Local and systemic toxicity of nanoscale debris particles in total hip arthroplasty

Cobalt and chromium exposure affects osteoblast function and impairs the mineralization of prosthesis surfaces in vitro

Cobalt (Co) and chromium (Cr) ions and nanoparticles equivalent to those released through tribo-corrosion of prosthetic metal-on-metal (MOM) bearings and taper junctions are detrimental to osteoblast activity and function in vitro when examined as individual species. Here we examined the effects of Co(2+):Cr(3+) and Co(2+):Cr(6+) combinations on osteoblast-like SaOS-2 cellular activity, alkaline phosphatase (ALP) activity and mineralization to better reflect clinical exposure conditions in vivo. We also assessed the effect of Co(2+):Cr(3+) combinations and Co:Cr nanoparticles on SaOS-2 cell osteogenic responses on grit-blasted, plasma-sprayed titanium-coated, and hydroxyapatite-coated prosthesis surfaces. Cellular activity and ALP activity were reduced to a greater extent with combination treatments compared to individual ions. Co(2+) and Cr(3+) interacted additively and synergistically to reduce cellular activity and ALP activity, respectively, while the Co(2+) with Cr(6+) combination was dominated by the effect of Cr(6+) alone. Mineralization by osteoblasts was greater on hydroxyapatite-coated surfaces compared to grit-blasted and plasma-sprayed titanium-coated surfaces. Treatments with Co(2+):Cr(3+) ions and Co:Cr nanoparticles reduced the percentage mineralization on all surfaces, with hydroxyapatite-coated surfaces having the least reduction. In conclusion, our data suggests that previous studies investigating individual metal ions underestimate their potential clinical effects on osteoblast activity. Furthermore, the data suggests that hydroxyapatite-coated surfaces may modulate osteoblast responses to metal debris.

Clinical relevance of corrosion patterns attributed to inflammatory cell-induced corrosion: A retrieval study

In vitro studies have shown that human osteoclasts can corrode stainless steel and titanium leading to the production of metal ions responsible for inflammatory reactions. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as inflammatory cell-induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing reactive oxygen species (ROS) through Fenton-like reactions. The extent and clinical relevance of this phenomenon has yet to be understood. We analysed a cohort of 100 CoCr alloy hips collected at our retrieval centre; we performed macroscopic and microscopic screening and used statistical analysis to correlate our findings with implant and clinical variables. We found that 59% of our implants had evidence of surface damage consistent with what has previously been described as cell-induced corrosion. There was a significant association between the patterns and aseptic loosening for the ASR modular (r = -0.488, p = 0.016) and the Durom modular (r = 0.454, p = 0.026). This is the largest implant retrieval study to examine the phenomena of so-called ICI corrosion and is the first to investigate its clinical relevance. We recommend further work to determine the role of cells in the damage patterns observed. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 155-164, 2017.

Innate immunity sensors participating in pathophysiology of joint diseases: a brief overview

The innate immune system consists of functionally specialized "modules" that are activated in response to a particular set of stimuli via sensors located on the surface or inside the tissue cells. These cells screen tissues for a wide range of exogenous and endogenous danger/damage-induced signals with the aim to reject or tolerate them and maintain tissue integrity. In this line of thinking, inflammation evolved as an adaptive tool for restoring tissue homeostasis. A number of diseases are mediated by a maladaptation of the innate immune response, perpetuating chronic inflammation and tissue damage. Here, we review recent evidence on the cross talk between innate immune sensors and development of rheumatoid arthritis, osteoarthritis, and aseptic loosening of total joint replacements. In relation to the latter topic, there is a growing body of evidence that aseptic loosening and periprosthetic osteolysis results from long-term maladaptation of periprosthetic tissues to the presence of by-products continuously released from an artificial joint.

The effect of frictional torque and bending moment on corrosion at the taper interface : an in vitro study

The aim of this study was to assess the effect of frictional torque and bending moment on fretting corrosion at the taper interface of a modular femoral component and to investigate whether different combinations of material also had an effect. The combinations we examined were 1) cobalt-chromium (CoCr) heads on CoCr stems 2) CoCr heads on titanium alloy (Ti) stems and 3) ceramic heads on CoCr stems. In test 1 increasing torque was imposed by offsetting the stem in the anteroposterior plane in increments of 0 mm, 4 mm, 6 mm and 8 mm when the torque generated was equivalent to 0 Nm, 9 Nm, 14 Nm and 18 Nm. In test 2 we investigated the effect of increasing the bending moment by offsetting the application of axial load from the midline in the mediolateral plane. Increments of offset equivalent to head + 0 mm, head + 7 mm and head + 14 mm were used. Significantly higher currents and amplitudes were seen with increasing torque for all combinations of material. However, Ti stems showed the highest corrosion currents. Increased bending moments associated with using larger offset heads produced more corrosion: Ti stems generally performed worse than CoCr stems. Using ceramic heads did not prevent corrosion, but reduced it significantly in all loading configurations.

Endoplasmic reticulum stress-mediated inflammatory signaling pathways within the osteolytic periosteum and interface membrane in particle-induced osteolysis

Aseptic loosening secondary to periprosthetic inflammatory osteolysis results from the biological response to wear particles and is a leading cause of arthroplasty failure. The origin of this inflammatory response remains unclear. We aim to validate the definite link between endoplasmic reticulum (ER) stress and particle-induced inflammatory signaling pathways in periprosthetic osteolysis. We examine the histopathologic changes of osteolysis and the expression of specific biomarkers for ER-stress-mediated inflammatory signaling pathways (IRE1α, GRP78/Bip, c-Fos, NF-κB, ROS and Ca(2+)). Moreover, pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) and osteoclastogenic molecules (VEGF, OPG, RANKL and M-CSF) were assessed in clinical interface membranes and murine periosteum tissues. We found wear particles to be capable of inducing ER stress in macrophages within clinical osteolytic interface membranes and murine osteolytic periosteum tissues and to be associated with the inflammatory response and osteoclastogenesis. Blocking ER stress with sodium 4-phenylbutyrate (4-PBA) results in a dramatic amelioration of particle-induced osteolysis and a significant reduction of ER-stress intensity. Simultaneously, this ER-stress blocker also lessens inflammatory cell infiltration, diminishes the capability of osteoclastogenesis and reduces the inflammatory response by lowering IRE1α, GRP78/Bip, c-Fos, NF-κB, ROS and Ca(2+) levels. Thus, ER stress plays an important role in particle-induced inflammatory osteolysis and osteoclastogenic reactions. The pharmacological targeting of ER-stress-mediated inflammatory signaling pathways might be an appealing approach for alleviating or preventing particle-induced osteolysis in at-risk patients.

Toxicology of wear particles of cobalt-chromium alloy metal-on-metal hip implants Part II: Importance of physicochemical properties and dose in animal and in vitro studies as a basis for risk assessment

The objective of the Part II analysis was to evaluate animal and in vitro toxicology studies of CoCr particles with respect to their physicochemistry and dose relevance to metal-on-metal (MoM) implant patients as derived from Part I. In the various toxicology studies, physicochemical characteristics were infrequently considered and administered doses were orders of magnitude higher than what occurs in patients. Co was consistently shown to rapidly release from CoCr particles for distribution and elimination from the body. CoCr micron sized particles appear more biopersistent in vivo resulting in inflammatory responses that are not seen with similar mass concentrations of nanoparticles. We conclude, that in an attempt to obtain data for a complete risk assessment, future studies need to focus on physicochemical characteristics of nano and micron sized particles and on doses and dose metrics relevant to those generated in patients or in properly conducted hip simulator studies.

Systemic toxicity related to metal hip prostheses

INTRODUCTION

One in eight of all total hip replacements requires revision within 10 years, 60% because of wear-related complications. The bearing surfaces may be made of cobalt/chromium, stainless steel, ceramic, or polyethylene. Friction between bearing surfaces and corrosion of non-moving parts can result in increased local and systemic metal concentrations.

OBJECTIVES

To identify and systematically review published reports of systemic toxicity attributed to metal released from hip implants and to propose criteria for the assessment of these patients.

METHODS

Medline (from 1950) and Embase (from 1980) were searched to 28 February 2014 using the search terms (text/abstract) chrom* or cobalt* and [toxic* or intox* or poison* or adverse effect or complication] and [prosthes* or 'joint replacement' or hip or arthroplast*] and PubMed (all available years) was searched using the search term (("Chromium/adverse effects"[Mesh] OR "Chromium/poisoning"[Mesh] OR "Chromium/toxicity"[Mesh]) OR ("Cobalt/adverse effects"[Mesh] OR "Cobalt/poisoning"[Mesh] OR "Cobalt/toxicity"[Mesh])) AND ("Arthroplasty, Replacement, Hip"[Mesh] OR "Hip Prosthesis"[Mesh]). These searches identified 281 unique references, of which 23 contained original case data. Three further reports were identified from the bibliographies of these papers. As some cases were reported repeatedly the 26 papers described only 18 individual cases. Systemic toxicity. Ten of these eighteen patients had undergone revision from a ceramic-containing bearing to one containing a metal component. The other eight had metal-on-metal prostheses. Systemic toxicity was first manifest months and often several years after placement of the metal-containing joint. The reported systemic features fell into three main categories: neuro-ocular toxicity (14 patients), cardiotoxicity (11 patients) and thyroid toxicity (9 patients). Neurotoxicity was manifest as peripheral neuropathy (8 cases), sensorineural hearing loss (7) and cognitive decline (5); ocular toxicity presented as visual impairment (6). All these neurological features, except cognitive decline, have been associated with cobalt poisoning previously. Type of prosthesis and blood metal concentrations. Where blood or serum metal concentrations were reported (n = 17 for cobalt and n = 14 for chromium), the median cobalt concentration was 398 (range, 13.6-6521) μg/L and the median chromium concentration was 48 μg/L (in whole blood) (range, 4.1-221 μg/L including serum and blood values). Those patients reported to have systemic features who had received a metal-on-metal prosthesis (n = 8) had a median peak blood cobalt concentration of 34.5 (range, 13.6-398.6) μg/L; those with a metal-containing revision of a failed ceramic prosthesis (n = 10) had a median blood cobalt concentration of 506 (range, 353-6521) μg/L. Management. The most common treatment was removal of the metal-containing prosthesis, undertaken in all but 2 patients. This was usually associated with a fall in circulating cobalt concentration and improvement in some or all features. Clinical and toxicological assessment of systemic features. We propose the following criteria for assessing the likelihood that clinical features are related to cobalt toxicity: clinical effects consistent with the known neurological, cardiac, or thyroidal effects of cobalt, and for which any other explanation is less likely; increased blood cobalt concentrations (substantially higher than those in patients with well-functioning prostheses) several months after hip replacement; a fall in the blood cobalt concentration, usually accompanied by signs of improvement in features. When judged by these criteria, the systemic features in 10 of the reported cases are likely to be related to cobalt exposure from a metal-containing hip prosthesis.

CONCLUSIONS

Rarely, patients exposed to high circulating concentrations of cobalt from failed hip replacements develop neurological damage, hypothyroidism and/or cardiomyopathy, which may not resolve completely even after removal of the prosthesis. The greatest risk of systemic cobalt toxicity seems to result from accelerated wear of a cobalt-containing revision of a failed ceramic prosthesis, rather than from primary failure of a metal-on-metal prosthesis.

Nanotechnology: current concepts in orthopaedic surgery and future directions

Nanotechnology is the study, production and controlled manipulation of materials with a grain size < 100 nm. At this level, the laws of classical mechanics fall away and those of quantum mechanics take over, resulting in unique behaviour of matter in terms of melting point, conductivity and reactivity. Additionally, and likely more significant, as grain size decreases, the ratio of surface area to volume drastically increases, allowing for greater interaction between implants and the surrounding cellular environment. This favourable increase in surface area plays an important role in mesenchymal cell differentiation and ultimately bone-implant interactions. Basic science and translational research have revealed important potential applications for nanotechnology in orthopaedic surgery, particularly with regard to improving the interaction between implants and host bone. Nanophase materials more closely match the architecture of native trabecular bone, thereby greatly improving the osseo-integration of orthopaedic implants. Nanophase-coated prostheses can also reduce bacterial adhesion more than conventionally surfaced prostheses. Nanophase selenium has shown great promise when used for tumour reconstructions, as has nanophase silver in the management of traumatic wounds. Nanophase silver may significantly improve healing of peripheral nerve injuries, and nanophase gold has powerful anti-inflammatory effects on tendon inflammation. Considerable advances must be made in our understanding of the potential health risks of production, implantation and wear patterns of nanophase devices before they are approved for clinical use. Their potential, however, is considerable, and is likely to benefit us all in the future.

Clinical features, testing, and management of patients with suspected prosthetic hip-associated cobalt toxicity: a systematic review of cases

Safety concerns regarding cobalt-containing metal alloy hip prosthetics (Co-HP) have resulted in product recalls, a medical device alert, and issuance of guidance for clinicians. Recently, cases of suspected prosthetic hip-associated cobalt toxicity (PHACT) from Co-HP have been reported. Although little is known about suspected PHACT, these patients may be referred to medical toxicologists for evaluation and management recommendations. We searched MEDLINE, EMBASE, and unpublished abstracts from toxicology scientific meetings for references relevant to PHACT. Authors independently screened publications for inclusion criteria: publication in English, human study population, subject(s) are symptomatic (except for isolated hip pain), and cobalt values in any matrix (blood, serum, urine, CSF, synovial fluid) available for review. Data from 10 cases are reviewed. Patients with suspected PHACT had findings consistent with cobalt toxicity, including thyroid, cardiac, and neurologic dysfunction. Signs and symptoms appeared between 3 and 72 months after arthroplasty (median 19 months). Neurologic symptoms were most common. Ancillary testing varied considerably. All patients had elevated cobalt levels in one or more matrices. Enhanced elimination was attempted in 27 % of patients. At this time, the information currently available regarding patients with suspected PHACT is inadequate to guide clinical decision making. No consensus has been reached regarding the management of Co-HP patients with systemic symptoms. Indications for chelation have not been established and require further study. Improved case definitions, improved surveillance, and controlled studies are needed to elucidate the scope of this problem and guide future investigations.

Adverse local tissue reaction associated with a modular hip hemiarthroplasty

BACKGROUND

The local and systemic effects of wear debris and corrosion products remain a concern in arthroplasty and reaction to corrosion or wear products from modular junctions has been reported in primary and revision total joint arthroplasties. These effects have not been reported previously for unipolar hemiarthroplasties where there is no prosthetic bearing surface to contribute to the phenomenon. This may have implications for clinical surveillance and implant design.

CASE DESCRIPTION

We report the case of a 72-year-old man who had symptomatic pseudotumor formation, confirmed by pathologic examination of the excised pseudotumor, with a large-head modular hip hemiarthroplasty. Metallosis and corrosion of the modular head/neck taper junction were noted at the time of revision surgery.

LITERATURE REVIEW

To our knowledge, this is the first report of pseudotumor formation where the corrosion or wear products arose from the modular junction of the implant with no bearing couple present to contribute wear debris that may influence the formation of the pseudotumor.

PURPOSES AND CLINICAL RELEVANCE

Adverse tissue reactions to wear debris generated at prosthetic articulating surfaces and corrosion and wear products from nonarticulating prosthetic junctions have been reported. The problem has been reported to be higher in metal-on-metal bearing couples and in large-diameter hip arthroplasties. Mixed-alloy junctions appear to be more susceptible to corrosion. We believe that corrosion should be considered a possible diagnostic entity when investigating persistent symptoms after hemiarthroplasty and may be avoided with the use of monoblock components.

An overview of recent advances in designing orthopedic and craniofacial implants

Great deal of research is still going on in the field of orthopedic and craniofacial implant development to resolve various issues being faced by the industry today. Despite several disadvantages of the metallic implants, they continue to be used, primarily because of their superior mechanical properties. In order to minimize the harmful effects of the metallic implants and its by-products, several modifications are being made to these materials, for instance nickel-free stainless steel, cobalt-chromium and titanium alloys are being introduced to eliminate the toxic effects of nickel being released from the alloys, introduce metallic implants with lower modulus, reduce the cost of these alloys by replacing rare elements with less expensive elements etc. New alloys like tantalum, niobium, zirconium, and magnesium are receiving attention given their satisfying mechanical and biological properties. Non-oxide ceramics like silicon nitride and silicon carbide are being currently developed as a promising implant material possessing a combination of properties such as good wear and corrosion resistance, increased ductility, good fracture and creep resistance, and relatively high hardness in comparison to alumina. Polymer/magnesium composites are being developed to improve mechanical properties as well as retain polymer's property of degradation. Recent advances in orthobiologics are proving interesting as well. This paper thus deals with the latest improvements being made to the existing implant materials and includes new materials being introduced in the field of biomaterials.

The effects on bone cells of metal ions released from orthopaedic implants. A review

The increasing use of orthopedic implants and, in particular, of hip and knee joint replacements for young and active patients, has stimulated interest and concern regarding the chronic, long-term effects of the materials used. This review focuses on the current knowledge of the adverse biologic reactions to metal particles released from orthopaedic implants in vivo and in vitro. More specifically, the purpose of this article is to provide an overview of the current literature about the adverse effects of metal particles on bone cells and peri-implant bone.

Combining ageing and wear to assess the durability of zirconia-based ceramic heads for total hip arthroplasty

The degradation of zirconia-based ceramic components for total hip arthroplasty (head and cup) has been the topic of many works. However, the correlation between what is measured in vivo and what is expected from in vitro simulations on hip simulators may be sometimes feeble, especially where zirconia component are concerned, mainly due to a lack of representativeness of in vitro experiments. The present study seeks to explore the combined effects of hydrothermal ageing and wear on zirconia components. It shows that hydrothermal ageing increases the roughness of zirconia components, which in turn might increase the wear rate of the polyethylene counterparts. Moreover, the friction during hip simulation increases the ageing rate of the zirconia components. This auto-accelerating degradation may explain some of the poor long-term in vivo results of zirconia hip prostheses reported in the literature. Finally, it is shown that zirconia-toughened alumina components may be free from this combined degradation.

Survivorship, patient reported outcome and satisfaction following resurfacing and total hip arthroplasty

Resurfacing (RA) and total hip arthroplasty (THA) are options in the treatment of debilitating hip pathology. 381 patients that had undergone arthroplasty with a BHR RA, ASR RA, metal-on-metal (MoM) THA or ceramic-on-ceramic (CoC) THA were reviewed for satisfaction, function, health and survivorship at a median follow up of 50 months. Significantly lower survivorship for revision and reoperation was observed in the ASR group. The BHR and CoC demonstrated better outcome scores than the ASR (OHS and SAPS) and the BHR better scores than the MoM (OHS and SF12 PCS). In the short to medium term, survivorship and outcomes for the best performing RA (BHR) and THA (CoC) were comparable. There was a non-significant trend towards poorer outcome scores in the MoM THA group.

Risk of cancer following primary total hip replacement or primary resurfacing arthroplasty of the hip: a retrospective cohort study in Scotland

Background:

Release and dispersion of particles arising from corrosion and wear of total hip arthroplasty (THA) components has raised concerns about a possible increased risk of cancer. Concerns have been heightened by a recent revival in the use of metal-on-metal (MoM) hip prostheses.

Methods:

From a linked database of hospital discharge, cancer registration, and mortality records, we selected a cohort of patients who underwent primary THA (1990–2009) or primary resurfacing arthroplasty (mainly 2000–2009) in Scotland, with follow-up to the end of 2010. Available operation codes did not enable us to distinguish MoM THAs. Indirectly standardised incidence ratios (SIRs) were calculated for selected cancers with standardisation for age, sex, deprivation, and calendar period.

Results:

The study cohort included 71 990 patients yielding 547 001 person-years at risk (PYAR) and 13 946 cancers diagnosed during follow-up. For the total period of observation combined, the risks of all cancers (SIR: 1.05; 95% CI: confidence interval 1.04–1.07), prostate cancer (SIR: 1.07; 95% CI: 1.01–1.14), and multiple myeloma (SIR: 1.22; 95% CI: 1.06–1.41) were increased. These modest increases in risk emerged in the context of effectively multiple tests of statistical significance, and may reflect inadequate adjustment for confounding factors. For 1317 patients undergoing primary resurfacing arthroplasty between 2000 and 2009 (PYAR=5698), the SIR for all cancers (n=39) was 1.23 (95% CI: 0.87–1.68).

Conclusion:

In the context of previous research, these results do not suggest a major cause for concern. However, the duration of follow-up of patients receiving recently introduced, new-generation MoM prostheses is too short to rule out a genuinely increased risk of cancer entirely.

Particle-induced osteolysis mediated by endoplasmic reticulum stress in prosthesis loosening

We hypothesized that endoplasmic reticulum (ER) stress in macrophages induced by wear particles was one of the reasons for particle-induced osteolysis (PIO) in total hip arthroplasty (THA) failure. In the present study, the expression of ER stress markers was examined by Western blot in macrophages treated with particles from materials used in prosthetics, specimens from PIO animal models and patients suffering from aseptic loosening. To address whether ER stress triggers these inflammatory responses, the effect of an ER stress blocker on the expression of inflammatory cytokines in particle-treated macrophages and PIO animal models was tested. The results demonstrated that ER stress markers were significantly upregulated in particle-treated macrophages, periosteum tissues from PIO animal models and clinical specimens of prosthesis loosening. Blocking ER stress with a specific inhibitor dramatically reduced the particle-induced expression of inflammatory cytokines in vitro and in vivo. Furthermore, in PIO animal models, this ER stress blocker dramatically suppressed the differentiation of osteoclasts and reduced the severity of osteolysis. Thus, the results of the present study suggest that ER stress plays a key role in particle-induced osteolysis and that targeting the ER stress pathway may lead to novel therapeutic approaches for the treatment of aseptic prosthesis loosening.

Cobalt metabolism and toxicology--a brief update

Cobalt metabolism and toxicology are summarized. The biological functions of cobalt are updated in the light of recent understanding of cobalt interference with the sensing in almost all animal cells of oxygen deficiency (hypoxia). Cobalt (Co(2+)) stabilizes the transcriptional activator hypoxia-inducible factor (HIF) and thus mimics hypoxia and stimulates erythropoietin (Epo) production, but probably also by the same mechanism induces a coordinated up-regulation of a number of adaptive responses to hypoxia, many with potential carcinogenic effects. This means on the other hand that cobalt (Co(2+)) also may have beneficial effects under conditions of tissue hypoxia, and possibly can represent an alternative to hypoxic preconditioning. Cobalt is acutely toxic in larger doses, and in mammalian in vitro test systems cobalt ions and cobalt metal are cytotoxic and induce apoptosis and at higher concentrations necrosis with inflammatory response. Cobalt metal and salts are also genotoxic, mainly caused by oxidative DNA damage by reactive oxygen species, perhaps combined with inhibition of DNA repair. Of note, the evidence for carcinogenicity of cobalt metal and cobalt sulfate is considered sufficient in experimental animals, but is as yet considered inadequate in humans. Interestingly, some of the toxic effects of cobalt (Co(2+)) have recently been proposed to be due to putative inhibition of Ca(2+) entry and Ca(2+)-signaling and competition with Ca(2+) for intracellular Ca(2+)-binding proteins. The tissue partitioning of cobalt (Co(2+)) and its time-dependence after administration of a single dose have been studied in man, but mainly in laboratory animals. Cobalt is accumulated primarily in liver, kidney, pancreas, and heart, with the relative content in skeleton and skeletal muscle increasing with time after cobalt administration. In man the renal excretion is initially rapid but decreasing over the first days, followed by a second, slow phase lasting several weeks, and with a significant long-term retention in tissues for several years. In serum cobalt (Co(2+)) binds to albumin, and the concentration of free, ionized Co(2+) is estimated at 5-12% of the total cobalt concentration. In human red cells the membrane transport pathway for cobalt (Co(2+)) uptake appears to be shared with calcium (Ca(2+)), but with the uptake being essentially irreversible as cobalt is effectively bound in the cytosol and is not itself extruded by the Ca-pump. It is tempting to speculate that this could perhaps also be the case in other animal cells. If this were actually the case, the tissue partitioning and biokinetics of cobalt in cells and tissues would be closely related to the uptake of calcium, with cobalt partitioning primarily into tissues with a high calcium turn-over, and with cobalt accumulation and retention in tissues with a slow turn-over of the cells. The occupational cobalt exposure, e.g. in cobalt processing plants and hard-metal industry is well known and has probably been somewhat reduced in more recent years due to improved work place hygiene. Of note, however, adverse reactions to heart and lung have recently been demonstrated following cobalt exposure near or slightly under the current occupational exposure limit. Over the last decades the use of cobalt-chromium hard-metal alloys in orthopedic joint replacements, in particular in metal-on-metal bearings in hip joint arthroplasty, has created an entirely new source of internal cobalt exposure. Corrosion and wear produce soluble metal ions and metal debris in the form of huge numbers of wear particles in nanometric size, with systemic dissemination through lymph and systemic vascular system. This may cause adverse local reactions in peri-prosthetic soft-tissues, and in addition systemic toxicity. Of note, the metal nanoparticles have been demonstrated to be clearly more toxic than larger, micrometer-sized particles, and this has made the concept of nanotoxicology a crucial, new discipline. As another new potential source of cobalt exposure, suspicion has been raised that cobalt salts may be misused by athletes as an attractive alternative to Epo doping for enhancing aerobic performance. The cobalt toxicity in vitro seems to reside mainly with ionized cobalt. It is tempting to speculate that ionized cobalt is also the primary toxic form for systemic toxicity in vivo. Under this assumption, the relevant parameter for risk assessment would be the time-averaged value for systemic cobalt ion exposure that from a theoretical point of view might be obtained by measuring the cobalt content in red cells, since their cobalt uptake reflects uptake only of free ionized cobalt (Co(2+)), and since the uptake during their 120 days life span is practically irreversible. This clearly calls for future clinical studies in exposed individuals with a systematic comparison of concurrent measurements of cobalt concentration in red cells and in serum.

Dose-response relationships for blood cobalt concentrations and health effects: a review of the literature and application of a biokinetic model

Cobalt (Co) is an essential component of vitamin B(12). As with all metals, at sufficiently high doses, Co may exert detrimental effects on different organ systems, and adverse responses have been observed in animals, patients undergoing Co therapy, and workers exposed to respirable Co particulates. Although blood Co concentrations are postulated to be the most accurate indicator of ongoing Co exposure, little is known regarding the dose-response relationships between blood Co concentrations and adverse health effects in various organ systems. In this analysis, the animal toxicology and epidemiology literature were evaluated to identify blood Co concentrations at which effects have, and have not, been reported. Where necessary, a biokinetic model was used to convert oral doses to blood Co concentrations. Our results indicated that blood Co concentrations of 300 μg/L and less have not been associated with adverse responses of any type in humans. Concentrations of 300 μg/L and higher were associated with certain hematological and reversible endocrine responses, including polycythemia and reduced iodide uptake. Blood Co concentrations of 700-800 μg Co/L and higher may pose a risk of more serious neurological, reproductive, or cardiac effects. These blood concentrations should be useful to clinicians and toxicologists who are attempting to interpret blood Co concentrations in exposed individuals.