Feasibility evaluation of low-crystallinity β -tricalcium phosphate blocks as a bone substitute fabricated by a dissolution–precipitation reaction from α -tricalcium phosphate blocks

Although sintered β-tricalcium phosphate blocks have been used clinically as artificial bone substitutes, the crystallinity of β-tricalcium phosphate, which might dominate biocompatibility, is extremely high. The objective of this study is to evaluate the feasibility of fabricating low-crystallinity β-tricalcium phosphate blocks, which are expected to exhibit good biocompatibility via a dissolution–precipitation reaction of α-tricalcium phosphate blocks as a precursor under hydrothermal conditions at 200°C for 24 h. Although β-tricalcium phosphate is a metastable phase, the presence of Mg²⁺ in the reaction solution inhibits the formation of its corresponding stable phase and induces β-tricalcium phosphate formation under acidic conditions. It was found that low-crystallinity β-tricalcium phosphate blocks could be fabricated from α-tricalcium phosphate blocks immersed in 1.0 mol/L MgCl₂ + 0.1 mol/L NaH₂PO₄ solution while maintaining the shape of the α-tricalcium phosphate blocks. The crystallite size of the fabricated β-tricalcium phosphate blocks was 42 nm, which was substantially smaller than that of the sintered β-tricalcium phosphate blocks. When the fabricated β-tricalcium phosphate blocks were implanted into bone defects in rabbit femurs, they exhibited excellent tissue responses. In particular, the initial osteoconductivity (two and four weeks) was substantially greater than that of sintered β-tricalcium phosphate blocks.