| Summary: | 博士 === 國立中正大學 === 機械工程學系暨研究所 === 99 === Abstract
Human teeth exhibit many levels of hierarchical structure from macroscopic to microscopic length scales. Though their composition is mainly hydroxyapatite and some minor organic substances, they possess extraordinary mechanical properties and unusual fine structures. Dental caries is a process of demineralization. Fluoride is an effective anti-caries agent and has been widely applied for caries prevention.
Accordingly, my thesis aimed to investigate the mechanical and tribological properties of intact tooth, the fluoride and laser treatment enamel surface using a combination of various techniques including nanoindentation, nanoscratch, nanowear, Atomic Force Microscopy (AFM), SEM and electron probe microanalysis (EPMA). We would like to observe the ultrastructure of enamel rod from various section plans and positions while probing their mechanical and tribological properties of the specific area at nano scale according to their relative positions to the enamel rod geometry. We would also investigate the mechanical property of tooth enamel affected by topical fluoride and laser treatment. EPMA was used to detect the calcium composition along longitudinal axis of the enamel rod and the fluorine element composition.
In the ultrastructure of enamel rod, the nanohardness and elastic modulus of the head of the enamel rods are significantly higher than that of the tail region in the axial-section plane. Both nanohardness and elastic modulus gradually decrease from enamel surface toward dentino-enamel junction. Such a variation correlates well with the decreasing trend of calcium composition from our element analysis. The friction coefficient and the nanowear of the enamel showed an inversed trend to the hardness in respect to their relative topological position in the long axis of enamel rod toward DEJ. The relationship between the nanowear depth and the distance from the outer enamel surface to DEJ presented as an exponential function.
The enamel surface treated by topical fluoride combined with laser irradiation at an energy density of 732 mJ/cm2 per pulsed for 5s and 10s showed reduced surface roughness of the CaF2-like deposits and a significant increase in the nanohardness and elastic modulus. In addition, the wear resistance of the CaF2-like deposits is improved by 34% following laser irradiation for 5s and 40% for 10s irradiation. Interestingly, the laser treatments increase the fluoride uptake by 23% in the enamel.
This study clarified the fundamental nanomechanical and nanotribological properties of human enamel rods and provides a useful reference for the future development of biomimetic and dental restorative materials. In the prevention dental caries, this study showed that addition of laser irradiation yields a significant improvement in the effectiveness of topical fluoride treatment.
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