The purpose of this study was to understand the effects of terminal sterilisation and residual calcium on human demineralised bone matrix (DBM) in ectopic bone formation in nude rat.

The intramuscular implantation of human DBM prepared by the Queensland Bone Bank (QBB) from four donors into eight male athymic rats was used to assess osteoinductivity. The DBM contained different levels of residual calcium and treated with or without gamma-irradiation at 11kGy. At 6 weeks post-implantation, calcium deposition was assessed by manual palpitation and radiological imaging. Tissue morphology and cellular interactions was analysed using various histological staining methods whilst protein expression of anabolic and catabolic biomarkers were examined through immunohistochemistry. All results were then analysed in qualitative, semi-quantitative and quantitative manners and tested for statistical significance.

Bone formation was observed in all specimens at the gross level. This was confirmed by histology which revealed bony capsules surrounded by soft tissue in the muscle pockets and differences in tissue components. On a cellular level, variations in osteoclast expression were found between the two groups as well as amongst individual donors through statistical analysis which resulted in an imbalance of the expression of anabolic and catabolic markers. Furthermore, a positive relationship between residual calcium and new bone formation in gamma irradiated DBM samples was found. To date, no studies have compared the effect of calcium in gamma irradiated DBM.

Our results suggest that gamma irradiation even at low doses and residual calcium may affect new bone formation. Taken together, this study stresses the importance of selecting ideal conditions for graft processing and the need to identify an optimal level of irradiation and remaining calcium levels that confers a balance between osteoinductivity and sterility.