Short-term in vivo evaluation of zinc-containing calcium phosphate using a normalized procedure

The effect of zinc-substituted calcium phosphate (CaP) on bone osteogenesis was evaluated using an in vivo normalized ISO 10993-6 protocol. Zinc-containing hydroxyapatite (ZnHA) powder with 0.3% by wt zinc (experimental group) and stoichiometric hydroxyapatite (control group) were shaped into cylindrical implants (2×6 mm) and were sintered at 1000 °C. Thermal treatment transformed the ZnHA cylinder into a biphasic implant that was composed of Zn-substituted HA and Zn-substituted β-tricalcium phosphate (ZnHA/βZnTCP); the hydroxyapatite cylinder was a highly crystalline and poorly soluble HA implant. In vivo tests were performed in New Zealand White rabbits by implanting two cylinders of ZnHA/βZnTCP in the left tibia and two cylinders of HA in the right tibia for 7, 14 and 28 days. Incorporation of 0.3% by wt zinc into CaP increased the rate of Zn release to the biological medium. Microfluorescence analyses (μXRF-SR) using synchrotron radiation suggested that some of the Zn released from the biomaterial was incorporated into new bone near the implanted region. In contrast with previous studies, histomorphometric analysis did not show significant differences between the newly formed bone around ZnHA/βZnTCP and HA due to the dissolution profile of Zn-doped CaP. Despite the great potential of Zn-containing CaP matrices for future use in bone regeneration, additional in vivo studies must be conducted to explain the mobility of zinc at the CaP surface and its interactions with a biological medium.