Age-related transparent root dentin: mineral concentration, crystallite size, and mechanical properties.

Many fractures occur in teeth that have been altered, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of these altered dentins. One such altered form of dentin is transparent (sometimes called sclerotic) dentin, which forms gradually with aging. The present study focuses on differences in the structure and mechanical properties of normal versus transparent dentin. The mineral concentration, as measured by X-ray computed microtomography, was significantly higher in transparent dentin, the elevated concentration being consistent with the closure of the tubule lumens. Crystallite size, as measured by small angle X-ray scattering, was slightly smaller in transparent dentin, although the importance of this finding requires further study. The elastic properties were unchanged by transparency; however, transparent dentin, unlike normal dentin, exhibited almost no yielding before failure. In addition, the fracture toughness was lowered by roughly 20% while the fatigue lifetime was deleteriously affected at high stress levels. These results are discussed in terms of the altered microstructure of transparent dentin.

Intrafibrillar mineral may be absent in dentinogenesis imperfecta type II (DI-II).

High-resolution synchrotron radiation computed tomography (SRCT) and small-angle x-ray scattering (SAXS) were performed on normal and dentinogenesis imperfecta type II (DI-II) teeth. The SRCT showed that the mineral concentration was 33% lower on average in the DI-II dentin with respect to normal dentin. The SAXS spectra from normal dentin exhibited low-angle diffraction peaks at harmonics of 67.6 nm, consistent with nucleation and growth of the apatite phase within gaps in the collagen fibrils (intrafibrillar mineralization). In contrast, the low-angle peaks were almost non-existent in the DI-II dentin. Crystallite thickness was independent of location in both DI-II and normal dentin, although the crystallites were significantly thicker in DI-II dentin (6.8 nm [SD = 0.5] vs. 5.1 nm [SD = 0.6]). The shape factor of the crystallites, as determined by SAXS, showed a continuous progression in normal dentin from roughly one-dimensional (needle-like) near the pulp to two-dimensional (plate-like) near the dentin-enamel junction. The crystallites in DI-II dentin, on the other hand, remained needle-like throughout. The above observations are consistent with an absence of intrafibrillar mineral in DI-II dentin.

X-ray Tomographic Study of Chemical Vapor Infiltration Processing of Ceramic Composites.

The fabrication of improved ceramic-matrix composites will require a better understanding of processing variables and how they control the development of the composite microstructure. Noninvasive, high-resolution methods of x-ray tomography have been used to measure the growth of silicon carbide in a woven Nicalon-fiber composite during chemical vapor infiltration. The high spatial resolution allows one to measure the densification within individual fiber tows and to follow the closure of macroscopic pores in situ. The experiments provide a direct test of a recently proposed model that describes how the surface area available for matrix deposition changes during infiltration. The measurements indicate that this surface area is independent of the fiber architecture and location within the preform and is dominated by large-scale macroporosity during the final stages of composite consolidation. The measured surface areas are in good agreement with the theoretical model.

Application of x-ray microtomography in materials science illustrated by a study of a continuous fiber metal matrix composite.

The advantages of the use of x-ray microtomography in materials science are discussed, and illustrated by the nondestructive study of the mechanical damage in a continuous fiber SiC/Al composite at a resolution of about 25 µm. A laboratory x-ray source was used, and it was shown that quantitative measurements of the linear absorption coefficient at this resolution are possible, even though the AgKα radiation used is accompanied by a considerable amount of white radiation, provided that the counter system is properly corrected for pulse pile up and dead-time.