Effect of sintering temperature rise from 870 to 920 °C on physicomechanical and biological quality of nano-hydroxyapatite: An explorative multi-phase experimental in vitro/vivo study

Hydroxyapatite (HA) is a proper scaffold for bone repair, however, it is not of excellent mechanical properties. Most previous studies on the effect of temperature increases were in vitro and had assessed merely improvements of HA's physicomechanical quality. This in vitro/vivo study investigated the effect of temperature increases from 870 to 920 °C on physicomechanical and biological quality of Nano-HA. Forty experimentally produced HA disks sintered at 870 to 920 °C were prepared (n = 20 × 2). Disks were subjected to Vickers microindentation test (1 disk from each group divided into 4 quarters), Fourier transform infrared spectroscopy (1 disk), X-ray diffraction (XRD) [1 disk together with non-sintered HA], field emission scanning electron microscopy (FSEM, 1 disk from each group together with non-sintered HA), cell seeding and SEM assessment (2 disks), MTT assay over 4 different time periods (16 quadrants of 4 disks from each group), 6 one-thirds of 2 disks from each group for immunocytochemical (ICC) assay, and 8 disks from each group [as well as non-sintered HA] for the animal study (implantation in 4 sockets in 8 rabbits [32 specimens], histomorphometry, and computerized tomography) over two time periods. Quantitative data were analyzed statistically (α = 0.05). Vickers microhardness increased from 63.7 ± 11.9 in the 870 group to 153.4 ± 104.7 in the 920 group (P = 0.057). XRD indicated more regular crystal patterns in sintered groups compared to non-sintered nanoHA. FSEM showed larger crystals in the 920 group compared to 870 and non-sintered nanoHA. Expression of osteocalcin, osteonectin, and RUNX2 genes were more visible in ICC samples of the 920HA group. In MTT, cell numbers increased in all groups significantly (P = 0.000), with no between-group differences (P > 0.3). In rabbit experiments, the extent of 'newly formed bone' increased significantly over time (two-way ANOVA, P = 0.000), reaching 39.5%, 46.4%, and 77.5% in the groups non-sintered HA, 870, and 920, respectively. The 920 °C-sintered nanoHA induced the highest bone formation (P = 0.000). Increasing the temperature of nanoHA sintering from 870 to 920 °C can improve its physicomechanical properties and bone formation potential.