In vivo corrosion of modular endoprostheses remains a great concern, as the release of heavy metal ions can impair the implant's service life and the wellbeing of the patient. The detailed corrosion mechanisms that occur in vivo are so far not completely understood. In this context, the effects of implant released cobalt (Co) and chromium (Cr) ions on osteoblast mineralization and gene expression have not been investigated extensively.
This comprehensive study aimed at furthering the understanding of in vivo implant corrosion from the clinical signs via prosthesis retrievals and histology of the synovial membranes down to the molecular processes instigated by corrosion products and its effects on bone mineralization. A detailed in vivo failure analysis was performed investigating 22 retrieved hip endoprostheses from different manufacturers and taper material combinations. The aim was to find a correlation of taper damage and especially corrosion to susceptible biomedical alloys and its effect on periprosthetic tissue as well as the clinical implant performance with regard to revision diagnosis and presence of radiolucent lines (RLL). A second part investigated the effects of Co and Cr ions on the in vitro mineralization process of osteoblasts. Cell cultures were exposed to relevant concentrations of CoCl₂ and CrCl₃ (0 μM, 100 μM, 200 μM) with and without addition of phosphate. Mineralization behavior was analyzed with Alizarin Red assay and Von Kossa staining of calcium depots, alkaline phosphatase activity of osteoblasts and gene expression was analyzed with real time quantitative PCR.
The retrieval study provides evidence of in vivo fretting and crevice corrosion on all metallic tapers combined with either ceramic or metal femoral heads. Within the modular taper junctions, selective dissolution of the α phase occurred in wrought TiAl6V4 alloys, and etching of the fine-grained wrought CoCr28Mo6 alloy implants was observed in formed crevices. In addition, significant amounts of wear particles and corrosion products were detected in retrieved synovial membranes. An increased risk for the occurrence of a RLL in the proximal zones was determined for patients with a corroded mixed metal taper.
Whereas Co ions have hardly any effects on mineralization, Cr ions cause a significant concentration dependent decrease in mineralization rate of osteoblasts. However, this effect is alleviated by addition of a phosphate source. Our data reveal that Cr ions depleted dissolved phosphates by forming an insoluble complex (CrPO₄), which inhibits the phosphate dependent mineralization process. No significant effect of the heavy metal ions on osteoblast activity by means of alkaline phosphate activity as well as on gene expression is determined.
This study broadens the understanding of in vivo corrosion of metallic modular implants and its clinically relevant effects on mineralization. Based on these findings, in vivo corrosion of CoCr28Mo6 endoprostheses should be limited to avoid inhibitory effects of Cr³⁺ on bone mineralization which can contribute to premature implant failure.