Elsevier

Dental Materials

Volume 20, Issue 3, March 2004, Pages 236-243
Dental Materials

Polymerization shrinkage: effects of constraint and filling technique in composite restorations

https://doi.org/10.1016/S0109-5641(03)00098-8Get rights and content

Abstract

Objectives. To evaluate the linear polymerization shrinkage (LPS) and its effect upon mean gap width, bond strength and cohesive strength of a composite placed under different constraints (C-factors—CF) and filling techniques.

Methods. Composite was placed in cavities sized 4×4×2 mm3 (CF=3) or on flat dentin surfaces (CF=0.3) of bovine incisors, after adhesive application. They were inserted in one or three increments, and light cured (600 mW/cm2) for 80 s. The LPS was measured by placing a probe on the top surface of the composite in order to measure its dislodgment in the top–bottom direction. Half of the sample was sectioned to obtain composite resin sticks subjecting them to tensile forces at 0.5 mm/min. The other half of the sample was sectioned and the mean gap width was measured in both sides of the sections. Then the sections were sliced again to obtain composite/dentin sticks. The mean gap width in the sticks was performed before subjecting them to tensile forces at 0.5 mm/min. Data was analyzed by a two-way ANOVA and the correlation between the bond strength and gap width was analyzed by simple linear regression.

Results. (1) Linear polymerization shrinkage: significant differences were observed for the interaction (p<0.05). Under the low constraint, the LPS were similar for both filling techniques. Under higher constraint, polymerization shrinkage was lower for the incremental technique. (2) Gap width and bond strength: no difference was detected either for interaction, or for technique (p>0.05). Under higher constraint, the gap width was higher and the bond strength lower. (3) The cohesive strength of composite resin was similar for all groups (p>0.05). No correlation between bond strength and gap width was found (p=0.17).

Significance. The effects of polymerization shrinkage were not reduced by the filling technique under the different cavity constraints tested.

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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