This research explored the biological effects of wear from cobalt-chromium (CoCr) hip implants on joint tissues. Such wear may cause implant failure and various adverse effects in patients, resulting in reduced usage of CoCr implants in arthroplasty procedures. CoCr prostheses allow for broader motion, and are more cost-effective than ceramic-based alternatives. CoCr implants release cytotoxic wear comprising ions, nanoparticles (NP), and microparticles (MP), believed to contribute majorly to implant complications. Existing literature provides varying cytotoxicity assessments of these debris components. This work presents a novel perspective on this issue using an innovative combination of three-dimensional dielectrophoresis (3DEP), inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy/energy-dispersive spectroscopy (SEM-EDX) instruments, assessing cell electrophysiology, ion content, and debris features respectively.

Based on cell damage and 3DEP analyses, high levels of Cr6+ and Co2+ ions were comparably cytotoxic to those of Cr3+ over a 72h culture of red blood cells (RBC), contradicting most reports. Cr3+ was shown by 3DEP and ICP-MS to enter cells through membrane interaction, and unexpectedly more than Cr 6+ at high levels (from 1 mg.L-1 onwards). This prompted a re-evaluation of Cr 3+ contribution to joint tissue damage. 3DEP and cell morphometry revealed that Co2+ deformed RBC membranes, consistent with its entry kinetics reminiscent of endocytosis. Unlike Cr 3+ and Co2+, Cr6+ damaged intracellular targets as documented, and by haemoglobin oxidation in RBC culture.

The nature of NP/MP cytotoxicity was tentatively inferred by characterising their features through a novel non-invasive enzymatic method of extraction from the human hip fluid. This method was successfully coupled with an automated screening of NP in high volumes (SEM-EDX) and confirmed their reported features. SEM-EDX and 3DEP could establish a speculativecytotoxicity hierarchy for CoCr debris based on size, shape and composition. It designated needle-shaped Co NP as potentially the most cytotoxic, thereby causing implant failure.