Comparison of small-angle neutron and X-ray scattering for studying cortical bone nanostructure

Abstract

in this study, we present a combined small-angle neutron and X-ray scattering (SAnS and SAXS) study of the nanoscale structure of cortical bone specimens from three different species. The variation of the scattering cross section of elements across the periodic table is very different for neutrons and X-rays. For X-rays, it is proportional to the electron density while for neutrons it varies irregularly with the atomic number. Hence, combining the two techniques on the same specimens allows for a more detailed interpretation of the scattering patterns as compared to a single-contrast experiment. The current study was performed on bovine, porcine and ovine specimens, obtained in two perpendicular directions with respect to the main axis of the bone (longitudinal and radial) in order to maximise the understanding of the nanostructural organisation. The specimens were also imaged with high resolution micro-computed tomography (micro-ct), yielding tissue mineral density and microstructural orientation as reference. We show that the SANS and SAXS patterns from the same specimen are effectively identical, suggesting that these bone specimens can be approximated as a two-component composite material. Hence, the observed small-angle scattering results mainly from the mineral-collagen contrast, apart from minor features associated with the internal collagen structure.

Bone: An Outstanding Composite Material

“…The organic phase of bone composite consists primarily of type I collagen (90% by weight), some other minor collagen types (III and VI), and a variety of non-collagenous proteins such as laminin, fibronectin, vitronectin, osteocalcin, osteonectin, osteopontin, and bone sialoprotein [11]. The basic structural unit of collagen is called tropocollagen (see Figure 1), which is a very rigid linear molecule with a diameter and length of 1.1 nm and 300 nm, respectively [12]. This molecule possesses a rearranged, right-handed, triple-helix structure formed by polypeptide chains with a highly repetitive amino acid sequence, glycine-X-Y, where glycine is being used in every third residue and X is often proline and Y is frequently hydroxyproline [13,14].…”

“…The triple-helix structures are associated with microfibrils, where five tropocollagens are assembled in parallel (D-period) [13]. At the next level of the hierarchy, the tropocollagen molecules congregate into fibrils by being covalently cross-linked at their tips and their axial staggering, forming overlap zones and gaps with a specific periodicity of approximately 67 nm [12,15]. should be noted that this ratio can vary depending on the age and health conditions of the human being [10].…”

“…The organic and inorganic components of bone blend together to generate mineralized collagen fibrils, as demonstrated in Figure 1. The mineral component in bone occurs as stacks of thin polycrystalline sheets, which surround and lie between the collagen fibrils [12]. The collagen-mineral system is arranged in parallel arrays within thin sheets called lamellae.…”

“…It was noticed that water mainly occupies the regions in intercollagen molecular spaces, terminal segments (Dspacing) within collagen microfibrils, and interface spacing between collagen and mineral structures. Water was also identified as being loosely bound to the surfaces of collagen fibrils [12]. The bone water content is an essential component since it confers much of bone's unique strength and resilience by stress reduction during dynamic loading [40,41].…”