Electron microscopic study on bone formation and bioresorption after implantation of β-tricalcium phosphate in rabbit models

The role of bone formation and bioresorption in an early stage after implantation of beta-tricalcium phosphate (beta-TCP) was investigated using scanning and transmission electron microscopy (SEM, TEM). The ceramic beta-TCP cylinders were implanted into cavities drilled in the femoral condyles of eight NZW rabbits. Four of the rabbits were sacrificed at 2 weeks and four at 4 weeks after implantation, respectively. The femoral condyles were excised to prepare the specimens for SEM and TEM. SEM showed giant cells of more than 20 mu m in diameter were observed on the surface of beta-TCP at 2 weeks after implantation. TEM demonstrated that collagen fibrils secreted from the monocytic cells invaded beta-TCP micropores at 2 weeks. Multinucleated giant cells (MNGCs) were in contact with the surface of beta-TCP at 2 weeks. Some of them had a ruffled border (RB) at the cell-substrate interface, characteristic of osteoclasts. These findings suggest that cell-mediated disintegration by osteoclasts played a role in the bioresorption of beta-TCP at an early stage after implantation. In addition, the micropores of beta-TCP ceramic may provide an environment for collagen formation, leading to the deposition of apatite crystals. Therefore, the micropores facilitate bone ingrowth as well as ceramic resorption.