Fabrication and evaluation of interconnected porous carbonate apatite from alpha tricalcium phosphate spheres

Carbonate apatite (CO₃Ap) blocks have attracted considerable attention as an artificial bone substitute material because CO₃Ap is a component of and shares properties with bone, including high osteoconductivity and replacement by bone similar to autografts. In this study, we fabricated an interconnected porous CO₃Ap block using α-tricalcium phosphate (TCP) spheres and evaluated the tissue response to this material in a rabbit tibial bone defect model. Interconnected porous α-TCP, the precursor of interconnected porous CO₃Ap, could not be fabricated directly by sintering α-TCP spheres. It was therefore made via a setting reaction with α-TCP spheres, yielding interconnected porous calcium-deficient hydroxyapatite that was subjected to heat treatment. Immersing the interconnected porous α-TCP in Na–CO₃–PO₄ solution produced CO₃Ap, which retained the interconnected porous structure after the dissolution–precipitation reaction. The diametral tensile strength and porosity of the porous CO₃Ap were 1.8 ± 0.4 MPa and 55% ± 3.2%, respectively. Both porous and dense (control) CO₃Ap showed excellent tissue response and good osteoconductivity. At 4 weeks after surgery, approximately 15% ± 4.9% of the tibial bone defect was filled with new bone when reconstruction was performed using porous CO₃Ap; this amount was five times greater than that obtained with dense CO₃Ap. At 12 weeks after surgery, for porous CO₃Ap, approximately 47% of the defect was filled with new bone as compared to 16% for dense CO₃Ap. Thus, the interconnected porous CO₃Ap block is a promising artificial bone substitute material for the treatment of bone defects caused by large fractures or bone tumor resection.