Reducing the radiation sterilization dose improves mechanical and biological quality while retaining sterility assurance levels of bone allografts

Background: Bone allografts carry a risk of infection, so terminal sterilization by gamma irradiation at 25 kGy is recommended; but is deleterious to bone quality. Contemporary bone banking significantly reduces initial allograft bioburden, questioning the need to sterilize at 25 kGy. Methods: We inoculated allograft bone with Staphylococcus epidermidis and Bacillus pumilus, then exposed them to gamma irradiation at 0, 5, 10, 15,20 and 25 kGy. Mechanical and biological properties of allografts were also assessed. Our aim was to determine an optimal dose that achieves sterility assurance while minimizing deleterious effects on allograft tissue. Results: 20-25 kGy eliminated both organisms at concentrations from 10(1) to 10(3) CFU, while 10-15 kGy sterilized bone samples to a bioburden concentration of 10(2) CFU. Irradiation did not generate pro-inflammatory bone surfaces, as evidenced by macrophage activation, nor did it affect attachment or proliferation of osteoblasts. At doses >= 10 kGy, the toughness of cortical bone was reduced (P < 0.05), and attachment and fusion of osteoclasts onto irradiated bone declined at 20 and 25 kGy (P < 0.05). There was no change in collagen cross-links, but a significant dose-response increase in denatured collagen (P < 0.05). Conclusions: Our mechanical and cell biological data converge on 15 kGy as a threshold for radiation sterilization of bone allografts. Between 5 and 15 kGy, bone banks can undertake validation that provides allografts with an acceptable sterility assurance level, improving their strength and biocompatibility significantly. Clinical relevance: The application of radiation sterilization doses between 5 and 15 kGy will improve bone allograft mechanical performance and promote integration, while retaining sterility assurance levels. Improved quality of allograft bone will promote superior clinical outcomes. (C) 2013 Elsevier Inc. All rights reserved.