Propriétés physico-chimiques et ostéogéniques d'un biociment hydraulique à base de phosphates de calcium
Biocements made from β-TCP - H3PO4 - H2O and β-TCP - MCPM - H2O mixtures were studied in order to obtain a better control of their setting time and mechanical strength (β-TCP = Ca3(PO4)2; MCPM = Ca(H2PO4)2.H2O). The effects of factors like the purity of the β-TCP powder, its particle size distribution, the carnet composition or the presence of additives were investigated. More fundamental studies were also done on the reactions controlling the setting time, i.e. β-TP dissolution and DCPD (CaHPO4.2H2O) precipitation. The influence of additives on the kinetics of these controlling reactions were studied in order to establish how they influenced the setting time. To complete these experiments, cements made from β-TCP - MCPM - CSH - H2O mixtures were implanted in rabbit tibias (CSH = CaSO4.l/2H2O). The aging behavior in vivo and in vitro were compared. Results show that many factors affect the physico-chemical properties of the cements, particularly setting time and tensile strength. However, these factors have only a very limited number of pathways in which they can act. For example, setting time can only change when either β-TCP dissolution rate, DCPD germination rate or interparticular free volume are modified. Tensile strength only depends on DCPD weight fraction (binder for β-TCP powder), porosity and microstructure. Sulfate, pyrophosphate and citrate ions delay the cement's setting time in the following order: sulfate < citrate < pyrophosphate. These three ions inhibit DCPD crystal growth in the same order. Therefore, these results suggest that all the ions which inhibit DCPD crystal growth are potential setting time delaying additives. The concentration range in which sulfate ions act on setting time is limited between 0 and 0.1 M; beyond this concentration sulfate ions precipitate as CSD (CaSO4.2H2O). As DCPD and CSD structures are nearly identical, the presence of CSD crystals speeds up the setting reaction by acting as seeds for DCPD crystals. This phenomenon provokes a decrease in the setting time and a refinement of the microstructure with a consequent increase in the tensile strength. Accurate control of the cement composition is very important, i.e. volume and concentration of the phosphoric acid solution. Results show that an excess of phosphoric acid provokes the recrystallization of DCPD into DCP, a process which strongly decreases tensile strength. A modification of the microstructure is also observed when initial β-TCP specific surface area is changed. An increase in the surface area gives an increase in the tensile strength and a decrease in the setting time. In our experiments, a fivefold increase of specific surface area decreases the setting time by a factor of three and doubles the tensile strength. These results show that β-TCP particle size distribution has a very strong effect on the physico-chemical properties of our cements. The comparison between in vitro and in vivo tests prove that in vivo results cannot be anticipated by in vitro experiments. However, in vivo results are extremely positive: our cement is biocompatible, bioresorbable and osteoconductive. These results show that after one month, our cements are closely bonded to living bone, and after four months, our cements are nearly completely resorbed and replaced by new bone (except for dense and large β-TCP particles).