3.1 Physical properties of universal cements

Given that the majority of the universal cements have been marketed relatively recently, the reports on their laboratory and clinical performances are scarce.

Recently, a series of 3 papers has been published on the physical properties of RXU compared to several self-adhesive cements (including the direct predecessor of RXU – RelyX Unicem2, 3M ESPE), and several multi-step cement systems.^{34, 35} Higher shrinkage of RXU compared to other investigated cements was reported, possibly due to lower inorganic filler content.³⁴ In terms of hardness, RXU demonstrated lower values compared to other investigated cements, with the values always being higher in the dual-cure mode for all the cements.³⁵ The total elution of monomers from RXU cement was lower or comparable to the investigated self-adhesive and multi-step cements.³⁶ This was supposedly related to the absence of bisphenol A glycidyl methacrylate (Bis-GMA) in both RXU and Scotchbond Universal Plus adhesive. In particular, bis-GMA is manufactured from bisphenol A (BPA), that was shown to activate estrogen receptors and act as a risk factor for fertility, and even induce deterioration of psychological health in children.³⁷ It is important though to mention that all the cements eluted monomers well below the half-maximal-effect concentration levels. The water sorption of RXU was found to be comparable or lower than several self-adhesive cements, but higher than the investigated adhesive multi-step cements.³⁶ The same study demonstrated that the solubility of RXU was comparable to, or lower, than the other investigated cements. Since water sorption is related to color stability of resin-based materials,³⁸ these findings are in accordance with the results of another recent study on the color stability of resin cements.³⁹ This group of authors found that the color stability of RXU was comparable to Panavia SA and lower than Panavia V5 (tested also in the previously mentioned study). However, RXU demonstrated higher color change compared to other investigated self-adhesive cements. This could be due to differences in the composition of the resin matrix and filler content and size.³⁸

Polymerization stress to feldspathic ceramics of SOC compared to 8 different self-adhesive cements (all employed after a silane pretreatment) was demonstrated to be comparable or lower when the universal cement was used.⁴⁰ This cement also demonstrated higher water sorption compared to 7 self-adhesive cements.⁴¹ The same study affirmed, however, that this material is more stable than the majority of the other investigated cements color-wise after 1 year of storage in different dying solutions,⁴¹ which could be contradictory. Another study investigated the color stability of adhesive and self-adhesive cements after an accelerated aging protocol (2 weeks of consecutive UV light at 60°C for 4 h and vapor condensation at 50°C for 4 h).⁴² All the investigated cements exhibited significant color changes after accelerated aging, apart from Panavia F 2 (Kuraray Noritake), which performed significantly better. In terms of mechanical properties, SOC showed similar hardness and indentation modulus to the majority of other investigated cements (7 self-adhesive cements).⁴³ After 180 days of storage in a NaCl solution, the mechanical properties of SOC did not decrease, while there was a significant decrease in the properties of the cement stored in physiological saliva, artificial saliva and distilled water. In general, the materials with a higher filler content showed higher hardness.⁴³

To the best of our knowledge, there is currently no available data in the literature on the physical properties of PSAU.

3.2 Interaction of universal cements with dental tissues

Universal cements are required to bond predictably to tooth substrates. Therefore, functional acidic monomers are incorporated into their composition. These monomers are intended to etch and infiltrate the dental substrate, forming chemical bonds with Ca²⁺ ions from hydroxyapatite, as well as with methacrylate monomers, performing somewhat a “silanization” of the tooth surface. Although several functional acidic monomers are currently used in dental cements, such as 4-methacryloxyethyl trimellitic acid (4-MET), 4-methacryloxyethyl trimellitic anhydride (4-META), and dipentaerythritol penta-acrylate phosphate (PENTA), the gold standard functional monomer at present is surely 10-methacryloyloxydecyl dihydrogen thiophosphate (10-MDP). This phosphoric acid-spacer-methacrylate group molecule forms very strong and stable bonds with hydroxyapatite,⁴⁴ forming 10-MDP-Ca salts, as well as hydrogen bonds with collagen molecules.⁴⁵ On the other end, this molecule bonds with the carbon chain of the resin monomers.⁴⁵ Additionally, 10-MDP and 10-MDP-Ca salts inhibit endogenous dentinal matrix-metalloproteinases shown to degrade the hybrid layer and diminish the longevity of resin-dentin bonds.^{46, 47}

All three universal resin cements, or the universal adhesives they are associated with, contain 10-MDP, and/or other functional monomers. This surely impacts their bonding performance to tooth tissues and restorative materials. RXU combined with the pertinent adhesive demonstrated bond strength to dentin and enamel comparable to Panavia V5 (Kuraray Noritake)⁴⁸ and G-Cem One self-adhesive cement (GC, Tokyo, Japan)⁴⁹ placed with the G-Cem One Adhesive Enhancing Primer (GC), while in enamel, the universal system yielded lower results or at baseline (compared to G-Cem One, GC), or after thermocycling (compared to Panavia V5, Kuraray Noritake).⁴⁸ Bond strength of RXU comparable or better than several self-etch and multi-step cements was also demonstrated in root dentin.⁵⁰

PSAU, on the other hand, demonstrated comparable shear bond strength to dentin as its predecessor Panavia SA Plus (Kuraray Noritake), both employed in self-adhesive mode.⁵¹ Another group of authors partially confirmed these results, as microtensile bond strength to dentin was comparable when these two cements were applied directly on dentin in dual-cure mode, while in the self-cure mode and in the groups where prior resin coating was applied on dentin, Panavia SA Plus demonstrated better bonding performance.⁵² Bond strength on tooth tissues using PSAU with Universal Bond Quick has still not been tested but might further improve bond strength values, as indicated in patent literature. A proprietary amide monomer was introduced into the formulation of this adhesive resin, which supposedly lowers the surface tension of the bonding substrate and allows for an immediate impregnation with the adhesive resin, without the need to wait or active application. Our internal research confirmed that the shear bond strength to dentin of both RXU and PSAU benefits from the application of the pertinent universal adhesive (unpublished results). These results need to be further pursued and replicated.

Although SOC is the first marketed universal cement, to the best of our knowledge there are no reports in the literature on bonding performances of this cement to tooth tissues, or restorative materials.

3.3 Interaction of universal cements with the restorative materials

It is naturally equally important for universal cements to bond predictably to restorative materials. Bond strength of RXU cement system on composite, polymer-infiltrated ceramic, lithium disilicate, feldspar ceramic, and zirconia was higher than that of Panavia V5 coupled with the ceramic primer (Kuraray Noritake)⁴⁸ and comparable to lithium-disilicate to that of G-Cem One used with the G-MultiPrimer (GC).⁴⁹ PSAU demonstrated comparable bond strength to ceramics to Panavia SA Plus (Kuraray Noritake) used with a separate ceramic primer.⁵¹ However, it seems that the application of a separate silane further improves bond strength of PSAU to ceramics.⁵³ In fact, the hydrophobicity of the polished or HF-etched lithium-disilicate surfaces was higher in groups treated with a silane-containing primer compared to the PSAU-treated groups, possibly due to the better rheological properties of the silane in the liquid primer compared to the one in the cement, which has a higher viscosity.⁵³ Furthermore, when bonding to composite materials, a system that employs a primer showed better bonding performance compared to the PSAU universal cement used in self-adhesive mode.⁵⁴ The apparent efficacy of these two novel universal luting systems to silanize glass ceramics was made possible through adjustments in their chemical composition. In the RXU + Scotchbond Universal Plus system, it is the adhesive resin that contains a mixture of silanes and is recommended for use as a primer for ceramic and composite-based restorations. However, it was reported in a recent meta-analysis that universal adhesives advocated as ceramic primers are less effective compared to separate silanes.⁵⁵ This is probably due to the acidic and hydrophilic nature of the universal adhesives which likely cause premature condensation of the most commonly used silane monomers, such as methacryloxypropyl trimethoxysilane (γ-MPTS). Nevertheless, the Scotchbond Universal Plus adhesive contains alternative silane molecules, 3-(aminopropyl)triethoxysilane and γ-methacryloxypropyltriethoxysilane, that might be more resistant to the hydrophilic acidic environment, as demonstrated by the previously mentioned studies.^{48, 49} On the other hand, a proprietary long-chain silane monomer was added directly to Panavia SA Universal cement, in the more hydrophobic and pH neutral paste. This seems to have prevented the premature hydrolysis of the silane molecule. Since separate silane primers are probably activated by atmospheric water,⁵⁶ there might be concerns that a highly viscous mixture of cement pastes could not effectively use water from the atmosphere to hydrolyze the silane-coupling monomer.⁵¹ However, it was clearly demonstrated by means of an NMR analysis that the silane-coupling monomer undergoes hydrolysis upon mixing of the two pastes, possibly due to water absorbed onto glass,⁵¹ as well as to the water molecules released in the setting mechanism of the cement. As for the SOC, although the manufacturer recommends luting of composite, ceramic and zirconia restorations without any pretreatment, it is not clear whether this universal cement contains a silane. This information is not provided by the manufacturer and according to the SDS information, it does not seem to be the case.

It should be noted that the bonding performance of resin cements can be greatly influenced by the restoration's mechanical and chemical surface treatment. For instance, shear bond strength of PSAU to differently pretreated translucent zirconia (alumina sandblasting, several protocols for silica coating) was recently tested.⁵⁷ In this study, at baseline, PSAU performed better or equally well than the other luting protocols (Panavia SA Plus with or without a silane/universal adhesive pretreatment) regardless of the chemo/mechanical treatment of the translucent zirconia. After thermocycling, nearly all the tested groups had a significant decrease in bond strength. PSAU still performed better on silica-coated surfaces than the protocols that did not contain silane, but worse than those that entailed a separate silane application.⁵⁷

It is indeed tempting to simplify the luting procedures, and adhesive procedures in general, and preferably blend in all the different components that might come in handy during this process, such as a silane and 10-MDP, but these must be performed with caution, since, for instance, 10-MDP might cover some of the γ-MPTS free methacrylate groups, and therefore these molecules could cancel each other's effect.⁵⁸ Moreover, although the silane formulation within the universal adhesive or universal cement seem to provide a rather efficient chemical bond strength to different types of ceramics, it is paramount that these materials are resistant to hydrolytic degradation. It was recently reported that the application of Scotchbond Universal Plus + RXU on several novel lithium disilicate glass ceramics yielded bond strength at baseline comparable to the groups where an additional silane was used.⁵⁹ However, after thermocycling, the bond strength in the groups pretreated only with the universal adhesive decreased significantly more. This could indicate that the silane-containing universal adhesive is more prone to hydrolytic degradation, which could have led to plasticization of the adhesive layer, and monomer leaching due to breaking of the polymer covalent bonds.

3.4 Considerations on polymerization efficacy

Universal cements are dual-cure materials, meaning that free radical polymerization is initiated both by a redox chemical and a photo-initiation route. This enables their polymerization in the areas of difficult access to curing light. Adequate polymerization of resin-based cements is essential for their mechanical properties and long-term bonding performances.⁶⁰ This was also demonstrated in the reports from the previous sections, where dual-cure groups nearly always demonstrated superior physical properties as well as bonding performance compared to their chemical-cure counterparts. In the dual-cure products, the different polymerization initiation and acceleration components are divided into separate pastes to prevent their premature activation. As the components introduced in the acidic paste need to be acid-resistant, the polymerization initiators added in self-adhesive and universal cements had to be modified accordingly. Hence, benzoyl peroxide and tertiary amines needed to be replaced with other oxidants, and reductants, such as cumene hydroperoxide and various thioureas.¹ As regards the photo-initiation, apart from the traditional camphorquinone (1,7,7-trimethylbicylo[2.2.1]heptane-2,3-dione, CQ)/amine complex, other, more efficient initiators, such as diphenyl-(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) have been added in self-adhesive and universal cements (Table 3). To improve the acid-resistance of chemical and photoinitiators, aryl borate salts, as well as sodium aryl sulfates, have been introduced.⁶ Aryl borate salts can also contribute to the curing activation of dual-cure resin-based cements, as part of the “touch-cure” polymerization strategy, intended to improve the monomer network formation in areas with limited exposure to the curing light.⁶¹

The polymerization initiates once the two pastes are mixed and the inhibitor, added to slow down the setting and enable an adequate working time, has been consumed. During the setting of the cement, which is initially hydrophilic and acidic, the functional acidic monomers react both with hydroxyapatite and alkaline fillers. Consequently, the tooth tissues are slightly demineralized, and the functional monomers are bound chemically to hydroxyapatite.¹¹ The remaining portion of the acidic monomers is neutralized through a reaction with cement fillers.⁶² As the reaction proceeds, the cement's pH increases, and the water molecules released in the reaction are reused, supposedly leading to the formation of a more hydrophobic material.⁶³ It is important to emphasize that very often clinically the cementation substrate is not dentin or enamel, but a composite build-up material, meaning that the setting and neutralization reaction of the acidic monomers relies solely on their reaction with the ion-leachable glass fillers.¹⁶ This could lead to inadequate polymerization of the cement, and deterioration of its mechanical properties. Namely, acidic monomers can inhibit the amine accelerators necessary for photopolymerization.⁶⁴ Further, unreacted hydrophilic acidic monomers can determine higher water sorption and solubility of the cements.⁶⁴

The information on the curing efficacy of universal cements is scarce. A recent study comparing the RXU to several one- and multi-step cement systems reported that DC of RXU was comparable to other investigated cements. Interestingly, the DC in self-cure mode reached that of the dual-cure mode after 24 h and the DC was higher in RXU compared to its self-adhesive predecessor.³⁴ These results could corroborate the manufacturer's claims on introducing a novel amphiphilic polymerization initiator system that should supposedly initiate polymerization effectively both in the segments of the cement layer that are in contact with dentin, and the more hydrophobic portion of the cement layer. Furthermore, in order to avoid incompatibility of the polymerization initiation between the adhesive resin and the cement and to enable chemical curing initiation of universal adhesives during the luting procedures, polymerization accelerators (Scotchbond Universal Plus for RXU), or dual-cure activators (Clearfil Bond Quick for PSAU) were introduced in their formulations, precluding the need for polymerization before the application of the cement. OneCoat 7 Universal is the only adhesive in the universal cement/adhesive systems that requires polymerization prior to luting. The efficiency of the polymerization activation of PSAU by Clearfil Bond Quick was demonstrated in a recent report,⁵³ since the application of Clearfil Bond Quick before the cement significantly increased (p < 0.05) the degree of conversion of the cement in self-cure mode. In dual-cure mode however, the DC was not influenced by the adhesive resin application and was comparable to the DC of PSAU's predecessor in the dual-cure mode, and higher in the self-cure mode.