P. A. Revell

The expression of osteoclast markers on foreign body giant cells.

The expression of some candidate osteoclast markers, tartrate-resistant acid phosphatase (TRAP), macrophage associated antigens (M phi Ag), and vitronectin receptor (VNR) on foreign body giant cells (FBGCs) was investigated in peri-implant tissues of loosened total joint arthroplasties. Osteoclasts showed distinct staining characteristics. They were strongly TRAP-positive at tartrate concentrations of 50-200 mM and expressed VNR and a restricted range of M phi Ag. In contrast, FBGCs were shown to be significantly heterogeneous. Significant numbers of FBGCs were TRAP-positive at a 100 mM tartrate concentration and some were more intense than osteoclasts. A population of FBGCs did not express M phi Ag such as CD11b, but expressed VNR. It was demonstrated that these candidate osteoclast markers were also positive on FBGCs. These results have highlighted the difficulty in distinguishing these two cell lineages and suggested that there might be some uncertainty in defining osteoclast-like cells in culture studies.

Assessment of the role of cytokines in bone resorption in patients with total joint replacement

Periprosthetic osteolysis is known to be a consequence of a local chronic inflammatory reaction in the synovial tissue and the bone-implant interface membrane, and is mediated by macrophages (Mϕ) and foreign body multinucleated giant cells (MNGC) in these tissues. Activated Mϕ produce major classes of cytokines which have been documented in the regulation of bone cell formation, function and activity. In rheumatoid arthritis, inflammatory mediators released by Mϕ participate significantly in articular tissue destruction. In this study we have analysed the production and tissue distribution of 4 cytokines in the interface membranes obtained from patients with osteolysis associated aseptic loosening and in rheumatoid synovium to determine their role in the functional transformation of effector cells in these two conditions. The production of IL-1, GM-CSF, IL-6 and TNF-α was assessed using immunohistochemistry on cryostat sections of the interface and the synovial tissues. IL-1 and GM-CSF were detected in significantly high numbers of the inflammatory Mϕ in both RA and aseptic loosening. A specific pattern of expression was noted in the interface. IL-1 production was sporadic throughout the sections, while GM-CSF was immunolocalized in a distinct subset of phagocytic macrophages on the implant side. IL-6 showed moderate expression in both conditions and was more widely produced at sites near the bone side in the interface. TNF-α expression was absent or reduced in the interface but was more abundant in RA synovial Mϕ. The differential expression of cytokines indicates that bone lysis in these two pathological conditions is mediated by different mechanisms and regulated by different cytokines.

Modulation of the phenotypic and functional properties of phagocytic macrophages by wear particles from orthopaedic implants

An attempt was made to assess the local chronic inflammatory response in patients with failed orthopaedic implant that is clinically associated with osteolysis, bone and bone marrow necrosis. The main objective was to analyse the heterogeneity of the macrophage functional subsets in the bone–implant interface membrane and to evaluate their possible role in the development of an erosive inflammatory lesion within the bone. Immunohistology was performed on 21 specimens of the bone–implant interface obtained from 17 patients during revision arthroplasty, and synovial membranes from rheumatoid (RA, n=4), and osteoarthritis (OA, n=4) patients. Three well-characterized monoclonal antibodies (MAb) recognizing antigenic determinants on specific functional subsets of macrophages (Mφ) were used. RFD1 (interdigitating reticulum cells/antigen presenting cells, (APC), RFD7 (mature phagocytic macrophages), and RFD9 epithelioid cells and foreign body giant cells (FBGC). RFD1 was expressed on a variable number of perivascular and synovial lining Mφ in both RA and OA synovia, at a frequency of 25%–40%. In cases with total joint replacements, the interface showed a marked increase in the expression of RFD1 (20%–90%). A considerably greater percentage of RFD1 positive Mφ and FBGC was noted in the interfaces from cases with a high level of detectable metal particulate wear debris (mean 80%, range 60%–90%) than in cases with polyethylene wear debris (mean 30%, range 0%–50%), p 0.0001. RFD7 labelled most tissue Mφ in each group. Immunoreactivity for RFD9 was restricted to FBGC in all cases analysed. The finding of elevated expression of RFD1 on metal-containing Mφ and FBGC in the bone-implant interface suggests an increase in antigen-presenting phenotype and indicates that metal particles have more impact in the induction of immune-mediated responses. Such responses are characterized by sustained cellular hyperreactivity and phenotypic changes in Mφ subsets.

Analysis of push-out test data based on interfacial fracture energy

Push-out testing is frequently used to assess the interfacial shear strength developed at a bone–biomaterial interface during in vivo experiments. The aim of the present research was to assess the in vivo performance of a novel substrate/coating combination and to introduce a more rigorous fracture mechanics analysis of the push-out test data. An adhesively bonded hydroxyapatite (HA), and a Ti-6Al-4V alloy plasma sprayed with HA, were implanted in female New Zealand white rabbits for up to 6 months in duration. After death, push-out tests were carried out and the shear strength was calculated in the conventional way, together with microscopical examination of crack paths. A finite element model was drawn up representing four potential failure mechanisms. The measured “failure shear strengths” in conventional analysis were approximately equal for the two coatings. However, JC at failure calculated from the model was 210 J m-2 at the novel adhesively bonded HA/bone interface and 5 J m-2 at a conventional titanium/plasma-sprayed HA interface. The conventional shear strength approach is strongly test dependent, and we believe that the fracture energy approach represents a more rigorous analysis of the real failure criterion in the implant/host tissue structure.

Interleukin 15 production by macrophages in the implant interface membrane of aseptically loosened joint replacements.

The T lymphocytes are cells involved in immunologically mediated hypersensitivity reactions. They are found in the cellular infiltrate present in the interface membrane of aseptically loosened joint prostheses. Activation of these cells has been demonstrated but not the important cytokine, interleukin 2 (IL2) required for such activation. This study describes the localization of IL15 and its mRNA in macrophages in interface membranes associated with activated T lymphocytes which are proliferating (as is evident from HLA-DR and Ki67 expression, respectively). The findings provide further evidence for immune mediated processes in aseptic loosening of orthopaedic implants because IL15 has similar activities to IL2 in T lymphocyte activation.

The role of newly formed vessels and cell adhesion molecules in the tissue response to wear products from orthopaedic implants

Neovascularization and activation of endothelial cells play an important role in recruitment of blood leucocytes at sites of inflammation. This study aimed to assess the pattern of vascular growth and the expression of cell adhesion molecules on vascular endothelium and inflammatory macrophages and T cells in the bone-implant interface from patients with aseptically loosened orthopaedic prostheses. ELAM-1, VCAM-1, ICAM-1 and the receptors LFA-1 and CR3 were immunolocalized on cryostat sections of the interface obtained during revision arthroplasty. The results showed that ELAM-1 was restricted to endothelium and was upregulated on different vessels in 21 cases. Its expression correlated strongly with the presence of metal wear debris. VCAM-1 was less frequently expressed (n=6 cases), and was co-expressed with ELAM-1 in three cases with metal debris. ICAM-1 was detected on a large number of vessels on the bone side in 13 cases, but was more strongly expressed on macrophage subsets and foreign body giant cells (FBGCs) on the lining layer at the implant side. This study indicates the contribution of three different pathways in the migration of inflammatory cells to the bone-implant interface in response to phagocytosis of implant degradation products. Upregulated ELAM-1 expression may suggest a role in hypersensitivity reactions. Finally the persistent expression of VCAM-1 and ICAM-1 on macrophages and FBGCs in the lining layer indicates possible cellular interactions with the extracellular matrix proteins.