[Osseointegration of 2 different types of calcium phosphate materials: ceramics and ionic cements]

Different kinds of calcium phosphate biomaterials can be used as bone substitutes. Ceramics are constituted by HA or TCP grains linked by grain boundaries. Their porosity depends on the powder characteristics and the sintering temperature. It can be very low with a pore size inferior to one micron. The setting of calcium phosphate hydraulic cements results from the precipitation of a calcium phosphate phase different from the one in suspension in the paste. The strength of the cement is given by the entanglement of the growing mineral crystals. Calcium phosphate hydraulic cements and ceramics have very different physico-chemical characteristics. We have studied the histological integration of both kinds of material. The first material was constituted by macroporous ceramics composed of 75% HA and 25% beta-TCP, the cement was made of beta-TCP grains dispersed in a DCPD matrix. The sequence of events which leads to the ceramic integration is always the same: a/ ingrowth of a loose connective tissue; b/ osteoblast differentiation from fibroblast-like cells of the connective tissue in close proximity to the implant surface; c/ osteoid synthesis at the ceramic surface toward the pore center; d/ remodeling of the immature bone and the ceramic itself. The cement is differently integrated. The osteoblasts differentiate at some distance from the implant and there is a trabeculae ingrowth toward the material.

The early stages of both materials osteointegration are different. The integration is centrifugal for ceramics and centripetal for the cement.