| Abstract|| |
Aim: Organic acids have the potential to decrease the strength of mineral trioxide aggregate (MTA) during retreatment. However, this might cause alterations in dentin composition and structure. The aim of this study was to assess the efficacy of chemicals on surface microhardness of white MTA (WMTA) and dentin.
Materials and Methods: White MTA-Angelus® was mixed and packed into tooth molds. Six experimental groups (n = 15) were formed and exposed to 2% carbonic acid, 2% chlorhexidine gluconate, 5.25% sodium hypochlorite, 10% citric acid, 20% tartaric acid, and normal saline for 10 min and 20 min intervals on 1 and 21 days of setting, respectively. Vickers microhardness of WMTA and dentin for each specimen was measured before and after exposure. Data were subjected to repeated-analysis of variance (ANOVA) and post-hoc tests.
Results: Carbonic acid was effective in significantly reducing the surface hardness of WMTA on both 1 and 21 days; followed by citric and tartaric acid (P < 0.05). Two percent chlorhexidine gluconate and NaOCl were effective on 1-day set WMTA. All chemicals, except 2% chlorhexidine, decreased microhardness of dentin significantly (P < 0.05) at 20 min interval.
Conclusion: Cautious use of these chemicals not exceeding 10 min is mandatory to prevent significant alterations in mechanical properties of tooth during MTA retrieval.
Keywords: Carbonic acid; chlorhexidine gluconate; citric acid; tartaric acid; white mineral trioxide aggregate
|How to cite this article:|
Butt N, Talwar S. In-vitro evaluation of various solvents for retrieval of mineral trioxide aggregate and their effect on microhardness of dentin. J Conserv Dent 2013;16:199-202
|How to cite this URL:|
Butt N, Talwar S. In-vitro evaluation of various solvents for retrieval of mineral trioxide aggregate and their effect on microhardness of dentin. J Conserv Dent [serial online] 2013 [cited 2019 Dec 11];16:199-202. Available from: http://www.jcd.org.in/text.asp?2013/16/3/199/111313
| Introduction|| |
Mineral trioxide aggregate (MTA) has shown potential as an endodontic material in several clinical applications , since its introduction by Torabinejad et al., in 1993. It has been reported that the compressive strength of MTA after 1 day was 40.0 MPa, which increases to 67.3 MPa after 21 days in presence of moisture.  Despite the superior characteristics of MTA, its retrievability from the root canal has been regarded as one of its main disadvantages. Although initial root canal therapy has been shown to be a predictable procedure with a high degree of success, failures can occur after treatment.  Rotary and ultrasonic instruments alone have not been shown to be efficient in the removal of MTA from root canal.  The need to have a solvent for MTA has been stressed upon in several previous studies. ,
MTA is a mechanical mixture of Portland cement, bismuth oxide, and gypsum.  Because of its chemical similarity to MTA, some investigators suggested Portland cement as a substitute material for MTA.  Studies have shown that the surface hardness of Portland cement is reduced by organic acids ,, like carbonic acid, citric acid, and tartaric acid. When these acids come into contact with the cement based matrix, the reactions with the hydrates of the cement paste (portlandite, calcium silicate hydrate (C-S-H), and hydrated aluminates) produce mainly calcium and aluminum salts, whose solubility varies from high to very high in water. Consequently, the porosity increases, and in the long term, the mechanical strength of the immersed part of the structure drops. Since the chemical compositions of MTA and Portland cement are similar; MTA may also interact with acids or chemicals and show disintegration. This may aid in its retrieval from the root canals due to loss of strength and integrity.
Dentin composition has been described based on its organic and inorganic components. Calcium (Ca) and phosphorus (P) present in hydroxyapatite crystals are the major inorganic components of dental hard tissue. Changes in the mineral content ratio may alter the original proportion of organic and inorganic components, which in turn reduces the microhardness, increases the permeability and solubility of the root canal dentin, and inhibits resistance to bacterial ingress, hence permitting leakage.  During the use of these solvents, radicular and coronal dentin are also exposed to them which may cause alterations in its microhardness. Thus, it is of interest to investigate to what extent the dentin of the root canal is affected by the use of various irrigating solutions.
The purpose of this study was to evaluate the efficacy of various chemicals when used as irrigants on dissolution of partially and completely set mineral trioxide aggregate and their effect on microhardness of dentin. We hypothesized that the tested solvents may induce changes in the surface microhardness of white MTA (WMTA) and dentin.
| Materials and Methods|| |
One hundred eighty sound human maxillary and mandibular molars extracted for periodontal reasons were used in this study. All teeth were sectioned transversely to obtain a tooth slice of 5 mm height. White MTA powder (Angelus, Londrina, PR, Brazil) was mixed with sterile water at a powder to liquid ratio of 3:1, to a thick creamy consistency. The cement was incrementally compacted in the molds with a stainless steel condenser (Hu-Friedy, Chicago, IL) to minimize voids. A pressure of 3.22 MPa was applied by using a custom-made device with a stainless steel piston to condense the samples. A moist cotton pellet was placed on top of the condensed WMTA and stored at 100% humidity. All specimens were examined under the optical microscope (LABOMED CX RII). Any specimen found to have cracks, defects or gaps between the material and tooth structure were excluded from this study.
Ninety specimens were tested for hardness after 1-day set MTA by using Vickers microhardness testing machine (Leica VMHT Auto, Version 10, UK) and a square-based pyramid-shaped diamond indenter on the polished surface of the cement and dentin. Indentations were made at a minimum of three widely similarly positioned locations at 0.5 mm level, along lines parallel to the edge of the root canal lumen using 100 g and a dwell time of 5 s. In each sample, three indentations were made and the average length of the two diagonals was used to calculate the microhardness value (Vickers hardness number [VHN]). Vickers microhardness value was displayed on the digital readout of the microhardness tester.
These specimens were then randomly divided into six groups with 15 teeth in each group and exposed to experimental chemicals as follows: Group 1: 2% carbonic acid (CAG); Group 2: 2% chlorhexidine gluconate solution (CHX) (Dentochlor, Ammdent, India); Group 3: 5.25% sodium hypochlorite solution (NaOCl) (Clorox; Clorox Co., Oakland, CA); Group 4: 10% citric acid (CA); Group 5: 20% tartaric acid (TA); Group 6 (control): normal saline (CG). The preparation of carbonic, citric, and tartaric acids at required concentrations were carried out in Department of Biochemistry, Maulana Azad Medical College, New Delhi.
The first 90 samples received irrigation continuously for 10 min and 20 min followed by microhardness testing of WMTA and dentin at each time interval. Similar procedures were repeated for the remaining 90 samples at the end of 21 days of setting of WMTA. From the data collected, the mean microhardness values before and after the exposure to chemicals for all groups were calculated. Comparison of mean change in microhardness of WMTA and dentin values within the groups was done with repeated analysis of variance (ANOVA) test and intergroup comparison was performed with post-hoc test. A probability level of P < 0.05 was defined as statistically significant. Statistical analysis was performed using Statistical Product and Service Solutions (SPSS) 18.0 for Windows (SPSS Inc, Chicago, IL).
| Results|| |
The results of the microhardness testing for WMTA and dentin before and after exposure to chemicals after 1 and 21 days of setting are shown in [Table 1], [Table 2], [Table 3] and [Table 4]. The difference between the Vickers microhardness values after the exposure to chemical between the groups was statistically significant (P < 0.05).
|Table 1: Intergroup comparison of the mean Vickers hardness values of WMTA samples after the use of chemicals at day 1|
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|Table 2: Intergroup comparison of the mean Vickers hardness values of root canal dentin after the use of chemicals at day 1|
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|Table 3: Intergroup comparison of the mean Vickers hardness values of WMTA samples after the use of chemicals at day 21|
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|Table 4: Intergroup comparison of the mean Vickers hardness values of root canal dentin after the use of chemicals at day 21|
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Two percent CAG exposure significantly decreased the surface hardness of WMTA after 10 min and 20 min exposure on both 1 day (49.38 VHN, 41.54 VHN, respectively) and 21 days (55.47 VHN, 54.39 VHN, respectively) of setting. The reduction in hardness of WMTA was greater in CAG as compared with the other test groups on both the days.
In 2% CHX group, the reduction in surface hardness on partial set WMTA (1 day) was found to be significant at both 10 min and 20 min exposure (53.34 VHN, 51.49 VHN), whereas it had negligible effect on dentin.
5.25% NaOCl effect on WMTA microhardness was comparable to 2% CHX on partial set WMTA (1 day) with significant reduction of dentin microhardness at 20 min exposure.
Ten percent CA and 20% TA significantly reduced the microhardness of WMTA at 20 min exposure for both partial set (45.49 VHN, 44.6 VHN, respectively) and completely set WMTA (62.48 VHN, 60.52 VHN, respectively). They also significantly reduced the microhardness of dentin after 20 min exposure on both the days.
| Discussion|| |
This study was conducted to evaluate the effect of various chemicals on the dissolution of MTA by microhardness determination of the concerned material, which provides indirect evidence of mineral loss or gain in the dental hard tissues. MTA is a hydraulic cement (i.e. sets and is stable under water), relying primarily on hydration reactions for setting.  Hydration reactions and maturation of MTA continue well beyond clinically observed setting time, mostly occurring within the first week of setting.  Calcium hydroxide (CH) and C-S-H are the main hydration products of the reaction between tricalcium silicate (C 3 S) and dicalcium silicate (C 2 S), present in MTA, with water. , It has been observed that carbonation results in a drop in tensile strength and resiliency of PC.  Carbonic acid is a weak acid with a pH of 5.48. In Portland cement-based materials, carbonation results primarily in the conversion of portlandite, Ca(OH) 2 , to calcite, CaCO 3 . Long-term attack by carbonic acid decomposes the CSH gel into calcium carbonate, acid-insoluble silica gel and water. In general, complete carbonation results in decalcification of the CSH gel and an overall decrease in strength. The low standard deviations in all groups indicate that the material was homogenous enough for the study.
The results of the microhardness test showed that the citric and tartaric acid-treated specimens had microhardness of partial and completely set MTA significantly lower than CHX and NaOCl groups; this may relate to the poorly crystallized C-S-H in the former groups. Citric acid is a frequently recommended irrigant for endodontic use.  Both, 10% citric and 20% tartaric acids are calcium depleting organic acids with a pH of 2.15. The calcium-depleting nature of these acids may have the potential to decrease the strength of MTA by disrupting the crystallization of C-S-H. Smith et al.,  stated that calcium depleting agents have a negative effect on setting of MTA as it hampers the formation of CSH gel. Rai et al.,  studied the effect of 20 wt% tartaric acid during the hydration of Portland cement and proved that tartaric acid is a strong retarder for the hydration of Portland cement. It interacts chemically with the cement mineral phases forming calcium tartarate hydrate with considerable evolution of heat and decrease in amounts of C 3 S, C 2 S, and C 3 A phases. Due to these chemical interactions, it might be possible to use these organic acids for dissolving the obturated calcium silicate based material in the retreatment cases.
CHX and NaOCl are most commonly used endodontic irrigants.  NaOCl has an alkaline pH of 9-10.5. CHX is a cationic bisguanide having a neutral pH. Results of the present study indicate that NaOCl and CHX cause significant decrease of surface microhardness of partial set MTA at both intervals. Thus, the use of these irrigants within 24 hours of placement of WMTA should be avoided and when indicated, can be an adjunct for the retrieval of partial set WMTA.
The second objective of this study was also to evaluate the effect of these chemicals on the microhardness of dentin. In the present study, to measure the Vickers hardness values for dentin, indentations were made at the 0.5 mm level from the root canal walls with a load of 100 g for 5 seconds. The pilot studies that preceded the present experiment showed that the application of this load on the root canal lumen dentin was sufficient to promote good visualization of the pre-and post-treatment indentations. The decalcifying efficacy of acids and chelating agents depends largely on application time, the pH and concentration of the solution, and the hardness of dentin.  The results of the present study indicate that all chemical solutions, except 2% chlorhexidine, decreased microhardness of root canal dentin significantly (P < 0.05). Amongst the test groups, NaOCl caused maximum reduction in the microhardness of dentin followed by carbonic, citric, and tartaric acids. This may be attributed to degradation of organic dentine components by NaOCl.  According to a previous study, citric acid solutions have the strongest effect on reducing dentin microhardness because of its chelating property.  In dentin, calcium is not available in the form of ion but rather as a complex within the hydroxyapatite crystals, which impedes its complete reaction with the acid. The other organic acids like carbonic and tartaric acids thus also have some calcium depleting role and their carboxyl groups possess tendency of reducing dentin microhardness.
| Conclusion|| |
The results of the present study indicate that 2% carbonic acid has maximum efficacy in reducing the surface microhardness of partial and completely set MTA, followed by 10% citric acid and 20% tartaric acid. 5.25% NaOCl and 2% chlorhexidine are effective on partial set MTA (1 day) with negligible effect on completely set MTA (21 days). Consequently, the results obtained supported the hypothesis tested. Since the test solutions induced significant reduction in the surface microhardness of dentin (except 2% chlorhexidine P > 0.05), cautious use of these chemicals is mandatory to prevent significant alterations in the mechanical and physical properties of the tooth. Further studies are needed to confirm in vivo effects of the tested solvents on retrievability of MTA.
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Department of Conservative Dentistry and Endodontics, Maulana Azad Institute of Dental Sciences, Maulana Azad Medical College Complex, Bahadur Shah Zafar Marg, New Delhi-110 002
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4]