Elsevier

Biomaterials

Volume 24, Issue 21, September 2003, Pages 3795-3803
Biomaterials

In vitro degradation of resin–dentin bonds analyzed by microtensile bond test, scanning and transmission electron microscopy

https://doi.org/10.1016/S0142-9612(03)00262-XGet rights and content

Abstract

Our knowledge of the mechanisms responsible for the degradation of resin–dentin bonds are poorly understood. This study investigated the degradation of resin–dentin bonds after 1 year immersion in water. Resin–dentin beams (adhesive area: 0.9 mm2) were made by bonding using a resin adhesive, to extracted human teeth. The experimental beams were stored in water for 1 year. Beams that had been stored in water for 24 h were used as controls. After water storage, the beams were subjected to microtensile bond testing. The dentin side of the fractured surface was observed using FE-SEM. Subsequently, these fractured beams were embedded in epoxy resin and examined by TEM. The bond strength of the control specimens (40.3±15.1 MPa) decreased significantly (p<0.01) after 1 year of water exposure (13.3±5.6 MPa). Loss of resin was observed within fractured hybrid layers in the 1 year specimens but not in the controls. Transmission electron microscopic examination revealed the presence of micromorphological alterations in the collagen fibrils after 1 year of water storage. These micromorphological changes (resin elution and alteration of the collagen fibrils) seem to be responsible for the bond degradation leading to bond strength reduction.

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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