The influence of mineral trioxide aggregate (MTA) thickness on its microhardness properties — an in vitro study

Drs. Iris Slutzky-Goldberg, Lea Sabag, and David Keinan test the effect of the MTA thickness on its microhardness properties


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.


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).

MTA was found suitable for filling the entire root canal space in compromised cases on the basis of its microhardness.


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:

140329 C Keinan 02 

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%. 


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).

140329 C Keinan 01

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).


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. 


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. 



1. Lee SJ, Monsef M, Torabinejad M. Sealing ability of a mineral trioxide aggregate for repair of lateral root perforations. J Endod. 1993;19(11):541–544.

2. Sarkar NK, Caicedo R, Ritwik P, Moiseyeva R, Kawashima I. Physicochemical basis of the biologic properties of mineral trioxide aggregate. J Endod. 2005;31(2):97−100.

3. Gandolfi MG, Van Landuyt K, Taddei P, Modena E, Van Meerbeek B, Prati C. Environmental scanning electron microscopy connected with energy dispersive x-ray analysis and Raman techniques to study ProRoot mineral trioxide aggregate and calcium silicate cements in wet conditions and in real time. J Endod. 2010;36(5):851–857.

4. Zhao W, Wang J, Zhai W, Wang Z, Chang J. The self-setting properties and in vitro bioactivity of tricalcium silicate. Biomaterials. 2005;26(31):6113–6121.

5. Torabinejad M, Watson TF, Pitt Ford TR. Sealing ability of a mineral trioxide aggregate when used as a root end filling material. J Endod. 1993;19(12):591–595.

6. Torabinejad M, Hong CU, McDonald F, Pitt Ford TR. Physical and chemical properties of a new root-end filling material. J Endod. 1995;21(7):349-353.

7. Fischer EJ, Arens DE, Miller CH. Bacterial leakage of mineral trioxide aggregate as compared with zinc-free amalgam, intermediate restorative material, and Super-EBA as a root-end filling material. J Endod. 1998;24:176–179.

8. Adamo HL, Buruiana R, Schertzer L, Boylan RJ. A comparison of MTA, Super-EBA, composite and amalgam as root-end filling materials using a bacterial microleakage model. Int Endod J. 1999;32(3):197–203.

9. Torabinejad M, Hong CU, Pitt Ford TR, Kettering JD. Cytotoxicity of four root end filling materials. J Endod. 1995;21(10):489–92. 

10. Camargo SE, Camargo CH, Hiller KA, Rode SM, Schweikl H, Schmalz G. Cytotoxicity and genotoxicity of pulp capping materials in two cell lines. Int Endod J. 2009;42(3):227–237.

11. Torabinejad M, Smith PW, Kettering JD, Pitt Ford TR. Comparative investigation of marginal adaptation of mineral trioxide aggregate and other commonly used root-end filling materials. J Endod. 1995;21(6):295–299.

12. Ford TR, Torabinejad M, Abedi HR, Bakland LK, Kariyawasam SP. Using mineral trioxide aggregate as a pulp capping material. J Am Dent Assoc. 1996;127(10):1491–1494.

13.  Main C, Mirzayan N, Shabahang S, Torabinejad M.  Repair of root perforations using mineral trioxide aggregate: a long-term study. J Endod. 2004;30(2):80–83.

14. Pitt Ford TR, Torabinejad M, McKendry DJ, Hong CU, Kariyawasam SP. Use of mineral trioxide aggregate for repair of furcal perforations. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1995;79:756–763.

15. Holland R, de Souza V, Murata SS, Nery MJ, Bernabé PF, Otoboni Filho JA, Dezan Júnior E. Healing process of dog dental pulp after pulpotomy and pulp covering with mineral trioxide aggregate or Portland cement. Braz Dent J. 2001;12(2):109–113.

16.  Kratchman SI. Perforation repair and one-step apexification procedures. Dent Clin North Am. 2004;48(1):291-307. 

17. El-Meligy OA, Avery DR. Comparison of apexification with mineral trioxide aggregate and calcium hydroxide. Pediatr Dent. 2006;28(3):248-253.

18. Bortoluzzi EA, Souza EM, Reis JM, Esberard RM, Tanomaru-Filho M. Fracture strength of bovine incisors after intra-radicular treatment with MTA in an experimental immature tooth model. Int Endod J. 2007;40(9):684–691.

19. Kayahan MB, Nekoofar MH, Kazandağ M, Canpolat C, Malkondu O, Kaptan F, Dummer PM. Effect of acid-etching procedure on selected physical properties of mineral trioxide aggregate. Int Endod J. 2009;42(11):1004–1014.

20. Nekoofar MH, Adusei G, Sheykhrezae MS, Hayes SJ, Bryant ST, Dummer PM. The effect of condensation pressure on selected physical properties of mineral trioxide aggregate. Int Endod J. 2007;40(6):453-461.

21. Nekoofar MH, Aseeley Z, Dummer PM. The effect of various mixing techniques on the surface microhardness of mineral trioxide aggregate. Int Endod J. 2010;43(4):312–320.

22. Saghiri MA, Lotfi M, Saghiri AM, Vosoughhosseini S, Aeinehchi M, Ranjkesh B. Scanning electron micrograph and surface hardness of mineral trioxide aggregate in the presence of alkaline pH. J Endod. 2009;35(5):706-710.

23. Shie MY, Huang TH, Kao CT, Huang CH, Ding SJ. The effect of a physiologic solution pH on properties of white mineral trioxide aggregate. J Endod. 2009;35(1):98-101. 

24. Hachmeister DR, Schindler WG, Walker WA 3rd, Thomas DD. The sealing ability and retention characteristics of mineral trioxide aggregate in a model of apexification. J Endod. 2002;28(5):386–390.

25. Lawley GR, Schindler WG, Walker WA III, Kolodrubetz D. Evaluation of ultrasonically placed MTA and fracture resistance with intracanal composite resin in a model of apexification. J Endod. 2004;30(3):167-172.

26. Lamb EL, Loushine RJ, Weller RN, Kimbrough WF, Pashley DH. Effect of root resection on the apical sealing ability of mineral trioxide aggregate. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003; 95(6):732-735.

27. Valois C, Costa ED Jr. Influence of the thickness of mineral trioxide aggregate on sealing ability of root-end fillings in vitro. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2004;97(1):108-111.

 28. Matt GD, Thorpe JR, Strother JM, McClanahan SB. Comparative study of white and gray mineral trioxide aggregate (MTA) simulating a one- or two-step apical barrier technique. J Endod. 2004;30(12):876-879.

29. Witherspoon DE, Small JC, Regan JD, Nunn M. Retrospective analysis of open apex teeth obturated with mineral trioxide aggregate. J Endod. 2008;34(10):1171–1176.

30. Bakland LK. Management of traumatically injured pulps in immature teeth using MTA. J Calif Dent Assoc. 2000;28(11):855–858.

31. Maltese C, Pistolesi C, Bravo A, Cella F, Cerulli T, Salvioni D. Effects of setting regulators on the efficiency of an inorganic acid based alkali-free accelerator reacting with a Portland cement. Cements and Concrete Research. 2007;37:528–3536.

32. Asgary S, Parirokh M, Eghbal MJ, Brink F. Chemical differences between white and gray mineral trioxide aggregate. J Endod. 2005;31(2):101–103.

33. Asgary S, Parirokh M, Eghbal M, Stowe S, Brink F. A qualitative X-ray analysis of white and grey mineral trioxide aggregate using compositional imaging. J Mater Sci Mater Med. 2006;17(2):187−191.

34. Giuliani V, Baccetti T, Pace R, Pagavino G. The use of MTA in teeth with necrotic pulps and open apices. Dent Traumatol. 2002;18(4):217-221. 

35. Budig CG, Eleazer PD. In Vitro Comparison of the Setting of dry ProRoot MTA by moisture absorbed through the Root. J Endod. 2008;34(6):712-714.

36. Erdem AP, Ozdas DO, Dincol E, Sepet E, Aren G. Case Series: root healing with MTA after horizontal fracture. Eur Arch Paediatr Dent. 2009;10(2):110–113.

37. Mohammadi Z, Yazdizadeh M. Obturation of immature non-vital tooth using MTA. Case report. NY State Dent J. 2011;77(1):33-35.

38. Al-Kahtani A, Shostad S, Schifferle R, Bhambhani S. In-vitro evaluation of microleakage of an orthograde apical plug of mineral trioxide aggregate in permanent teeth with simulated immature apices. J Endod. 2005;31(2):117–119.

39. Bogen G, Kuttler S. Mineral trioxide aggregate obturation: a review and case series. J Endod. 2009;35(6):777–790.

40. Jacobovitz M, de Lima RK. Treatment of inflammatory internal root resorption with mineral trioxide aggregate: a case report. Int Endod J. 2008;41(10):905-912.

41. Cauwels RG, Pieters IY, Martens LC, Verbeeck RM. Fracture resistance and reinforcement of immature roots with gutta percha, mineral trioxide aggregate and calcium phosphate bone cement: a standardized in vitro model. Dent Traumatol. 2010;26(2):137-142.

 42. Kang JS, Rhim EM, Huh SY, Ahn SJ, Kim DS, Kim SY, Park SH. The effects of humidity and serum on the surface microhardness and morphology of five retrograde filling materials. Scanning. 2012;34(4):207-214.

43. Boutsioukis C, Noula G, Lambrianidis T. Ex vivo study of the efficiency of two techniques for the removal of mineral trioxide aggregate used as a root canal filling material. J Endod. 2008,34(10):1239-1242.


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