Nanomechanical properties of tooth surface upon anti-caries treatment

Objectives: The tooth is an anisotropic composite material containing hydroxyapatite nanocrystals. Calcium-fluoride-like deposits from topical fluoride treatment play a key role in caries prevention. Previous microhardness analyses of teeth and their surface deposits introduced errors due to substrate effect and thereby could not justify the mechanical properties of tooth surfaces and the early loss of caries-preventive fluoride-containing deposits. This study aimed to explore the nano-mechanical properties of the tooth upon anti-caries treatment.

Methods: Atomic Force Microscopy (AFM) and a nano-indentation technique combined with electron probe microanalysis (EPMA) were applied to characterize the nano-mechanical properties and topographic structure of the enamel surface and the effect of topical fluoride treatment.

Results: The nanohardness in the center of an enamel rod is significantly higher than that in the tail portion. A trend toward decrease in nanohardness and increase in friction was observed when the tooth surface was probed toward the dentino-enamel junction. The fluoride-containing surface deposits were found to have low nanohardness and high nano-wear depth. However, a 22% increase in fluoride concentration could still be detected on the treated enamel surface following the removal of the surface deposits.

Conclusion: These results revealed the nanomechanical properties of the native tooth surface. The results also explain the early loss of calcium-fluoride-like deposits and justify the long-term effectiveness of topical fluoride treatment when the surface deposits erode.