Adherent endotoxin on orthopedic wear particles stimulates cytokine production and osteoclast differentiation

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.

Diagnosis of infection after total hip replacement

Subclinical infection in patients with pain following total hip replacement (THR) is an underestimated condition that needs consideration because it mimics aseptic loosening, contributes to periprosthetic osteolysis, and necessitates proper treatment. We aimed to define the reliability of diagnostic parameters that are routinely used before revision surgery for the assessment of infection. A continuous series of 26 subjects who underwent THR revision surgery was considered, including 21 cases diagnosed as aseptic loosening (group A) and 5 hip revisions with a clinical diagnosis for infection (group B). Seven subjects at the time of the primary arthroplasty were used as negative controls (group C). Technetium-99m labeled hydroxymethylene diphosphonate [(99m)Tc-HDP]- and technetium-99m hexamethylpropyleneamine oxide [(99m)Tc-HMPAO)]-labeled granulocyte scintigraphy, histology of peri-implant tissues, laboratory tests for inflammation, and microbiology were performed. Scintigraphy was positive for loosening [positive (99m)Tc-HDP scan] but negative for infection [negative (99m)Tc-HMPAO-labeled granulocyte scan] in all group A patients, whereas in 11 cases (52%) a positive culture was unexpectedly obtained. Histology showed conflicting results: Polymorphonuclear cells (PMNs) were found only in 5 of 11 culture-positive patients, whereas in 2 cases the presence of PMNs did not correspond to a positive culture. In group B patients, both isotope scans and microbiology were found to be positive. All control subjects (group C) had negative cultures. In our opinion, smoldering infection could be present in a significant proportion of cases of failed hip implants currently diagnosed as "nonseptic." The inflammatory response to wear debris and the presence of superimposed, slowly growing bacteria could act synergically, both contributing to the pathogenesis of periprosthetic osteolysis.

Fibroblasts express RANKL and support osteoclastogenesis in a COX-2-dependent manner after stimulation with titanium particles

Synovial fibroblasts are possible mediators of osteolysis. Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Fibroblasts pretreated with titanium or PGE2 stimulated osteoclast formation, showing the functional importance of RANKL induction. Synovial fibroblasts and their activation pathways are potential targets to prevent osteolysis.

INTRODUCTION

Bone loss adjacent to the implant is a major cause of joint arthroplasty failure. Although the cellular and molecular response to microscopic wear debris particles is recognized as causative, little is known concerning role of synovial fibroblasts in these events.

MATERIALS AND METHODS

Murine embryonic fibroblasts and knee synovial fibroblasts in culture stimulated with titanium particles were examined by FACS, real time RT-PCR, Northern blot, and Western blot for expressions of vascular cell adhesion molecule (VCAM)1, RANKL, cyclooxygenase (COX)-1, and COX-2, and the four prostaglandin E2 (PGE2) receptor isoforms. Experiments were performed in the presence and absence of COX inhibitors, protein kinase A (PKA) and protein kinase C (PKC) inhibitors, and various EP receptor agonists. Osteoclast formation was examined in co-cultures of pretreated glutaraldehyde-fixed fibroblasts and primary murine spleen cells treated with macrophage-colony stimulating factor (M-CSF) for 7-days.

RESULTS

TNF-alpha stimulated VCAM1 expression, consistent with a synovial fibroblast phenotype. Titanium particles stimulated RANKL gene and protein expressions in fibroblasts in a dose-dependent manner. Gene expression was increased 5-fold by 4 h, and protein levels reached a maximum after 48 h. Within 1 h, titanium particles also induced COX-2 mRNA and protein levels, whereas both indomethacin and celecoxib blocked the stimulation of RANKL, suggesting a COX-2-mediated event. Furthermore, PGE2 induced RANKL gene and protein expression and rescued RANKL expression in titanium-treated cultures containing COX-2 inhibitors. Fibroblast cultures pretreated with either PGE2 or titanium particles enhanced osteoclast formation, indicating the functional importance of RANKL induction. EP4 was the most abundant PGE2 receptor isoform, EP1 and EP2 were expressed at low levels, and EP3 was absent. The EP1 selective agonist iloprost and the EP2 selective agonist butaprost minimally stimulated RANKL. In contrast, the EP2 and EP4 agonist misoprostol induced RANKL to a magnitude similar to PGE2. Finally, PKA antagonism strongly repressed RANKL stimulation by PGE2.

CONCLUSION

Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Thus, the synovial fibroblast is important mediator of osteolysis and target for therapeutic strategies.

The role of macrophages in osteolysis of total joint replacement

The osteolysis associated with conventional polyethylene on metal total joint replacements is associated with the formation of an inflamed periprosthetic membrane rich in macrophages, cytokines and implant-derived wear particles. There is a wealth of evidence to indicate that the presence and activation of macrophages in the periprosthetic tissues around joint replacements is stimulated by UHMWPE particles. Particles within the size range 0.1-1.0 microm have been shown to be the most reactive. Animal studies have provided increasing evidence that, of the milieu of cytokines produced by particle-stimulated macrophages, TNF-alpha is a key cytokine involved in osteolysis. Recent advances in the understanding of the mechanisms of osteoclastogenesis and osteoclast activation at the cellular and molecular level have indicated that bone marrow-derived macrophages may play a dual role in osteolysis associated with total joint replacement. Firstly, as the major cell in host defence responding to UHMWPE particles via the production of cytokines and secondly as precursors for the osteoclasts responsible for the ensuing bone resorption.

Polyethylene and titanium particles induce osteolysis by similar, lymphocyte-independent, mechanisms

Periprosthetic osteolysis is a major clinical problem that limits the long-term survival of total joint arthroplasties. Osteolysis is induced by implant-derived wear particles, primarily from the polyethylene bearing surfaces. This study examined two hypotheses. First, that similar mechanisms are responsible for osteolysis induced by polyethylene and titanium particles. Second, that lymphocytes do not play a major role in particle-induced osteolysis. To test these hypotheses, we used the murine calvarial model that we have previously used to examine titanium-induced osteolysis. Polyethylene particles rapidly induced osteolysis in the murine calvaria 5-7 days after implantation. The polyethylene-induced osteolysis was associated with large numbers of osteoclasts as well as the formation of a thick periosteal fibrous tissue layer with numerous macrophages containing phagocytosed polyethylene particles. Polyethylene-induced osteolysis was rapidly repaired and was undetectable by day 21 after implantation. Lymphocytes were noted in the fibrous layer of wild-type mice. However, the amount of osteolysis and cytokine production induced by polyethylene particles was not substantially affected by the lack of lymphocytes in Pfp/Rag2 double knock out mice. All of these findings are similar to our observations of osteolysis induced by titanium particles. These results provide strong support for both of our hypotheses: that similar mechanisms are responsible for osteolysis induced by polyethylene and titanium particles and that lymphocytes do not play a major role in particle-induced osteolysis.

Osteoclast signalling pathways

The osteoclast is a monocyte-derived cell with complex regulatory control due to its role, balancing calcium homeostasis with skeletal modelling and repair. Normal differentiation requires tyrosine kinase- and tumor necrosis-family receptors, normally fms and RANK. Ligands for these receptors plus unidentified serum or cell-presented factor(s) are needed for in vitro differentiation, possibly signalling via an immune-like tyrosine kinase acceptor molecule. Osteoclast development and activity are increased by cytokines signalling through GP130, such as IL-6, by TGF-beta, and by IL-1, although these cannot replace serum. Other tyrosine kinase receptors including kit and met can augment fms signalling, and TNFs other than RANKL, including TNFalpha and TRAIL, modify RANK signalling, which is also susceptible to interference by interferons. The situation is further complicated by G-protein coupled receptors including the calcitonin receptor, by integrin or calcium-mediated signals, and by estrogen receptors, which operate in bone largely via NO downstream signals. Differentiation, activity, and survival signals merge in intracellular second messengers. These include cytoplasmic kinases of several families; differentiation pathways often terminate in Erk/Jun kinases or NF-kappaB. Key regulatory intermediates include TRAF6, src, Smad3, phosphatidylinositol-3-kinase, Jak/Stat, and the cGMP-dependent protein kinase I. There are substantial uncertainties regarding how intracellular agents connect to primary signals. The frontier includes characterization of how scaffolding/adapter proteins, such as cbl, gab, grb, p130Cas, and shc, as well as itam-containing proteins and nonreceptor tyrosine kinase adapters of the src and syk families, delimit and integrate signals of multiple receptors to bring about specific outcomes.

TNF-alpha expression is transcriptionally regulated by RANK ligand

Tumor necrosis factor (TNF)-alpha is known for its osteoclastogenic and resorptive activities. Induction of osteoclastogenesis by receptor activator of NF-kappaB ligand (RANKL) is accompanied by increased TNF-alpha expression. In the present study we investigated the mechanism by which RANKL induces expression of TNF-alpha in osteoclast precursors. The macrophage-like cell-line, RAW 264.7 was used as a model for osteoclast precursors. To examine if RANKL-mediated increase in TNF-alpha expression involves increased stability of its transcript, RAW264.7 cells were treated with or without RANKL, and then a transcription inhibitor was added. At different time points, TNF-alpha and L32 mRNA levels were examined. TNF-alpha mRNA stability was not altered by RANKL. We next measured directly the transcription rate of TNF-alpha by a run-on assay and found that RANKL increases TNF-alpha transcription rate by 2.9-fold in RAW264.7 cells. We further characterized this transcriptional induction of TNF-alpha by RANKL. Gel shift assays using nuclear extracts derived from RANKL-treated RAW264.7 cells show increased specific NF-kappaB binding activity on the murine TNF-alpha promoter. Gliotoxin, known for its ability to inhibit NF-kappaB activation blocked RANKL-induced TNF-alpha expression. We finally used 1,260 bp of the murine TNF-alpha promoter fused to luciferase, as well as four mutants of this promoter carrying mutations in each of the four NF-kappaB sites to stably transfect RAW 264.7 cells. Reporter activity was increased in response to RANKL in wild type promoter transfected cells, whereas treatment of the mutants' transfected cells did not elicit reporter activity. In conclusion, RANKL induces TNF-alpha expression via a transcriptional mechanism, depending on the NF-kappaB sites in the TNF promoter.

Particle bioreactivity and wear-mediated osteolysis

This review focuses on wear debris-mediated osteolysis, a major factor compromising the long-term success of total joint arthroplasty. Studies on retrieved implants and animal models, as well as in vitro studies on particle bioreactivity, suggest that wear-mediated periprosthetic osteolysis is unlikely to be caused solely by 1 particular cell type or particulate species, but is rather the cumulative consequence of a number of biological reactions. Our recent findings suggest 3 novel mechanisms of particle bioreactivity that may contribute to osteolysis: 1) exacerbated inflammation caused by elevated reactive oxygen species production by activated macrophages and osteoclasts, (2) impaired periprosthetic bone formation secondary to disrupted osteogenesis, and (3) compromised bone regeneration resulting from increased cytotoxic response of mesenchymal osteoprogenitor cells. Understanding the pathogenesis of wear-mediated osteolysis is needed to improve orthopedic implant biocompatibility and wear reduction, and to develop effective pharmacotherapies.

Toll-like receptor 9 regulates tumor necrosis factor-alpha expression by different mechanisms. Implications for osteoclastogenesis

CpG oligodeoxynucleotides (CpG-ODNs), mimicking bacterial DNA, stimulate osteoclastogenesis via Toll-like receptor 9 (TLR9) in receptor activator of NF-kappa B ligand (RANKL)-primed osteoclast precursors. This activity is mediated via tumor necrosis factor (TNF)-alpha induction by CpG-ODN. To further reveal the role of the cytokine in TLR9-mediated osteoclastogenesis, we compared the ability of CpG-ODN to induce osteoclastogenesis in two murine strains, BALB/c and C57BL/6, expressing different TNF-alpha alleles. The induction of osteoclastogenesis and TNF-alpha release by CpG-ODN was by far more noticeable in BALB/c-derived than in C57BL/6-derived osteoclast precursors. Unexpectedly, as revealed by Northern analysis, CpG-ODN induction of TNF-alpha mRNA increase was more efficient in C57BL/6-derived cells. The cytokine transcript abundance was increased due to both increased message stability and rate of transcription. The difference between the two cell types was the result of a higher transcription rate in CpG-ODN-induced C57BL/6-derived cells caused by a single nucleotide polymorphism in kappa B2a site within the TNF-alpha promoter sequence. CpG-ODN enhanced the rate of the cytokine translation in BALB/c-derived cells. Thus, CpG-ODN modulated both transcription and translation of TNF-alpha. The induction of transcription was more evident in C57BL/6-derived cells, while the induction of translation took place only in BALB/c-derived osteoclast precursors. Altogether the cytokine was induced to a larger extent in BALB/c-derived osteoclast precursors, consistent with the increased CpG-ODN osteoclastogenic effect in these cells.

Detection of bacterial endotoxin in human tissues

Detection of bacterial lipopolysaccharide (LPS) in the absence of overt infection is a challenging problem in tissue homogenates and other complex samples. We found that conventional Limulus amebocyte lysate (LAL) assays are not suitable for this purpose due to interference from β-glucan-like molecules. In contrast, a modified LAL assay that is unaffected by β-glucan-like molecules was able to detect LPS in infected tissue and in a subset of clinically aseptic tissues. A two-step LAL assay was used to exclude the possibility of false positives due to nonspecific amidases. False positives due to sample color were also excluded, as were false negatives due to assay inhibition. This is the first report to successfully detect LPS in tissue in the absence of overt infection. This approach may be extremely useful in assessing recent hypotheses that subclinical levels of bacteria contribute to a wide range of chronic diseases.

The effectiveness of polyethylene versus titanium particles in inducing osteolysis in vivo

Bearing surface wear and periprosthetic osteolysis due to wear particles are among the most common reasons for joint replacement failure. A murine calvarial model of wear particle-induced osteolysis has been used to identify different biologic factors associated with this problem and to test nonsurgical methods of modulating the host response to particulate debris. This model has utilized titanium particles, however, in clinical practice the most common source of particulate debris is polyethylene particles from bearing surface wear. We now report a calvarial model of wear particle-induced osteolysis based on commercially available polyethylene particles. We found that compared to sham surgery osteoclast recruitment and bone resorption can be induced by introduction of the titanium particles or polyethylene particles. However, bone resorption was significantly higher with polyethylene particles compared to titanium particles (p=0.02). We consider the polyethylene based murine calvarial model of wear particle-induced osteolysis a reliable and clinically relevant tool to understand the host factors and potential pharmacologic interventions that can influence wear debris generated osteolysis. This model might serve as an extension of the well-established titanium based bone resorption model.

Antibiotic prophylaxis in total hip arthroplasty: effects of antibiotic prophylaxis systemically and in bone cement on the revision rate of 22,170 primary hip replacements followed 0-14 years in the Norwegian Arthroplasty Register

We studied the effects of antibiotic prophylaxis, systemically and in bone cement, on the revision rate of cemented total hip arthroplasties (THAs) in data from the Norwegian Arthroplasty Register during the period 1987-2001. To have comparable groups, only THAs performed because of primary osteoarthritis, using cemented implants with documented good results, and high-viscosity cement were included. If systemic antibiotic prophylaxis had been given, only operations with cephalosporin or penicillin were selected. Cox-estimated survival relative revision risks (RR) are presented with adjustment for differences among groups in gender, age, cement brand, type of systemic antibiotic prophylaxis, type of prosthesis, type of operating room, and duration of the operation. Of 22,170 THAs studied, 696 THAs (3.1%) were revised, 440 (2.0%) for aseptic loosening and 102 (0.5%) for deep infection. We found the lowest risk of revision when the antibiotic prophylaxis was given both systemically and in the cement (15,676 THAs). Compared to this combined regime, patients who received antibiotic prophylaxis only systemically (5,960 THAs) had a 1.4 times higher revision rate with all reasons for revision as endpoint (p = 0.001), 1.3 times higher with aseptic loosening (p = 0.02) and 1.8 times higher with infection as the endpoint (p = 0.01). With the combined antibiotic regime, the results were better if antibiotics were given 4 times on the day of surgery (2,194 THAs), as compared to once (1,424 THAs) (p < 0.001), twice (2,680 THAs) (p < 0.001), or 3 times (5,522 THAs) (p = 0.02). Those who received systemic prophylaxis a single day 1, 2 or 3 times, as compared to 4 times, had a revision rate 1.8-3.5 times higher with all reasons for revision as endpoint, 1.5-3.1 times higher with aseptic loosening, and 2.7-6.8 times higher with infection. When we compared systemic prophylaxis 4 times in 1 day, no further improvement resulted in those given systemic prophylaxis for 2 days (1,928 THAs) or 3 days (717 THAs). In a subset of data including only the Charnley prosthesis, we obtained similar results. This observational study shows that the best results were recorded when antibiotic prophylaxis was given both systemically and in the bone cement, and if the systemic antibiotic was given 4 times on the day of surgery.

Does endotoxin contribute to aseptic loosening of orthopedic implants?

Aseptic loosening of orthopedic implants caused by wear particles is a major clinical problem. This review examines the hypothesis that bacterial endotoxin contributes to aseptic loosening. Clinical findings support this hypothesis: bacterial biofilms exist on many implants from patients with aseptic loosening and antibiotics in bone cement reduce the rate of aseptic loosening. Three approaches were used to demonstrate that adherent endotoxin increases bioactivity of titanium particles. These experiments measured cytokine production and osteoclast differentiation in vitro and murine calvarial osteolysis in vivo. First, removal of >99.9% of the adherent endotoxin from titanium particles significantly ablates their biological activity. Second, adding lipopolysaccharide back to these "endotoxin-free" particles restores their biological activity. Third, cells or mice that are genetically hyporesponsive to endotoxin are significantly less responsive to titanium particles than are wild-type controls. Other investigators have confirmed and extended these results to include virtually all orthopedically relevant types of particles, including authentic titanium alloy particles retrieved from patients with loosening. Our recent studies suggest that adherent endotoxin on orthopedic implants may also inhibit initial osseointegration of the implants. Taken together, these studies suggest that bacterial endotoxin may have a significant role in induction of aseptic loosening.

Attachment and proliferation of neonatal rat calvarial osteoblasts on Ti6Al4V: effect of surface chemistries of the alloy

This study examined the cell attachment and proliferation of neonatal rat calvarial osteoblasts on Ti6Al4V alloy as affected by the surface modifications. The modifications could alter simultaneously the surface chemistries of the alloy (elemental difference of Ti, Al, V, Cu and Ni about 300-600mum thick examined by EDS) as well as the XPS nano-surface characteristics of oxides on the metal surface (chemistries of oxides, amphoteric OH group adsorbed on oxides, and oxide thickness). Three materials including two from modifications and a control were examined. It is argued that a slight change of the nano-surface characteristics of oxides as a result of the modifications neither alters the in vitro capability of Ca and P ion adsorption nor affects the metal ion dissolution behavior of the alloy. This implies that any influence on the cytocompatibility of the materials should only be correlated to the effect of surface chemistries of the alloy and the associated metal ion dissolution behavior of the alloy. The experimental results suggest that the cell response of neonatal rat calvarial osteoblasts on the Ti6Al4V alloy should neither be affected by the variation of surface chemistries of the alloy in a range studied.

Quantification of inflammatory cellular responses using real-time polymerase chain reaction

The introduction of tissue engineering strategies for the repair and replacement of human body components extends the application and importance of biomaterials. Implanted biomaterials frequently evoke inflammatory responses that are complex and not well understood at present. The goals of this work were to develop improved measurement methods for the quantification of cellular inflammatory responses to biomaterials and obtain data that lead to an enhanced understanding of the ways in which the body responds to the introduction of biomaterials. To evaluate the biocompatibility of materials, we established a system that allows for the analysis and quantitation of cellular inflammatory responses in vitro. In this study, the inflammatory responses of murine macrophages (RAW 264.7) were analyzed. The cells were incubated with polymethylmethacrylate (PMMA) microspheres in the presence and absence of lipopolysaccharide (LPS) at 8 and 18 h. The analysis of the genetic material obtained from the cells was quantitated using real-time reverse transcription polymerase chain reaction (RT-PCR). The cell populations treated with LPS or PMMA microspheres singly resulted in an elevation of cytokine levels compared to the untreated control. LPS resulted in a 258-fold increase, while PMMA resulted in an 87.9-fold increase at 8 h. RAW 264.7 cells incubated with LPS and PMMA particles demonstrated a synergistic effect by producing a marked increase in the level of cytokine expression, 336-fold greater than that of the untreated control at 8 h. Fluorescence microscopy studies that assessed cellular viability were also performed and are consistent with the RT-PCR results.

Pathogenesis of prosthesis-related infection

In spite of its incidence decreasing to 1% nowadays, prosthesis-related infection remains a research, diagnostic, therapeutic and cost-related problem. It can be defined as a presence of bacteria in the artificial joint space, which is significantly associated with evident laboratory and/or tissue markers, and clinical signs of running infection. We believe that the more precise understanding of pathogenesis, the more effective preventative and therapeutic measures, and the lower infection rate. The implants are colonized by airborne, skin-, and/ or surgeon-related bacteria during surgery despite being operated in closely respected operating regime. Some prosthetic characteristics are advantageous and may play important roles in the process of bacterial adherence. After successful attachment on the biomaterial surface bacteria multiply and physiologically transform into a "biofilm" community, making them much more resistant to antibiotic therapy and host immunity. Bacterial resistance is a complex phenomenon influenced by intrinsic and extrinsic factors, including the cell configuration in the biofilm community. So the cure of periprosthetic sepsis without removing of all foreign bodies and necrotic bone fragments is often ineffective. Acute hematogenous sepsis is suggestive of a distortion of a previously aseptic joint space by invasion of bacteria through the vessels.

Inhibitory effect of titanium particles on osteoclast formation generated by treatment of mouse bone marrow cells with PGE2

OBJECTIVE

The present study was designed to evaluate the effect of titanium (Ti) particles with no endotoxin on osteoclast differentiation and osteoclast activity in in vitro experiments.

METHODS

Osteoclast formation as well as osteoclastic bone resorbing activity were examined using the mouse bone marrow culture system and purified rabbit osteoclasts treated with Ti particles (2.5-20 microgram cm-2).

RESULTS

Ti particles, with no adherent endotoxin, inhibited osteoclastogenesis and receptor activator of NF-kappaB ligand (RANKL) expression in bone marrow cells treated with prostaglandin E2 (PGE2) (100 nM). The inhibitory effect of Ti particles was concentration-dependent (5-20 microgram cm-2), and was observed only on the generation of osteoclasts by PGE2, but not by 1,25-dihydroxyvitamin D3 or soluble RANKL. This suggests that Ti particles did not act uniformly on a common process in the generation of osteoclasts, but specifically on signal transduction for PGE2 in generating osteoclasts. In highly purified osteoclasts, Ti particles showed no effect on survival and bone resorbing activity.

CONCLUSION

Ti particles inhibited osteoclast differentiation and RANKL expression in mouse bone marrow cells treated with PGE2, without affecting mature osteoclast survival or activity. Thus, Ti particles may alter the osteoclastogenetic action of PGE2, which is one of the regulatory factors of bone remodeling.

Particles and periimplant bone resorption

There is compelling evidence that the most important factor in late periprosthetic bone resorption is an inflammatory reaction to debris. Based on results from several laboratories, it seems likely that opsonized particles activate the macrophage nuclear factor-kappa B signal transduction system via membrane receptors, leading to release of tumor necrosis factor-alpha and other cytokines and growth factors. Tumor necrosis factor stimulates osteoblasts to release cytokines that recruit inflammatory cells and osteoclast precursors to the site and promote the differentiation of early osteoclasts. Tumor necrosis factor influences fibroblasts to release tissue metalloproteinases, and induces c-src in osteoclast precursors, the expression of which is necessary for additional bone resorption. Phagocytosis of debris by osteoblasts may reduce collagen synthesis, whereas phagocytosis by fibroblasts may induce chemokines that amplify inflammation. Bone has been partially protected from particle-induced resorption in animals with defective or inhibited tumor necrosis factor or nuclear factor-kappa B signaling. Many aspects of this inflammatory reaction require clarification, including identifying the factors that influence variability among patients, and testing the importance of costimulatory molecules such as bacterial endotoxin, but the fundamental importance of particles in most cases of aseptic loosening seems certain.

Particle disease. A comprehensive theory of periprosthetic osteolysis: a review

Aseptic loosening and osteolysis are considered the main long-term problems of hip arthroplasty. Pathogenesis of periprosthetic osteolysis is multifactorial, and both the biological and mechanical factors seem to play an important role. Bearing surfaces continuously generate excessive amounts of micron and submicron particles provoking an adverse inflammatory response of periprosthetic connective tissues. In general, a key role has been attributed to macrophages. Cytokines, growth factors, PGE2, and enzymes are secreted with activated periprosthetic cells resulting in formation of osteolytic granulomas. The final osteolytic step is taken predominantly by osteoclasts which are getting ready for action mainly by an osteoprotegerin ligand (RANKL) and TNFalpha. Rankl is expressed by activated macrophages, osteoblasts, and lymphocytes. In parallel, a repetitive hydraulic effect of the joint fluid is manifested on the susceptible bone.

Dual modulation of osteoclast differentiation by lipopolysaccharide

Lipopolysaccharide (LPS) modulates bone resorption by augmentation of osteoclastogenesis. It increases in osteoblasts the production of RANKL, interleukin (IL)-1, prostaglandin E2 (PGE2), and TNF-alpha, each known to induce osteoclast activity, viability, and differentiation. We examined the role of direct interactions of LPS with osteoclast precursors in promoting their differentiation. To this end, we have used bone marrow mononuclear cell preparations in the absence of osteoblasts or stromal cells. We found that LPS does not induce osteoclast differentiation in these cells. Moreover, the inclusion of LPS blocked the osteoclastogenic activity of RANKL. However, LPS is a potent inducer of osteoclastogenesis in RANKL-pretreated cells, even if present in the absence of exogenous RANKL. Osteoprotegerin (OPG), does not affect the stimulatory phase of LPS modulation of osteoclastogenesis, ruling out involvement of endogenous RANKL. LPS induces the expression of TNF-a and IL-1beta in osteoclast precursors, regardless if they were or were not pretreated with RANKL. These two cytokines induced osteoclast differentiation in RANKL-pretreated cells. To examine if these cytokines mediate LPS effect in an autocrine mechanism, we measured the effect of their neutralization on LPS osteoclastogenic activity. Although neutralization of IL-1beta did not affect LPS activity, a marked inhibition was observed when TNF-alpha was neutralized. However, TNF-a expression was increased also in conditions in which LPS inhibited RANKL osteoclastogenic activity. We found that LPS reduces the expression of RANK and macrophage colony-stimulating factor (M-CSF) receptor. In summary, LPS impacts on osteoclastogenesis also via its interactions with the precursor cells. LPS inhibits RANKL activity by reducing the expression of RANK and M-CSF receptor and stimulates osteoclastogenesis in RANKL-pretreated cells via TNF-alpha.

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

Aseptic loosening of orthopaedic implants is thought to be primarily due to stimulation of cytokine production by wear particles from the implants. The cytokines increase osteoclast differentiation, leading to osteolysis and implant loosening. Accumulating evidence indicates that adherent endotoxin mediates the biological responses induced by the wear particles. One mechanism by which adherent endotoxin may act is by increasing phagocytosis of the wear particles. To test this hypothesis, the effect of adherent endotoxin on phagocytosis of titanium particles was determined. First, we developed reliable confocal and fluorescence microscopy methods to examine both the attachment and internalization steps of phagocytosis. Use of these methods showed that adherent endotoxin does not detectably alter the rate or the extent of phagocytosis of titanium particles by RAW 264.7 cells. Despite this lack of an effect on phagocytosis, adherent endotoxin dramatically increases the ability of RAW 264.7 cells to produce TNF-alpha and induce osteoclast differentiation. Thus, adherent endotoxin mediates these biological responses by a mechanism that does not rely on increased phagocytosis. These results also demonstrate that phagocytosis is not sufficient to induce cytokine production and osteoclast differentiation but do not rule out the possibility that phagocytosis is required for induction of these responses by titanium particles with adherent endotoxin.