DELAYED IMPLANTATION OF DEMINERALIZED BONE POWDER AFTER LOCAL IRRADIATION IN RATS

Previous studies have shown that osteoinduction is inhibited when demi neralized bone powder (DBP) is implanted within 48 hours of local irra diation (i.e., when the acute effects of radiation injury are present) . This study sought to determine whether normal osteoinduction can pro ceed if the implantation of DBP is delayed until 21 days postirradiati on. At 21 days, the acute effects of radiation injury are likely to ha ve subsided, but the long-term effects have not yet appeared. Twenty-e ight-day-old rats were administered a radiation dose of either 7 or 20 Gy over a localized area of one thigh. The contralateral, nonirradiat ed thigh served as a control. Demineralized bone powder was subcutaneo usly implanted 21 days later. Subcutaneous pellets were retrieved at v arious intervals thereafter (up to Day 45) and were histologically gra ded for evidence of osteoinduction. No difference in osteoinduction wa s detected at any time interval in pellets from the 7-Gy-treated sites , compared with controls. Pellets from the 20-Gy-treated sites, howeve r, demonstrated significantly a lower osteoinductive response at each interval. These latter pellets showed small scattered areas of osteoin duction and reduced formation of marrow elements. Thus, although osteo induction proceeds normally when the implantation of DBP is delayed fo r 21 days after irradiation with 7 Gy, this is not the case when 20 Gy are administered. Given the absence of gross impairment of tissue vas cularity in irradiated sites, it is possible that inhibition of osteoi nduction after higher doses of radiation results from permanent damage to mesenchymal precursor cells.