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Researchers found traces of metal in bone samples from patients whose hip or knee joints had to be replaced. (Image: Albina Glisic/Shutterstock)

Endoprostheses constantly release metal particles into the body

By Charité – Universitätsmedizin Berlin
September 24, 2020

BERLIN, Germany: Using highly complex analytical techniques, researchers from Charité – Universitätsmedizin Berlin have been able to observe how different metals were released from joint implants and accumulated in the surrounding bone tissue. Contrary to previous assumptions, a constant release of metals from various implant components occurred independently of mechanical stress. The study findings will help to optimise the materials used in implants and enhance their safety.

Modern joint implants restore pain-free mobility of patients with chronic degenerative joint disease, thereby drastically enhancing their quality of life. To ensure long-term mechanical stability, artificial joints are made from materials containing a range of different metal alloys. A crucial factor in determining an implant’s long-term effectiveness, however, is its integration into the surrounding bone tissue. Previous studies on implant stability showed that friction between the articulating surfaces (bearing surfaces) can result in the formation of metal debris. These metal residues can lead to osteolysis and thus to premature loosening of the implant. However, a possible permanent release of metals from other parts of the prosthesis has been disregarded so far.

Analyses of spatially resolved synchrotron X-ray fluorescence images of human bone and bone marrow biopsies: characteristic distribution patterns for specific metals (cobalt, chromium and titanium) released from knee and hip arthroplasty implants. (Image: Naujok/Charité)

The research group at the Julius Wolff Institute of Biomechanics and Musculoskeletal Regeneration of the Charité and BIH Center for Regenerative Therapies has now investigated the spatial distribution and local toxicokinetics of released metallic wear and corrosion products in the surrounding bone tissue using a unique synchrotron-based X-ray fluorescence imaging system.

“Our work has enabled us to show, for the first time, that both particulate and dissolved metals released from arthroplasty implants are present in the surrounding bone and bone marrow at supraphysiological levels,” said co-author Dr Sven Geißler from the Julius Wolff Institute. “Therefore, the collagen-rich layer which encapsulates the implant after surgery does not separate these metals from human tissue to the extent previously assumed.”

The researchers examined very small bone samples from 14 patients undergoing either a hip or knee arthroplasty procedure. They used X-ray fluorescence analysis to determine the elemental composition of the samples qualitatively and quantitatively. This technique allows unique insights into the concentration, distribution, localisation and accumulation of metallic degradation products such as cobalt, chromium or titanium in the adjacent bone and bone marrow.

“Our study has made a major contribution to the improvement of the risk–benefit evaluation of medical devices”
– Dr Sven Geißler, Charité Berlin

The extremely bright and intensively focused X-ray beam required was achieved by the synchrotron radiation source at the European Synchrotron Radiation Facility in Grenoble in France, and is the only particle accelerator in the world to offer a spatial resolution of up to 30 μm. “Our work therefore addresses an issue of enormous clinical relevance with a highly complex experimental setup,” said lead-author Dr Janosch Schoon from the Julius Wolff Institute.

“Our study has made a major contribution to the improvement of the risk–benefit evaluation of medical devices. It has shown that these evaluations should not only comprise biocompatibility testing of raw materials; rather, biocompatibility testing should also extend to wear and corrosion products. The data from this study will therefore prove instrumental in keeping implant safety at the highest possible level,” explained Geißler. Based on their findings, the researchers plan to conduct additional studies which will investigate the biological consequences of metal release on bones and bone marrow. At the same time, the researchers will develop new approaches which will facilitate the reliable preclinical testing of implant materials using both human cells and engineered tissues.

The study, titled  “Metal‐specific biomaterial accumulation in human peri‐implant bone and bone marrow”, was published online on 3 August 2020 in Advanced Science.

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