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Xia J, Tian ZR, Hua L, Chen L, Zhou Z, Qian L, Ungar PS. Enamel crystallite strength and wear: nanoscale responses of teeth to chewing loads. J R Soc Interface 2018; 14:rsif.2017.0456. [PMID: 29070592 DOI: 10.1098/rsif.2017.0456] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2017] [Accepted: 09/26/2017] [Indexed: 11/12/2022] Open
Abstract
The nanoscale responses of teeth to chewing loads are poorly understood. This has contributed to debate concerning the aetiology of enamel wear and resistance to fracture. Here we develop a new model for reactions of individual hydroxyapatite nanofibres to varying loads and directions of force. Hydroxyapatite nanofibres, or crystallites, composed of chains of bonded nanospheres, are the fundamental building blocks of enamel. This study indicates that these nanofibres respond to contact pressure in three distinct ways depending on force magnitude and direction: (i) plucking (nanosphere loss when the strength of the bonding protein 'glue' is exceeded), (ii) plastic deformation (compression to gradually bend nanofibres and squeeze the protein layer), and (iii) fragmentation (nanofibres fracture when the strength of H-bonds that bind smaller nanoparticles into nanospheres is exceeded). Critical contact pressure to initiate plucking is the lowest, followed by plastic deformation, and then fragmentation. Further, lower contact pressures are required for a response with shear forces applied perpendicular to the long axes of crystallites than with crushing forces parallel to them alone. These nanoscale responses are explained as a function of the interfacial nanochemical bonding between and within individual crystallites. In other words, nanochemistry plays a critical role in the responses of enamel to varying chewing loads.
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Affiliation(s)
- Jing Xia
- Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, People's Republic of China
| | - Z Ryan Tian
- Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA.,Institute of Nanoscale Science and Engineering, University of Arkansas, Fayetteville, AR 72701, USA
| | - Licheng Hua
- Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, People's Republic of China.,Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA.,Institute of Nanoscale Science and Engineering, University of Arkansas, Fayetteville, AR 72701, USA
| | - Lei Chen
- Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, People's Republic of China
| | - Zhongrong Zhou
- Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, People's Republic of China
| | - Linmao Qian
- Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Southwest Jiaotong University, Chengdu 610031, People's Republic of China
| | - Peter S Ungar
- Department of Anthropology, University of Arkansas, Fayetteville, AR 72701, USA
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Bartlett JD, Simmer JP. Kallikrein-related peptidase-4 (KLK4): role in enamel formation and revelations from ablated mice. Front Physiol 2014; 5:240. [PMID: 25071586 PMCID: PMC4082239 DOI: 10.3389/fphys.2014.00240] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2014] [Accepted: 06/10/2014] [Indexed: 12/26/2022] Open
Abstract
Enamel development occurs in stages. During the secretory stage, a soft protein rich enamel layer is produced that expands to reach its final thickness. During the maturation stage, proteins are removed and the enamel matures into the hardest substance in the body. KLK4 is expressed during the transition from secretory to the maturation stage and its expression continues throughout maturation. KLK4 is a glycosylated chymotrypsin-like serine protease that cleaves enamel matrix proteins prior to their export out of the hardening enamel layer. Mutations in KLK4 can cause autosomal recessive, non-syndromic enamel malformations in humans and mice. Klk4 ablated mice initially have normal-looking teeth with enamel of full thickness. However, the enamel is soft and protein-rich. Three findings are notable from Klk4 ablated mice: first, enamel rods fall from the interrod enamel leaving behind empty holes where the enamel fractures near the underlying dentin surface. Second, the ~10,000 crystallites that normally fuse to form a solid enamel rod fail to grow together in the ablated mice and can fall out of the rods. Third, and most striking, the crystallites grow substantially in width and thickness (a- and b-axis) in the ablated mice until they almost interlock. The crystallites grow in defined enamel rods, but interlocking is prevented presumably because too much protein remains. Conventional thought holds that enamel proteins bind specifically to the sides of enamel crystals to inhibit growth in width and thickness so that the thin, ribbon-like enamel crystallites grow predominantly in length. Results from Klk4 ablated mice demonstrate that this convention requires updating. An alternative mechanism is proposed whereby enamel proteins serve to form a mold or support structure that shapes and orients the mineral ribbons as they grow in length. The remnants of this support structure must be removed by KLK4 so that the crystallites can interlock to form fully hardened enamel.
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Affiliation(s)
- John D Bartlett
- Harvard School of Dental Medicine Boston, MA ; Department of Mineralized Tissue Biology, The Forsyth Institute Cambridge, MA
| | - James P Simmer
- Department of Biological and Material Sciences, University of Michigan School of Dentistry Ann Arbor, MI, USA
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