Metal Wear Debris Research Articles

Exposure of articular chondrocytes to wear particles induces phagocytosis, differential inflammatory gene expression, and reduced proliferation.

The production of wear debris particulate remains a concern due to its association with implant failure through complex biologic interactions. In the setting of unicompartmental knee arthroplasty (UKA), damage and wear of the components may introduce debris particulate into the adjacent, otherwise, healthy compartment. The purpose of this study was to investigate the in vitro effect of polymeric and metallic wear debris particles on cell proliferation, extracellular matrix regulation, and phagocytosis index of normal human articular chondrocytes (nHACs). In culture, nHACs were exposed to both cobalt-chromium-molybdenum (CoCrMo) and polymethyl-methacrylate (PMMA) wear debris particulate for 3 and 10 days. At 3 days, no significant difference in cell proliferation was found between control cells and cells exposed to both CoCrMo or PMMA particles. However, cell proliferation was significantly decreased for CoCrMo exposed nHACs at both 6 (P < 0.001) and 10 days (P < 0.001) and PMMA at 10 days (P < 0.001). Target gene expression displayed both a time- and material-dependent response to CoCrMo and PMMA particles. Significant differences in COL10A1, ACAN, VCAN, IL-1β, TNF-α, MMP3, ADAMTS1, CASP3, and CASP9 regulation were found between CoCrMo and PMMA exposed nHACs at day 3 with gene regulation returning to near baseline at 10 days. Results from our study indicate a role of wear debris induced cartilage degeneration after exposure to polymeric and metallic wear debris particulate, suggesting an additional pathway of cartilage breakdown, potentially manifesting in traditional clinical symptoms.

CoCr micro-particles induce a robust type 2 inflammation in knee synovial tissues leading to fibrosis and tissue damage

Abstract Cobalt chrome (CoCr) is a major component of orthopaedic implants and a source of metallic wear debris, which may contribute to harmful inflammation and implant failure. In this study we show that the presence of M2 macrophages (CD68+, CD206+, and CD163+) and tissue fibrosis is observed in human peri-prosthetic tissue samples obtained from patients undergoing revision joint arthroplasty. We further examined the molecular mechanisms that drive the sterile inflammatory response to CoCr microparticles in 2 clinically relevant murine articular models. Bilateral intra-articular knee injections of CoCr microparticles were given to C57BL/6 mice and innate immune cell infiltrates were examined in both acute (2 day) and chronic (2 week) models. At the acute time point, CoCr microparticles induce inflammation characterized by an innate type 2 response in synovial tissues with marked increases in neutrophils, eosinophils, and M2 macrophages. This response in articular tissues is dependent on both SYK and BTK signaling as pharmacologic blockade of either pathway abrogates the innate response. Studies in our chronic model demonstrates that repeated exposure of articular tissues to CoCr microparticles triggers an inflammatory response that is characterized by persistent innate cell infiltrates of neutrophils and macrophages, as well as the development of tissue fibrosis. Additionally we show that the innate response and generation of tissue fibrosis in our chronic model is independent of the Caspase-1 inflammasome. Taken together, our findings suggest that CoCr mediates a robust type 2 inflammation that leads to fibrosis and identifies novel potential targets for control of microparticle induced sterile inflammatory responses.

Histological Findings in Six Failed Metal-on-Metal Implants.

Although joint replacement surgery to relieve pain due to osteoarthritis is generally a successful operation, adverse local tissue reactions can occur in hip arthroplasty in patients who receive metal-on-metal (MoM) implants and lead to early failure. This has led to revisions, lawsuits, and manufacturing recalls. An understanding of the pathological process initiated by metal wear debris is essential in clinical surveillance of cases. We retrospectively reviewed six cases of patients who underwent MoM hip arthroplasties and required early revision. Tissue removed at revision surgery was analyzed histologically by two independent reviewers. All six patients (four males, two females) underwent revision hip arthroplasty after early failure. Revision occurred between 18 and 56 months after the index procedure. Four patients received MoM implants from Depuy, one from Stryker, and one from Wright Medical. A consistent array of pathological findings was associated with these failed MoM implants. Beginning with the development of metal debris, a histiocytic response and proliferation occurred with subsequent corrosion product formation. Membranes were formed, many with cellular infiltrates and pseudosynovial linings and often with ulceration and bleeding. The tissue also demonstrated lymphocytosis and perivascular infiltrates with atypical changes of the endothelial lining cells. Our findings document a predictable cascade of harmful local tissue changes initiated by metal debris in failed MoM hip arthroplasties.

Study of wear-corrosion resistance of Co-based biomaterial

The longevity and the safety of artificial joint implants are determined by wear and corrosion properties. For many clinical cases, the released metallic ions and wear debris can cause adverse tissue reactions. Eventually, failure of such medical devices can be induced. Therefore, it is of great interest to increase the wear-corrosion resistance of implant materials. The release of metallic ions is related to the damage on the passive film which can spontaneously form on the surface of the cobalt–chromium–molybdenum (CoCrMo) alloy. In order to prevent the tribocorrosion reaction (the interaction between tribology and corrosion), a titanium nitride (TiN) film was applied on the surface by magnetron sputtering technology. The tribocorrosion behaviours of the titanium nitride and cobalt–chromium–molybdenum combination were studied. It has been shown that titanium nitride can greatly increase tribocorrosion resistance. However, if the titanium nitride film is damaged by tribological contact or the impact of the contact, the corrosion rate would dramatically increase.

Metal in Total Hip Arthroplasty: Wear Particles, Biology, and Diagnosis

Total hip arthroplasty (THA) has been performed for nearly 50 years. Between 2006 and 2012, more than 600,000 metal-on-metal THA procedures were performed in the United States. This article reviews the production of metal wear debris in a metal-on-metal articulation and the interaction of cobalt and chromium ions that ultimately led to a dramatic decline in the use of metal-on-metal THA articulations. Additionally, the article reviews mechanisms of metal wear, the biologic reaction to cobalt and chromium ions, the clinical presentation of failing metal-on-metal articulations, and current diagnostic strategies. Further, the article discusses the use of inflammatory markers, metal ion levels, radiographs, metal artifact reduction sequence magnetic resonance imaging, and ultrasound for failed metal-on-metal THA procedures. When adopting new technologies, orthopedic surgeons must weigh the potential increased benefits against the possibility of new mechanisms of failure. Metal-on-metal bearings are a prime example of the give and take between innovation and clinical results, especially in the setting of an already successful procedure such as THA. [Orthopedics. 2016; 39(6):371-379.].

A novel method for isolation and recovery of ceramic nanoparticles and metal wear debris from serum lubricants at ultra-low wear rates

Ceramics have been used to deliver significant improvements in the wear properties of orthopaedic bearing materials, which has made it challenging to isolate wear debris from simulator lubricants. Ceramics such as silicon nitride, as well as ceramic-like surface coatings on metal substrates have been explored as potential alternatives to conventional implant materials. Current isolation methods were designed for isolating conventional metal, UHMWPE and ceramic wear debris. In this paper, we describe a methodology for isolation and recovery of ceramic or ceramic-like coating particles and metal wear particles from serum lubricants under ultra-low and low wear performance. Enzymatic digestion was used to digest the serum proteins and sodium polytungstate was used as a novel density gradient medium to isolate particles from proteins and other contaminants by ultracentrifugation. This method demonstrated over 80% recovery of particles and did not alter the size or morphology of ceramic and metal particles during the isolation process. Statement of SignificanceImprovements in resistance to wear and mechanical damage of the articulating surfaces have a large influence on longevity and reliability of joint replacement devices. Modern ceramics have demonstrated ultra-low wear rates for hard-on-hard total hip replacements. Generation of very low concentrations of wear debris in simulator lubricants has made it challenging to isolate the particles for characterisation and further analysis. We have introduced a novel method to isolate ceramic and metal particles from serum-based lubricants using enzymatic digestion and novel sodium polytungstate gradients. This is the first study to demonstrate the recovery of ceramic and metal particles from serum lubricants at lowest detectable in vitro wear rates reported in literature.

CoCr microparticles induce a robust type 2 innate immune response that is dependent on SYK and BTK signaling

Abstract Cobalt chrome (CoCr) is a major component of orthopaedic implants and a source of metallic wear debris, which may contribute to harmful inflammation and implant failure. Intra-peritoneal inoculation of CoCr induces a robust innate type 2 response with increases in M2 macrophages, eosinophils, and neutrophils and this response is dependent on Spleen tyrosine kinase (SYK) and Bruton’s tyrosine kinase (BTK) signaling, but not TLR, Caspase-1, or Cyclooxygenase signaling. We have also shown that this robust response is independent of T and B cell signaling. To examine this response in a more clinically relevant murine model, bilateral intra-articular knee injections of CoCr microparticles were given to C57BL/6 mice and changes in different innate immune cells were assessed from knee synovial cell isolates. Administration of the SYK inhibitor BAY 61-3606, or the BTK inhibitor Ibrutinib, abrogated innate immune cell recruitment and M2 macrophage polarization which is otherwise observed 2 days after inoculation with CoCr microparticles. The presence of M2 macrophages (CD68+, CD206+, and CD163+) is also observed in human peri-prosthetic tissue samples obtained from patients undergoing revision joint arthroplasty. Taken together, our findings suggest that BTK signaling is required for the type 2 inflammatory response to CoCr microparticles, providing a potential novel target for control of harmful inflammation leading to fibrosis and aseptic loosening.

Trunnion Corrosion causing Failure in Metal-on-Polyethylene Total Hip Arthroplasty with Monolithic Femoral Components

We describe nine patients who had total hip arthroplasty failure [titanium alloy monolithic stem, cobalt-chromium head (32 mm or 36 mm), highly cross-linked polyethylene liner, metal socket] related to metal wear debris generated at the trunnion. Symptoms included pain with onset 2.9 years after THA. Preoperative serum cobalt metal ion levels were elevated [mean 8.8 ng/ml (normal &lt; 0.9 ng/ml)] and were higher than chromium levels [mean 1.2 ng/ml (normal &lt; 0.3 ng/ml)]. All patients had debridement of the periarticular soft tissues, stem retention, revision to ceramic head and new liner; two patients had acetabular revision. At early follow-up, 7 of 8 available patients did well, with improved cobalt (0.6 ng/ml) and little change in chromium levels. We recommend heightened awareness regarding this mode of failure.

Trojan-Like Internalization of Anatase Titanium Dioxide Nanoparticles by Human Osteoblast Cells.

Dentistry and orthopedics are undergoing a revolution in order to provide more reliable, comfortable and long-lasting implants to patients. Titanium (Ti) and titanium alloys have been used in dental implants and total hip arthroplasty due to their excellent biocompatibility. However, Ti-based implants in human body suffer surface degradation (corrosion and wear) resulting in the release of metallic ions and solid wear debris (mainly titanium dioxide) leading to peri-implant inflammatory reactions. Unfortunately, our current understanding of the biological interactions with titanium dioxide nanoparticles is still very limited. Taking this into consideration, this study focuses on the internalization of titanium dioxide nanoparticles on primary bone cells, exploring the events occurring at the nano-bio interface. For the first time, we report the selective binding of calcium (Ca), phosphorous (P) and proteins from cell culture medium to anatase nanoparticles that are extremely important for nanoparticle internalization and bone cells survival. In the intricate biological environment, anatase nanoparticles form bio-complexes (mixture of proteins and ions) which act as a kind of ‘Trojan-horse’ internalization by cells. Furthermore, anatase nanoparticles-induced modifications on cell behavior (viability and internalization) could be understand in detail. The results presented in this report can inspire new strategies for the use of titanium dioxide nanoparticles in several regeneration therapies.

Diagnosis and Treatment of Adverse Local Tissue Reactions at the Head-Neck Junction

Modular junctions in total hip arthroplasty are susceptible to mechanically assisted crevice corrosion, leading to the release of metal wear debris. Adverse local tissue reactions result from an immune-mediated biological reaction to this debris and can have a profound effect on the surrounding periarticular soft tissue envelope. Patients often present with pain or muscle weakness and demonstrate elevated serum cobalt and chromium levels. Serum inflammatory markers and synovial fluid tests help distinguish these reactions from deep infection in the majority of cases; however, the presence of amorphous material or fragmented cells can lead to difficulty in some cases. Advanced cross-sectional imaging is essential in establishing the diagnosis. Early revision surgery is generally the treatment of choice for symptomatic adverse local tissue reaction from corrosion at the modular head-neck junction. The existing stem is retained, and a new ceramic head is placed on the existing trunnion whenever possible. This strategy generally leads to short-term improvement of symptoms with reliable clinical outcomes; however, longer term results are presently lacking.

Systemic cobalt toxicity from total hip arthroplasties: review of a rare condition Part 1 - history, mechanism, measurements, and pathophysiology.

Recently, the use of metal-on-metal articulations in total hip arthroplasty (THA) has led to an increase in adverse events owing to local soft-tissue reactions from metal ions and wear debris. While the majority of these implants perform well, it has been increasingly recognised that a small proportion of patients may develop complications secondary to systemic cobalt toxicity when these implants fail. However, distinguishing true toxicity from benign elevations in cobalt ion levels can be challenging. The purpose of this two part series is to review the use of cobalt alloys in THA and to highlight the following related topics of interest: mechanisms of cobalt ion release and their measurement, definitions of pathological cobalt ion levels, and the pathophysiology, risk factors and treatment of cobalt toxicity. Historically, these metal-on-metal arthroplasties are composed of a chromium-cobalt articulation. The release of cobalt is due to the mechanical and oxidative stresses placed on the prosthetic joint. It exerts its pathological effects through direct cellular toxicity. This manuscript will highlight the pathophysiology of cobalt toxicity in patients with metal-on-metal hip arthroplasties. Patients with new or evolving hip symptoms with a prior history of THA warrant orthopaedic surgical evaluation. Increased awareness of the range of systemic symptoms associated with cobalt toxicity, coupled with prompt orthopaedic intervention, may forestall the development of further complications.

No association between pseudotumors, high serum metal-ion levels and metal hypersensitivity in large-head metal-on-metal total hip arthroplasty at 5-7-year follow-up.

The relationship between metal wear debris, pseudotumor formation and metal hypersensitivity is complex and not completely understood. The purpose of this study was to assess the prevalence of pseudotumor formation in a consecutive series of metal-on-metal (MoM) total hip arthroplasty (THA) and to investigate its relationship to serum metal-ion levels and hypersensitivity to metal. Forty-one patients (31 males), mean age 52 (28-68) years, with a total of 49 large-head MoM THA participated in a 5-7-year follow-up study. Patients underwent ultrasonography, serum metal-ion concentrations were measured, metal allergy and atopic dermatitis were evaluated, and the questionnaires of the Oxford Hip Score (OHS), Harris Hip Score (HHS) and the Short-Form Health Survey (SF-36) were completed. Pseudotumors were found in eight patients, but they were asymptomatic and their serum metal-ion levels were similar to those observed in patients with no pseudotumors (p > 0.36). The capsule-stem distance of mean 8.6 mm (SD 3.82, 95% CI: 5.40-11.79) was wider (p = 0.02) in patients with pseudotumours than in patients without pseudotumors of mean 5.6 mm (SD 2.89, 95% CI: 4.68-6.58). Positive patch test reactions were seen in three patients. Higher serum metal-ion levels of chromium and cobalt were significantly correlated with steeper cup inclination and smaller femoral head sizes, and were associated with female gender (p < 0.04). We found no association between pseudotumor formation, serum metal-ion levels, metal patch test reactivity, and atopic dermatitis. However, clinicians should be aware of asymptomatic pseudotumors, and we advise further exploration into the mechanisms involved in the pathogenesis of pseudotumors.

In Vitro Analyses of the Toxicity, Immunological, and Gene Expression Effects of Cobalt-Chromium Alloy Wear Debris and Co Ions Derived from Metal-on-Metal Hip Implants

Joint replacement has proven to be an extremely successful and cost-effective means of relieving arthritic pain and improving quality of life for recipients. Wear debris-induced osteolysis is, however, a major limitation and causes orthopaedic implant aseptic loosening, and various cell types including macrophages, monocytes, osteoblasts, and osteoclasts, are involved. During the last few years, there has been increasing concern about metal-on-metal (MoM) hip replacements regarding adverse reactions to metal debris associated with the MoM articulation. Even though MoM-bearing technology was initially aimed to extend the durability of hip replacements and to reduce the requirement for revision, they have been reported to release at least three times more cobalt and chromium ions than metal-on-polyethylene (MoP) hip replacements. As a result, the toxicity of metal particles and ions produced by bearing surfaces, both locally in the periprosthetic space and systemically, became a concern. Several investigations have been carried out to understand the mechanisms responsible for the adverse response to metal wear debris. This review aims at summarising in vitro analyses of the toxicity, immunological, and gene expression effects of cobalt ions and wear debris derived from MoM hip implants.

A computational approach to fretting wear prediction at the head–stem taper junction of total hip replacements

Wear is one of the main reasons for failure of modular total hip replacements. Recent evidence suggests that fretting wear occurs at the taper junction which provides fixation between the prosthesis femoral head and stem components. The fine metallic wear debris that is released can lead to adverse soft-tissue reactions which can necessitate a revision surgery. The present study proposes a computational methodology utilising an energy wear law and a 3D finite element model to predict fretting wear at the taper junction. The method is novel in that it simulates the weakening of the initial taper ‘fixation’ (created at impaction of the head onto the stem in surgery) due to the wearing process. The taper fixation is modelled using a contact analysis with overlapped meshes at the taper junction. The reduction in fixation is modelled by progressive removal of the overlap between components based on calculated wear. The fretting wear analysis approach has been shown to model the evolution of wear effectively; however, it has been shown that accurate, quantitative values for wear are critically dependant on mesh refinement, wear scaling factor and fraction, wear coefficient used and knowledge of the device loading history. The method has been implemented with a 3D finite element model of the taper junction of a commercial total hip replacement. This has been used to determine taper wear patterns, wear damage and wear rates which have been shown to be consistent with those found from observation and measurement of retrieved prostheses. The numerical method could be used to consider the effect of design changes and clinical technique on subsequent fretting wear in modular prosthetic devices.

Particle characterisation and cytokine expression in failed small-diameter metal-on-metal total hip arthroplasties.

The peri-prosthetic tissue response to wear debris is complex and influenced by various factors including the size, area and number of particles. We hypothesised that the 'biologically active area' of all metal wear particles may predict the type of peri-prosthetic tissue response. Peri-prosthetic tissue was sampled from 21 patients undergoing revision of a small diameter metal-on-metal (MoM) total hip arthroplasty (THA) for aseptic loosening. An enzymatic protocol was used for tissue digestion and scanning electron microscope was used to characterise particles. Equivalent circle diameters and particle areas were calculated. Histomorphometric analyses were performed on all tissue specimens. Aspirates of synovial fluid were collected for analysis of the cytokine profile analysis, and compared with a control group of patients undergoing primary THA (n = 11) and revision of a failed ceramic-on-polyethylene arthroplasty (n = 6). The overall distribution of the size and area of the particles in both lymphocyte and non-lymphocyte-dominated responses were similar; however, the subgroup with lymphocyte-dominated peri-prosthetic tissue responses had a significantly larger total number of particles. 14 cytokines (interleukin (IL)-1ß, IL-2, IL-4, IL-5, IL-6, IL-10, IL-13, IL-17, interferon (IFN)-γ, and IFN-gamma-inducible protein 10), chemokines (macrophage inflammatory protein (MIP)-1α and MIP-1ß), and growth factors (granulocyte macrophage colony stimulating factor (GM-CSF) and platelet derived growth factor) were detected at significantly higher levels in patients with metal wear debris compared with the control group. Significantly higher levels for IL-1ß, IL-5, IL-10 and GM-CSF were found in the subgroup of tissues from failed MoM THAs with a lymphocyte-dominated peri-prosthetic response compared with those without this response. These results suggest that the 'biologically active area' predicts the type of peri-prosthetic tissue response. The cytokines IL-1ß, IL-5, IL-10, and GM-CSF are associated with lymphocyte-dominated tissue responses from failed small-diameter MoM THA.

Repeated low dose injection of cobalt chromium microparticles in a mouse knee model induces robust inflammation with cartilage and bone loss (INC1P.345)

Abstract Total joint arthroplasty is a well accepted orthopaedic procedure for the treatment of many pathological joint conditions, but the release of metallic wear debris such as cobalt chromium (CoCr) microparticles can lead to pain, osteolysis, and implant failure. Our studies of periprosthetic tissue samples from patients undergoing revision surgery have shown that these particles induce an M2 macrophage phenotype as evidenced through gene expression analysis and CD68 immunostaining. We used an intra-articular murine knee model to further examine the synovial tissue response to CoCr and have shown that a single high dose (10mg) inoculation of CoCr microparticles triggers a robust type 2 immune response characterized by the presence of alternatively activated macrophages, neutrophils, and eosinophils 2 days after injection. We further examined the long term effects of repeated alternate day low dose (0.5mg) injection of CoCr microparticles on synovial tissues over 14 days. Tissues exposed to CoCr microparticles were found to have increased clinical scores for inflammation with loss of cartilage and bone tissue at day 14 when compared to controls. Together these studies indicate that exposure of synovial tissues to CoCr induces an innate type 2 immune response with long term hard tissue damage. These studies may ultimately lead to the identification of targeted therapeutic options to locally treat implant associated pain and potentially increase implant longevity.

Volumetric assessment of material loss from retrieved cemented metal hip replacement stems

The aim of this study was to investigate the scale of metallic wear debris generation at the cement-stem interface of polished cobalt-chrome implants. Thirty-one Zimmer CPT cemented femoral stems were retrieved; mean time in vivo was 77.8 months (range 38–97 months), with 70% (n=21) of the stems considered to be well-fixed at the time of revision surgery. Volumetric loss was measured using optical microscopy, with focus variation technology capable of 3D reproduction of the surfaces. The scale of loss was found to be pronounced (mean: 3.1mm3, 0.02–11.4mm3), with a mean rate of 0.5mm3/year (0.003–1.9mm3/year). These results demonstrate that material loss from the cement-stem interface is comparable to that of a taper interface, even in apparently well-fixed stems.

Surgical Modification of the Murine Calvaria Osteolysis Model.

The murine calvaria model has been adopted for evaluation of osteolysis and inflammation induced by polyethylene (PE) or metal wear debris. However, this model suffers from several complications. The purpose of our study is to introduce a surgical modification with lower complication rates, thus providing more accurate results. Forty C57/BL6 mice were divided into two groups, both receiving polyethylene particles. Surgical modifications were performed in group 1, and group 2 underwent traditional surgeries. The incidence of fluid leakage was recorded on the operative day. Curst formation, wound dehiscence, and bone exposure were recorded on day 7. Histological osteolysis was demonstrated by HE staining of tissue slices. Micro-CT was used for quantifying evaluation of osteolysis in two groups. Intraoperative fluid leakage was significantly reduced in group 1. Postoperative crust formation, wound dehiscence, and bone exposure were also significantly decreased in group 1. HE staining results revealed obvious osteolysis in group 1 and more obvious osteolysis in group 2. Bone volume fraction (BVF) was (0.32 ± 0.03) in group 1 compared to group 2 (0.24 ± 0.05). Bone mineral density (BMD) was (1.11 ± 0.03) in group 1 compared to group 2 (1.01 ± 0.02). Surgical modifications provide a reliable way for establishment of the murine calvaria osteolysis model.

Effects of CoCr metal wear debris generated from metal-on-metal hip implants and Co ions on human monocyte-like U937 cells

Hip resurfacing with cobalt–chromium (CoCr) alloy was developed as a surgical alternative to total hip replacement. However, the biological effects of nanoparticles generated by wear at the metal-on-metal articulating surfaces has limited the success of such implants. The aim of this study was to investigate the effects of the combined exposure to CoCr nanoparticles and cobalt ions released from a resurfacing implant on monocytes (U937 cells) and whether these resulted in morphology changes, proliferation alterations, toxicity and cytokine release. The interaction between prior exposure to Co ions and the cellular response to nanoparticulate debris was determined to simulate the situation in patients with metal-on-metal implants receiving a second implant. Effects on U937 cells were mainly seen after 120h of treatment. Prior exposure to Co ions increased the toxic effects induced by the debris, and by Co ions themselves, suggesting the potential for interaction in vivo. Increased TNF-α secretion by resting cells exposed to nanoparticles could contribute to osteolysis processes in vivo, while increased IFN-γ production by activated cells could represent cellular protection against tissue damage. Data suggest that interactions between Co ions and CoCr nanoparticles would occur in vivo, and could threaten the survival of a CoCr metal implant.

Determining inductive sensor wear debris limits for rolling contact fatigue of bearings

A relatively recent development for the condition monitoring of oil-wetted machinery has been the in-line full-flow inductive wear debris sensor (hereafter referred to as inductive wear debris sensors). These sensors detect a disturbance to a magnetic field caused by metallic wear debris shed from deteriorating dynamic components that is entrained in the lubricant. Applications for these sensors currently include (but are not limited to) aviation machinery, wind-turbine generators, marine propulsion systems, and locomotives. Inductive wear debris sensors can distinguish between ferromagnetic and nonferromagnetic particles as well as providing size and other related information. One of the primary advantages of this sensor type is that the detectable size range is broad and may be used to track the progress of an incipient failure such as the rolling contact fatigue of a bearing or gear as it occurs. One aspect that has received little attention in the literature is the methodology for determining a suitable limit for a particular application. Limits are a critically important aspect of machinery condition monitoring and need to be established by a robust and reliable method otherwise unnecessary maintenance can occur or an incipient fault may be missed. This paper describes a generic method for determining a physically meaningful debris limit for a deteriorating rolling element bearing when utilizing an inductive wear debris sensor.