Comparative study on the sintered porous A‐type carbonate apatite, B‐type carbonate apatite, and hydroxyapatite

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AbstractThere are two types of carbonated apatite (cap): A‐type and B‐type, where carbonate ions substitute for hydroxyl sites in A‐CAp and phosphate sites in B‐CAp. Despite the known capacity of CAp in supporting the activities of bone cells, the distinct potential of A‐CAp and B‐CAp as porous bone grafts is unclear. This study aimed to fabricate porous A‐CAp and B‐CAp and evaluate their biological activities in vitro and in vivo as compared to stoichiometric hydroxyapatite (HAp). Gelatin‐casting and freeze‐drying followed by sintering were used to produce A‐ and B‐CAp with high porosity ∼70%, pore sizes ∼100 µm, and high strength < 3 MPa. The carbonate contents of A‐ and B‐CAp were 3.83 and 4.5 wt%, respectively, as determined by thermal gravimetric analysis and calculations based on thermal decomposition reactions. A‐ and B‐CAp dissolved faster than HAp. A‐CAp had a higher affinity for albumin but poorer osteoblastic cell attachment, while both A‐ and B‐CAp had lower proliferation rates than HAp. After 2 weeks implantation in vivo, A‐CAp and HAp showed limited bone formation, whereas B‐CAp demonstrated bone ingrowth. Notably, A‐CAp showed enhanced osteoclast maturation. Both A‐CAp and B‐CAp were degradable after 8 weeks post‐implantation. Structural features and carbonate substitution types elicit different output on physicochemical, osteoconductivity, and degradability.