Self-dissolving implants Cohesion for a while: bone screws

Author / Editor: Christoph Wöhrle * / Christian Lüttmann

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

Companies on the topic

The screws examined are tiny: their diameter is 2 millimeters.

(Image: Hereon / Björn Wiese)

Geesthacht – broken bones do not always heal by themselves, sometimes medicine has to help: so that they can grow together better, the damaged bones are fixed with screws, nails or plates. Up to now, such implants have generally consisted of durable materials such as stainless steel or titanium. After the bone has healed, the screws often need to be removed. Another operation is then pending for those who have actually recovered.

An alternative are implants that dissolve in the body by themselves while the bones grow back together. Magnesium is seen as a promising candidate for such soluble implants: The light metal is body-friendly, everyone carries around 25 grams of it anyway. In addition, it is continuously dissolved by body fluids. So far, two companies in Germany have offered such soluble implants for two different clinical applications: for vascular supports (stents) and for bone fixation. Research is about understanding the principle more precisely in order to transfer it to other cases, in particular the therapy of bone fractures.

Magnesium alloy with that certain something

The implants are not made of pure magnesium, but of alloys in which other substances are added to the light metal, e.g. B. gadolinium. “This allows you to specifically set how long a bone screw remains stable in the body before it gradually disappears,” explains Dr. Björn Wiese from the Hereon Institute for Metallic Biomaterials. The question is how the alloy content affects: How in detail does the gadolinium content influence the strength and degradation behavior of the implant? A good compromise has to be found: if an alloy decomposes too quickly, it would not stabilize the bone long enough. If it stays in the body for too long, there is a risk of later complaints.

In order to clarify this question, the researchers at the Helmholtz Center Hereon started an extensive series of experiments at the German electron psychrotron in Hamburg. The study was financed by the BMBF projects SynchroLoad and MgBone. In addition to the Hereon scientists, the Karlsruhe Institute of Technology (KIT) was also involved in the study.

On the left you can see the screw before the attempt to dismantle it.  On the right in the picture the same screw after 56 days in vitro without the degradation products.  The degradation due to corrosion can be clearly seen here.  The surface has changed.
On the left you can see the screw before the attempt to dismantle it. On the right in the picture the same screw after 56 days in vitro without the degradation products. The degradation due to corrosion can be clearly seen here. The surface has changed.

(Image: Krüger et. Al 2021)

“We made screws from different alloys, one with five percent and the other with ten percent gadolinium,” says Wiese. The researchers placed these samples in Petri dishes filled with body fluid-like mixtures of salts, vitamins and proteins and exposed them to body-like conditions in incubators for up to 56 days. In doing so, they tested several times with two methods how the screws change over time: on the one hand, they precisely measured the progressive weight loss, on the other hand they took pictures using micro-computed tomography. This procedure works in a similar way to a CT scanner in a hospital, but delivers significantly more detailed images with a resolution of a few micrometers.

Broken bones?  No problem.  At least if it is up to Jena researchers who have developed a new, self-healing bone cement (symbol picture).

More additive can double removal time

As expected, the working group observed through the micro-CT examination that the first thing to do is to loosen the tips or teeth of the thread and gradually round them off. On the other hand, the corrosion in the thread valleys does not progress as strongly. Another result, which Wiese cites: “The magnesium alloy with five percent gadolinium corroded significantly faster than that with ten percent.” Extrapolated, a screw with a little gadolinium would have completely dissolved in the body after a little more than four years. On the other hand, one with a lot of gadolinium would only have completely disappeared after about eight years. This means: If long-term stabilization is important after a bone fracture, it is better to implant magnesium screws with a higher gadolinium content.

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

(Image: Krüger et. Al 2021)

The scientists also recognized how the manufacturing process – turning the screw from a magnesium-gadolinium blank – affects the corrosion behavior. “In the case of the stronger alloy with the higher gadolinium content, the deformation layer was less deep, which has an influence on the corrosion,” explains Wiese. However, it remains questionable what effect the abrasion from the turning tool has on the corrosion behavior of the screws: For example, iron could detach from the tool during turning, stick to the screw surface and lead to rapid degradation. Other research questions are also still open: How in detail do the magnesium and gadolinium atoms, which are gradually detaching from the implant, influence the formation of new bone? And is faster or slower corrosion an advantage for this? Questions that the Hereon experts will soon be tackling with new series of tests.

Originalpublikation: Diana Krüger, Berit Zeller-Plumhoff, Björn Wiese, Sangbong Yi, Marcus Zuber, Florian Wieland, Julian Moosmann, Regine Willumeit-Römer: Assessing the microstructure and in vitro degradation behavior of Mg-xGd screw implants using μCT, Journal of Magnesium and Alloys, online: 25 September 2021; DOI: 10.1016/j.jma.2021.07.029

* C. Wöhrle, Helmholtz Center, 21502 Geesthacht

(ID:47732182)

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