J. Lutz

Co(II)‐mediated effects of plain and plasma immersion ion implanted cobalt‐chromium alloys on the osteogenic differentiation of human mesenchymal stem cells

Medical CoCr is one of the main alloys used for metal‐on‐metal prosthesis in patients with total hip arthroplasty. CoCr surfaces modified by nitrogen plasma immersion ion implantation (PIII) are characterized by improved wear resistance but also showed increased Co(II) ion release under in vitro conditions. For the first time, CoCr modified by nitrogen PIII was evaluated with regard to its effect on the osteogenic differentiation of MSC. The activity of alkaline phosphatase, the expression of the osteogenic genes Runt‐related transcription factor 2, osteopontin as well as integrin‐binding bone sialoprotein and the production of osteocalcin and hydroxyapatite were determined. The results of our study demonstrate that Co(II) ions released from the alloy affected the osteogenic differentiation of MSC. Distinct differences in differentiation markers were found between pristine and modified alloys for osteocalcin but not for integrin‐binding sialoprotein and hydroxyapatite. Interestingly, osteopontin was upregulated in naive and differentiated MSC by Co(II) ions and modified CoCr, likely through the induction of a cellular hypoxic response. The findings of this study contribute to a better understanding of possible risk factors with regard to a clinical applicability of surface modified CoCr implant materials.

Cytocompatibility of nitrogen plasma ion immersed medical cobalt-chromium alloys.

Wear particles and ion release from medical CoCr contribute to the risk for aseptic loosening. Nitrogen plasma immersion ion implantation (PIII) has been shown to reduce wear of CoCr but is associated with increased Co ion release. This work investigated the cytocompatibility of CoCr modified by nitrogen PIII at different temperatures (mCoCr). The osteosarcoma cell line Saos-2, mesenchymal stem cells (MSCs), and mononuclear cells (MNCs) were grown directly on CoCr/mCoCr discs or treated with CoCl2. Proliferation and metabolic activity of Saos-2 and MSC were decreased on mCoCr in correlation with increasing implantation temperature. The elevated Co ion release seemed to play a decisive role since analog effects were observed by treatment of cells with CoCl2. Proliferation of phytohemagglutinin-stimulated MNC was reduced in the presence of CoCr discs or CoCl2. For MNC-alloy cultures we observed an increase in interleukin 2 (IL-2) and a decrease in interferon γ (IFN-γ) and IL-10 secretion compared to cultures without alloys. The results of this study demonstrate that PIII process temperature and corresponding Co ion release correlate with material cytocompatibility. Therefore, the two competing parameters, reduction of wear and increase in Co ions, have to be taken into consideration for the evaluation of the clinical applicability of nitrogen-implanted CoCr.