Scanning electron microscopic evaluation of the material interface of adjacent layers of dental materials

Abstract

Objective

Many dental materials are used in contact with each other in sandwich techniques. Liners, bases and permanent restorative materials are placed adjacent to each other and allowed to set under the same conditions. The same applies for endodontic materials where irrigating solutions and root canal dressings come in contact with root canal obturating materials and root-end fillers. The aim of this research was to investigate the material interface of mineral trioxide aggregate (MTA) in contact with non-setting calcium hydroxide paste (CH), glass ionomer cement (GIC) and intermediate restorative material (IRM).

Methods

All materials were mixed according to manufacturer’s instructions. Freshly mixed MTA (Dentsply) was placed in a plastic container and another dental material was compacted on it while still unset. These materials included GIC (Fuji IX), non-setting calcium hydroxide (Calasept) and IRM (Dentsply). The materials were allowed to set for 28 days at 37 °C and 100% humidity. The layered materials were sectioned longitudinally embedded in resin and polished to expose the interface between the two materials. Scanning electron microscopy (SEM) was performed of the interface and X-ray energy dispersive analysis (EDX) was conducted at 50 μm intervals to establish elements present at specific distances from the material interface.

Results

The calcium hydroxide paste did not affect the hydration of MTA. Migration of silicon, aluminum and bismuth from the MTA to the CH occurred. The GIC exhibited a high degree of micro-cracking and some porosity in the interfacial region. Strontium from the GIC was detected at 200 μm within the MTA. The zinc from the IRM cement was detected at 100 μm within the MTA. The zinc affected the hydration of the MTA leading to retardation of setting and increased porosity.

Conclusions

MTA interacts with other dental materials with resultant elemental migration in adjacent materials. Zinc oxide eugenol based cements should be avoided in the presence of MTA as zinc causes retardation of cement hydration with increased porosity. Glass ionomer cements absorb the water of hydration from the MTA also resulting in increased porosity and incomplete hydration of MTA.

Introduction

Mineral trioxide aggregate (MTA) has been developed to be used primarily to seal lateral root perforations and as a root-end filling material following apicectomy . It can be used for a variety of applications including pulp capping and as a dressing over pulpotomies of permanent teeth . In primary teeth MTA has demonstrated similar treatment outcomes to the formocresol pulpotomy procedure . The versatile use of MTA makes the material come in contact with blood and tissue fluids and with other dental materials.

The setting of MTA consists of two separate reactions; an initial reaction between tricalcium aluminate and water which in the presence of gypsum, results in the production of ettringite which later forms monosulfate once the gypsum is depleted. The main reaction between the tricalcium and dicalcium silicate and water results in the production of calcium silicate hydrate gel which is poorly crystalline and calcium hydroxide . Set MTA is reported to be composed of numerous residual un-hydrated cement grains which, had a dense rim of hydration product made up of pure calcium silicate hydrate. There was very little ettringite or monosulfate present. Un-reacted bismuth oxide particles and calcium hydroxide were also detected . The hydraulic nature of the cement led to the manufacturer’s recommendation to cover the freshly placed material with a wet gauze to avoid desiccation . The presence of gypsum and production of ettringite leads to an extended setting time. The manufacturer thus recommends leaving the material undisturbed for at least 4 h .

The production of calcium hydroxide makes the MTA very reactive and thus susceptible to reaction when in contact with other dental materials, body fluids and blood. A hydroxyapatite layer is deposited over the cement surface when MTA used as a root-end filling material is in contact with tissue fluids . The hydroxyapatite is formed by reaction of the calcium hydroxide with phosphates present in tissue fluids. Blood contamination has no effect on the sealing ability of MTA but adversely affects the displacement of the material when used to repair furcal perforations . A biphasic surface containing large hexagonal crystalline plates composed of calcium embedded in a pool of globular crystals was reported in contact with fetal bovine serum . Contact of MTA with irrigating solutions results in a reduced Ca/Si molar ratio and a reduced calcium hydroxide production .

The manufacturer’s recommendation of leaving the material undisturbed for at least 4 h and covering with a moist cotton pellet necessitates multiple appointments with an increase in chair-side time. Single visit treatments using MTA as a pulp capping agent , for pulpotomy procedures and perforation repair material have been reported with glass ionomer cement and zinc-oxide eugenol placed over unset MTA. Placement of glass ionomer cement over MTA resulted in the deposition of calcium salts at the interface of the two materials . The setting time of the glass ionomer cement and the shear bond strength was not affected by contact with partially set mineral trioxide aggregate. No crazing of glass ionomer occurred when in contact with MTA , thus it has been recommended to place the glass ionomer over partially set MTA in a single visit avoiding the need of multiple visits that are usually necessary when using MTA due to its extended setting time . The use of resins and etch over MTA has also been investigated. Etching systems subject the MTA to acidic conditions. The total etch technique was demonstrated to give a higher interfacial bond strength then self-etching bonding agents . The acidic pH results in a reduced push-out bond strength with the failure being mostly adhesive . Acidic pHs have resulted in an increased microleakage while conversely previous research had indicated that the acidic pH did not affect the microleakage . The difference in the microleakage pattern could result from lack of consideration of the dye uptake by the materials since in most studies only the micro-leakage at the tooth to material interface is assessed . Acids have a deleterious effect on the morphology of the set material . The increasing acidity resulted in extensive porosity , a reduction in surface hardness and an increased compressive strength of the set material .

The layering of temporary and base materials on freshly placed MTA and contact of MTA with materials used routinely as inter-appointment dressings in endodontics effects the physical properties of MTA. The effect of contact of dental materials on the hydration mechanism of MTA has never been reported. The aim of this research was to evaluate the hydration kinetics of freshly mixed MTA in contact with non-setting calcium hydroxide paste, glass ionomer cement and intermediate restorative material.

Materials and methods

The materials used in this study included mineral trioxide aggregate (ProRoot MTA; Dentsply, Tulsa Dental Products, Tulsa, OK, USA), glass ionomer cement (Fuji IX GP, GC America, Alsip IL, USA), non-setting calcium hydroxide (Calasept, Nordiska Dental, Angelholm, Sweden) and Intermediate Restorative Material (IRM, Dentsply DeTrey, Konstanz, Germany). Three composite mixes were prepared: MTA with glass ionomer cement (GIC); MTA with IRM and MTA with calcium hydroxide (CH). All the materials were mixed according to manufacturer’s instructions. Approximately 1.5 g of MTA was placed in a plastic tube 48 mm long and 12 mm internal diameter together with a similar quantity of one of the other three cements so that the two materials in each tube were in layers 3–4 mm thick with an approximately horizontal interface. The MTA was mixed to a stiff paste and leveled with a glass spatula. The 2nd material was layered on top taking care not to interfere with the MTA layer. The tubes were sealed at both ends. The use of glass tubes was specifically avoided in order to eliminate the risk of alkali-silica reaction between the MTA and the glass . Materials were also prepared and placed individually in plastic tubes for examination of the surfaces without contamination by other materials. The containers were placed vertically in an incubator and the materials were allowed to cure for 28 days at 37 °C and 100% humidity. A drop of water was placed on the MTA surface after 24 h to avoid desiccation.

Microscopy of cured cements

After the mixes were cured, they were removed from the plastic tubes and dried by evacuation in a vacuum desiccator for 8 h. Polished sections were prepared by embedding the mixes in epoxy resin, orientated so that the interfaces could be examined in cross-section. The hardened epoxy resin blocks were ground and polished using diamond polishing compound in successively finer grades from 25 μm to 0.25 μm. A thin conductive coating of evaporated carbon was applied to the polished sections prior to examination in the scanning electron microscope (SEM). The scanning electron microscope used was an ISI SS40 (ISI, Tokyo, Japan), with an energy-dispersive X-ray system (SAMx Numerix, Levens, France) and a standard beryllium window X-ray detector. The beryllium window was used to absorb X-rays emitted from light elements. Quantitative analyses were carried out using X-ray standards obtained from minerals for each element, with the exception of bismuth. A bismuth standard was obtained using particles of bismuth oxide in the uncured MTA. Oxygen was calculated by stoichiometry . The sections were examined using backscattered electron imaging.

The microstructure of the interfacial region of MTA in contact with the three different dental materials was analyzed by identifying and labeling of hydration products viewed under the scanning electron microscope in back scatter mode and by elemental analysis of the different hydration products by X-ray energy dispersive analysis. Lines parallel to the interface at increasing incremental distances of 50 μm up to 600 μm were scanned and spectra collected were used to draw atomic ratio plots.

Materials and methods

The materials used in this study included mineral trioxide aggregate (ProRoot MTA; Dentsply, Tulsa Dental Products, Tulsa, OK, USA), glass ionomer cement (Fuji IX GP, GC America, Alsip IL, USA), non-setting calcium hydroxide (Calasept, Nordiska Dental, Angelholm, Sweden) and Intermediate Restorative Material (IRM, Dentsply DeTrey, Konstanz, Germany). Three composite mixes were prepared: MTA with glass ionomer cement (GIC); MTA with IRM and MTA with calcium hydroxide (CH). All the materials were mixed according to manufacturer’s instructions. Approximately 1.5 g of MTA was placed in a plastic tube 48 mm long and 12 mm internal diameter together with a similar quantity of one of the other three cements so that the two materials in each tube were in layers 3–4 mm thick with an approximately horizontal interface. The MTA was mixed to a stiff paste and leveled with a glass spatula. The 2nd material was layered on top taking care not to interfere with the MTA layer. The tubes were sealed at both ends. The use of glass tubes was specifically avoided in order to eliminate the risk of alkali-silica reaction between the MTA and the glass . Materials were also prepared and placed individually in plastic tubes for examination of the surfaces without contamination by other materials. The containers were placed vertically in an incubator and the materials were allowed to cure for 28 days at 37 °C and 100% humidity. A drop of water was placed on the MTA surface after 24 h to avoid desiccation.

Microscopy of cured cements

After the mixes were cured, they were removed from the plastic tubes and dried by evacuation in a vacuum desiccator for 8 h. Polished sections were prepared by embedding the mixes in epoxy resin, orientated so that the interfaces could be examined in cross-section. The hardened epoxy resin blocks were ground and polished using diamond polishing compound in successively finer grades from 25 μm to 0.25 μm. A thin conductive coating of evaporated carbon was applied to the polished sections prior to examination in the scanning electron microscope (SEM). The scanning electron microscope used was an ISI SS40 (ISI, Tokyo, Japan), with an energy-dispersive X-ray system (SAMx Numerix, Levens, France) and a standard beryllium window X-ray detector. The beryllium window was used to absorb X-rays emitted from light elements. Quantitative analyses were carried out using X-ray standards obtained from minerals for each element, with the exception of bismuth. A bismuth standard was obtained using particles of bismuth oxide in the uncured MTA. Oxygen was calculated by stoichiometry . The sections were examined using backscattered electron imaging.

The microstructure of the interfacial region of MTA in contact with the three different dental materials was analyzed by identifying and labeling of hydration products viewed under the scanning electron microscope in back scatter mode and by elemental analysis of the different hydration products by X-ray energy dispersive analysis. Lines parallel to the interface at increasing incremental distances of 50 μm up to 600 μm were scanned and spectra collected were used to draw atomic ratio plots.

Results

Microscopy of uncontaminated materials

The back scattered electron images and the X-ray energy dispersive analysis of the calcium hydroxide, glass ionomer cement and IRM are shown in Fig. 1 . The non setting calcium hydroxide was composed of a matrix of particles of varying size the largest being 10 μm in diameter ( Fig. 1 a). The elemental composition included calcium, barium and sulfur ( Fig. 1 b). The glass ionomer cement microstructure was composed of glass particles interspersed in a matrix. Numerous cracks were visible in the specimen passing within the matrix around the glass particles ( Fig. 1 c). The elemental constitution consisted of silicon, aluminum and strontium ( Fig. 1 d). IRM was composed of a dense matrix made up of very fine particles ( Fig. 1 e) with zinc as the main constituent of this material ( Fig. 1 f).

Fig. 1
Back scatter electron images and X-ray energy dispersive analysis of (a and b) non-setting calcium hydroxide, (c and d) glass ionomer cement, and (e and f) intermediate restorative material.

Microscopy of interfacial regions of MTA and other dental cements

The back scattered electron images of the interfacial region of MTA in contact with non-setting calcium hydroxide, glass ionomer cement and IRM are shown in Fig. 2 . The calcium hydroxide paste did not show any changes in microstructure when in contact with MTA ( Fig. 2 a and b). Both materials had a similar microstructure mainly composed of a cement matrix with different sized particles and radiopacifier interspersed within this matrix. The bismuth oxide in MTA and barium sulfate in CH appeared shiny in the micrograph and the radiopacifiers were equally distributed within the material. There was no sign of segregation or any changes in the interfacial region. EDX analysis at the interfacial region to 600 μm away from this area ( Fig. 3 ) showed the presence of silicon and aluminum which are part of the constituents of MTA at 100 μm within the calcium hydroxide ( Fig. 3 b and c respectively). The bismuth migrated to 50 μm into the calcium hydroxide paste ( Fig. 3 d). There was no migration of barium from the CH to the MTA ( Fig. 3 e).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Scanning electron microscopic evaluation of the material interface of adjacent layers of dental materials
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