The evaluation of interfaces between MTA and two types of GIC (conventional and resin modified) under an SEM: An in vitro study.

CONTEXT: Mineral trioxide aggregate (MTA) is a biocompatible repair material that is often used along with glass ionomer cement (GIC) in many clinical situations. AIMS: In this study, the interface of GIC and MTA was examined, and the effect of time on this interface was tested.
MATERIALS AND METHODS: Forty 9-mm hollow cylindrical glass molds were filled with MTA and then according to the group either conventional GIC or resin-modified GIC (RMGIC) is filled immediately or after 45 min. The specimens were then sectioned, carbon coated, and examined using a scanning electron microscope (SEM) and the elemental analysis was done. STATISTICAL ANALYSIS: Observational study, no statistical analysis done.
RESULTS: The SEM showed that both the groups underwent adhesive separation and gap formation at the interface. The specimens in which GIC was condensed over freshly mixed MTA (group IIA and group IIB) also showed cohesive separation in MTA; however, it was more in the GIC condensed after 45 min over MTA groups (group IA and group IB). The results were better for conventional GIC than RMGIC.
CONCLUSIONS: GIC can be applied over freshly mixed MTA with minimal effects on the MTA, but this effect decreases with time.

INTRODUCTION

Mineral trioxide aggregate (MTA) was introduced as a retrograde filling material in 1993 by Torabinejad. It is composed of tricalcium silicate, dicalcium silicate, tricalcium aluminate, calcium sulfate, bismuth oxide, and small quantities of other oxides that alter its mechanical properties.[1] Initially, it was used to seal off all pathways of communication between the root canal and the exterior of a tooth.[2] It is now popular as a root-end filling material,[12] in vital pulp therapy including direct pulp capping and pulpotomy of immature teeth[3] and as an apical barrier in immature teeth with necrotic pulps. It is now also productively used in regenerative endodontic procedures in immature teeth with apical periodontitis.[45]

Glass ionomer cement (GIC) has been suggested as an effective intracanal barrier to prevent coronal microleakage.[6] It has established itself as a good sealing material with proven antibacterial properties.[7]

GIC is placed over MTA in cases of perforation repair, pulp capping, and external cervical resorption.

But the placement of GIC over MTA has always been a question in the mind of the clinician. Should the permanent restoration with GIC over the MTA be done in the same sitting or do you have to wait for a few days for the MTA to set.

It was thus the aim of our study to analyze the interface of conventional GIC and resin-modified GIC (RMGIC) with white MTA using scanning electron microscopy.

MATERIALS AND METHODS

The materials used in this study were ProRoot white MTA (Dentsply Tulsa Dental, Johnson City, TN, USA), type II glass ionomer (Fuji II, GC, Tokyo, Japan), and light cured GIC (Fuji II LC, GC, Tokyo, Japan).

Specimen fabrication

Forty 9-mm hollow cylindrical plastic molds with both ends open were taken. Gelfoam, of 1 mm thickness, was placed from one open end of a glass mold sealed with modeling wax. The gelfoam was moistened with saline to simulate clinical conditions.

Then ProRoot white MTA was mixed according to the manufacturer's instructions. When the mixture exhibited a thick creamy consistency, it was immediately placed into the glass mold with a plastic instrument. A plugger followed by a wet cotton pellet was used to condense the material into the glass mold over the moist gelfoam for a thickness of 4 mm. Moist gauze was placed into the mold over the MTA for a thickness of 2 mm and was temporized with IRM for 2 mm. These measurements were marked with a marker pen before the placement of the material.

The specimens were divided into two groups of 10 specimens each.

Group I

Subgroup I-A

The temporary filling and the gauze were removed after 45 min. Type II GIC was mixed according to the manufacturer's instruction and was condensed over MTA with a plastic instrument. GIC was protected with petroleum jelly.

Subgroup I-B

The temporary filling and the gauze were removed after 45 min. Light cured (resin modified) GIC was mixed according to the manufacturer's instruction and was condensed over MTA with a plastic instrument and cured.

Group II

Subgroup II-A

Type II GIC was mixed according to the manufacturer's instruction and was condensed over the freshly placed MTA with a plastic instrument. GIC was protected with petroleum jelly.

Subgroup II-B

Light cured GIC was mixed according to the manufacturer's instruction and was condensed over the freshly placed MTA with a plastic instrument and cured.

The specimens were allowed to set for 10 min within the plastic tubes to ensure completion of the initial setting reaction of the GIC. Then the plastic tubes were removed carefully, and the specimens were stored at 37°C and 100% humidity for 24 h to encourage setting. All samples were prepared by the same operator.

Examination of interface

The specimens were vertically sectioned using diamond disk and an interface was observed under a scanning electron microscope (SEM) and by energy dispersive x-ray analysis (EDAX).

RESULTS

Microstructural analysis of the interface

The SEM showed that all the groups underwent adhesive separation and gap formation at the interface. This could have arisen out of the processing of the samples. The immediately condensed GIC group (group II-A and group II-B) also showed cohesive separation in MTA; however, it was more in the GIC condensed after 45 min over MTA group (group I-A and group I-B). All the groups showed vertical and horizontal cracks in GIC that were interconnected with each other in the internal voids within the GIC. These changes were contained in the outermost interfacial layer of the MTA, and the inner layers of MTA and GIC seemed affected.

The adhesion was better for the conventional GIC with MTA [Figure 1] than with the light cure GIC with MTA [Figure 2].

Figure 1

The interface between conventional GIC and MTA at 200x magnification

Figure 2

The interface between resin modified GIC and MTA at 400x magnification

As for the EDAX elemental analysis, calcium appeared to be evenly dispersed as densely packed fine particles predominantly on the MTA side [Figures 3 and 4].

Figure 3

EDAX report showing more Ca penetration at the interface between conventional GIC and MTA

Figure 4

EDAX report showing Ca penetration at the interface between resin modified GIC and MTA

DISCUSSION

The various advantageous properties of MTA include high biocompatibility, radiopacity, low solubility, and high alkalinity (pH value of MTA), which is 10.2 immediately after mixing, and rises to 12.5 and remains constant after 3 h (Torabinejad et al. 1995) (pH = 12.5) that gives it its antimicrobial properties.[8] MTA is a hydraulic type of cement, meaning that it sets by reacting with water, and is then stable in water. When mixed with water, it forms via an exothermic reaction. The setting reactions in MTA are approximated to be similar to those in Portland cement, which are best studied by analyzing the hydration of its individual components. The composition of MTA was verified by experimentation by Camelleri.[9] The two most important hydration reactions are those of the greatest constituents, tricalcium silicate, and dicalcium silicate. Tricalcium silicate sets via the following reaction (Bhatty 1991; Ramachandran et al. 2003):

2(3CaO-SiO2) + 6H2O→3CaO - 2SiO2 - 3H2O + 3Ca(OH)2

Directions for use of MTA specify a working time of 5 min and state that it will “set over a period of 4 h”. Alternatively, the setting time of MTA has been quoted by different researchers as 165 min (Torabinejad et al. 1995), 45-140 min for initial and final setting (Chng et al. 2005), 40-140 min for initial and final setting (Islam et al. 2006), 50 min (Kogan et al. 2006), 220-250 min (Ding et al. 2008), 151 min (Huang et al. 2008), and 150 min (Porter et al. 2010).

One of the main disadvantages of using MTA is its long setting time (2 h, 45 min). MTA sets into a hard mass, in the presence of moisture by forming calcium hydroxide and silicate hydrate gel.[1011] But the complete setting of MTA occurs after about 21 days.[12] Manufacturers claim that a moist cotton pellet should be placed over MTA for 4 h to allow its setting; thus, a second clinical sitting becomes mandatory for the placement of a final restoration over MTA. In the recent time, various modifications have been done to decrease the setting time of MTA such as MTA Plus. To complete the final restoration in a single visit, a material that is compatible with MTA can be applied over partially set MTA.[13] Therefore, it is important to identify materials that can be applied over MTA that can allow for immediate final restoration placement.

Resin composites cannot be placed directly over freshly mixed MTA because they can affect the MTA setting, and the etching and rinsing of unset MTA can dislodge the material. However, placing a GIC material over partially set MTA as a part of a permanent restoration or provisional has to be considered.[13] It has been claimed that GICs can be layered over partially set MTA after 45 min, which might enable single-visit procedures.[1314]

Nandini et al.[15] did a study in 2007 to check the influence of GIC on the setting of MTA when used as a furcal repair material and said that GIC could be placed over MTA after 45 min for a single visit procedure. RMGICs can be placed on MTA after conditioning of the surface.[16] But they did not compare its placement with that of conventional GIC or the chemical interaction. Also Yavari et al.[17] found that an acidic environment increased the solubility of white MTA, this needs to be considered during pulp capping procedures.

However, the mixing of MTA and GIC is not advocated. According to a study conducted by Yu-Na Jeong,[18] the mixture of MTA and GIC not only showed lower setting time but also showed decreased compressive strength and increased solubility.

The observation of the formation of calcium salt crystals at the interface can be attributed to the normal maturation process of MTA in the presence of sufficient moisture as described in the previous studies, a fact that might explain its absence in the dry condition groups. It was also reported earlier that the presence of calcium salts at the MTA GIC interface was a result of the interaction of the negatively charged carboxylate anion (RCOO-) in the polyacrylic acid with the calcium in the MTA. It is not clear at this point if the presence of these crystals at the interface would affect the clinical performance of MTA or GIC however this was beyond the scope of this study.

Adhesive separation and gap formation were seen at the interface in all groups.

A reason for this observation is the setting contraction of RMGIC, which is similar to the contraction of resin composites.[19] Also the vacuum related dehydration shrinkage of GIC that is required for SEM procedures, might be a reason.[20]

Plastic molds were used to aid in studying the interface without sectioning of specimen and were standardized to 1.4 mm diameter based on previous studies.[21] A thickness of 4 mm of MTA was selected because this thickness is needed to achieve a good seal. GIC (2 mm thickness) was used based on Davidson and Mjör's[22] suggestion for bilayered restorations.

The cohesive separation on the MTA side of the dry condition groups may be related to the incomplete setting of interfacial MTA in subgroup A of group I.

These results are in agreement with the results of Ashraf et al.[23] who also found changes in the interfacial layer of MTA as well as GIC.

So to minimize the changes in physical properties of the MTA it would be advised to place the conventional GIC over the MTA after 45 min. This would be very useful in cases of pulp capping and pulpotomies. Placing the final restoration over the MTA will save not only money for the patient but also valuable time for the clinician.

CONCLUSION

According to the results obtained in this study, the placement of MTA and GIC can be done in the same appointment without the risk of adverse interactions between the MTA and GIC.

The GIC placed after 45 min had good result over the immediately placed GIC. Among the conventional and RMGICs, the conventional GIC showed a better result than the RMGIC.

So with the results obtained, we can conclude that GIC can be placed over MTA in the same sitting with no loss of strength or detriment in their properties.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest