Characterization of bone mineral crystals in horse radius by small-angle X-ray scattering

The size and the orientation of the bone salt (mineral) crystals in the cranial and caudal zones in the transverse midshaft section of the equine radius were investigated by small-angle X-ray scattering (SAXS). The results are interpreted as indicating that the crystals had an elongated shape with an average thickness of T = 3.17 +/- 0.15 nm in the caudal region and T = 3.79 +/- 0.20 nm in the cranial region. Their orientation was predominantly in the longitudinal direction of the bone. There was no preferential orientation within the transverse plane. The distribution of tilt angles with respect to the longitudinal direction was determined directly from the SAXS data: the average angle was about 30 degrees for the cranial region and 45 degrees for the caudal region. Assuming that the needle-like crystals are parallel with the collagen fibrils, the angular distribution of the crystals is in good agreement with previous measurements of collagen orientation using circularly polarized light microscopy.