ULTRASTRUCTURAL AND ELECTRON-DIFFRACTION OF THE BONE-CERAMIC INTERFACIAL ZONE IN CORAL AND BIPHASIC CAP IMPLANTS

We investigated the influence of natural coral implants used as a bone substitute on the quality of bone ingrowth in rabbits 2, 3, and 6 wee ks after implantation. Explants were characterized by transmission ele ctron microscopy and electron diffraction. Bone ingrowth has been prev iously demonstrated by light microscopy, however, few have been perfor med in electron microscopy to compare mineralized tissue ingrowth in c oral implants which occurs at the expense of calcium carbonate to that of calcium phosphate (CaP) implants. The interface between coral arag onite and mineralized tissue or bone was abrupt, with no invasion of t he aragonite structure by newly formed crystals, as occurs in micropor es when biphasic CaP (BCP) ceramics were used. The restoring process a ppears to be different from that induced by BCP implants. Precipitatio n of needle-like apatite crystals on the CaCO3 implant surface was not observed. Instead, apatitic smooth-shaped crystals formed in aggregat es. The coral dissolution process does not release phosphate and so pr ecipitation of apatite does not occur in the micropores of the coral i mplant, thereby limiting the formation of an apatite layer and hence b one bonding to the outer surface of the implant. In addition, on the o uter surface of the implant, close to bone and a phosphorus source, th e CaP crystals that do form are in aggregates presumably due to the ca rbonate and mismatch between the aragonite and the apatite. This seems to result in a delayed bone attachment or weaker bone bonding than Ca P implants which encourage an epitaxial biological crystal deposition.