In vitro degradation of resin–dentin bonds analyzed by microtensile bond test, scanning and transmission electron microscopy
Introduction
Significant advances in adhesive dentistry have occurred over the past two decades. Increased clinical success followed improvements in the formulations of dental resin adhesives, but little detailed information is available on the durability of such bonds. Recently developed adhesives that contain resin monomers with hydrophilic functional groups have improved bond strengths. This has improved the clinical applications of adhesive dentistry that include pit and fissure sealants, orthodontic brackets, adhesive bridges, laminate veneers, and direct resin restorations. However, undesirable consequences (i.e. recurrent caries or marginal discoloration) are often found in resin restorations following long-term clinical use. Thus, the study of bond durability and degradation mechanisms is an important issue in restorative dentistry.
Defects in resin impregnation and imperfect polymerization of the adhesive resin can create bond defects that result in the creation of demineralized dentin zones [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Demineralized dentin zone are created by a discrepancy between the depth of demineralization and the depth of resin infiltration. This zone theoretically exists at the border between the hybrid layer and mineralized dentin. The morphological manifestation of this zone is thought to consist of exposed, naked collagen fibrils surrounded by nanometer-sized, water-filled interfibrillar spaces. Thus, this zone permits silver nitrate uptake into these nanometer-sized voids, a phenomenon that is termed, nanoleakage [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Nanoleakage is a term that indicates the uptake a tracer molecules into nanometer-sized spaces that are present at the top of, within, and beneath the hybrid layer or bonding resin layer in the absence of an interfacial gap (i.e. microleakage). Tay et al. [19] reported additional nanometer-sized defects such as water trees or a reticular mode of nanoleakage, within bonded interphases using ammoniacal silver nitrate as a tracer. In their study, they speculated that domains of more hydrophilic oligonomers or regions of incompletely polymerized resin lead to nanometer-sized defects, regardless of the degree of resin infiltration. Evidence of imperfect resin infiltration of demineralized dentin was also confirmed from the failure patterns of fractured surfaces after bond testing [21], [22]. These defects are created either during resin bonding or following long-term water immersion, and probably contribute to the loss of adhesive resin and/or collagen fibrils within the hybrid layers.
Therefore, the objective of this study was to determine if there was any biodegradation of resin–dentin bonds after 1 year of water exposure, using the combined methodologies of microtensile bond testing, correlated with SEM observations of the fractured surfaces and interfacial observations by TEM. The null hypothesis tested was that there is no alteration in either the tensile bond strengths or micromorphology of resin–dentin bonds after 1 year of water storage.
Section snippets
Tooth preparation
Twenty-four noncarious human premolars were extracted for orthodontic reasons with patients’ informed consent under a protocol approved by the appropriate institutional review board. The teeth were stored in distilled water at 4°C containing 1% chloramine T solution and were used within 1 month after extraction. Each tooth was sectioned using a slow-speed diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) under water coolant/lubrication, to expose the mid-coronal dentin surface. Each
Bond strength results
A significant difference (p<0.01) was found in the bond strength of resin to dentin between 24 h controls (40.3±15.1 MPa: n=20) and specimens stored in water for 1 year (13.3±5.6 MPa: n=16), when the data were examined using the Wilcoxon rank-sum test as shown in Fig. 2. The fall in bond strength was approximately 67% after 1 year of water exposure.
Scanning electron microscopy analysis
Fig. 3 shows the dentin side of control (Fig. 3a) and aged specimens (Figs. 3b and c) that fractured through the hybrid layer. The bonding resin
Discussion
The bond strength of experimental specimens fell approximately 67%, compared to the 24 h controls, after 1 year of water exposure (Fig. 2). In this study, interfacial TEM micrographs were prepared from the fractured beams of the dentin side of the bonds that fail during microtensile bond testing. The up-right orientation of collagen fibrils found at the top of the fractured hybrid layer was probably caused by the tensile stress developing during bond strength testing. Thus, it is possible that
Acknowledgments
This work was supported, in part, by Grant-in-Aid for Scientific Research No. 11470401, and for High-Performance Biomedical Materials Research from the Ministry of Education, Science, Sports and Culture, Japan, and by fellowship awards of the Japanese Society of Pediatric Dentistry, and the Futokukai Foundation. The authors thank Mr. S. Hayashi and Mr. K. Yoshida (the Center for Electron Microscopy and Bio-Imaging Research of Iwate Medical University) and Mr. N. Nodasaka (the Center for
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2022, International Journal of Adhesion and AdhesivesCitation Excerpt :However, the specimens bonded to ACAD and NCAD had reduced mean values for the immediate evaluation, with no significant differences compared with longer storage time. According to several authors, direct storage significantly reduces bond strength and increases nano-infiltration when compared with indirect storage [54–56]. The indirect storage technique consists of performing the adhesive and restorative procedures, storing them for a period, and then sectioning them into sticks to submit to the test [54,57,58].