The effect of sterilization on the mechanical properties of intact rabbit humeri in three-point bending, four-point bending and torsion

Load bearing bone allografts are used to replace the mechanical function of bone that has been removed or to augment bone that has been damaged in trauma. In order to minimize the risk of infection and immune response, the bone is delipidated and terminally sterilized prior to implantation. The optimal method for bone graft sterilization has been the topic of considerable research. Recently, supercritical carbon dioxide (SCCO₂) treatments have been shown to terminally sterilize bone against a range of bacteria and viruses. This study aimed to evaluate the effect of SCCO₂ treatment compared with two doses of gamma irradiation, on the mechanical properties of whole bone. Paired rabbit humeri were dissected and randomly assigned into either SCCO₂ control, SCCO₂ additive or gamma irradiation at 10 or 25 kGy treatment groups. The bones were mechanically tested in three-point and four-point bending and torsion, with the lefts acting as controls for the treated rights. Maximum load, energy to failure and stiffness were evaluated. This study found that SCCO₂ treatment with or without additive did not alter maximum load, energy to failure or stiffness significantly under any loading modality. Gamma irradiation had a deleterious dose dependant effect, with statistically significant decreases in all mechanical tests at 25 kGy; while at 10 kGy there were reductions in all loading profiles, though only reaching statistical significance in torsion. This study highlights the expediency of SCCO₂ treatment for bone allograft processing as terminal sterilization can be achieved while maintaining the intrinsic mechanical properties of the graft.