Nanomechanics and Chemistry of Caries-Like Lesions in Dental Enamel
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Nanomechanics and Chemistry of Caries-Like Lesions in Dental Enamel Michelle E. Dickinson and Adrian B. Mann Department of Ceramics & Materials Engineering and Department of Biomedical Engineering, Rutgers University, Piscataway, NJ 08854, USA ABSTRACT Dental enamel lesions are formed by the demineralisation of dental enamel due to dietary and bacterial factors which lower the local pH. If progression of this demineralisation continues the enamel structure eventually fails giving rise to a cavity (carie). Fortunately, in the earliest stages this process is reversible and if the pH of the environment increases, ions such as calcium, phosphate and fluoride can diffuse back into the enamel to give an arrested lesion. Hence, the tooth’s structural integrity is preserved. The use of artificially produced enamel caries allows the study of lesion formation under conditions of highly controlled demineralisation. This provides a fundamental insight into the process of caries formation in enamel. In this study human premolars have been treated with a lactic acid solution to create artificial “caries-like” lesions in the enamel on the buccal side of the teeth. In the test samples the lesions penetrated around 100 µm into the enamel structure, accounting for approximately one tenth of the thickness of the enamel. Cross sections through the lesion were characterized with nanoindentation, electron probe micro-analysis and time of flight secondary ion mass spectrometry. From the data obtained maps of both mechanical and chemical properties were plotted across the entire width of the lesions. The results show that the lesions have a significantly reduced hardness and elastic modulus in comparison to sound enamel. These changes in mechanical properties were found to correlate with a loss of calcium and phosphate from the structure. There was also evidence of a stronger, less demineralised layer of enamel close to the lesion’s surface. This surface zone is suggestive of remineralisation within the lesion which is of importance with regards to preventing the lesion developing into a carie. INTRODUCTION Human dental enamel has a complex hierarchical structure consisting of nanoscale hydroxyapatite (Ca10(PO4)6OH2) prisms arranged into prismatic clusters. These clusters are long rods measuring a few microns in width, but up to millimeters in length. The prisms and prismatic clusters are closely packed into an organic matrix to give the enamel’s macroscale structure [1]. In the oral cavity the exposed surface of the enamel is under conditions of continually fluctuating pH due to dietary effects and the action of bacteria. Under acidic conditions the enamel loses ions such as calcium and phosphate. In localised areas the pH cycle may be acidic for prolonged periods of time, for instance due to the presence of acidogenic bacteria on the enamel’s surface. This leads to the creation of a demineralised region called a white spot lesion if it occurs in vivo or a caries-like lesion if it is in vitro. Continued exposure to low pH causes t
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