P. Fratzl

Nucleation and Growth of Mineral Crystals in Bone Studied by Small-Angle X-Ray Scattering

SummaryThe mechanism of calcification in bone and related tissues is a matter of current interest. The mean size and the arrangement of the mineral crystals are important parameters difficult to obtain by electron microscopy. Furthermore, most studies have been carried out on poorly calcified model systems or chemically treated samples. In the work presented here, native bone was studied as a function of age by a quantitative small-angle X-ray scattering method (SAXS). Bone samples (calvariae and ulnae) from rats and mice were investigated. Measurements were performed on native bone immediately after dissection for samples up to 1 mm thick. The size, shape, and predominant orientation of the mineral crystals in bone were obtained for embryonal, young, and adult animals. The results indicate that the mineral nucleates as thin layers of calcium phosphate within the hole zone of the collagen fibrils. The mineral nuclei subsequently grow in thickness to about 3 nm, which corresponds to maximum space available in these holes.

Mineralization of cancellous bone after alendronate and sodium fluoride treatment: A quantitative backscattered electron imaging study on minipig ribs

Fluoride stimulates bone formation, whereas bisphosphonates reduce bone resorption. In clinical trials, both treatments increase bone density, although sodium fluoride (NaF) increases and alendronate (bisphosphonate, ALN) decreases bone turnover. In a comparative study using minipigs an inverse correlation has been reported between bone turnover and elastic modulus. Small-angle X-ray scattering (SAXS) measurements of these bones revealed no structural deterioration of the collagen/mineral composite at the nanometer range for ALN-treated vertebra, whereas a slight increase of the average thickness of the mineral crystals as well as changes of the structure of the collagen/mineral composite were found in the bones of NaF-treated animals. In this study we used quantitative backscattered electron imaging (qBSE) to investigate the cancellous bones from ribs of minipigs treated with vehicle, NaF, or ALN. This method provides information on the local mineral concentration in the micrometer range. Mineralization spectra were obtained from each treatment group, and statistically significant differences between ALN and controls were found for the peak position, the peak height, the peak width, and the average calcium (Ca) concentration of the mineral distribution. The results reveal that the cancellous bone matrix was more uniformly mineralized after ALN treatment. The reduced bone turnover induced by ALN, documented histomorphometrically could be at the origin of this phenomenon. No significant differences were detected between NaF and control. Together with the earlier SAXS data these results may explain in part the increase in bone density and the improvement of biomechanical properties observed after ALN treatment in animals and in osteoporotic patients.

Kinetics of Phase Separation Along Stable Trajectories

The phase separation kinetics of dilute Al-(Zn,Mg) alloys were studied by small-angle and diffuse neutron scattering techniques and the mean cluster radius R as well as the volume fraction V of the solute-rich phase were determined. It could be shown that, within (R,V)-space, the decomposition corresponds to a trajectory, which is stable against small perturbations. Using a simple two-phase model, this trajectory appears as a “valley” of the free energy of the system. When this model is extended to concentrated alloys such a valley still exists in the free energy as function of the different system parameters. During evolution along this valley a time-scaling of the system is obtained. In fact all parameters of the two-phase model, like composition and volume fraction of the phases or the total interphase-surface must be functions of a scaling parameter R, which for dilute alloys, can be identified as the average cluster radius.