Osteoclastic differentiation by mononuclear phagocytes containing biomaterial particles

Aseptic loosening of implant components is a common and important complication of both cemented and uncemented prosthetic joint replacements. Wear particles derived from organic polymer and metal implant biomaterials are commonly found within macrophages and macrophage polykaryons in the fibrous membrane between loose implant components and the host bone undergoing resorption. In order to determine whether biomaterial particle-containing, foreign-body macrophages may contribute to periprosthetic bone resorption, we cultured murine monocytes that had phagocytosed particles of biomaterials commonly employed in bone implant surgery [polymethylmethacrylate (PMMA), ultra-high molecular weight polyethylene (PE), titanium and chromium-cobalt] on bone slices and glass coverslips with UMR 106 osteoblast-like stromal cells in the presence of 1,25-dihydroxy-vitamin D₃. Under these conditions, all biomaterial particle-containing, foreign-body macrophages differentiated into osteoclastic cells, i.e. tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells capable of extensive lacunar bone resorption. This study shows that particle phagocytosis by macrophages does not abrogate the ability of these cells to undergo osteoclast differentiation. These findings emphasise the importance of the foreign-body macrophage response to biomaterial wear particles in the pathogenesis of aseptic loosening.     

Bisphosphonate Inhibition of bone resorption induced by particulate biomaterial-associated macrophages

Aseptic loosening of total joint replacements is associated with bone resorption. A heavy infiltrate of foreign body macrophages in response to biomateri-al wear particles is commonly found in the fibrous membrane surrounding loose components. It has recently been shown that foreign body macrophages can differentiate into osteoclastic cells. To determine whether pharmacological inhibitors of bone resorption have a role to play in controlling the osteolysis of aseptic loosening, we analyzed the effect of a bisphosphonate, disodium ethane-1, 1-diphos-phonate (EHDP) on this process. Murine monocytes and foreign body macrophages (derived from granulomas formed by subcutaneous implantation of particles of prosthetic biomaterials) were co-cultured with UMR106 osteoblast-like cells in the presence of 1, 25 dihydroxyvitamin D₃ for 14 days on glass cover-slips and bone slices. EHDP significantly inhibited bone resorption in these co-cultures. There was little or no expression of the osteoclast-associated enzyme, tartrate-resistant acid phosphatase (TRAP) in EHDP-treated co-cultures. Addition of EHDP to monocyte-UMR106 co-cultures after the appearance of TRAP-positive cells did not abolish bone resorption, indicating that EHDP, in addition to its known inhibitory effect on osteoclast function, suppresses differentiation of osteoclast precursors. EHDP inhibition of the osteolysis induced by particulate biomaterial-associated macrophages shows that pharmacological inhibition of bone resorption might be used to control the osteolysis of aseptic loosening.     

Resorption of bone by inflammatory cells derived from the joint capsule of hip arthroplasties.

The role of inflammatory cells in aseptic loosening and failure of cemented joint replacements is unclear. Inflammatory cells from the revision joint capsule of four failed hip arthroplasties were examined to determine their nature and resorptive capacity. The capsules contained numerous macrophages and abundant foreign-body macrophage polykaryons, distinguished from osteoclasts by their antigenic phenotype and lack of response to calcitonin. When cultured on cortical bone slices in vitro, both macrophages and macrophage polykaryons produced small resorption pits and were associated with areas of superficial resorption of the bone surface. These results indicate that foreign-body induced macrophages and macrophage polykaryons are capable of a type of low-grade bone resorption which may be of pathogenic significance in the loosening of cemented joint prosthetic components.     

Cellular reactions in hip prosthesis loosening.

Based on the results obtained in histological examinations carried out on periprosthetic tissues in a large series of cases of prosthetic hip joint explants, the authors analyze the cellular events that may occur in loosening phenomena as compared to what occurs in the paraphysiological repair process that is observed in the stable prosthesis. Like other authors, they believe that the principal role in the mechanism of loosening is played by macrophages which are recalled in a large number, at times together with multinucleate giant cells, at the bone-implant interface, after micromovements of the prosthesis and the formation of wear particles have occurred. The macrophages would be capable of favoring resorption of the periprosthetic bone tissue, producing areas of osteolysis in which the transmission of the mechanical stress of loading is modified. The ensuing prosthetic instability increases wear phenomena, causes a greater amount of osteolysis, and, in a vicious cycle, loss of the relationship between bone and implant, and, thus, prosthetic loosening. Finally, the authors report a hypothesis on the pathogenesis of the phenomenon, based on which non-physiological stress, associated with wear and eventually infection, leads to loosening.     

Cytokine receptor profile of arthroplasty macrophages, foreign body giant cells and mature osteoclasts

In the arthroplasty pseudomembrane surrounding a loose prosthesis there is a marked macrophage and foreign body giant cell (FBGC) response to implant-derived wear particles. These cells contribute to the osteolysis of loosening by releasing cytokines and growth factors which influence the formation and activity of osteoclasts. Using a panel of monoclonal antibodies directed against known cytokine/growth factor receptors, we have determined by immunohis-tochemistry whether arthroplasty macrophages, FB-GCs and osteoclasts express receptors for cytokines and growth factors that are known to modulate osteolysis.

All these cell types reacted with antibodies directed against the following cytokine/growth factor receptors: gp130, IL-1R type 1, IL-2R, IL-4R, IL-6R, TNFR, M-CSFR, GM-CSFR and SCFR but not with antibodies directed against IL-3R and IL-8R. Arthroplasty macrophages, FBGCs and osteoclasts thus show a similar pattern of cytokine/growth factor receptor expression. This reflects the fact that arthroplasty macrophages are capable of osteoclast differentiation and that these cell types form part of the mononuclear phagocyte system. As regards the osteolysis of aseptic loosening, it also indicates that these cells are targets for numerous cytokines and growth factors which influence osteoclast formation and bone resorption.     

Polymethylmethacrylate-stimulated macrophages increase rat osteoclast precursor recruitment through their effect on osteoblasts in vitro

An in vitro rat osteoclast precursor model was employed to study the role of macrophages in the osteolysis associated with aseptic loosening of cemented total joint replacements. Bone resorption at the bone-bone cement interface may involve the release of mediators by macrophages in response to phagocytosis of polymethylmethacrylate particles. Two potential pathways for the macrophage-directed bone resorption were studied. An indirect pathway was investigated in which the macrophage response to cement particles was used to stimulate rat osteosarcoma (ROS) 17/2.8 osteoblasts. Osteoblast-soluble factors then were added to osteoclast precursors. In the direct pathway, osteoclast precursors were exposed directly to macrophage-soluble factors released in response to phagocytosis of cement particles. Osteoclast precursors were identified after adherence to polished human dentin slices. Acid phosphatase-positive osteoclasts were counted using light microscopy at ×200 magnification. In the indirect pathway, where the macrophage response was mediated through the rat osteosarcoma osteoblasts, a significant increase in the recruitment of osteoclast precursors was observed. In the direct pathway, when the macrophage-conditioned medium was allowed to interact directly with osteoclast precursors, the adherence of the precursors was significantly decreased. This demonstrates that the macrophage mediators released following phagocytosis of polymethylmethacrylate particles affect the release of soluble factors from osteoblasts. In turn, these osteoblast factors stimulate recruitment of osteoclast precursors to calcified tissue. Evidence from this in vitro model reveals that macrophage-soluble factors, in the absence of an osteoblast contribution, decrease the adherence of osteoclast precursors to calcified substrate. We propose that bone resorption at the aseptically loose interface of cemented arthroplasty may be mediated, at least in part, by soluble factors secreted by osteoblasts responding to macrophages that have phagocytosed particles of polymethylmethacrylate cement.     

Cytokine receptor profile of arthroplasty macrophages, foreign body giant cells and mature osteoclasts

In the arthroplasty pseudomembrane surrounding a loose prosthesis there is a marked macrophage and foreign body giant cell (FBGC) response to implant-derived wear particles. These cells contribute to the osteolysis of loosening by releasing cytokines and growth factors which influence the formation and activity of osteoclasts. Using a panel of monoclonal antibodies directed against known cytokine/growth factor receptors, we have determined by immunohistochemistry whether arthroplasty macrophages, FB-GCs and osteoclasts express receptors for cytokines and growth factors that are known to modulate osteolysis. All these cell types reacted with antibodies directed against the following cytokine/growth factor receptors: gp130, IL-1R type 1, IL-2R, IL-4R, IL-6R, TNFR, M-CSFR, GM-CSFR and SCFR but not with antibodies directed against IL-3R and IL-8R. Arthroplasty macrophages, FBGCs and osteoclasts thus show a similar pattern of cytokine/growth factor receptor expression. This reflects the fact that arthroplasty macrophages are capable of osteoclast differentiation and that these cell types form part of the mononuclear phagocyte system. As regards the osteolysis of aseptic loosening, it also indicates that these cells are targets for numerous cytokines and growth factors which influence osteoclast formation and bone resorption.     

Inflammatory Response to Implant Particulates in a Macrophage/Osteoblast Coculture Model

The purpose of this study was to further define the cellular response to titanium and polymethylmethacrylate (PMMA) particles in aseptic loosening, and to determine if the use of pamidronate may be effective in inhibiting bone resorption associated with this response. Macrophages and osteoblasts were cocultured to simulate the environment around an aseptically loose prosthesis. Macrophages were plated on the bottom of six well plates and osteoblasts were plated on culture dish inserts, and placed into the wells with the macrophages. Incubation of macrophages with PMMA in this system led to release of prostaglandin E (PGE₂), granulocyte macrophage-colony stimulating factor (GM-CSF), and interleukin-6 (IL-6). Incubation with titanium led to release of tumor necrosis factor (TNF) and IL-6. Exposure of calvaria to media from cells exposed to either PMMA or titanium led to release of calcium 45. Incubation of calvaria with pamidronate was able to inhibit release of calcium 45 associated with exposure to the macrophage/osteoblast/particle conditioned medium. Bone resorption at the interface between implant and bone is a consistent feature leading to loosening of orthopedic implants. By inhibiting bone resorption associated with the inflammatory response to implant particulates, pamidronate or other bisphosphonates may have clinical utility in the treatment or prevention or aseptic loosening. Received: 22 December 1995 / Accepted: 3 May 1996     

Localized osteolysis in stable, non-septic total hip replacement

We are reporting four cases of extensive, localized bone resorption adjacent to a rigidly anchored, cemented total hip replacement. None of these hips showed evidence of infection on clinical, bacteriological, or pathological evaluation. The tissue from the regions of osteolysis showed sheets of macrophages and foreign-body giant cells invading the femoral cortices. Abundant methylmethacrylate particulate debris was present in the tissues, but polyethylene wear debris was absent. The histological appearance of this tissue resembled that reported about loosened total hip implants with the exception of the synovial-like layer at the cement surface. The cases reported here show that aggressive bone lysis may occur around stable cemented total hip arthroplasties without the presence of sepsis or malignant disease.     

Human mesenchymal tumour-associated macrophages differentiate into osteoclastic bone-resorbing cells

The cellular mechanisms which account for the formation of osteoclasts and bone resorption associated with enlarging benign and malignant mesenchymal tumours of bone are uncertain. Osteoclasts are marrow-derived, multinucleated, bone-resorbing cells which express a macrophage phenotype. We have determined whether tumour-associated macrophages (TAMs) isolated from benign and malignant mesenchymal tumours are capable of differentiating into osteoclasts. Macrophages were cultured on both coverslips and dentine slices for up to 21 days with UMR 106 osteoblastic cells in the presence of 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) and human macrophage colony-stimulating factor (M-CSF) or, in the absence of UMR 106 cells, with M-CSF and RANK ligand. In all tumours, the formation of osteoclasts from CD14-positive macrophages was shown by the formation of tartrate-resistant-acid-phosphatase and vitronectin-receptor-positive multinucleated cells which were capable of carrying out lacunar resorption. These results indicate that the tumour osteolysis associated with the growth of mesenchymal tumours in bone is likely to be due in part to the differentiation of mononuclear phagocyte osteoclast precursors which are present in the TAM population of these lesions.     

Efficacy of etanercept for wear debris-induced osteolysis.

A major limitation of total joint arthroplasty is that up to 20% of patients require revision surgery to correct prosthetic loosening. Aseptic loosening is believed to result from the phagocytosis of wear debris particles by macrophages, which secrete proinflammatory cytokines that stimulate osteolysis. Tumor necrosis factor alpha (TNF-alpha) has been shown to be one of the prominent cytokines in this cascade and to be involved critically in the generation of particle-induced osteolysis. Etanercept is a soluble inhibitor of TNF-alpha, which is widely used for the treatment of rheumatoid arthritis. Here, we show this agent's ability to prevent wear debris-induced osteolysis. In vitro we show that Etanercept can inhibit directly osteoclastic bone resorption in a bone wafer pit assay, as well as cytokine production from titanium (Ti)-stimulated macrophages. Using a quantitative in vivo model of wear debris-induced osteolysis, we show that Etanercept prevents bone resorption and osteoclastogenesis. In mice treated with Etanercept at the time of osteolysis induction, bone resorption and osteoclast numbers were reduced to background levels in both normal and human TNF-alpha (hTNF-alpha) transgenic mice. In an effort to evaluate its effect on established osteolysis, Etanercept was administered 5 days after Ti implantation, and we observed that further osteolysis was prevented. These data support the concept that TNF-alpha is involved critically in osteoclastogenesis and bone resorption during periprosthetic osteolysis and suggest that soluble TNF-alpha inhibitors may be useful as therapeutic agents for the treatment of prosthetic loosening in humans.     

Osteolysis in alloarthroplasty of the hip. The role of bone cement fragmentation

Movement at the interface between bone and cement and fractures of the cement can cause fragmentation of the polymethylmethacrylate (PMMA) bone cement implant. In order to obtain further information about the effect of PMMA fragments on the surrounding tissue and the role of such particles in the development of bone resorption, the authors investigated 17 patients with cemented total hip endoprostheses showing osteolysis and implant loosening in the femoral shaft with (Group B) and without (Group A) involvement of the acetabulum. The roentgenographic follow-up examinations revealed an initially slow and later more rapid extension of the endosteal bone erosions, with a predilection for the tip of the stem, the lesser trochanter, and laterally for the middle of the stem. At revision surgery, tissue samples were taken from the joint capsule and the bone-cement interface, in particular from the osteolysis in the femoral shaft and the acetabulum. The tissue samples were processed for histology, microscopically examined, and semiquantitatively evaluated. The retrieved devices were also carefully inspected. Large foreign-body granulomas were found at the bone-cement interface and in the joint capsule. Histiocytes and foreign-body giant cells stored particles of PMMA and polyethylene, of which fragmented bone cement predominated. Granulomatous tissue invaded bone canals and marrow spaces and induced resorption of the surrounding bone. In four cases in Group A, tissue from the osteolysis contained only fragmented bone cement, demonstrating that PMMA particles alone may be responsible for triggering focal bone resorption. Osteolysis seems to begin at the site where disintegration of bone cement begins. In cases in which polyethylene particles were found in the tissue in addition to fragmented bone cement, wear from the ultrahigh molecular weight polyethylene socket has been increased by entrapment of PMMA particles between the joint surfaces. Thus, fragmentation of bone cement and abrasion of polyethylene enhance each other. Bone cement particles promote polyethylene wear, which in turn promotes granuloma formation, bone resorption, and subsequent bone cement disintegration.     

Periprosthetic osteolysis: genetics, mechanisms and potential therapeutic interventions

Aseptic loosening and periprosthetic osteolysis occur as a result of the biological response to particulate wear debris and are one of the leading causes of arthroplasty failure. Periprosthetic osteolysis originates from chronic inflammatory responses triggered by implant-derived particulate debris, which cause recruitment of cells, including macrophages, fibroblasts, lymphocytes and osteoclasts. These cells secrete proinflammatory and osteoclastogenic cytokines, exacerbating the inflammatory response. In addition to their direct activation by phagocytosis, there are contributing autocrine and paracrine effects that create a complex milieu within the periprosthetic space, which ultimately governs the development of osteolysis. Chronic cell activation may upset the delicate balance between bone formation and bone resorption leading to periprosthetic osteolysis. This article summarizes the genetic mechanisms underlying periprosthetic loosening and identifies potential therapeutic agents.     

Polymethylmethacrylate-induced release of bone-resorbing factors

A pseudomembranous structure that has the histological characteristics of a foreign-body-like reaction invariably develops at the bone-cement interface in the proximity of resorption of bone around aseptically loosened cemented prostheses. This study was an attempt to implicate polymethylmethacrylate in this resorptive process. Unfractionated peripheral-blood mononuclear cells (consisting of lymphocytes and monocytes) and surface-adherent cells (monocyte-enriched) were prepared from control subjects who did and did not have clinical evidence of osteoarthrosis and from patients who had osteoarthrosis and were having a revision for failure of a cemented hip or knee implant. Cells were cultured for varying periods in the presence and absence of nonpolymerized methacrylate (one to two-micrometer spherules), pulverized polymerized material, or culture chambers that were pre-coated with polymerized cement. Conditioned media that were derived from both methacrylate-stimulated cell populations were shown to contain specific bone-resorbing mediators (interleukin-1, tumor necrosis factor, or prostaglandin E2) and to directly affect bone resorption in 45Ca-labeled murine limb-bone assays.     

人工全髋聚乙烯假体磨损及其生物学效应

目的 探讨人工关节聚乙烯磨损与假体松动的关系。方法 采用光镜与电镜对15例人工髋关节翻修病例的聚乙烯假体臼杯内表面及假体周围软组织作形态学观察。进而在动物实验中观察微小聚乙烯磨损颗粒促使骨吸收性细胞因子增加，植入物周围骨溶解以及植入物-骨整合强度下降的生物效应。结果 聚乙烯假体在人工关节松动形成时均明显磨损，聚乙烯磨损颗粒诱发机体单核-巨噬细胞增殖并分泌骨吸收性细胞因子。结论 聚乙烯磨损与假体松动密度相关。其中生物因素是引起假体松动的主要途径。     

High-turnover periprosthetic bone remodeling and immature bone formation around loose cemented total hip joints.

Aseptic loosening and periprosthetic osteolysis are the major problems awaiting solution in total hip surgery. The clinical investigation focused on the analysis of periprosthetic bone remodeling to clarify one important key event in the cascade of periprosthetic connective tissue weakening and osteolysis around loose artificial hip joints. Twelve acetabular bone samples adjacent to granulomatous synovial-like membrane of loose hip prosthesis were retrieved at revision surgery and processed for Villanueva bone staining for morphological observation and bone histomorphometric analysis. Eight well-fixed bony samples were used as control. Although osteoclastic surface and eroded surface by osteoclasts were evident in the periprosthetic bone from loose hip joints (p = 0.003 and p = 0.027), increased osteoid/low-mineralized bone matrix (p < 0.001) and osteoid width (p < 0.001) also were significant findings in structural analysis. In addition, not only elevated mineral apposition rate (MAR; p = 0.044) but also increased mineralizing surface (p = 0.044) and bone formation rate (BFR; p = 0.002) in loose periprosthetic bones were shown in dynamic data analysis. These results were confirmed by precise morphological observation by confocal laser scanning microscopy. Active coupling of bone formation and resorption and increased osteocytes with abundant bone canalicular projections were found in combined with the presence of immature bone matrices (osteoid and low-mineralized bone areas) in periprosthetic bones from loose hip joints. These results indicated that active osteoclastic bone resorption and/or defective bone formation are coupled with monocyte/macrophage-mediated foreign body-type granuloma in the synovial-like interface membrane of loose hip joints. Thus, this unique high-turnover periprosthetic bone remodeling with bad bone quality probably is caused by the result of cellular host response combined with inappropriate cyclic mechanical loading. The fragile periprosthetic bone may contribute to hip prosthesis loosening.     

人工关节松动病因的研究

目的：探讨人工关节松动的病因。方法：选择７例松动人工髋关节，翻修手术时取松动关节周围的界膜组织；同时选择１０例骨折内固定患者，拆除内固定物时取内固定物周围瘢痕组织。标本做组织学检查和肿瘤坏死因子（ＴＮＦ）测定。选择１０只成年兔，将２０只模拟假体分别置入双侧股骨远端。分别于术后第６、８、１０、１２、１４周向右侧膝关节腔注射聚乙烯微粒悬液，作为实验侧；左侧膝关节腔注射生理盐水，作为对照侧。第１６周取股骨远端标本，做组织学检查。结果：松动人工髋关节周围的界膜组织主要含大量的组织细胞和聚乙烯微粒，而骨折内固定物周围的瘢痕组织主要为纤维成分，无聚乙烯微粒。松动关节周围界膜组织中的ＴＮＦ浓度明显高于骨折内固定物周围的瘢痕组织（Ｐ＜０．０１）。动物实验发现，实验侧模拟假体周围有一层充满组织细胞的纤维结缔组织界膜，并有明显骨吸收和骨溶解现象，而对照侧无明显纤维结缔组织界膜，也无骨破坏现象。结论：人工关节磨损后，产生大量的磨损微粒，微粒刺激组织细胞分泌ＴＮＦ等溶骨性因子，这些溶骨性因子直接或间接地激活破骨细胞，从而引起假体周围骨吸收、骨溶解，最终导致假体松动。假体松动后又可加重磨损，产生更多的微粒，形成恶性循环     

Lymphocyte response to polymethylmethacrylate in loose total hip prostheses.

We investigated the lymphocyte-mediated immune response to polymethylmethacrylate bone cement in 26 patients who had revision surgery for aseptic loosening of cemented total hip arthroplasties, at a mean time of seven years after the first replacement. We studied eight patients with cemented total hip arthroplasties which were not loose as controls. Patch tests to polymethylmethacrylate bone cement were positive in 13 patients with loosening, and these patients had higher lymphoblast transformation values against polymethylmethacrylate bone cement patients with a negative skin reaction (p < 0.01) or those in the control group (p < 0.001). Specific monoclonal antibodies were used to assess the percentage of certain cells of the immune system according to their cluster of differentiation (CD). There was a higher number of total T and B lymphocytes (CD2 and CD22) and interleukin-2 receptor-positive lymphocytes (activated cells, CD25) in patients with loose prostheses. More CD25 lymphocytes were found in patients with positive patch tests. The activation of the lymphocyte-mediated immune response was not related to the presence or absence of aggressive granulomatous lesions at the cement-bone interface.     

Mechanisms of cellular recruitment in aseptic loosening of prosthetic joint implants

The association of macrophages engaged in polymethylmethacrylate (PMMA) particle phagocytosis with pockets of inflammatory cells is a pathognomonic feature of the aseptically loose interface not present at the well-fixed interface. The mechanism by which the presence of PMMA particles leads to cellular recruitment, bone resorption, and ultimate loosening is poorly understood. Granulocyte macrophage colony stimulating factor (GM-CSF) and interleukin 6 (IL-6), cytokines released by osteoblasts, stimulate the recruitment of macrophages into sites of inflammation. We show that exposure of macrophages to PMMA particles stimulated release of tumor necrosis factor (TNF), but no increase in prostaglandin E2 (PGE-2) or interleukin 1. Incubation of osteoblasts with conditioned medium from macrophages exposed to PMMA particles led to release of GMCSF, IL-6, and PGE-2. Incubation of the PMMA/macrophage medium with antibodies to TNF prior to osteoblast exposure inhibited release of GM-CSF, IL-6, and PGE-2 by the osteoblasts. Our data demonstrate that exposure of macrophages to PMMA particles leads to the release of TNF which then stimulates osteoblasts to produce GMCSF, IL-6, and PGE-2. Based upon the results of this study, we propose that the process of cellular recruitment in aseptic loosening is initiated when the mechanical failure of the cement mantle leads to the production of PMMA particles. These particles are phagocytized by macrophages leading to the production of TNF. TNF stimulates surrounding osteoblasts to produce GM-CSF, IL-6, and PGE-2 which leads to recruitment of macrophages and osteoclasts into the area of the bone-cement interface. The recruitment of these cells potentiates this process leading to bone resorption and ultimately, clinical loosening of prosthetic joint implants.     

Quantitative small-animal surrogate to evaluate drug efficacy in preventing wear debris-induced osteolysis.

Individuals who suffer from severe joint destruction caused by the various arthritidies often undergo total joint arthroplasty. A major limitation of this treatment is the development of aseptic loosening of the prosthesis in as many as 20% of patients. The current paradigm to explain aseptic loosening proposes that wear debris generated from the prosthesis initiates a macrophage-mediated inflammatory response by resident macrophages, leading to osteoclast activation and bone resorption at the implant interface. No therapeutic interventions have been proved to prevent or inhibit aseptic loosening. The development of therapeutic strategies is limited due to the absence of a quantitative surrogate in which drugs can be screened rapidly in large numbers of animals. We have previously described a model in which titanium particles implanted on mouse calvaria induce an inflammatory response with osteolysis similar to that observed in clinical aseptic loosening. Here, we present new methods by which the osteolysis in this model can be quantified. We determined that 6-8-week-old mice in normal health have a sagittal suture area of 50 (+/-6) microm2, which contains approximately five osteoclasts. As a result of the titanium-induced inflammation and osteolysis, the sagittal suture area increases to 197 (+/-27) microm2, with approximately 30 osteoclasts, after 10 days of treatment. The sagittal suture area and the number of osteoclasts in the calvaria of sham-treated mice remained unchanged during the 10 days. We also determined the effects of pentoxifylline, a drug that blocks the responses of tumor necrosis factor-alpha to wear debris, and the osteoclast inhibitor alendronate. We found that both drugs effectively block wear debris-induced osteolysis but not osteoclastogenesis. In conclusion, we found the measurements made with this model to be reproducible and to permit quantitative analysis of agents that are to be screened for their potential to prevent aseptic loosening.