Clinical Manipulation of Mineral Trioxide Aggregate: Lessons from the Construction Industry and Their Relevance to Clinical Practice

Abstract

Mineral trioxide aggregate (MTA) is based on ordinary Portland cement (with added radiopaque agents) and, thus, shares many of its features. Although MTA is reported to be difficult to handle clinically, concrete materials made using Portland cement are the foundation of the construction industry. In this paper, we summarize important lessons from the construction literature that are relevant to the successful use of MTA in clinical practice, including behaviour during storage, susceptibility to acidic environments, the effects of exposure of the setting material to moisture and interactions with substances that may interfere with the speed of setting and the quality of the end product.

The use of mineral trioxide aggregate (MTA) in endodontics and restorative dentistry has become popular despite its high cost. MTA contains 80% ordinary Portland cement (OPC), to which is added 20% bismuth trioxide to make the material radiopaque for subsequent identification on dental radiographs. Although MTA has become a well-recognized material in endodontics, restorative dentistry and pediatric dentistry, training in its use is not common outside postgraduate continuing education courses and endodontic specialist training programs. Despite accepted indications for its use in the primary dentition, MTA techniques are not taught universally at dental schools, the greatest barrier being its high cost.¹ Thus, information on handling and use is limited to the supplied "instructions for use" and various published case reports.²

Although case reports and clinical trials guide clinicians on where MTA can be used, they do not provide practical information on the rationale for the individual steps taken to manipulate the material and how these affect clinical success. Because MTA is primarily OPC, it is insightful to assess literature related to the use of OPC in the construction industry to identify key factors that relate to performance and draw parallels between them and clinical practice. The clinical issues discussed below are, therefore, intended to complement manufacturers' directions for use.

Water and the Setting Reaction

In the construction industry, OPC is commonly combined with sand or gravel and water to produce concrete. The sand or gravel filler, which is termed aggregate, provides additional strength to the final set product, making it better suited to situations where heavy loads will be applied, such as in buildings, roads and bridges. During setting, OPC reacts with water to form calcium silicate/aluminate hydrates (such as (CaO)₃· (Al₂O₃)·6H₂O and (CaO)₃(SiO₂)₂·4H₂O), calcium hydroxide and water.

The final set cement is a crystalline structure with voids containing water and calcium hydroxide. Despite appearances, the set material is not fully solid; rather there is an associated fluid state, much like water held within in a wet sponge. During the setting reaction, needle-like crystalline hydrates form a framework connecting all the particles together and, effectively, turning the original powder–liquid mixture into a solid-like colloidal gel. If water is lost to the atmosphere during the setting reaction, this will significantly weaken the set material. Hence, loss of moisture while MTA is setting must be avoided.

Exposure of Set Material to Acids

When set cement is exposed to acids, the saturation of its contained fluids with calcium hydroxide is lost, as hydroxide ions are consumed in acid–base reactions. This leads to loss of some of the hydrate structure, creating a surface etching effect. A single treatment with hydrochloric acid can be used both to clean and etch concrete, exposing the aggregate as well as the matrix. Any other exposure of concrete to acidic environments is avoided.

Likewise, exposure of MTA to strong acids will cause surface etching resulting from the loss of calcium hydroxide from the set cement, as hydroxyl ions react with the acid, and consequent dissolution of calcium silicate hydrates. Fluids from the surrounding environment may enter the cement to replace the lost calcium hydroxide or hydrates, depending on the ions present in those fluids.³

Presence of Acids during Mixing

In the construction industry, acidic water is never used in mixing concrete, because it will cause the formation of intermediate compounds that retard hydration of the cement and limit the production of calcium hydroxide.^4,5 Furthermore, acids will decompose both calcium silicate hydrate structures and calcium hydroxide. In the presence of acids, the compounds that form during setting are likely to be more soluble; this disrupts the formation of the mesh of interlocking crystals and also causes them to leach out of the set material.⁶ Before concrete is poured onto acidic soil, a process known as chemical stabilization or soil conditioning is performed.⁷ The soil is mixed with an alkaline material (such as calcium oxide or calcium hydroxide) and allowed to reach a neutral pH before placement of cement.

The dental parallels to acidic soil are  the presence of deep caries (indicating organic acids in the dentin), bacteria in large numbers, such as in infected root canals (accompanied by acidic waste products and metabolites) and inflammation, such as in periapical regions. An acidic pH can be expected at sites of necrosis and inflammation.⁸

As in construction, the presence of acids in the environment where MTA is to be placed will adversely affect the setting reaction. As the environmental pH falls from 7.4 to 4.4, greater leakage can be expected at the margins of the set MTA, and its adhesion to the tooth structure will decrease.^9,10 The micro-hardness of the set MTA is reduced and its microstructure changes from cubic and needle-like crystals to eroded cubic crystal structures.¹¹

Thus, pH should be increased back to physiological normal before placing MTA. For example, dressing the root canal of an abscessed tooth with calcium hydroxide for 1–2 weeks before MTA placement will improve the properties of the set MTA.¹² Moreover, in a vital tooth undergoing apexification, a short treatment with calcium hydroxide can stimulate repair at the apex of the tooth, as well as help to disinfect the canal.^13,14

It has been suggested that pretreatment with calcium hydroxide paste may adversely affect the sealing ability of MTA, as it may be difficult to remove and, thus, remnants might act as a barrier to the adaptation of MTA to the root canal walls or become involved in the MTA setting reaction.¹⁵ The latter point is at odds with the literature from the construction industry, which advocates the use of calcium hydroxide to condition acidic soil. However, calcium hydroxide dressings used in dentistry may contain various additives, such as methylcellulose and carboxymethylcellulose, which are known to retard the setting of OPC.^16,17 Therefore, if a dressing of calcium hydroxide paste is used, extensive irrigation should be carried out to ensure that no remaining dressing material is present, as remnants of the cellulose thickener will retard the setting of MTA.

Similarly, acidic irrigants, etching solutions and conditioners must be adequately washed away before MTA placement. Sodium hypochlorite (NaOCl) irrigants, which have pH values above 11, will neutralize any remaining acids when used to rinse root canals.¹⁸ As discussed earlier, the presence of acids can modify the hydration of MTA, resulting in the formation of new compounds within the matrix structure which may inhibit the hydration reactions.^19-21 NaOCl reacts with bismuth oxide, which turns the yellow powder a dark brown; therefore, the preparation should be adequately irrigated with saline to avoid unnecessary extra darkening of the MTA.²² As darkening of MTA may be expected, Belobrov and Parashos suggest white MTA should not be used in the esthetic zone, rather, calcium hydroxide should be considered for Cvek pulpotomies.²³

Interactions with EDTA

Some common irrigating solutions are not acidic (e.g., disodium edetate has a pH of 7.0–7.4; and tetrasodium edetate has a pH up to 11.3). The issue with ethylenediaminetetraacetic acid (EDTA) is not primarily the pH, but rather chelation. EDTA has 6 potential sites for binding positively charged ions, such as metal ions. Calcium ions are important reactants in the setting of OPC and MTA. If EDTA solutions used to remove the smear layer in endodontics are not rinsed away properly, residual EDTA will chelate calcium ions and disturb the precipitation of hydration products during the setting reaction.²⁴ This explains the finding of Lee and colleagues²⁴ that MTA stored in EDTA solution had no crystalline structure and a low Ca:Si molar ratio. Furthermore, EDTA-treated MTA has been shown to have reduced micro-hardness and to be less biocompatible, as gauged by reduced adhesion of fibroblasts, compared with MTA that has not been treated with EDTA.²⁴

Interactions with Phosphoric Acid

From the above discussion, it follows that phosphoric acid used for etching should be washed away thoroughly before MTA is placed. This situation is particularly relevant in a deep cavity or pulp capping application, where other acids, such as organic acids from bacteria, are also likely present. Even in small amounts, phosphoric acid will alter the MTA setting reaction and reduce the micro-hardness of the set material.²⁵ Thus, acid etchant should be washed from the walls of the cavity preparation with water before MTA placement in a deep cavity. Alternatively, MTA can be covered with a glass ionomer cement (GIC) before etching of the cavity margins in the final phases of restoration placement.²⁶ In many cases, the easiest way to address the influence of both acid etchants and EDTA will be to irrigate or rinse the area with water before placement of MTA.²⁷

Contaminants such as Blood

A general principle in the construction literature is that the higher the level of chemical impurities in the mixing water, the greater the likelihood that 1 or more of these impurities will interfere with the OPC setting reaction, resulting in reduced compressive strength.²⁸ MTA set in the presence of blood has inferior physical properties, i.e., reduced compressive strength and micro-hardness and less resistance to displacement.^29-31 Likewise, in the presence of serum, the MTA setting process is altered with a changed surface morphology and reduced micro-hardness³² and the reaction may be retarded.³³

Although MTA is often described or marketed as being able to set in a wet and possibly bloody environment, it is important to minimize the ingress of any tissue fluids or blood into MTA during placement or as it is setting. This is particularly important along the margins of the preparation where leakage would occur if the MTA sets with inferior properties. Clinicians should minimize hemorrhagic contamination, as excessive blood will not only impair vision and access but will also affect the setting reaction and the quality of the end product.

Variations in the Liquid Component of MTA

In concrete, various additives and impurities in the mixing water are known to alter the setting reaction and affect the end product. Sodium chloride (as found in saline) and many other inorganic and organic materials will likely result in slower setting due to the formation of alternative products.³⁴

Manufacturers' instructions for use of MTA typically recommend sterile or distilled water as the liquid to be mixed with the MTA powder. For both quality control and convenience, this is often included with the MTA powder. Although MTA powder will set if mixed with local anesthetic solutions, the reaction is slower and the set material has less compressive strength.^35,36

NaOCl solutions will allow MTA to set faster than distilled water, but once again at the expense of compressive strength.^35,37 However, in a confined situation where the cement is placed in a non-loading area, this would not be an issue. In many cases, the benefits of an accelerated setting reaction, i.e., less opportunity for dislodgement and disintegration of the restoration being placed, must be balanced carefully against reduced physical properties.^35,37,38

Chlorhexidine gluconate as an alternative to sterile water is not suitable because it completely inhibits the setting reaction of MTA.³⁷

Curing of the Cement

The reaction of OPC with water is dynamic, and water must be retained within the cement during curing to maintain the structure and ensure strength of the final product. If water is lost to evaporation, the strength of the set cement will be reduced. In the construction industry, a range of methods are used to minimize water loss during curing, including wet curing (e.g., sprinkling water on the cement to replace water that has evaporated) and membrane curing (e.g., covering the cement with a water-tight membrane to prevent evaporation).

In clinical practice, the technique corresponding to wet curing is to place a damp cotton pellet on the MTA as it begins to set and leave it in place. However, if the cotton pellet is too dry, water will be drawn out of the cement, weakening it; if the pellet is too wet and is placed too soon, this will also weaken the cement. Using a cotton pellet also delays completing the clinical procedure and may compromise the quality of the seal.³⁹

Following the industry approach of membrane curing, a material, such as GIC or resin-modified GIC liner, can be placed over the MTA. Once this has been placed, the MTA is stable in terms of water loss or gain from the surface, and the clinician can proceed to restore the tooth or obturate the canal. This concept has been tested with white ProRoot MTA (Dentsply, Johnson City, USA), which has an initial setting time of 45 minutes.  GIC placed over the MTA after 45 minutes gives a shear bond strength to dentin which is the same as that seen for any more than waiting 72 hours to place the GIC once the MTA has set completely.²⁶ Therefore, there appears to be no advantage in leaving MTA to set over a period of a few days compared to a one-visit restoration.  However, there do not appear to be any adequate studies that assess the implications of waiting less than 45 minutes before placing GIC onto MTA.

An alternative to covering MTA with GIC is to use a self-etching bonding agent system as a waterproof layer above the material. In one study,⁴⁰ after allowing 10 minutes for the MTA to set, a bonding agent was placed without significantly affecting the final Vickers micro-hardness or distance between the MTA and the bonding agent, compared with waiting 1 day or 7 days before placing the bonding agent. Again, this illustrates that allows that a single-visit restoration with composite resin can be placed over MTA.⁴⁰

Storage of MTA

Both OPC and MTA powders are highly hygroscopic and, when exposed to the atmosphere, will absorb moisture and begin to hydrate. Although OPC can be packaged in airtight bags and containers of appropriate size, the issue of packaging arises with MTA, as some products are sold in multiple-use bottles. Recent research has shown that opening the container causes changes in the particle size of the remaining MTA, which likely has implications in terms of delayed setting and inferior resulting material. Single-use packaging is ideal. Alternatively, airtight jars allow fewer changes in stored material.⁴¹

The OPC setting reaction is retarded in cold temperatures. Likewise, MTA that has been refrigerated shows a significant reduction in surface hardness, greater porosity and leakage; therefore, refrigeration of MTA should be avoided.^42,43

Summary

This analysis of certain aspects of industrial concrete provides insight into and sound principles for the clinical manipulation of MTA. The practical points from this discussion are summarized in Table 1.

THE AUTHORS

	Dr. Ha is a PhD student, University of Queensland, School of Dentistry, Brisbane, Australia.
	Dr. Kahler is a senior lecturer, University of Queensland, School of Dentistry, Brisbane, Australia.
	Prof. Walsh is a professor, University of Queensland, School of Dentistry, Brisbane, Australia.

Correspondence to: Dr. William N. Ha, University of Queensland, School of Dentistry, UQ Oral Health Centre, 288 Herston Road, Herston QLD 4006, Australia. Email: w.ha@uq.edu.au

Acknowledgements: This work was supported in part by the Australian Dental Research Foundation.

The authors have no declared financial interests in any company manufacturing the types of products mentioned in this article.

This article has been peer reviewed.