Residual shrinkage stress distributions in molars after composite restoration

doi:10.1016/j.dental.2003.05.007

Dental Materials

Volume 20, Issue 6, July 2004, Pages 554-564

https://doi.org/10.1016/j.dental.2003.05.007 Get rights and content

Abstract

Objective. Experimental measurements on various restoration configurations have shown that restored teeth deform under the influence of polymerization shrinkage, but actual residual stresses could not be determined. The purpose of this study was to calculate and validate shrinkage stresses associated with the reported tooth deformations.

Methods. Three different restoration configurations were applied in a finite element model of a molar. The composite properties were based on experimentally determined composite behavior during polymerization. The occlusal deformation pattern and the residual stress states of the tooth, restoration, and tooth-restoration interface were calculated using a polymerization model based on the post-gel shrinkage concept. Reported strain gauge measurements and occlusal deformation patterns were used for validation.

Results. The shrinkage stresses depended on the configuration and size of the restorations. The tooth's resistance against polymerization shrinkage diminished with loss of dental hard tissue. Larger restorations resulted in lower stress levels in the restoration and tooth-restoration interface, but increased stresses in the tooth. The maximum stress values found for different configurations were not decisively different.

Significance. The validated model indicated that shrinkage stress cannot be based on composite properties or restoration configuration alone, but has to be approached as a distributed pattern that depends on the location and on the properties of tooth and restoration, geometry, constraints, and restoration procedures. Tooth deformation was indicative of stresses in the tooth rather than in the restoration or across the tooth-restoration interface.

Introduction

It is widely accepted that polymerization shrinkage of current restorative composites cause residual stresses in restored teeth. These stresses are called residual because after curing a restored tooth is left under stress even when there is no functional loading. The presence of residual stresses results in a changed behavior of the restored tooth, which may become evident in its clinical performance. Clinical symptoms associated with residual shrinkage stresses are inadequate adaptation, microcrack propagation, marginal loss, post-operative sensitivity, microleakage, and secondary caries.1., 2., 3., 4., 5.

Stress is not a material property, but a local physical state that is derived from the combination of material properties, geometry, boundary conditions, and history. Since residual shrinkage stress is not a straightforward property that can be measured directly, its quantification in restorations remains a source of controversy. Various methods have been used to estimate residual shrinkage stresses, ranging from extrapolated shrinkage or load measurements in vitro to stress analyses in tooth shaped anatomies using photoelastic or finite element methods.6., 7. Calculation of shrinkage stresses in a tooth-restoration complex is not trivial. One challenge is the description of the sequence of changes that take place in composite during polymerization. Another challenge is that stress depends on the geometry and mechanical properties of surrounding tissues.

Many intricacies of the biomechanical manifestations of polymerization processes are still not well understood. As a result the development of a residual shrinkage stress model (i.e. the expression of our understanding of the event) is an interactive process with experimental observations. Deformation patterns of occlusal surfaces for restored molars due to shrinkage stresses have been determined and observed in a series of experiments.⁸ While deformation could be measured experimentally, the stress distribution must still be calculated using transient composite properties determined from another series of experiments.⁹ The purpose of this study was to develop and validate a polymerization model for the calculation of residual shrinkage stress distributions associated with the reported deformations, using the determined transient properties distributions. The validated model can consequently be used to investigate the shrinkage stresses in restored teeth, and study factors that affect them.

Section snippets

Material and methods

Deformations of occlusal surfaces have been reported for a successive range of cavity preparations in extracted human molars:⁸ Class I, small Class II OM, large Class II OM, and Class II MOD. The deformation due to polymerization shrinkage was quantified by comparing the digitized occlusal surfaces before and after restoration (Fig. 1). To calculate corresponding residual stresses in the tooth, a finite element simulation was carried out for a similar combination of geometry, boundary

Results

To validate the polymerization modeling procedures, deformation was calculated in two finite element simulations (strain gauge simulation and occlusal deformation of a tooth crown), using the equations for light intensity, elastic modulus, and post-gel shrinkage derived from experimental data.

The response of the strain gauge is not solely caused by post-gel shrinkage, but also by development of elastic modulus and distribution of properties due to the light attenuation in the composite. Results

Discussion

The large number of publications about polymerization shrinkage of restorative composites indicates that it is still considered a serious clinical concern. Shrinkage stress has been associated with clinical symptoms such as microfracture, microleakage and post-operative sensitivity. None of these symptoms, however, are direct measures of shrinkage stress. The alleged presence of shrinkage stresses is thus only known through indirect manifestations. One of them, tooth deformation, is often not

Conclusions

This study used deformation patterns—measured experimentally—to validate a biomechanical shrinkage model that was based on experimental polymerization data. Although more has to be done to unravel remaining less-understood polymerization sequences, the current study confirmed the validity of a linear elastic approach based on the post-gel shrinkage concept for the calculation of residual stresses in a restored tooth. It was shown that tooth deformation indicated the stress levels in the tooth,

Acknowledgements

Based in part on abstract No. 498, presented at the 80th IADR meeting in San Diego, March 6–9, 2002. The authors thank Iryna B. Olson for her assistance with the Cumulus analysis. This study was supported by the Minnesota Dental Research Center for Biomaterials and Biomechanics, a Faculty Development Grant from Chulalongkorn University, and NIH/NIDR Grant R01-DE12225.

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Residual shrinkage stress distributions in molars after composite restoration

Abstract

Introduction

Section snippets

Material and methods

Results

Discussion

Conclusions

Acknowledgements

Dent Mater

J Dent

J Dent

J Biomech

Dent Mater

Dent Mater

J Dent

J Am Dent Assoc

J Am Dent Assoc

Polymerization shrinkage and microleakage

Polymerization shrinkage of posterior composite resins and its possible influence on postoperative sensitivity

Quintessence Int

Effect of composite type, light intensity, configuration factor and laser polymerization on polymerization contraction forces

Am J Dent