Utilizing x-ray diffraction tomography to study the effect of biodegradable magnesium implants on the bone ultrastructure

Magnesium-based alloys are suitable materials for biodegradable bone implants due to their high biocompatibility and mechanical properties similar to bone. Diffraction tomography (DCT) is an imaging technique based on acquiring positionresolved diffraction patterns at multiple angular orientations. DCT is able to provide information about the crystal structure of hydroxyapatite (HAp) in bone, which is necessary for understanding the influence of magnesium degradation on the bone tissue. This article presents a study of a sheep bone explant containing a biodegradable magnesium-zinc-calcium (BRI.Mag, Mg-Zn-Ca) alloy implant. The DCT data were reconstructed and analyzed using the MATLAB computing environment and ASTRA Toolbox. Thus, the DCT images with 200 μm pixel size were reconstructed and, therefore, a DCT study workflow was developed. That allowed to retrieve the crystal lattice parameters of HAp such as full width at half maxima (FWHM) and reflection position around the implant surface of a selected (002) diffraction peak. Based on these parameters, the corresponding d-spacing and crystal size of HAp were determined. It was shown that the degradation of the implant does not notably affect the lattice spacing of the bone within a distance of 3 mm from the implant surface. At the same time, the crystal size decreases closer to the implant (1.5 mm). The developed workflow for the reconstruction of a volume-resolved diffraction experiment of a bone explant containing metal implants is a convenient approach to investigate the crystal structure of the sample on exact locations in a non-invasive way. The workflow will be further applied for a broader range of samples containing Mg-based biodegradable implants.