics and §Section of Dental Materials, Southern Illinois University School of Dental Medicine, Alton, Illinois. Address requests for reprints to John F. Hatton, DMD, Director, Graduate Endodontics, Saint Louis University Center for Advanced Dental Education, 3320 Rutger St, St Louis, MO 63104. E-mail address: [email protected]. 0099-2399/$0 - see front matter Copyright © 2007 by the American Association of Endodontists. doi:10.1016/j.joen.2007.06.012
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ineral trioxide aggregate (MTA) (ProRoot; Dentsply, Tulsa Dental, Tulsa, OK) fulfills many of the ideal properties of a root-end filling material as described by Gartner and Dorn (1). However, the composition of the material often makes it difficult to use, for example, in apical surgeries with limited access. MTA is a difficult material to handle because of its granular consistency (2), slow setting time (3), and initial looseness (2, 4, 5). Once the mixture starts to dry, it loses its cohesiveness and becomes hard to handle (6). This problem is highlighted by the development and introduction of special delivery systems to facilitate the placement of MTA. Even with the advantages that some of these delivery systems offer, MTA has a tendency to washout in the presence of excess moisture (2). MTA is very similar in composition to that of Portland cement (PC) (7) and appears to have the same difficulties with handling and washout as PC placed in a wet environment (8). It was the intent of this research to incorporate additives used by the cement industry to MTA to try and improve two of the main criticisms of MTA, poor handling and long setting time. Two additives that we believed would minimally affect the biocompatibility were chosen for testing, methylcellulose (MC), an anti-washout ingredient; and calcium chloride (CaCl2), an accelerator of the setting time. The MC anti-washout admixture binds water molecules within the cement, accomplishing two things. First, the addition of MC to cement increases the cohesiveness and plasticity (or moldability) of the material, making it easier to handle. Our goal was to modify the plasticity of MTA to create a material that handled similarly to a reinforced zinc oxide– eugenol filling material. Second, using an admix of MC should increase the washout resistance, a benefit when placed in a contaminated (wet) site. This rate of washout resistance has been shown to increase as the amount of anti-washout admixture is increased. In addition, the viscosity of the mix increases as the concentration of MC increases. However, the addition of percentages of MC greater than 3% can significantly extend the setting time of PC (9). The normal setting time of PC ranges from 3– 4 hours. This property can also be modified. There are several ways that the setting time of PC can be accelerated, thereby increasing the initial strength and decreasing early washout. One way is by the addition of chemicals such as CaCl2. The addition of accelerators increases the rate of hydration, thereby accelerating the setting reaction (10). The purpose of the study was to determine whether additives could be used with MTA to improve the material’s poor handling characteristics and long setting time.
Materials and Methods A series of studies were initially performed to determine the most appropriate concentrations of two additives, CaCl2 and MC, to improve the handling characteristics of MTA. The initial studies were conducted with PC because of its similarity in properties and reduced cost as compared with MTA. The goal was to obtain a handling property similar to a zinc oxide– eugenol type root-end filling material. Once the proper concentration of additives that improved the cement’s handling characteristics was determined, the additives were tested on gray MTA. Concentrations of MC (Sigma, St Louis, MO) ranging from 1%–3% by weight were tested to find the concentration that best improved the cohesiveness and workability of the material. These percentages were then combined with a 2% CaCl2 (PCCA, Houston, TX) solution. Similar to manufacturer’s recommendations for MTA, a 3:1 powder to liquid ratio was used for all PC samples. To evaluate all combinations, 8 groups were tested in the preliminary study with the PC.
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Chemical Modification of MTA
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Basic Research—Technology The additive solutions were prepared in the following way. Calcium chloride equal to 2% of the sample weight was added to distilled water and mixed into solution proportional to the water needed for a 3:1 powder to water ratio. Half of the solution was placed on a hot plate on which the temperature of the solution was raised to 80°C to get the MC into solution (per manufacturer’s instructions). The appropriate amount of MC to create the test concentrations was added to the warmed solution and stirred to wet the particles. The remainder of the room temperature solution was added and then stirred until all of the chemicals were in solution. It was then stored at 0°C for 20 minutes to allow for the mixture to thicken. The solution was then mechanically stirred with a magnetic stirrer for 30 minutes to create a homogenous gel. These solutions in a 3:1 powder to liquid ratio were then added to the following test groups. The PC groups were Group 1, PC, no additives; Group 2, 1% MC; Group 3, 2% MC; Group 4, 3% MC; Group 5, CaCl2 (2%); Group 6, 1% MC/CaCl2 (2%); Group 7, 2% MC/CaCl2 (2%); and Group 8, 3% MC/CaCl2 (2%). Each group was evaluated for compressive strength (n ⫽ 6) and setting time (n ⫽ 3). As a result of the study with PC, the following groups with MTA were tested. The MTA groups were Group 9, positive control, MTA mixed according to manufacturer’s instructions by using saline; Group 10, 1% MC/CaCl2 (2%); and Group 11, 2% MC/CaCl2 (2%). Six samples of each group were measured for compressive strength, and 3 were analyzed for setting times. For the compressive strength testing samples, the material was placed into Teflon tube molds 8 mm high and 4 mm in diameter with a glass slab beneath to ensure a flat surface for condensation. The material was added in increments and condensed with an amalgam plugger to minimize air entrapment. Molds were filled to excess, and the excess material was removed by using a glass slide to ensure level samples. Specimens were handled in a similar matter as reported by Torabinejad et al. (11). Specimens were suspended on a metal grate above a closed hot water bath (37°C) and covered to provide 100% humidity without submerging the samples to allow for the initial set. After 3 hours, the samples were removed from the Teflon tube and examined for voids and irregularities. If voids were detected, the sample was discarded. The samples were then immersed in distilled water (23°C) for the duration of the test period. Each group was tested at 24 hours and 21 days. The samples were then removed from the distilled water and allowed to dry. The flat surfaces of each sample were sanded smooth with 600 grit sandpaper. The samples were tested by using an Instron (Instron Co, Park Ridge, IL) materials testing machine until failure. A crosshead speed of 2 mm/sec with a load cell of 5 kN was used. All measurements were recorded in kilograms and converted to megapascals (MPa). To determine setting time, a standard Vicat needle apparatus (Shambhavi Impex, Maharashtra, India) was used. The molds used to test the samples were 10 mm wide and 2 mm deep. Spatulation was carried out on a glass slab by using a cement spatula until all particles were adequately incorporated into the mixture. After a homogenous mix
TABLE 2. Compressive Strength of MTA Groups Group
24-Hr (mean)
3-Week (mean)
MTA 1% MC/CaCl2 in MTA 2% MC/CaCl2 in MTA
26.4 ⫾ 6.6 25.5 ⫾ 4.4 20.8 ⫾ 1.9
30.4 ⫾ 12.8 29.1 ⫾ 6.4 26.5 ⫾ 6.6
Units in MPa; n ⫽ 6.
was achieved, the cement was transferred into the molds, supported by glass slabs beneath to ensure adequate control of the material, condensed with an amalgam plugger, and filled to excess. The excess was removed with a glass slab until the surface was smooth and level. After the preparation of each sample, it was suspended above a hot water bath at a temperature of 37°C and covered with a lid to achieve 100% humidity. The samples were then removed from the hot water bath container and tested every 5 minutes with a Vicat needle apparatus (needle mass, 300 ⫾ 0.5 g with a flat diameter of 1 ⫾ 0.5 mm). The needle was cleaned between each test. Setting was judged to be complete when the needle did not penetrate the surface of the material after being allowed to settle for 30 seconds. Statistical analysis was carried out by using analysis of variance with post hoc Tukey B test, with significance set at P ⬍ .05.
Results The results for the compressive strength of the PC groups at 24 hours and 3 weeks are found in Table 1. At 24 hours, addition of 2% CaCl2 alone resulted in a significantly increased compressive strength compared with all other groups. Also at 24 hours, there was a tendency for decreased compressive strength because the concentration of MC was increased, although this was not significant. At 3 weeks, all groups demonstrated a significantly decreased compressive strength compared with the PC control. There also was a significant difference between the CaCl2 only and the MC and MC/CaCl2 additives. There were no significant differences between any of the MC and MC/CaCl2 additives at 3 weeks. The results for the compressive strength testing of MTA are found in Table 2. There was no significant difference between the MTA control and the groups with the additives at both 24 hours and 3 weeks. The addition of 2% MC demonstrated the greatest reduction in compressive strength, but this was not significant (P ⬍ .05). The results of the setting time for the PC group are reported in Table 3. There was a significant decrease in setting time between the PC control and the addition of CaCl2 and the MC/CaCl2 additives. The setting times of the 1%, 2%, and 3% MC/CaCl2 additives were significantly different from each other, with higher concentrations of MC resulting in longer setting times. There was no significant difference between the PC control and the MC additive only. Table 4 reports the setting time of MTA compared with the groups containing the additives. There was a significant decrease in setting time between the MTA only control and both MC/CaCl2 groups. In addition,
TABLE 1. Compressive Strength of PC Groups Group
24-Hr (mean)
3-Week (mean)
PC 1% MC in PC 2% MC in PC 3% MC in PC CaCl2 (2% solution) 1% MC/CaCl2 in PC 2% MC/CaCl2 in PC 3% MC/CaCl2 in PC
16.8 ⫾ 4.5 15.8 ⫾ 1.8 9.6 ⫾ 3.4 10.1 ⫾ 1.8 35.6 ⫾ 4.9 24.1 ⫾ 4.6 22.3 ⫾ 4.4 18.1 ⫾ 5.0
52.9 ⫾ 6.3 31.3 ⫾ 6.5 31.7 ⫾ 4.4 33.3 ⫾ 5.8 41.5 ⫾ 11.1 33.8 ⫾ 7.6 33.5 ⫾ 7.5 35.2 ⫾ 9.8
Units in MPa; n ⫽ 6.
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TABLE 3. Setting Time of PC Groups Group
Minutes (mean)
PC 1% MC in PC 2% MC in PC 3% MC in PC CaCl2 (2% solution) 1% MC/CaCl2 in PC 2% MC/CaCl2 in PC 3% MC/CaCl2 in PC
183 ⫾ 8 215 ⫾ 5 182 ⫾ 3 202 ⫾ 8 83 ⫾ 3 60 ⫾ 5 107 ⫾ 3 125 ⫾ 5
n ⫽ 3.
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Basic Research—Technology TABLE 4. Setting Time of MTA Groups Group
24-Hr (mean)
MTA 1% MC/CaCl2 in MTA 2% MC/CaCl2 in MTA
202 ⫾ 3 57 ⫾ 3 105 ⫾ 5
n ⫽ 3.
the setting time for the 1% MC/CaCl2 group was significantly less than the time for the 2% MC/CaCl2 group.
Discussion The purpose of this study was to improve on the handling characteristics and long setting times of MTA. The material is very grainy and has a poor consistency, making it difficult to use in some clinical situations. Our intent was to give MTA a more cohesive, more moldable consistency similar to a reinforced zinc oxide– eugenol, eg, IRM (Dentsply Caulk, Milford, DE). Because PC and MTA are very similar in composition, we selected two additives used by the cement industry, MC and CaCl2. MC is used as an additive to improve the performance of PC in a wet environment. MC is composed of nonionic water-soluble cellulose ether, which has an OH base and is almost like water. It increases viscosity and dispersion resistance (9). CaCl2 is used to accelerate the setting reaction of PC. The mechanism of action is not fully understood, but it is believed that CaCl2 is partially consumed during hydration, reacting with tricalcium aluminate and forming chloroaluminate (10). MC was tested by using 1%, 2%, and 3% concentrations by weight and CaCl2 in a 2% by weight concentration. These concentrations mimic those used by the cement industry (10). Higher concentrations of MC have the capacity to entrap air and retain higher amounts of H2O, altering PC strength and retarding the setting reaction. Both of these effects are a function of concentration (9). Greater than 2% CaCl2 adversely affects the cement by increasing the risk of drying shrinkage and reducing ultimate strength (10). These additives were first tested on PC, and the most promising combinations were then tested on MTA. There are no standardized tests for evaluating handling characteristics. We mixed and compacted the cements on a glass slab. Several clinicians rated each cement and cement combinations for cohesiveness and ability to be condensed. PC and MTA by themselves were rated poor. All of the MC and MC/CaCl2 concentrations greatly improved the handling ratings. The CaCl2 by itself had no effect on handling characteristics. The MC/CaCl2 combinations all resulted in cement rated similar to our handling control, IRM. The compressive strength of the cements was evaluated first to determine whether the additives had any adverse effects on the cements’ mechanical properties. Compressive strength is commonly used in the cement industry to evaluate cement mixes. The results are shown in Table 1 for PC and Table 2 for MTA. The compressive strengths were measured at 24 hours and 3 weeks. The PC’s compressive strength was similar to that reported in the cement literature for an approximate 3/1 P/L ratio (10). The CaCl2 significantly increased the 24-hour compressive strength, as would be expected as a result of its acceleration of the setting reaction. The addition of MC in any amount did not significantly alter the 24-hour strength, but it did offset the 24-hour strength increase as a result of the CaCl2. At 3 weeks, the addition of MC with or without the CaCl2 resulted in a significant decrease in compressive strength from the PC control. The compressive strength of the MTA was lower than that reported by Torabinejad et al. (11). Their values for the compressive strength of MTA were 40 MPa at 24 hours and 67.3 MPa after 21 days. The compressive strength of our MTA control was 26.4 MPa after 24 hours and 30.4 MPa at 3 weeks. These values are similar to those reported by
JOE — Volume 33, Number 10, October 2007
Kogan et al. (2), 28.4 MPa at 1 week. The explanation for the lower strengths we reported for MTA is that the composition of the MTA of Torabinejad et al. and our MTA might be different. We believe that our MTA contains more bismuth oxide to increase the radiopacity, and that this is inert filler that might lower the strength of the cement. Coomaraswamy et al. (5) have shown that bismuth oxide acts as an internal flaw and results in more porosities as a result of more unused water in the setting reaction. Bismuth oxide drastically affected the strength of PC, varying the compressive strength from 82.1 to 28.7 MPa as the bismuth concentration increased from 0% to 40% by weight. The MTA control with 20% exhibited a compressive strength of 33 MPa, similar to our results. Unlike the results from the PC tests, the addition of 1% or 2% MC with CaCl2 had no significant effect on the 24-hour or 3-week compressive strength of MTA. We believe this is due to the fact that MTA has already had its compressive strength lowered compared with PC by the addition of the bismuth and the additional plasticizing effect of the MC is negligible. The effect of the additives on the setting time of the cements was evaluated next. It would be beneficial to shorten the setting time of the cement, which is exceedingly long for clinical use. A shorter setting time would be beneficial because it would allow less time for contaminants in the oral environment to adversely affect it, allow safer placement of a restorative material over it (pulp capping), and also shorten the period when washout of the cement could occur. The results for PC are shown in Table 3, and the results for MTA are shown in Table 4. MC had a tendency to retard the setting reaction of PC. This was significant at 1% and 3% but not at the 2% addition of MC. We have no explanation for this. The CaCl2 significantly shortened the setting time as expected. The combination of MC and CaCl2 significantly reduced the setting time of the PC. Most importantly, the 1% MC/CaCl2 combination had a significantly shorter setting time than the 2% and 3% combinations. This would indicate that the 1% MC/CaCl2 combination would be the most appropriate combination for use with MTA. Because of the fact that the setting times were decreased as MC concentration decreased, setting times were measured for MTA only for the 1% MC/CaCl2 and 2% MC/CaCl2 combinations. Our setting of 202 minutes for MTA was greater than that reported by Torabinejad et al. (11). They showed the setting time of MTA with the Gilmore needle (ELE International, Inc, Loveland, CO) to be 2 hours and 45 minutes compared with our results of 3 hours and 22 minutes with the Vicat needle. Deal et al. (8) (methodology not reported) also determined the setting time of MTA to be 2 hours and 39 minutes. Kogan et al. (2) found the setting time of MTA to be 50 minutes (Vicat needle), whereas Islam et al. (7) (Gilmore needle) found the setting time of MTA of white MTA to be 140 minutes with grey gray setting at 175 minutes. A possible explanation for the differences in setting times is the use of different needles with differing weights (300 g Vicat vs 453.6 g Gilmore needle) and the amount of time the needle rests on the surface to produce the indentation. None of these investigators used the same technique. Determination of setting times is merely an estimation of when a given probe fails to indent a sample, and there is no uniformly accepted test procedure for all cements. The addition of both the 1% and 2% MC/CaCl2 combinations significantly shortened the setting time, with the 1% being significantly shorter than the 2%. The MC/CaCl2 combinations all improved the handling characteristics of both PC and MTA. Because all the MC/CaCl2 combinations had equal compressive strengths and the 1% MC/2% CaCl2 had the shortest setting time, this is the combination we recommend for further testing with MTA. This combination needs to be further tested clinically to determine whether indeed it allows for easier placement and whether it significantly reduces cement washout.
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Basic Research—Technology Acknowledgments The authors wish to acknowledge the support of Dentsply Tulsa Dental for their donation of ProRoot MTA.
References 1. Gartner AH, Dorn SO. Advances in endodontic surgery. Dent Clin North Am 1992;36:357–78. 2. Kogan P, He J, Glickman GN, Watanabe I. Comparison of the physical properties of MTA and Portland cement. J Endod 2006;32:569 –72. 3. Camilleri J, Monstein FE, Di Silvio I, Pitt Ford TR. The chemical constitution and biocompatibility of accelerated Portland cement. Int Endod J 2005;38:834 – 42. 4. Fridland M, Rosado R. Mineral trioxide aggregate (MTA) solubility and porosity with different water-to-powder ratios. J Endod 2003;29:814 –7.
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5. Coomaraswamy KS, Lumley PJ, Hofmann MP. Effect of bismuth oxide radioopacifier content on the material properties of an endodontic Portland cement-based (MTAlike) system. J Endod 2007;33:295– 8. 6. Lee ES. A new mineral trioxide aggregate root-end filling technique. J Endod 2000;26:764 –5. 7. Islam I, Chng HK, Yap AU. Comparison of the physical and mechanical properties of MTA and Portland cement. J Endod 2006;32:193–7. 8. Deal B, Wenkus C, Johnson B, Fayad M. Chemical and physical properties of MTA, Portland cement, and a new experimental material, fast-set MTA. J Endod 2002;28:252. 9. E Nawy, ed. Concrete construction engineering handbook. New York: CRC Press, 1997. 10. Kosmatka S, Panarese W. Design and control of concrete mixtures. 13th ed. Skokie, IL: Portland Cement Association, 1988. 11. Torabinejad M, Hong CU, McDonald F, Pitt Ford TR. Physical and chemical properties of a new root-end filling material. J Endod 1995;21:349 –53.
JOE — Volume 33, Number 10, October 2007
Chemical Modification of ProRoot MTA to Improve Handling Characteristics and Decrease Setting Time Benjamin S. Ber, DDS, MS,*† John F. Hatton, DMD,†‡ and Gregory P. Stewart, PhD§ Abstract Mineral trioxide aggregate (MTA) fulfills many of the ideal properties of a root-end filling material. However, the composition of this material often makes MTA difficult to use, a direct result of its granular consistency, slow setting time, and initial looseness. Additives used by the Portland cement (PC) industry to increase PC’s plasticity and decrease its setting time were added first to PC and then to gray MTA in an attempt to improve MTA’s handling characteristics, with the combination providing the best handling characteristics tested for its effect on compressive strength (for changes in the original material’s properties) and decrease in setting time. An admix of 1% methylcellulose and 2% calcium chloride resulted in a mix of chemically modified MTA that, when compared with unmodified MTA, (1) handled similarly to a reinforced zinc oxide– eugenol cement, (2) gave an approximately equal compressive strength, and (3) set one third faster (57 ⫾ 3 minutes). (J Endod 2007;33:1231–1234)
Key Words Compressive strength, handling characteristics, MTA, setting time
From *Private Practice, Houma, Louisiana; †Department of Endodontics, Saint Louis University Center for Advanced Dental Education, St Louis, Missouri; and ‡Section of Endodontics and §Section of Dental Materials, Southern Illinois University School of Dental Medicine, Alton, Illinois. Address requests for reprints to John F. Hatton, DMD, Director, Graduate Endodontics, Saint Louis University Center for Advanced Dental Education, 3320 Rutger St, St Louis, MO 63104. E-mail address: [email protected]. 0099-2399/$0 - see front matter Copyright © 2007 by the American Association of Endodontists. doi:10.1016/j.joen.2007.06.012
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ineral trioxide aggregate (MTA) (ProRoot; Dentsply, Tulsa Dental, Tulsa, OK) fulfills many of the ideal properties of a root-end filling material as described by Gartner and Dorn (1). However, the composition of the material often makes it difficult to use, for example, in apical surgeries with limited access. MTA is a difficult material to handle because of its granular consistency (2), slow setting time (3), and initial looseness (2, 4, 5). Once the mixture starts to dry, it loses its cohesiveness and becomes hard to handle (6). This problem is highlighted by the development and introduction of special delivery systems to facilitate the placement of MTA. Even with the advantages that some of these delivery systems offer, MTA has a tendency to washout in the presence of excess moisture (2). MTA is very similar in composition to that of Portland cement (PC) (7) and appears to have the same difficulties with handling and washout as PC placed in a wet environment (8). It was the intent of this research to incorporate additives used by the cement industry to MTA to try and improve two of the main criticisms of MTA, poor handling and long setting time. Two additives that we believed would minimally affect the biocompatibility were chosen for testing, methylcellulose (MC), an anti-washout ingredient; and calcium chloride (CaCl2), an accelerator of the setting time. The MC anti-washout admixture binds water molecules within the cement, accomplishing two things. First, the addition of MC to cement increases the cohesiveness and plasticity (or moldability) of the material, making it easier to handle. Our goal was to modify the plasticity of MTA to create a material that handled similarly to a reinforced zinc oxide– eugenol filling material. Second, using an admix of MC should increase the washout resistance, a benefit when placed in a contaminated (wet) site. This rate of washout resistance has been shown to increase as the amount of anti-washout admixture is increased. In addition, the viscosity of the mix increases as the concentration of MC increases. However, the addition of percentages of MC greater than 3% can significantly extend the setting time of PC (9). The normal setting time of PC ranges from 3– 4 hours. This property can also be modified. There are several ways that the setting time of PC can be accelerated, thereby increasing the initial strength and decreasing early washout. One way is by the addition of chemicals such as CaCl2. The addition of accelerators increases the rate of hydration, thereby accelerating the setting reaction (10). The purpose of the study was to determine whether additives could be used with MTA to improve the material’s poor handling characteristics and long setting time.
Materials and Methods A series of studies were initially performed to determine the most appropriate concentrations of two additives, CaCl2 and MC, to improve the handling characteristics of MTA. The initial studies were conducted with PC because of its similarity in properties and reduced cost as compared with MTA. The goal was to obtain a handling property similar to a zinc oxide– eugenol type root-end filling material. Once the proper concentration of additives that improved the cement’s handling characteristics was determined, the additives were tested on gray MTA. Concentrations of MC (Sigma, St Louis, MO) ranging from 1%–3% by weight were tested to find the concentration that best improved the cohesiveness and workability of the material. These percentages were then combined with a 2% CaCl2 (PCCA, Houston, TX) solution. Similar to manufacturer’s recommendations for MTA, a 3:1 powder to liquid ratio was used for all PC samples. To evaluate all combinations, 8 groups were tested in the preliminary study with the PC.
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Basic Research—Technology The additive solutions were prepared in the following way. Calcium chloride equal to 2% of the sample weight was added to distilled water and mixed into solution proportional to the water needed for a 3:1 powder to water ratio. Half of the solution was placed on a hot plate on which the temperature of the solution was raised to 80°C to get the MC into solution (per manufacturer’s instructions). The appropriate amount of MC to create the test concentrations was added to the warmed solution and stirred to wet the particles. The remainder of the room temperature solution was added and then stirred until all of the chemicals were in solution. It was then stored at 0°C for 20 minutes to allow for the mixture to thicken. The solution was then mechanically stirred with a magnetic stirrer for 30 minutes to create a homogenous gel. These solutions in a 3:1 powder to liquid ratio were then added to the following test groups. The PC groups were Group 1, PC, no additives; Group 2, 1% MC; Group 3, 2% MC; Group 4, 3% MC; Group 5, CaCl2 (2%); Group 6, 1% MC/CaCl2 (2%); Group 7, 2% MC/CaCl2 (2%); and Group 8, 3% MC/CaCl2 (2%). Each group was evaluated for compressive strength (n ⫽ 6) and setting time (n ⫽ 3). As a result of the study with PC, the following groups with MTA were tested. The MTA groups were Group 9, positive control, MTA mixed according to manufacturer’s instructions by using saline; Group 10, 1% MC/CaCl2 (2%); and Group 11, 2% MC/CaCl2 (2%). Six samples of each group were measured for compressive strength, and 3 were analyzed for setting times. For the compressive strength testing samples, the material was placed into Teflon tube molds 8 mm high and 4 mm in diameter with a glass slab beneath to ensure a flat surface for condensation. The material was added in increments and condensed with an amalgam plugger to minimize air entrapment. Molds were filled to excess, and the excess material was removed by using a glass slide to ensure level samples. Specimens were handled in a similar matter as reported by Torabinejad et al. (11). Specimens were suspended on a metal grate above a closed hot water bath (37°C) and covered to provide 100% humidity without submerging the samples to allow for the initial set. After 3 hours, the samples were removed from the Teflon tube and examined for voids and irregularities. If voids were detected, the sample was discarded. The samples were then immersed in distilled water (23°C) for the duration of the test period. Each group was tested at 24 hours and 21 days. The samples were then removed from the distilled water and allowed to dry. The flat surfaces of each sample were sanded smooth with 600 grit sandpaper. The samples were tested by using an Instron (Instron Co, Park Ridge, IL) materials testing machine until failure. A crosshead speed of 2 mm/sec with a load cell of 5 kN was used. All measurements were recorded in kilograms and converted to megapascals (MPa). To determine setting time, a standard Vicat needle apparatus (Shambhavi Impex, Maharashtra, India) was used. The molds used to test the samples were 10 mm wide and 2 mm deep. Spatulation was carried out on a glass slab by using a cement spatula until all particles were adequately incorporated into the mixture. After a homogenous mix
TABLE 2. Compressive Strength of MTA Groups Group
24-Hr (mean)
3-Week (mean)
MTA 1% MC/CaCl2 in MTA 2% MC/CaCl2 in MTA
26.4 ⫾ 6.6 25.5 ⫾ 4.4 20.8 ⫾ 1.9
30.4 ⫾ 12.8 29.1 ⫾ 6.4 26.5 ⫾ 6.6
Units in MPa; n ⫽ 6.
was achieved, the cement was transferred into the molds, supported by glass slabs beneath to ensure adequate control of the material, condensed with an amalgam plugger, and filled to excess. The excess was removed with a glass slab until the surface was smooth and level. After the preparation of each sample, it was suspended above a hot water bath at a temperature of 37°C and covered with a lid to achieve 100% humidity. The samples were then removed from the hot water bath container and tested every 5 minutes with a Vicat needle apparatus (needle mass, 300 ⫾ 0.5 g with a flat diameter of 1 ⫾ 0.5 mm). The needle was cleaned between each test. Setting was judged to be complete when the needle did not penetrate the surface of the material after being allowed to settle for 30 seconds. Statistical analysis was carried out by using analysis of variance with post hoc Tukey B test, with significance set at P ⬍ .05.
Results The results for the compressive strength of the PC groups at 24 hours and 3 weeks are found in Table 1. At 24 hours, addition of 2% CaCl2 alone resulted in a significantly increased compressive strength compared with all other groups. Also at 24 hours, there was a tendency for decreased compressive strength because the concentration of MC was increased, although this was not significant. At 3 weeks, all groups demonstrated a significantly decreased compressive strength compared with the PC control. There also was a significant difference between the CaCl2 only and the MC and MC/CaCl2 additives. There were no significant differences between any of the MC and MC/CaCl2 additives at 3 weeks. The results for the compressive strength testing of MTA are found in Table 2. There was no significant difference between the MTA control and the groups with the additives at both 24 hours and 3 weeks. The addition of 2% MC demonstrated the greatest reduction in compressive strength, but this was not significant (P ⬍ .05). The results of the setting time for the PC group are reported in Table 3. There was a significant decrease in setting time between the PC control and the addition of CaCl2 and the MC/CaCl2 additives. The setting times of the 1%, 2%, and 3% MC/CaCl2 additives were significantly different from each other, with higher concentrations of MC resulting in longer setting times. There was no significant difference between the PC control and the MC additive only. Table 4 reports the setting time of MTA compared with the groups containing the additives. There was a significant decrease in setting time between the MTA only control and both MC/CaCl2 groups. In addition,
TABLE 1. Compressive Strength of PC Groups Group
24-Hr (mean)
3-Week (mean)
PC 1% MC in PC 2% MC in PC 3% MC in PC CaCl2 (2% solution) 1% MC/CaCl2 in PC 2% MC/CaCl2 in PC 3% MC/CaCl2 in PC
16.8 ⫾ 4.5 15.8 ⫾ 1.8 9.6 ⫾ 3.4 10.1 ⫾ 1.8 35.6 ⫾ 4.9 24.1 ⫾ 4.6 22.3 ⫾ 4.4 18.1 ⫾ 5.0
52.9 ⫾ 6.3 31.3 ⫾ 6.5 31.7 ⫾ 4.4 33.3 ⫾ 5.8 41.5 ⫾ 11.1 33.8 ⫾ 7.6 33.5 ⫾ 7.5 35.2 ⫾ 9.8
Units in MPa; n ⫽ 6.
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Ber et al.
TABLE 3. Setting Time of PC Groups Group
Minutes (mean)
PC 1% MC in PC 2% MC in PC 3% MC in PC CaCl2 (2% solution) 1% MC/CaCl2 in PC 2% MC/CaCl2 in PC 3% MC/CaCl2 in PC
183 ⫾ 8 215 ⫾ 5 182 ⫾ 3 202 ⫾ 8 83 ⫾ 3 60 ⫾ 5 107 ⫾ 3 125 ⫾ 5
n ⫽ 3.
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Basic Research—Technology TABLE 4. Setting Time of MTA Groups Group
24-Hr (mean)
MTA 1% MC/CaCl2 in MTA 2% MC/CaCl2 in MTA
202 ⫾ 3 57 ⫾ 3 105 ⫾ 5
n ⫽ 3.
the setting time for the 1% MC/CaCl2 group was significantly less than the time for the 2% MC/CaCl2 group.
Discussion The purpose of this study was to improve on the handling characteristics and long setting times of MTA. The material is very grainy and has a poor consistency, making it difficult to use in some clinical situations. Our intent was to give MTA a more cohesive, more moldable consistency similar to a reinforced zinc oxide– eugenol, eg, IRM (Dentsply Caulk, Milford, DE). Because PC and MTA are very similar in composition, we selected two additives used by the cement industry, MC and CaCl2. MC is used as an additive to improve the performance of PC in a wet environment. MC is composed of nonionic water-soluble cellulose ether, which has an OH base and is almost like water. It increases viscosity and dispersion resistance (9). CaCl2 is used to accelerate the setting reaction of PC. The mechanism of action is not fully understood, but it is believed that CaCl2 is partially consumed during hydration, reacting with tricalcium aluminate and forming chloroaluminate (10). MC was tested by using 1%, 2%, and 3% concentrations by weight and CaCl2 in a 2% by weight concentration. These concentrations mimic those used by the cement industry (10). Higher concentrations of MC have the capacity to entrap air and retain higher amounts of H2O, altering PC strength and retarding the setting reaction. Both of these effects are a function of concentration (9). Greater than 2% CaCl2 adversely affects the cement by increasing the risk of drying shrinkage and reducing ultimate strength (10). These additives were first tested on PC, and the most promising combinations were then tested on MTA. There are no standardized tests for evaluating handling characteristics. We mixed and compacted the cements on a glass slab. Several clinicians rated each cement and cement combinations for cohesiveness and ability to be condensed. PC and MTA by themselves were rated poor. All of the MC and MC/CaCl2 concentrations greatly improved the handling ratings. The CaCl2 by itself had no effect on handling characteristics. The MC/CaCl2 combinations all resulted in cement rated similar to our handling control, IRM. The compressive strength of the cements was evaluated first to determine whether the additives had any adverse effects on the cements’ mechanical properties. Compressive strength is commonly used in the cement industry to evaluate cement mixes. The results are shown in Table 1 for PC and Table 2 for MTA. The compressive strengths were measured at 24 hours and 3 weeks. The PC’s compressive strength was similar to that reported in the cement literature for an approximate 3/1 P/L ratio (10). The CaCl2 significantly increased the 24-hour compressive strength, as would be expected as a result of its acceleration of the setting reaction. The addition of MC in any amount did not significantly alter the 24-hour strength, but it did offset the 24-hour strength increase as a result of the CaCl2. At 3 weeks, the addition of MC with or without the CaCl2 resulted in a significant decrease in compressive strength from the PC control. The compressive strength of the MTA was lower than that reported by Torabinejad et al. (11). Their values for the compressive strength of MTA were 40 MPa at 24 hours and 67.3 MPa after 21 days. The compressive strength of our MTA control was 26.4 MPa after 24 hours and 30.4 MPa at 3 weeks. These values are similar to those reported by
JOE — Volume 33, Number 10, October 2007
Kogan et al. (2), 28.4 MPa at 1 week. The explanation for the lower strengths we reported for MTA is that the composition of the MTA of Torabinejad et al. and our MTA might be different. We believe that our MTA contains more bismuth oxide to increase the radiopacity, and that this is inert filler that might lower the strength of the cement. Coomaraswamy et al. (5) have shown that bismuth oxide acts as an internal flaw and results in more porosities as a result of more unused water in the setting reaction. Bismuth oxide drastically affected the strength of PC, varying the compressive strength from 82.1 to 28.7 MPa as the bismuth concentration increased from 0% to 40% by weight. The MTA control with 20% exhibited a compressive strength of 33 MPa, similar to our results. Unlike the results from the PC tests, the addition of 1% or 2% MC with CaCl2 had no significant effect on the 24-hour or 3-week compressive strength of MTA. We believe this is due to the fact that MTA has already had its compressive strength lowered compared with PC by the addition of the bismuth and the additional plasticizing effect of the MC is negligible. The effect of the additives on the setting time of the cements was evaluated next. It would be beneficial to shorten the setting time of the cement, which is exceedingly long for clinical use. A shorter setting time would be beneficial because it would allow less time for contaminants in the oral environment to adversely affect it, allow safer placement of a restorative material over it (pulp capping), and also shorten the period when washout of the cement could occur. The results for PC are shown in Table 3, and the results for MTA are shown in Table 4. MC had a tendency to retard the setting reaction of PC. This was significant at 1% and 3% but not at the 2% addition of MC. We have no explanation for this. The CaCl2 significantly shortened the setting time as expected. The combination of MC and CaCl2 significantly reduced the setting time of the PC. Most importantly, the 1% MC/CaCl2 combination had a significantly shorter setting time than the 2% and 3% combinations. This would indicate that the 1% MC/CaCl2 combination would be the most appropriate combination for use with MTA. Because of the fact that the setting times were decreased as MC concentration decreased, setting times were measured for MTA only for the 1% MC/CaCl2 and 2% MC/CaCl2 combinations. Our setting of 202 minutes for MTA was greater than that reported by Torabinejad et al. (11). They showed the setting time of MTA with the Gilmore needle (ELE International, Inc, Loveland, CO) to be 2 hours and 45 minutes compared with our results of 3 hours and 22 minutes with the Vicat needle. Deal et al. (8) (methodology not reported) also determined the setting time of MTA to be 2 hours and 39 minutes. Kogan et al. (2) found the setting time of MTA to be 50 minutes (Vicat needle), whereas Islam et al. (7) (Gilmore needle) found the setting time of MTA of white MTA to be 140 minutes with grey gray setting at 175 minutes. A possible explanation for the differences in setting times is the use of different needles with differing weights (300 g Vicat vs 453.6 g Gilmore needle) and the amount of time the needle rests on the surface to produce the indentation. None of these investigators used the same technique. Determination of setting times is merely an estimation of when a given probe fails to indent a sample, and there is no uniformly accepted test procedure for all cements. The addition of both the 1% and 2% MC/CaCl2 combinations significantly shortened the setting time, with the 1% being significantly shorter than the 2%. The MC/CaCl2 combinations all improved the handling characteristics of both PC and MTA. Because all the MC/CaCl2 combinations had equal compressive strengths and the 1% MC/2% CaCl2 had the shortest setting time, this is the combination we recommend for further testing with MTA. This combination needs to be further tested clinically to determine whether indeed it allows for easier placement and whether it significantly reduces cement washout.
Chemical Modification of MTA
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Basic Research—Technology Acknowledgments The authors wish to acknowledge the support of Dentsply Tulsa Dental for their donation of ProRoot MTA.
References 1. Gartner AH, Dorn SO. Advances in endodontic surgery. Dent Clin North Am 1992;36:357–78. 2. Kogan P, He J, Glickman GN, Watanabe I. Comparison of the physical properties of MTA and Portland cement. J Endod 2006;32:569 –72. 3. Camilleri J, Monstein FE, Di Silvio I, Pitt Ford TR. The chemical constitution and biocompatibility of accelerated Portland cement. Int Endod J 2005;38:834 – 42. 4. Fridland M, Rosado R. Mineral trioxide aggregate (MTA) solubility and porosity with different water-to-powder ratios. J Endod 2003;29:814 –7.
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5. Coomaraswamy KS, Lumley PJ, Hofmann MP. Effect of bismuth oxide radioopacifier content on the material properties of an endodontic Portland cement-based (MTAlike) system. J Endod 2007;33:295– 8. 6. Lee ES. A new mineral trioxide aggregate root-end filling technique. J Endod 2000;26:764 –5. 7. Islam I, Chng HK, Yap AU. Comparison of the physical and mechanical properties of MTA and Portland cement. J Endod 2006;32:193–7. 8. Deal B, Wenkus C, Johnson B, Fayad M. Chemical and physical properties of MTA, Portland cement, and a new experimental material, fast-set MTA. J Endod 2002;28:252. 9. E Nawy, ed. Concrete construction engineering handbook. New York: CRC Press, 1997. 10. Kosmatka S, Panarese W. Design and control of concrete mixtures. 13th ed. Skokie, IL: Portland Cement Association, 1988. 11. Torabinejad M, Hong CU, McDonald F, Pitt Ford TR. Physical and chemical properties of a new root-end filling material. J Endod 1995;21:349 –53.
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