Purpose: Heterotopic ossification (HO) is a common complication following total hip replacement. Clinical studies showed the effectiveness of irradiation for prevention of heterotopic ossification. The mechanism of radiotherapy responsible for the reduction of heterotopic ossification is unclear. The purpose of this study was to find a suitable cell system, which can reproduce in-vitro data resulting from clinical in-vivo studies. The establishment of such a cell model allows detailed analyses of the mechanism of radiotherapy.
Method: The chicken limb bud test was used as an in-vitro model. The cells acquired by the limb bud test were irradiated with different doses (0 Gy, 3 Gy, 7 Gy, 10 Gy, 20 Gy). Irradiation was set either 1 hour before, or 1 or 3 days after BMP-2 incubation. The synthesis of proteoglycans (PGS) upon treatment with bone morphogenetic protein (BMP)-2 was measured in cells incubated with BMP-2 for 4 days followed by 35SO4(2-) labeling for 6 hours. Labeled proteoglycans were precipitated using Alcian blue and measured in a raytest radio-TLC analyzer. The incubation with BMP-2 was defined to correlate the in-vivo stimulus meaning the operation.
Results: The proteoglycan synthesis was significantly reduced by irradiation 1 hour before or 1 day after BMP-2 incubation, if the dosage was at least 7 Gy. Higher doses than 7 Gy did not lead to lower proteoglycan levels. There was only a trend for a reduction of proteoglycan synthesis by 3 Gy irradiation, but no significant difference compared to the non-irradiated control. An irradiation 3 days after BMP-2 incubation had no effect on proteoglycan.
Conclusion: A dose and time dependent effect of radiation on BMP-2-induced proteoglycan synthesis was observed. Therefore the results of clinical in-vivo studies were reproduced exactly by the limb bud test. We established an in-vitro cell model to analyze the mechanism of the prevention of heterotopic ossification by radiotherapy on cellular or sub-cellular level.