Fabrication and mechanical properties of calcium phosphate cements (CPC) for bone substitution

Calcium phosphate cements have been used in medical and dental applications for many years. However, their low strength and their high brittleness prohibit their use in many stress-bearing locations, which would require an improvement in mechanical properties. The influence of microstructural parameters on the latter has nevertheless barely been investigated in a systematic manner. To this aim, apatite cements have been fabricated through α-TCP (α-tricalcium phosphate) hydrolysis, and their mechanical properties have been measured (Young's modulus, fracture toughness and compressive strength), as a function of various parameters (particle size, liquid-to-powder ratio, amount and morphology of porosity, including macropores created by mannitol particles used as porogen). Five days following the mixing of phases, identification and microstructural observation indicated the presence of unreacted α-TCP particles, exhibiting very weak links with the apatite matrix and often surrounded by microcracks. The latter provoke a decrease in Young's modulus. The coarser the microstructure, the larger the critical flaw size causing fracture. In the case of macroporous materials, the critical flaw size increases with macroporosity. The knowledge gained should allow, in the end, to improve mechanical properties by controlling the microstructure, and to find a better compromise between strength and biological behaviour.