K.L. Van Landuyt

A Critical Review of the Durability of Adhesion to Tooth Tissue: Methods and Results

The immediate bonding effectiveness of contemporary adhesives is quite favorable, regardless of the approach used. In the long term, the bonding effectiveness of some adhesives drops dramatically, whereas the bond strengths of other adhesives are more stable. This review examines the fundamental processes that cause the adhesion of biomaterials to enamel and dentin to degrade with time. Non-carious class V clinical trials remain the ultimate test method for the assessment of bonding effectiveness, but in addition to being high-cost, they are time- and labor-consuming, and they provide little information on the true cause of clinical failure. Therefore, several laboratory protocols were developed to predict bond durability. This paper critically appraises methodologies that focus on chemical degradation patterns of hydrolysis and elution of interface components, as well as mechanically oriented test set-ups, such as fatigue and fracture toughness measurements. A correlation of in vitro and in vivo data revealed that, currently, the most validated method to assess adhesion durability involves aging of micro-specimens of biomaterials bonded to either enamel or dentin. After about 3 months, all classes of adhesives exhibited mechanical and morphological evidence of degradation that resembles in vivo aging effects. A comparison of contemporary adhesives revealed that the three-step etch-and-rinse adhesives remain the ‘gold standard’ in terms of durability. Any kind of simplification in the clinical application procedure results in loss of bonding effectiveness. Only the two-step self-etch adhesives approach the gold standard and do have some additional clinical benefits.

Monomer-Solvent Phase Separation in One-step Self-etch Adhesives:

One-step adhesives bond less effectively to enamel/dentin than do their multi-step versions. To investigate whether this might be due to phase separation between adhesive ingredients, we characterized the interaction of 5 experimental and 3 commercial self-etch adhesives with dentin using transmission electron microscopy. All adhesives were examined for homogeneity by light microscopy. Bonding effectiveness to dentin was determined with the use of a micro-tensile bond-strength protocol. The lower bond strength of the one-step adhesives was associated with light-microscopic observation of multiple droplets that disappeared slowly. Interfacial analysis confirmed the entrapment of droplets within the adhesive layer. The prompt disappearance of droplets upon application of a small amount of HEMA (2-hydroxyethyl methacrylate) or a HEMA-containing bonding agent, as well as the absence of droplets at the interface of all HEMA-containing adhesives, strongly suggests that the adhesive monomers separate from water upon evaporation of ethanol/acetone. Upon polymerization, the droplets become entrapped within the adhesive, potentially jeopardizing bond durability. This can be avoided by strong air-drying of the adhesive, thereby removing interfacial water and thus improving bonding effectiveness.

How much do resin-based dental materials release? A meta-analytical approach

OBJECTIVES Resin-based dental materials are not inert in the oral environment, and may release components, initially due to incomplete polymerization, and later due to degradation. Since there are concerns regarding potential toxicity, more precise knowledge of the actual quantity of released eluates is necessary. However, due to a great variety in analytical methodology employed in different studies and in the presentation of the results, it is still unclear to which quantities of components a patient may be exposed. The objective of this meta-analytical study was to review the literature on the short- and long-term release of components from resin-based dental materials, and to determine how much (order of magnitude) of those components may leach out in the oral cavity. METHODS Out of an initial set of 71 studies, 22 were included. In spite of the large statistical incertitude due to the great variety in methodology and lack of complete information (detection limits were seldom mentioned), a meta-analytical mean for the evaluated eluates was calculated. To relate the amount of potentially released material components with the size of restorations, the mean size of standard composite restorations was estimated using a 3D graphical program. RESULTS While the release of monomers was analyzed in many studies, that of additives, such as initiators, inhibitors and stabilizers, was seldom investigated. Significantly more components were found to be released in organic than in water-based media. Resin-based dental materials might account for the total burden of orally ingested bisphenol A, but they may release even higher amounts of monomers, such as HEMA, TEGDMA, BisGMA and UDMA. Compared to these monomers, similar or even higher amounts of additives may elute, even though composites generally only contain very small amounts of additives. A positive correlation was found between the total quantity of released eluates and the volume of extraction solution. SIGNIFICANCE There is a clear need for more accurate and standardized analytical research to determine the long-term release from resin-based materials. Several guidelines for standardization are proposed.

Inhibition of Enzymatic Degradation of Adhesive-Dentin Interfaces

Adhesive procedures activate dentin-associated matrix metalloproteinases (MMPs), and so iatrogenically initiate bond degradation. We hypothesized that adding MMP inhibitors to adhesive primers may prevent this endogenous enzymatic degradation, thereby improving bond durability. A non-specific MMP inhibitor (chlorhexidine) and a MMP-2/9-specific inhibitor (SB-3CT) were admixed to the primers of an etch & rinse and a self-etch adhesive, both considered as gold-standard adhesives within their respective categories. For dentin powder exposed to the adhesives under clinical application conditions, gelatin zymography revealed the release of MMP-2 (not of MMP-9) by the etch & rinse adhesive, while no release of enzymes could be detected for the mild self-etch adhesive, most likely because of its limited dentin demineralization effect. The built-in MMP inhibitors appeared effective in reducing bond degradation only for the etch & rinse adhesive, and not for the self-etch adhesive. Water sorption of adhesive interfaces most likely remains the principal mechanism of bond degradation, while endogenous enzymes appear to contribute to bond degradation of only etch & rinse adhesives.

Eight-year clinical evaluation of a 2-step self-etch adhesive with and without selective enamel etching

OBJECTIVES The objective of this randomized controlled clinical trial was to evaluate the 8-year clinical performance of a mild 2-step self-etch adhesive in non-carious Class-V lesions with and without prior selective phosphoric acid-etching of the enamel cavity margins. METHODS A total of 100 non-carious Class-V lesions in 29 patients were restored with Clearfil AP-X (Kuraray). The composite restorations were bonded following two different approaches: (1) application of Clearfil SE (Kuraray) following a self-etch approach (control group; C-SE non-etch), (2) selective phosphoric acid-etching of the enamel cavity margins before application of Clearfil SE (experimental group; C-SE etch). The restorations were evaluated after 6 months, 1, 2, 3, 5 and 8 years of clinical service regarding their retention, marginal integrity and discoloration, caries occurrence, preservation of tooth vitality and post-operative sensitivity. RESULTS The recall rate at 8 years was 76%. Only two restorations, one of the C-SE non-etch group and one of the C-SE etch group, were clinically unacceptable due to loss of retention leading to a retention rate and a clinical success rate of 97% in both groups. Aging of the restorations was characterized by an increase in the percentage of restorations with a small but clinically acceptable marginal defect (C-SE non-etch: 92%; C-SE etch: 84%) and/or a superficial marginal discoloration (C-SE non-etch: 44%; C-SE etch: 28%). At the enamel side, the presence of small marginal defects (C-SE non-etch: 86%; C-SE etch: 65%) and superficial marginal discoloration (C-SE non-etch: 11%; C-SE etch%) was more frequently noticed in the control group than in the experimental group. The difference, however, was only statistically significant for the presence of superficial marginal discoloration (McNemar, p=0.01). SIGNIFICANCE After 8 years of clinical functioning, the clinical effectiveness of Clearfil SE remained excellent, with selective acid-etching of the enamel cavity margins only having some minor positive effect on marginal integrity and absence of marginal discoloration at enamel.

Current aspects on bonding effectiveness and stability in adhesive dentistry

Improved dental adhesive technology has extensively influenced modern concepts in restorative dentistry. In light of minimal-invasive dentistry, this new approach promotes a more conservative cavity design, which basically relies on the effectiveness of current enamel-dentine adhesives. Nowadays, the interaction of adhesives with the dental substrate is based on two different strategies, commonly described as an etch-and-rinse and a self-etch approach. In an attempt to simplify the bonding technique, manufacturers have decreased the number of steps necessary for the accomplishment of the bonding procedure. As a consequence, two-step etch-and-rinse and one-step (self-etch) adhesives were introduced and gained rapid popularity in the dental market due to their claimed user-friendliness and lower technique sensitivity. However, many concerns have been raised on the bonding effectiveness of these simplified adhesives, especially in terms of durability, although this tends to be very material dependent. In order to blend all the adhesive components into one single solution, one-step adhesives were made more acidic and hydrophilic. Unfortunately, these properties induce a wide variety of seemingly unrelated problems that may jeopardize the effectiveness and stability of adhesion to the dental substrate. Being more susceptible to water sorption and thus nanoleakage, these adhesives are more prone to bond degradation and tend to fail prematurely as compared to their multi-step counterparts. Incidentally, another factor that may interfere with the bonding effectiveness of adhesives is the technique used for caries removal and cavity preparation. Several tools are on the market today to effectively remove carious tissue, thereby respecting the current trend of minimum intervention. Despite their promising performance, such techniques modify the tooth substrate in different aspects, possibly affecting bonding effectiveness. Altogether, we may conclude that not only the adhesive formulation, but also substrate nature must be taken into account to achieve a stable bonding interface, rendering the restorative treatment more predictable in terms of clinical performance. In this review, we analyse the current theoretical and clinical aspects of adhesion to enamel and dentine, and discuss the diverse possibilities to overcome problems which nowadays still challenge clinicians in their achievement of a more stable and effective bond to tooth enamel and dentine.

Influence of the Chemical Structure of Functional Monomers on Their Adhesive Performance

Functional monomers in adhesive systems can improve bonding by enhancing wetting and demineralization, and by chemical bonding to calcium. This study tested the hypothesis that small changes in the chemical structure of functional monomers may improve their bonding effectiveness. Three experimental phosphonate monomers (HAEPA, EAEPA, and MAEPA), with slightly different chemical structures, and 10-MDP (control) were evaluated. Adhesive performance was determined in terms of microtensile bond strength of 4 cements that differed only for the functional monomer. Based on the Adhesion-Decalcification concept, the chemical bonding potential was assessed by atomic absorption spectrophotometry of the dissolution rate of the calcium salt of the functional monomers. High bond strength of the adhesive cement corresponded to low dissolution rate of the calcium salt of the respective functional monomer. The latter is according to the Adhesion-Decalcification concept, suggestive of a high chemical bonding capacity. We conclude that the adhesive performance of an adhesive material depends on the chemical structure of the functional monomer.

The role of HEMA in one-step self-etch adhesives.

UNLABELLED In spite of its high allergenic potential, 2-hydroxyethyl methacrylate (HEMA), a low-molecular-weight monomer, is frequently used in adhesives for its positive influence on the bond strength. In addition, the presence of HEMA in one-component one-step adhesives can prevent phase separation. OBJECTIVES In search of improved bonding effectiveness, the 24-h bond strength of four experimental one-step self-etch adhesives with different concentrations of HEMA to bur-cut enamel and dentin was determined using a micro-tensile bond strength protocol. METHODS The tested experimental adhesives (Exp-0, Exp-10, Exp-19 and Exp-36) only differed in their concentration of HEMA, which was 0, 10, 19 and 36%, respectively. With an increasing concentration of HEMA, the concentration of acetone was decreased. Besides bond strength, the adhesives were also examined by light-microscopy for phase separation. The interface was investigated by SEM and TEM. RESULTS Regarding bond strength, Exp-10 performed best. Even though Exp-36 was the only adhesive formulation that did not exhibit phase separation on a glass plate, it yielded the lowest bond strength. Accordingly, droplets could be observed by SEM and TEM in the adhesive layers of all adhesives, except for Exp-36 on enamel. CONCLUSION A small amount of HEMA (10%) improved the bond strength of a one-step self-etch adhesive. When added in higher concentrations, this beneficial effect of HEMA on the bond strength is lost due to increased osmosis, which resulted in many droplets; due to reduced polymerization conversion; and sub-optimal physico-mechanical properties of the resultant poly-HEMA containing adhesive interface.

Origin of Interfacial Droplets with One-step Adhesives

Contemporary one-step self-etch adhesives are often documented with interfacial droplets. The objective of this study was to research the origin of these droplets. Two HEMA-rich and one HEMA-free adhesive were applied to enamel and dentin, with the lining composite either immediately cured or cured only after 20 min. All one-step adhesives exhibited droplets at the interface; however, the droplets had two different origins. With the HEMA-free adhesives, droplets were located throughout the adhesive layer and were stable in number over time. With the HEMA-rich adhesives, the droplets were observed exclusively at the adhesive resin/composite interface, and their number increased significantly when the composite was delay-cured. Only the latter droplets caused a significant drop in bond strength after delayed curing. While the droplets in the HEMA-free one-step adhesives should be ascribed to phase separation, those observed with HEMA-rich adhesives resulted from water absorption from dentin through osmosis.

Meta-analytical Review of Parameters Involved in Dentin Bonding

Bond-strength testing is the method most used for the assessment of bonding effectiveness to enamel and dentin. We aimed to disclose general trends in adhesive performance by collecting dentin bond-strength data systematically. The PubMed and EMBASE databases were used to identify 2,157 bond-strength tests in 298 papers. Most used was the micro-tensile test, which appeared to have a larger discriminative power than the traditional macro-shear test. Because of the huge variability in dentin bond-strength data and the high number of co-variables, a neural network statistical model was constructed. Variables like ‘research group’ and ‘adhesive brand’ appeared most determining. Weighted means derived from this analysis confirmed the high sensitivity of current adhesive approaches (especially of all-in-one adhesives) to long-term water-storage and substrate variability.