Effects of Micropore Size Distribution in Carbonate Apatite Honeycomb Granules on Bone Replacement

Successful bone defect repair requires understanding both the role of micropores smaller than 10 μm and that of macropores. Although the effects of micropore volume have previously been reported, the influence of micropore size distribution remains unclear owing to the difficulty of independently varying the distribution of the total micropore volume. In this study, carbonate apatite (CAp) granules were synthesized from calcium sulfate (CS) and calcium hydroxide (CH) precursors, yielding distinct micropore size distributions while maintaining an equivalent overall micropore volume among the materials. The granules exhibited a honeycomb (HC) macrostructure that facilitated cellular and tissue infiltration. CS- and CH-derived CAp HC granules (CS-CAp and CH-CAp) exhibited identical carbonate content (12%), micropore volume (0.2 cm³/g), and macropore size (135 μm), differing only in their micropore size distribution. Specifically, CS-CAp exhibited three distinct modes in pore size distribution, with the first, second, and third most abundant peaks at ≈350, 900, and 7 nm, respectively, whereas CH-CAp showed peaks at ≈100, 200, and 20 nm. When implanted into critical-sized defects in rabbit femurs, CS-CAp induced more than fourfold greater new bone formation than CH-CAp at both 4 and 12 weeks. Although no significant difference in material resorption was observed at 4 weeks, CS-CAp showed significantly less residual material at 12 weeks. Moreover, CS-CAp was replaced predominantly by bone, whereas CH-CAp was primarily replaced by adipose tissue. These findings demonstrate that micropore size distribution determines the type and extent of tissue regeneration and the rate of material replacement, providing valuable insight for the micropore design of synthetic bone grafts.