A new fix
for broken bones

Analysis of the bone ultrastructure around biodegradable magnesium implants

When surgeons repair bone fractures, they often use plates, screws, or nails for fixation. These temporary implants are commonly made of metals or polymers and need to be surgically removed after the fracture has healed. Biodegradable implants, which decompose after a certain time, can spare patients the second surgery. On top of that, they can support the healing process. Magnesium alloys are promising candidates, because magnesium occurs naturally within the body; it has a stimulating effect on bone growth, and elastic properties that are similar to bone tissue. To safely apply such implants, scientists need to understand the interaction of the bone with the degrading implant. This is done by looking at the nanoscale building blocks of the bone tissue – so-called fibrils – that form at the interface between bone and implant. The fibrils consist of collagen and hydroxyapatite platelets (HAP), whose thickness, orientation, and crystal lattice parameters can be studied with X-rays.

Schematic showing a bone on the left handside, a close-up reveals cylindric structures called the osteons. They consist of different layers called lamellae, which are again shown in a close-up to be composites of extrafibrillar matrices, containing fibrils. Those fibrils consist of HAP crystals which are depicted as black hexagons connected by collagen.

Ultrastructure of cortical bone. The osteons, which form the cortical bone, are composed of a Harvesian canal surrounded by lamellae. These consist of bundles of fibrils, so-called extrafibrillar matrices. Fibrils, in turn, are composite structures of collagen and hydroxyapatite platelets (HAP).

CC by-sa 4.0 Denise Müller-Dum & Jens Kube, awk/jk

Experimental Setup

The goal of the study by Berit Zeller-Plumhoff from the Helmholtz Zentrum Geesthacht, Germany, and her international team was to compare the bone ultrastructure formed around two non-degradable implants that are already used in the clinic (titanium and polyether ether ketone, PEEK) with the tissue that developed around two magnesium-gadolinium alloys (Mg-10Gd and Mg-5Gd, with 10 or 5 weight percent Gd, respectively). They surgically implanted screws of the materials into rats and sampled thin sections along the implant axis after 4, 8, and 12 weeks of healing.

The samples were analyzed using small angle X-ray scattering (SAXS), and X-ray diffraction (XRD) experiments set up at the P03 nano-focus end station at PETRA III, Deutsches Elektronen-Synchrotron (DESY). For the XRD experiments, they placed a LAMBDA 750k detector next to the flight tube, 18–19 cm from the focal point. “We chose the LAMBDA detector for our XRD experiments, because the space around the sample was very confined due to the flight tube for the SAXS setup. The combination of a high spatial resolution (small detector pixels) and a compact detector design enabled us to position the detector close to the sample. Thus, we could measure the X-ray diffraction signal of the hydroxyapatite crystals at a high resolution and with good quality”, said Florian Wieland, one of the authors of the study. From the collected images, the team determined crystal size and crystal lattice spacing for different regions around the implant; the SAXS results were used to retrieve HAP orientation and thickness.

SetupPETRA III, Deutsches Elektronen-Synchrotron (DESY), P03 nano-focus end-station
CameraLAMBDA 750k Si detector
Resolution786,432 pixels
Acquisition frequency0.2 Hz
Photon energy12.8 keV & 13.7 keV
A flight tube in the foreground of the picture directs the observer's eye toward the position of the sample stage, an orange detector is placed next to it.

Experimental setup of the XRD experiment at the PETRA III P03 beamline. The current form factor of LAMBDA 750k offers an even more compact design than the one shown here.

© Berit Zeller-Plumhoff, with kind approval
Three images are shown. The biggest one is a microscopic image of a screw in bone, the second image is a close up of the interface screw thread and bone and the third one is a color map showing the lattice spacing calculated from the x-ray experiments.

Exemplary thin section of a Mg-5Gd screw 12 weeks after implantation imaged using light microscopy. The red box displays a close-up of a corresponding thin section, prepared and after laser cutting. The map (bottom right) shows the calculated lattice spacing for the highlighted 90 µm x 90 µm region based on the XRD measurements.

© CC-BY 4.0 Zeller-Plumhoff et al. (2020)

Results

Based on these experiments, Zeller-Plumhoff and her colleagues were the first to publish a quantitative comparison of the bone ultrastructure around biodegradable and commonly used implants. They found that HAP orientation and thickness of the newly formed tissue are very similar for the four materials. However, the XRD experiments revealed some differences in lattice spacing and the crystal size of HAP. In the bone tissue, spacing was tighter in the case of Mg-xGd implants if compared to titanium, and crystals were smaller. The researchers inferred that magnesium from the decomposing alloy is incorporated into the newly formed fibrils, which is in agreement with previous studies. However, they caution that the apatite which formed in the so-called degradation layer may also contain gadolinium. The health impacts of gadolinium are still being debated, which is why further study of the elemental composition of the apatite in the different regions is required.

Reference

B. Zeller-Plumhoff, C. Malich, D. Krüger, G. Campbell, B. Wiese, S. Galli, A. Wennerberg, R. Willumeit-Römer, D.C. F. Wieland (2020): Analysis of the bone ultrastructure around biodegradable Mg–xGd implants using small angle X-ray scattering and X-ray diffraction, Acta Biomaterialia 101: 637-645. https://doi.org/10.1016/j.actbio.2019.11.030