Polymerization shrinkage: effects of constraint and filling technique in composite restorations
Introduction
Light cured composite resins are now widely used in clinical practice because of their esthetic advantages, ease of use, good bonding to tooth structure and improved mechanical properties. However, the polymerization reaction of light cured composites leads to the development of higher stresses when the composite resin is bonded to the cavity walls.7 The role of the bonded to unbonded surface area ratios (constraint or C-factor index) on the development of polymerization stresses with composite resins was demonstrated by Feilzer et al.7 They described an in vitro model in which restorations with C-factor <1 are the only ones likely to survive polymerization shrinkage stresses. Thus, early bond strengths are crucial since they are responsible for preserving the adhesive interface during development of stress from polymerization.
Haller et al.10 reported a reduction of the bond strength to dentin of some adhesive systems when applied to 3D cavities in comparison with flat surfaces. Yoshikawa et al.33 were the first to evaluate resin–dentin bond strength using the microtensile test in class I cavities and they found decreased bond strength for all adhesive systems tested under high C-factors. However, this study disregarded the role of the filling technique on the stress relief since different C-factor cavities were not filled under the same technique.
Other studies have reached similar conclusions, regarding the reduction of bond strength values under high C-factor cavities.3., 2., 32. However, to the extent of our knowledge no study has effectively compared the effects of constraints restored under different filling techniques.
It is of common belief that widespread incremental filling techniques are capable of reducing the concentration of stresses arising from the cure of resins at the tooth interface when light activated composite resin are employed.12., 15. However, Versluis et al.27 reported in a theoretical study using Finite Element Analysis methods (FEA), that incremental filling techniques could produce higher polymerization stresses at the restoration interface compared with bulk filling. On the other hand, other laboratorial studies have not detected any difference among filling techniques,25., 30. thus requiring further studies on this field.
Bond strength tests and microleakage studies are used as in vitro indicators of the strength and integrity of the marginal seal of composite resin restorations. Several studies have attempted to correlate the results of these tests but none have succeeded.19., 24., 1. However, most of them were not performed in the same specimen and under the same C-factor.
Therefore, the objective of this study was: (1) to measure the linear polymerization shrinkage, gap width, adhesive bond strength and the cohesive strength of the composite resin employed for restoration of different C-factor cavities under different filling techniques and; (2) the relationship between marginal adaptation and microtensile bond strength in the same specimen.
Section snippets
Selection and teeth preparation
Twenty sound bovine incisors were used. Teeth free from cracks or any other kind of structural defect were selected under magnification of ×20. The teeth were disinfected in 0.5% chloramine for 15 days and stored for less than 6 months in 0.9% saline solution. All buccal surfaces were ground and flattened under water refrigeration with a 180 grit SiC paper (Fig. 1a). On each tooth, two standardized cavities were prepared in the buccal surfaces with C-factor=3 (4 mm wide, 4 mm high and 2 mm
Results
The results and their respective standard deviations are shown in Table 1, Table 2.
Linear polymerization shrinkage. Two-way ANOVA (C-factor vs. filling technique) showed that significant differences were observed for the interaction of the factors (p<0.05). Under the low C-factor, linear polymerization shrinkage was similar for both filling techniques. Conversely, under the high C-factor, the incremental technique (1.57%) reduced the shrinkage compared to the bulk filling (1.95%).
Gap width. The
Discussion
The experimental design of this study was conducted in bovine teeth due to ethical concerns that are related to the use of human teeth. The histological features of human and bovine permanent dentin seem to be similar23 and no difference in bond strength values has been detected.22 Thus the bovine incisor dentin is a suitable substitute for human molar dentin.23
Although several studies on polymerization shrinkage and its harmful effects have been conducted, little attention was given to the
Acknowledgements
This study is part of the thesis of Alessandro Dourado Loguercio (University of São Paulo) for partial fulfillment of the PhD degree in Dental Materials. We thank Dr Ricardo M Carvalho and Dr Mário F. Góes for their expert advice. This investigation was supported in part by FAPESP 99/05124-0 (Fundação de Apoio à Pesquisa do Estado de São Paulo). The authors are also grateful to Paulo Eduardo dos Santos for graphic illustration support.
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