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

Carbonate apatite (CO3 Ap) blocks have attracted considerable attention as an artificial bone substitute material because CO3 Ap 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 CO3 Ap 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 CO3 Ap, 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 CO3 Ap, which retained the interconnected porous structure after the dissolution-precipitation reaction. The diametral tensile strength and porosity of the porous CO3 Ap were 1.8 ± 0.4 MPa and 55% ± 3.2%, respectively. Both porous and dense (control) CO3 Ap 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 CO3 Ap; this amount was five times greater than that obtained with dense CO3 Ap. At 12 weeks after surgery, for porous CO3 Ap, approximately 47% of the defect was filled with new bone as compared to 16% for dense CO3 Ap. Thus, the interconnected porous CO3 Ap block is a promising artificial bone substitute material for the treatment of bone defects caused by large fractures or bone tumor resection.