Towards more durable bonding to dentin

Dental adhesives are materials most often used in daily dental practice, especially because of their primary bonding property to tooth tissue as a principal function for several adhesive restorative and luting procedures. In light of their further optimization, the newest generation of dental adhesives, being referred to as universal adhesives (UAs), cannot only be used to bond to tooth tissue (enamel/dentin), optionally in either a self-etch (SE), etch-and-rinse (E&R) or combined selective enamel E&R with dentin SE bonding mode, but they are also applicable to bond to indirect glass-rich/poor ceramics.

The primary aims of this PhD project were (1) to explore a selection of (pre-)commercial UAs (knowledge phase), identifying their strengths and shortcomings, (2) to overcome the too thin film thickness of UAs (UA shortcoming 1), and (3) to find a suitable alternative/analogue for/of the hydrophilic co-monomer 2-hydroxyethyl methacrylate HEMA, being a main constituent of many UAs despite having many disadvantages (UA shortcoming 2). This PhD work involved (1) measurement of bonding effectiveness and bond durability of several UAs to dentin, also when combined with a beforehand employed (enamel/)dentin conditioner as alternative to the classic but aggressive phosphoric-acid etchant, (2) assessment of the benefit of a high- and low-filled extra bonding layer (EBL) as remedy of UA shortcoming 1 to increase their film thickness as well as better seal the adhesive-dentin interface, and (3) testing several acrylamide monomers, namely DEAA, HEAA and FAM-201, as HEMA-alternative monomers in experimental 2-step UA formulations, and in (partial) replacement of HEMA and TEGDMA in experimental 1-step UA formulations as a remedy of UA shortcoming 2.

In the first part of this PhD thesis (CHAPTER 2), we investigated in PAPER 1 the bonding effectiveness and bond durability of a newly released UA to be (clinically) applied following a quick bonding protocol, as recommended by its manufacturer, this measured versus an extended/conventional 20-sec application time. A second UA and the 2-step gold-standard SE adhesive were included as references, all applied in SE and E&R bonding mode. The resultant adhesive-dentin interfaces were characterized using TEM. In brief, the longer 20-s application time solely improved immediate bonding effectiveness of the new UA, but not its bond durability. Rapid bonding technology should perhaps be regarded more as a marketing advantage than a true benefit, as this relatively short timesaving may clinically be less relevant. In PAPER 2, we tested if a new experimental metal-salt (ZrO(NO3)2) conditioner could replace classic phosphoric acid as part of an E&R bonding mode and so provide durable bonding to dentin when employed with a UA applied in E&R and SE bonding mode. The resultant adhesive-dentin interfaces were again ultrastructurally characterized using S/TEM. This study revealed that the bond durability of the conditioner/UA combination, while being artificially aged by 50k thermo-cycles, was equivalent to that obtained when the UA was pre-ceded by phosphoric-acid etching or applied in SE mode. TEM of the adhesive-dentin interface pre-treated by the metal-salt conditioner revealed abundant hydroxyapatite (HAp) at the hybrid-layer bottom, while the conditioner exposed less collagen and resulted in less distinct resin-tag formation. This PhD project part concluded that the metal-based conditioner should be regarded as a valid alternative to phosphoric-acid etching gel.

The second part of this thesis (CHAPTER 3) focused on the bond-promoting effect of an EBL with hydrophobic properties to test the hypothesis that a well-polymerized hydrophobic resin layer would better seal/protect the adhesive-dentin interface against water sorption and related hydrolytic bond-degradation effects. Three representative commercial UAs were experimentally used as primers, without being light cured, and followed by the application of the low-filled adhesive resin of (1) the considered gold-standard 2SE adhesive in PAPER 3, and the high-filled adhesive resin of (2) the considered gold-standard 3E&R adhesive in PAPER 4, hence transforming the 1/2-step UA into a 2/3-step UA. The gold-standard adhesives served again as reference/controls. In general, the UAs benefited from EBL, with significantly better performance being recorded when a high-filled than a low-filled EBL was applied. Nevertheless, the individual benefit was product- and EBL-dependent.

In the third part of this PhD thesis (CHAPTER 4), a series of new experimentally designed and developed acrylamide monomer-based 2- and 1-step UAs were investigated. In PAPER 5, we investigated two acrylamide co-monomers, namely diethyl acrylamide (DEAA) and hydroxyethyl acrylamide (HEAA), as monomer alternatives for HEMA in two-step UA formulations, when employed in 3E&R and 2SE bonding modes and subjected to substantial aging. A HEMA-free and HEMA-containing adhesive were additionally formulated and employed as controls. The experimentally prepared adhesives were subjected to multi-parameter evaluation, including measurement of bonding effectiveness and bond durability to dentin using a micro-tensile bond-strength (µTBS) approach, measurement of three-point bending strength, contact-angle wetting, viscosity, and watersorption, as well as their adhesive-dentin interfacial interaction was characterized by TEM. The data were further correlated to the molecular weights and partition coefficients (LogP) of the monomers investigated. The acrylamide monomer DEAA revealed promising data regarding µTBS, watersorption, and physico-mechanical properties, which qualifies this co-monomer as a potential HEMA-alternative to be employed in 2-step UAs.

In PAPER 6, we investigated the new acrylamide cross-linking monomer namely, N,N'-diacryloyl-4,7,10-trioxa-1,13-tridecanediamine (FAM-201), as well as again HEAA, both in full and partial replacement, respectively, of HEMA and TEGDMA in 1-step UA formulations, when employed in 2E&R and 1SE bonding modes and subjected to substantial aging. A HEMA-free and HEMA-containing adhesive were additionally formulated and employed as controls. The experimentally prepared adhesives were tested for their bonding effectiveness and bond durability to dentin using µTBS, potential 10-MDP_Ca-salt nano-layering at the adhesive interface by thin-film XRD, cytotoxicity of the pure monomers, and characterization of the resultant adhesive-dentin interfacial interaction by TEM. The data were further correlated to the molecular weights and partition coefficients (LogP) of the monomers investigated. The acrylamide cross-linking monomer FAM-201 revealed the most promising µTBS and least cytotoxicity, when compared to the other monomers investigated, and presented with an intermediary favorable LogP, while HEAA presented with the highest LogP, indicating high hydrophilicity and watersorption. This PhD study concluded that the acrylamide co-monomer FAM-201 could replace HEMA in 1-step UAs.

Based on the aforementioned results, the following main conclusions of this PhD project were drawn:

1.The different (pre-)commercial UAs tested in this PhD research showed a comparably effective immediate bonding performance. However, long-term bond-durability testing revealed that the UAs’ aged μTBS underscored that of the gold-standard SE and E&R adhesives, by which further improvement in the bonding performance of UAs remains needed.

2.Ultra-structural interfacial analysis of the tested UAs generally disclosed tight adhesive-dentin interfaces with a well-defined hybrid layer, which was thicker when the UA was applied in E&R (5-6 μm) than in SE (1-2 μm) bonding mode. However, a more important finding was their thin film thickness, which at least in part explains their impaired long-term bond durability.

3.The optional applicability of UAs in SE or E&R mode is an additional (clinical) advantage, but no conclusion could be drawn which of the two UA bonding modes could be considered as more durable.

4.The alternative metal-based (enamel/)dentin conditioner revealed a more HAp-protected hybrid layer with less exposure of collagen fibrils along with a bond strength comparable to that obtained with a phosphoric-acid E&R bonding mode. These findings confirm that the metal-based conditioner can be considered as a suitable alternative (enamel/)dentin conditioner to classic phosphoric acid employed in an E&R bonding mode.

5.The UAs investigated in this PhD research benefited, in general, from the application of a hydrophobic and well-polymerized extra bonding layer (EBL), with a significantly higher benefit being recorded for the high-filled EBL. However, the EBL benefit appeared not only product dependent but also depending on the EBL kind and the bonding mode employed.

6.The acrylamide co-monomers DEAA and FAM-201 can be considered as HEMA-alternative co-monomers in 2- and 1-step adhesives, respectively.