Shear bond strength evaluation of an alkasite restorative material to three different liners with and without using adhesive system: An in vitro study.

AIMS: The aim of the study is to compare and evaluate the bonding ability of alkasite restorative material to TheraCal LC™ (TLC), Biodentine™ (BD), resin-modified glass ionomer cement (RMGIC) using an universal adhesive and characterizing their failure modes. SUBJECTS AND METHODS: Ninety extracted intact human molars were divided into three groups of (n = 30) as Group I (TLC), Group II (RMGIC), and Group III (BD). Each group was subdivided into two based on application of universal adhesive. Cention N was bonded to each sample. Shear bond strength analysis was performed. The data were analyzed using SPSS version 22 software.
RESULTS: No significant difference was observed between Group I and Group II (P < 0.05) while Group III showed the least bond strength (P < 0.05). The modes of failure were predominantly cohesive in Groups I and III (TLC and BD) while RMGIC showed mixed and adhesive failures.
CONCLUSIONS: The bond strength of Cention N to TLC and RMGIC was similar and significantly higher than that of BD following application of universal adhesive. Copyright:

INTRODUCTION

Failure of restorations is a major concern for a dental practitioner.[1] A restorative material which provides the best sealing capacity at the interface, i.e., between the restoration and tooth structure has an enhanced clinical longevity.[2] To tackle these existing hurdles, a new alkasite restorative has been introduced to the class of leading restorative materials known as Cention N.

The filler content of Cention N (Ivoclar Vivadent) is an alkaline filler, capable of releasing acid-neutralizing ions. Adhesive may or may not be used with this material. According to the manufacturer, Cention N has to be placed in combination with a liner or a base in deep carious lesions.[3] Widely used liners include conventional glass ionomer (GI) and resin-modified GI cement (RMGIC).

Recently, there has been an incline toward the use of bioactive materials.[4] Biodentine™(BD) (Septodont, Saint-Maur-des-Fossés, Creteil, France) and TheraCal LC™(TLC) (Bisco Inc., Schaumburg, IL, USA) are calcium silicate-based bioactive liners that are proposed as alternatives to GIs. The use of bioactive liners beneath final restoration would clinically be more advantageous than using GI liners as they are biologically well-tolerated by the pulp tissue[5] and have comparatively higher remineralizing ability.[6]

TLC (Bisco Inc., Schaumburg, IL, USA) is a light-cured mineral trioxide aggregate (MTA) - filled, resin-modified calcium silicate cement.[7] At present, there is limited information in the literature on the bonding ability of TLC to composite in comparison to other liners.

To simplify the bonding procedure, recently, a new single bottle universal or multimode adhesive with silanes (Single Bond Universal™, 3M ESPE, St. Paul, MN, USA) was introduced. It can be used in self-etch or total etch or selective etch mode and on any surface without additional primer.[7]

No study to date has compared the bonding ability between TLC, BD, and RMGIC to Cention N with or without the use of universal adhesive.

Hence, in the present study, the shear bond strength (SBS) of BD/TLC/RMGIC to alkasite restorative material using universal adhesive was compared and the null hypothesis was that there is no dissimilarity in the SBS within each substrate (TLC/BD/RMGIC). The study also targeted to identify the specific modes of failure.

SUBJECTS AND METHODS

A total of 90 human molars which were extracted for periodontal reasons were selected for the study. The teeth were cleaned with ultrasonic scalers. The teeth were stored in 0.1% thymol until further use. Using a high-speed diamond disc under water cooling, the occlusal surfaces were ground perpendicular to the long axis of the tooth to obtain a flat surface.

Then, a cavity of 6 mm × 4 mm × 2 mm was prepared using round diamond bur to retain the liner. The liners were manipulated as per the manufacturer's instructions. These teeth were mounted in acrylic resin blocks using a rectangular aluminum mold that was 15 mm × 15 mm × 25 mm in dimension such that the occlusal surfaces were flush with the resin surface. The study design as followed by Velagela et al.[7] was adopted in this study.

Experimental groups

A total of 90 samples were randomly divided into three groups of 30 samples each, based on the type of liner used:

Group I: TLC

Group II: RMGIC

Group III: BD.

Each group had two subgroups:

Subgroup A: Cention N without adhesive application

Subgroup B: Cention N with adhesive application.

Adhesive application

Samples from subgroup B, universal adhesive (Single Bond Universal TM, 3M ESPE, St. Paul, MN, USA) was applied on TLC/BD/RM-GIC surface with a bristle brush, rubbed for 20 s followed by gentle air drying with oil-free compressed air for approximately 5 s to evaporate the solvent and was light-cured for 10 s after placing the polyethylene tube as per the manufacturer's instructions.

Cention N placement

For all the samples, Cention N (Ivoclar Vivadent Bendererstr Schaan Liechtenstein) was placed in the tube and light-cured with a light-curing unit (Bluephase, Ivoclar Vivadent, Schaan, Lichtenstein) with an intensity of 1200 mV/cm2 for 20 s. For the manipulation, one scoop of powder is used per 1 drop of liquid [Figure 1]. After the completion of Cention N build-up, the polyethylene tubes tube (3-mm diameter, 5-mm height) was removed with a sharp knife. All specimens were stored at 37°C in distilled water for 24 h.

Figure 1

Manipulation and placement of Cention N

Measurement of macro shear bond strength

The specimens were attached to the Universal Testing Machine (Model No. UNITEST – 10, ACME, India). A chisel with knife edge was gently held flush against the Cention - TLC/RM-GIC/BD interface and loaded at a cross-head speed of 0.5 mm/min until bond failure occurred [Figure 2]. The load at failure was recorded in Newtons (N). SBS was calculated and expressed in Megapascals (MPa) by dividing the peak load at failure to the specimen surface area (7.07 mm2) according to the formula.[21]

Figure 2

Specimen mounting procedure

SBS (MPa) = Load (N)/Area πr2 (mm2).

Fracture analysis

The fractured test specimens were examined under a stereomicroscope (Swift Stereo SM80, Tokyo, Japan) at a magnification of ×25 and fractures were classified as follows and fractures were classified as follows:

Cohesive - Failure with in TLC/BD/RMGIC or Cention N

Adhesive - Failure at Cention N-TLC/BD/RMGIC interface

Mixed - When two modes of failure occur simultaneously.

Fracture analysis was performed by a single observer who was completely uninformed about the experimental groups.

Statistical analysis

The data were analyzed statistically by comparison of mean microleakage and post hoc test using SPSS version 22 software.

RESULTS

From the results of the present study, it was observed that there was a statistically significant difference (P < 0.05) found between the groups as well as within the groups [Table 1].

Table 1

Descriptive statistics showing mean, standard deviation of shear bond strength for the three liners with and without adhesive application

Group	Subgroup	n	Mean±SD
I	No adhesive application	15	9.6000±2.64818
	With adhesive application	15	14.0800±4.03105
II	No adhesive application	15	19.9900±5.11348
	With adhesive application	15	20.2800±6.81295
III	No adhesive application	15	6.7800±2.62224
	With adhesive application	15	6.6900±1.16201

SD: Standard deviation

The mean SBS of Group I A and B (TLC) was 9.6 ± 2.64 MPa and 14.08 ± 4.03 MPa, respectively, Group II A and B (RMGIC) was 19.99 ± 5.11 MPa and 20.28 ± 6.81 MPa, respectively, Group III A and B (BD) was 6.78 ± 2.62 MPa and 6.69 ± 1.16 MPa, respectively.

The mean SBS values and standard deviations were analyzed using ANOVA test; the post hoc test was used for intergroup comparison (P < 0.05). Highest SBS value was shown by RMGIC (20.28 MPa) followed by TLC (14.08 MPa) and the least SBS value was shown by BD (6.69 MPa) [Table 2]. Most commonly seen mode of failure was cohesive with TLC and BD and mixed failure mode was seen with RMGIC.

Table 2

One way analysis of variance for intragroup comparison

Subgroup	Sum of squares	df	Mean square	F	Significant
No adhesive application
Between groups	1452.043	2	726.022	54.402	<0.001*
Within groups	560.513	42	13.346
Total	2012.556	44
With adhesive application
Between groups	1388.701	2	694.351	32.540	<0.001*
Within groups	896.222	42	21.339
Total	2284.923	44

*P<0.05 is statistically significant

DISCUSSION

The biggest challenge in dentistry is to find a restorative material with physical properties at par with tooth structure.[7] Keeping this in mind, technological advancements have led to the current development of adhesive restorative materials such as GI cement, ormocers, and resin composite.

Cention N an alkasite restorative is a tooth-colored, resin-based, self-curing material. The powder consists of various glass fillers, initiators, and pigments whereas the liquid comprises dimethacrylates and initiators.[3] A patented isofiller, partially functionalized by silanes, has tried to overcome the hurdle of polymerization shrinkage. This isofiller acts as a shrinkage stress reliever which minimizes shrinkage force.[3]

In comparison to conventional GIC, RMGIC has a formulation of 80% GIC combined with 20% visible light polymerized resin components.[8] It allows the acid–base reaction to take its course even after light polymerization and overcomes the inherent drawback of the conventional GIC by showing higher early strength, less moisture sensitivity, and more resistance to solubility and disintegration.[9]

With the introduction of calcium silicate cements, a unique category of materials was developed which combined bioactivity, biocompatibility, and strength. Because of their remineralizing property, they have been advocated to be used as liners. Two such materials are TLC and BD. Both are calcium silicate-based materials but are rather diverse.

TLC is a light-curable resin-modified calcium silicate-based MTA used as a liner under composite restorations. It was developed to overcome, the main disadvantages of MTA, i.e., long setting time, high solubility, and its poor handling characteristic.[10] It releases significantly more calcium ions than either MTA or a calcium hydroxide-based liner.[11]

TLC does not include water for material hydration, and thus it depends on the water taken up from the environment and its diffusion within the material. The hydration of this material may be compromised when compared with water-based materials such as BD. The manufacturer suggests placement of permanent restoration immediately after curing.[12]

BD powder consists of tricalcium silicate, calcium carbonate, and zirconium oxide. The liquid consists of calcium chloride and a water-reducing agent.[13] BD has good sealing ability, high compressive strength, a short setting time, biocompatibility, bioactivity, and remineralization properties. Thus, it is recommended as a dentin substitute for resin composite restorations.[1415]

To mimic intraoral conditions, the restorative material should be able to transfer biting loads over a large area of tooth structure, thus avoiding local stresses.[16] Therefore, it is important that it has adhesion to dentine. The bond strength between restoration and liners is critical for even distribution of stresses, thus providing longevity and success of restorations.[17]

In this study, Groups I and II (TLC and RMGIC) showed significantly higher SBS values than Group III (BD) (P < 0.05). TheraCal LC and RMGIC are resin-based light cure cements that attain early cohesive strength. Since alkasite restorative materials are also resin based, similar chemistry of these materials leads to a strong bonding between them. Second, these materials are cured by a free radical initiator system, which provides a potential for the chemical bonding between these materials.[18] Moreover, TheraCal LC contains a hydrophilic resin monomer that makes it an excellent adhesion promoter enhancing bond strength.[7] This finding is supported by a recent study done by Singla and Wahi.[19]

Group III (BD) showed the lowest SBS value, which may be due to its low early strength of the material per se and this was in agreement with previous studies by Deepa et al.,[7] Abdelmegid et al.,[13] and Meraji and Camilleri.[20] Odabaş et al.[21] evaluated the SBS of composite to BD after 2 time intervals (i.e., 12 min and 24 h) and obtained an increased SBS value for the 24-h period. Bachoo et al.[22] reported that the initial setting reaction of BD takes approximately 12 min after mixing the powder and the liquid. However, it takes up to 2 weeks to achieve complete maturation of BD. BD is a porous material that needs time for crystallization of hydrated calcium silicate gel to attain bulk strength adequate to withstand the polymerization stresses.[7] Gjorgievska et al.[10] evaluated the interfacial properties of BD and found that its adhesion to composite is mainly micromechanical and not ion exchange based because of the lack of resin content, which could have been the second reason for poor bond strength.

In earlier studies, when the degree of conversion was assessed for the liners, it was noted that they required some time to attain complete maturation. Although RMGIC appears to set clinically within minutes, a continuing maturation phase occurs over several months through posthardening reactions. However, 90% of the conversion developes within the first 10 min in light-activated dual-cure resin-based materials.[23] BD on the other hand required a longer time (2 weeks) to complete its maturation.[24] In the present study, bonding was performed immediately after liner placement to depict a single appointment clinical procedure. This could be the reason for low bond strength and high cohesive failures in BD (60%).

In the present study, effect of application of an adhesive agent on bond strength was also evaluated. A significant increase in bond strength value was seen in TheraCal LC and RMGIC groups (P < 0.05). Whereas, no significant difference was seen in the BD group. This self-etch 10-MDP-based adhesive shows chemical bonding to Ca ions and Al and zirconium oxides. The bifunctional silane molecule bonds chemically to silica-containing materials and has methacrylate functionality that allows chemical union with resinous substrate. Silanes also act as adhesion promoters by enhancing the wetting ability of the adhesive system.[7] This adhesive was selected in the study, aiming for additional chemical bonding with Ca releasing bioactive liners.

Though the SBS of Groups I and II (TheraCal LC and RMGIC) were higher, the failure modes were predominantly cohesive (70%) in Group I (TheraCal LC) while Group II (RMGIC) showed 60% mixed failures, 20% adhesive, and 20% cohesive failures. Cohesive failure in TheraCal LC could have been due to its low bulk strength. TheraCal LC, resin-modified calcium silicate cement is a combination of a HEMA/TEGDMA-based resin and calcium-silicate powder. On light activation, HEMA and TEGDMA monomers create a polymeric network that is able to stabilize the outer surface of the cement. Thus formed poly-HEMA is hydrophilic and favors the absorption of moisture and triggers a second setting reaction that is hydration of calcium silicate particles with liberation of calcium ions. TheraCal LC releases more Ca ions than RMGIC. Hence, a strong chemical bonding among adhesive and Ca, Al, Zr, and silicon ions of TheraCal LC could have resulted in higher bond strength values in spite of its low bulk strength.[725]

The limitation of this in vitro study includes the difficulty to accurately simulate the oral condition, which could have led to variations due to effect of forces of mastication and tooth flexure. Further in vitro studies comparing it to the standard composite resin restoration needs to be done. Cention N belongs to a new class of restorative material. Surface characteristics and its interaction with different liners can be evaluated in future using elemental mapping. Effect of aging and different bonding protocol has to be evaluated to find the best adhesive agent for Cention N.

The investigations of the present study have revealed that SBS of Cention N to RMGIC was the highest followed by TheraCal LC. The least value was shown by BD. Bond strength significantly improved on application of Universal adhesive to TheraCal LC and RMGIC whereas no significant difference was seen for BD.

CONCLUSIONS

Within the limitations of the present study, it can be concluded that:

TheraCal LC and RMGIC achieved adequate bond strength to Cention N

Application of Universal adhesive improved the bonding of Cention N to TheraCal LC and RMGIC

Most commonly seen mode of failure was cohesive with TheraCal LC and BD and mixed failure mode was seen with RMGIC

Cention N restoration can be placed immediately over TheraCal LC and RMGIC, completing the procedure in a single appointment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.