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Author: Andrea Six, Swiss Federal Laboratory for Materials Science and Technology

Where bones are broken, surgeons usually have to connect the fragments with implants. Over time, the magnesium orthopedic screws will dissolve in the body, saving the patient from reoperation after the healing is completed, and reducing the risk of infection. However, what happens inside the body during this process is still largely unknown. In order to develop optimized alloys and orthopedic screws with functionalized surfaces, Empa researchers are studying magnesium corrosion.

When a surgeon wants to repair bone fragments after a fracture, the key question is which type of implant to use: screws and steel plates made of titanium or steel, which are very mechanically and chemically stable in the body, but later Must go through another surgical procedure? Or implants made of organic materials will dissolve over time, but may have some other disadvantages, such as lack of mechanical strength or unfavorable degradation products? Empa researchers are currently working on solving this problem: miniature magnesium implants and screws. They are mechanically strong at first, but then dissolve in the body in a controlled way that does not cause tissue damage.

This kind of magnesium implant is particularly suitable for medical orthopedic applications in children with rapid bone growth. The biodegradable screws will not damage the child's bone growth and save a second operation for small patients. In addition, the risk of infection can be minimized and costs can be reduced. "Magnesium is generally considered a white powder and is often used as a dietary supplement," said Arie Bruinink of "Connecting Technology and Corrosion" at Empa Labs. However, implants made of magnesium alloys are not only biocompatible; they also have bone-like mechanical properties in the first delicate healing stage and are therefore more suitable than titanium.

The benefit of absorbable screws may also be its curse. After all, dissolution is related to complex corrosion processes that change the surface structure and produce many products-these products may or may not be harmful. Depending on the type of magnesium alloy, due to insufficient corrosion resistance, hydrogen gas is generated during the degradation process-even forming an air cushion under the patient's skin. Although the surgeon’s intention is that magnesium screws will degrade due to corrosion, during which time the magnesium will oxidize and generate hydrogen gas, the formation of air cushions should be avoided. If more hydrogen is formed suddenly than the body can expel, the healing process of fragile bones may be disturbed.

However, it is this kind of bio-corrosion that exposes magnesium screws, and so far little is known about this kind of corrosion. This is where the corrosion researchers at Empa come in. They use specially developed analytical methods to describe biological corrosion in the human body under as realistic conditions as possible. Goal: The best alloy of magnesium and other biocompatible elements, and the new surface characteristics that can absorb magnesium screws. Ultimately, the researchers’ goal is to slowly and controllably degrade the implant without causing air pockets to form in the tissue.

"So far, it has been clear that the response will vary depending on the level of acidity in the tissue," Bruinink explained. In a slightly acidic environment, a large amount of hydrogen will be generated when magnesium corrodes; at a pH in the alkaline range, carbonate-containing products will be produced, which can even inhibit the required magnesium degradation. In a neutral environment with a pH of 7.4, such as in blood, magnesium hydroxide and phosphate products are formed, which at least slows down further corrosion. Blood-acting as an effective buffer-can keep its pH within a constant range. According to Bruinink, to date, magnesium implants have been analyzed in a relatively effective but non-physiological cushioning system. He thinks this procedure is unrealistic.

"Blood is a very special juice"-this is what Goethe's restless scholar Faust said. Whether Dr. Faust understands the so-called interstitial fluid is still unknown. About ten liters of salt water far exceeds the amount of blood in the human body. This underestimated "juice" moves slowly between tissues and cells, a hundred times slower than a snail. If new implants are to be developed, it is this interstitial fluid that is essential. The healing process of fractures is controlled by immune cells, which aim to produce a well-balanced bone resorption and remodeling structure, which is mainly embedded in the interstitial fluid.

However, the acidity of tissue fluid changes more than the acidity of blood. Depending on the body part and tissue condition, a variety of parameters will affect the inserted screw. In order to provide a true prediction of the biological corrosion process in the body, Bruinink has developed experimental analysis technology and flow cell, in which the pH adjustment is modeled on the human body. For example, in a battery pack consisting of 10 flow cells, researchers inserted magnesium alloy samples, which were washed with artificial tissue fluid-at the same speed as a human body.

In addition to pH measurement, detailed electrochemical characterization of small flow cells is currently underway. Evaluate electrochemical potential, changes in interfacial electrical impedance as a corrosion characteristic, and hydrogen production. "The flow cell is a miniature laboratory that simulates the reality of biological corrosion," Bruinink said. In the next step, the alloy samples will be combined with living cells in a miniature laboratory to simulate events in the body in more detail. Bruinink: "Once it is clear what actually happens in the biological corrosion process of magnesium alloys, we will be able to produce suitable implants with functionalized surfaces, for example, to promote beneficial reactions in the biological environment." Further explore and monitor the corrosion of bioabsorbable magnesium Citation provided by the Swiss Federal Laboratory of Materials Science and Technology: Progress in Orthopedic Surgery: Dissolved Screws (2019, November 6) Retrieved on December 14, 2021 from https://medicalxpress.com/news/2019 -11-orthopedic -surgery-advances-dissolves.html This document is protected by copyright. Except for any fair transaction for private learning or research purposes, no part may be copied without written permission. The content is for reference only.

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