Determination of polymerization shrinkage stress by means of a photoelastic investigation
Introduction
Photopolymerizable composite resins are used worldwide as well established restorative systems.1 Despite successful improvements in adhesive systems, the problem of tight interfaces between cementum or dentin and adhesive filling materials appears to remain unsolved because of the continuing existence of polymerization shrinkage in composite resin materials. The polymerization shrinkage of dental composite resins is influenced by numerous parameters affecting the polymerization shrinkage itself or the polymerization shrinkage stress. Each of these parameters has been subjected to alterations, improvements or innovations during the last decade.
Several attempts have been made to minimize polymerization shrinkage by altering the filler load. Aw and Nicholls2 showed a moderate correlation between filler volume and shrinkage. But they also came to the conclusion that other factors such as filler size and resin chemistry may also affect shrinkage. A higher filler load and therefore a reduction in matrix does not automatically result in less polymerization shrinkage stress.3 Different packable composites had maximum force rates that were statistically similar to the conventional hybrid composite control. Price et al.4 showed a significant influence of the filler weight on polymerization shrinkage. Therefore, the filler load may have a positive influence on polymerization shrinkage, but may on the other hand affect polymerization shrinkage strain negatively because of the higher modulus of highly filled composite resins.
Another attempt to reduce the polymerization stress was by influencing the stress development with soft start or pulse-delay curing techniques to prolong the setting of the gel-point.5., 6., 7., 8., 9., 10. But curing technique will only have a limited influence on polymerization shrinkage stress; if the material is cured properly, the curing technique will not significantly affect polymerization shrinkage.4 A positive influence of curing technique on polymerization shrinkage itself is very often due to a lower degree of conversion. This observation may lead to a misinterpretation of positive results in investigations on polymerization stress or marginal integrity!11 The degree of conversion and volumetric shrinkage shows a non-linear relationship with energy density and a pronounced influence on stress.12 Therefore, information on depth of cure or degree of conversion is crucial for providing truthful statements on shrinkage stress influences of curing techniques.5., 13., 14. Other authors completely deny the possibility of even reducing polymerization shrinkage strains with particular curing modes.15
Several investigations show that the configuration of the cavity has an influence on shrinkage stress development.16., 17., 18., 19., 20. It is discussed that the way composite resins are applied can lower the C-factor of composite resin increments17., 21. and therefore polymerization shrinkage stress. But those results often differ from finite element models, showing lowest maximum normal transient stresses in bulk fill class V cavities compared to three horizontal increments and three wedge increments.22 Therefore, the advantages of an incremental filling technique in polymerization stress reduction are discussed controversely.23
Since the claims that the effect of influencing polymerization shrinkage strain by altering curing techniques had only little or no effect, investigations were made with new monomers providing less shrinkage and a lower modulus.15 These requirements and announcements have existed for decades,24., 25. and new monomers like stereoisomeric alicyclic spiroorthocarbonates, which expand during polymerization, or other cyclopolymerizable monomers have been introduced.26., 27., 28.
Several efforts have been made to reduce polymerization shrinkage by substituting high shrinkage monomers such as TEGDMA with different new and experimental comonomers, providing lower polymerization shrinkage29 or by synthesizing other new monomers in the meantime.30 At the present time, the most promising technology for the reduction of polymerization shrinkage is silorane technology.31
The determination of polymerization shrinkage is influenced by a multitude of factors; even the storage temperature and storage time as well as the testing temperature have a significant influence on polymerization shrinkage.32 A modified mercury dilatometer33., 34. was used for numerous investigations on polymerization shrinkage.18., 32. Measurements of polymerization shrinkage in water instead of mercury were carried out by several authors with a water dilatometer.35., 36., 37. Goldman35 himself claimed that the verification of water absorption in the test material was one of the main problems in his investigation. Rees and Jacobsen37 solved the problem of a possible water uptake during the test period in their water-filled dilatometer by taking the diffusion coefficient of the materials into consideration. The main problem of those devices remains the possibility that the resin sample may not polymerize completely in these devices33., 38. or may absorb water during the polymerization process.35., 36. The same problem occurs with methods for the determination of specific densities in pycnometers or hydrometers.39., 40., 41., 42. On the other hand, Rueggeberg and Tamareselvy40 found no difference in polymerization shrinkage data obtained with an infrared spectrometer.
Other described methods are a laser interferometric method for measuring linear polymerization shrinkage in light-cured dental restoratives,43 laser beam scanning,44 or the use of a gas pycnometer.45
Attempts to measure the polymerization shrinkage stress5 address the demands of the clinical situation better because of the fact that it is not shrinkage itself but rather polymerization shrinkage stress that actually affects the bonded interface to the tooth structure. Feilzer et al.38 introduced a system for evaluating polymerization shrinkage with a wall-to-wall curing contraction stress. The comparison between the testing device and clinical situations was difficult because the testing device had restrained axial walls whereas clinical situations have stable cavity walls. The linometer, introduced by De Gee et al.46 is a modification of the wall-to-wall curing contraction device of Feilzer et al.,38 in which the chamber of the dilatometer is equipped with air vents to prevent excessive lift of the disc. De Gee et al.46 found no statistical differences in polymerization shrinkages measured with the linometer or with the dilatometer. The devices used by Bouschlicher et al.,21 Davidson et al.16 and Feilzer et al.17 were almost comparable to the tensometer. Therefore, their results are influenced mostly by the polymerization shrinkage stress. The authors, mentioned above, mostly determined polymerization shrinkage stress, even if they often named the determinant investigated in their papers as polymerization shrinkage.
A very suitable method to determine polymerization shrinkage stress using a deflecting disc method was introduced in 1991 by Watts and Cash47 and was also used by other investigators.48 This method combines the measurement of the volumetric polymerization shrinkage with a strong influence of the polymerization shrinkage stress.
So far, photoelastic methods49 were used to investigate polymerization shrinkage stress development.50 However, in their investigation a particular transparent composite resin was used to visualize the development of the observed strain. A transparent material is crucial for obtaining photoelastic images. Therefore, this method was not suitable for the investigation of different composite resin materials with different translucencies. The purpose of this study was also to determine the polymerization shrinkage stress out of visualized polymerization strain with a photoelastic investigation, but in contrast to the investigation of Kinomoto and Torii,50 a photoelastic material was employed as the cavity material, which allows the use of real resin composite restoratives for the photoelastic investigation of polymerization shrinkage stress.
Section snippets
Materials and methods
Conventional PMMA acrylic materials using a photoelastic constant of fσ=230 N/mm51 has been used in a photoelastic investigation of visible light curing.49 Specially designed photoelastic materials with a lower photoelastic constant (fσ=10.5 N/mm) and a modulus of 3.400 MPa51 such as Araldit B (Tiedemann & Betz GmbH, Garmisch-Partenkirchen, Germany) allow for the determination of isochromatic rings, which visualize the strain in the material. From this measured strain, stress values (MPa) are
Results
In this photoelastic investigation, polymerization shrinkage stress was determined at 4 min and 24 h post exposure. Right after exposure (after 4 min), the mean polymerization stress values calculated were 3.8±0.1 MPa for Tetric Ceram, 3.9±0.1 MPa for EsthetX, 3.1±0.1 MPa for Z 250, 4.1±0.2 MPa for Clearfil AP-X, 3.5±0.1 MPa for Prodigy Condensable, 3.3±0.1 MPa for Filtek P 60, 3.6±0.1 MPa for Surefil, 3.9±0.1 MPa for Clearfil Photo Posterior, 4.4±0.1 MPa for Solitaire 2, 2.7±0.1 MPa for In
Discussion
Photoelastic images are suitable to visualize contraction strains resulting from polymerization shrinkage of resin-based composites.49., 51. In obtaining data, a complete cavity design cannot be used because of the varying thicknesses of the photoelastic material in different parts of the cavity.51 By cutting samples out of an entire cavity, artifacts may arise at the interface between the restorative and the photoelastic material. Therefore, a continuous flat cavity design was selected for
Conclusion
This study demonstrates the advances in reducing polymerization shrinkage strain by the use of new monomer compositions or modifications of the filler and monomer ratios. This finding does not mean that those early experimental materials will serve as appropriate restorative materials for dentistry due to the fact that other factors beyond shrinkage stress are important, such as physical properties, biocompatibility and handling properties, all of which have to be considered. From the chemical
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