Influence of C-factor and Light-curing Mode on Gap Formation in Resin Composite Restorations
To investigate the influence of the C-factor (Cf) and light-curing mode (LCM) on gap formation in resin composite (RC) restorations. Cylindrical Class I cavities with a 5.0 mm diameter and three different depths (1.0, 2.0 and 3.0 mm) were prepared in the occlusal surfaces of 60 human molars and restored with P60 (P) and Supreme (Su). RCs were light-cured in accordance with two modes: Standard (S)—850mW/cm2/20 seconds and Ramp (R)–100 up to 1000mW/cm2/10 seconds +1000mW/cm2/10 seconds. After storage in distilled water, the restorations were cut into three slices and the gap widths were analyzed in a 3D-scanning system. The data were analyzed by ANOVA and Student-Newman-Keul's test (α=0.05). ANOVA detected significant influence for the RC, Cf and LCM independent factors and for the double interactions RC vs Cf and LCM vs Cf. Smaller gap formation was found for cavities restored with Su. R was responsible for the smaller gap formation. The highest gap formation was found for cavities with Cf=3.4, followed by Cf=2.6 and 1.8 without statistical differences between them. These findings suggest that Cf played an essential role in gap formation. R LCM may allow RC relaxation during polymerization reaction. Finally, nanocomposites (Su) may lead to less gap formation at the resin-dentin interface.SUMMARY
INTRODUCTION
The optical characteristics and mechanical properties of light-curing resin composites are responsible for their safe use in both anterior and posterior restorations. Nevertheless, one of the problems that could interfere with their clinical performance is the shrinkage stress generated during their polymerization reaction.1
Rupture of the double carbon bonding of methacrylate monomers present in polymeric matrixes results in a reduction of 0.3 to 0.4 nm of the space maintained between polymer ranges by Van der Waals attraction forces and establishes 0.15 nm-long covalent bondings.2 As a result, the material undergoes a decrease in volume, which can be interpreted as densification.3 The polymerization shrinkage generated could lead to gap development, fluid penetration and bacterial presence at the tooth/composite interface and to post-operative sensitivity.4–5 Variables, such as resin monomer, type and concentration of filler particles and photo initiators, influence this phenomenon.6 One of the most promising contributions to the restorative material field currently is the use of nanotechnology in new resin composite formulations. Obtaining materials with a greater amount of charge decreases polymerization shrinkage.7
Clinically, light-curing mode and energy density provided by the light source can influence polymerization shrinkage stress developed by the material.8 Activation with a high intensity light creates a rapid light-curing process, leading to higher shrinkage stress in the composite.9 On the other hand, although activation with lower light intensity may reduce leakage development at the tooth-restorative material interface,10 it could affect the degree of conversion and mechanical properties of composites.11 Several recent studies have shown that the use of techniques in which the composite is first submitted to low light irradiance, followed by an increase in light intensity, are able to promote decreased shrinkage stress without interfering with the degree of conversion and mechanical properties of the material.12–13
The cavity shape is considered to be of great importance in conserving the composite-dentin bond.1 Feilzer and others established the configuration factor concept (C-factor=bonded to unbonded surfaces), and it was demonstrated that, in most of the clinically relevant cavity configurations, the stress relieving flow is not sufficient to preserve adhesion to dentin by dentin-bonding agents.14
Considering the importance of the multifactorial aspect of the polymerization shrinkage phenomenon, the current study investigated influence of the C-factor and light-curing mode on restoration interface sealing using light-curing resin composites.
METHODS AND MATERIALS
Two commercially available resin composites, chosen in accordance with their different types of filler particles, were tested: a mini-filled hybrid resin composite (P60 [P]) and a nanofilled resin composite (Supreme [Su]). Both materials have the same polymeric matrix. Their compositions are described in Table 1.
All the specimens in the current study were light-cured with a quartz-tungsten-halogen unit (Optilux 501, Kerr, Danbury, CT, USA). Two light-curing modes (LCM) were used: Standard (S)–850mW/cm2 for 20 seconds (17J/cm2) and Ramp (R)–100 up to 1000mW/cm2 for 10 seconds + 1000mW/cm2 for 10 seconds (≅17J/cm2). The energy density (ED) was calculated by using a radiometer (Demetron Inc, Danbury, CT, USA). For the R mode, ED was obtained by the sum of mean irradiance over the first 10 seconds multiplied by 10 seconds with 10J/cm2, corresponding to the ED in the last 10 seconds of light exposure.
Selection and Preparation of Teeth/Restorative Procedure
The gap analysis steps are illustrated in Figure 1. Sixty human molars free of structural defects (0.5% chloramine solution/15 days) were used in the current study. All occlusal surfaces were wet ground in a polishing machine with 150 and 600-grit SiC papers until flat dentin surfaces were obtained. The roots were embedded in polyester resin inside PVC cylinders (0.5 inch in diameter) with the flat dentin surfaces parallel to the cylinder borders. The cylinders were fixed in a special sample aligning device, and cylindrical Class I cavities 5.0 mm in diameter having three different depths (1.0, 2.0 and 3.0 mm) were prepared in all flat dentin surfaces with a diamond bur (#4054, KG Sorensen, SP, Brazil) in a high-speed handpiece. The cavity depths were controlled by using a digital caliper (MPI/E-101, Mitutoyo, Tokyo, Japan). The C-factor (Cf) was obtained using the following formula:
where r is the cavity radius and h is the cavity depth. Therefore, Cf for the three cavities was: A=1.8; B=2.6 and C=3.4.
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
The cavities were bonded with the Single Bond2 adhesive system (3M ESPE, St Paul, MN, USA) in accordance with the manufacturer's instructions and restored in bulk with P or Su. The resin composites were covered with a polyester strip and light-cured. Twelve experimental groups, in accordance with resin composite LCM and Cf, were produced (n=5). After storage in distilled water (37°C/seven days), finishing and polishing procedures were performed with sequential Sof-Lex discs (3M ESPE). The restorations were cut into three slices in a buccolingual direction and spaced around 1.0 mm using a diamond disk (KG Sorensen, São Paulo, SP, Brazil). The sectioned surfaces were embedded in poly-ester resin, polished with 600/1200-grit SiC abrasive paper and sonicated in distilled water for five minutes.
Gap Measurement/Statistical Analysis
The sectioned surfaces were analyzed in a 3D-scanning system (Talyscan 150, Taylor Hobson, Leicester, England). The GAP width was analyzed in 10 different positions for each slice. The statistical analyses were done with Statgraphics 5.1 (Manugistics, Rockville, MD, USA). The data were analyzed by Multifactor ANOVA and one-way ANOVA with Student-Newman-Keuls' test for multiple comparisons (α=0.05).
RESULTS
The results are shown in Figures 2, 3, 4 and 5. Multifactor ANOVA detected a significant influence for the resin composite LCM and Cf independent factors (p<0.0001) and for the double interactions resin composite vs Cf and LCM vs Cf (p<0.0001). Conversely, no significant differences were found for double interaction resin composite vs LCM (p=0.2122) and for the triple interaction among the three factors studied (p=0.6731). Smaller gap formation was found for the cavities restored with Su. Regarding LCM, R was responsible for the smaller gap formation. The Student-Newman-Keuls' test showed that the highest gap formation was found in cavities with Cf=3.4, followed by Cf=2.6 and 1.8 without statistical difference between them (Figure 6).
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
Citation: Operative Dentistry 34, 5; 10.2341/08-035-L
DISCUSSION
Shrinkage stress generated during resin composite polymerization can be responsible for maintaining the interface tooth-restorative material and for the consequential failure of the restoration.1 Light-curing initiates the conversion of monomer molecules to a polymer network, a process that leads to resin composite shrinkage because of closer packing of the molecules and transformation of the resin composite from a viscous-plastic state to a rigid-plastic state.15 Initially, shrinkage stresses in a cavity are compensated by viscous flow of the resin composite, but within a very short time after light-curing begins, viscous flow is reduced and the resin composite starts to transfer stresses to the cavity walls.14–15
Filler content, type of organic matrix and flexural modulus have a direct influence on shrinkage stresses and marginal adaptation in cavities restored with light-curing resin composites.16 Due to the composition of the materials used in this experiment, a difference in gap width among composite factor groups was expected. In spite of their similar organic matrix composition, there are differences in filler content and flexural modulus (e) of P and Su(ep=11700 MPa and eSu=10500 MPa).17 This may justify the different behavior with regard to shrinkage stress and gap formation values that were observed (Figure 2). Su is a resin composite with nanometric filler that involves the lower e of the material and would possibly allow for greater reduction in shrinkage stresses during light-curing and a smaller gap formation. On the other hand, a resin composite with a higher e, such as P, would cause higher, and therefore more extensive stress formation. This assumption was confirmed in an in vitro study of the marginal integrity of Class V restorations.18 From another viewpoint, it is also possible that nanometric fillers promote a light scattering effect (similar to mini-filled particles) that would not permit light energy to activate the deepest resin composite layers.19 A recent study of the polymerization depth of dental composites observed lower microhardness values at a polymerization depth of more than three millimeters.20 This was explained by the light scattering effect that decreases the degree of conversion of the material. Further investigations are necessary to support this hypothesis.
The polymerization reaction comprises three phases: pre-gel, gel and post-gel. In the pre-gel phase, a viscous behavior is presented by the composite, and shrinkage stresses generated during the polymerization reaction can be released by the material flow.121–23 Polymer chains are distributed in a linear mode and have mobility that permits tensions induced by polymerization shrinkage to be dissipated by flowing.24 As the reaction progresses, the post-gel phase starts the first crosslinks between chains, making flow difficult and simultaneously promoting the increase of mechanical properties and e that involve inducing tensions in the restoration.24 But light-curing the material in accordance with protocols that use lower irradiance at the beginning can extend the pre-gel phase, allowing polymerization shrinkage stress relief.1025 Retarding the resin composite from reaching its gel point could explain the smaller gap formation when R polymerization is used (Figure 3).26 Previous studies have demonstrated that the highest stress development associated with polymerization shrinkage occurs during the first 30 to 40 seconds of light irradiation.1627 In agreement with these findings, Feilzer and others demonstrated that light-curing with low irradiance was related to better marginal sealing of cavities restored with light-curing resin composites.10 On the other hand, light-curing with low irradiance may promote a decrease in the degree of conversion and negatively influence the mechanical properties of the material.1328 Despite the differences in LCM, the final ED used in the current study for both techniques was the same: 17J/cm2.12–13 According to Rueggeberg and others, this ED would be adequate to light-cure restorative resin composites.11
In particular, influence of the confinement conditions imposed on the resin composite (usually expressed as the bonded to unbonded ratio, known as Cf) plays an essential role in gap formation.14 In the current study, cylindrical cavities with different Cfs were produced by varying the depth and keeping to the same diameter. The results obtained demonstrated that high Cf values (3.4) indicated high gap formation (Figures 4 and 6). This can be explained, because the stress relieving flow was not sufficient in this case to preserve adhesion to dentin by dentin-bonding agents. On the other hand, lower Cf values (1.8 and 1.6) permitted more resin composite relaxation.22 High gap formation values were commonly observed at the internal angles in all groups tested, and this could be related to high shrinkage stress in these areas.29 As shown in Figure 6, in the cavo-surface region, restoration sealing was improved by the proximity of free surfaces that allowed stress relief by resin composite flow.2229 This explains why gap measurement was analyzed only on the cavity floor. In microleakage studies, cavity depth was found to have a stronger influence than diameter.29 In accordance with these findings, the current study always used the same diameter and different depths to create experimental groups with different Cf values.
In summary, based on the results obtained in this in vitro study, which simulated clinical restorative procedures in a tooth cavity, gap formation is a multifactor phenomenon that depends on several intrinsic factors315 related to restorative materials and extrinsic factors, such as Cf14–16 and LCM.1013 Furthermore, it is also important to study other factors, such as the incremental technique and use of liner materials in order to improve restoration sealing.
CONCLUSIONS
Considering the limitations of this in vitro study, it was possible to conclude that:
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High C-factor values produced the highest gap formation.
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The ramp light-curing mode was efficient for reducing gap formation.
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Nanofilled composite was more efficient for cavity sealing than mini-filled hybrid composites.
The results suggest that further investigations should be conducted in order to promote the better sealing of cavities restored with light-curing resin composites.
Schematic illustration of the gap analysis.
Mean gap width for independent factor resin composites (vertical bar represents the standard deviation).
Mean gap width for independent factor LCM (vertical bar represents the standard deviation).
Mean gap width for independent factor Cf (vertical bar represents the standard deviation).
Mean gap width for all experimental groups (vertical bar represents the standard deviation). Columns under the horizontal line are not statistically different (p>0.05).
Representative image of gap measurement (cavities with Cf =1.8, 2.6 and 3.4) (D=dentin; CR=composite resin; and I=interface).
Contributor Notes
Gustavo Oliveira dos Santos, Department of Restorative Dentistry, School of Dentistry, State Rio de Janeiro University, Rio de Janeiro, Brazil
Maria Elisa Oliveira dos Santos, Department of Dental Materials, School of Dentistry, Federal Fluminense University, Rio de Janeiro, Brazil
Eduardo Martins Sampaio, Department of Mechanical Engineering, School of Engineering, State Rio de Janeiro University, Rio de Janeiro, Brazil
Kátia Regina Hostílio Cervantes Dias, Department of Restorative Dentistry, School of Dentistry, State Rio de Janeiro University, Rio de Janeiro, Brazil
Eduardo Moreira da Silva, Department of Restorative Dentistry, School of Dentistry, Federal Fluminense University, Rio de Janeiro, Brazil