A novel three-dimensional tissue equivalent model to study the combined effects of cyclic mechanical strain and wear particles on the osteolytic potential of primary human macrophages in vitro

The effects of cyclic mechanical strain and challenge with physiologically relevant doses of submicrometre-size polyethylene (PE) particles on the ost eolytic potential of primary human mononuclear phagocytes were investigated . Cells were seeded into a three-dimensional tissue matrix and co-cultured with particles (mean size 0.21 mum) at particle volume to cell number ratio s of 7.5. 15, 30 and 100 mum(3)/cell. Matrices were then either cultured st atically or subjected to 20 per cent cyclic compressional strain in the 'Co mCell' for 16 h prior to the assessment of cell viability and quantificatio n of the pro-inflammatory cytokine tumour necrosis factor alpha (TNF alpha) . The MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazdium bromide) ass ay was shown to be too insensitive to detect changes in cell viability. How ever, when quantified by the adenosine triphosphate (ATP) assay, cell viabi lity was demonstrated to be reduced following exposure to cyclic strain. Ma crophages cultured in the static three-dimensional tissue equivalent model produced very high levels of TNF alpha in response to submicrometre PE part icles at a ratio of 100 mum(3)/cell. Cyclic strain in the absence of partic les gave only a small increase in TNF alpha production. However. the combin ed effects of strain and particle stimulation at a ratio of 30 mum(3)/cell resulted in the secretion of significantly more TNF alpha than was produced by macrophages subjected to strain alone, or the cells-only control. This synergy between cyclic strain and PE particle stimulation was only evident when the volume of particles was reduced below the volume that maximally st imulated cells. These results suggest that while cyclic strain may not be t he primary factor responsible for macrophage activation and periprosthetic osteolysis, at low particle load, it may contribute significantly to the os teolytic potential of macrophages in vitro or in vivo.