Influence of Freezing Temperature During Irradiation on the Biomechanical Properties of Irradiated Frozen Cortical Allograft

Gamma irradiation minimizes the risk of infectious disease transmission and is the terminal sterilization method of choice for human cortical allografts. Its application is simple, and its results are reliable. Its more prominent indirect effect is strongly influenced by three main factors - the presence of oxygen, water and by temperature. There are, however, enduring concerns regarding its undesired adverse effects on the biomechanical properties of bone allografts. Even so, the available literature mentioning these effects had conflicting conclusions especially pertaining to the effect of temperature during the irradiation process. The purpose of this study was to examine the influence of freezing temperatures during gamma irradiation on the biomechanical properties of human cortical allografts. Five mid shaft femurs from cadaveric donors were cut into bone cubes measuring 4mm3. Each cube was assigned to one of 3 experimental groups. The first group was subjected to gamma irradiation at deep-freezing temperatures below -40°C (the Radiated in Dry ice / RD group), i.e. the recommended temperature for long-term storage of bone allografts. The second group was subjected to irradiation within a freezing temperature range of -40°C to 0°C (the Radiated in Gel ice / RG group) whilst the last group, acting as control (C group), did not receive irradiation treatment. Irradiation dose was set between 25 and 35 kGy. All samples were subsequently subjected to compressive load to failure to determine their biomechanical properties. Allografts irradiated at a higher freezing temperature range (RG, -40°C to 0°C) were observed to have a significantly lower yield point, ultimate tensile strength, resilience and toughness in comparison to those which were not irradiated (Control) and those irradiated at deep freezing temperatures (RD, below -40°C). The above four parameters of the Control and RD group samples were comparable. No difference in the Young’s modulus parameter was observed in all three groups. The results suggest that irradiation at deep freezing temperatures confers protection to the biomechanical properties of cortical femoral allografts, which may be the result of the complete immobilization of water molecules when deep frozen. With its inability to participate in the formation of free radicals via its radiolysis in the frozen state, the deleterious effects of radiation on collagen architecture are hence attenuated. The apparent loss of this protective effect when a higher freezing temperature range was employed during irradiation was substantiated.