Effect of sterilization and storage treatments on screw pullout strength in human allograft bone

Many conflicting studies have been performed evaluating the effects of sterilization and storage treatments on the mechanical properties of allograft bone. In the current study, four right and left matched, sterile, deep-frozen, tibial pairs from human donors with an average age of 32 years were tested. One tibia from each pair served as a matched control for the opposite side. Each tibia was cut into four equal segments. One segment of each tibia underwent no treatment; the other three underwent one of the following treatments: irradiation, freeze drying, or ethylene oxide (ETO). Screw pullout tests were performed using four 3.5-mm cortical screws per segment. Sixteen screw pullout tests were performed for each allograft treatment. The freeze-dried specimens required significantly less force for screw pullout. The screw pullout force for the irradiated specimens and the ETO specimens did not significantly differ from their controls. These results indicate that freeze drying affected screw pullout strength. This method of processing should be questioned for structural allografts in which screw fixation is mandatory. The use of irradiation or ETO for sterilization may not have an adverse effect on screw pullout strength.