Influence of surface energy parameters of dental self-adhesive resin cements on bond strength to dentin

The purpose of this study was to determine the surface energy parameters of dental self-adhesive resin cements (SRCs) and to measure their bond strength to dentin. Six dental SRCs (RelyX Unicem Clicker, RU; Maxcem Elite, ME; BisCem, BC; Clearfil SA Luting, SA; Multilink Speed, MS; seT PP, SP) and one resin-modified glass ionomer cement (RelyX Luting 2, RL; control) were tested. Smear layer-covered bovine dentin was used as bonding substrate. Using the dynamic sessile drop method, surface energy, surface energy components, degree of hydrophobicity/hydrophilicity (expressed as ΔG_sws using thermodynamic notation), and apparent surface energies for each material were calculated. The luting cements were bonded to the dentin and stored in water at 37 °C for 24 h prior to shear bond strength test (n = 10). Pearson correlation analysis was applied to detect possible correlations between surface energy parameters and measured shear bond strength (α = 0.05). RU, SA, and MS produced negative ΔG_sws values (hydrophobic), whereas ME, BC, SP, and RL yielded positive ones (hydrophilic). RU had the highest value among all six SRCs tested, the value for MS being statistically equivalent (p = 0.785). The base component, ΔG_sws, and surface energy determined with water showed significant negative linear correlations with dentin bond strength (r/p = −0.801/0.030, −0.900/0.006, and −0.892/0.007, respectively). These results suggest that bonding to smear layer-covered bovine dentin was governed by the base component and the hydrophobicity/hydrophilicity of the SRCs.

Keywords: dental self-adhesive resin cement; bond strength; surface energy parameters; hydrophobicity/hydrophilicity

Recently, dental self-adhesive resin cements (SRCs) have been developed in an attempt to simplify the cementation procedure and thus to reduce application time and technique sensitivity.[1], [2] These materials incorporate some acidic methacrylate monomers to condition the tooth surface [3], [4] and to potentially induce chemical adhesion to hydroxyapatite.[5] Although quite versatile in their use and indicated for final adhesive cementation of various dental restorations, these materials are not generally indicated in the cementation of veneers due to their excessive enamel microleakage and lower dentin bond strengths compared with traditional resin luting cements.[4], [6], [7] SRCs have recently won over dental clinicians due to benefits such as the simplification of clinical steps, low incidence of post-operative sensitivity, and early clinical success.[4], [8]

Numerous previous studies have compared the adhesive qualities of SRCs to tooth [5], [9], [10], [11], [12] and to various dental restorative materials [13], [14] with those of traditional resin luting cements and water-based cements. However, studies on the bond strength of SRCs to dentin appear inconsistent. De Munck et al. [5] reported that the microtensile bond strength of the SRC RelyXTM Unicem (3 M ESPE, Seefeld, Germany) to human dentin was statistically similar to that of the traditional self-etching adhesive resin cement PanaviaTM F (Kuraray Medical Inc., Okayama, Japan). Walter et al. [12] found that RelyXTM Unicem showed higher microtensile bond strengths to bovine dentin than PanaviaTM F and a resin-modified glass ionomer cement (RMGIC) (FujiCEMTM, GC Corp., Tokyo, Japan). On the other hand, Viotti et al. [7] demonstrated that although the microtensile bond strengths of SRCs to human dentin varied greatly among materials, most of them were significantly lower than the etch-and-rinse resin cement (RelyXTM ARC, 3 M ESPE) and PanaviaTM F. Fuentes et al. [10] also reported lower bonding efficacy of three commercial SRCs to human dentin in comparison with RelyXTM ARC.

Although numerous dental SRCs are currently available, their precise bonding characteristics are not known. The bonding capability of luting cements can be estimated based on their surface energy parameters.[15] It can be assumed that surface energy parameters of monomer and polymer are the same or close,[15], [16], [17] because all groups and segments of the functional monomer molecules, which are related to the wetting properties,[18] are also present in the polymeric material.[15], [17] Thus, although uncured cements are applied to bonding substrates clinically, the surface energy parameters can be determined on cured luting cements experimentally. Using this approach, Asmussen and Peutzfeldt [17] have demonstrated the influence of composite and adhesive-treated dentin surface energy parameters on bond strength. Kim et al. [15] reported that a surface energy component of various luting cements significantly affected bond strength to zirconia ceramic. However, there seem to have been few studies on surface characteristics of SRCs and their influence on adhesion to the tooth surface.

The surface energy of a solid can be estimated by contact angle measurements. Among various methods for determining contact angles, the sessile drop method – in which a liquid drop sits on a flat surface – is used to measure static and dynamic contact angles of solids.[19] The purpose of the present in vitro study was thus to determine the surface energy parameters of several dental SRC products using the dynamic sessile drop methods and to measure their bond strength to smear layer-covered dentin. The surface roughness of the materials was also assessed. Relationships between surface energy parameters and measured shear bond strengths were investigated using the Pearson correlation test.

Six commercially available dental SRCs (RelyXTM Unicem Clicker, RU; Maxcem EliteTM, ME; BisCem®, BC; Clearfil® SA Luting, SA; Multilink® Speed, MS; seT PP, SP) were tested. One RMGIC (RelyXTM Luting 2, RL) was used as the control. Their codes, manufacturers, and compositions are summarized in Table 1. Freshly extracted bovine incisors were used as the bonding substrate, because of their convenient size for contact angle measurement. Bovine teeth have been demonstrated to be a good substitute for human teeth.[20]

Table 1. Dental luting cements used in this study.

Cylindrical molds (10 mm in diameter; 1 mm in height) were placed on a polyester strip over a glass slide. Mixed cement was filled into the mold, covered with another polyester strip and glass slide, and gently pressed to expel the excess material. RL was then allowed to self-cure; all the other cements were additionally irradiated for 40 s according to the manufacturers' recommendations by placing the tip of the light guide of a quartz–tungsten–halogen light-curing unit (Elipar TriLight, 3 M ESPE; standard mode, output intensity = 750 mW cm−2) against the upper glass slide. After 30 min,[15] the strips were removed from the specimen and the surface roughness Ra of each specimen was measured using a previously calibrated profilometer (Surftest SV-400, Mitutoyo Corp., Kawasaki, Japan) at a stylus speed of 0.1 mm s−1, a cutoff of 0.8 mm, and a range of 600 μm. Ten specimens per luting cement were prepared and the Ra of each specimen was recorded as the average of the five readings.

The labial surface of each tooth was wet abraded with 120- and then 600-grit SiC paper to create a flat and standardized smear layer-covered dentin.[21] The prepared dentin surface was cleaned with pumice slurry, rinsed with a water spray, lightly air-dried, and subjected to surface roughness measurement as described above (n = 10).

The contact angle method was used to calculate the surface energy parameters of the luting cements and smear layer-covered dentin, utilizing the well-known Young's equation [22]: γs = γsl + γl cosΘ, where Θ is the contact angle and γs, γsl, and γl are the surface tensions of the solid, solid–liquid, and liquid surfaces, respectively. The Young-Dupré equation allows calculation of the thermodynamic work of adhesion Wa of the solid/liquid [22]: Wa = γl(1 + cosΘ). Combining the equation with the Lifshitz-van der Waals/Lewis acid-base (LWAB) theory yields the following equation (Equation (1)) [15], [23]:

(1)

Graph

where the superscripts LW, +, and − refer to the Lifshitz-van der Waals, acid, and base components, respectively. Hence, by measuring the contact angles of a liquid triplet with known surface energy parameters on solid surfaces, the solid surface free energy, and its components can be calculated using the least square method.[15] In addition, the total surface energy γs of the materials is derived by the equation [15], [17], [24]: , in which is the acid/base component.

To measure contact angles, 10 specimens per material were prepared in the same way as for surface roughness measurement. The advancing contact angle (Θa) and receding one (Θr) of each of three different test liquids were determined on the surfaces of the luting cements and dentin by the dynamic sessile drop method at room temperature using a contact angle measurement apparatus (OCA 15 plus, DataPhysics Instrument GmbH, Filderstadt, Germany). Although solid surface energy parameters calculated from the LWAB approach are theoretically similar irrespective of the probe liquid used,[25] Combe et al. [26] clearly showed that the choice of liquid triplets remains critical even in applying the LWAB method. In this study, water (W: γ: 72.8;γLW: 21.8;γ+: 25.5;γ−: 25.5), glycerol (G: γ: 64;γLW: 34;γ+: 3.92;γ−: 57.4), and methylene iodide (MI: γ: 50.8;γLW: 50.8) were used as the test liquids (all in mJ m−2).[27] This liquid triplet seems appropriate due to its low condition number (6.29) and high (13.6).[26], [28] The advancing contact angle of each liquid was measured after settling 6 μL droplets on the material surface. The receding contact angle was then measured after sucking 2 μL from the droplet into the syringe (Figure 1).[25], [29] In the LWAB approach, only the advancing contact angles were employed.[25]

Graph: Figure 1 Photographs of liquid droplets. (a) dispensing the liquid; (b) droplet at advancing contact angle measurement; (c) reverse dispensing the liquid; (d) droplet at receding contact angle measurement.

If the solid shows contact angle hysteresis (CAH, the difference between advancing and receding contact angles), the CAH phenomenon can be utilized to characterize the surface.[30] This study thus also employs the CAH approach, which consists of measuring both the advancing contact angle and the receding one using the same measuring liquid of a known value of γl.[29], [31] The surface free energy of the studied materials was then calculated from the following equation (Equation (2)) [32]:

(2)

Graph

Tooth specimens whose dentin surfaces were prepared in the same way as for surface roughness measurement were embedded in round silicone rubber molds using acrylic resin, ensuring that the prepared surface remained uncovered, and placed into a cool water bath to minimize any effect of heat from the acrylic's exothermic setting reaction. The dentin surfaces to be bonded were isolated using a custom-manufactured bonding jig (Ultradent Products Inc., South Jordan, UT, USA). Each prepared luting cement was bonded to the surface by packing the material into cylindrical-shaped plastic matrices (Ultradent Products Inc.) with an internal diameter of 2.38 mm.[33] Excess cement was carefully removed from the periphery of the matrix with an explorer and then irradiated using the light-curing unit. RL injected into the matrices was allowed to self-cure for 10 min before removal.[33] In this manner, two or three bonded cement cylinders were made on one dentin surface and a total of 10 cement cylinders prepared for each material. The specimens were then stored in distilled water at 37 °C for 24 h before bond strength testing.

Specimens were then perpendicularly engaged at their bonded cement cylinder bases with a round-notched custom shear blade (Ultradent Products Inc.) in a universal testing machine (Model 3343, Instron Inc., Canton, MA, USA) at a crosshead speed of 1.0 mm min−1 until bonding failure occurred.[33] Bond strengths (MPa) were calculated from the peak load of failure (N) divided by the bonded surface area. Following debonding, all fractured interfaces were examined under an optical microscope (SMZ800, Nikon Corp., Tokyo, Japan) at 10 × magnification to determine the mode of fracture. These were classified into one of three types: A, adhesive failure at the dentin–cement interface; C, cohesive failure within cement; and AC, a combination of these failure modes.

When the surface roughness or bond strength data were normally distributed and exhibited equal variances, the means of the different groups were compared using a one-way analysis of variance (ANOVA) with Tukey's post hoc test; in case of lack of homogeneity of variances, the Games-Howell post hoc test was adopted (α = 0.05).

The surface roughness of the luting cements and bovine dentin is presented in Figure 2. The mean Ra values for the six SRCs ranged from 0.066 to 0.123 μm, with significant differences among them (one-way ANOVA, p < 0.001). Tukey's post hoc test revealed the surface of the RL cement to be significantly rougher than that of the SRCs (p < 0.001).

Graph: Figure 2 Ra surface roughness of luting cements and dentin tested (n = 10). Black squares denote mean Ra, boxes represent standard deviation, and whiskers define the minimum and maximum Ra values. Means with the same letters are not significantly different (p > 0.05). Dentin data were not included in the statistical analysis.

Figure 3 shows the advancing and receding contact angles of the materials tested as measured with the three probe liquids. The surface energy parameters of the materials, which were calculated from the contact angle data, are shown in Table 2. All materials showed large and small values although the difference was relatively small in RL and dentin. In all materials, values were greater than values. Apparent surface energies , , and determined from the CAH model using water, glycerol, and methylene iodide, respectively, were not always consistent with the surface energies γs derived from the LWAB approach.

Graph: Figure 3 Advancing (Θa) and receding (Θr) contact angles measured with water (W), glycerol (G), and methylene iodide (MI) on dental luting cements and bovine dentin (n = 10).

Table 2. Surface energy parameters of the luting cements and bovine dentin calculated from the contact angle data.

The shear bond strengths of the luting cements to bovine dentin are shown in Table 3. One-way ANOVA showed statistically significant differences among the test groups (p < 0.001). The Games-Howell post hoc test further revealed that RU had the highest value among the six SRCs tested, the value for MS being statistically equivalent (p = 0.785). SA produced a statistically similar bond strength to MS (p = 0.776). The other three SRCs and RL cements exhibited lower bond strengths, with no significant differences among the groups (p > 0.05).

Table 3. Shear bond strengths of the luting cements to bovine dentin (n = 10).

Figure 4 summarizes the results of the Pearson correlation analyses between individual surface characteristics and bond strength to dentin. Among the surface energy parameters, only , ΔGsws, and showed significant negative linear correlations with dentin bond strength (r/p = −0.801/0.030, −0.900/0.006, and −0.892/0.007, respectively). No statistically significant correlation was found between surface roughness and bond strength (p = 0.239).

Graph: Figure 4 Scatter plot of Pearson correlation analysis results: (a) γs− (base component), (b) ΔGsws (degree of hydrophobicity/hydrophilicity), and (c) γsW (surface energy determined using the water advancing and receding contact angles) vs. shear bond strength to dentin.

As shown in Figure 3, CAH phenomena were observed in all measurements. Surface roughness and surface chemical heterogeneity reportedly constitute the two main causes of CAH.[34], [35] The former would account for the CAH on the rough surfaces (Ra > 0.1 μm), whereas the latter would account for the remaining hysteresis on the smooth surfaces (Ra > 0.1 μm).[34] In fact, CAH is observed even for a solid with very low surface free energy.[36] The SRCs tested in this study showed Ra values of around 0.1 μm, whereas the Ra of the RL cement was around 0.2 μm (Figure 2). Busscher et al. [34] reported that changes in solid surface Ra below 0.1 μm have little effect on contact angle. Despite significant differences in the Ra among the SRCs, there were only small variations (below 0.1 μm) in the value. It can thus be assumed that the surface roughness of the SRCs did not influence the contact angle, which was used to calculate the surface energy parameters.[15] In RL and dentin, on the contrary, actual contact angles could have deviated from the estimated contact angles obtained from Young's equation, mainly due to surface roughness.[23] Nonetheless, RL and dentin did not generally produce greater hysteresis than the SRCs (Figure 3), implying that surface roughness-induced CAH was not great.

The smear layer-covered dentin and all luting materials showed large and small values (Table 2), suggesting their hydrophobic characteristics.[24] The degree of hydrophobicity/hydrophilicity of the materials was further investigated using thermodynamic notation. The boundary between hydrophobicity and hydrophilicity of a solid material (s) in the presence of water (W) is equal to the cohesive polar attraction between the water molecules.[37] The work of cohesion, Wc, can be expressed in terms of the free energy, G, so that ΔGc = −2γ = −Wc.[38] The degree of hydrophobicity/hydrophilicity of a material is linked to the magnitude of ΔGsws = −2γsw, where γsw =  + . The LW interfacial tension and the AB interfacial tension are calculated using the following equations, respectively [37]: and . Taking ΔGsws = 0 as the boundary between hydrophobicity and hydrophilicity,[37] the smear layer-covered bovine dentin surface was found to present a mainly hydrophilic character (ΔGsws = 18.73 mJ m−2), its total surface energy moreover being higher than those of SRCs due to its large (Table 2). Classifying the six SRC according to the ΔGsws, RU, SA, and MS are hydrophobic whereas the ME, BC, and SP cements are hydrophilic. The RMGIC RL showed a higher surface energy than the other SRCs due to its higher , , and (LWAB approach). This finding suggests that the RL cement should be classified differently from SRCs, although both are potentially self-adhesive to tooth substrate.[5]

The hydrophobic cements in this study tended to produce higher shear bond strengths to dentin than the hydrophilic ones (Tables 2 and 3). The Pearson correlation analyses results (Figure 4) clearly show that the bond strength to dentin decreased with increasing hydrophilicity of the luting cements. In addition, the base components of the material appeared to govern the hydrophobicity/hydrophilicity. All the luting cements tested in this study showed adhesive failure between luting agent and bovine dentin, indicating their limited self-adhesive capacity to dentin.[1] In particular, RL, the most hydrophilic among the materials tested (ΔGsws = 38.23 mJ m−2), is not recommended when high dentin bond strength is required (Table 3).

Conventional resin cements based on crosslinking monomers such as bisphenol A diglycidyl methacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA) are basically hydrophobic, and do not naturally adhere to dental substrates without the aid of a primer.[39], [40] The SRCs tested in this study all contained acidic and hydrophilic organophosphate ester monomers (Table 1), which probably accounted for their high values (Table 2).[15] Acidic methacrylate monomers incorporated in SRCs are believed to impart adhesive properties mainly through (1) micromechanical retention by simultaneous demineralization and infiltration of the tooth substrate and (2) potential additional chemical bonding to the tooth substrate.[41] The former may be limited by the relatively high viscosity of the materials and the limited interaction time. An electron microscopic study [5] proved that SRCs do not deeply demineralize dentin surface and thus only interact superficially with dentin. The latter may be due to a chemical interaction between the acidic monomers and the calcium in the hydroxyapatite, as proposed in the case of mild self-etch dentin adhesives.[42] A strong bond would be achieved only when functional groups in the acidic monomers produce an optimal interaction with hydroxyapatite on the dentin surface, otherwise unreacted functional groups could serve as sites for water absorption. In reality, however, intimate contact between the two bonding sites may be impaired by the relatively high viscosity of the materials and the lack of fully exposed hydroxyapatite on the dentin surface.[5], [18] Thus, hydrophilicity of SRCs may decrease the bond strength to dentin, notwithstanding the need for a certain amount of acidic and hydrophilic resin monomers for bonding. In fact, the crosslinking monomers Bis-GMA, TEGDMA, and UDMA also have polar hydrophilic functional groups (hydroxyl, ethylene oxide, and urethane groups, respectively), that can serve as water absorption sites.[43]

Although the apparent surface energies were not consistent with those from the LWAB approach (Table 2), a significant negative correlation was observed between the and the bond strength to dentin (Figure 4(c)). Thus, taking into account the receding contact angles in addition to the advancing ones may help better characterize the solid surface than considering advancing or static angles only when water is used as a single probe liquid. Also, high γs, , and values imply that the thermodynamic state of the smear layer-covered bovine dentin is favorable for bonding.[26]

Varying greatly in composition, dental SRCs have a range of surface characteristics and, as a result, different bonding capabilities. Their chemical surface characteristics can easily be determined using the contact angle method and calculation of surface energy parameters. The current study suggests that surface energy parameters derived from contact angle measurements of SRCs account for their bonding efficacy to dentin. Not only is the incorporation of acidic methacrylate monomers into SRCs essential for effective bonding to smear layer-covered dentin, but their final hydrophobicity/hydrophilicity should be optimized. Further studies are required to clarify the relationship between surface characteristics of SRCs and their long-term bonding durability to enamel and various restorative materials.

The newest type of dental luting cement is the SRC designed for adhesive luting of ceramic, metal or composite indirect restorations. The elimination of a separate etching step and the resulting low incidence of post-operative sensitivity are main advantages of dental SRCs. Nonetheless, their indications remain restricted, mainly due to their relatively limited self-adhesive capability. Further research into optimal formulations and clinical application techniques is needed to enhance the adhesion to various substrates. The findings of this study may be helpful in the optimal formulation of SRCs.

This study was supported by a grant from the Korea Healthcare Technology R&D Project, Ministry of Health, Welfare and Family Affairs, Republic of Korea (A091074). Authors are grateful to SDI Ltd for donating their materials. However, there are no conflicts of interest.