Substrate geometry directs the in vitro mineralization of calcium phosphate ceramics

Repetitive concavities on the surface of bone implants have recently been demonstrated to foster bone formation when implanted at ectopic locations in vivo. The current study aimed to evaluate the effect of surface concavities on the surface mineralization of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) ceramics in vitro. Hemispherical concavities with different diameters were prepared at the surface of HA and β-TCP sintered disks: 1.8 mm (large concavity), 0.8 mm (medium concavity) and 0.4 mm (small concavity). HA and β-TCP disks were sintered at 1100 or 1200 °C and soaked in simulated body fluid for 28 days at 37 °C; the mineralization process was followed by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction and calcium quantification analyses. The results showed that massive mineralization occurred exclusively at the surface of HA disks treated at 1200 °C and that nucleation of large aggregates of calcium phosphate started specifically inside small concavities instead of on the planar surface of the disks. Regarding the effect of concavity diameter size on surface mineralization, it was observed that small concavities induce 124- and 10-fold increased mineralization compared to concavities of large or medium size, respectively. The results of this study demonstrated that (i) in vitro surface mineralization of calcium phosphate ceramics with surface concavities starts preferentially within the concavities and not on the planar surface, and (ii) concavity size is an effective parameter to control the spatial position and extent of mineralization in vitro.