Ultrastructural study of apatite precipitation in implanted calcium phosphate ceramic: influence of the implantation site.

Macroporous biphasic calcium phosphate (MBCP) blocks were implanted into rabbit trabecular bone and muscle, recovered 18 weeks later, and then observed and analyzed by transmission electron microscopy (TEM), electron diffraction, and electron microprobes. The results showed that (1) apatitic microcrystals appeared by secondary nucleation in both bone and muscle sites; (2) precipitated microcrystals were aggregated around ceramic crystals in bone sites but distributed randomly and without orientation in micropores in muscle sites; (3) the ratio of calcium to phosphorus was higher for microcrystals in bone than muscle sites; and (4) precipitated microcrystals around beta-tricalcium phosphate (beta-TCP) crystals were less aggregated and dense than those around hydroxyapatite (HA). These findings suggest that microenvironmental parameters such as fluid circulation and the interaction of ceramics with proteins or cells affect the physicochemical dissolution/reprecipitation process. Epitaxic growth of apatitic microcrystals seems more favorable from HA than beta-TCP.