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

Macroporous biphasic calcium phosphate (MBCP) blocks were implanted into ra bbit trabecular bone and muscle, recovered 18 weeks later, and then observe d and analyzed by transmission electron microscopy (TEM), electron diffract ion, and electron microprobes. The results showed that(1) apatitic microcry stals appeared by secondary nucleation in both bone and muscle sites; (2) p recipitated microcrystals were aggregated around ceramic crystals in bone s ites but distributed randomly and without orientation in micropores in musc le sites; (3) the ratio of calcium to phosphorus was higher for microcrysta ls in bone than muscle sites; and (4) precipitated microcrystals around bet a-tricalcium phosphate (beta-TCP) crystals were less aggregated and dense t han those around hydroxyapatite (HA). These findings suggest that microenvi ronmental parameters such as fluid circulation and the interaction of ceram ics with proteins or cells affect the physicochemical dissolution/reprecipi tation process. Epitaxic growth of apatitic microcrystals seems more favora ble from HA than beta-TCP.