The cooperative effect of size and crystallinity degree on the resorption of biomimetic hydroxyapatite for soft tissue augmentation

Two kinds of hydroxyapatite (HA) crystals were studied as subcutaneous fillers in order to evaluate how their different degree of crystallinity and dimensions influence in vivo resorption. By appropriately adjusting the synthesis temperature, poorly crystalline HA (HApc) and highly crystalline HA (HAhc) were synthesized into clusters of needle-shaped crystals of about 50 nm and plate-shaped crystals of about 100 nm, respectively. The clusters of HApc had larger dimensions (30 µm) than those of HAhc (3 µm). Subcutaneous in vivo inoculations were performed in ten 6-month-old FVB female mice. HAhc underwent complete macroscopic resorption already 4 weeks after the implantation while HApc still showed agglomerates at the eighth week. This unexpected finding may be ascribed to the different size and morphology of the HAhc nanocrystals responsible for a lower aggregation and microcluster dimension than HApc nanocrystals.