The influence of sterilization on octacalcium phosphate for clinical applications

Octacalcium phosphate (OCP) is regarded as an in vivo precursor of hydroxyapatite (HAp) with similar crystalline structures. It is important to understand the mechanism of transformation of OCP to HAp in order to reveal the mechanism of mineralization and help in the development of artificial bone-repairing materials. For orthopedic use, it is of vital importance that the key attributes of OCP are not compromised by radiation sterilization. Therefore the aim of this study is to determine the influence of gamma sterilization and various environmental conditions on the physiochemical characteristics of OCP. Electron spin resonance confirmed the presence of entrapped free radicals in the structural lattice of the OCP following irradiation. The inclusion of a 24 h annealing step following sterilization successfully reduced the number of free radicals generated during the process. Furthermore, this technique established that the degree of radiation-induced free radicals that reduced significantly over time following 1 year of accelerating aging. Other interesting findings included the results for X-ray diffraction, whereby it was established that selected samples had significant transformation from OCP to HAp. For example, the wt.% of OCP was reduced from 71 to 57, while the HAp content increased from 29 to 43. However, it was apparent from the results that the transformation occurred without any change in overall chemistry, that is, change in molar Ca:P ratio. This suggests that the overall conversion was related to a gradual shift of the Ca deficiency in the HAp phase. Overall, this study determined that although free radicals were detected in the OCP material following sterilization, this had no significant impact on the stability and performance of the material over time. Lastly, by exposing the OCP material to accelerated aging at 55°C and 60% relative humidity, this promoted the transformation rate of OCP to HAp.