Establishing freeze drying process for cortical and cancellous bone allograft cubes

Abstract

Freeze drying is a dehydration method to dry bone under freezing environment, enabling removal of water with no or minimial effects on bone strength and durability. Larger size bones obviously require longer freeze drying time to reduce water content to the required level for long term storage at room temperature. For small size bone cubes or chips, it is a normal practice to pool cortical and cancellous bones for freeze drying. The study was aimed at determining if different type of bones of the same size influence the drying time. Human bone cubes of 10 mm x 10 mm x 10 mm were prepared from cortical bone of tibiae and cancellous bone from femoral heads. The bone cubes were freeze dried to reduce water content to less than 6%. Moisture content was monitored using gravimetric method.Weight and density of cortical bone were significantly higher than cancellous bone despite of having similar small size (p<0.05). Cortical bones (density 2.05 ± 0.35 g/cm3) with initial water content of 10.93% required 5 hours to freeze dry, while cancellous bone cubes (density 0.72 ± 0.44 g/cm3) with initial water content of 78.95% required only 1.87 hours. This study confirmed that the structure hence density of human bone cubes determine the freeze drying time. Therefore in the standard operating procedure for freeze drying of bone allograft cubes, high density cortical bone cubes and low density cancellous bone cubes must be freeze dried separately despite being of similar small size.