Gamma sterilization is usually used to minimize the risk of infection 
transmission through bone allografts. However, it is believed that gamma 
irradiation affects the mechanical properties of allografts and free radical 
scavengers can be used to alleviate the radiation-induced degradation of these 
properties. The aim of this study was to investigate the radioprotective effects 
of N-Acetyl-L-Cysteine (NAC) free radical scavenger on the material properties 
of sterilized bovine cortical bone at microstructure level. Forty-two cortical 
tissue specimens were excised from three bovine femurs and irradiated to 35 and 
70 kGy gamma rays in the presence of 5, 50, and 100 mM concentrations of NAC. 
The localized variations in microhardness were evaluated via indentation in the 
radial and longitudinal directions to examine different regions of the 
microstructures of the specimens, including the osteonal and interstitial 
tissues. A significant increase was observed in the hardness of osteonal, 
interstitial, and longitudinal combined microstructures exposed to 35 and 70 kGy 
radiations (P < 0.05), whereas a relative reduction of the hardness was observed 
in the radial direction. Furthermore, it was found that the application of 50 
and 100 mM NAC during gamma irradiation significantly subsided the hardening in 
longitudinal combined microstructure. Moreover, the reduction of hardness in 
radial direction was suppressed in the presence of 100 mM of NAC. In conclusion, 
the results indicated that NAC free radical scavenger can protect the cortical 
bone against deteriorative effects of ionizing radiation and can be used to 
improve the material properties of sterilized allografts.