Differential bonds degradation of two resin-modified glass-ionomer cements in primary and permanent teeth
Introduction
There are substantial microstructural differences between permanent and deciduous dentin. Primary dentin has higher tubule density and diameter, resulting in a reduced area of intertubular dentin available for bonding.1 Chemically, primary dentin seems to be more reactive to the acidic conditioners,2, 3 which could be explained by the reduced degree of mineralization observed in primary dental hard tissues.4, 5 Due to these differences, most of the used protocols for dentin bonding are not as effective when applied to primary dentin as they are on permanent dentin.6, 7
Glass ionomer cements are usually selected for restoring primary teeth. The application of weak acids is required, in order to clean the tooth surface, remove the smear layer, and expose collagen fibrils, allowing glass-ionomer components to inter-diffuse and establish a micro-mechanical entanglement with dentin.8 Furthermore, chemical bonding may be attained, by ionic interaction of the carboxyl groups of the polyalkenoic acid with calcium ions of remnant hydroxyapatite that remain attached to the collagen fibril.9, 10, 11 Resin-modified glass-ionomer cements (RMGIC) are particularly promising for restorative indications.12 Chemical and/or hydrolytic degradation has been previously reported for resin–dentin bonded interfaces.13, 14, 15, 16, 17, 18 If RMGIC–dentin bonded interfaces are also prone to these degradation processes remains to be ascertained.19
The purpose of this study was to ascertain RMGICs–dentin bonds resistance to degradation. The null hypothesis to be tested is that there are no differences in bond strength resistance to degradation when using two different RMGICs on primary or on permanent dentin.
Section snippets
Specimens preparation
Twelve of each primary and permanent non-carious human molars were used in this study. The human molars were obtained after the institutional informed consent from all donators. The research was approved by the Research Ethics Commission. The teeth were cleaned with pumice/water slurry, rinsed and stored in distilled water in a refrigerator (4 °C) until use. The pulp chambers of crowns were sealed with composite resin and their cusps flattened with 220-grit abrasive paper. Occlusal Class I
Results
Mean bond strength values (MPa), obtained for primary and permanent dentin, are shown in Table 2. Bond strength was affected by dentin type (F = 71.00; p < 0.001) and NaOCl challenge (F = 33.21, p < 0.001). Interaction between RMGIC and NaOCl immersion was also significant (F = 11.95, p = 0.001).
At 24 h evaluation, dentin bond strengths were similar for both materials in permanent dentin. Bond strength values to primary dentin were higher than to permanent dentin when restored with Fuji II LC; while using
Discussion
In vitro dentin bond strength studies commonly use flat surfaces to test the bonding effectiveness of dental materials. Clinically, however, restorative materials are inserted into cavities. Two major considerations should be taken into account when using cavities instead of flat surfaces: (1) tensions are created at the material/dentin interfaces due to high polymerization contraction stress; such pre-stressed interfaces may be more susceptible to failure and degradation20, 21; and (2) the
Conclusions
Vitremer–dentin bonds are prone to degradation. Restoring primary dentin with Fuji II LC is preferred due to the particular tolerance to major water presence of this substrate. Chemical degradation of Fuji II LC–dentin bonds did not occur.
Acknowledgments
This investigation was supported by Grants CICYT/FEDER #MAT 2008-02347\MAT, JA P07-CTS-2568, JA P08-CTS-3944.
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