Evaluation of bond strength of one step and two steps self-etch adhesive agents with two different pulp-capping materials

Primary purpose of restorative dentistry is to preserve pulp vitality.
 Besides calcium hydroxide, the application of calcium silicate cements as a
 material for direct pulp capping has become used recently. The aim of our
 study was to investigate the influence of materials for direct and indirect
 pulp capping on the bond strength of composite restorations using two
 different self etch dentin adhesives. The test was performed on 60 intact
 molars extracted for periodontal or orthodontic reasons. The prepared
 samples were divided into the two groups and three subgroups depending on
 the adhesive and pulp cupping material. Two different calcium silicate based
 materials were used for pulp cupping. One step and two steps self etch
 dentin adhesive was applied to prepared specimens depending on the group and
 with the help of a specially made metal mold set up a composite post. The
 share bond strength was assessed using a universal testing machine. The
 results showed significant difference in the share bond strength between the
 samples treated with TheraCal LC and Biodentine with TheraCal LC being
 superior to Biodentine.


INTRODUCTION
Calcium silicate based materials are bioactive materials capable of forming apatite using calcium silicate or calcium aluminates. These materials are also biointeractive and release ions needed to stimulate formation of a dentin bridge [1].
Biodentine is dentin substitute composed of powder of tricalcium silicate cement, zirconium oxide, calcium carbonate and liquid. Biodentine (BD) is a bioactive material, with mechanical properties similar to dentin and can be used as its replacement [2]. It has good mechanical properties, as well as excellent biocompatibility and bioactive behavior.
TheraCal LC is light-curing calcium silicate cement promoted by the manufacturer for direct pulp capping or as a liner under restorative materials. Studies show that TheraCal LC has calcium ions release properties in a concentration range that can stimulate activity of dental pulp and odontoblasts [3][4][5][6][7]. Bioavailability of calcium ions plays a key role in the proliferation caused by the material, differentiation of human dental pulp cells and new formation of mineralized hard tissues. Compared to other materials for direct and indirect pulp capping it has been proven that TheraCal LC releases higher concentration of calcium ions compared to Dycal, but has lower level of released ions than Biodentine [1]. In clinical practice, self-etch adhesives are currently widely used. They are based on the use of acidic functional monomers that can simultaneously demineralize and prime dentin. Self-etch adhesives eliminate the rinsing phase, significantly reducing clinical application time, technique sensitivity, and risk errors during application [8]. Self-etch adhesives can be classified into two-step self-etch adhesives that include the application of an additional layer of solvent-free hydrophobic resin creating stronger adhesive layers, and one step self-etch adhesives which contain hydrophilic monomers, water, and volatile solvents [9,10]. Bond strength between dentin liners and composite depends on their physicochemical properties, nature of the bond between liner and RC, and the types of adhesive used.
The aim of our study was to investigate the influence of materials for direct and indirect pulp capping on the bond strength of composite restorations using two different self etch dentine adhesives.

MATERIAL AND METHOD
The study included 60 intact molars extracted for periodontal or orthodontic reasons. After removal of soft tissues, the teeth were stored in distilled water (ISO 3696 grade 3) at room temperature. The teeth were then molded in self-adhesive acrylate using 2.5 cm × 2.5 cm × 2.5 cm plastic molds and cut at the level of the occlusal surface with a high speed diamond disc with water cooling to obtain smooth dentinal surface. This way prepared samples were divided into the two groups (1 and 2), depending on the adhesive used (One Coat 7 Universal-Coltene and Clearfil SE Bond Kuraray Noritake).
Each group was further subdivided into the three subgroups of 10 teeth each, with A, B and E subgroups from the first group where adhesive One Coat 7 Universal was used and C, D, F subgroups from the second group where Clearfil SE Bond was used. The specimens from subgroups A, B, C and D received cavity preparation on the occlusal surface using high speed handpiece and diamond bur with water cooling in 6 mm diameter and 2 mm depth to provide liner retention. Biodentine (Septodont, Saint-Maur-des-Fosses, France) was applied in cavities from subgroups A and C while Theracal LC (Bisco Inc., Schaumburg, IL, USA) was applied in the subgroups B and D. The teeth from subgroups E and F, where adhesive were applied directly to dentin served as control samples.
Dentin adhesive was applied to prepared specimens using a metal mold made specifically for this purpose; we set up the composite post (3 mm diameter and 3 mm height). The materials used in our research are shown in Table 1.

Measurement of bond strength
The specimens were then mounted on a universal Instron 4301 bond strength-testing machine. A force of 1 mm/ min was applied to each specimen by applying a notched edge blade with curved edge up to breaking of the bond between the composite and TLC/BD ( Figure 1). Share bond strength values were measured in Newtons (N) and then converted to Mega pascals (MPa). Fracture type analysis was performed using a stereomicroscope with magnification ×25, and fracture was classified as cohesive (fracture in the material itself), adhesive (fracture of the interface between the composite and the liner) and mixed (when both types of fracture are present at the same time).

RESULTS
Descriptive analysis of the results obtained for the first and second groups is shown in Tables 2 and 3. Analysis between groups A and B indicated that, for p<0.05, there was a significant difference between the two groups in relation to the average values of bond strength (Mann-Whitney U Test: Z=-2.8111; p=0.0049) in favor of a significantly greater bond strength in group B. Further analysis between groups A/E and B/E indicated that, for p<0.05, there was significant difference between the two groups in relation to the average values of bond strength (Mann-Whitney U Test: Z=-2.8419; p=0.0045/Z=-2.1923; p=0.0283) in favor of significantly greater bond strength in Group E.
Analysis between groups C and D indicated that, for p<0.05, there was a significant difference between the two groups in relation to the average values of the bond strength (Mann-Whitney U Test: Z=-2.3638; p=0.0181) in favor of a significantly greater bond strength in group D. Additional analysis between the C/F and D/S groups indicated that, for p<0.05, there was a significant difference between the groups with respect to the average values of the bond strength (Mann-Whitney U Test: Z=-2.6796; p=0.0074/Z=-1.7052; p=0.0882) in favor of a significantly greater bond strength in group F.
Analysis of the average bond strength between the A/C and B/D groups indicated that for p>0.05, there was no significant difference between the two groups with respect to the average bond strength values (Mann-Whitney U Test: Z=-0.8305; p=0.4062 and Z=0.0112; p=1.0000).
The observed modes of failure in the BD group were predominantly cohesive, whereas the TheraCal LC group had a mixed fracture besides the cohesive one.

DISCUSSION
Primary purpose of restorative dentistry is to preserve pulp vitality [11]. Calcium hydroxide has been the gold standard for a long time in direct pulp capping [12]. In addition to this material, the application of calcium silicate cements as a material for direct pulp capping has become more present recently. Calcium silicate based materials are bioactive materials capable of forming apatite using calcium silicate or calcium aluminates. These materials are also bio interactive and release ions needed to stimulate dentin bridge formation [13]. Biodentine is a bioactive material, with mechanical properties similar to dentin and can be used as its replacement to cause dentin bridge formation. It has good mechanical properties, as well as excellent biocompatibility and bioactive behavior. In ad-  dition, initial setting time of BD is about 12 minutes and it does not cause tooth changes [2,14,15]. A major drawback of Biodentine is its water-based chemistry that affects the bond with both dentin and composite, since the bond is mainly micromechanical. To overcome this limitation, tri calcuim silicate (TheraCal LC), modified with resin, was introduced as a material for direct pulp capping. Light-curable resin-modified products have the advantages of precise placement, command set, superior physical strength, less solubility, and reduced heavy metal release. These products with hydrophilic polymer matrix allowed the high release of calcium and hydroxide ions. They are promising materials for dental treatment of direct pulp capping [16].
TheraCal LC is a light-cured polymerizing hydraulic silicate material that sets by hydration. Bonding begins with the contact of material and water. Unlike Biodentine, TheraCal LC does not include water to hydrate the material, and the bonding process depends on the water captured by the environment and its diffusion into the material [17].
Bond strength between restorative materials and materials for direct and indirect pulp capping is also important for the success of restorations. It depends on their physicochemical characteristics, nature of the bond between the liner and the composite material and the type of adhesive system.
Presence of the resin component in TheraCal LC facilitates the placement of the final restoration in the same session. Various research have been done on the impact of these materials on bond strength. Deepa et al. compared and evaluated share bond strength of the composite with three different liners: TheraCal LC, Biodentine, and Fuji II LC after applying universal adhesive (Single Bond Universal). They found that bond strength of the composite with TheraCal LC and Fuji II LC was similar and significantly higher than that with Biodentine after application of universal adhesive [18]. Meraji and Camilleri's researches also show higher bond strengths of TheraCal LC than Biodentine, whether on composite material or Glass Ionomer Cements. The same authors concluded that bond strength between TheraCal LC and composite when applying totaletch adhesives was significantly better than bond strength when applying self-etch dentin adhesives [19].
The results of our study also showed that there was a significant difference in the share bond strength between the samples treated with TheraCal LC and Biodentine in the two tested dentin adhesives in favor of significantly greater bond strength in the TheraCal LC group. This may be due to the fact that the TheraCal LC exhibits early cohesive force upon photo-activation. In contrast, Biodentine shows low hardness of the material itself, at an early stage of application [20].
After initial setting, Biodentine is still a porous material that needs at least 2 weeks for complete polymerization of the hydrated calcium silicate gel and to form a solid mesh that will attain strength sufficient to withstand the stresses of polymerization [15,18]. In our study, we applied the adhesive and composite material in the same session shortly after the initial setting of BD. This may be the reason for the low bond strength, as well as the cohesive type of material fracture.
The effectiveness of bonding of current commercial dentin adhesives is variable. Karadas et al. investigated bond strength of different dentin adhesives with TheraCal LC. They reported that etch and rinse adhesives provided better bond strength restoration than self-etch adhesives [21]. The study also showed that two-step self-etch adhesives had higher bond strength values than one-step selfetch adhesives, with the exception of Clearfil S3, probably due to the fact that it contains 10-Methacryloyloxydecyl dihydrogen phosphate (MDP), which is known to bond chemically to the tooth calcium [21,22]. In contrast, our study showed that there was no significant difference in the bond strength between one-step and two-step self-etch adhesives in the two examined liners. The application of one-step dentin adhesives provides optimal bonding efficiency with a simplified application protocol.
It has been proven that in order to resist the material's contraction forces and provide good enamel and dentin retention, bond strength of the composite material should be minimum 17-20 MPa [23]. This bond strength was achieved in both tested dentin adhesives only in the samples where we used TheraCal LC as the liner. Cantekin showed in his research that light-cured MTA showed clinically acceptable and higher shear bond scores compared to MTA when used with Methacrylate-based composite, as composite material used in our research. According to the same authors, Methacrylate-based composites achieved greater bond strength than silorane-based composites and GI Cement [24].
Despite the high bond strength values of the TheraCal LC samples, the results of our study showed that this bond strength was significantly lower than that of the control group in the two tested dentin adhesives, indicating that despite the presence of the resin component, TheraCal LC as well as Biodentine have an effect on the bond strength of the composite material with dentin.

CONCLUSION
TLC has achieved bond strength sufficient to resist material's contraction forces and provide good enamel and dentin retention at an early stage after application. However, the application of TheraCal LC has an impact on the bond strength of the composite material to the dentin.
BD showed significantly lower bond strength in the early stage after the initial bonding of the material, as well as cohesive fracture confirming the fact that, prior to placement of definitive restoration, this material should be allowed to mature long enough to achieve the required hardness to withstand the forces of the contraction of the restorative material and to provide good retention. The type of dentin adhesive (one-step and two-step self-etch adhesives) had no effect on the bond strength of the two examined liners.