Developing a more complete understanding of stresses produced in dental composites during polymerization
Introduction
The polymerization of dental composites is accompanied by a volumetric contraction, typically on the order of 1.5–5%, which results in the development of internal stresses. The stresses are a product of the non-yielding, or rigid nature of the reinforced cross-linked polymer network formed in the reaction. The stresses have been implicated in the imperfect margins formed around composite restorations, which may lead to a reduced service life. The generation, measurement and characterization of these stresses has been the subject of numerous investigations in the past 35–40 years, beginning with studies by Bowen [1], [2] and proliferating after the appearance of work by Davidson et al. [3] and Feilzer et al. [4]. The origin of the stresses, the means for measuring them, the relation between the stresses and clinical outcomes, the clinical factors that affect stress generation, and strategies to reduce them, have all been investigated. The objective of this article is to provide an overview of these topics, placing special emphasis on the studies originating in our laboratory.
Section snippets
Origin of stress in polymerizing dental composite
Dental composites harden by a chemical reaction involving the splitting of carbon–carbon double bonds on individual monomer molecules and the formation of carbon–carbon single bonds to form polymer chains. This reaction, which involves the evolution of a significant amount of heat, causes a volume reduction as covalent bonds are created and molecular distances and free volume are reduced.
Several factors contribute to the production of the stresses in resin composites during curing. The
Methods for measurement of stresses in composites
Numerous methods have been proposed for measuring or estimating the stress produced in dental composites during polymerization contraction. The most common method has been the use of a force transducer to record forces, typically uniaxial, from a composite disk or cylinder [1], [3], [4], [11], [12], [13], [14], [15], [16]. Though the premise underlying each of these testing methods is similar, the results for similar materials may vary greatly due to differences in the testing configuration,
Relationship between contraction stress in dental composites and outcomes
There are no data correlating contraction stress in dental composites and clinical success. Due to the complex nature of the residual stress in a composite restoration, it is actually unlikely that such a correlation can be made. Versluis et al. [25] have recently suggested that ‘shrinkage stress could not be expressed into a single average value based on composite properties or restoration configuration alone, but had to be approached as a distribution that depended on the location and
Factors affecting contraction stress in dental composites
Many factors affect the development of contraction stress in dental composites. These can be separated into material formulation factors (filler content, monomer chemistry, monomer structure, filler/matrix interactions, additives, etc.) and material polymerization factors (polymerization rate, i.e. catalyst and inhibitor concentration, external constraint conditions, cavity geometry, curing method, placement technique, etc.). Numerous articles have been written describing these factors, and
Strategies to reduce contraction stress in dental composites
Numerous strategies have been developed in an attempt to reduce the stress developed in dental composites as a result of polymerization shrinkage. These strategies involve modifications of the formulation of the material or the curing scheme. In our lab, we have completed studies on the inclusion of non-bonded silica nanofillers, polyethylene spheres, and multifunctional monomers to provide stress relief during curing contraction. In other studies, we have tested the effects of increased
Summary
While this paper has attempted to describe many of the factors that affect the development and management of internal stresses produced in dental composites during polymerization, it is by no means a complete treatise on this complex issue. One of the major difficulties presented during the study of this phenomenon is the control of the important variables. While it is known that DC, elastic modulus, curing rate and the boundary conditions all significantly affect the magnitude of the
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