Touch-cure activation by marketed universal resin luting cements of their associated primer to dentin

Abstract

Purpose: The aim of this study was to investigate the dentin shear bond strength (SBS) and bonding interface of three recently developed “universal” resin luting cements based on different modalities.

Methods: The dentin SBS and interfacial analysis of three recently launched “universal” resin luting cements, namely, G-Cem One, RelyX Universal and Panavia SA cement universal, were studied. All bonding protocols, including the previous use of their dedicated primer or universal adhesive in touch-cure mode or light-cure mode were performed. Variolink Esthetic LC used in conjunction with Scotchbond Universal Plus was used as a control group. For each group (n = 9), 10 specimens were tested for dentin SBS and two were examined by scanning electron microscopy. SBS were analyzed by two-way ANOVA followed by Dunnett’s test.

Results: SBS values showed that the three “universal” resin luting cements tested exhibit different adhesive behaviors. G-Cem One with its touch-cure activated primer had a greater SBS to dentin (25.5 MPa) than that of the control group (22.1 MPa).

Conclusion: “Universal” resin luting cements have variable efficacy when used in self-curing mode. The touch-curing mode is also of concern but may show high potential for some formulations.

Introduction

Over the last 10 years, adhesive dentistry has undergone profound changes. In the dental market, there has been a clear trend to simplify procedures by creating products that are easier to handle, less operator-dependent, and more versatile [1]. A major revolution following these simplifications may have been one-bottle universal adhesives [2]. These simplified adhesives, which contain functional monomers, including the best-known 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) [3], offer good medium-term clinical performance while drastically simplifying clinical procedures [3].

The conventional resin composites used to assemble indirect prosthetic elements classically include adhesive and self-adhesive resin luting cements. These two resin luting cements can be light cured, self-cured or dual cured but the best performance is obtained by light curing [4]. Multistep adhesive resin luting cements offer excellent clinical performance [5,6] but they require strict clinical implementation and are operator-dependent and time-consuming processes [7]. For this reason, single-step self-adhesive luting cements have been proposed for simplification purposes. Although these materials perform well in terms of assembling peripheral elements [8], their use in bonded partial restorations is limited by their reduced bond strength to dentin and enamel [9,10,11], as well as their long duration of aging [12].

Recently, resin-luting cements marketed under a new “universal” terminology have appeared on the market, offering a simplified solution. Although not consensual, the terms “universal cement” or “universal resin luting cements” have recently been discussed as definitions for classification [13]; however, the latter term is debatable. According to this article, the ability of a cement to be used in self-adhesive mode is associated with the application of a universal adhesive that characterizes a “universal” resin luting cement [13]. However, better bond strengths on enamel and dentin are achieved when self-adhesive cement is applied after a light-cured dental adhesive than when used alone [14,15]. The optional touch-cure polymerization of the universal adhesive or the primer associated with the luting cement has not been discussed, even though this process is very original and common to these new cements. Until now, no self-adhesive luting resin cement could trigger touch-cure activation by a primer or a dental adhesive, whereas some adhesive resin luting cements have been combined with a touch-cure primer but cannot be used directly on dentine in self-adhesive mode [16].

The aim of this study was to compare the immediate shear bond strength (SBS) and bonding interfaces to dentin of three of these “universal” resin luting cements according to their various possible self-curing adhesive protocols (including universal adhesive or primer touch-cure activation) in comparison to a reference light-curing adhesive resin luting cement. The null hypothesis was that there would be no difference in the SBS values obtained for the three “universal” resin luting cements in comparison to the control group, regardless of the dentin surface treatment performed.

Materials and Methods

Materials used and experimental procedures

Three recently marketed “universal” resin luting cements, namely, G-Cem One (GC Corp., Tokyo, Japan), RelyX Universal (3M Dental, St Paul, MN, USA), and Panavia SA Cement Universal (Kuraray-Noritake Dental Inc., Tokyo, Japan), were chosen due to their possible touch-cure activation of their associated primer or universal adhesive used in self-etch mode. These materials were tested for dentin bond strength and subjected to scanning electron microscope (SEM) interface analysis for all possible curing modes of primers/adhesives according to the manufacturer’s instructions, and the “universal” resin luting cements were self-cured. These materials were compared to the light-cure adhesive resin luting cement (Variolink Esthetic LC, Ivoclar, Schaan, Liechtenstein) used in conjunction with a light-cured universal adhesive applied to dentin in self-etch mode (Scotchbond Universal Plus, 3M Dental).

The materials, manufacturers, batch numbers and composition based on previous publications [13,17] are presented in Table 1.

Table 1 Materials, abbreviations, manufacturers, batch numbers and chemical compositions

Type of materials	Name	Abbreviation	Manufacturer	Batch number	Composition
Adhesive resin luting cement	Variolink Esthetic LC	VL	Ivoclar AG, Schaan, Liechtenstein	Z02BNV	UDMA, Bis-GMA, HEMA, TEGDMA, GDMA, barium glass, zirconia/silica filler, Ivocerin
“Universal” resin luting cements	G-Cem One	GCO	GC Corp., Tokyo, Japan	2207211	UDMA, 10-MDP, other dimethacrylates monomers, fluoroaluminosilicate glass, silicon dioxide, trimethoxysilane, 6-tert-butyl-2,4-xylenol, 2,6-di-tert-butyl-p-cresol, EDTA disodium salt dehydrate, vanadyl acetylacetonate, TPO, initiators, ascorbic acid, camphorquinone, MgO, pigments
	RelyX Universal	R-U	3M Dental, St Paul, MN, USA	9214626	UDMA, TEGDMA, HEMA, MMA, mixture of glycerol phosphate dimethacrylate, vitreous silica, γMPTES, tert-amylhydroperoxide, 2,6-di-tert-butyl-p-cresol, acetic acid, copper(2+) salt monohydrate, ytterbium trifluoride, L-ascorbic acid 6-hexadecanoate hydrate (1:2), titanium dioxide, triphenyl phosphite
	Panavia SA Universal	PSA-U	Kuraray-Noritake Dental Inc., Tokyo, Japan	3N0013	BisGMA, TEGDMA, 2-HEMA, 10-MDP, hydrophobic aliphatic dimethacrylate, hydrophobic aromatic dimethacrylate, silanated barium glass filler, silanated colloidal silicon dioxide, aluminum oxide filler, surface treated NaF, silane coupling agent, camphorquinone, peroxide, catalysts, accelerators, pigments
Specific primer and universal adhesives	Adhesive Enhancing Primer	AEP	GC Corp.	2207211	10-MDP, 4-META, 2-hydroxy-1,3 dimethoxypropane, vanadyl acetylacetonate, ethanol, 2,6-di-tert-butyl-p-cresol
	Scotchbond Universal Plus	SBU	3M Dental	7910510	brominated dimethacrylate, HEMA, 10-MDP, DEGDMA, 2-propenoic acid, 2-methyl-, 3-(triethoxysilyl)propylester reaction products with silica and 3-(triethoxysilyl)-1-propanamine, copolymer of acrylic and itaconic acid, synthetic amorphous fumed silica, γMPTES, APTES, acetic acid, copper(2+) salt monohydrate, ethanol, water, camphorquinone, dimethylbenzocaine
	Clearfil Universal Bond	CUB	Kuraray-Noritake Dental Inc.	3P0296	Bis-GMA, HEMA, 10-MDP, hydrophilic amide monomers, colloidal silica, sodium fluoride, silane coupling agent, ethanol, water, camphorquinone

Sample preparation

One hundred and eight freshly extracted human permanent molars were collected from adults after extraction, cleaned of soft tissues, stored at 4°C in a solution of 1% chloramine, and used within 3 months. Teeth were obtained from the dental departments of AP-HP, France. All the experiments were conducted in accordance with the Declaration of Helsinki. All teeth were collected with informed and oral consent from the patients according to ethical guidelines established by French law and with dedicated authorization from University of Paris-Cité Dental School (n°DC-2009-927, Cellule Bioéthique DGRI/A5, Paris, France). The criterion for tooth selection included the absence of cracks or decay. The greater portion of the roots was removed with sandpaper (80 grit) using a polishing machine. The occlusal surface of the crowns was then abraded with water-cooled sandpaper (800 grit) to expose a flat dentin surface (>7 mm²). The residual crowns were embedded in self-curing acrylic resin in a plastic cylinder with the flat dentin surface exposed. The surfaces were inspected under 40× magnification to ensure that the enamel was completely removed and that the dentin was cleared of debris.

These samples were randomly assigned to nine groups (n = 12) corresponding to the various adhesive protocols tested and handled according to the manufacturer’s recommendations. The samples were incubated in a dark room to prevent any impact from ambient light. A cylindrical Teflon mold was used on the samples to build a 2 mm-high cylinder of resin luting cement (diameter = 1.5 mm) with a flat base of 7 mm², and steps requiring light curing with a polywave LED lamp (Valo Grand, Ultradent, South Jordan, UT, USA) were used. The abbreviations, groups and detailed adhesive protocols used are presented in Table 2. After demolding, any excess material was gently removed from around the base of the material cylinder with a scalpel. All the samples were then stored in distilled water at 37°C for 48 h.

For each group, 10 teeth were used for the shear bond strength test, and two teeth were used for SEM interfacial observations. The number of samples was determined from other publications isolated in the literature using a similar protocol [15,18].

The shear bond strength (SBS) was determined with a universal testing machine (LRX, Lloyd Instruments, Fareham, UK). The shear force was applied at the resin luting cement/dentin interface, with a chisel-shaped blade parallel to the dentin surface. A cross-head speed of 0.5 mm/min was chosen.

The debonded specimens were observed under a binocular microscope (BZH10 Olympus, Hamburg, Germany) at ×30 magnification, and the failure modes were classified as the following four types:

1. •   Type CF-D: cohesive failure within the dentin.
2. •   Type AF: adhesive failure at the interface between the material and the dentin.
3. •   Type MF: mixed failure (adhesive and cohesive failure within the material/dentin).
4. •   Type CF-LC: cohesive failure within the resin luting cement.

Table 2 Bonding and curing protocols

Material and curing protocol tested	Bonding protocol
Group: SBU (LC) + VL (LC) Scotchbond Universal Plus in self-etch mode light cured associated with Variolink Esthetic LC light cured	For each sample, SBU was applied on dentin with a brushing motion for 20 s, then gently air dried for 5 s and light-cured for 10 s. After the Teflon mold was applied on the sample, a 2-mm-high cylinder of Variolink Esthetic LC was built and light-cured for 20 s.
Group: GCO Alone (SC) G-Cem One in self-cure mode	After Teflon mold was applied on the sample, a 2-mm-high cylinder of GCO was built for each sample. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group: AEP (TC) + GCO (SC) Adhesive Enhancing Primer in self-etch mode touch-cured associated with G-Cem One self-cured	For each sample, AEP was applied on dentin for 10 s and dried with maximum air pressure for 5 s. Then, the Teflon mold was applied on the sample to build a 2-mm-high cylinder of GCO. To self-cure the material, a pressure of 50 g was applied on the sample during 60 min.
Group: R-U Alone (SC) RelyX Universal in self-cure mode	After the Teflon mold was applied on the sample, a 2-mm-high cylinder of R-U was built for each sample. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group: SBU (TC) + R-U (SC) Scotchbond Universal Plus in self-etch mode touch-cured associated with RelyX Universal self-cured	For each sample, SBU was applied on dentin with a brushing motion for 20 s and then gently air dried for 5 s. Then, the Teflon mold was applied on the sample to build a 2-mm-high cylinder of R-U. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group: SBU (LC) + R-U (SC) Scotchbond Universal Plus in self-etch mode light cured associated with RelyX Universal self-cured	For each sample, SBU was applied on dentin with a brushing motion for 20 s, then gently air dried for 5 s and light-cured for 10 s. Then, the Teflon mold was applied on the sample to build a 2-mm-high cylinder of R-U. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group: PSA-U Alone (SC) Panavia SA Universal Cement in self-cure mode	After the Teflon mold was applied on the sample, a 2-mm-high cylinder of PSA-U was built for each sample. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group: CUQ (TC) + PSA-U (SC) Clearfil Universal Quick in self-etch mode touch-cured associated with Panavia SA Universal Cement self-cured	For each sample, CUQ was applied on dentin with a brushing motion for 20 s and then gently air dried for 5 s. Then, the Teflon mold was applied on the sample to build a 2-mm-high cylinder of PSA-U. To self-cure the material, a pressure of 50 g was applied on the sample for 60 min.
Group : CUQ (LC) + PSA-U (SC) Clearfil Universal Quick in self-etch mode light cured associated with Panavia SA Universal Cement self-cured	For each sample, CUQ was applied on dentin with a brushing motion for 20 s, then gently air dried for 5 s and light-cured for 10 s. Then, the Teflon mold was applied on the sample to build a 2 mm-high cylinder of PSA-U. To self-cure the material, a pressure of 50 g was applied on the sample during 60 min.

Scanning electron microscopy

For SEM examination, after re-inclusion in self-curing acrylic resin of the bonded samples after storage in water, the resin was sectioned perpendicularly to the bonded interface using a low-speed diamond saw (Isomet, Buehler, Coventry, UK) with water cooling, as close as possible to the center of the cylinder. The sections obtained were polished with abrasive disks of decreasing grit size (400, 800, 1,200, 2,400, and 4,000 SiC), followed by diamond particles of 3 and 1 µm, and finally etched with orthophosphoric acid for 10 s (Scotchbond Universal Etchant, 3M Dental) and then rinsed. The samples were cleaned by ultrasonication after each step. Finally, the sections were dehydrated in ethanol and metallized for microscopic examination (JSM-6400, Jeol Ltd., Tokyo, Japan).

Statistical analysis

A normal distribution was confirmed by the Shapiro-Wilk test, and the equality of variances was assessed using the Levene test before the tests were performed. The SBS data are expressed as the mean values and standard deviations. Two-way ANOVA followed by Dunnett’s post hoc test was used to investigate the difference in SBS between the tested materials and the adhesive resin luting cement SBU (LC) + VL, which was used as a control group. The chosen significance level was P < 0.05. Statistical calculations were performed with R software (version 4.3.1; R Foundation for Statistical Computing, Vienna, Austria).

Results

SBS and failure analysis

The shear bond strengths for all experimental groups are summarized in Table 3. The Shapiro-Wilk test confirmed that the SBS values were normally distributed among all the groups (P > 0.05). Levene’s test showed equal variance (P > 0.05). Two-way ANOVA followed by Dunnett’s post hoc test revealed that SBS was significantly influenced by the “resin luting cement” and the “mode of polymerization” (P < 0.05).

According to the results of Dunnett’s post hoc test, with SBU (LC) + VL serving as the control group, AEP (TC) + GCO was the only group that showed a significantly greater SBS to dentin than the control group (25.5 MPa vs. 20.9 MPa). Compared to the control group, all the other groups tested had significantly lower SBS values with the exception of the SBU (LC) + RU group (22.1 MPa).

The failure modes are listed in Table 4. The failure patterns were mostly adhesive for all the groups.

Table 3 Mean and standard deviations of the shear bond strength (SBS)

Groups	SBS (±SD)
SBU (LC) + VL (control group)	20.9 (± 1.1)
GCO alone	17.9* (± 1.4)
AEP (TC) + GCO	25.5* (± 1.4)
R-U alone	13.0* (± 1.5)
SBU (TC) + R-U	15.1* (± 1.7)
SBU (LC) + R-U	22.1 (± 1.4)
PSA-U alone	2.4* (± 2.6)
CUQ (TC) + PSA-U	9.2* (± 1.0)
CUQ (LC) + PSA-U	14.4* (± 1.1)

Values with an asterisk were significantly different from the control group at P < 0.05.

 

Table 4 Mode of failure

	Groups tested	Number of samples	CF-D	AF	MF	CF-LC
1	SBU (LC) + VL	10	3	7	0	0
2	GCO alone	10	0	10	0	0
3	AEP (TC) + GCO	10	1	9	0	0
4	RU alone	10	0	10	0	0
5	SBU (TC) + RU	10	0	10	0	0
6	SBU (LC) + RU	10	0	10	0	0
7	PSA-U alone	10	0	10	0	0
8	CUQ (TC) + PSA-U	10	0	10	0	0
9	CUQ (LC) + PSA-U	10	0	10	0	0

SEM examination

Figure 1 shows SEM images of the G-Cem One, RelyX Universal and Panavia SA Cement Universal interfaces to dentin without the application of their associated primer or universal adhesive at ×1,000 magnification. Figure 2 shows the SEM images of Variolink Esthetic LC, G-Cem One, RelyX Universal and Panavia SA Cement Universal associated with their primer or universal adhesive with possible curing modes at ×1,000 magnification.

PSA-U alone was the only group that did not show close contact with the dentin. Other groups exhibited close contact at all interfaces studied (“universal” resin luting cement/dentin, “universal” resin luting cement/adhesive and adhesive/dentin). No resin tag-like structures (not measurable) in the dentinal tubules were visible for R-U alone or PSA-U alone, whereas GCO alone showed deeper infiltration with resinous tags, such as structures in dentine tubules.

For SBU (LC) + VL or when a specific primer or adhesive was used before the application of “universal” resin luting cements, some resinous tags in dentine tubules were visible.

Finally, with regard to the adhesive layer thickness, it should be noted that for “universal” resin luting cements combined with dedicated light curing or touch-curing universal adhesive (R-U and PSA-U), a thinner layer can be measured when adhesive polymerization is triggered by touch-cure activation (4.1 µm for SBU [TC] + R-U vs. 6.3 µm for SBU [LC] + R-U and a nonmeasurable thickness for CUQ [TC] + R-U vs. 5.6 µm for CUQ [LC] + R-U).

Fig. 1 SEM images of GCO alone (a), R-U alone (b) and PSA-U alone (c) interfaced with dentin without the application of their associated primer or universal adhesive at ×1,000 magnification. The orange arrows show resinous tag-like structures.

Fig. 2 SEM images of the universal resin luting cements and the resin luting cement associated with their primer or universal adhesive at ×1,000 magnification. The orange arrows show resinous tag-like structures. (a) corresponds to SBU (LC) + VL, (b) corresponds to AEP (TC) + GCO, (c) corresponds to SBU (TC) + R-U, (d) corresponds to SBU (LC) + R-U, (e) corresponds to CUQ (TC) + PSA-U, (f) corresponds to CUQ (LC) + PSA-U.

Discussion

Conventional resin luting cements (i.e., adhesives and self-adhesives) can be light cured, dual cured or self-cured [13]. In many publications, dual-cure adhesive resin cements are recommended for indirect restoration luting to achieve the best clinical outcomes [5,6] while compensating for possible insufficient light irradiation [19]. However, when adequate light curing is possible, light-cure adhesive resin composites may improve the mechanical and adhesive properties of the materials due to the superior efficiency of this type of chemical reaction [4,20,21,22]. For this reason, the SBU (LC) + VL control group was tested using materials widely reported in the literature and was chosen for this study. It thus represents the association between a light-curing resin luting cement with a universal adhesive as described in several studies. This association was therefore created, and designated as a control group to represent what could be likened to the association offering the best possible adhesion values under in-vitro conditions [20,22].

SBS values to dentin of the three tested “universal” resin luting cements significantly differed from the control group in relation to the dentin surface treatment performed. Thus, the null hypothesis was rejected.

Currently, no studies have investigated the immediate SBS values and created interfaces of “universal” resin luting cements to dentin. These cements can evolve into self-adhesive resin luting cements with the possibility of using a primer or a universal adhesive to improve adhesion, depending on the complexity of the case or the need for retentivity. Touch-cure activation is also available for complex cases, further increasing the versatility of these cements. In this respect, they represent a major innovation. In this study, “universal” resin luting cements were studied in self-curing mode since numerous studies have shown that this polymerization mode is effective for dentin [23].

In this study, the three “universal” resin luting cements studied exhibited very disparate performances compared with those of the control groups. The overwhelming majority of adhesive ruptures at the interface for the samples tested show the relevance of the results tested [24]. The few cohesive failures in dentin observed for AEP + GCO and SBU (LC) + VL, i.e., the two groups with the highest adhesion values, reflect the high immediate adhesion values observed in these tissues.

When used directly on dentin with a self-adhesive protocol, these “universal” resin luting cements had significantly lower yields than did the control group, in line with the literature and expected results [9,10,11,23]. What may seem more surprising, however, was the extent of the difference between these three resin luting cements. GCO alone (17.9 MPa) and RU alone (13.0 MPa) outperformed PSA-U alone (2.4 MPa). However, low adhesion values were already reported for PSA-U [7], while another study showed low immediate adhesion values but greater adhesion at 24 h [18]. The difference with the current study, which carried out the tests at 48 h, probably stems from the sample storage environment. Whereas their samples were stored at 100% relative humidity, they were stored after 60 min in distilled water in this study. In this case, it is possible to imagine that PSA-U had only slightly developed SBS values in self-etching mode 60 min after mixing and that the samples’ interfaces were contaminated by the aqueous environment when stored for 48 h, leading to the extremely low adhesion values observed. This mechanism of contamination during the setting of a resinous material has already been described [25]. It was shown that PSA-U alone had a very low conversion rate in self-curing mode in the early stages of polymerization, reinforcing the theory of water contamination at the interface [26].

When used in conjunction with their associated primer or universal adhesive, all the “universal” resin luting cements achieved improved bond strength. However, AEP + GCO was the only treatment that had a significantly greater SBS (25.5 MPa) than the control treatment (20.9 MPa). Despite very promising results in some studies, touch-cure activation has long been under-researched, given the small number of products used by the method [27,28,29]. Touch polymerization works on the basis of co-initiators contained in the resin luting cement and in the primer or the universal adhesive to trigger a fast and efficient polymerization reaction at the resin luting cement/primer or resin luting cement/universal adhesive interface [27]. The mechanism proposed to explain this difference was that a single-bottle adhesive, due to its relative hydrophilicity necessary for self-etch properties, must be considered a semipermeable membrane [30]. Thus, for the vast majority of conventional resin luting cements used in dual-cure or self-cure mode without touch-cure activation, the slow polymerization kinetics due to the low or no irradiance transmitted through the prosthetic part leads to the formation of water droplets from dentin along the adhesive–cement interface, reducing the bonding values obtained [31]. When the polymerization reaction is touch-cure driven, the triggering process at the dentin interface is accelerated, avoiding the undesirable effects described above and leading to increased bond strength and degree of conversion [29,32]. The superiority of the AEP + GCO immediate SBS over the SBU (LC) + VL control group could therefore be based on the very high efficacy of this touch-cure intake, which, unlike self-cure intake, has already been shown to be superior to light-cure intake in some studies [26]. As shown in a previous study, the recent development of the most efficient primers for triggering touch-cure reactions relied on vanadium derivatives for initialization [29]. Based on the partially known composition of the products tested, it can be assumed that AEP contains these vanadium derivatives to activate touch-cure polymerization but not SBU or CUB.

In the other groups tested, “universal” resin luting cements combined with their universal adhesives showed no superiority over the control group. A limited conversion rate with touch-cure activation of these universal adhesives therefore appears to be the most likely hypothesis to explain the results. One of the major differences between a primer and a universal adhesive is that the latter generally contains compounds that initiate light curing, whereas the former is optimized for touch-curing. The qualitative and quantitative composition of polymerization initiators in an adhesive was shown to have a major impact on the polymerization of the monomers it contains [33]. This raises the question of whether a universal adhesive can have an effective touch-cure or whether an optimized primer is needed to be truly effective. To date, the only touch-cure protocols showing higher SBS values than light-cure protocols for dentin have been shown with dedicated primers [10,27]. This hypothesis is reinforced by the fact that for the RU (15.1 MPa vs. 22.1 MPa) and PSA-U (9.2 MPa vs. 14.4 MPa) groups, lower SBS values were obtained when their associated universal adhesives were used in touch-cure mode rather than in light-cure mode although these values cannot be directly compared with each other using the Dunnett test chosen for the analysis of experimental results. However, the way in which the samples were processed can also have an impact on the results observed. In this study, the “universal” resin luting cements were not light-cured, so as not to interfere with the touch-cure activation of the universal adhesives or primers studied. It has previously been shown for self-adhesive resin luting cements that their adhesive behavior can also be linked to their polymerization mode, in a material-dependent way. Some of them showed better bonding strength values after additional photopolymerization [34]. Different results could have been obtained if the “universal” resin luting cements had been light-cured in this study.

SEM revealed that all the tested groups except for the PSA-U-only group had close contact with the dentin, probably due to the retraction of the self-curing acrylic resin used to reinclude the samples before SEM analysis. The shrinkage stress of the resin should surpass the weak bonding properties of PSA-U to dentin, as demonstrated in other studies on the adhesive properties of low-adhesion materials [35]. The interaction of self-adhesive resin luting cements with dentin (and therefore “universal” resin luting cements used alone) has been described, with interdiffusion zones with smear layer infiltration and tag-like structures when the luting agent or its components partially or fully infiltrate open dentinal tubules close to the interface between the luting agent and dentin [36]. This interaction is different and less efficient than that obtained with self-etch adhesives since no demineralization or hybridization could be observed with transmission electron microscopy (TEM) interface analysis [37]. On SEM analysis, only the interdiffusion zone can be objective for R-U alone or for PSA-U, as demonstrated in earlier studies on self-adhesive resin cements [36,38], in which a smear layer was retained. In the current study, the specimens were polished at 800 grit, creating a smear layer comparable to that obtained with a diamond red ring bur [35]. However, in the GCO alone group, an interdiffusion zone and tag-like structures are identifiable with SEM analysis. This could explain why this group had significantly greater bonding strength values than those of the R-U-only and PSA-U-only groups, reflecting better interactions with the dentin.

When used with universal adhesives or their specific primers containing proven and highly investigated functional monomers (10-MDP for each formulation in this study), an interdiffusion zone and tag-like structures are visible in the tubules [39]. This leads to improved interaction with the smear layer and the underlying dentin and thus to increased bond strength [39] and reduced degradation over time [40]. All the tested resin luting cements therefore appear to benefit the use of their associated primer or universal adhesive both microscopically and in terms of bond strength values; however, when used on their own, GCO appears to have a different adhesive and microscopic behavior than that classically described for self-adhesive resin luting cements.

This in vitro study has a certain number of limitations due to its structure:

• •   It would be interesting to carry out bond strength tests with aging, in particular to evaluate the evolution over time. However, these results provide an overview of the little-known behavior of these new “universal” resin luting cements, provide scientific pathways for future research, and offer explanations for practitioners that already use these cements.
• •   Due to the in vitro nature of these materials, these results cannot be directly applied in the clinic. However, further studies are needed to draw clinical conclusions.

The three “universal” resin luting cements tested in this study differed greatly in terms of immediate SBS to dentine. It seems difficult to generalize the interface and adhesive behavior of this new family of resin luting cements.

However, when the tested materials were used as self-adhesive resin luting cements, the performance of all materials was immediately inferior to that of the adhesive resin luting cement control group. However, for one of them (G-Cem One), touch-cure activation with the primer (Adhesive Enhancing Primer) led to higher SBS values than those obtained from the control group; and doubts can be raised about the effectiveness of this touch-cure activation for the other “universal” resin luting cements tested.

However, further in vitro studies are needed to clarify the setting of these materials and evaluate the SBS values obtained after aging. Clinical studies are also needed to evaluate the performance of these materials.

Abbreviations

AEP: Adhesive Enhancing Primer; AF: adhesive failure; ANOVA: analysis of variance; APTES: 3-(aminopropyl)triethoxysilane; Bis-GMA: bisphenol A-glycidyl methacrylate; CF-D: cohesive failure in dentin; CF-LC: cohesive failure in resin luting cement; CUB: Clearfil Universal Bond quick; DEGDMA: diethylene glycol dimethacrylate; DUDMA: diurethane dimethacrylate; GCO: G-Cem One; GDMA: glycerol dimethacrylate; HEMA: hydroxyethyl methacrylate; LC: light-cure; MF: mixed failure; MMA: methyl methacrylate; PSA-U: Panavia SA Universal; R-U: RelyX Universal; SBU: Scotchbond Universal plus; SBS: shear bond strength; SC: self-cure; SEM: scanning electron microscope; TC: touch cure; TEGDMA: triethyleneglycol dimethacrylate; TPO: triphenylphosphine oxide; UDMA: urethane dimethacrylate; VL: Variolink Esthetic LC; 10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate; γMPTES: γ-methacryloxypropyltriethoxysilane; 4-META: 4-methacryloxyethyl trimellitate anhydride

Ethical Statements

Not applicable

Conflicts of Interest

The authors have no conflict of interest to declare.

Funding

Not applicable

Author Contributions

SAG conducted the experiments and wrote a part of the manuscript. ED, RC, TF and JPA provided expert opinions on resin luting cements, wrote a part of the manuscript and contributed to the proofreading of the manuscript. SLG realized SEM analysis and contributed to its evaluation. PF provided an expert opinion on resin luting cements, wrote a part of the manuscript, analyzed the data and conducted the primary literature research.

ORCID iD

¹⁾SAG: [e-mail], https://orcid.org/0000-0002-4646-5917

^1,2)ED: [e-mail], https://orcid.org/0000-0002-7704-5990

^1,3)RC: [e-mail], https://orcid.org/0000-0003-3411-9694

¹⁾SLG: [e-mail], https://orcid.org/0000-0003-1255-672X

^1,4)TF: [e-mail], https://orcid.org/0009-0002-9075-6292

^1,5)JPA: [e-mail], https://orcid.org/0000-0003-2187-2003

^1,4)PF*: [e-mail], https://orcid.org/0000-0002-2245-6290

Acknowledgments

Not applicable

Data Availability Statements

Data are available on request.

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