Zinc oxide eugenol-based cements are used as temporary filling materials and come into contact with MTA when it is used for pulp capping. Zinc is a retarder of cement hydration [47], and zinc salts form calcium hydrozincate (Ca(Zn(OH)₃H₂O)₂), an insoluble hydroxide in alkaline solution that creates a coating on MTA particles. In addition, zinc oxide retards the hydration of tricalcium silicate, although it does not interfere with the tricalcium aluminate/gypsum reaction. Zinc is incorporated into the calcium silicate hydrate gel phase [46], and the migration of zinc from zinc oxide cement into MTA has been reported. Furthermore, MTA in contact with intermediate restorative material (IRM) (Dentsply Caulk, Milford, DE, USA) exhibits a high degree of porosity resulting from incomplete hydration (Fig. 5.1b) [9].

The shear bond strengths of different composite adhesive systems to white MTA has been compared [6] with the conclusion that an etch-and-rinse adhesive system was preferred when placing compomer materials upon white MTA because it exhibited significantly higher shear bond strength values than self-etch adhesive systems. MTA used as pulp capping material necessitates the layering of MTA with composite for immediate restoration of the tooth. The use of bonding agent has been shown to reduce gaps at the material interface. Contact with both bonding agent and composite can reduce the micro-hardness of MTA in the early stages of its hydration [65].

When used inside the root canal for furcal repair and apexification procedures, MTA may come into contact with medicaments such as non-setting calcium hydroxide paste. No consensus has been reached on whether the calcium hydroxide in medicaments affects the sealing ability of MTA [20, 58]. However, calcium hydroxide does create an alkaline environment, which increases the porosity and un-hydrated microstructure of MTA [49], although no microstructural changes have been observed within MTA following contact with calcium hydroxide paste. On the other hand, migration of silicon and aluminium from MTA into calcium hydroxide medicaments has been observed [9].

MTA in contact with formaldehyde exhibits black discolouration Marciano MA, Hungaro Duarte MA University of Bauru (Personal Communication). Formaldehyde is released from resin-based sealers containing hexamethylenetetramine, such as AH 26, and Sealer 26 as a result of a chemical reaction between bisphenol A resin and hexamethylenetetramine.

It is well known that exposure of MTA to a low pH environment will influence its physical and chemical properties [14, 70]. It is therefore not surprising that in the laboratory environment application of acid etch has adverse effects in the short term, on the push-out bond strength [56], porosity, micro-hardness [37] and compressive strength [28] of MTA, suggesting it would be better to postpone restorative procedures to allow more advanced hydration of the cement. Over longer time periods after placement, the application of etchants on some brands of MTA does not appear to affect its compressive strength [29]; however, other brands were more susceptible to etchant and have a reduced strength.

Following the laboratory evaluation of acid etchant on various materials [44], it was concluded that etching of MTA did not improve its shear bond strength to composite resin and that the surface etching of MTA was not necessary prior to composite placement using a total-etch adhesive resin. Furthermore, it is advised that when MTA is used in vital pulp therapies, it is better to cover the material with glass ionomer cement. However, it must be emphasised that these reports are based on laboratory investigations and that clinical trials have not been reported on the use and effects of acid etchant on MTA in a clinical setting.

The effects of exposing MTA to acid etching have been demonstrated by SEM analysis. In general, a selective loss of matrix from around the crystalline structures of MTA has been observed resulting in a relatively uniform ‘honeycomb’ pattern without penetrating deeply or removing substantial amounts of cement. In addition, etching has revealed crystalline structures such as plate-shaped and laminated crystals on the MTA surface; however, needlelike crystals have been reported to be missing (Fig. 5.3a, b) [28]. Further investigation on the material microstructure after exposure to acid etch in order to identify crystal morphology is necessary. The significance of these morphological changes is unclear.

The contamination of MTA by blood has been investigated in a number of laboratory studies in terms of the effect on its physical properties, leakage, displacement, marginal adaptation and colour. There is little doubt that blood contamination on the surface of MTA and, in particular, when incorporated into the material is detrimental to its hydration and thus its ultimate physical properties and performance. Unfortunately, MTA is often placed in contact with vital tissues that ooze blood/serum (pulp capping) or in situations where blood pools on its surface (root-end filling), and the impact of blood contamination is important and must be minimised.

Demineralised and mineralised graft materials appear to have a differential effect on the micro-hardness of white MTA. White MTA micro-hardness values when in contact with Bio-Oss (Geistlich Pharma, Princeton, New Jersey, USA), MinerOss (BioHorizons, Markham, Ontario, Canada) and Puros (Zimmer Dental, Carlsbad, California, USA) have been reported to be lower than those for OraGraft (Salvin Dental Specialities, Charlotte, North Carolina, USA) and control groups regardless of incubation period [53].

A number of laboratory studies have evaluated the physical properties of MTA specimens following exposure to a range of acidic environments during hydration on the basis that in some situations the tissues or fluids that come into contact with MTA are acidic. The mean pH of pus from periapical abscesses was generally acidic, although some samples were neutral and some were alkaline [40]. It has been reported that surface hardness of MTA was reduced in an acidic environment [37] as is push-out strength [56]. Leakage of root-end fillings has also been reported to be affected by low pH [50]. The effect of acidic environment on the dislodgement resistance of MTA when used as a perforation repair material has been compared, and it was concluded that its dislodgment resistance was significantly reduced after exposure to acid [23]. Alkaline pH has also been reported to have a negative impact on push-out strength [51], surface hardness and porosity [49].

Since MTA is known to have relatively poor physical properties in terms of strength and wear resistance, it has never been recommended for use when in contact with saliva. Thus, the contamination of MTA with saliva is unlikely in most situations, but may occur if the material is used to repair perforations that communicate with the mouth, i.e. in perforations occurring within periodontal pockets. Despite this, a number of laboratory studies have been conducted on salivary contamination of MTA with conflicting results. It has been reported that saliva reduces [24], increases [36] or has no effect [25] on leakage of MTA when used as a root-end filling material. The leakage of MTA when exposed to saliva and used as an orifice barrier [71] and an orthograde filling material [3] has also been evaluated with conflicting results. Overall, it must be remembered that the validity of leakage studies has been questioned and the conclusions of these and other similar studies are likely to be meaningless.

Mineral trioxide aggregate used as a root-end filling comes into contact with tissue fluid before complete hydration is achieved. When MTA is in contact with tissue fluid, the setting time is extended and in certain cases the material may not set at all [11, 19]. Immersion in tissue fluid has been reported to result in incomplete setting of MTA as the presence of phosphates in solution retards its hydration. The retardation is induced by the formation of insoluble hydroxides in the alkaline solution. The insoluble hydroxides form a coating over the cement particles. The adsorption of phosphate ions on the surface of the clinker phase or on the hydration product is thought to result in the precipitation of calcium phosphates [46]. Glucose, which is present in Hank’s balanced salt solution, is also a known retarder of cement hydration [46].

The retardation of setting is apparent under the scanning electron microscope when the surface microstructure of MTA in contact with Hank’s balanced salt solution was investigated. MTA in contact with simulated body fluid exhibits no evidence of hydration (Fig. 5.5). The cement in contact with simulated body fluid exhibits micro-cracking, which is caused by expansion of the cement [11].

When set MTA is stored in Hank’s balanced salt solution, the pH of the storage solution becomes less alkaline. This could be due to the presence of buffers in simulated body fluids. These buffers are added to these solutions to maintain their pH. Regardless of the less alkaline pH, calcium ion release has been demonstrated in Hank’s balanced salt solution. The leaching of calcium is higher in simulated body fluid than in distilled water both when tested after 1 day of immersion and at 28 days of material contact with the solution [8, 17].

The compressive strength of MTA stored in Hank’s balanced salt solution is lower than when stored in a humid environment or immersed in water [18]. The dimensional stability of MTA in contact with different soaking solutions has been investigated and a net expansion has been reported when the materials were placed in contact with physiological solutions [8, 59]. MTA materials achieved approximately half of their final linear setting expansion by 300 min, with approximately 75 % of expansion occurring by 460 min and the final 25 % of total expansion occurring between 460 min and 24 h [59].

MTA releases calcium hydroxide as a by-product of hydration. The calcium hydroxide has a beneficial effect on the pulp, and dentine bridge formation has been reported. Random controlled clinical trials have shown that MTA is associated with the best clinical outcomes when used as a pulp capping material and for apexification/apexogenesis procedures (as discussed in Chap. 3; Table 3.​1). The metallic ions released by set and setting MTA when placed clinically, may release dentine matrix components that potentially influence cellular events for dentine repair and regeneration [61].

When in contact with dentine, MTA altered the toughness more than the strength and stiffness of dentine after ageing in 100 % relative humidity. Dentine toughness is attributed to its collagen matrix; thus, MTA seems to affect the dentine collagen matrix [54]. Prolonged contact of mineralised dentine with MTA has an adverse effect on the integrity of the dentine collagen matrix. However, the amount of collagen extracted was limited to the contact surface. Clinicians are thus advised to use MTA with caution when it is applied to thin dentinal walls [32].

Along the material-dentine interface, MTA forms a taglike structure that is composed of either Ca- and P-rich crystalline deposits or the material itself. The width of a Ca- and Si-rich layer detected along the dentine layer of the material-dentine interface increases over time [21]. The selective diffusion of silicon, calcium and phosphorous across the cement-dentine interface has been demonstrated [16]. The movement of calcium is difficult to show using scanning electron microscopy and elemental mapping since calcium is present in both the material and dentine. However, elemental migration is clearly shown across the interface (Fig. 5.6). Together with calcium, silicon and phosphorus, migration of bismuth is also evident. Migration of bismuth into dentine can be problematic as bismuth oxide has been shown to react with collagen resulting in a change in colour of both MTA and the tooth [35] (Fig. 5.7).