Determinants of in vitro gap formation of resin composites
Introduction
The shrinkage which accompanies the polymerization of resin composites, generates stress at the tooth-restoration interface and may lead to marginal gap formation, marginal discoloration, postoperative sensitivity, and secondary caries.1 These clinical consequences of polymerization shrinkage constitute the main reasons for replacement of resin composite restorations,2 which explains why polymerization shrinkage is regarded as the main limitation of present-day resin composites. The magnitude of the stress generated in a polymerizing resin composite restoration is influenced by numerous factors related to the materials, technique, and cavity preparation, and the interplay between these factors dictates the exact manifestation of the shrinkage for a given restoration. Factors that influence stress formation include volumetric polymerization shrinkage, elastic modulus, and flow of the resin composite, adherence of the resin composite to the cavity walls, and configuration factor of the restoration (the ratio of bonded to unbonded composite surfaces).3., 4., 5., 6.
For a butt-joint cavity, the following interplay between these factors may be hypothesized (Fig. 1). Initially, the shrinking resin composite exerts a force on the cavity walls that is resisted by the adherence mediated by the adhesive system. As polymerization proceeds, shrinkage stress increases and eventually becomes larger than the adherence, and a gap is formed. If the shrinkage stress surpasses the adherence late in the polymerization process, gap formation is limited, if early, gap formation is more extensive. In a resin composite with high volumetric polymerization shrinkage, high shrinkage stress develops. At the same volumetric polymerization shrinkage, a stiffer composite places higher stress on the adherence than does a resin composite of lesser stiffness. Further, the shrinkage stress is reduced by flow of the resin composite from the non-bonded surface.
The resin composite market is enormous and saturated with documented and undocumented theories and claims. If certain properties of resin composites would prove more important for gap formation than others, these properties could serve as selecting criteria and help dentists choose between the countless types and brands of resin composites. The hypothesis to be tested is that gap formation around butt-joint resin composite restorations is influenced by volumetric polymerization shrinkage, flow, flexural modulus, and bond strength of the resin composites.
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Materials and methods
Eleven materials representing different types of resin composite were tested (Table 1). All experiments were conducted at ambient temperature.
Results
The results are shown in Table 2. Regarding the polymerization shrinkage measurements, the Table shows the shrinkage at the end of the monitoring period (20 min after start of light-cure) as well as the shrinkage 10 s after start of light-cure. Polymerization shrinkage varied significantly between 0.66 and 2.03% at 10 s and between 1.74 and 3.97% at 20 min. The shrinkage was particularly high for the flowable resin composites. Viscous flow varied significantly between 8.5 and 26.5 mm and was
Discussion
Analyzing longitudinal 5-year data from a multicentre clinical trial of Occlusin, Hayashi and Wilson found that marginal deterioration and cavomarginal discoloration observed at previous reviews were significantly correlated with the failure rate at the 5-year review.12 This finding that marginal deterioration was a sensitive predictor of failure of a posterior composite corroborates the importance of minimizing gap formation. The extensive survey of Hickel and Manhart on the longevity of
Acknowledgments
The authors would like to thank Degussa-Hüls, Heraeus-Kulzer, Kerr, 3M Espe, and Vivadent for supplying materials for this study.
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