Abstract
The current in vitro study evaluated the Vickers hardness number (VHN) and hardness ratio of four bulk-fill composites (VisCalor bulk; Admira Fusion x-tra; x-tra fil; and GrandioSO x-tra-Voco, Cuxhaven, Germany) to assess the risk of bacterial colonization in comparison with standard composite materials. Thirty samples were prepared for each group. The VHN of both the external (top) and internal surface (bottom) was determined with a micro-hardness tester (200 g load for 15 s), and the hardness ratio was also calculated for each sample. Subsequently, storage in an acidic soft drink (Coca-Cola, Coca-Cola Company, Milano, Italy) was performed; for each group, 10 samples were stored for 1 day, while another 10 were stored for 7 days and the remaining 10 were kept in water as controls. A significant reduction in VHN was shown for all the groups when comparing the external versus internal side (P < 0.05), although the hardness ratio was greater than 0.80, resulting in an adequate polymerization. Regarding the acid storage, all the groups showed a significant decrease of VHN when compared with the controls, both after 1 day (P < 0.05) and after 7 days (P < 0.001). All the products showed adequate depth of cure without further risk of bacterial colonization. However, acid exposure negatively affected micro-hardness values, which might promote subsequent colonization.
Highlights
Resin-based composites (RBCs) are nowadays the most used materials for the direct restoration of teeth, mainly due to their aesthetic properties, which are extremely appreciated when compared to those of other materials historically used in dentistry [1]
Descriptive statistics of the tested materials’ micro-hardness after light curing are shown in Table
Studies assessing the depth of cure are essential, even in the lower surfaces of the materials, as the cure of composite restorations can only be evaluated by clinicians in the upper surface that are directly exposed to the curing light [26,27]
Summary
Resin-based composites (RBCs) are nowadays the most used materials for the direct restoration of teeth, mainly due to their aesthetic properties, which are extremely appreciated when compared to those of other materials historically used in dentistry [1]. RBCs require an incremental apposition of 2 mm-layers, both to allow the curing light to penetrate the material and to contrast the shrinkage that takes place during polymerization [2]. In recent years, manufacturers have tried to develop new products requiring single-layer stratification, reducing the operative steps. Materials 2020, 13, 1308 introduced with this purpose, and according to manufacturers, can be used in a single step to fill cavities of a 4 mm depth or greater [3]. When choosing composite resins, clinicians should consider the risks of secondary decay and cytotoxicity that are still associated with them [4,5]
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