Improvement of strength and porosity of β-TCP/HA composited bone filling material synthesized by Hydrothermal Hot-Pressing

At present, it is expected that we can use the β-tricalcium phosphate (β-Ca₃(PO₄)2, β-TCP) bone graft material with good bio absorbability for the treatment of bone defects. However, there is a problem that the strength of β-TCP applied clinically is low. It is because a porous body having a porosity of 70 % or more is required to speed up the reformation of the affected area in the scenes of medical treatment. The wet method is mainly used in the conventional β-TCP synthesis. Although the method can easily control porosity and pore shape, there is a limit to improvement of its strength. Therefore, we studied in order to synthesize a bone filling material that maintains a compressive strength of 10 MPa while having a porosity of 70 % by the Hydrothermal Hot-Pressing (HHP) method. Since the HHP makes a sample denser and generates pores at the same time, it is possible to synthesize a porous and high-strength sample. Furthermore, improvement in strength is expected by compositing Hydroxyapatite (Ca₁₀(PO₁₄)₆(OH)₂, HA) with β-TCP synthesis step. Whereas, although there are cases that β-TCP is composited in the HA sintered body in the previous study, there is no reports of compositing HA with β-TCP by the HHP method. Dividing the synthesis process of β-TCP into three steps of HHP, pre-sintering and sintering, we established the synthesis method of β-TCP / HA composited by controlling the reaction that occurs in each process. In addition, controlling the amount of water generated in each reaction, we reduced several millimeters of crucks to several micrometers order, and the porosity and strength were greatly improved. As a result, we were able to obtain sample with a porosity of 49 % and a compressive strength of 25 MPa. However, it was difficult for the molded body to maintain high strength for a long time, because cracks generated during sintering grew as time passes and it became powder (powdering). The sintered body kept its shape just after the sintering, but powdering occurred in seven days. In addition, powdering is more likely to occur as the porosity is higher. Thus we have to solve this problem to synthesize a sample with higher porosity. We analyzed the composition of the sintered body by XRD in order to investigate the cause of powdering. In consequence it became clear that α-tricalcium phosphate (α-Ca₃(PO₄)₂,a-TCP) is mixed in and disappears in 7 days. α-TCP is the high temperature phase of β-TCP. It is known to be soluble in water and to hydrolyze into HA. Thus, it is considered that powdering occurs in the following process. First, as the crystallinity of the synthesized at the time of HHP is lower, the reactivity in pre-sintering and sintering increases, and α-TCP is formed at a relatively low temperature. Second, powdering proceeds as α-TCP generated during sintering gradually hydrolyzes. Based on this result, we tried suppressing the formation of α-TCP. As the result of controlling the crystallinity of the synthesized at the time of HHP, we discovered the synthesis conditions under which pulverization was suppressed. Consequently, we succeeded in synthesizing the sintered body that has a porosity of 59.3% and a compressive strength of 10.4 MPa. In the future, if the pore shape is controlled by changing the pre-sintering and sintering conditions, it is expected that the porosity and the compressive strength can be increased.