Contribution à l'élaboration de biomatériaux phosphocalciques à usage médical : étude des phénomènes de prise et de durcissement de ciments hydrauliques à base de phosphate de calcium

Doctorat en sciences appliquées -- UCL, 199

Calcium phosphate cements: study of the beta-tricalcium phosphate--dicalcium phosphate--calcite cements.

The setting and strengthening properties of beta-tricalcium phosphate (beta-TCP)--dicalcium phosphate dihydrate (DCPD)--calcite blends upon admixture with water were investigated at 25 and 37 degrees C. Setting was accelerated by seeding the system with hydroxyapatite (HAp), and strengthening improved when the solids were mixed with a solution saturated with DCPD and HAp. The relationship strength versus ageing time in wet conditions was correlated with mineralogical changes of the material. X-Ray diffraction, thermal analysis and scanning electron microscopy observations showed that DCPD and calcite react together to form small HAp crystals acting as bridges between the beta-TCP aggregates present in the paste. Both gaseous CO2 released by the reaction of calcite and the conversion of lower (DCPD, calcite) to higher-density phases (HAp) contributed to increase the porosity of the material. Nevertheless, quite acceptable diametral strengths (around 1.5 MPa) could be achieved, despite the high porosity of the hardened product (up to 54 vol%). After exhaustion of DCPD, calcite can react with beta-TCP to form further HAp, but this process is detrimental to the strength of the material. Both the mineralogical transformations, and the strengthening of the material were accelerated considerably upon increasing the ageing temperature

Calcium phosphate cements: action of setting regulators on the properties of the beta-tricalcium phosphate-monocalcium phosphate cements.

Various additives were tested as setting retarders of the beta-tricalcium phosphate-monocalcium phosphate monohydrate (beta-TCP-MCPM) cements. Calcium pyrophosphate (CPP), calcium sulphate dihydrate (CSD) and calcium sulphate hemihydrate (CSH) were found to increase the setting time from 30 s to about 10 min. Moreover, the use of CSH resulted in a marked increase of the final diametral strength of the cement, which could be raised from 1 MPa to about 3 MPa. The best results were obtained when CSH and CPP were added together to the cement, while the addition of CSD and CPP alone only retarded the setting, without improving the final strength. A particular cement composition (64 wt% beta-TCP, 16 wt% MCPM, 15 wt% CSH and 5 wt% CPP), selected for its optimum final strength, was aged in vitro for 8 d at 37 degrees C in saline solution (0.9 wt% NaCl in water). After a moderate decrease, the diametral strength of the specimen was found to level off at about 60% of its initial value (3.2 MPa), for ageing times beyond 1 d. This behaviour has been ascribed to the progressive dissolution of the CSD fraction of the hardened cement, compensated by the crystallization of further amounts of DCPD

Calcium phosphate cements: study of the beta-tricalcium phosphate--monocalcium phosphate system.

The possibility of making cements based on beta-tricalcium phosphate (beta-TCP), a promising bone graft material, was investigated. Upon admixture with water, beta-TCP/monocalcium phosphate monohydrate (MCPM) mixtures were found to set and harden like conventional hydraulic cements. Beta-TCP powders with larger particle size, obtained by sintering at higher temperatures, increased the ultimate strength of the cement. Results show that setting occurs after dissolution of MCPM, as a result of the precipitation of dicalcium phosphate dihydrate (DCPD) in the paste. The ultimate tensile strength of the hardened cement is proportional to the amount of DCPD formed. Upon ageing above 40 degrees C, DCPD transforms progressively into anhydrous dicalcium phosphate (DCP), thereby decreasing the strength. Ageing of the pastes in 100% r.h. results in a decay of the mechanical properties. This can be ascribed to an intergranular dissolution of the beta-TCP aggregates as a result of the pH lowering brought about by the MCPM to DCPD conversion

Calcium phosphate cements: effect of fluorides on the setting and hardening of beta-tricalcium phosphate-dicalcium phosphate-calcite cements.

Increasing amounts of fluoride ions have been found to increase the setting and hardening rates of beta-tricalcium phosphate-dicalcium phosphate dihydrate-calcite cements. Thus, fluoride-containing cements reached a diametral strength of about 1.5 MPa after 15 d, but the fluoride-free reference sample reached only 0.45 MPa. The acceleration of setting and hardening is correlated to an increased rate of hydroxyapatite formation in the cement at the expense of dicalcium phosphate dihydrate and calcite. Adding monocalcium phosphate monohydrate and fluoride to the blends resulted in a marked decrease of their setting time from about 1 h down to 8 min, without greatly affecting their final strength

Bone repair of defects filled with a phosphocalcic hydraulic cement: an in vivo study

A quickly setting calcium phosphate-based hydraulic cement mixed with particles of tricalcium phosphate (TCP) ceramic was implanted in 56 metaphysial defects made in the long bones of ten adult mongrel dogs. Microradiography, histology and scanning electron microscopy (SEM) demonstrated the slow resorption of the cement and the bony incorporation of the calcium phosphate ceramic particles which were consistently embedded in bone. The original structural pattern of the bone tended to be restored 7 months after implantation. The cement did not hinder the incorporation of the calcium phosphate ceramic particles, neither did it elicit any inflammatory or foreign-body response. The cement was easily shaped and allowed a perfect filling of any defect, resulting in close contact of the whole implant surface with the host bone at the time of surgery, associated with appreciable mechanical strength. Most of the practical problems associated with the use of calcium phosphate ceramics in the repair of bone defects could be overcome with the cement

Evaluation of tricalciumphosphate/hydroxyapatite cement for tooth replacement; an experimental animal study.

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