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Cohesion for a period: bone screws

Auto-dissolving implants Cohesion for a period: bone screws

Author/Editor: Christoph Wehrl* / Christian Lotmann

Some broken bones need extra support to heal well. Self-dissolving implants are available so that used bone screws do not have to be removed again later during surgery. A research team led by the Helmholtz Center here examined various alloys for this purpose.

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The checked screws are extremely small: 2 millimeters in diameter.

(Photo: Hereon/Björn Wiese)

Geesthacht – broken bones do not always heal on their own, and sometimes medication should help: so that they can grow together better, the damaged bones are fixed with screws, screws or plates. Until now, these implants have generally consisted of durable materials such as stainless steel or titanium. After the bone has healed, the pins often need to be removed. Another operation is pending after that for those who have already recovered.

An alternative is implants that dissolve in the body on their own while the bones grow back together. Magnesium is a promising candidate for such soluble implants: the light mineral is body friendly, everyone carries around 25 grams of it anyway. In addition, it is constantly dissolved by body fluids. So far, two companies in Germany have offered such dissolvable implants in two different clinical applications: for vascular support (stents) and for bone stabilization. The research is about understanding the principle more accurately in order to transfer it to other cases, particularly the treatment of bone fractures.

Magnesium alloy with this specific thing

The implants are not made of pure magnesium, but of alloys in which other substances are added to the light metal, for example b. gadolinium; Dr. explains. Bjorn Wiese of the Heron Institute for Mineral Biomaterials. The question is how does alloy content affect: How in detail does gadolinium content influence the strength and deterioration of implant behavior? A good compromise must be found: if the alloy disintegrates too quickly, it will not hold the bone long enough. If it remains in the body for a long time, then there is a risk of subsequent complaints.

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In order to clarify this question, researchers at the Helmholtz Center here began an extensive series of experiments on the German Electron Sechrotron in Hamburg. The study was funded by the BMBF SynchroLoad and MgBone projects. In addition to the Heron scientists, the Karlsruhe Institute of Technology (KIT) was also involved in the study.

On the left you can see the screw before trying to take it apart.  On the right in the photo, the same screw after 56 days in the lab without decomposition products.  The deterioration due to corrosion can be clearly seen here.  The surface has changed.
On the left you can see the screw before trying to take it apart. On the right in the photo, the same screw after 56 days in the lab without decomposition products. The deterioration due to corrosion can be clearly seen here. The surface has changed.

(Photo: Krueger et al. 2021)

“We made screws out of different alloys, one with five percent and one with ten percent gadolinium,” Wiese says. The researchers placed these samples in Petri dishes filled with a body fluid-like mixture of salts, vitamins and proteins, and subjected them to bodily-like conditions in incubators for up to 56 days. In doing so, they tested several times in two ways on how the nails changed over time: on the one hand, they accurately measured gradual weight loss, and on the other hand, they took pictures using a CT scan. This procedure works in a similar way to a hospital CT scanner, but provides significantly more detailed images with a resolution of a few micrometers.

Broken bones?  Not a problem.  At least if it were up to Jena researchers who developed a new self-healing bone cement (avatar).

More additives can double the removal time

As expected, the working group noted through the CT scan that the first thing to do was loosen the tips or teeth of the floss and gradually bring them closer. On the other hand, the erosion in the valleys of the threads does not progress with the same force. Another result, Wiese cites: “A magnesium alloy containing five percent gadolinium corroded much faster than that of 10 percent.” In extrapolation, a nail containing a little gadolinium would have dissolved completely in the body after a little more than four years. On the other hand, a person with a lot of gadolinium may disappear completely after about eight years. This means: If long-term fixation is important after a bone fracture, it is better to implant magnesium screws with a higher gadolinium content.

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The tips and depths of the Mg-5Gd screw can be seen here through a special microscope image.
The tips and depths of the Mg-5Gd screw can be seen here through a special microscope image.

(Photo: Krueger et al. 2021)

The scientists also learned about how the manufacturing process – turning the screw from a magnesium-gadolinium vacuum – affects the corrosion behavior. “In the case of a stronger alloy with a higher gadolinium content, the deformation layer was less deep, which affects the corrosion,” Wiese explains. However, it is still questionable what effect the wear from the rotating tool has on the wear behavior of the bolt: for example, iron can detach from the tool during turning, stick to the surface of the bolt and lead to rapid deterioration. Other research questions remain open as well: How do the magnesium and gadolinium atoms, which gradually separate from the implant, affect the formation of new bone in detail? Is faster or slower wear an advantage? Questions that Hereon experts will soon address with a new series of tests.

original post: Diana Krueger, Berit Zeller Blumhof, Bjorn Wiese, Sangpong Yee, Marcus Zuber, Florian Welland, Julian Mosman, Regen Willumet Romer: Evaluation of the microstructure and in vitro degradation behavior of Mg-xGd screw implants using μCT, Journal of Magnesium and Alloys, online: Sep 25, 2021; DOI: 10.1016 / j.jma.2021.07.029

* C. Wöhrle, Helmholtz Center, 21502 Geesthacht

(No.: 47732182)