Effects of preservation on the mechanical strength and chemical composition of cortical bone: an experimental study in sheep femora

Preservation methods have enabled bone banks to furnish cortical bone grafts to orthopaedic surgeons. However, cortical bone preserved by freezing and autoclaving, may be weakened by these treatments. To test this hypothesis we compared the ultimate tensile strength of freshly harvested sheep femora with that of femora which were frozen at −20°C for 60 days, or autoclaved at 134°C for 8 min. We measured the collagen and mineral contents (calcium, phosphorus, magnesium) and hydroxyproline of the specimens and tested for changes induced by preservation. Mechanical three point tests showed that frozen femora were significantly stronger than either fresh or autoclaved femora (p<0.05). Frozen specimens also had the highest phosphorus level, indicating these measures are related to strength. Cortical bone is not significantly weakened by autoclaving or freezing. This result does not imply that preserved grafts are clinically interchangeable with fresh grafts, rather, it suggests that future studies should focus on post surgical issues, such as the rate of remodeling and integration, which may be sensitive to preservation technique.