In metal-on-metal pairings, both the shell and head of an implant¹ consist of a cobalt-chromium-molybdenum alloy². The release of metal ions into the body has been reported as a result of implant wear. Bone loss (osteolysis) was observed in many cases. Some implant manufacturers have withdrawn³ devices of this type from the market. Recently, physicians and researchers from Charité - Universitätsmedizin Berlin and DRK Klinikum Westend have been able to show that cobalt and chromium release contributes to bone loss. Their findings, which show that metal ions impair⁴ the progenitors⁵ of bone-forming cells, have been published in the current edition of the journal Biomaterials*. Total hip⁶ replacement⁷ has been hailed as the 'operation of the century', and approximately 220,000 such procedures are performed in Germany every year. Most hip replacement operations produce satisfactory results, allowing patients to regain⁸ their mobility⁹ and pain-free status. Most of the procedures performed today use metal-on-polyethylene or ceramic-on-ceramic implants¹⁰. Metal-on-metal pairings have been shown to be associated with additional bone loss, leading to premature¹¹ revision surgery.

 

The Berlin-based team of researchers tested adjacent tissues, joint¹² fluids and bone marrow¹³, looking for changes that might be triggered by exposure to chromium and cobalt. The research revealed that both, metal wear particles and also dissolved metals, play a crucial role in the patient's overall level of exposure. Dissolved metal ions were shown to reach the bone marrow, where they impair mesenchymal stromal cells (MSCs), the progenitors of osteoblasts, a type of cell that is responsible for bone mineralization. The study revealed that MSCs isolated¹⁴ from the bone marrow of patients with elevated bone marrow metal concentrations had lost the ability to differentiate¹⁵ into bone-forming osteoblasts. The researchers were then able to replicate¹⁶ this effect by exposing cell cultures derived¹⁷ from non-exposed patients to a relevant level of dissolved chromium and cobalt - with identical results.

 

"Local exposure assessment¹⁸ represents a significant contribution in our quest to better understand the biological responses to metal wear, and helps to identify causations of implant failure," says Anastasia Rakow, a physician and researcher at Charité's Center for Musculoskeletal Surgery. Janosch Schoon, researcher at the Julius Wolff Institute and member of the German Society of Toxicology (GT), adds: "According to our research, the products of corrosion¹⁹ and wear are caused by multiple factors. The properties of the materials involved play a central role, as do biomechanical and anatomical factors specific to the individual patient. We need a systematic²⁰ approach if we intend to be able to adequately estimate actual exposure levels arising from the various metals used in hip implants." Artificial hip joints²¹ are becoming increasingly more durable²², and there has been a significant drop in the incidence of complications such as aseptic loosening. "In order to ensure long-term success and an implant lifespan of more than 15 years, we need to further improve our understanding of the biological effects of the materials used, in particular those of the implanted metals" explains Prof. Dr. Carsten Perka, Medical Director of the Center for Musculoskeletal Surgery. "This is why we will further encourage and promote interdisciplinary collaborations between physicians, toxicologists and biologists at the Berlin-Brandenburg Center for Regenerative Therapies." The researchers conclude the findings of their study as follows: the risks associated with metal-on-metal pairings clearly exceed their benefits. The researchers' long-term aim is to optimize²³ patient safety by using their findings to improve the design and composition of future implants.

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