Gamma radiation has been widely used for sterilization of bone allografts due to its efficacy against both bacterial and viral disease transmission. During gamma irradiation sterilization highly reactive hydroxyl radicals are formed due to ionization of water molecules. It has been speculated that these free radicals impair the integrity of collagen molecules.48 In addition; radiation may also denature bone morphogenetic proteins, thereby reducing the osteoinductive potential of the allograft. Due to changes in mechanical properties of bone by gamma irradiation it has been used with some cautiousness in clinical practice, especially in load bearing applications. Gamma irradiation at clinically relevant levels (at least 25 kGy) has no effect on the mechanical properties of frozen unprocessed human cancellous bone but irradiation at a higher dose of 60 kGy was found to significantly reduce the compressive failure stress and the elastic modulus.49 The effect of irradiation on freeze dried bone is different. A dose of 25 kGy in combination with freeze drying has been shown to reduce the ultimate strength by 42.5%.50 However, it was also

shown that irradiated freeze-dried impacted grafts provide a more stable femoral reconstruction than fresh frozen grafts when tested in a hip simulator.51 On the other hand, the higher compactness caused by impaction of irradiated bone may reduce the speed of the incorporation process and reduce the cement penetration.

There are few clinical reports of impaction grafting with irradiated bone grafts in hip revision surgery on both the femoral and acetabular side, using radiological and clinical scores as outcome measures. A study evaluating femoral impaction grafting with the Exeter hip using non-rinsed irradiated (25 kGy) bone grafts showed a lacking radiologic incorporation and trabecular remodeling.52 In contrast, in a 2 to 7 years follow up study using rinsed fresh and irradiated (25-30 kGy) allograft bone in 41 revision femoral athroplasties no significant difference was found in clinical

or radiological outcome.53 Comparing fresh-frozen and irradiated allograft bone in impaction grafting of the acetabulum, no obvious difference in clinical performance with evidence of incorporation and remodeling after 6 months and 13 months for the irradiated graft, using two different radiologic criteria were found.54 Buckley et al.,55 using non washed irradiated (25 kGy) 19 bone for acetabular revision surgery, showed a 88% survival at a mean follow up of five years, which is comparable with that of studies using non irradiated bone. In addition, Hamer et al.56 histologically examined five cases of rerevisions where irradiated cortical allograft had been

used. All cases showed favorable histologic features, with soft tissue attachment by fibrous adhesion, union of graft to host and osseous remodeling. In addition, irradiation of unprocessed femoral heads has also been shown to produce altered lipids from the marrow that are cytotoxic to osteoblast cultures, which may have an impact on bony ingrowth into a graft and long-term survival.57