It is well known that some important, clinically-relevant physical and mechanical properties of resin composite such as marginal adaptation, hardness, strength, as well as wear resistance depend not only on material itself, but also on clinical manipulation[9-10]. Several studies found no statistical difference among polymerization techniques was observed for gap formation[11-12], some other studies compared soft with hard curing modes and reported that although the high-intensity curing lights provided rapid curing to resin-based composite increments, the two-step curing cycle (a period of low intensity followed by a period of high intensity) afforded significantly better marginal integrity[13-15]. The investigators deemed that the so-called “soft-start polymerization” characterized by using an initial low-power intensity of the curing light followed by higher-power intensity minimized internal stresses in composite and improved its marginal adaptation[5, 16]. This initial low-light intensity could facilitate a certain degree of polymer chain reaction so that a portion of shrinkage stress relaxes while the resin reaches its final hardening[6, 17-20]. In this study, the method that measuring the width of margin gap between the mold and specimen with scanning electron microscope is a commonly used way to explore the marginal quality[21-22]. Although the interface between composite and the mould of the specimen is not the same as the clinical interface between composite and the dentine/enamel, the condition of the interfaces of these two mode is the same. The polymerization of composite is the result of two photoactivation modes just with equal working time, but different light intensity. The present results of marginal adaptation are consistent with the findings of these investigations. However, the effect of soft-start curing mode may be material-depended. For Filtek P60 and Tetric Ceram HB, the soft-start curing mode afforded significantly higher marginal integrity, but for Ecusphere-Carat, no statistical difference between polymerization techniques was observed.
Hardness is the resistance of material to indentation and it correlates well to the material's strength and rigidity[20]. Vickers hardness testing was selected for this study because of its relative simplicity and favorable correlation to the degree of conversion[23]. Santos et al[24] found a correlation between the increase in surface hardness and an increase in the degree of conversion of resin materials. In the present study, the top hardness values of soft-start mode were significantly lower than the values of standard mode. Polymerization of resin associated with the initial cure may be sufficient to interfere with low-power intensity light transmission, but severely decrease the total amount of light reaching the surface of composite restorations in the same working time, compared to the standard mode[19]. The polymerization degree was degraded while using soft-start mode, the microhardness was degraded accordingly. Polymerization degree will raise while the amount of curing light that composite receive, and the amount of curing light depends on curing time and the light intensity. So some authors suggest that it should prolong the working time or raise the light intensity in the followed period to increase amount of the light, to raise the degree of polymerization and the hardness[10, 15]. In the polymerization, shrinkage stress will rise when the degree of conversion is increased[6]. The shrinkage stress will be kept in the restoration that destroys the restorations, or the integrity of the composite/tooth interface. This study did not design such an analogical experiment to explore the change of marginal gap and the hardness. That was the shortage of this investigation. In addition, the author found that the contrast of the surface hardness for the same material between these two curing modes was limited, but the contrast of the hardness among these three composites was much more significant. The effectiveness of curing modes to change the hardness was finite, and inferior to the materials themselves. The effect of materials themselves is the determinative factor for hardness, rather than the curing modes. It suggests that effectiveness of using different curing mode to elevate the hardness of the composite is limited.
Ideally, the degree of polymerization of composite should be the same throughout its depth, the hardness ratio between top and bottom surfaces should be very close or equal to each other. The results of this study showed that the hardness of bottom surface of composite resin cured by both standard and soft-start mode was significantly lower than that of the top surface. This might be resulted from the change of polymerization degree with curing depth. As light passes through the composite, the light intensity is greatly reduced due to light scattering. On the other hand, when the distance between the light tip and composite resin was increased, the depth of cure was decreased. All these factors decreased the cure effectiveness and polymerization degree of the resin composite at the bottom surface[25-26] and subsequently result in the decrease of bottom microhardness.
In general terms, the mechanical properties of composite show a significant correlation with the filler fraction[27]. Manhart et al [28] has found that the Vickers hardness numbers of the tested materials showed a positive correlation with the filler weight percent. In our study, no significant difference was observed for hardness between Tetric Ceram HB and Ecusphere-Carat though the filler load levels of Ecusphere-Carat was lower than Filtek P60 and Tetric Ceram HB (Filtek P60 84 wt%, Tetric Ceram HB 81 wt% and Ecusphere-Carat 77 wt%). Filtek P60 exhibited the highest hardness values. This seems to attribute to its filler load levels and inorganic filler of zirconium.
For this investigation, although scanning electron microscope and Vickers hardness testing are common methods used for resin composite to evaluate the polymerization shrinkage and mechanical properties, there are limitations for them. They are indirect methods, and the result contains randomness. For example, if the sites of the scanning electron microscope change, the width of marginal gap will change correspondingly. The filling moulds were not made with human tooth, and the condition was isolated, too. The investigation did not contain the study of microleakage or other relative study such as using FTIR analysis for the degree of conversion, so the evidence for the conclusion of soft-start for reducing the marginal adaptation may be not enough. So we deem that further investigation is needed for the effect of packable resin-based composites on tooth/restoration marginal integrity using different curing methods and curing time.
Within the limits of the present study the following can be concluded: the soft-start polymerization technique improve the marginal adaptation of packable resin-based composites, and decreases top surface hardness.
