Nano-insights into bone stability

When people fracture their hip in a fall, it is very often in the femoral neck – the narrow section of bone directly below the hip joint. This often happens with advanced age, when the bone has lost density. Most often, the femoral neck fractures from the top side, where it is generally much more porous than on the underside.

However, this correlation is not always present: sometimes a femoral neck fractures even though it is not porous. Researchers at PSI have now discovered the possible cause, through special X-ray analyses using the Swiss Light Source SLS at PSI and measurements at the Swedish synchrotron MAX IV: an altered nanostructure of the bone.

New X-ray technique offers detailed insights

The team, led by Marianne Liebi, a scientist in the PSI Center for Photon Science, used a new imaging technique to examine two bone samples each from 78 different femoral necks. In each case one sample was taken from the top and one from the underside of the same femoral neck. The team obtained the samples from the University of Bern, whose experts participated in the analysis as part of a joint research project. The method is called small-angle X-ray scattering tensor tomography, or SAXS-TT for short. It combines the analysis of so-called small-angle scattering signals from a high-resolution X-ray image with 3-D tomography, that is, imaging from different angles. This method has been developed at PSI over the last ten years and tested for the analysis of various materials, including bone.

Arrangement of collagen fibres becomes visible

The analysis of bone samples from 78 different femoral necks revealed that, in addition to the lower bone density on the upper side of the femoral neck, another factor stands out: the collagen fibres – which make up bones and are a thousand times finer than hairs – run differently on the upper side than on the underside. While on the underside they lie neatly parallel, allowing them to effectively cushion the forces acting on the femoral neck, they appear more disordered on the upper side, running at an angle or even crisscrossing. This makes them less flexible. “Furthermore,” says lead author Torne Tänzer, a doctoral candidate in Liebi’s research group, “the mineral platelets are less regularly arranged and differently shaped.” The mineral platelets of a bone are tiny lamellae of calcium phosphate that lie between the collagen fibres and stabilise them.

The arrangement of fibres and platelets, it is hypothesised, could influence the stability of bones. “We now want to investigate this hypothesis in further studies by conducting mechanical stress tests on femoral necks with different structures,” says Tänzer. This should reveal whether or not an irregular structure actually increases the risk of fractures. “We may then also be able to determine to what extent such changes in nanostructure are related to age.”

The researchers hope that their work will contribute to a deeper understanding of bone structure in general, as well as to analysis methods. Furthermore, it could advance fundamental research into bone mechanics. “Methods for examining biological materials at the nanoscale, both structurally and mechanically, are constantly being developed,” says Marianne Liebi. “We are demonstrating what these developments can already achieve today and what direction they can go in the future.”

Faster imaging thanks to SLS upgrade

In future studies, the researchers will benefit from the recent SLS upgrade. Its entire electron storage ring was replaced with more than a thousand new, high-precision magnets, thus increasing the intensity and brilliance of the X-ray light source many times over. This makes it possible to produce significantly more detailed images than before while considerably reducing measurement time.

“We were able to scan bone samples in full 3-D from only two of the 78 femoral necks because, with the previous technology, this was simply very time-consuming and incredibly complex,” Torne Tänzer says. The 3-D tomography required a full day per scan, while the 2-D thin-section measurements, which were performed during the SLS upgrade at the Swedish synchrotron MAX IV, took only 20 minutes. With these few 3-D examples, the researchers were able to draw conclusions about the other samples that were only viewed in two dimensions, thus improving interpretation of the 2-D data. “With the upgraded SLS, we will now be able to analyse many more samples in 3-D. This will significantly boost our ability to gain new insights.”