Mechanical Properties of Gamma Irradiated Versus Non-Gamma Irradiated Dense Cancellous Bone

Abstract

Category: Basic Sciences/Biologics Introduction/Purpose: Allograft bone is used for a myriad of purposes in the foot and ankle, not limited to: restoring bone length at an arthrodesis site, correction of an angular deformity or filling a bone defect. The processing techniques between tissue banks vary and may include gamma irradiation, use of hydrogen peroxide, ethylene oxide, or strict aseptic processing of allograft bone. The purpose of this pilot study is to determine if using gamma irradiation during processing will significantly decrease the modulus, maximum compression strain and stress and energy during static testing of dense cancellous bone. The purpose is to also determine if using gamma irradiation during processing will significantly decrease the strain energy of the allograft during dynamic testing in dense cancellous bone. Methods: Cylinders of bone were harvested from cadaveric male specimens age 18-55 with no history of skeletal disease, abnormality or trauma to the lower extremities. Bone cylinders were harvested from the femoral head, femoral condyle and proximal tibia and were separated into right and left. All right bone cylinder specimens were processed aseptically. All left bone cylinder specimens were processed using low-dose gamma irradiation (12.7-13 kGy). Following processing, 36 bone cylinder specimens underwent static testing of right (N=18, aseptic processing) versus left (N=18, low-dose gamma irradiated) in equal amounts to determine modulus, maximum compression stress and strain and energy. 36 bone cylinders underwent dynamic testing of right (N=18, aseptic processing) versus left (N=18, low dose gamma irradiated) for strain energy. Results: Static testing results demonstrated a 27% reduction in modulus (p=.18), a 26% reduction in maximum stress (p=.20), a 15% reduction in strain at maximum stress and a 37% reduction in energy (p=0.05) in the gamma irradiated specimens versus aseptically processed specimens. Energy is the area under a stress strain curve and represents the ability of a material to resist impact. Dynamic testing results include a 36% reduction in cumulative energy in gamma irradiated specimens versus aseptically processed specimens at the 1000th cycle (p=0.02), a 37% reduction at the 2500th cycle (p=0.03), and a 39% reduction at the 5000th cycle (p=0.05). Six samples in the irradiated group failed before 10,000 cycles and one sample in the aseptic group failed before 10,000 cycles. Conclusion: This pilot study confirms previous studies using cortical bone that gamma irradiation degrades the structural properties of cancellous allograft bone. Statistically significant differences in cumulative energy absorption were observed between treatment groups during dynamic testing. Energy absorption decreased by 36-39% in the gamma irradiated group. Differences in the average between the two treatment groups were observed for all static material properties, though not statistically significant: modulus, maximum stress, strain at maximum stress and energy. The most drastic difference was observed in energy, which represents the ability of allograft to plastically deform without fracturing or failing.