Shear bond strength of calcium silicate-based cements to glass ionomers

Vital pulp treatment is frequently used by pediatric dentists to eradicate bacteria within the dentin-pulp complex, preserve pulp vitality, and create a conducive environment for apexogenesis [8]. Successful VPT necessitates stable pulpal hemodynamics and the establishment of a hermetic coronal restoration to eliminate severe inflammatory reactions [9]. The ideal biomaterial utilized in VPT should promote dentin formation by stimulating residual pulp tissue and possess the ability to resist long-term bacterial infiltration when restorative materials are applied [10]. Hence, the bonding between restorative materials and biomaterials is of utmost importance. Failure to achieve hermetic occlusion at the interface between the biomaterial and restorative material would result in bacterial penetration into the pulp, ultimately compromising the success of the VPT procedure [11].

Biomaterials and restorative materials must possess favorable compressive strength to withstand masticatory forces [12]. To minimize microleakage beneath composite resin restorations, the use of glass ionomer cement or resin-modified glass ionomer cement as bases has frequently been suggested [13]. A review of the literature pertaining to the SBS of calcium silicate-based materials revealed that the majority of studies have focused on WMTA [4, 14‐17], with limited investigations conducted on MMTA and MPPC [18]. The objective of our research was to compare the SBS of WMTA, MMTA, and MPPC-calcium silicate-based biomaterials commonly employed in VPT with those of various restorative materials (EFHF, FIILC, and ABR).

One of the prevalent in vitro approaches employed to assess the adhesive characteristics of restorative materials is the examination of bond strength [19]. Adhesive assessments encompass both quantitative analyses and qualitative screening tests, which aid in predicting the load-bearing capacity and longevity of the bond while also facilitating the investigation of adhesive interfaces and adhesion failures [20]. MTA exhibits brittleness, rendering it unsuitable for tensile bond strength testing [21]. Consequently, in this investigation, the SBS test was employed to evaluate the bond strength of calcium silicate-based biomaterials with various restorative materials containing glass ionomers.

In this research, the group with the lowest average SBS value for all samples, without adhesive application, was observed in the WMTA + EFHF group. Likewise, Bicer et al. [22] observed that WMTA samples lacking any binding agent exhibited a comparatively lower SBS value (5 ± 0.50 MPa) to EFHF when compared to resin-modified glass ionomer cement and compomer. Similarly, in an investigation conducted by Cantekin and Avcı [4], the glass ionomer cement group had the lowest SBS within the WMTA group, where no adhesive was applied.

Tulumbacı et al. [23] reported a mean SBS of 2.84 ± 3.51 MPa in the resin modified glass ionomer cement (RMGIC) group (utilizing Photac Fil Quick Applicap) when combined with WMTA without adhesive. Interestingly, in this study, it was found that the mean SBS between WMTA without adhesive and FIILC was higher (7.41 ± 2.95 MPa). In this study, the average SBS value of the non-adhesive version of the WMTA + FIILC combination was determined to be statistically significantly lower than the version where the adhesive was applied. It is hypothesized that this disparity may be attributed to variations in the composition and particle size of different brands of glass ionomer cement. In a study conducted by Bicer et al. [22], which runs parallel to the aforementioned investigation, the mean SBS between WMTA without adhesive and FIILC was reported as 6.22 ± 0.84 MPa.

As ABR is a novel material, no existing literature has been identified that evaluated the SBS in relation to established biomaterials. Our study, however, revealed that ABR demonstrated significantly higher SBS values in various groups, except for the MPPC group without adhesive. ABR incorporates an ionic resin component containing phosphate acid groups [24]. We hypothesize that, through a water-dependent ionization process, the hydrogen ions dissociate from the phosphate groups and are substituted by calcium ions generated from the hydration of MTA. This phenomenon consequently enhances the strength of the bond.

In this research, the group with the lowest average SBS value for all samples without adhesive application was observed in the MMTA + EFHF group. Additionally, the lowest average SBS value for all adhesive-applied samples was detected in the MMTA + FIILC group. However, in this study, the mean SBS of the adhesive-free MMTA and EFHF group (21.48 ± 8.95 MPa) showed a significant increase compared to the values reported by Duman et al. (5.76 ± 3.63 MPa). Similarly, the mean SBS of the adhesive-free MMTA and FIILC group (9.79 ± 2.83 MPa) also exhibited higher values than those reported in the study conducted by Duman et al. (6.06 ± 5.75 MPa) [6]. The observed difference in the mean SBS between the glass ionomer cement and MMTA groups in this study may be attributed to various factors, including variations in research conditions, sample size, and differences among practitioners.

Duman et al. [6] reported the SBS value of 37.27 ± 18.81 MPa for the EFHF group with MPPC without adhesive, whereas our study yielded a value of 6.36 ± 4.95 MPa. It is important to note that the stress distribution in shear bond tests can be intricate, leading to variations in results among different researchers. The heterogeneity observed in these outcomes may also be attributed to several bonding variables, including sample storage conditions, sample characteristics, surface preparation techniques, thermal cycling, and film thickness. Furthermore, variations in the results may arise owing to the diverse physical properties of calcium silicate-based biomaterials, variations in radioactive components, discrepancies in the production process, purity of the components, and differences in hydration products [6, 25]. Although the primary constituent of the biomaterials utilized in our research is calcium silicate, the recently developed MMTA incorporates zirconium oxide instead of bismuth oxide, unlike WMTA. MPPC primarily comprises dicalcium and tricalcium silicates [6, 26]. The variability observed in the study results can be attributed to the aforementioned factors, which is consistent with the results of previous investigations [6, 25]. Moreover, the mean SBS of the MPPC and FIILC groups in the non-adhesive group (10.29 ± 11.64 MPa) was similar to that reported by Duman et al. (9.80 ± 5.33 MPa) [6].

Previous studies have explored the SBS of biomaterials and restorative materials bonded without the use of adhesive agents. In the second part of our research, we extended the investigation to include the application of adhesive agents in the bonding of biomaterials and glass ionomer cement to make a valuable contribution to the existing literature [6, 11, 22]. Irrespective of the adhesive application, it was observed that the WMTA biomaterial exhibited the highest average SBS (22.54 + 14.15 MPa) when applied in conjunction with the adhesive agent. The EFHF group (12.99 + 7.57 MPa) demonstrated the next highest SBS values within the adhesive group. Upon evaluating the bond strength of restorative materials with MMTA following the application of adhesive agents, it was observed that ABR exhibited significantly higher bond values than the other materials, similar to the group without adhesive application. Additionally, for EFHF and FIILC, the groups without adhesive treatment demonstrated superior performance compared with the adhesive-applied groups. This phenomenon may be attributed to the fact that both the micromechanical and chemical bonds contribute to the adhesion between these types of cement. Previous studies have reported that the high SBS of the EFHF group without adhesive application to MTA can be attributed to two factors. First, the surface of MTA contains metallic oxides that facilitate strong chemical bonding with glass ionomer cement through metallic bonds. Second, the presence of pores on the MTA surface increases the surface area for micromechanical bonding between MTA and the glass ionomer cement [21, 27].

When the SBS of all restorative material groups was evaluated with MPPC, it was found that the highest values were reached in the ABR group in which the adhesive was applied, similar to the findings with MMTA. However, unlike MMTA, the mean SBS after the adhesive application of EFHF was higher than that in the group without adhesive application. It was observed that adhesive application did not reduce or change the bond strength value in the bonding of FIILC with all three biomaterials. The lower bond strength observed between FIILC and the biomaterials can be attributed to the bonding mechanism of this material. Mitra et al. [28] also reported that stress resulting from polymerization shrinkage and dimensional changes can compromise the adhesion of RMGICs, which supports the findings of our study. In this study, lower SBS values were observed in all adhesive-applied biomaterial-RMGIC groups than in other restorative materials containing glass ionomers. Similarly, Ajami et al. [11] reported low bond strengths between RMGIC and pulp-capping agents. We believe that the weaker bond strength of RMGIC to calcium silicate-based materials may be attributed to polymerization shrinkage caused by monomers such adps hydroxyethyml metacrylate and urethane dimethacrylate present in the cement. The application of adhesive increased the average SBS value in all biomaterial-glass ionomer-containing restorative material groups, except for the FIILC and EFHF-MMTA groups. The higher SBS in the adhesive-applied groups may be attributed to the adhesive system utilized in this study, specifically the use of Clearfil SE Bond containing a 10-MDP functional monomer. In addition to micromechanical bonding, this monomer facilitates chemical adhesion through chemical binding with calcium in biomaterials [22, 29].

Based on a comprehensive analysis of the data obtained in this study, it can be concluded that the application of ABR with adhesive as a restorative material on biomaterials effectively enhanced the bond strength. Similar to our findings, François et al. [30] demonstrated that ABR applied with a bonding agent exhibited higher shear bond values than ABR without a bonding agent. The presence of methacrylate monomers in ABR, along with its compositional similarity to composite resins, may account for the observed high shear bond strength. Following the polymerization of the resin-containing materials, the presence of unpolymerized residual monomers leads to the formation of an oxygen inhibition layer on the surface. This layer, containing a greater number of unsaturated carbon double bonds available for chemical bonding, has been reported to enhance bonding by forming strong covalent bonds with the bonding system [31]. This explanation offers insight into the observed increase in the ABR bond strength after the application of the adhesive in this study.

A high bond strength between restorative materials and biomaterials is essential to minimize microleakage. It is generally considered that a bond strength ranging from to 17–20 MPa is required to achieve well-sealed restorations and sufficient resistance to contraction forces [32]. Based on the findings of this study, only the MMTA-EFHF group exhibited values higher than 17 MPa in the non-adhesive group. In the adhesive group, the ABR + MMTA, ABR + MPPC, and ABR + WMTA groups demonstrated bond strengths exceeding 17 MPa within the ABR group.

In this study, restorative materials were applied onto the surfaces of biomaterials after 4 h, according to the manufacturer’s recommendations. However, it is worth noting that in clinical settings, restorative material applications of calcium silicate-based biomaterials are typically performed after a minimum of 24 h. This disparity in timing may limit the generalizability of our findings to other clinical applications. It is important to recognize that while MTA undergoes initial setting within a short timeframe, it is recommended to delay the placement of permanent restorations for at least 72 h or longer. This allows for improved resistance to dislodgement, enhanced sealing, and optimal physical properties [33]. The results obtained in this study can serve as a valuable reference for the design of future in vivo and in vitro studies to address this limitation. Considering the influence of bonding variables, it is advisable to incorporate dentin and enamel surface properties as additional guiding factors in future investigations.

The null hypotheses formulated for this study are as follows: there is no statistically significant difference in the SBS between the tested biomaterials and restorative materials, and the adhesive application does not exert a statistically significant effect on the bond strength between the biomaterial and restorative material. Based on the findings of this study, it can be concluded that the SBS values of the biomaterial-glass ionomer-containing restorative material groups exhibited significant variability, thus rejecting the null hypotheses. However, it is important to note that in vitro studies cannot fully replicate all clinical aspects or accurately predict clinical behavior. Further prospective clinical studies are warranted to validate these findings. Additionally, there is a need for future studies to analyze the interfaces between new biomaterials and restorative cements in the presence of saliva contamination as well as to consider the completion of the setting reactions of calcium silicate-based biomaterials. These areas of investigation will contribute to a more comprehensive understanding of the behavior and performance of these materials in clinical scenarios.