Comparison of mechanical behaviors of enamel rod and interrod regions in enamel
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Michael V. Swain Biomaterials Science Research Unit, Faculty of Dentistry, University of Sydney, New South Wales 2006, Australia; and Department of Oral Sciences, University of Otago, Dunedin 9054, New Zealand
Arndt Klocke Division of Orthodontics, Department of Orofacial Sciences, University of California, San Francisco, California 94143; and Department of Orthodontics, University Medical Center Hamburg-Eppendorf, Hamburg 20246, Germany
Gerold A. Schneidera) Institute of Advanced Ceramics, Hamburg University of Technology, Hamburg 21073, Germany (Received 27 August 2011; accepted 14 October 2011)
Interrod regions exist between the enamel rods and are known to have different crystallite orientations and a higher organic content compared to the enamel rods (the intrarod regions). This study aims to characterize the mechanical properties of both regions especially the timedependent properties by using spherical indentation. Despite the very small amount of proteins, the interrod region shows statistically significantly higher inelastic energy dissipation than the intrarod region with increased deformation times. The total displacement under constant load (creep), viscosity, and stress relaxation behavior of both regions are also reported. Similar to the observation of previous studies, the elastic modulus and hardness in the intrarod region are significantly higher than in the interrod region. I. INTRODUCTION
Dental enamel is able to maintain its structural integrity under mechanical loading conditions in the oral environment. A better understanding of its structure–property relationships is a basis for improvements in restorative as well as preventive dentistry. Compositionally, enamel contains ;90% of apatite crystallites, ;8% of water, and ;2% of organic matrix by volume.1 Structurally, enamel consists of three levels of hierarchical structures. Level 1 consists of ;50-nm-diameter apatite fibers2,3 surrounded by 1- to 2-nm-thin organic layer3 that are closely bundled together (to form enamel rods at level 2). Level 2 consists of ;2- to 4-lm enamel rods that span from the dentino-enamel junction (DEJ) to near tooth surface.4 Groups of enamel rods form Hunter-Schreger bands at level 3.5 Within each band, the enamel rods have similar local orientations but different from the neighboring bands. This feature is prevailed in the inner enamel near DEJ. At level 2, “interrod” regions were observed between rods where the crystallite orientations are significantly different from those in the center of the rods6,7 and are relatively organic-rich.8 The cross sections of enamel rods and the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.409 448
J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012
http://journals.cambridge.org
Downloaded: 12 Jan 2015
accompanying interrod regions are shown in the scanning electron microscopy image in Fig. 1. It is often said that proteins and peptides contribute to the toughness of biomineralized materials (dentin, bone, enamel, nacre) through their
