Aims. Second-generation metal-on-metal (MoM) articulations in total hip arthroplasty (THA) were introduced in order to reduce wear-related complications. The current study reports on the serum cobalt levels and the clinical outcome at a minimum of 20 years following THA with a MoM (Metasul) or a ceramic-on-polyethylene (CoP) bearing. Methods. The present study provides an update of a previously published prospective randomized controlled study, evaluating the serum cobalt levels of a consecutive cohort of 100 patients following THA with a MoM or a CoP articulation. A total of 31 patients were available for clinical and radiological follow-up examination. After exclusion of 11 patients because of other cobalt-containing implants, 20 patients (MoM (n = 11); CoP (n = 9)) with a mean age of 69 years (42 to 97) were analyzed. Serum cobalt levels were compared to serum cobalt levels five years out of surgery. Results. The median cobalt concentration in the MoM group was 1.04 μg/l (interquartile range (IQR) 0.64 to 1.70) at a mean of 21 years (20 to 24) postoperatively and these values were similar (p = 0.799) to cobalt levels at five years. In the CoP control group, the median cobalt levels were below the detection limit (< 0.3 μg/l; median 0.15 μg/l, IQR 0.15 to 0.75) at 20 years. The mean Harris Hip Score was 91.4 points (61 to 100) in the MoM group and 92.8 points (63 to 100) in the CoP group. Conclusion. This study represents the longest follow-up series evaluating the serum cobalt levels after 28 mm head MoM bearing THA and shows that serum cobalt concentrations remain at low levels at a mean of 21 years (20 to 24) after implantation. Cite this article:Bone Joint Res. 2020;9(3):145–150

An increase in metal ion levels is seen after implantation of all MoM hip prosthesis due to release from the surface directly, more so during articulation and corrosion of the bearing surfaces. The bearing surfaces in MoM prosthesis consist of cobalt, chromium and molybdenum. Several case-reports of cobalt toxicity due to a MoM prosthesis have been published in the last decade. Cobalt intoxication may lead to a variety of symptoms: neuro-ocular toxicity (tinnitus, vertigo, deafness, blindness, convulsions, headaches and peripheral neuropathy), cardiotoxicity and thyroid toxicity. Nausea, anorexia and unexplained weight loss have been described. Systemic effects from metal ions even with well functioning implants or with ion concentrations lower than those associated with known adverse effects may exist and warrant investigation. The aim of this study is to investigate self-reported systemic complaints in association with cobalt ion concentrations in patients with any type of MoM hip prosthesis. A cohort study was conducted. Patients with both unilateral and bilateral, resurfacing and large head metal on metal total hip arthroplasties were included for the current study. Blood metal ion concentrations (cobalt and chromium) were measured by inductively coupled plasma mass spectrometry (ICP-MS). Based on the known cobalt toxicity symptoms of case-reports and toxicology reports a new non-validated questionnaire was developed. questions were subdivided in general questions/symptoms, vestibular symptoms, neurological symptoms, emotional health and cardio- and thyroid toxicity symptoms. Independent samples T test, Fishers Exact Test and Pearsons (R) correlation were used. Analysis was performed on two groups; a low cobalt ion concentration group and a high cobalt ion concentration group A total of 62 patients, 36 (58%) men and 26 (42%) women, were included with a mean age at surgery of 60.8 ± 9.3 years (41.6 – 78.1) and a mean follow up of 6.3 ± 1.4years (3.7 – 9.6). In these patients a total of 71 prosthesis were implanted: 53 unilateral and 9 bilateral. Of these, 44 were resurfacing and 27 large head metal on metal (LHMoM) total hip arthroplasties. Mean cobalt and chromium ion concentrations were 104 ± 141 nmol/L (9 – 833) and 95 ± 130nmol/L (6 – 592), respectively. Based on the different thresholds (120 – 170 or 220 nmol/L) the low cobalt ion concentration group consisted of 44 (71%), 51 (82%) or 55 (89%) subjects respectively. No differences were found in general characteristics, independently of the threshold. The composite score of vestibular symptoms (vision, hearing, tinnitus, dizziness) was significantly higher (p < .050) in all high cobalt ion concentrations groups, independent of the threshold value This study aimed to detect a trend in self-reported systemic complaints in patients with metal-on-metal hip arthroplasty due to raised cobalt ion concentrations. Vestibular symptoms were more common in high cobalt ion concentration groups independent of the three threshold levels tested. The upper limit of acceptable cobalt ion concentrations remains uncertain. With regards to proactively inquired, self-reported symptoms the threshold where effects may be present could be lower than values currently applied in clinical follow-up. It is unknown what exposure to elevated metal ion concentrations for a longer period of time causes with aging subjects. Further research with a larger cohort and a more standardized questionnaire is necessary to detect previously undiscovered or under-reported effects

Objectives. The cytotoxicity induced by cobalt ions (Co. 2+. ) and cobalt nanoparticles (Co-NPs) which released following the insertion of a total hip prosthesis, has been reported. However, little is known about the underlying mechanisms. In this study, we investigate the toxic effect of Co. 2+. and Co-NPs on liver cells, and explain further the potential mechanisms. Methods. Co-NPs were characterised for size, shape, elemental analysis, and hydrodynamic diameter, and were assessed by Transmission Electron Microscope, Scanning Electron Microscope, Energy Dispersive X-ray Spectroscopy and Dynamic Light Scattering. BRL-3A cells were used in this study. Cytotoxicity was evaluated by MTT and lactate dehydrogenase release assay. In order to clarify the potential mechanisms, reactive oxygen species, Bax/Bcl-2 mRNA expression, IL-8 mRNA expression and DNA damage were assessed on BRL-3A cells after Co. 2+. or Co-NPs treatment. Results. Results showed cytotoxic effects of Co. 2+. and Co-NPs were dependent upon time and dosage, and the cytotoxicity of Co-NPs was greater than that of Co. 2+. In addition, Co-NPs elicited a significant (p < 0.05) reduction in cell viability with a concomitant increase in lactic dehydrogenase release, reactive oxygen species generation, IL-8 mRNA expression, Bax/Bcl-2 mRNA expression and DNA damage after 24 hours of exposure. Conclusion. Co-NPs induced greater cytotoxicity and genotoxicity in BRL-3A cells than Co. 2+. Cell membrane damage, oxidative stress, immune inflammation and DNA damage may play an important role in the effects of Co-NPs on liver cells. Cite this article: Y. K. Liu, X. X. Deng, H.L. Yang. Cytotoxicity and genotoxicity in liver cells induced by cobalt nanoparticles and ions. Bone Joint Res 2016;5:461–469. DOI: 10.1302/2046-3758.510.BJR-2016-0016.R1

Background. Adverse reactions to metal debris are implicated in the failure of metal-on-metal hip arthroplasty. The peri-implant tissues are often infiltrated by leukocytes which may cause observed immunological effects, including soft tissue necrosis and osteolysis. Cobalt ions from orthopaedic implants aberrantly activate the innate immune receptor human toll-like receptor-4 (TLR4), leading to inflammatory cytokine release including interleukin-8 (IL-8). IL-8 has been shown to increase expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). These factors are essential for leukocyte adhesion to endothelium, which is required for leukocyte migration into tissues. This study investigates cobalt's effect on gene and protein changes in IL-8, ICAM-1 and VCAM-1 to determine their potential role in immune cell infiltration of peri-implant tissues. Methods. TLR4-expressing human dermal microvascular endothelial cells (HMEC-1) were treated with a range of clinically relevant cobalt ion concentrations. IL-8 protein secretion was measured by enzyme-linked immunosorbent assay (ELISA). Gene expression changes were quantified by TaqMan-based real time polymerase chain reaction. Results. Stimulation with cobalt ions significantly increases IL-8 secretion (n=3) in HMEC-1 cells. This is a TLR4-specific effect as a small molecule TLR4 antagonist inhibited cobalt-induced IL-8 secretion. Following cobalt treatment (0.75mM cobalt chloride) there is a 12-fold increase in ICAM-1 (p-value=0.0004) and a 6-fold increase in VCAM-1 (p-value<0.0001) gene expression. Work will be undertaken to determine the role of TLR4 in these responses. Conclusion. Cobalt increases IL-8 secretion and adhesion molecule gene expression in HMEC-1 cells. This in vitro finding demonstrates the potential for cobalt ions to increase leukocyte adhesion to the endothelial surface. This may contribute to leukocyte infiltration of peri-implant tissues in metal-on-metal hip arthroplasty failure

Background. Increased revision rates and early failure of Metal-on-Metal (MoM) hip replacements are often due to adverse reaction to metal debris (ARMD). ARMD describes numerous symptoms in patients such as pain, osteolysis and soft tissue damage. 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 e.g. interleukin-8 (IL-8). This study investigates whether TLR4-specific antagonists inhibit the inflammatory response to cobalt using IL-8 gene expression and protein secretion as a marker of TLR4 activation. Methods. MonoMac 6 (MM6) cells, a human macrophage cell line, were treated with TLR4-specific antagonists followed by 0.75mM of cobalt chloride. Lipopolysaccharide (LPS), a known TLR4 agonist was used as a positive control. Enzyme-linked immunosorbent assay (ELISA) was used to assess IL-8 protein secretion and real time- polymerase chain reaction (RT-PCR) allowed quantification of IL-8 gene expression. Results. MM6 cells treated with cobalt and LPS up-regulate IL-8 gene expression and protein secretion (n=3). The addition of TLR4-specific antagonists significantly inhibits this up-regulation suggesting the observed effects are TLR4-mediated. MM6 cells stimulated with cobalt (0.75mM) for 16 hours demonstrated a 27-fold increase in IL-8 gene expression (p-value = < 0.0001). When pre-treated with 10μg/ml of a TLR4-specific antagonist fold increase decreased to 6-fold (p-value = < 0.0001). IL-8 secretion decreased from 5000pg/ml to 3000pg/ml (p-value = < 0.0001). Conclusion. TLR4-specific antagonists inhibit cobalt-mediated IL-8 gene expression and protein secretion in MM6 cells. This finding demonstrates the potential to exploit this inhibition in the context of MoM joint replacements by contributing to the development of novel therapeutics designed to improve MoM implant longevity, reduce the incidence of ARMD and prevent subsequent revision surgery

Background. Metal-on-metal (MoM) hip arthroplasty has been associated with adverse reactions including pseudotumours, and osteolysis. Tissues surrounding failed MoM hip implants are often infiltrated by inflammatory cells such as monocytes and neutrophils. The mechanisms by which these cells are recruited to the tissues remain unclear. Cobalt from MoM implants activates Toll-like receptor 4 (TLR4), an immune cell surface receptor usually responsible for recognition of bacteria and prevention of sepsis. Activation by bacteria leads to secretion of pro-inflammatory cytokines which guide other immune cells to the site of inflammation. The effect of cobalt on this response is unknown and therefore this study aims to determine the effect of cobalt-mediated TLR4 activation on the migration of inflammatory cells. Methods. A human macrophage cell line (MonoMac 6) was stimulated with a physiologically-relevant range of cobalt ions for 24h with or without pre-treatment with a TLR4 antagonist. Conditioned media was collected and used in a trans-well migration assay to determine its effect on migration of primary monocytes and neutrophils isolated from whole human blood. Migrated cells were stained with haematoxylin and counted at ×40 magnification. Results. Conditioned media from cobalt-treated macrophages caused elevated monocyte and neutrophil migration across all concentrations. Pre-treatment of MonoMac 6 cells with a TLR4 antagonist significantly decreased the response. This suggests that the cytokine profile produced in response to cobalt-mediated TLR4 activation is pro-migratory for immune cells. Conclusions. Cobalt activation of TLR4 leads to secretion of inflammatory cytokines that attract monocytes and neutrophils. This work highlights a potential mechanism by which cobalt ions from failed MoM joints could be involved in inflammatory cell recruitment to the surrounding tissues. The TLR4 signalling pathway represents an exciting area for further investigation as a therapeutic target in the prevention of adverse reactions to cobalt ions. Disclosure. This work is funded by DePuy Synthes Ltd and the Newcastle NIHR Biomedical Research Centre

Objectives. Recently, high failure rates of metal-on-metal (MOM) hip implants have raised concerns of cobalt toxicity. Adverse reactions occur to cobalt nanoparticles (CoNPs) and cobalt ions (Co. 2+. ) during wear of MOM hip implants, but the toxic mechanism is not clear. Methods. To evaluate the protective effect of zinc ions (Zn. 2+. ), Balb/3T3 mouse fibroblast cells were pretreated with 50 μM Zn. 2+. for four hours. The cells were then exposed to different concentrations of CoNPs and Co. 2+. for four hours, 24 hours and 48 hours. The cell viabilities, reactive oxygen species (ROS) levels, and inflammatory cytokines were measured. Results. CoNPs and Co. 2+. can induce the increase of ROS and inflammatory cytokines, such as tumour necrosis factor α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6). However, Zn pretreatment can significantly prevent cytotoxicity induced by CoNPs and Co. 2+. , decrease ROS production, and decrease levels of inflammatory cytokines in Balb/3T3 mouse fibroblast cells. Conclusion. These results suggest that Zn pretreatment can provide protection against inflammation and cytotoxicity induced by CoNPs and Co. 2+. in Balb/3T3 cells. Cite this article: Y. Liu, H. Zhu, H. Hong, W. Wang, F. Liu. Can zinc protect cells from the cytotoxic effects of cobalt ions and nanoparticles derived from metal-on-metal joint arthroplasties? Bone Joint Res 2017;6:649–655. DOI: 10.1302/2046-3758.612.BJR-2016-0137.R2

Summary. Metal-on-metal hip replacements have been associated with adverse reactions including inflammatory pseudotumours and soft tissue necrosis. We have shown that cobalt can directly activate toll-like receptor 4, an immune receptor causing pro-inflammatory interleukin-8 secretion. This may contribute to adverse reaction development. Introduction. Metal-on-metal hips have the highest failure rate of any joint arthroplasty material. Reasons for failure include the development of pseudotumours, soft tissue necrosis and pain around the affected joint. The adverse reactions appear to be inflammatory as failing joints are often infiltrated by immune cells such as lymphocytes. However the exact cellular and biological mechanisms underlying this inflammation are unknown. Toll-like receptor 4 (TLR4) is found on the surface of immune cells including macrophages and dendritic cells. It is activated by lipopolysaccharide (LPS) from Gram negative bacteria, inducing an immune response against the pathogen through increased secretion of pro-inflammatory cytokines. It has recently been shown that nickel can activate TLR4, causing inflammation. Cobalt, a component of many metal-on-metal joints, is adjacent to nickel in the periodic table and shares a number of nickel's properties. Consequently we hypothesised that cobalt ions from metal-on-metal joints can activate TLR4. Methods. An in vitro cell culture model was developed using human and murine TLR4 reporter cell lines to investigate the effects of metal ions, including cobalt, on TLR4. Real-time PCR was used to examine the effect of cobalt on inflammatory gene expression, including IL-8, CCL-2 and IRAK-2, while an ELISA assay was conducted to investigate IL-8 protein expression in a human macrophage cell line (MonoMac 6). The TLR4 agonist LPS was included as a positive control and as a negative control TLR4 activation was blocked using the chemical agonist CLI-095 (Invivogen, UK). Results. Using human TLR4 reporter cells we show that cobalt at clinically-relevant concentrations can activate human TLR4. This effect appears unique to humans as murine TLR4 is unresponsive to cobalt but still responds to LPS. We also demonstrate that in human macrophages physiologically-relevant concentrations of cobalt cause increased pro-inflammatory IL-8 secretion (p<0.001). IL-8 is involved in perpetuating the immune response by recruiting more inflammatory cells to the site of inflammation. Cobalt-induced IL-8 secretion can be blocked using a TLR4 antagonist (p<0.001) showing that the effect is due to cobalt activation. Cobalt ions also alter gene expression in human macrophages. Cobalt upregulates expression of IL-8 and IRAK2 genes; IRAK2 is a key component of the TLR4 signalling pathway. Interestingly, cobalt causes downregulation of the CCL2 gene whereas it is upregulated in response to LPS. Discussion. In this study we have demonstrated that cobalt ions can activate human TLR4 signalling and in human macrophages this can increase expression of pro-inflammatory IL-8. We have also developed a robust series of assays for determining the effects of metal ions and other orthopaedic materials on the TLR4 signalling pathway. These methods will be used to investigate the immunological effects of additional orthopaedic metals (e.g. chromium, titanium and molybdenum). This work has identified a key pathway involved in the immune response to metal ions which can now be investigated for genetic variability and as a potential therapeutic target

We determined serum cobalt levels in 55 patients by atomic absorption spectrophotometry before and after implantation of uncemented total hip arthroplasties. In a randomised, prospective trial 27 wrought Co-28Cr-6Mo-0.2C metal-on-metal articulations were compared with 28 ceramic-on-polyethylene hips which did not contain cobalt. Other sources of iatrogenic cobalt loading were excluded. The metal-on-metal group produced detectable serum cobalt levels (median 1.1 μg/l after one year) which were significantly different (p < 0.0001) from those of the ceramic-on-polyethylene control group (median below detection limit of 0.3 μg/l after one year). Our findings indicate that metal-on-metal bearings generate some systemic release of cobalt

The mechanism of adverse tissue reaction to implant derived cobalt and chromium is unknown. It is possible that only one of these metals, cobalt, plays critical role in the failure of MOM implant. Cobalt ions are known to stabilize hypoxia inducible factor (HIF) 1α, which is involved in inflammatory pathway involving upregulation of BNIP3, GLUT1, HO-1 and COX-2 genes. This study used human monocytic cell line U937 to test the cytotoxic and inflammatory response to cobalt and chromium in form of ions and nanoparticles (NP) at clinically relevant doses. MTT assay was used to assess cytotoxic potential of metals for up to 24 hours. Gene expression was studied using qPCR and protein expression using Western Blot technique. Inflammatory cytokine release was studied using ELISA assay. Cytotoxicity study showed similar toxicity cobalt NP throughout the range of concentration 5–100μg/ml. Stabilization of HIF1α protein was observed after stimulation with cobalt ions and NP. This resulted in upregulation of GLUT1, BNIP3, HO-1 and COX-2 genes. Stimulation caused increased release in TNFα and inhibition of IL-10. No significant release of IL-1β was observed. Stimulation with chromium ions or NP did not cause any changes in cell viability, stabilization of HIF or cytokine release profile. Chromium NP caused upregulation of COX-2 after 6 hours of exposure. These results indicate significant role of cobalt in the inflammatory process and its potential as the cause of failure of MOM implants

Metal and their alloys have been widely used as implantable materials and prostheses in orthopaedic surgery. However, concerns exist as the metal nanoparticles released from wear of the prostheses cause clinical complications and in some cases result in catastrophic host tissue responses. The mechanism of nanotoxicity and cellular responses to wear metal nanoparticles are largely unknown. The aim of this study was to characterise macrophage phagocytosed cobalt/chromium metal nanoparticles both in vitro and in vivo, and investigate the consequent cytotoxicity. Two types of macrophage cell lines, murine RAW246.7 and human THP-1s were used for in vitro study, and tissues retrieved from pseudotumour patients caused by metal-on-metal hip resurfacing (MoMHR) were used for ex vivo observation. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) in combination with backscatter, energy-disperse X-ray spectrometer (EDS), focused ion beam (FIB) were employed to characterise phagocytosed metal nanoparticles. Alamar blue assay, cell viability assays in addition to confocal microscopy in combination with imaging analysis were employed to study the cytotoxiticy in vitro. The results showed that macrophages phagocytosed cobalt and chromium nanoparticles in vitro and the phagocytosed metal particles were confirmed by backscatter SEM+EDS and FIB+EDS. these particles were toxic to macrophages at a dose dependent manner. The analysis of retrieved tissue from revision of MoMHR showed that cobalt/chromium metal nanoparticles were observed exclusively in living macrophages and fragments of dead macrophages, but they were not seen within either live or dead fibroblasts. Dead fibroblasts were associated with dead and disintegrated macrophages and were not directly in contact with metal particles; chromium but not cobalt was the predominant component remaining in tissue. We conclude that as an important type of innate immune cells and phagocytes, macrophages play a key role in metal nanoparticles related cytotoxicity. Metal nanoparticles are taken up mainly by macrophages. They corrode in an acidic environment of the phagosomes. Cobalt that is more soluble than chromium may release inside macrophages to cause death of individual nanoparticle-overloaded macrophages. It is then released into the local environment and results in death of fibroblasts and is subsequently leached from the tissue

We carried out a cross-sectional study with analysis of the demographic, clinical and laboratory characteristics of patients with metal-on-metal hip resurfacing, ceramic-on-ceramic and metal-on-polyethylene hip replacements. Our aim was to evaluate the relationship between metal-on-metal replacements, the levels of cobalt and chromium ions in whole blood and the absolute numbers of circulating lymphocytes. We recruited 164 patients (101 men and 63 women) with hip replacements, 106 with metal-on-metal hips and 58 with non-metal-on-metal hips, aged < 65 years, with a pre-operative diagnosis of osteoarthritis and no pre-existing immunological disorders. Laboratory-defined T-cell lymphopenia was present in13 patients (15%) (CD8. +. lymphopenia) and 11 patients (13%) (CD3. +. lymphopenia) with unilateral metal-on-metal hips. There were significant differences in the absolute CD8. +. lymphocyte subset counts for the metal-on-metal groups compared with each control group (p-values ranging between 0.024 and 0.046). Statistical modelling with analysis of covariance using age, gender, type of hip replacement, smoking and circulating metal ion levels, showed that circulating levels of metal ions, especially cobalt, explained the variation in absolute lymphocyte counts for almost all lymphocyte subsets

We measured the levels of cobalt and chromium in the serum in three groups of patients after uncemented porous-coated arthroplasty. Group 1 consisted of 14 consecutive patients undergoing revision for aseptic loosening. Group 2 comprised 14 matched patients in whom the arthroplasty was stable and group 3 was 14 similarly matched patients with arthritis awaiting hip replacement. Specimens were analysed using atomic absorption spectrophotometry. Aseptic loosening of a component resulted in a significant elevation of serum cobalt (p < 0.05), but not of serum chromium. The relative risk of a component being loose, if the patient had a serum cobalt greater than 9.0 nmol/l, was 2.8

Commonly used alterations of prosthetic surfaces include grit-blasting (GB), plasma-sprayed titanium (Ti) or hydroxyapatite (HA) coating. Systemic concentrations of cobalt (Co) and chromium (Cr) are elevated in patients with metal-on-metal hip replacement, but can occur for all modular hip replacements. Here, we use whole genome microarrays to assess differential gene expression in primary human osteoblasts grown in vitro and on these prosthesis surfaces following exposure to clinically relevant concentrations of Co and Cr. Mesenchymal cells obtained from bone-fragments of 3 patients undergoing joint replacement surgery were differentiated into osteoblasts. Subsequently, cells were cultured in vitro on tissue-culture plates (TCP), or on GB, Ti and HA surfaces (JRI Orthopaedics Ltd, Sheffield, UK). Following 24hr exposure to a combination of clinically equivalent concentrations of Co2+:Cr3+, RNA was extracted and hybridized to SurePrint-G3 Gene Expression Microarray. Probe signals were normalised using ‘Limma’ package on R-Bioconductor and differential gene expression assessed with empirical Bayes approach (Log2FC>1.00, P<0.001 for differentially expressed genes). For cells grown on TCP, 11 genes were upregulated with 500μg/L Co2+:Cr3+. Of these, 4 were associated to HIF-1 signalling based on KEGG pathway analysis (P=5.4e-5). Exposure to 1000μg/L Co2+:Cr3+ altered expression at 164 loci for HA surfaces, and a separate 50 loci for Ti surfaces compared to GB surfaces. Genes for osteoblast differentiation (BMP2 and RGS2) were downregulated on HA surfaces compared to GB, whilst genes for cell-adhesion (ESAM), vesicular trafficking (RAB37) and protection against oxidative damage (NRF2) were upregulated. Ti surfaces caused an upregulation in ERBB3 and CNTF, which are associated with inhibition of osteoblast differentiation and mineralisation, when compared to GB surfaces. This study confirms the role of HIF-1 signalling in response to prosthesis generated metal ions, and is the first to provide a comprehensive genome-wide insight into transcriptional response of osteoblasts at prosthesis surface to clinically equivalent metal exposure

Particulate wear debris can induce the release of bone-resorbing cytokines from cultured macrophages and fibroblasts in vitro, and these mediators are believed to be the cause of the periprosthetic bone resorption which leads to aseptic loosening in vivo. Much less is known about the effects of particulate debris on the growth and metabolism of osteoblastic cells. We exposed two human osteoblast-like cell lines (SaOS-2 and MG-63) to particulate cobalt, chromium and cobalt-chromium alloy at concentrations of 0, 0.01, 0.1 and 1.0 mg/ml. Cobalt was toxic to both cell lines and inhibited the production of type-I collagen, osteocalcin and alkaline phosphatase. Chromium and cobalt-chromium were well tolerated by both cell lines, producing no cytotoxicity and no inhibition of type-I collagen synthesis. At the highest concentration tested (1.0 mg/ml), however, chromium inhibited alkaline phosphatase activity, and both chromium and cobalt-chromium alloy inhibited osteocalcin expression. Our results clearly show that particulate metal debris can modulate the growth and metabolism of osteoblastic cells in vitro. Reduced osteoblastic activity at the bone-implant interface may be an important mechanism by which particulate wear debris influences the pathogenesis of aseptic loosening in vivo

Abstract

Background

Investigations into the response to implant debris tend to concentrate on how a population of cells proliferate in the presence of implant material, and how the regulation of cytokines change. For the problem of cobalt-chromium (CoCr) implants this has been done for osteoblasts and osteoclasts to understand how bone resorption, leading to aseptic loosening, is occurring. However, investigating the formation of the extracellular-matrix (ECM) may give a better indication of the mechanisms occurring. ECM is excreted from cells and is important for adhesion, structure, signaling and growth. Type I collagen is the most abundant protein in the ECM and is known to direct tissue development and is therefore a key part of understanding the mechanism behind aseptic loosening.

Systemic concentrations of metal ions (cobalt and chromium) are persistently elevated in patients with metal-on-metal hip resurfacing (MOMHR) compared to conventional total hip arthroplasty (THA). Several studies by us and others have described the detrimental effects of metal exposure on survival and function of various cell types in-vitro, but the mechanisms for these effects remain unclear. Epigenetic modifications following chronic metal exposure is a possible mechanism that could mediate these effects. Here we test the methylation status in genomic DNA from MOMHR (“cases”) and THA (“controls”) patient-groups, and its correlation with circulating metal levels.

The cohort consisted of 34 patients with a well-functioning MOMHR at a median follow-up of 9.75 years. These were individually matched for gender, age and time-since-surgery to a non-exposure group consisting of patients with THA. Genomic DNA was isolated from blood samples and cell composition estimated using the ‘estimateCellCounts’ function in ‘minfi R-package’. Methylation was assessed using the Illumina 450k BeadChip array analysing 426,225 probes. Logit model was fitted at each probe with case/control status as independent variable and covariates of gender, age, time-since-surgery, smoking, non-arthroplasty metal exposure, and cell composition. DNA methylation age was assessed using an online calculator (https://dnamage.genetics.ucla.edu/) and comparisons made between cases and controls, and correlated with circulating metal levels.

Cell distributions did not differ between the cases and controls (Wilcoxon test p<0.17) with no probe having an association at 5% FDR. Circulating metal levels and LVEDD also had no association with any probe at 5% FDR. There was no preferential age acceleration between cases and controls (Wilcox p<0.7), and it had no correlation with plasma-chromium or blood-cobalt levels (p<0.9).

In summary, large methylation changes following MOMHR seem to be absent, compared to THA. Future research with larger samples will be needed to clarify the presence and extent of small methylation changes.

The most common reason for revision surgery of total hip replacements is aseptic loosening of implants secondary to osteolysis, which is caused by immune-mediated reactions to implant debris. These debris can cause pseudotumour formation. As revision surgery is associated with higher mortality and infection, it is important to understand the pro-inflammatory process to improve implant survival. Toll-like receptor 4 (TLR4) has been shown to mediate immune responses to cobalt ions. Statin use in epidemiological studies has been associated with reduced risk of revision surgery. In-vitro studies have demonstrated the potential for statins to reduce orthopaedic debris-induced immune responses and there is evidence that statins can modulate TLR4 activity. This study investigates simvastatin's effect on orthopaedic biomaterial-mediated changes in protein expression of key inflammatory markers and soluble-ICAM-1 (sICAM-1), an angiogenic factor implicated in pseudotumour formation. Human macrophage THP-1 cells were pre-incubated with 50µM simvastatin for 2-hours or a vehicle control (VC), before being exposed to 0.75mM cobalt chloride, 50μm3 per cell zirconium oxide or LPS as a positive control, in addition to a further 24-hour co-incubation with 50µM simvastatin or VC. Interleukin −8 (IL-8), sICAM-1, chemokine ligand 2 (CCL2), CCL3 and CCL4 protein secretion was measured by enzyme-linked immunosorbent assay (ELISA). GraphPad Prism 10 was used for statistical analysis including a one-way ANOVA. Pre-treatment with simvastatin significantly reduced LPS and cobalt-mediated IL-8 secretion (n=3) and sICAM-1 protein secretion (n=2) in THP-1 cells. Pre-treatment with simvastatin significantly reduced LPS-mediated but not cobalt ion-mediated CCL2 (n=3) and CCL3 protein (n=3) secretion in THP-1 cells. Simvastatin significantly reduced zirconium oxide-mediated CCL4 secretion (n=3). Simvastatin significantly reduced cobalt-ion mediated IL-8 and sICAM-1 protein secretion in THP-1 cells. This in-vitro finding demonstrates the potential for simvastatin to reduce recruitment of leukocytes which mediate the deleterious inflammatory processes driving implant failure

Critical-sized bone defects remain challenging in the clinical setting. Autologous bone grafting remains preferred by clinicians. However, the use of autologous tissue is associated with donor-site morbidity and limited accessibility to the graft tissue. Advances in the development of synthetic bone substitutes focus on improving their osteoinductive properties. Whereas osteoinductivity has been demonstrated with ceramics, it is still a challenge in case of polymeric composites. One of the approaches to improve the regenerative properties of biomaterials, without changing their synthetic character, is the addition of inorganic ions with known osteogenic and angiogenic properties. We have previously reported that the use of a bioactive composite with high ceramic content composed of poly(ethyleneoxide terephthalate)/poly(butylene terephthalate) (1000PEOT70PBT30, PolyActive, PA) and 50% beta-tricalcium phosphate (β-TCP) with the addition of zinc in a form of a coating of the TCP particles can enhance the osteogenic differentiation of human mesenchymal stromal cells (hMSCs) (3). To further support the regenerative properties of these scaffolds, inorganic ions with known angiogenic properties, copper or cobalt, were added to the coating solution. β-TCP particles were immersed in a zinc and copper or zinc and cobalt solution with a concentration of 15 or 45 mM. 3D porous scaffolds composed of 1000PEOT70PBT30 and pure or coated β-TCP were additively manufactured by 3D fibre deposition. The osteogenic and angiogenic properties of the fabricated scaffolds were tested in vitro through culture with hMSCs and human umbilical vein endothelial cells, respectively. The materials were further evaluated through ectopic implantation in an in vivo mini-pig model. The early expression of relevant osteogenic gene markers (collagen-1, osteocalcin) of hMSCs was upregulated in the presence of lower concentration of inorganic ions. Further analysis will focus on the evaluation of ectopic bone formation and vascularisation of these scaffolds after implantation in a mini-pig ectopic intramuscular model