Comparative Effectiveness Research on Macro/nano Fillers in Dental Resins

Objectives: Dental composites consist of two phases: the synthetic polymer matrix and the inorganic filler. They play a significant role in improving the strength and elastic stiffness of the composite. A very high inorganic filler content can grant the proper degree of strength and hardness. The inorganic component is much harder than the organic network: a large filler content is, therefore, advantageous to enhance strength and stability and to reduce shrinkage in the polymerization. Innovative fillers have been considered for a long a very promising route to fulfill the thermomechanical requirements of resins for clinical use in dentistry. Most of these new fillers employ to some extent nanostructured characteristics that are expected to overcome previously adopted dispersions of macro and micro fillers.
Methods: Inorganic components are selected and treated in three categories: macroparticles, nanoparticles, and fibers. The Vickers index and the ultimate tensile strength are measured in samples prepared by the three groups and their suitable mixing. All the composites are prepared by the same protocol following the standard polymerization procedure in use in the clinical practice.
Results: Our interest to nanofillers has fallen on the use of titania and alumina since that have well-established applications in orthopedic surgery. The success is largely due to the excellent biocompatibility, the low friction, and a wear rate. The good properties of strength and durability can be achieved only when the grains/tubes are very small (<4 µm) and have a uniform distribution. Grains larger than 7 µm decrease the mechanical properties of about 20%. Titania acts in a very different route exploiting its needle-like shape that organizes itself in filamentary interconnections among other grains and the polymeric matrix. This mechanism doubles the Vickers hardness. Further, the final appearance of the polished surfaces is largely improved by the titania nanotubes.
Conclusions: The innovative fillers mostly based on nanoparticles and nanotubes are compared with the commercial resins in current use in various fields of odontoiatric practice. We emphasize the relevance of mixing, in particular in the attempts to mimic the behavioral properties of enamel, overcoming the limits of the present composites.