The mechanical anisotropy on a longitudinal section of human enamel studied by nanoindentation.

Nanoindentation has been widely used for probing the mechanical properties of tooth, especially for characterizing its complex hierarchical structures. Previous studies have confirmed the anisotropic mechanical behaviors caused by the alternated orientations of enamel rods and the alignment of fibril-like hydroxyapatite crystals, but the longitudinal section of enamel, which was composed of parallel-arranged rods, was regarded as a homogeneous continuum as always. In this study, nanoindentation combined with SEM was carried out with the indenter rotating on the longitudinal section of enamel to evaluate the relativity between the nano-mechanical properties and the orientation of indentation impressions. It has been shown that the enamel presented different elastic modulus and hardness with different angles of indenter on its longitudinal section, and its anisotropy was also confirmed by the remarkable asymmetric morphologies of impressions. We observed that the parallel arrangement of crystal fibrils and enamel rods might trigger the expansion of the micro-cracks in preferred orientation, and result in scalene triangle indentation impressions, altering contact areas as well as inconsistent mechanical behaviors. Consequently, it is considered that the longitudinal sections of enamel should be modeled as anisotropic.

The enamel softening and loss during early erosion studied by AFM, SEM and nanoindentation.

Enamel dissolution occurs when it contacts with acids produced by plaque bacteria, foods or drinks. There have been numerous and varied studies quantifying and characterizing the rate, extent and chemical aspects of enamel erosion; however, there is still hot debate about the amounts of enamel softening and loss. The objective of this study was to measure the enamel erosion process with high accuracy. Native third molars were partly eroded in citric acid (pH = 3.8) with some domains protected by a Ti coating layer. The surface morphology and structures before and after exposure to citric acid for different time periods were studied by AFM, SEM and nanoindentation, respectively. Based on this, the functions between the amounts of enamel softening, loss and erosion time were established for the first time. It was demonstrated that AFM, SEM and nanoindentation were suitable for measuring the early stages of enamel demineralization qualitatively and quantitatively.

[The comparative study of the effect of silica and calcium carbonate based dentifrice on dentine abrasives in vitro].

PURPOSE: To evaluate the effect of Silica (SiO(2)) and Calcium Carbonate (CaCO(3)) based dentifrice on their dentine abrasives. METHODS: Microscope, environmental electron scanning electron microscope (SEM) and radioactive Dentine Abrasion (RDA) assay were used to quantitatively measure the wear of dentine. RESULTS: The mechanical brushing assay showed severe dentine abrasion with CaCO(3) based dentifrices. There were only minor scratches on the specimens brushed with SiO(2) dentifrices. Analysis of the thin section by SEM after exposure to dentifrices showed that NaF/SiO(2) produced significant remineralization whereas MFP/CaCO(3) system resulted in little remineralization. RDA values were 86.17 and 98.61 for SiO(2) based dentifrices and 222.85 and 253.84 for CaCO(3) based dentifrices, respectively. The morphology of the irregular shaped CaCO(3) with sharp points/edges further explained the brushing and RDA results. CONCLUSION: These results indicate that abrasive in dentifrice, e.g. CaCO(3), can cause severe abrasion of dentine in vitro.