Bond strength of composite resin to white mineral trioxide aggregate: Effect of different surface treatments.

BACKGROUND: The main aim of restorative dentistry is to restore and preserve dental health with the use of appropriate restorative modalities to protect the pulp and restore its function. This study compared the effect of different surface treatments of mineral trioxide aggregate (MTA) on the bond strength of composite resin to MTA.
MATERIALS AND METHODS: Forty cylindrical acrylic blocks with a hole were prepared and filled by ProRoot MTA. The samples were assigned to four groups: Group 1 - no surface treatment; Group 2 - phosphoric acid etching; Group 3 - sandblasting; and Group 4 - hydrofluoric acid (HF) etching, rinsing, and silane application. OptiBond Solo Plus adhesive was utilized in all the groups. Then, composite resin cylinders were bonded to sample surfaces. The samples were thermocycled and tested for microshear bond strength using a universal testing machine at a crosshead speed of 1 mm/min. Data were analyzed with Kruskal-Wallis and Mann-Whitney tests. Scanning electron microscopy images were prepared for each study group after surface treatments.
RESULTS: Means and standard deviations of bond strength values in study groups 1-4 were 14.83 ± 7.76, 21.85 ±7.99, 6.48 ± 3.89, and 26.01 ± 11.09 Mpa, respectively.
CONCLUSIONS: Within the limitations of this study, phosphoric acid etching or HF etching plus silanization was preferred to surface treatment of MTA before composite resin bonding.

INTRODUCTION

Mineral trioxide aggregate (MTA) has been confirmed and used as an apical barrier in cases of open apices, repair of perforations, treatment of internal/external root resorption, and direct pulp capping (DPC).[1]

Provision of a coronal seal during restorative procedures of root-filled teeth, especially in cases of perforation or DPC, is very important. In such cases, the use of a secondary intracoronal seal has been suggested with the application of adhesive materials due to inadequate sealing of the perforations or exposure areas.[23]

Furthermore, in most cases with furca perforations or DPC, it is not possible to achieve retention from the intracanal post or secondary retention features. As a result, the advantages of applying an adhesive restoration over MTA consist of achieving secondary retention and secondary seal. In many cases, composite resin is recommended because it exerts lower forces on the pulp capping biomaterial during placement of the final restoration.[4] Since the goal is to obtain a monolithic restoration and to have a more durable, sealed, and successful restoration, the bond strength between the composite resin and MTA is of utmost importance. The composite resin with favorable bond strength can establish the MTA.

There are little data available on the bond strength between MTA and adhesives or various surface treatments.[567] Therefore, this study was undertaken to compare the effect of different surface treatments on the microshear bond strength of composite resin to the MTA.

MATERIALS AND METHODS

Forty cylindrical acrylic blocks with a hole as mold were prepared.[89] Holes with 4 mm in diameter and 2 mm in height were filled with ProRoot MTA (ProRoot MTA, Dentsply Tulsa Dental, USA). The blocks were stored in an incubator at 37°C under 100% humidity for 72 h for final setting. The specimens were divided into four groups, with a specific surface treatment in each performed as follows:

Group 1: No surface treatment was performed on MTA

Group 2: The surface of MTA was etched with 37.5% phosphoric acid gel (Kerr, Karlsruhe, Germany) for 20 s

Group 3: The surface of MTA was sandblasted using an intraoral sandblasting device, Dento-Prep (DK-8721, Daugaard, Denmark), at 7 mm from the surface using 50-μ Al2O3powder under 30 Psi air pressure for 15 s. The surface was rinsed with air/water syringe and then dried

Group 4: The surface of MTA was treated with 9% hydrofluoric acid (HF) (Ultradent, USA) for 90 s. Then, one layer of silane (Ultradent, USA) was applied and allowed to dry.

In all the groups, OptiBond Solo Plus (OBSP) adhesive resin (Kerr, Karlsruhe, Germany) was applied to the specimen and then light-cured for 20 s, using an light-emitting diode light-curing unit (Ivoclar Vivadent, FL-9494 Schaan, Liechtenstein) at an intensity of 650 mW/cm2. Then, 1 mm length of prepared Tygon tube, 0.7 mm internal diameter, was placed on the bonding area, and resin composite (Point 4, 3M ESPE, USA) was packed into the tube and light-cured for 40 s.

After being stored at 37°C distilled water for 24 h, all the specimens underwent a 1000-cycle thermocycling procedure at 5/55°C.

Microshear bond strength tests were carried out using a universal testing machine (Zwick, Roell Z020, Germany), with a shearing force at a strain rate of 1 mm/min and a 0.5-mm wide chisel.

The failure modes were then evaluated under a stereomicroscope (Nikon, Tokyo, Japan) at ×30. Data were analyzed with Kruskal–Wallis and Mann–Whitney tests. Statistical significance was defined at P < 0.05.

Scanning electron microscopy analysis

Two specimens from each group were prepared using the same special surface treatment for scanning electron microscope (SEM) evaluation. Subsequent to surface treatment, the specimens were evaluated under SEM (XL30, PHILIPS, Netherlands) after gold-sputtering.

RESULTS

Mean values and standard deviations of microshear bond strength are presented in Table 1. Kruskal–Wallis test showed a statistically significant difference in microshear bond strength values between the four study groups (P < 0.05). The maximum microshear bond strength was observed in Group 4, which was prepared with HF and silane (26.0 ± 11.0), and the minimum value was observed in Group 3, where sandblasting was used for surface treatment (6.4 ± 3.8).

Table 1

Mean values, standard deviations, and 95% confidence intervals of microshear bond strength

The results of Mann–Whitney test are presented in Table 1. There were significant differences in bond strength values between Group 1 and Groups 3 and 4. In addition, there were significant differences in bond strength values between Group 2 and Group 3 and Group 3 and Group 4.

The majority of fracture modes in the experimental groups were adhesive (82.5%); other fracture modes were cohesive (MTA) in Groups 2 and 4. SEM images of specimens with different surface treatments are shown in Figure 1.

Figure 1

(a) SEM of MTA (no surface treatment). (b) SEM of treatment with phosphoric acid. (c) SEM of sandblasted MTA. (d) SEM of MTA treated with hydrofluoric acid (×1000) (SEM: Scanning electron micrograph, MTA: Mineral trioxide aggregate)

DISCUSSION

In addition to biocompatibility, bioactivity, and remineralization properties of pulp capping materials, the bond strength of these materials to restorative materials is important. Proper bonding of composite resins to pulp capping biomaterials produces an adhesive interface, which is capable of distributing stresses relatively evenly over the entire bonding area.[10] In this study, different surface treatments were carried out on MTA to bond the composite resin.

The principal ingredients of white MTA include calcium oxide, silica, and bismuth oxide;[11] phosphorous is present in very small amounts in MTA.[12] However, hydroxyapatite crystals are precipitated in phosphate-buffered solution.[13]

Although the chemical composition of MTA precipitates was not examined in the present study, the composition of crystalline structures created on MTA surfaces could be deduced. It was assumed that they are calcium hydroxide and calcium silicate hydrate, which are the principal bonding phases in a hydrated Portland cement-based material.[1415]

Under SEM, Group 1 with no surface treatment showed two structural phases, including a crystal phase of calcium phosphate and an amorphous phase of calcium oxide with a granular view [Figure 1a], which is consistent within the results of some previous studies.[11161718] The mean microshear bond strength in this group was significantly higher than that in Group 3 (sandblasting and OBSP). The presence of intracrystalline spaces on the MTA surface results in a spongy appearance, which helps in infiltration of resin and results in micromechanical bonding.

In this study, application of phosphoric acid etch on the MTA surface resulted in the removal of crystals and amorphous structures, creating a rough and porous surface that improved adhesion of the resin materials [Figure 1b]. The microshear bond strength in Group 2 was higher than that in Groups 1 and 3. There were no statistically significant differences between phosphoric acid-etched and nonetched groups (Groups 1 and 2). In a study by Oskoee et al.,[10] no significant differences were reported between the shear bond strength of etched and nonetched groups.

A study used scanning electron microscopy to evaluate the effects of acid etching on surface characteristics of MTA and showed that the disordered structure and spindle-shaped crystals were removed during the process;[19] therefore, the selective removal of the matrix surrounding the crystals results in a sponge-like surface suitable for bonding to composite resins with no significant effect on MTA structure.

Phosphoric acid treatment eroded the crystalline structure on the white MTA surface, creating a cracked surface that contained internal pores. Consistent with the results of the present study, the characteristic etching pattern on MTA as a result of phosphoric acid treatment was reported previously,[19] suggesting that phosphoric acid might contribute to a reliable micromechanical bonding of the etch-and-rinse adhesive system to MTA.[7]

Based on the results of this study, the minimum bond strength was recorded in Group 3. Sandblasting of the surface with 50-μ alumina resulted in a homogeneous appearance with lower pore depths compared to acid-etching under SEM [Figure 1c]. The low bond strength value in this group was attributed to fewer pores, which can be a result of sandblasting without the use of silane because as noted previously, application of silane for the preservation of bonding is critical after sandblasting.[202122]

Maximum bond strength was recorded in Group 4 in the present study. Application of 9% HF for 90 s resulted in a homogenously porous appearance under SEM [Figure 1d]. HF can remove all the smear layers on the surface and etch the silica and the boundaries between the phases. Application of silane after etching improved the wetting and served as a chemical bonding agent. It is a bifunctional molecule, reacting with the hydroxyl groups of the silica phases on one side and bonding to copolymerized resin from the other side.[23]

It seems that the stronger acidic treatment resulted in a more destructive surface than the weaker acidic treatment, and the eroded surface enhanced the bond strength.[7]

Several studies have reported the presence of different amounts of silica in the MTA phase. Torabinejad et al. showed 2.47% and 6% silica in the crystal and amorphous phases, respectively.[17] In addition, Dammaschke et al. reported the presence of tricalcium and dicalcium silicate in MTA crystals.[16] In the present study, the highest bond strength value (26.01 MPa) was recorded in Group 4, which was attributed to the combination of the chemical bond between silane and the etched silica groups on MTA, as well as the micromechanical bonding caused by the rough MTA surface.

In this study, cohesive (MTA) fracture modes were seen in Groups 2 and 4. A previous study reported that the bond is acceptable when fracture occurs within each material rather than in the bonded interface (i.e., cohesive rather than adhesive).[24]

CONCLUSIONS

High values of microshear bond strength means obtained in this study can be explained by surface porosities and micromechanical bonding mechanism. Maximum bond strength in HF and silane treated group could be related to the chemical bonding between silica groups on MTA surface and the silane molecules.

Financial support and sponsorship

This report is based on a Grant (#385071) submitted to the Vice Chancellery for Research, Isfahan University of Medical Sciences, Isfahan, Iran.

Conflicts of interest

There are no conflicts of interest.