Enamel prism orientation within human teeth

L. Raue1 and H. Klein1

1GZG, Abt. Kristallographie, Georg-August-Universität Göttingen, Goldschmidtstrasse 1, 37077 Göttingen, Germany

Dental enamel is the most highly mineralised and hardest biological tissue in human body [16]. It is made of hydroxylapatite (HAP) - Ca5(PO4)3(OH), which is hexagonal (6/m). The lattice parameters are a = b = 9.418 Å and c = 6.875 Å. About the inner structure of dental enamel it’s known, that it is composed of elongated “enamel prisms” of about 20µm diameter [1]. The crystallographic structure, respectively the orientation of the HAP-crystals in the prisms has not been investigated in detail up to now [2-4]. It is known, that the prisms penetrate through the enamel undisturbed from the dentine to the surface [1,5]. Structural built-up information about the prisms like the often described feather-like spreading [6] originate from light optical and electron microscopy descriptions about 160 respectively 50 years ago [7-10]. Thereby the real spatial orientation of the crystallites can differ in contrast to these shape based observations. The exact crystallographic orientation of the crystallites within the prisms has not been described. In this study, the spatial orientation of of the HAP crystallites was determined using X-ray texture analysis. The method is described in detail elsewhere [11]. Source of the X-ray beam was the wiggler BW5 at the synchrotron DORIS, Hamburg, Germany. A monochromatic beam with a wavelength of 0.12155 A°and a rectangular size of 5 00 µm x 500 µm was used. The textures have been examined in more then 150 local positions within all kind of teeth. For each position 33 diffraction images at different orientation angles were taken. The images were evaluated with the program MAUD [12] finally providing information of the orientation g of the lattice planes (h k l). Exemplary the data for molar tooth 11 and incisor tooth 17 (FDI Notation) is shown here (cf. Fig 1). All data points from other teeth show comparable trends, supporting the results presented here. For interpreting textures of human dental enamel, it is sufficient to look at the (0 0 1) pole figures which likewise show the orientation of the c-axis of the unit cell, since the present texture type originates from a free rotation around the normal of the tilted (0 0 1) crystal planes [13-15].

Fig. 1 – Orientation of the HAP crystallites within the respective local sample position shown by the (0 0 1) pole figures. The local examined positions are marked with squares. The corresponding (0 0 1) pole figures show the maximum degree of preferred orientation by the given numbers, e.g. a 7.6-times. The higher this number is, the more preferred oriented the crystals are. Molar: looking at the orientation of the maxima and going from A to F, the maximum changes continuously from the upper left side to the upper middle. Going from G to P, the position changes only little at the upper middle having the change of tendency from top right to top left at point K. Between point Q and U the position of the maximum changes continuously from the upper middle

to the upper right side. Summing up, one can say that the numeric values are highest at the cusps and lowest at the sides and in the middle between the cusps. The orientation changes continuously from flat inclination to vertical direction following the morphology of the tooth at both sides (A–E and Q–U). The data points between (F–P) show a general vertical direction, having the maximum at the upper middle. Incisor: the pure numbers increase in vertical direction up to the absolute maximum (v1–v6). In horizontal direction, values first increase, then stay stable and afterwards again decrease going from left (h1) to right (h8). The intermediate data points (i1–i3) show values of the respective mixture from horizontal and vertical direction. Orientation changes in all cases constantly. In vertical direction the maximum changes from the middle to the upper middle, in horizontal direction, the maximum changes from the left upper middle to the right upper middle, respectively going in increasing numbering direction. The intermediate data points show again a mixture of both behaviours. Combining the information about the spatial orientation of the maximum values and the type of orientation (tilted fibre texture [13-15]), one can deduce the orientation of the enamel prisms directly from the pole figures by looking at the position of the maximum in the stereographic projections, since the HAP crystallites link up along their c-axis [16]. With the carried out experiments it was possible to characterize the orientation of the HAP crystallites in detail for the different locally measured points within all examined teeth. During an literature review, it was discovered that some schematic drawings of enamel prism orientation should be revised (esp. showing the region between the cups of molars; prisms have a nearly vertical orientation and do not follow the morphology of the surface orthogonal) [17-20]. References [1] Avery, J. K. (1994). Oral development and histology, Thieme Medical Pub. Inc., New York. [2] Al-Jawad, M., Steuwer, A., Kilcoyne, S. H., Shore, R.C., Cywinski, R. & Wood, D. J. (2007). 2D mapping of

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[15] Raue, L., Gersdorff, N., Rödiger, M. & Klein, H. (2011). New insights in prism orientation within human enamel, accepted (in print) at the Archives of Oral Biology. - DOI 10.1016/j.archoralbio.2011.08.015

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