Effect of Streptococcus mutans on mechanical properties of human dental structures
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Ren-Jei Chung Graduate Institute of Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
Fang-Cheng Chou, Hsiang-Long Hsiao, and Hung-Bin Hsu Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan (Received 22 January 2008; accepted 9 March 2009)
In this study, the mechanical properties of human dental structures have been investigated by using instrumented nanoindentation. Immersion in solutions containing Streptococcus mutans, which is the principal cause of dental caries, was applied to tooth specimens to clarify its effect on the microstructure and mechanical properties of the dental structures. With an extended time of up to 16 h, the pH value of the S. mutans solutions dropped from 7.3 to 5.8. Therefore, after immersion in the S. mutans solutions for 16 h, slight erosions of the dental structures began; after 64 h, severe tooth decay occurred with obviously etched dental features. After 128 h, the elastic modulus of enamel and dentine dropped to 85 and 67%, respectively, of the original values of untreated specimens, and the hardness dropped to 88 and 55%, respectively.
I. INTRODUCTION
Many biological materials, such as teeth and bones, are complex structures composed of hard minerals and ductile proteins.1–5 For example, teeth, the strongest tissue in the human body, are constructed from hard, but brittle enamel consisting mostly of hydroxylapatite crystals and ductile but soft dentine, which contains a relatively large amount of organic proteins.3–9 The mechanism for the combination of the enamel and dentine to form a perfect composite material is of great interest, and quantifying the mechanical properties of biological materials will benefit the developments of better biotechnology and biomedical materials. Moreover, the mechanical properties of biological tissues under disease infections or medical treatments are believed to change. Therefore, the analyses of mechanical property variations will contribute to the research in medical therapy. Researchers in previous studies have tried to clarify the variations in the mechanical properties of dental structures under some popular solution treatments. It has been reported that the mechanical properties of dental structures may degrade when exposed to tooth-bleaching agents.10–14 In addition, acidic soft drinks or other artificial systems are also believed to demineralize dental a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0275 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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structures and to reduce strengths.15–19 However, the other important factor for the degradation of dental mechanical properties by the infection of Streptococcus mutans, which is considered to be the principal cause of dental caries in humans, has never been realized. The S. mutans, first described by Clark in 1924, sticks to tooth surfaces and subsists on carbohydrates. When carbohydrates are metabolized
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