Drs. Iris Slutzky-Goldberg, Lea Sabag, and David Keinan test the effect of the MTA thickness on its microhardness properties
Abstract
Aim: The purpose of this study was to test the effect of the MTA thickness on the microhardness properties.
Materials and method: A total of 30 roots from extracted single canal human teeth were divided into 3 groups of 4-mm, 6-mm, and 10-mm long root sections. After canal preparation, white MTA (ProRoot®, DENTSPLY Tulsa Dental Specialties) was delivered into the root canal space using an MTA carrier.
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The microhardness was measured after 4 weeks using a Vickers Diamond Microhardness Test for each sample. Statistical analysis included one-way analysis of variance and the t-test at a 5% level of significance.
Results: The 10-mm thick ProRoot MTA was significantly harder than the 6-mm or 4-mm material (p < 0.0001); there was no statistical difference in microhardness between the 4-mm thick and the 6-mm thick material (p > 0.05).
Conclusions:
MTA was found suitable for filling the entire root canal space in compromised cases on the basis of its microhardness.
Introduction
Mineral trioxide aggregate (MTA) was first described in the dental literature by Lee, et al.1 MTA is composed of three powdered ingredients, which are 75% Portland cement, 20% bismuth oxide, 5% gypsum, and trace amounts of SiO2, CaO, MgO, K2SO4, and Na2SO4.1 There are four major components in Portland cement: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The self-setting properties of calcium silicate cements are attributed to the progressive hydration reaction of the orthosilicate ions.3 Calcium silicate hydrate gel polymerizes and hardens over time, forming a solid network, which is associated with an increased mechanical strength.4
MTA proved to be superior to materials, such as amalgam, IRM, and Super-EBA, in both biocompatibility and sealing ability.5-10 Many applications were suggested for the clinical use of this material, among them as a root end-filling material,11 root perforation repair,12,13 and a direct pulp capping following pulpotomy.14-15 MTA is also commonly used for one-step apexification.16 In an in vivo study, which compared MTA and calcium hydroxide ability to stimulate root-end closure in necrotic permanent teeth with immature apices, none of the MTA-treated teeth showed any clinical or radiographic pathosis.17 It was also shown that root canal-treated teeth obturated with MTA exhibit higher fracture resistance than untreated teeth.18
The compressive strength and surface microhardness of calcium silicate cements tend to increase with time.19 The effect of condensation pressure on the surface hardness of ProRoot demonstrate a negative correlation, in which higher condensation pressures produce lower surface hardness values.20 This may be related to forcing the liquid out of the mix prior to setting resulting in alteration to the powder liquid ratio. However, higher condensation pressures resulted in fewer voids and micro-channels when analyzed by SEM.20
The microhardness of MTA can be influenced by the pH values of the mixing medium, and even the mixing techniques used.21 More porosity and unhydrated structure were observed in White MTA (WMTA) exposed to low pH values.22,23 The placement of MTA is technique-sensitive, and according to several studies, the application of ultrasonic energy may improve its sealing properties.24,25 It was also demonstrated that acid etch applied 4 hours after mixing MTA with water significantly reduced its resultant compressive strength compared with the controls, but these differences were not significant after 24 and 96 hours.19 However, newer formulas of trioxide aggregate may set even after 15 minutes that may make it more resistant to acid etching.
The manufacturer recommends to place 3- to 5-mm thick MTA. This is in accordance with the results of previous studies, which suggested a minimum thickness of 3-4 mm when the material was used as a root-end filling material26 to prevent apical leakage.27 In another in vitro study, which tested white and gray MTA for microhardness, a 5-mm thick barrier was significantly harder than a 2-mm barrier,28 regardless of the type of MTA used. This may be attributed to a sufficient bulk of material that can also get hydration through all its thickness.
The purpose of this study was to measure the microhardness of MTA in vitro in relation to its thickness and to compare the microhardness when the material was used only as root-end filling (4- or 6-mm thick) or for obturation of the entire root canal (10 mm).
Materials and methods
An in vitro examination of MTA microhardness in extracted teeth was carried out using the technique previously described by Valois and Costa.27
Thirty extracted, single-canal human teeth, stored in tap water at 4° C, were used for this study. The crowns were separated from the roots at the cemento-enamel junction and were divided according to length into three groups of 10 teeth each: standardized to 10 mm, 6 mm, and 4 mm.
The canals were instrumented with Gates Glidden burs No. 1-No. 4 (Dentsply Maillefer Switzerland) in a crown-down manner until the No. 1 size bur could pass through the apical foramen. The specimens were then prepared with K-files until an ISO size 90 file could be visualized 1 mm past the apex. Irrigation with 10 ml 3% sodium hypochlorite was used throughout the instrumentation, followed by a final flush of 5 ml. To provide a simulated periapical environment, the root segments were placed in saline, as previously described by Lee, et al.1
Following previously described procedures white MTA (Dentsply Maillefer Switzerland) was delivered into to the canal space by using ultrasonically vibrated pluggers,29 and the teeth were sealed with Coltosol® F (Coltène Whaledent), a premixed non-eugenol provisional filling material. The MTA was allowed to set at 37° C and 100% humidity for 24 hours. All samples were stored in tap water for 4 weeks at 37° C and 100% humidity. The samples were then removed, sectioned longitudinally with a diamond bur, and embedded in resin. The samples were then polished with a variable speed grinder polisher (IsoMet®-6 Buehler Düsseldorf, Germany).
Microhardness measurements were carried out using a Vickers Diamond Microhardness Tester MHT-1 (Matsuzawa, Tokyo, Japan) on each sample. The indenter exerted 500g pressure for 15 seconds on the set material, producing one impression with two orthogonal diagonals. The samples were evaluated under an optical microscope (Olympus Optical Microscope, Hamburg, Germany) at 10X magnification; digital images were captured and imported into a Photo Shop Pro version 5.01 (Jasc Software, Inc., Minneapolis, Minnesota). Indentation size was measured in microns.
Microhardness was calculated according to the following equation:
F-Pressure in kg applied to the material
d-average of the two diagonals in millimeters
The results were statistically analyzed by one-way analysis of variance and the t-test. Significance was set at 5%.
Results
More measurements were carried out in the 10-mm samples (N = 37) than in the 4-mm (N = 15) or 6-mm (N = 15) samples, as the 10-mm length roots allowed more indentation sites (Table 1).
As can be seen in the 10-mm thickness group (N = 37), the indentation size was between 76-146 microns (average, 92 ±16 microns). In the 6-mm thickness group (N = 15), indentation size was 90-123 microns (average 107 ±12 microns). In the 4-mm thickness group (N = 15), the indentation size was 96.5-133.5 microns (average 110 ±11 microns).
The microhardness in the 10-mm group was an average 1131 ±254 MPa; for the 6-mm group,823 ± 182 MPa; and for the 4-mm group, 760 ± 146 MPa.
Statistical analysis showed that the 10-mm thick MTA was significantly harder than the 6-mm or 4-mm thick MTA (p < 0.0001); no statistical difference in microhardness was found between the two other groups (p > 0.05).
Discussion
Initially, mineral trioxide aggregate was introduced for the repair of root perforations.1 As hard tissue induction is one of its exceptional properties, it has been recommended for use as an apical barrier in the treatment of immature teeth with necrotic pulps and open apices.30
The setting and hardening of calcium silicate cements are hydration reactions and require water.31 We used ProRoot white MTA, since gray MTA may cause discoloration when placed in the coronal area or near the CEJ in anterior teeth.32 There are several composition differences between gray MTA and white MTA. White MTA contents of Al2O3, MgO, and Fe2O3 are much less than in gray MTA.33 The particle size distribution of white MTA is approximately 8 times smaller than that of gray MTA, and this could provide more surface area for hydration reactions and greater early strength.33
The minimal thickness recommended in the literature for ProRoot MTA when used as root-end filling material is 3 mm26 and; for apexification, 4 mm.34 Five-mm thick ProRoot MTA was recommended as an apical barrier, based on findings that showed that 5-mm MTA was significantly harder than 2-mm thick MTA.28 The results of our study did not show any statistical difference between the 6-mm and 4-mm thick samples, suggesting that with regard to microhardness, a minimum MTA thickness of 4 mm may be sufficient for apical closure.
The higher microhardness demon-strated in the 10-mm group as compared with the 4-mm group or the 6-mm group was a surprising finding of the study. MTA requires moisture for setting.9 An in vitro study by Budig and Eleazer35 had shown that even dry MTA packed into the root canal space can set by outside moisture penetrating through the root when soaked in saline for 72 hours. Therefore, it should have been expected that there will not be any statistical difference in microhardness between the 4- and 6-mm long samples. One possible explanation for the better results of the 10-mm long samples can be related to a higher pH remaining in the longer sample following irrigation with the basic sodium hypochlorite. Nekoofar, et al.,21 had already demonstrated the effect of the pH on the physical properties of MTA.
The results of this study imply that MTA can be used for obturation of the entire root canal, as previously suggested by Whiterspoon, et al.29 This holds true for the coronal fragment of a horizontally fractured tooth,36 for short-length canals as well as in compromised cases, such as treatment of a necrotic immature tooth37 or young permanent teeth after traumatic injury.38 The superior healing properties of MTA, which are attributed to its osteoconductive and cementogenic properties, appear to render the use of MTA for filling of the entire root canal system with improved healing rate,39 and in compromised cases, such as internal root resorption.40 Furthermore, it was also found that MTA resisted bacterial leakage to a higher degree than did gutta percha and sealer when used as an obturation material.38 The use of MTA for filling the entire root canal system may also serve to reinforce the root.41 Furthermore, sealing the entire root canal with MTA will enable completion of the root filling in one visit, a reduction in treatment time, thereby facilitating the timely restoration of the tooth.29
The microhardness test is non-destructive, and any further consequences of any changes of strength in the superficial layers will affect the possibility of the material to fail over time.42 One should also bear in mind that microhardness is only one of the physical properties that should be examined when considering the ability of MTA to serve as a total root filling material in compromised cases. The prudent clinician has also to recognize the fact that removal of the set material, especially in curved canals may be impossible.43
Further study, including long-term success, is required to determine the suitability of MTA as a root canal obturation material.
Conclusions
Based on the results obtained from this in vitro study, the 10-mm thick ProRoot MTA exhibits greater microhardness than the 4-mm thick or 6-mm thick material. No statistical difference in microhardness was observed between the 4-mm and the 6-mm thick groups. On the basis of its microhardness, it appears that ProRoot MTA is suitable for root canal obturation in selected compromised cases.
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