Adherent endotoxin mediates biological responses of titanium particles without stimulating their phagocytosis

Ibudilast Mitigates Delayed Bone Healing Caused by Lipopolysaccharide by Altering Osteoblast and Osteoclast Activity

Bacterial infection in orthopedic surgery is challenging because cell wall components released after bactericidal treatment can alter osteoblast and osteoclast activity and impair fracture stability. However, the precise effects and mechanisms whereby cell wall components impair bone healing are unclear. In this study, we characterized the effects of lipopolysaccharide (LPS) on bone healing and osteoclast and osteoblast activity in vitro and in vivo and evaluated the effects of ibudilast, an antagonist of toll-like receptor 4 (TLR4), on LPS-induced changes. In particular, micro-computed tomography was used to reconstruct femoral morphology and analyze callus bone content in a femoral defect mouse model. In the sham-treated group, significant bone bridge and cancellous bone formation were observed after surgery, however, LPS treatment delayed bone bridge and cancellous bone formation. LPS inhibited osteogenic factor-induced MC3T3-E1 cell differentiation, alkaline phosphatase (ALP) levels, calcium deposition, and osteopontin secretion and increased the activity of osteoclast-associated molecules, including cathepsin K and tartrate-resistant acid phosphatase in vitro. Finally, ibudilast blocked the LPS-induced inhibition of osteoblast activation and activation of osteoclast in vitro and attenuated LPS-induced delayed callus bone formation in vivo. Our results provide a basis for the development of a novel strategy for the treatment of bone infection.

Quercetin inhibits macrophage polarization through the p-38α/β signalling pathway and regulates OPG/RANKL balance in a mouse skull model

Aseptic loosening caused by wear particles is a common complication after total hip arthroplasty. We investigated the effect of the quercetin on wear particle‐mediated macrophage polarization, inflammatory response and osteolysis. In vitro, we verified that Ti particles promoted the differentiation of RAW264.7 cells into M1 macrophages through p‐38α/β signalling pathway by using flow cytometry, immunofluorescence assay and small interfering p‐38α/β RNA. We used enzyme‐linked immunosorbent assays to confirm that the protein expression of M1 macrophages increased in the presence of Ti particles and that these pro‐inflammatory factors further regulated the imbalance of OPG/RANKL and promoted the differentiation of osteoclasts. However, this could be suppressed, and the protein expression of M2 macrophages was increased by the presence of the quercetin. In vivo, we revealed similar results in the mouse skull by μ‐CT, H&E staining, immunohistochemistry and immunofluorescence assay. We obtained samples from patients with osteolytic tissue. Immunofluorescence analysis indicated that most of the macrophages surrounding the wear particles were M1 macrophages and that pro‐inflammatory factors were released. Titanium particle‐mediated M1 macrophage polarization, which caused the release of pro‐inflammatory factors through the p‐38α/β signalling pathway, regulated OPG/RANKL balance. Macrophage polarization is expected to become a new clinical drug therapeutic target.

Inflammation and Bone Repair: From Particle Disease to Tissue Regeneration

When presented with an adverse stimulus, organisms evoke an immediate, pre-programmed, non-specific innate immune response. The purpose of this reaction is to maintain the organism's biological integrity and function, mitigate or eradicate the injurious source, and re-establish tissue homeostasis. The initial stage of this protective reaction is acute inflammation, which normally reduces or terminates the offending stimulus. As the inflammatory reaction recedes, the stage of tissue repair and regeneration follows. If the above sequence of events is perturbed, reconstitution of normal biological form and function will not be achieved. Dysregulation of these activities may result in incomplete healing, fibrosis, or chronic inflammation. Our laboratory has studied the reaction to wear particles from joint replacements as a paradigm for understanding the biological pathways of acute and chronic inflammation, and potential translational treatments to reconstitute lost bone. As inflammation is the cornerstone for healing in all anatomical locations, the concepts developed have relevance to tissue engineering and regenerative medicine in all organ systems. To accomplish our goal, we developed novel in vitro and in vivo models (including the murine femoral continuous intramedullary particle infusion model), translational strategies including modulation of macrophage chemotaxis and polarization, and methods to interfere with key transcription factors NFκB and MyD88. We purposefully modified MSCs to facilitate bone healing in inflammatory scenarios: by preconditioning the MSCs, and by genetically modifying MSCs to first sense NFκB activation and then overexpress the anti-inflammatory pro-regenerative cytokine IL-4. These advancements provide significant translational opportunities to enhance healing in bone and other organs.

Periprosthetic Joint Infection Caused by Gram-Positive Versus Gram-Negative Bacteria: Lipopolysaccharide, but not Lipoteichoic Acid, Exerts Adverse Osteoclast-Mediated Effects on the Bone

Periprosthetic joint infection (PJI)-the most common cause of knee arthroplasty failure-may result from Gram-positive (GP) or Gram-negative (GN) bacterial infections. The question as to whether PJI due to GP or GN bacteria can lead to different rates of aseptic loosening after reimplantation remains open. We have investigated this issue through a retrospective review of clinical records obtained from 320 patients with bacterial PJI. The results revealed that, compared with GP infections, GN infections were associated with an increased risk of aseptic loosening. In animal studies, mice underwent intrafemoral injection of lipopolysaccharide (LPS) from GN bacteria or lipoteichoic acid (LTA) from GP bacteria. We demonstrate that LPS-but not LTA-reduced both the number of trabeculae and the bone mineral density in mice. In addition, LPS-treated mice exhibited a reduced body weight, higher serum osteocalcin levels, and an increased number of osteoclasts. LPS accelerated monocyte differentiation into osteoclast-like cells, whereas LTA did not. Finally, ibudilast-a toll-like receptor (TLR)-4 antagonist-was found to inhibit LPS-induced bone loss and osteoclast activation in mice. Taken together, our data indicate that PJI caused by GN bacteria portends a higher risk of aseptic loosening after reimplantation, mainly because of LPS-mediated effects on osteoclast differentiation.

Sterile particle-induced inflammation is mediated by macrophages releasing IL-33 through a Bruton's tyrosine kinase-dependent pathway

Initiation of the innate sterile inflammatory response that can develop in response to microparticle exposure is little understood. Here, we report that a potent type 2 immune response associated with the accumulation of neutrophils, eosinophils and alternatively activated (M2) macrophages was observed in response to sterile microparticles similar in size to wear debris associated with prosthetic implants. Although elevations in interleukin-33 (IL-33) and type 2 cytokines occurred independently of caspase-1 inflammasome signalling, the response was dependent on Bruton's tyrosine kinase (BTK). IL-33 was produced by macrophages and BTK-dependent expression of IL-33 by macrophages was sufficient to initiate the type 2 response. Analysis of inflammation in patient periprosthetic tissue also revealed type 2 responses under aseptic conditions in patients undergoing revision surgery. These findings indicate that microparticle-induced sterile inflammation is initiated by macrophages activated to produce IL-33. They further suggest that both BTK and IL-33 may provide therapeutic targets for wear debris-induced periprosthetic inflammation.

Cobalt Alloy Implant Debris Induces Inflammation and Bone Loss Primarily through Danger Signaling, Not TLR4 Activation: Implications for DAMP-ening Implant Related Inflammation

Cobalt alloy debris has been implicated as causative in the early failure of some designs of current total joint implants. The ability of implant debris to cause excessive inflammation via danger signaling (NLRP3 inflammasome) vs. pathogen associated pattern recognition receptors (e.g. Toll-like receptors; TLRs) remains controversial. Recently, specific non-conserved histidines on human TLR4 have been shown activated by cobalt and nickel ions in solution. However, whether this TLR activation is directly or indirectly an effect of metals or secondary endogenous alarmins (danger-associated molecular patterns, DAMPs) elicited by danger signaling, remains unknown and contentious. Our study indicates that in both a human macrophage cell line (THP-1) and primary human macrophages, as well as an in vivo murine model of inflammatory osteolysis, that Cobalt-alloy particle induced NLRP3 inflammasome danger signaling inflammatory responses were highly dominant relative to TLR4 activation, as measured respectively by IL-1β or TNF-α, IL-6, IL-10, tissue histology and quantitative bone loss measurement. Despite the lack of metal binding histidines H456 and H458 in murine TLR4, murine calvaria challenge with Cobalt alloy particles induced significant macrophage driven in vivo inflammation and bone loss inflammatory osteolysis, whereas LPS calvaria challenge alone did not. Additionally, no significant increase (p<0.05) in inflammation and inflammatory bone loss by LPS co-challenge with Cobalt vs. Cobalt alone was evident, even at high levels of LPS (i.e. levels commiserate with hematogenous levels in fatal sepsis, >500pg/mL). Therefore, not only do the results of this investigation support Cobalt alloy danger signaling induced inflammation, but under normal homeostasis low levels of hematogenous PAMPs (<2pg/mL) from Gram-negative bacteria, seem to have negligible contribution to the danger signaling responses elicited by Cobalt alloy metal implant debris. This suggests the unique nature of Cobalt alloy particle bioreactivity is strong enough to illicit danger signaling that secondarily activate concomitant TLR activation, and may in part explain Cobalt particulate associated inflammatory and toxicity-like reactions of specific orthopedic implants.

TNF-α suppression and osteoprotegerin overexpression inhibits wear debris-induced inflammation and osteoclastogenesis in vitro

PURPOSE

Periprosthetic osteolysis, involving RANK/RANKL/osteoprotegerin (OPG) and TNF-α/NFκB signaling, contributes to bone resorption and inflammation. We constructed lentivirus vectors to inhibit TNF-α and enhance OPG expression and assessed their impacts on wear debris-induced inflammation and osteoclastogenesis in an osteoclast/osteoblast coculture system.

METHODS

We transduced mouse osteoblastic MC3T3-E1 cells with Lenti-negative control (Lenti-NC), Lenti-OPG or Lenti-siTNFα-OPG, and murine macrophage/monocyte RAW264.7 cells with Lenti-NC, Lenti-TNF-α siRNA or Lenti-siTNFα-OPG. Then, TNF-α and OPG protein levels were evaluated by enzyme-linked immunosorbent assay. We cocultured transduced MC3T3-E1 and RAW264.7 cells in transwell chambers in the presence of 0.1 mg/mL Ti particles to investigate the capacity of TNF-α inhibition to reduce wear debris-induced inflammation. We also assessed mRNA levels TNF-α, IL-1β, IL-6 and OPG by RT-PCR as well as osteoclastogenesis by tartrate-resistant acid phosphatase.

RESULTS

Lenti-siTNFα-OPG ameliorated Ti-particle-induced expression of TNF-α, IL-1β, IL-6 in MC3T3-E1/RAW264.7 cocultures, while enhancing mRNA and protein levels of OPG, and reducing the fraction of tartrate-resistant acid phosphatase (TRAP)+ cells.

CONCLUSIONS

Lenti-siTNFα-OPG can inhibit the wear debris-induced inflammatory responses and osteoclastogenesis in vitro, and may represent a promising therapeutic candidate for the treatment or prevention of wear particle-induced osteolysis.

Innate immune reactions in septic and aseptic osteolysis around hip implants

According to the long-standing definition, septic and aseptic total joint replacement loosening are two distinct conditions with little in common. Septic joint replacement loosening is driven by bacterial infection whereas aseptic loosening is caused by biomaterial wear debris released from the bearing surfaces. However, recently it has been recognized that the mechanisms that drive macrophage activation in septic and aseptic total joint replacement loosening resemble each other. In particular, accumulating evidence indicates that in addition to mediating bacterial recognition and the subsequent inflammatory reaction, toll-like receptors (TLRs) and their ligands, pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPS), play a key role in wear debris-induced inflammation and macrophage activation. In addition, subclinical bacterial biofilms have been identified from some cases of seemingly aseptic implant loosening. Furthermore, metal ions released from some total joint replacements can activate TLR signaling similar to bacterial derived PAMPs. Likewise, metal ions can function as haptens activating the adaptive immune system similar to bacterial derived antigens. Thus, it appears that aseptic and septic joint replacement loosening share similar underlying pathomechanisms and that this strict dichotomy to sterile aseptic and bacterial-caused septic implant loosening is somewhat questionable. Indeed, rather than being two, well-defined clinical entities, peri-implant osteolysis is, in fact, a spectrum of conditions in which the specific clinical picture is determined by complex interactions of multiple local and systemic factors.

Joint replacement surgery and the innate immune system

Total joint replacement is a highly successful, cost-effective surgical procedure that relieves pain and improves function for patients with end-stage arthritis. The most commonly used materials for modern joint replacements include metal alloys such as cobalt chrome and titanium alloys, polymers including polymethylmethacrylate and polyethylene, and ceramics. Implantation of a joint prosthesis incites an acute inflammatory reaction that is regulated by the innate immune system, a preprogrammed non-antigen specific biological response composed of cells, proteins, and other factors. This "frontline" immune mechanism was originally designed to combat invading microorganisms, but now responds to both pathogen-associated molecular patterns or PAMPS (by-products from microorganisms), and damage associated molecular patterns or DAMPS (molecular by-products from cells), via pattern recognition receptors (PRRs). In this way, potentially injurious stimuli that might disrupt the normal homeostatic regulatory mechanisms of the organism are efficiently dealt with, ensuring the survival of the host. Initial surgical implantation of the joint replacement, as well as ongoing generation of wear debris and byproducts during usage of the joint, activates the innate immune system. Understanding and potentially modulating these events may lead to improved function and increased longevity of joint replacements in the future.

Antibiotic prophylaxis for preventing infectious complications in orthognathic surgery

BACKGROUND

Orthognathic surgery (OS) is a term that refers to many elective surgical techniques to correct facial deformity; the associated malocclusion and functional disorders related to the stomatognathic system. Whilst such surgery is classed as "clean-contaminated", the usefulness of and the most appropriate regimen for antibiotic prophylaxis in these patients are still debated.

OBJECTIVES

To assess the effects of antibiotic prophylaxis for preventing surgical site infection (SSI) in people undergoing orthognathic surgery.

SEARCH METHODS

In June 2014, we searched the Cochrane Wounds Group Specialised Register; the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library); Ovid MEDLINE; Ovid MEDLINE (In-Process & Other Non-Indexed Citations); Ovid EMBASE; and EBSCO CINAHL. We also searched Google Scholar and performed manual searches in journals relevant to the topic, conference proceedings and lists of references of potentially included articles. We did not restrict the search and study selection with respect to language, date of publication or study setting.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) involving people undergoing orthognathic surgery comparing one regimen of antibiotic prophylaxis with any other regimen or placebo. The primary outcome was SSI, and secondary outcomes were systemic infections, adverse events, duration of hospital stay and health-related quality of life. Two review authors screened articles independently.

DATA COLLECTION AND ANALYSIS

Data were abstracted independently by two review authors, and agreement was checked. Risk of bias was assessed using the Cochrane risk of bias tool. Antibiotic regimens were classified as preoperative (one dose before surgery), short-term (before or during surgery and/or during the same day of surgery) and long-term (before or during surgery and longer than one day after surgery) antibiotic prophylaxis. Random-effects meta-analyses using inverse variance methods were undertaken when possible. We report risk ratios (RRs) and their corresponding 95% confidence intervals (CIs).

MAIN RESULTS

A total of 11 trials were included in this review. Most of the studies had an unclear risk of bias prompting us to downgrade the quality of evidence for our outcomes. Seven of these trials provided evidence for the main comparison and the primary outcome and these were pooled. Overall, long-term antibiotic prophylaxis probably reduces the risk of SSI (plausible effects range between a 76% to a 0.26% relative reduction in SSI with long-term antibiotic prophylaxis) (472 participants; RR 0.42, 95% CI 0.24 to 0.74; moderate-quality evidence). There is uncertainty surrounding the relative effects of short-term antibiotics compared with a single dose (220 participants; RR 0.34, 95% CI 0.09 to 1.22; low-quality evidence). No reports described adverse effects associated with the drugs in those trials that reported in this outcome. None of these trials assessed or reported data regarding other outcomes, and information was insufficient to show whether a specific antibiotic is better than another.

AUTHORS' CONCLUSIONS

For people undergoing orthognathic surgery, long term antibiotic prophylaxis decreases the risk of SSI compared with short-term antibiotic prophylaxis and the is uncertainty of whether short-term antibiotic prophylaxis decreases SSi risk relative to a single pre-operative dose of prophylactic antibiotics.

Particle-induced osteolysis is not accompanied by systemic remodeling but is reflected by systemic bone biomarkers

Particle-induced osteolysis is caused by an imbalance in bone resorption and formation, often leading to loss of implant fixation. Bone remodeling biomarkers may be useful for identification of osteolysis and studying pathogenesis, but interpretation of biomarker data could be confounded if local osteolysis engenders systemic bone remodeling. Our goal was to determine if remote bone remodeling contributes to biomarker levels. Serum concentrations of eight biomarkers and bone remodeling rates at local (femur), contiguous (tibia), and remote (humerus and lumbar vertebra) sites were evaluated in a rat model of particle-induced osteolysis. Serum CTX-1, cathepsin K, PINP, and OPG were elevated and osteocalcin was suppressed in the osteolytic group, but RANKL, TRAP 5b, and sclerostin were not affected at the termination of the study at 12 weeks. The one marker tested longitudinally (CTX-1) was elevated by 3 weeks. We found increased bone resorption and decreased bone formation locally, subtle differences in contiguous sites, but no differences remotely at 12 weeks. Thus, the skeletal response to local particle challenge was not systemic, implying that the observed differences in serum biomarker levels reflect differences in local remodeling.

Contributions of human tissue analysis to understanding the mechanisms of loosening and osteolysis in total hip replacement

Aseptic loosening and osteolysis are the most frequent late complications of total hip arthroplasty (THA) leading to revision of the prosthesis. This review aims to demonstrate how histopathological studies contribute to our understanding of the mechanisms of aseptic loosening/osteolysis development. Only studies analysing periprosthetic tissues retrieved from failed implants in humans were included. Data from 101 studies (5532 patients with failure of THA implants) published in English or German between 1974 and 2013 were included. "Control" samples were reported in 45 of the 101 studies. The most frequently examined tissues were the bone-implant interface membrane and pseudosynovial tissues. Histopathological studies contribute importantly to determination of key cell populations underlying the biological mechanisms of aseptic loosening and osteolysis. The studies demonstrated the key molecules of the host response at the protein level (chemokines, cytokines, nitric oxide metabolites, metalloproteinases). However, these studies also have important limitations. Tissues harvested at revision surgery reflect specifically end-stage failure and may not adequately reveal the evolution of pathophysiological events that lead to prosthetic loosening and osteolysis. One possible solution is to examine tissues harvested from stable total hip arthroplasties that have been revised at various time periods due to dislocation or periprosthetic fracture in multicenter studies.

Controlled-release of bone morphogenetic protein-2 from a microsphere coating applied to acid-etched Ti6AL4V implants increases biological bone growth in vivo

A central clinical challenge regarding the surgical treatment of bone and joint conditions is the eventual loosening of an orthopedic implant as a result of insufficient bone ingrowth at the bone-implant interface. We investigated the in vivo effectiveness of a coating containing recombinant human bone morphogenetic protein-2 (rhBMP-2)-loaded microspheres applied to acid-etched Ti6Al4V cylinders for implantation. Three groups of rabbits (24 per group) were used for implantation: (1) acid-etched Ti6Al4V implants coated with a mixture of rhBMP-2-loaded microspheres (125 ng rhBMP-2/mg microspheres) and α-butyl cyanoacrylate; (2) acid-etched, uncoated implants; and (3) bare, smooth uncoated implants. After implantation, 12 rabbits from each group were used for bone ingrowth determination at 4, 5, 6, 7, 8, and 12 weeks (2 rabbits per time point), while the remainder were used for histological analysis and push-out testing at 12 weeks. Scanning electron microscopy showed significant improvement in bone growth of the rhBMP-2 microspheres/α-butyl cyanoacrylate group compared with the other groups (p<0.01). Histological analysis and push-out testing also demonstrated enhanced bone growth of the rhBMP-2 group over that in the other two groups (p<0.01). The rhBMP-2 group showed the most significant bone growth, suggesting that coating acid-etched implants with a mixture of rhBMP-2-loaded microspheres and α-butyl cyanoacrylate may be an effective method to improve the osseointegration of orthopedic implants.

Orthopaedic implant failure: aseptic implant loosening–the contribution and future challenges of mouse models in translational research

Aseptic loosening as a result of wear debris is considered to be the main cause of long-term implant failure in orthopaedic surgery and improved biomaterials for bearing surfaces decreases significantly the release of micrometric wear particles. Increasingly, in-depth knowledge of osteoimmunology highlights the role of nanoparticles and ions released from some of these new bearing couples, opening up a new era in the comprehension of aseptic loosening. Mouse models have been essential in the progress made in the early comprehension of pathophysiology and in testing new therapeutic agents for particle-induced osteolysis. However, despite this encouraging progress, there is still no valid clinical alternative to revision surgery. The present review provides an update of the most commonly used bearing couples, the current concepts regarding particle–cell interactions and the approaches used to study the biology of periprosthetic osteolysis. It also discusses the contribution and future challenges of mouse models for successful translation of the preclinical progress into clinical applications.

Integrin-directed modulation of macrophage responses to biomaterials

Macrophages are the primary mediator of chronic inflammatory responses to implanted biomaterials, in cases when the material is either in particulate or bulk form. Chronic inflammation limits the performance and functional life of numerous implanted medical devices, and modulating macrophage interactions with biomaterials to mitigate this response would be beneficial. The integrin family of cell surface receptors mediates cell adhesion through binding to adhesive proteins nonspecifically adsorbed onto biomaterial surfaces. In this work, the roles of integrin Mac-1 (αMβ2) and RGD-binding integrins were investigated using model systems for both particulate and bulk biomaterials. Specifically, the macrophage functions of phagocytosis and inflammatory cytokine secretion in response to a model particulate material, polystyrene microparticles were investigated. Opsonizing proteins modulated microparticle uptake, and integrin Mac-1 and RGD-binding integrins were found to control microparticle uptake in an opsonin-dependent manner. The presence of adsorbed endotoxin did not affect microparticle uptake levels, but was required for the production of inflammatory cytokines in response to microparticles. Furthermore, it was demonstrated that integrin Mac-1 and RGD-binding integrins influence the in vivo foreign body response to a bulk biomaterial, subcutaneously implanted polyethylene terephthalate. A thinner foreign body capsule was formed when integrin Mac-1 was absent (~30% thinner) or when RGD-binding integrins were blocked by controlled release of a blocking peptide (~45% thinner). These findings indicate integrin Mac-1 and RGD-binding integrins are involved and may serve as therapeutic targets to mitigate macrophage inflammatory responses to both particulate and bulk biomaterials.

Elevated cytokine expression of different PEEK wear particles compared to UHMWPE in vivo

Due to their mechanical properties, there has been growing interest in poly-ether-ether-ketone (PEEK) and its composites as bearing material in total and unicompartmental knee arthroplasty. The aim of this study was to analyze the biological activity of wear particles of two different (pitch and PAN) carbon-fiber-reinforced- (CFR-) PEEK varieties in comparison to ultra-high-molecular-weight-polyethylene (UHMWPE) in vivo. The authors hypothesized no difference between the used biomaterials. Wear particle suspensions of the particulate biomaterials were injected into knee joints of Balb/c mice, which were sacrificed after seven days. The cytokine expression (IL-1β, IL-6, TNF-α) was analyzed immunohistochemically in the synovial layer, the adjacent bone marrow and the articular cartilage. Especially in the bone marrow of the two CFR-PEEK varieties there were increased cytokine expressions compared to the control and UHMWPE group. Furthermore, in the articular cartilage the CFR-PEEK pitch group showed an enhanced cytokine expression, which could be a negative predictor for the use in unicondylar knee systems. As these data suggest an increased proinflammatory potential of CFR-PEEK and its composites in vivo, the initial hypothesis had to be refuted. Summarizing these results, CFR-PEEK seems not to be an attractive alternative to UHMWPE as a bearing material, especially in unicompartmental knee arthroplasty.

Macrophages-Key cells in the response to wear debris from joint replacements

The generation of wear debris is an inevitable result of normal usage of joint replacements. Wear debris particles stimulate local and systemic biological reactions resulting in chronic inflammation, periprosthetic bone destruction, and eventually, implant loosening, and revision surgery. The latter may be indicated in up to 15% patients in the decade following the arthroplasty using conventional polyethylene. Macrophages play multiple roles in both inflammation and in maintaining tissue homeostasis. As sentinels of the innate immune system, they are central to the initiation of this inflammatory cascade, characterized by the release of proinflammatory and pro-osteoclastic factors. Similar to the response to pathogens, wear particles elicit a macrophage response, based on the unique properties of the cells belonging to this lineage, including sensing, chemotaxis, phagocytosis, and adaptive stimulation. The biological processes involved are complex, redundant, both local and systemic, and highly adaptive. Cells of the monocyte/macrophage lineage are implicated in this phenomenon, ultimately resulting in differentiation and activation of bone resorbing osteoclasts. Simultaneously, other distinct macrophage populations inhibit inflammation and protect the bone-implant interface from osteolysis. Here, the current knowledge about the physiology of monocyte/macrophage lineage cells is reviewed. In addition, the pattern and consequences of their interaction with wear debris and the recent developments in this field are presented.

Endotoxins-the invisible companion in biomaterials research

Metal implants and polymeric devices for the application in the clinical treatment of orthopedic tissue injuries are increasingly coated with bioactive biomaterials derived from natural substances to induce desirable biological effects. Many metals and polymers used in biomaterials research show high affinity for endotoxins, which are abundant in the environment. Endotoxin contamination is indicated in the pathology of periodontitis and aseptic implant loosening, but may also affect the evaluation of a biomaterial's bioactivity by inducing strong inflammatory reactions. In this review, we discuss the high affinity of three commonly used implant biomaterials for endotoxins and how the contamination can affect the outcome of the orthopedic fixation. The chemical nature of bacterial endotoxins and some of the clinical health implications are described, as this knowledge is critically important to tackle the issues associated with the measurement and removal of endotoxins from medical devices. Commonly used methods for endotoxin testing and removal from natural substances are examined and the lack of standard guidelines for the in vitro evaluation of biomaterials is discussed.

Cytokine secretion from human peripheral blood mononuclear cells cultured in vitro with metal particles

The failure of implanted medical devices can be associated with changes in the production of cytokines by cells of the immune system. Cytokines released by peripheral blood mononuclear cells upon contact with metal particles were quantified to understand their role in implantation intergration and their importance as messengers in the recruitment of T-lymphocytes at the implantation site. Opsonization was utilised to understand the influence of serum proteins on particle-induced cytokine production and release. Different metal compositions were used in the particulate format, Titanium (Ti), Titanium alloy (Ti6Al4V), and Stainless Steel 316L (SS), and were cultured in vitro with a mixed population of monocytes/macrophages and lymphocytes. The cells were also exposed to an exogenous stimulant mixture of phytohemagglutinin-P and interferon-gamma (IFN-γ) and opsonized particles with human serum. Interleukins, IL-1α, IL-1β, IL-2, IL-4, IL-6, IL-8, IFN-γ, and tumor necrosis factor-alpha (TNF-α) were investigated using enzyme-linked immunosorbent assay as they are an indicator of the inflammation evoked by particulate metals. It has been experimentally evidenced that metal particles induced higher amounts of IL-6 and IL-1 but very low amounts of TNF-α. T-lymphocyte activation was evaluated by the quantification of IL-2 and IFN-γ levels. The results showed that nonopsonized and opsonized metal particles did not induce the release of increased levels of IL-2 and IFN-γ.

Protein polymer hydrogels: effects of endotoxin on biocompatibility

Protein polymer-based hydrogels have shown potential for tissue engineering applications, but require biocompatibility testing for in vivo use. Enzymatically crosslinked protein polymer-based hydrogels were tested in vitro and in vivo to evaluate their biocompatibility. Endotoxins present in the hydrogel were removed by Trition X-114 phase separation. The reduction of endotoxins decreased TNF-α production by a macrophage cell line in vitro; however, significant inflammatory response was still present compared to collagen control gels. A branched PEG molecule and dexamethasone were added to the hydrogel to reduce the response. In vitro testing showed a decrease in the TNF-α levels with the addition of dexamethasone. In vivo implantations into the epididymal fat pad of C57/BL6 mice, however, indicated a decreased inflammatory mediated immune response with a hydrogel treated with both PEGylation and endotoxin reduction. This study demonstrates the importance of endotoxin testing and removal in determining the biocompatibility of biomaterials.

Revision joint replacement, wear particles, and macrophage polarization

Currently, younger, more active patients are being offered total joint replacement (TJR) for end-stage arthritic disorders. Despite improved durability of TJRs, particle-associated wear of the bearing surfaces continues to be associated with particulate debris, which can activate monocyte/macrophages. Activated macrophages then produce pro-inflammatory factors and cytokines that induce an inflammatory reaction that activates osteoclasts leading to bone breakdown and aseptic loosening. We hypothesized that activated macrophages in tissues harvested from revised joint replacements predominantly express an M1 pro-inflammatory phenotype due to wear-particle-associated cell activation, rather than an M2 anti-inflammatory phenotype. We further questioned whether it is possible to convert uncommitted monocyte/macrophages to an M2 phenotype by the addition of interleukin-4 (IL-4), or whether it is necessary to first pass through an M1 intermediate stage. Retrieved periprosthetic tissues demonstrated increased M1/M2 macrophage ratios compared to non-operated osteoarthritic synovial tissues, using immunohistochemical staining and Western blotting. Uncommitted monocyte/macrophages with/without polymethyl-methacrylate particles were transformed to an M2 phenotype by IL-4 more efficiently when the cells were first passed through an M1 phenotype by exposure to endotoxin. Wear particles induce a pro-inflammatory microenvironment that facilitates osteolysis; these events may potentially be modulated favorably by exposure to IL-4.

Micrometer-sized titanium particles can induce potent Th2-type responses through TLR4-independent pathways

Wear debris in joint replacements has been suggested as a cause of associated tissue-damaging inflammation. In this study, we examined whether solid titanium microparticles (mTi) of sufficient size to accumulate as wear debris could stimulate innate or adaptive immunity in vivo. mTi, administered in conjunction with OVA, promoted total and Ag-specific elevations in serum IgE and IgG1. Analysis of transferred transgenic OVA-specific naive T cells further showed that mTi acted as an adjuvant to drive Ag-specific Th2 cell differentiation in vivo. Assessment of the innate response indicated that mTi induced rapid recruitment and differentiation of alternatively activated macrophages in vivo, through IL-4- and TLR4-independent pathways. These studies suggest that solid microparticles alone can act as adjuvants to induce potent innate and adaptive Th2-type immune responses and further suggest that wear debris in joint replacements may have Th2-type inflammatory properties.

PI3Kγ deletion reduces variability in the in vivo osteolytic response induced by orthopaedic wear particles

Orthopedic wear particles activate a number of intracellular signaling pathways associated with inflammation in macrophages and we have previously shown that the phosphoinositol-3-kinase (PI3K)/Akt pathway is one of the signal transduction pathways that mediates the in vitro activation of macrophages by orthopedic wear particles. Since PI3Kγ is primarily responsible for PI3K activity during inflammation, we hypothesized that PI3Kγ mediates particle-induced osteolysis in vivo. Our results do not strongly support the hypothesis that PI3Kγ regulates the overall amount of particle-induced osteolysis in the murine calvarial model. However, our results strongly support the conclusion that variability in the amount of particle-induced osteolysis between individual mice is reduced in the PI3Kγ(-/-) mice. These results suggest that PI3Kγ contributes to osteolysis to different degrees in individual mice and that the mice, and patients, that are most susceptible to osteolysis may be so, in part, due to an increased contribution from PI3Kγ.

Relationship between the degree of osteolysis and cultures obtained by sonication of the prostheses in patients with aseptic loosening of a hip or knee arthroplasty

INTRODUCTION

The pathogenesis of prosthesis loosening is not well understood. The aim of our study was to sonicate components of joint prostheses removed due to aseptic loosening, culture the sonicate fluid, and to correlate these results with the degree of radiological osteolysis.

METHODS

From January 2008 to June 2009 all consecutive patients who underwent a revision of hip or knee prosthesis due to aseptic loosening were included in the study. Aseptic loosening was established when the patient had radiological signs of loosening without symptoms or signs of infection. The diagnosis was confirmed when histology was negative, and ≥ 5 out of 6 standard cultures of periprosthetic tissue were negative. Bone lysis was measured according to the Paprosky or Engh classifications without knowing the result of sonication cultures. Removed components were placed in sterile bags and immediately transported to the microbiology laboratory and sonicated. Sonicate fluid was cultured and the results were correlated with the degree of bone lysis. The proportion of components with positive sonication culture according to the bone lysis classification was compared using χ(2) test.

RESULTS

A total of 52 patients were included and 123 components were sonicated. In 30 patients at least 1 sonicated component was positive (57.7%) and 44 out of 123 (35.8%) components were positive. The proportion of positive sonication cultures was significantly higher in the group of components with a higher degree of bone lysis of 3 (76.5%) than in those with lower degrees (33.9% for 1 and 24% for 2) (χ(2) test, p = 0.0004).

CONCLUSIONS

Sonication cultures were positive in 57% of patients who underwent revision arthroplasty for aseptic loosening. The percentage of positive sonication cultures was significantly higher in patients with severe osteolysis.

LEVEL OF EVIDENCE

level I of Prognostic Studies-Investigating the Effect of a Patient Characteristic on the Outcome of Disease.

Macrophage response to high number of titanium particles is cytotoxic and COX-2 mediated and it is not affected by the particle's endotoxin content or the cleaning treatment

Periprosthetic osteolysis is a progressive deterioration of bone around prostheses resulting primarily from the presence of wear debris. Particulate material, number, and their interactions with environmental factors play important roles in macrophage activation around implants. We have previously shown that macrophages cultured in the presence of high numbers of cleaned titanium (Ti) particles released significant amounts of PGE₂ that is potentially detrimental for bone. Cleaning of particles has become routine in most studies of macrophage/particle interactions as contaminating endotoxin elicits a macrophage cytokine response and since numerous studies have suggested that endotoxins may be present on implant materials. However, the strenuous cleaning procedure itself represents a possible source of other contaminants (such as material by-products) that may be relevant to the prostanoid response of macrophages. To analyze this hypothesis, the macrophage response to high numbers of cleaned Ti particles was compared to that of unclean particles and to particles that were subjected to a short version of the cleaning procedure. It was found that neither the high amount of endotoxin on the unclean particles nor the duration of the cleaning procedure had an effect on the release of PGE₂ and the decrease in cell viability in response to high number of Ti particles. Evidence of a possible relationship between these two variables is presented.

Toll-like receptors and their adaptors are regulated in macrophages after phagocytosis of lipopolysaccharide-coated titanium particles

Macrophages phagocytose metallic wear particles and produce mediators, which can induce cellular host response and aseptic implant loosening. Lipopolysaccharide (LPS) on the wear debris can stimulate macrophages via Toll-like receptor 4 (TLR4) and enhance the response. However, the precise functional role and interaction of TLRs and their adaptor molecules is still unclear. Rat bone marrow macrophages were stimulated with titanium particle (Ti) coated by LPS (Ti/LPS+) and LPS-free Ti (Ti/LPS-). mRNA levels of cytokines, TLRs and their adaptor molecules were measured using real time PCR. mRNA levels of TNF-α, IL-1β, and IL-6 increased in Ti/LPS+ than Ti/LPS-. In contrast, mRNA levels of TLR4, TLR5, and TLR9 decreased in Ti/LPS+ compared to Ti/LPS-. mRNA levels of MyD88, IRAK1, IRAK4 decreased gradually, and TRAF6 underwent an initial transient increase, followed by suppression in Ti/LPS+. However, mRNA levels of TLR2 and IRAK2 increased after phagocytosis of Ti/LPS+ than Ti/LPS-. The increased expressions of proinflammatory cytokines found in Ti/LPS+ indicated that their productions cytokines could be enhanced by phagocytosis of LPS-coated particles. Subsequent down-regulation of TLR4, TLR5, TLR9, MyD88, IRAK1, and IRAK4 suggests that self-protective mechanisms to regulate excessive host responses are activated in macrophages. Increase of TLR2 and IRAK2 and a transient increase of TRAF6 in Ti/LPS+ suggest that another possible pathway to modulate TLR-mediated cellular response to prolong inflammatory response in foreign body reaction of aseptic loosening. This down- and/or up-regulation of the potential TLR-mediated responses to LPS-coated particles reflects the proactive behavior of effector cells.

Bacterial pathogen-associated molecular patterns stimulate biological activity of orthopaedic wear particles by activating cognate Toll-like receptors

Aseptic loosening of orthopaedic implants is induced by wear particles generated from the polymeric and metallic components of the implants. Substantial evidence suggests that activation of Toll-like receptors (TLRs) may contribute to the biological activity of the wear particles. Although pathogen-associated molecular patterns (PAMPs) produced by Gram-positive bacteria are likely to be more common in patients with aseptic loosening, prior studies have focused on LPS, a TLR4-specific PAMP produced by Gram-negative bacteria. Here we show that both TLR2 and TLR4 contribute to the biological activity of titanium particles with adherent bacterial debris. In addition, lipoteichoic acid, a PAMP produced by Gram-positive bacteria that activates TLR2, can, like LPS, adhere to the particles and increase their biological activity, and the increased biological activity requires the presence of the cognate TLR. Moreover, three lines of evidence support the conclusion that TLR activation requires bacterially derived PAMPs and that endogenously produced alarmins are not sufficient. First, neither TLR2 nor TLR4 contribute to the activity of "endotoxin-free" particles as would be expected if alarmins are sufficient to activate the TLRs. Second, noncognate TLRs do not contribute to the activity of particles with adherent LPS or lipoteichoic acid as would be expected if alarmins are sufficient to activate the TLRs. Third, polymyxin B, which inactivates LPS, blocks the activity of particles with adherent LPS. These results support the hypothesis that PAMPs produced by low levels of bacterial colonization may contribute to aseptic loosening of orthopaedic implants.

Individual susceptibility to periprosthetic osteolysis is associated with altered patterns of innate immune gene expression in response to pro-inflammatory stimuli

Susceptibility to osteolysis after total hip arthroplasty (THA) varies between individuals. We examined whether patients susceptible to osteolysis (group I, n = 34 subjects) after cemented Charnley THA have quantitatively different innate immune responses to pro-inflammatory stimuli versus patients without this susceptibility (group II, n = 28 subjects) at a mean of 14 years after primary surgery. Extracted peripheral blood mononuclear cells were stimulated for 3 h using endotoxin (lipopolysaccharide-LPS, 100 ng/mL), endotoxin-stripped titanium particles (Ti) or endotoxin-stripped particles with adherent LPS added-back (TI + LPS). Subjects returned 1 week later and the experimental protocol was repeated. Assays for mRNA induction for interleukin (IL)-1alpha, IL-1beta, IL-1Ra, IL-6, IL-10, IL-18, and tumor necrosis factor (TNF) were made using quantitative real-time PCR. Although baseline levels of mRNA expression were slightly lower in group I, inducibility of mRNA expression was markedly greater in group I versus group II for all cytokines in response to LPS or Ti + LPS, and for IL-1alpha in response to Ti (P < 0.05). LPS or Ti + LPS stimulation also resulted in an increase in the IL-1/IL-1Ra mRNA ratio in group I versus group II (P < 0.05). mRNA induction was highly reproducible between subject visits (r > 0.7, P < 0.001). Osteolysis-susceptible patients show repeatable, quantitatively different patterns of innate cytokine gene expression in response to pro-inflammatory stimuli versus THA patients who do not display this susceptibility. These innate immune differences may contribute to the variation in osteolysis-susceptibility observed clinically between individuals.

In vitro macrophage response to polyethylene and polycarbonate-urethane particles

This study was undertaken to compare macrophage response to polycarbonate-urethane (PCU), a proposed alternative material to polyethylene in acetabular components of total hip arthroplasty to cross-linked ultra-high molecular weight polyethylene (xUHMWPE) in the presence or absence of endotoxin. Polyethylene wear debris that is generated by total hip and knee replacements has been linked to osteolysis and limiting the lifespan of the implant. We added both lipopolysaccharide (LPS)-free and endotoxin-associated xUHMWPE and PCU particles to a human monocyte cell line (TH1) in culture and measured cell viability and tumor necrosis factor (TNF)alpha, interleukin (IL)-1beta, and prostaglandin E(2) (PGE(2)) in the medium after 24 h. Results indicate that particles (both xUHMWPE and PCU) free of endotoxin did not significantly induce secretion of TNFalpha, IL-1beta, or PGE(2) above basal levels. However, endotoxin-exposed PCU particles induced significantly less TNFalpha and IL-1beta than endotoxin-exposed xUHMWPE particles. This indicates that if endotoxin is available for binding to particles in vivo, then xUHMWPE may be more inflammatory to periprosthetic tissue and bone in part because of its affinity/reactivity with endotoxin when compared with PCU.

Toll-like receptors and aseptic loosening of hip endoprosthesis-a potential to respond against danger signals?

Bacterial remnants and subclinical biofilms residing on prosthesis surfaces have been speculated to play a role in hip implant loosening by opsonizing otherwise relatively inert wear particles. The innate immune system recognizes these microbial pathogen-associated molecular patterns (PAMPs) using Toll-like receptors (TLRs). Our objective was to evaluate the possible presence of TLRs in aseptic synovial membrane-like interface tissue. Bacterial culture-negative, aseptic (n = 4) periprosthetic synovial membrane-like tissue was compared to osteoarthritis synovial membrane (n = 5) for the presence of cells positive for all known human functional TLRs, stained using specific antibodies by immunohistochemistry, and evaluated using morphometry. In comparison to osteoarthtritic synovium, the number of TLR-positive cells was found to be increased in the aseptic setting, reflecting the considerable macrophage infiltration to the tissues investigated. Thus aseptic periprosthetic tissue seems to be very reactive to PAMPs. It has been recently recognized that TLR do not only respond to traditional PAMPs, but also to endogenous alarmings or danger signals released from necrotic and activated cells. Alarming-TLR interaction in the periprosthetic tissue might be a novel mechanism of aseptic loosening of endoprosthesis.

Efficacy of colistin-impregnated beads to prevent multidrug-resistant A. baumannii implant-associated osteomyelitis

Osteomyelitis (OM) from multidrug-resistant (MDR) Acinetobacter has emerged in >30% of combat-related injuries in Iraq and Afghanistan. While most of these strains are sensitive to colistin, the drug is not available in bone void fillers for local high-dose delivery. To address this, we developed a mouse model with MDR strains isolated from wounded military personnel. In contrast to S. aureus OM, which is osteolytic and characterized by biofilm in necrotic bone, A. baumannii OM results in blastic lesions that do not contain apparent biofilm. We also found that mice mount a specific IgG response against three proteins (40, 47, and 56 kDa) regardless of the strain used, suggesting that these may be immuno-dominant antigens. PCR for the A. baumannii-specific parC gene confirmed a 100% infection rate with 75% of the MDR strains, and in vitro testing confirmed that all strains were sensitive to colistin. We also developed a real-time quantitative PCR (RTQ-PCR) assay that could detect as few as 10 copies of parC in a sample. To demonstrate the efficacy of colistin prophylaxis in this model, mice were treated with either parenteral colistin (0.2 mg colistinmethate i.m. for 7 days), local colistin (PMMA bead impregnated with 1.0 mg colistin sulfate), or an unloaded PMMA bead control. While the parenteral colistin failed to demonstrate any significant effects versus the placebo, the colistin PMMA bead significantly reduced the infection rate such that only 29.2% of the mice had detectable levels of parC at 19 days (p < 0.05 vs. i.m. colistin and placebo).

Highly crosslinked vs conventional polyethylene particles: relative in vivo inflammatory response

Crosslinked polyethylenes have been introduced to reduce wear and osteolysis. The osteolysis rate depends on many factors including the biologic activity of the wear particles. This study examines the relative inflammatory potential of highly crosslinked and non-crosslinked polyethylene particles. Polyethylene particles were crosslinked and characterized. Dose-response curves were generated for endotoxin-positive and endotoxin-negative particles at each degree of cross-linking using an in vivo model. The 10-MRad crosslinked polyethylene was more inflammatory than an identical dose (25 mg/mL) of non-crosslinked polyethylene (P = .05). Endotoxin increased the inflammatory response to crosslinked and non-crosslinked polyethylene in a similar fashion. These data suggest that the improved wear characteristics of highly crosslinked polyethylenes may be offset somewhat by the modestly increased inflammatory profile of the highly crosslinked compared with non-crosslinked particles.

Polymorphisms in the interleukin-1 receptor antagonist and interleukin-6 genes affect risk of osteolysis in patients with total hip arthroplasty

OBJECTIVE

To determine whether osteolysis after total hip arthroplasty (THA) is associated with common polymorphisms within the genes encoding the interleukin-1 (IL-1) family and IL-6, and to determine whether polymorphisms that are associated with osteolysis affect in vitro messenger RNA (mRNA) expression in human peripheral blood mononuclear cells (PBMCs) in response to wear particles.

METHODS

Unrelated white subjects of North European descent (n=612) were recruited a mean of 11 years after cemented THA for primary osteoarthritis. Of these subjects, 272 had previous osteolysis and 340 had no radiographic evidence of osteolysis (control group). Genomic DNA was genotyped for the following single-nucleotide polymorphisms (SNPs): IL1A +4845, IL1B +3954, IL1B -3737, IL1B -511, IL1RA +2018, IL6 -174, IL6 -572, and IL6 -597. In a subset of 60 subjects, PBMCs were extracted and stimulated with titanium particles and/or endotoxin, and cytokine mRNA expression was measured using quantitative real-time reverse transcriptase-polymerase chain reaction.

RESULTS

The odds ratio (OR) for osteolysis associated with carriage of the IL1RA +2018C allele was 0.66 (95% confidence interval [95% CI] 0.48-0.91) (P=0.012). The remaining SNPs were not individually associated with osteolysis. The uncommon IL6 haplotype -174G/-572G/-597A (osteolysis group frequency 2.4%, control group frequency 0.8%) was associated with osteolysis (P=0.02, calculated using Haploview software). The IL1RA +2018CC genotype was associated with increased mRNA expression compared with the +2018TT genotype in both unstimulated and stimulated PBMCs (P=0.01 by analysis of variance, after Bonferroni correction).

CONCLUSION

The IL1RA +2018C allele is associated with a decreased risk of osteolysis after THA and with increased IL-1 receptor antagonist mRNA expression in vitro. An uncommon haplotype within the promoter region of the gene for IL-6 is positively associated with osteolysis.

What other biologic and mechanical factors might contribute to osteolysis?

An overwhelming consensus exists that wear particles are the primary driving force in aseptic loosening of orthopaedic implants. Nonetheless, considerable evidence has emerged demonstrating that various other factors can modulate the biologic activity of orthopaedic wear particles. Two of the most studied modulating factors are bacterial endotoxins and implant motion.

Upregulation of LITAF mRNA expression upon exposure to TiAlV and polyethylene wear particles in THP-1 macrophages

Tumor necrosis factor alpha (TNFalpha) plays a fundamental role in the pathogenesis of wear particle-induced periprosthetic osteolysis. However, particle-induced mechanisms that control TNFalpha gene expression are not yet well characterized. LITAF [lipopolysaccharide (LPS)-induced TNFalpha factor] is a novel transcription factor that regulates expression of the TNFalpha gene, but nothing is known about its role in wear particle-induced osteolysis. We evaluated the effect of titanium aluminum vanadium (TiAlV) and polyethylene particles on mRNA expression of LITAF. A human monocytic leukemia cell line (THP-1) was used in this in vitro study. THP-1 monocytes were differentiated to macrophage-like cells and exposed to LPS-detoxified polyethylene particles and prosthesis-derived TiAlV particles. Supernatant was used for TNFalpha protein measurement and total RNA was extracted from cells. LITAF was analyzed at the mRNA level using semiquantitative RT-PCR. Both polyethylene and TiAlV particles induced significant upregulation of LITAF mRNA that was followed by a significant TNFalpha response. These effects were dependent on the particle dose. Low particle concentrations exhibited no significant effect on expression of TNFalpha and LITAF mRNA. In comparison to exposure to polyethylene and TiAlV particles, LPS stimulation exhibited similar upregulation of LITAF mRNA, but led to an overwhelming TNFalpha response. Our findings provide evidence that LITAF is implicated in the pathogenesis of wear particle-induced osteolysis.

Inhibition of the PI3K-Akt signaling pathway reduces tumor necrosis factor-alpha production in response to titanium particles in vitro

BACKGROUND

Wear debris contributes to implant loosening after total joint arthroplasty, and few advances have been made in our ability to inhibit the biological response to wear particles. Bacterial endotoxins augment the effects of wear particles in vitro and in vivo. The cytokine, tumor necrosis factor-alpha (TNF-alpha), is produced by macrophages in response to bacterial endotoxins and wear particles, and it increases osteoclast activity resulting in bone resorption and implant loosening. The phosphoinositol-3-kinase (PI3K)-Akt intracellular signal transduction pathway contributes to cytokine production in response to soluble endotoxin. We investigated the role of the PI3K-Akt pathway in the production of TNF-alpha in response to wear particles with adherent endotoxin and so-called endotoxin-free wear particles.

METHODS

Cultured RAW264.7 murine macrophages were incubated with titanium particles with adherent endotoxin or with endotoxin-free titanium particles in the presence and absence of specific inhibitors of PI3K (LY294002) or Akt (SH-5). Akt activation was assessed with use of Western blot. TNF-alpha production was measured with use of enzyme-linked immunosorbent assay. Cytotoxicity was determined by measuring lactic dehydrogenase release.

RESULTS

Titanium particles with adherent endotoxin increased Akt activation, whereas endotoxin-free titanium particles did not. The PI3K inhibitor reduced TNF-alpha production by 70% in response to titanium with adherent endotoxin without increasing cytotoxicity. Similarly, the Akt inhibitor reduced TNF-alpha production by 83% in response to titanium particles with adherent endotoxin without increasing cytotoxicity. High concentrations of endotoxin-free titanium particles resulted in a small delayed increase in TNF-alpha production that was completely blocked by the PI3K inhibitor.

CONCLUSIONS

Inhibition of the PI3K-Akt pathway reduces macrophage TNF-alpha production in response to titanium particles with adherent endotoxin and endotoxin-free particles in vitro.

Peri-implant bone formation and implant integration strength of peptide-modified p(AAM-co-EG/AAC) interpenetrating polymer network-coated titanium implants

Interpenetrating polymer networks (IPNs) of poly (acrylamide-co-ethylene glycol/acrylic acid) functionalized with an -Arg-Gly-Asp- (RGD) containing 15 amino acid peptides, derived from rat bone sialoprotein (bsp-RGD(15), were grafted to titanium implants in an effort to modulate bone formation in the peri-implant region in the rat femoral ablation model. Bone-implant contact (BIC) and bone formation within the medullary canal were determined using microcomputed tomography at 2 and 4 weeks postimplantation. BIC for bsp-RGD(15)-IPN implants was enhanced relative to hydroxyapatite tricalcium phosphate (HA-TCP) coated implants, but was similar to all other groups. Aggregate bone formation neither indicated a dose-dependent effect of bsp-RGD(15) nor a meaningful trend. Mechanical testing of implant fixation revealed that only the HA-TCP coated implants supported significant (>1 MPa) interfacial shear strength, despite exhibiting lower overall BIC, an indication that bone ingrowth into the rougher coating was the primary mode of implant fixation. While no evidence was found to support the hypothesis that bsp-RGD(15)-modified IPN coated implants significantly impacted bone-implant bonding, these results point to the lack of correlation between in vitro studies employing primary osteoblasts and in vivo wound healing in the peri-implant region.

The balance between endotoxin accumulation and clearance during particle-induced osteolysis in murine calvaria

Bacterial endotoxin may contribute to aseptic loosening of orthopedic implants even in the absence of clinical or microbiological evidence of infection. One potential source of endotoxin during aseptic loosening is systemically circulating endotoxin, derived from intestinal flora, minor infections, or dental procedures, that may bind to wear particles. The current study demonstrates that systemically derived endotoxin accumulates when 'endotoxin-free' titanium and polyethylene particles are implanted on murine calvaria. Time-course experiments and experiments using germ-free mice rule out the possibility that the observed endotoxin accumulation may be due to bacterial contamination. In contrast, endotoxin is cleared from titanium particles that originally carry high amounts of adherent endotoxin. The mechanism of endotoxin clearance is not dependent on induction of a respiratory burst. Taken together, these results indicate that a balance between endotoxin accumulation and endotoxin clearance controls the steady-state level of endotoxin surrounding orthopedic wear particles implanted on murine calvaria. This balance may regulate the rate of osteolysis in the murine calvaria model as well as in patients with aseptic loosening.

Synergistic effects of mixed TiAlV and polyethylene wear particles on TNFα response in THP-1 macrophages / Synergistische Effekte gemischter TiAlV- und Polyethylen-Abriebpartikel auf die TNFα-Antwort in THP-1 Makrophagen

TNFalpha is a potent osteoclastogenic cytokine that has a fundamental role in the pathogenesis of wear particle-induced osteolysis. Wear particles of one composition and their biological effects are well characterised. In contrast, little is known about the effects of mixed particles with respect to mix ratio and particle concentration. We evaluated the effects of different mix ratios of polyethylene and TiAlV particles on TNFalpha response. We used a human monocytic cell line (THP-1) in this in vitro study. THP-1 monocytes were differentiated to macrophage-like cells and exposed to different mixtures of lipopolysaccharide-detoxified polyethylene and TiAlV particles. TNFalpha was analysed in culture supernatants using ELISAs. Both polyethylene and TiAlV particles induced a dose- and time-related release of TNFalpha, with maximum levels after 6 h. A PE/TiAlV mix ratio of 36:1 at 10(8) particles/ml induced significantly higher TNFalpha concentrations compared to equal particle concentrations of isolated TiAlV (p=0.047) or PE (p=0.044), indicating the synergistic effect of mixed particles. These results provide evidence that TiAlV and polyethylene particles have significant synergistic effects, depending on the mix ratio and particle concentrations. This supra-additive effect can contribute substantially to the pathogenesis of implant particle-induced osteolysis.

Differential inflammatory macrophage response to rutile and titanium particles

Titanium and its alloys are widely used as implant materials for dental and orthopaedic applications due to their advantageous bulk mechanical properties and biocompatibility, compared to other metallic biomaterials. In order to improve their wear and corrosion resistance, several surface modifications that give rise to an outer ceramic layer of rutile have been developed. The ability of rutile wear debris to stimulate the release of inflammatory cytokines from macrophages has not been addressed to date. We have compared the in vitro biocompatibility of sub-cytotoxic doses of rutile and titanium particles in THP-1 cells driven to the monocyte/macrophage differentiation pathway as well as in primary cultures of human macrophages. Confocal microscopy experiments indicated that differentiated THP-1 cells and primary macrophages efficiently internalised rutile and titanium particles. Treatment of THP-1 cells with rutile particles stimulated the release of TNF-alpha, IL-6 and IL-1beta to a lesser extent than titanium. The influence of osteoblasts on the particle-induced stimulation of TNF-alpha and IL-1beta was analysed by co-culturing differentiated THP-1 cells with human primary osteoblasts. Under these conditions, secretion levels of both cytokines after treatment of THP-1 cells with rutile particles were lower than after exposure to titanium. Finally, we observed that primary macrophages released higher amounts of TNF-alpha, IL-6 and IL-1beta after incubation with titanium particles than with rutile. Taken together, these data indicate that rutile particles are less bioreactive than titanium particles and, therefore, a higher biocompatibility of titanium-based implants modified with an outer surface layer of rutile is expected.

Titanium particles and surface-bound LPS activate different pathways in IC-21 macrophages

It is still unknown if wear-debris particles themselves induce osteolysis or if they serve a functional role as receptors for ligands that incite an inflammatory response that ultimately leads to bone resorption. In this study, commercially pure titanium particles (cpTi) were subjected to a serial combination of different cleaning methods to remove Lipopolysaccharide (LPS) or were incubated in LPS solutions of known concentrations. Then, the response of the macrophage cell line IC-21 to the cleaned particles, LPS-bound Ti particles, and soluble LPS was examined. It was found that cleaned particles up to 1000 particles per cell did not stimulate macrophages to release Tumor necrosis factor-alpha (TNF-alpha) or Interleukin 6 (IL-6), but they significantly increased the release of Prostaglandin E(2) (PGE(2)) when the particle concentration was higher than 500 particles per cell. At one particle per cell, Ti particles bound with LPS stimulated the release of IL-6 and TNF-alpha by macrophages. The level of released cytokines was dependent on, and correlated with, the amount of LPS present on the particles. The macrophages were more sensitive to soluble LPS than to particle-bound LPS, and the simultaneous addition of cleaned Ti particles did not have additional effects on the effects of soluble LPS. This study shows evidence that, cpTi particles and LPS have distinct mechanisms of action on the IC-21 macrophages, but that both contribute to the development of an inflammatory response.

Lipopolysaccharide found in aseptic loosening of patients with inflammatory arthritis

Aseptic loosening of orthopaedic implants occurs in the absence of clinical signs of infection. Nevertheless, bacterial endotoxins derived from subclinical infections, systemic sources, or the implant manufacturing process may contribute to aseptic loosening. Also, the rate of implant infection is greater in patients with inflammatory arthritis than in patients with osteoarthritis. We hypothesized that lipopolysaccharide, the classic endotoxin derived from gram-negative bacteria, is more prevalent in periprosthetic tissue surrounding aseptically loose implants in patients with inflammatory arthritis than in patients with osteoarthritis. To test this, we used a modified Limulus amebocyte assay not affected by beta-glucan-like molecules in mammalian tissues. Lipopolysaccharide rarely was detected in periprosthetic tissue from patients with osteoarthritis and aseptic loosening (one of six patients). In contrast, lipopolysaccharide was detected despite the absence of any clinical signs of infection in peri-prosthetic tissue from all four patients with inflammatory arthritis (rheumatoid arthritis, juvenile rheumatoid arthritis, and systemic lupus erythematosus). Lipopolysaccharide also was detected in two patients with gram-negative infections, who were included as positive control subjects. Endotoxins derived from low-grade or systemic bacteremia may be important contributors to aseptic loosening particularly in patients with autoimmune conditions such as inflammatory arthritis.

Endotoxin: The uninvited guest

In the laboratory environment where biomaterials are synthesized and their biocompatibility assessed, we find that endotoxin contamination is hard to avoid and must not be ignored. In those relatively few cases where endotoxin was known to be present, it has been clearly shown that endotoxin can significantly affect the biological response observed and hence confound any effect of the material. This short review explains what endotoxin is, how to test for it and remove it and what its effect on the biological response to biomaterials is. We advocate routine testing of endotoxin on biomaterials and of reagents used in experimental evaluation of biomaterials and this should be the responsibility of every scientist to ensure the validity of any biomaterial study.

Activation of NF-kappaB signalling and TNFalpha-expression in THP-1 macrophages by TiAlV- and polyethylene-wear particles

Wear particles are believed to induce periprosthetic inflammation which contributes to periprosthetic osteolysis. TNFalpha plays a pivotal role in the pathogenesis of this process. The molecular mechanisms leading to the development of periprosthetic inflammation with upregulated TNFalpha expression in monocytic cells in response to different wear particles have yet to be defined. In this study we evaluated the effects of polyethylene- and TiAlV-particles on activation of NF-kappaB signalling pathways and TNFalpha biosynthesis and release in monocytic cells with respect to periprosthetic osteoclastogenesis. THP-1 monocytic cells were differentiated to macrophage-like cells and exposed to LPS-detoxified polyethylene and prosthesis-derived TiAlV-particles. TNFalpha release was analyzed in culture supernatant by ELISA. NF-kappaB activation was examined by electrophoretic mobility shift assay (EMSA), and NF-kappaB target promoter activities including transactivation of the TNFalpha promoter were determined by luciferase reporter gene assays. Differentiated THP-1 macrophages were exposed to increasing numbers of particles for 0, 60, 180 and 360 min. Both, polyethylene- and TiAlV-particles induced a significant activation of both NF-kappaB and TNFalpha promoters at 180 min. A significant TNFalpha release was detected after 360 min exposure to polyethylene- and TiAlV-particles in a dose dependent manner. In comparison, LPS induced a much greater activation of NF-kappaB and TNFalpha promoters, and TNFalpha secretion into the supernatant was strongly induced. These results provide evidence that induction of the NF-kappaB signal transduction pathway in macrophages plays a major role in initiating and mediating the inflammatory response leading to periprosthetic osteolysis.