INTRODUCTION

Although adhesive restorative techniques have evolved in the past few years, the polymerization shrinkage of resin composites still poses a challenge to the bonded interface. Because the reaction takes place under confinement, the shrinkage inherent to conversion transfers stress to the adhesive layer,1 which may lead to cuspal deflection,2 tooth fracture3 or gap formation.4 Moreover, once rigidity starts to build up within the material, the magnitude of such stresses increases exponentially.5 Indeed, the correlation between the elastic modulus and polymerization stress has been well established.6

The reduction of the rate at which the reaction develops facilitates chain rearrangement due to flow before the material becomes a viscoelastic solid. Consequently, stress buildup within the composite will be lower, improving the marginal adaptation and longevity of the restoration. One way to obtain this effect in reaction kinetics is through the use of light attenuation, because the low photon flux will generate fewer polymerization growth centers.7 This is the principle of alternative photoactivation protocols, generally referred to as “soft-start” methods, which consist of an initial low irradiance followed by a second exposure with higher output to complete the radiant exposure so that the conversion is not jeopardized.8 Indeed, some studies have shown reduced marginal leakage with the use of ramp curing (150 to 800 mW/cm² in the first 15 seconds of a 40-second exposure9) or soft-start curing.10 However, some authors reported that composites photoactivated with a pulse-delay method (five seconds at 150 mW/cm², followed by 19 seconds at 935 mW/cm²) were more prone to degradation in ethanol when compared to the control specimens cured in continuous mode.11 This was credited to reduced crosslink density,12 which may negatively affect mechanical properties.13 In some studies, the stress reduction achieved with these photoactivation methods was associated with a reduced degree of conversion.14–15 On the other hand, stress reductions of up to 34% were reported with the use of pulse-delay methods816 without compromising microhardness before and after immersion in ethanol.16

Reductions in the irradiance reaching the composite were also observed in clinical practice for deep preparations, where the light guide is hardly ever in close contact with the first increment of the composite. As demonstrated by Hansen and Asmussen,17 the distance between the light guide and composites can be greater than 7 mm. Indeed, in a study using FTIR, increasing the distance between the light guide and composite from 0 to 10 mm caused a 23% reduction in irradiance related to a significant decrease in conversion on the bottom of 2-mm thick composite specimens.18 However, some authors did not find differences in conversion19–20 or microhardness,21 regardless of the distance between the light guide and composite.

In clinical practice, the soft-start methods are likely to be used to photoactivate composites in preparations with very distinct sizes and geometries. In the literature, however, the studies that analyzed low irradiance techniques generally performed light curing with the composite in direct contact with the photoactivator tip, ignoring the conjugated effect of these two light attenuating forms on the restorative cavity. For recent commercial composites in use, requiring a relatively low radiant exposure to reach a high degree of conversion, the association of soft-start methods and light distancing may reduce polymerization stress in a manner that these attenuating methods cannot promote separately, with a light dosage sufficiently low to modify reaction kinetics but enough to guarantee composite cure. Therefore, the current study evaluated the degree of conversion achieved with a micro-hybrid composite photoactivated using three different methods from three levels of distance. Since a correlation between polymerization stress and microleakage was established in some studies,22–23 the current study evaluated microleakage in restorations with the same conditions applied to the degree of conversion test as an in vitro predictor of the effects of stress development on marginal integrity. The tested hypotheses were: 1) the photoactivation methods (continuous or stepped) do not affect the degree of conversion; 2) microleakage is lower for the stepped method and 3) increasing the distance between the light guide and composite reduces the degree of conversion and microleakage.

METHODS AND MATERIALS

In all tests, a micro-hybrid composite (Esthet-X; Dentsply Caulk, Milford, DE, USA; batch #0109266, shade A2) was used and photoactivated with a quartz-tungsten-halogen light curing unit (VIP—variable intensity polymerizer; BISCO Inc, Schaumburg, IL, USA). Irradiance was checked daily with a built-in radiometer and with the Model 100 Optilux Radiometer (SDS Kerr, Danbury, CT, USA). The composites were photoactivated according to the protocols described in Table 1 at 0, 3 or 7 mm from the light guide. These distances were standardized with the use of opaque ring spacers placed between the light guide and the Mylar strip that covered the material.

Table 1 Groups According to the Photoactivation Method

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RESULTS

Results for the degree of conversion tests are shown in Tables 2 and 3. Two-way ANOVAs were performed for data obtained from the top and bottom surfaces. For data collected at the top of the specimen, only the pooled average of 7 mm distance was significant (p=0.015). When data was pooled for this factor, the degree of conversion was statistically lower for the 7 mm groups compared to the 0 and 3 mm groups (Table 2), which were equivalent to each other. For data collected at the bottom of the specimen (Table 3), none of the factors or the interactions was significant (p>0.05). T-tests performed for degree of conversion achieved in analogous groups at the top or bottom of the specimen were not able to find significant differences (p>0.05).

Table 2 Degree of Conversion (mean and standard deviation in %) for All Experimental Groups Assessed at the Top of the Specimen

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Table 3 Degree of Conversion (mean and standard deviation in %) for All Experimental Groups Assessed at the Bottom

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Figure 1 illustrates microleakage scores for the nine experimental groups. The Kruskal-Wallis test was able to detect statistical differences among the groups at a global level of significance of 1%. For the highest irradiance (600mW/cm²), a significant reduction in microleakage was observed when the distance from the light guide increased to 3 mm (p<0.05), which was statistically similar to the results observed at 7 mm. For the intermediate irradiance (400 mW/cm²), the microleakage was similar for all light guide distances (p>0.05). When the soft-start method was used, no statistical difference was observed between the groups irradiated at 0 and 3 mm and both presented higher scores than the group irradiated at 7 mm (p<0.01). Comparing exposure protocols within the same irradiation distance, no statistical differences were observed among groups at 3 mm (p>0.05). When the 0 mm groups were considered, soft-start irradiation caused a decrease in microleakage scores when compared to the continuous high irradiance mode (p<0.05). The 400 mW/cm² irradiance showed results similar to both. At 7 mm, a significant reduction in microleakage scores was only observed with the soft-start method (p<0.01).

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Figure 2 shows that microleakage and degree of conversion at the top of the specimen presented a positive linear correlation (r²=0.5884), while the correlation with the bottom results was only poor (r²=0.1713).

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DISCUSSION

As the distance between composite and light guide increases, severe light attenuation is observed,24 which may jeopardize the extent of polymerization.18 A number of studies have indirectly analyzed the effect of light guide distance on marginal integrity through the observation of microleakage at the cavosurface margin in the proximal box of Class II restorations.25–26 However, direct attempts to correlate this important clinical aspect to the degree of conversion have not yet been made. The current study proposes a systematic approach to verify how a controlled increase in this distance influences the degree of conversion and microleakage in a composite restoration material. Moreover, the influence of different photoactivation protocols in relation to the increase in light guide distance was also evaluated, both on the top and bottom of 2-mm thick specimens.

The distance between the composite and light guide significantly influenced the degree of conversion only at the irradiated surface (p<0.05). When the composite was photoactivated at 7 mm, the irradiance decreased 40% on average (Table 2), causing the conversion to decrease, as previously demonstrated.27–28 The decrease in irradiance at 3 mm (around 15%) was not enough to produce deleterious effects on the conversion. Regarding the photoactivation protocols, no statistical difference was observed in conversion, which is probably due to the fact that the radiant exposure employed in all methods was similar, within the same distance from the light guide. In fact, it has been reported that, at the range of irradiance tested in the current study, the conversion is determined by the radiant exposure.29 Other authors have demonstrated that the reciprocity between irradiance and exposure (in seconds) does not always hold true.30 In their study, it was demonstrated that irradiances higher than 1000 mW/cm², combined with exposure times shorter than five seconds, resulted in composites with degree of conversion and mechanical properties inferior to those obtained at 600mW/cm² × 40 seconds.30 In the current study, intermediate irradiances were used to assure even polymerization, which appears to have been the case, since the conversion did not change regardless of the protocol used.

At the bottom of the specimen, the light attenuation observed with the 7 mm distance did not influence the degree of conversion, unlike what was observed at the irradiated surface (Table 3). Although a t-test applied within the same group to the top and bottom results was not able to find statistical differences between them, a trend towards lower conversion was observed at the bottom. It can be hypothesized that light scattering through the composite and the shift in the material's refractive index during polymerization31 leveled off the irradiance at the bottom.

The microleakage test revealed a trend for reduced dye penetration at the groups irradiated at 7 mm when compared to groups irradiated at 0 mm, regardless of the photoactivation protocol (Figure 1). At the irradiated surface, this can be explained by the reduced degree of conversion observed for the 7 mm groups (in the pooled average). Indeed, the correlation plot drawn with these two parameters showed a positive linear relationship (r²=0.598). A lower degree of conversion is expected to be accompanied by reduced volumetric shrinkage,32 which, in turn, has been related to reduced polymerization stress.33 Since a positive relationship has already been established between microleakage and stress,23 it is possible that the reduced conversion led to reduced microleakage. However, another hypothesis may help to explain the reduced microleakage with the 7 mm groups. The reduction in irradiance achieved at this level might have caused the reaction rate to be slower, providing extended opportunity for chain rearrangement by flow, which relaxed some of the stress developed within the bonded interface.34 Indeed, the use of higher initial irradiances has been related to greater polymerization stress.35–36 If the irradiance is higher, a greater number of polymerization growth centers is activated at the same time, leading the reaction to auto-acceleration at an earlier time during conversion.37 The viscosity then increases earlier as well, which, in turn, reduces the possibility for chain rearrangement by flow, as previously mentioned.34

Regarding the different photoactivation protocols, except for the 3 mm groups, the microleakage was reduced with the soft-start methods in comparison to the higher irradiance (600 mW/cm²) continuous protocol. It is likely that, at 3 mm, the 15% reduction on the irradiance values was not enough to produce differences in microleakage as previously discussed for the degree of conversion results. However, comparing the results obtained with 0 or 7 mm, the reduction in irradiance from 200 mW/cm² to 130 mW/cm² (35%) on the first step of the soft-start method reduced the microleakage. Indeed, Lim and others12 suggested that irradiances no higher than 150mW/cm² should be used in pulse-delay methods. They found reductions of 26.5%, 15% and 0%, respectively, on polymerization stress with the use of 60, 100 and 150 mW/cm² irradiances, followed by a second pulse of 330 mW/cm² applied for 60 seconds, which led to a radiant exposure similar to the control group (±0.5 J/cm²). This could also explain why, at 0 mm, the irradiation at 400mW/cm² produced the same microleakage values compared to the soft-start technique. It is likely that the initial irradiance of 200 mW/cm² was already too high to allow for a prolonged viscous state in which chain rearrangement by flow would still be possible. Therefore, the light attenuation caused by a photoactivation protocol may be insufficient to promote better marginal adaptation, making necessary its association with light-tip distancing to reduce even more photon flux and the resulting polymerization stress. In a recent clinical trial, authors who also used 200mW/cm² as the initial irradiance for the soft-start technique did not observe a significant reduction in post-operative sensitivity or visible signs of marginal stress.38

The positive correlation between the degree of conversion and microleakage found for the irradiated surface (Figure 2) implies that the better performance on the microleakage test provided by the soft-start methods was due to a lower conversion. However, the reduction in conversion, despite being statistically significant, was only 4%. This may not be enough to jeopardize the mechanical properties of a composite restoration, as previously reported39 and still led to a reduction of 57.5% in microleakage.

The first hypothesis was confirmed, since the degree of conversion achieved by all methods was similar, provided that the radiant exposure was kept constant. The second hypothesis was partially accepted, as the reduction in microleakage achieved with the soft-start method was only observed for the 7-mm distance. The third hypothesis was also partially accepted, because the reduction in both the degree of conversion and microleakage depended on the irradiation method tested.

CONCLUSIONS

Within the limitations of the current study, it can be concluded that soft-start methods were able to reduce microleakage when the initial irradiance was low due to the 7-mm distance from the light source. This was accompanied by only a slightly lower conversion.