Dentin-Composite Interfaces: Static and Viscoelastic Properties Measured with Nanoindentation

The biomechanics of tooth-resin based restorative material interfaces can be accurately characterized by nanoindentation; but to properly assess the reliability of the data collected and the comparison of that data with differing nanoindentation forces and roughnesses standards are necessary. The research presented within this paper tested the hypothesis that the force of nanoindentation and the surface roughness of the sample will affect the measured elastic modulus of the dentin-composite interface. Two different restorative adhesive systems were used to assess material-dependent effects. Four different surface roughnesses were evaluated by sequentially polishing each specimen with grit sizes of 6, 3, 1, and 0.05 µm. One-way ANOVA was used to test the effect of grit size on surface roughness (RA) and elastic modulus of the dentin, hybrid, adhesive and composite layers. Individual comparisons were made using post hoc t-tests with Bonferroni adjustments. Grit size significantly affected RA (p<0.001), where the average surface roughness decreased with decreasing polishing particle size and 0.05 µm grit producing the smoothest surface (RA=8.4 ± 4.0 nm); however, it was found that grit size non-systematically affected the elastic modulus (p<0.001) and that elastic modulus was not correlated to the average surface roughness. Based on these results, the final polishing step of 0.05 µm grit size was selected to test the effect of force indentation load on the measured elastic modulus. The force was varied with the following values: 100, 200, 300, 600, 1200, 1800, 2400, and 3000 µN. Here, it was found that indentation force had a significant effect on the elastic modulus (p<0.001); however, there were no significant differences between moduli measured with indentation forces of 600, 1200, 1800 or 2400 µN. Based on these findings, the force used for nanoindentation should range between 600 and 2400 µN. There has been very little research done to date on the viscoelastic properties of dentin and the dentin-composite interface in particular. The hypothesis tested is that the dentin-composite interface has viscoelastic characteristics. Initial research suggested that the boundary conditions imposed on the interface affected the measurements for the hybrid layer, thus each layer (dentin, hybrid, adhesive, and composite) was tested separately. Two different adhesive systems were used to minimize experimental error. Each layer showed viscoelastic characteristics, notably the storage modulus increased with frequency for all layers. The most significant change in storage modulus was between 1 and 10 Hz; however, for the loss modulus of dentin at 1 Hz, the modulus was 3.14 ± 1.39 GPa at 10 Hz the loss modulus dropped to 1.06 ± 1.45 GPa and then remained relatively constant. For the hybrid layer, the loss modulus at 1 Hz was 0.12 ± 0.08 GPa and at10 Hz, was 0.27 ± -0.16 GPa. For the adhesive layer, the loss modulus at 1 Hz was 0.29 ± 0.30 GPa and at 10 Hz was 0.64 ± 0.44 GPa. For the composite layer the loss modulus at 1 Hz was 3.64 ± -3.15 GPa and at 10 Hz was 7.58 ± -3.81 GPa. From this, it is clear that from 1 to 10 Hz the loss modulus for dentin dropped by about one third while that of the macro-hybrid layer, macro-adhesive layer and composite layer approximately doubled. For values larger than 10 Hz, the storage and loss modulus values generally increased at a small rate. From the loss modulus results, it appears that the viscoelastic response of the composite layers differ from that of the dentin, hybrid and adhesive regions.