Supraphysiological loading induces osteocyte‐mediated osteoclastogenesis in a novel in vitro model for bone implant loosening

We aimed to develop an in vitro model for bone implant loosening, allowing analysis of biophysical and biological parameters contributing to mechanical instability‐induced osteoclast differentiation and peri‐implant bone loss. MLO‐Y4‐osteocytes were mechanically stimulated for 1 h by fluid shear stress using regimes simulating: (i) supraphysiological loading in the peri‐prosthetic interface (2.9 ± 2.9 Pa, 1 Hz, square wave); (ii) physiologic loading in the cortical bone (0.7 ± 0.7 Pa, 5 Hz, sinusoidal wave); and (iii) stress shielding. Cellular morphological parameters, membrane‐bound RANKL expression, gene expression influencing osteoclast differentiation, nitric oxide release and caspase 3/7‐activity were determined. Either Mouse bone marrow cells were cultured on top of loaded osteocytes or osteocyte‐conditioned medium was added to bone marrow cells. Osteoclast differentiation was assessed after 6 days. We found that osteocytes subjected to supraphysiological loading showed similar morphology and caspase 3/7‐activity compared to simulated physiological loading or stress shielding. Supraphysiological stimulation of osteocytes enhanced osteoclast differentiation by 1.9‐fold compared to physiological loading when cell‐to‐cell contact was permitted. In addition, it enhanced the number of osteoclasts using conditioned medium by 1.7‐fold, membrane‐bound RANKL by 3.3‐fold, and nitric oxide production by 3.2‐fold. The stimulatory effect of supraphysiological loading on membrane‐bound RANKL and nitric oxide production was higher than that achieved by stress shielding. In conclusion, the in vitro model developed recapitulated the catabolic biological situation in the peri‐prosthetic interface during instability that is associated with osteoclast differentiation and enhanced RANKL expression. The model thus provides a platform for pre‐clinical testing of pharmacological interventions with potential to stop instability‐induced bone implant loosening.