Tooth enamel, the outermost layer of human teeth, is a complex, hierarchically structured biocomposite. The details of this structure are important in multiple human health contexts, from understanding the progression of dental caries (tooth decay) to understanding the process of amelogenesis and related developmental defects. Enamel is composed primarily of long, nanoscale crystallites of hydroxyapatite that are bundled by the thousands to form micron-scale rods. Studies with transmission electron microscopy show the relationships between small groups of crystallites and X-ray diffraction characterize averages over many rods, but the direct measurement of variations in local crystallographic structure across and between enamel rods has been missing. Here, we describe a synchrotron X-ray-based experimental approach and a novel analysis method developed to address this gap in knowledge. A ~500-nm-wide beam of monochromatic X-rays in conjunction with a sample section only 1 μm in thickness enables 2D diffraction patterns to be collected from small well-separated volumes within the enamel microstructure but still probes enough crystallites (~300 per pattern) to extract population-level statistics on crystallographic features like lattice parameter, crystallite size, and orientation distributions. Furthermore, the development of a quantitative metric to characterize relative order and disorder based on the azimuthal autocorrelation of diffracted intensity enables these crystallographic measurements to be correlated with their location within the enamel microstructure (e.g., between rod and interrod regions). These methods represent a step forward in the characterization of human enamel and will elucidate the variation of the crystallographic structure across and between enamel rods for the first time.

中文翻译：

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一种绘制用于人类牙釉质的亚微米级晶体有序/无序的方法

牙釉质是人类牙齿的最外层，是一种复杂的、分层结构的生物复合材料。这种结构的细节在多种人类健康环境中都很重要，从了解龋齿（蛀牙）的进展到了解牙釉质形成和相关发育缺陷的过程。搪瓷主要由长的、纳米级的羟基磷灰石微晶组成，这些微晶由数千个捆绑在一起形成微米级的棒。透射电子显微镜研究表明，小群微晶和 X 射线衍射表征了许多棒的平均值之间的关系，但是缺少对釉棒之间和釉棒之间局部晶体结构变化的直接测量。这里，我们描述了一种基于同步加速器 X 射线的实验方法和一种新的分析方法，旨在解决这一知识差距。约 500 nm 宽的单色 X 射线束与厚度仅为 1 μm 的样品部分相结合，可以从牙釉质微结构内分离良好的小体积中收集 2D 衍射图案，但仍能探测到足够多的微晶（每模式）以提取晶格参数、微晶尺寸和取向分布等晶体学特征的群体级统计数据。此外，基于衍射强度的方位角自相关，开发了一种定量度量来表征相对有序和无序，这使得这些晶体学测量能够与它们在牙釉质微结构中的位置相关联（例如，棒和棒间区域之间）。这些方法代表了人类牙釉质表征的一个进步，并将首次阐明牙釉质棒之间和之间晶体结构的变化。