Key words: particles, cytokines, wear and degradation, macrophage, standards, research
Corrosion and wear of implanted medical devices (e.g., orthopedic prostheses) may produce particulate debris, leading to acute and chronic inflammatory responses in the host. Additionally, polymeric particles such as polytetrafluoroethylene (PTFE) may be injected directly into patients for clinical indications. In the presence of the biomaterial and/or wear particles host monocytes/macrophages are activated, synthesize or secrete mediators of inflammation, and phagocytize particles. In order to understand the mechanisms underlying the host immune response to particulates and device-associated infections, OST investigators have focused on the impact of medical device particles on macrophage function, because these cells play a pivotal role in the body's response to foreign bodies and interact with other cellular components of the immune system. OST has developed three murine test systems a) in vitro using established macrophages, b) in vitro (primary cells), and c) in vivo (mice). These were used to evaluate the inflammatory potential of particles and chemicals prepared from medical device materials, such as PTFE, titanium oxide, hydroxyapatite (HA), polymethylmethacrylate, and cadmium (both soluble and particulate forms), SiO2, and fumed silica, diamond particles, and particulate dental materials (Bis MGA).
OST scientists have assessed the effects of several particles and chemicals used in implanted medical devices (such as particles from metals and polymers) on macrophage activity. Macrophage cells were exposed to particles or chemicals with and without bacterial lipopolysaccharide (LPS; a component of bacterial cell walls to model bacterial infections), and were evaluated for cytotoxicity, and production of nitric oxide (NO) and TNF-alpha, an inflammatory cytokine. NO production is induced by LPS and is critically important in infections. NO is involved in both beneficial (e.g. antimicrobial activity, cytoprotection, immunoregulation) and harmful (e.g. tissue injury, vascular collapse) action in an infection. OST studies demonstrated that a minute amount of LPS, such as would be found associated with a bacterial infection at the site of implanted biomaterials, induces a significant release of both TNF-alpha and NO by macrophages.
The in vitro studies indicated that TNF-alpha was induced by PTFE, PMMA, TiO2, HA, and SiO2 particles and by LPS. Addition of particles alone did not stimulate NO production. The production of NO in response to LPS was affected by the added material. NO production increased in a dose-response manner with LPS plus PMMA, increased but not in a dose-dependent response with HA, and was inhibited by increasing concentrations of TiO2. This in vitro system may be capable of detecting differences in inflammatory potential to different materials, and is being considered for incorporation into developing the ASTM standard on Biological Response to Particles (F04.16.01).
A second test system utilizes primary cells to evaluate the effects of particles of titanium (Ti), cobalt-chrome (Co-Cr), stainless steel (SS), polyethylene, and hydroxyapatite (HA) on macrophage and lymphocyte function in vitro. Ti particles and TiO2 are phagocytized by macrophages. Dark deposits of Ti are clearly evident in the cultures. These results may help to explain why the titanium debris in human tissue is black even though it is expected to be TiO2. Assays for NO revealed that NO is being produced in response to Ti particles but NO is not being produced with the other particles. Thus the primary cell test system is consistent with the in vitro established cell results.
The third test system examined in mice the response to particulate debris at the site of an implant. Injected polymethylmethacrylate (PMMA) and polystyrene (PS) particles on immune cells in mice was examined. Splenic and peritoneal exudate cells were harvested 3-42 days post injection and examined for phagocytic activity and mitogen stimulation. Results suggested that chemistry of the particles of the same size may affect macrophage function and distribution of particles in tissues. The observed effects were time dependent and may have an impact on other immune functions. Studies examining effects of materials and particles are continuing using these three test systems.