Increased revision rates and early failure of Metal-on-Metal (MoM) hip replacements are often due to adverse reaction to metal debris (ARMD). Cobalt is a major component of MoM joints and can initiate an immune response via activation of the innate immune receptor Toll-like receptor 4 (TLR4). This leads to increased secretion of inflammatory cytokines/chemokines e.g. CCL3 and CCL4. The aim of this study was to evaluate whether TLR4-specific neutralising antibodies can prevent cobalt-mediated activation of TLR4. MonoMac 6 (MM6) cells, a human macrophage cell line, were treated with two different TLR4-specific monoclonal antibodies followed by 0.75mM of cobalt chloride (CoCl2). Lipopolysaccharide (LPS), a known TLR4 agonist was used as a positive control. Enzyme-linked immunosorbent assay (ELISA) was used to assess CCL3/CCL4 protein secretion and real time- polymerase chain reaction (RT-PCR) allowed quantification of CCL3/CCL4 gene expression. MM6 cells treated with cobalt and LPS up-regulate CCL3 and CCL4 gene expression and protein secretion. MM6 cells pre-treated with both monoclonal antibodies prior to stimulation with 0.75mM CoCl2 for 16 hours demonstrated significant inhibition of both CCL3 and CCL4 secretion as well as gene expression (both p=<0.0001). One of the antibodies failed to inhibit chemokine expression and secretion in LPS treated cells. This study identifies for the first time the use of TLR4-specific monoclonal antibodies to prevent cobalt activation of TLR4 and subsequent inflammatory response. This finding demonstrates the potential to exploit TLR4 inhibition in the context of MoM joint replacements by contributing to the development of novel therapeutics designed to reduce the incidence of ARMD.
Since 2010, there has been a sharp decline in the use of metal-on-metal joint replacement devices due to adverse responses associated with the release of metal wear particles and ions in patients. Surface engineered coatings offer an innovative solution to this problem by covering metal implant surfaces with biocompatible and wear resistant materials. The present study tests the hypothesis whether surface engineered coatings can reduce the overall biological impact of a device by investigating recently introduced silicon nitride coatings for joint replacements. Biological responses of peripheral blood mononuclear cells (PBMNCs) to Si3N4 model particles, SiNx coating wear particles and CoCr wear particles were evaluated by testing cytotoxicity, inflammatory cytokine release, oxidative stress and genotoxicity. Clinically relevant wear particles were generated from SiNx-on-SiNx and CoCr-on-CoCr bearing combinations using a multidirectional pin-on-plate tribometer. All particles were heat treated at 180°C for 4 h to destroy endotoxin contamination. Whole peripheral blood was collected from healthy donors (ethics approval BIOSCI 10–108, University of Leeds). The PBMNCs were isolated using Lymphoprep (Stemcell) and incubated with particles at various volumetric concentrations (0.5 to 100 µm3 particles/cell) for 24 h in 5% (v/v) CO2 at 37°C. After incubation, cell viability was measured using the ATPlite assay (Perkin Elmer); TNF-alpha release was measured by ELISA (Invitrogen); oxidative stress was measured using H2DCFDA (Abcam); and DNA damage was measured by comet assay (Tevigen). The results were expressed as mean ± 95% confidence limits and the data was analysed using one-way ANOVA and Tukey-Kramer post-hoc analysis. No evidence of cytotoxicity, oxidative stress, TNF-alpha release, or DNA damage was observed for the silicon nitride particles at any of the doses. However, CoCr wear particles caused cytotoxicity, oxidative stress, TNF-alpha release and DNA damage in PBMNCs at high doses (50 µm3 particles per cell). This study has demonstrated the in-vitro biocompatibility of SiNx coatings with primary human monocytic cells. Therefore, surface engineered coatings have potential to significantly reduce the biological impact of metal components in future orthopaedic devices.
Total ankle replacement (TAR) has a mean survivorship of 77% at 10 years which is poor compared to other types of joint arthroplasty. Osteolysis and aseptic loosening are commonly cited TAR failure modes, the mechanisms of which are unknown. Retrieval analyses of TAR devices may reveal mechanisms of failure similar or dissimilar to other joint replacements. This study investigated whether TAR explants exhibit similar damage modes to those recognised in other total joint replacements. 22 Ankle Evolution System TARs (Transystème, Nimes, France) were implanted and retrieved by the same surgeon. Mean implantation time was 7.8 yrs (5.3 to 12.1 range). Pain and/or loosening were the indications for revision. Macro photography, an Alicona Infinite microscope and the Hood/Wasielewski scale were used to classify damage modes on the polyethylene insert. Scanning electron microscopy with energy dispersive X-ray spectroscopy was used to determine the composition of third body debris and to image the fixation surface of the tibial components. Mean damage score was 185.4 (± 40.0 SD). Damage modes common to total knee replacements were identified on both the superior and inferior insert surfaces, these included: burnishing, scratching, pitting and abrasion. Titanium particles, hydroxyapatite fragments and bone debris were embedded in the insert surfaces. Fixation surface delamination was identified by the ongrowth of tissue between the cobalt chromium substrate and titanium alloy coating. Damage modes indicative of high levels of wear and deformation were evident. Pitting caused by third body debris was abundant and suggested fixation surface wear and failure.
