Micro- and nanostructure of human teeth : a synchrotron radiation-based X-ray study

The anisotropic composite nature of human teeth guarantees their function for decades under high mechanical loads and adverse chemical conditions. Even more so since only marginal remodelling and repair mechanisms take place in adult teeth. While the macroscopical anatomy of the tooth has been well understood, long range ordering of the tooth’s micro and nano components is still matter of research. Tooth micro- and nanostructure has been extensively studied, mainly with two-dimensional approaches as, for example, electron microscopy. The ultrastructural organization over a whole tooth is, however, not readily accessible with these approaches, because they only permit a very localized observation and often even remove the investigated structures from their natural three-dimensional organization.
The high degree of anisotropy in both dentin and enamel on the micro- and nanometer scale has a strong impact on the tooth’s mechanical properties. For example, the Young’s modulus and crack resistance of dentin are different parallel and perpendicular to the dentin tubules. Synchrotron radiation-based micro computed tomography with pixel sizes in the sub-micrometer range allows to three-dimensionally image dentin tubules, however only over restricted specimen sizes below one millimetre in diameter. To map the tubular network over an entire tooth, multiple scans are necessary. Given the generally limited beamtime available at synchrotron sources, a method has to be identified that allows for the visualization of dentin tubules with high accuracy and within reasonable time. Single distance phase retrieval, multiple distance phase retrieval and absorption contrast datasets, acquired at the beamline ID 19 at ESRF, were compared concerning their spatial and density resolution as well as their suitability for tubule rendering, and single distance phase retrieval, with a specimen detector distance of 75% of the critical value d^2/lambda, was found to yield optimal results.
The knowledge of tooth ultrastructure is of particular interest when dealing with carious lesions. The treatment of carious lesions is nowadays accompanied by the removal of affected hard tissues and their replacement with isotropic restoration materials. Despite their high performance, these restorations have limited life span. As a result, additional clinical interventions and the replacement of the restoration are often necessary. An alternative would be the fabrication of anisotropic fillings, which mimic the natural organization of the tooth. Such structures are speculated to exhibit properties similar to those of healthy teeth and thus to be superior to the isotropic materials currently in use, thus extending restoration lifetime. For this purpose an extensive mapping of tooth ultrastructure is necessary. Small-angle X-ray scattering (SAXS) in scanning setup allows for the investigation of tooth nanostructural organization over extended areas. Scanning SAXS measurements of micrometer-thin tooth slices were performed at the cSAXS beamline at the Paul Scherrer Institut, revealing a high degree of structural organization of the tooth’s nanometer-sized components. Based on this knowledge, a model for bio-mimetic fillings was proposed.
Nonetheless, bio-inspired fillings would still require costly interventions, and their superior performance has not been demonstrated yet. As alternative to restorations, where the affected tissue is removed and replaced with man-made materials, a treatment based on the re-mineralization of the carious lesion could be performed. The aim is not only the re-mineralization of de-mineralized tissues, but also the re-establishment of tooth morphology including its nanostructure, which in return will ensure mechanical properties comparable to those of healthy tissue. The morphology of the carious tissue is crucial for this procedure, as the structures retained in the lesion can act as nucleation sites for the re-mineralizing crystallites. Carious dentin and enamel were examined with scanning SAXS to determine whether the organization on the nanometer level is retained to some extent. In dentin, a significant part of the collagen network is retained concerning orientation and abundance after mild demineralization has taken place. In enamel, the overall orientation of the hydroxyapatite crystallites is unaltered, despite the complex organization of enamel lesions, consisting of alternating layers of de- and re-mineralized tissue.